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High-cycle fatigue properties of two ferritic-pearlitic medium-carbon forging steels 30MnVS and 49MnVS with smooth and notched (stress concentration factor Kt=2.1 and 3.9) specimens were studied by using rotating bending fatigue and fatigue crack growth (FCG) rate testing. Structural steels 40Cr and 30MnVS with quenched & tempered microstructures (40Cr-QT, 30MnVS-QT) were used for comparison. It was found that both fatigue strength and fatigue strength ratio at each Kt of the as-forged 30MnVS steel (30MnVS-AF), which are comparable to those of 30MnVS-QT and 40Cr-QT, are notably higher than those of the as-forged 49MnVS steel (49MnVS-AF), and this difference tends to decrease with an increase in Kt. However, 30MnVS-AF exhibits slightly higher fatigue notch sensitivity and FCG rate than 49MnVS-AF, and the steels with ferritic-pearlitic microstructure also have higher fatigue notch sensitivity than the steels with QT microstructure. A simple and practical method which incorporates simple microstructural parameters, and hardness is proposed to estimate the smooth fatigue strength (σw) of ferritic-pearlitic forging steels, while notched fatigue strength can be expressed by a power function of Kt with its coefficient equals to σw, which can be further classified into two groups corresponding to forging steels with relatively lower and higher C contents, respectively.

It is often unavoidable that automotive steel sheet parts might undergo plastic deformation during forming and service life. The present research explored the effect of pre-strain up to 15% on hydrogen embrittlement (HE) behavior of a cold-rolled and intercritically annealed Al-containing medium-Mn steel by electrochemical hydrogen charging and permeation testing, thermal hydrogen desorption spectrometry and slow strain rate tensile testing. The results show that the primary microstructural changes after pre-strain are the significant increase in dislocation density and the transformation of a slight part of retained austenite into martensite, which cause a decrease in hydrogen diffusion coefficient and a significant increase in absorbed reversible hydrogen concentration. Therefore, the HE susceptibility increases notably with an increase in pre-strain. It is thus concluded that pre-straining not only enhance the strength level but also remarkably increase the HE susceptibility of medium-Mn steel.

To obtain a refined and homogenized microstructure as well as enhanced combination of strength and toughness, the influence of controlled forging on the microstructure and mechanical properties of a Mn–Cr-type bainitic forging steel microalloyed with V + Ti elements was explored. Microstructural characterization shows that the final microstructure of the as-forged steel changed from primarily granular bainite (GB) to primarily lower lath-bainite (LLB) through controlled forging of mainly finish-forging temperature and post-forging cooling rate. The result of physical–chemical phase analysis shows that almost all the added Ti is in the (V,Ti)(C,N) precipitates, while only ~ 7% of the added V is in the precipitates, and there is only a slight increase in the amount of the (V,Ti)(C,N) precipitates with changing forging condition, and their precipitation strengthening is ~ 40 MPa. The tensile properties except the total elongation increase significantly from the GB sample to the LLB sample, and the latter exhibits identical or even higher mechanical properties than conventional quenched and tempered 42CrMo steel except the yield strength. Instrumental impact tests revealed that the crack initiation energy, propagation energy and the ability of arresting crack growth of the LLB sample are significantly higher than those of the GB sample. The reduction and/or even elimination of large blocky M/A, the increase in high-angle boundaries fraction and microstructure refinement are primarily responsible for this enhancement of doubled impact toughness. It is thus suggested that suitable controlled forging is a promising strategy to regulate and control the microstructure and mechanical properties of bainitic forging steel.

The microstructure and mechanical properties of a V-microalloyed Al-containing medium-Mn steel after hot rolling and intercritical annealing (IA) are explored. The tested steel exhibits a fine multiphase microstructure consisting of bimodal sizes of ferrite and retained austenite plus considerable amount of fine VC and/or M3C precipitates. Physical–chemical phase analysis shows that about 71.0% of the total V is in VC phase and more than 93% of VC particles is less than 5 nm. The calculated precipitation strengthening values of VC are ~ 347 and ~ 234 MPa for the specimens intercritically annealed at 625 and 750 °C, respectively. An excellent combination of strength and ductility as high as ~ 50 GPa% and yield strength (YS) of 890 MPa was obtained at intercritical temperature (TIA) of 725 °C, although it does not correspond to the maximum precipitation strengthening of VC phase. Therefore, it is suggested that an optimization of TIA corresponding to both excellent combination of strength and ductility and high YS should be further explored through chemical composition and IA process optimization.

Microstructural characteristics and tensile behavior of a novel hot-rolled medium-Mn steel, 0.25C-8.5Mn-0.5Si-2.5Al (wt.%), under different intercritical annealing temperatures (TIA) were evaluated. The results show that an ultrafine duplex lamellar microstructure with a maximum volume fraction of retained austenite (RA) of 65 vol.% was obtained at TIA of 750 °C. It was found that both the stability of the RA and its stability range play critical role in controlling the sustainable TRIP effect. The product of ultimate tensile strength and total elongation as high as ~ 66 GPa% was obtained at TIA of 725 °C, which is much higher than those of previously investigated medium-Mn steels with similar Mn contents, indicating a great potential for the tested steel. The analysis of the fracture surface revealed that the steel exhibited a kind of micro-delamination fracture, which was found to be primarily nucleated at the interfaces of the transformation-induced martensite and ferrite, and the micro-delamination fracture became more significant with increasing TIA.

Automobile steel sheet parts often experience unavoidable plastic deformation during cold stamping forming and service life. In this study, we explored the microstructure variation and tensile deformation behaviour of a cold-rolled Al-containing medium Mn steel (0.25C–8.67Mn–0.54Si–2.69Al, wt%) subjected to different degrees of pre-straining by tensile testing, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction and X-ray diffraction. The cold-rolled steel sheet processed by intercritical annealing at 725 °C for 60 min exhibited a superior combination of strength and ductility [the product of ultimate tensile strength (UTS) and total elongation (TEL), UTS × TEL] as high as 69 GPa% due to the TWIP (twinning-induced plasticity) and TRIP (transformation-induced plasticity) plasticity-enhancing mechanism during tensile deformation. The tensile behaviour, which originally exhibited notable Luders plus Portevin–Le Châtelier (PLC) banding phenomenon, was significantly changed by pre-straining. The strength increased, while the TEL and UTS × TEL decreased linearly with an increase in the pre-strain. A pre-strain equalling to the Luders strain, i.e. 3%, could cause an elimination of the Luders bands, while it had almost no influence on the PLC bands. The increase in the dislocation density, the enhanced stability of retained austenite and the strain-induced martensite and mechanical twins caused the reduction and elimination of the Luders bands with increasing amounts of pre-strain.

The present investigation attempts to evaluate the influence of intercritical annealing temperature (TIA) on the hydrogen embrittlement (HE) of a cold-rolled Al-containing medium-Mn steel (Fe-0.2C-4.88Mn-3.11Al-0.62Si) by using electrochemical hydrogen-charging, slow strain rate tensile test and scanning electron microscope. The results show that an excellent combination of strength and ductility (the product of ultimate tensile strength and total elongation) up to ∼53 GPa·% was obtained for the specimen intercritically annealed at an intermediate temperature of 730 °C, whereas the HE index increases significantly with an increase in TIA up to 850 °C. Being different from the typical dimple ductile fracture for the uncharged specimen, the hydrogen-charged specimen exhibits a mixed brittle interface decohesion and ductile intragranular fracture mode in the crack initiation region and the brittle fracture fraction increases with increasing TIA. Both the stability and amount of austenite play a critical role in governing the HE behavior of TRIP-assisted medium-Mn steel. Thus, it is suggested that suitable TIA should be explored to guarantee the safety service of automotive parts made of this type of steel in addition to acquiring excellent mechanical properties.

High-cycle fatigue behavior of vacuum-carburized 20Cr2Ni4 steel with three different case depths was studied by using rotating bending fatigue test. The results show that fatigue strength exhibits an increase–decrease pattern with increasing effective case dept, which is regarded to be mainly due to the unavoidable change in other parameters such as residual stress and grain size in case layer besides case depth. The present data and data in the available literature suggest that a dimensionless parameter of relative case depth is more appropriate to reflect the influence of case depth on fatigue strength. Further fracture surface analysis revealed that the fatigue failure was dominated by surface crack initiation mainly due to the extremely high cleanness of the tested steel with inclusion control. It is thus concluded that both relative case depth and other parameters should be comprehensively considered in obtaining optimized fatigue performance of vacuum-carburized steel.

This investigation attempted to enhance the resistance to hydrogen embrittlement (HE) of an Al-containing medium-Mn steel through heavy warm rolling (WR) directly after intercritical annealing. It is found that the HE index expressed by total elongation loss drops from ~78% to ~28% after heavy WR. This significantly enhanced resistance to HE is primarily associated with the well developed lamellar structure consisting of lathy ferrite and retained austenite after WR. It is thus suggested that WR is a promising and feasible approach to significantly enhance the HE resistance of medium-Mn steel while maintaining excellent strength-ductility balance.

The present work was attempted to evaluate the influence of reverted austenite (RA) fraction on hydrogen embrittlement (HE) of a hot rolled transformation-induced plasticity (TRIP)-aided medium Mn steel (0.1C-5Mn) by using electrochemical charging, slow strain rate tensile (SSRT) test and thermal desorption spectrometry (TDS) analysis. Different volume fractions of RA (~10–30 vol%) in the tested steel sheet were obtained by changing intercritical annealing time. The result of TDS analysis demonstrated that the charged hydrogen is primarily diffusible hydrogen corresponding to low-temperature hydrogen desorption peak and its concentration increases almost linearly with an increase in the volume fraction of RA. It was found that the intercritically annealed specimen exhibits an increasing susceptibility to HE with an increase in the volume fraction of RA primarily due to the increased RA transformation to martensite during tensile deformation. It is thus regarded that the TRIP effect is harmful to the HE resistance of TRIP-aided steels, and therefore both the mechanical stability and amount of RA have a strong influence on the HE behavior of TRIP-aided medium Mn steel.

The present study was attempted to evaluate the effect of tempering treatment on the hydrogen embrittlement (HE) behavior of a novel bainitic rail steel by using slow strain rate test (SSRT) with notched round bar specimens. The microstructure of the as-hot rolled rail steel mainly consists of granular bainite and ~9 vol% martensite. It was found that tempering treatment could increase the HE resistance and decrease the susceptibility to HE of the as-hot rolled bainitic rail steel except at temper embrittlement temperature of ~400 °C, and excellent HE properties could be obtained when tempered at 500 °C at less expense of strength and ductility. The enhanced HE properties are ascribed mainly to the gradual decomposition of blocky M/A constituents as well as the tempering of the martensite. It is thus suggested that suitable tempering after hot rolling could be applied to enhance the HE properties as well as to obtain maximum mechanical properties, and thus to guarantee the safety of bainitic steel rails in service.

The susceptibility to hydrogen embrittlement (HE) of warm-rolled medium Mn steel (0.2C-5Mn-0.6Si-3Al) with different microstructure characteristics obtained by changing intercritical annealing time at 750 °C was investigated by carrying out slow strain rate test and hydrogen thermal analysis on electrochemically hydrogen charged samples. The results show that the increase in intercritical annealing time leads to an increase of the susceptibility to HE regardless of the decrease of both strength and diffusible hydrogen content. Unlike that of typical ductile fracture characteristic of uncharged sample, the hydrogen-charged sample shows a mixed fracture characteristic of ductile intragranular and brittle interface decohesion fracture mode in the crack initiation region. The susceptibility to HE was correlated mainly to the mechanical stability of retained austenite. These results suggest that suitable intercritical annealing treatment should be performed to ensure safety service of components made of this kind steel besides obtaining excellent combination of strength and ductility.

The present study was attempted to evaluate the microstructural evolution and mechanical properties of a 3 wt%Al containing medium Mn steel which was warm rolled with different thickness reductions directly after intercritical annealing at 750 °C by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction and tensile tests. The results show that the laths of ferrite and retained austenite (RA) not only gradually rotated their longitudinal axis to the rolling direction but also were refined with increasing rolling thickness reduction, and thus a well developed lamellar structure of bimodal size distributed lathy ferrite and RA was obtained after heavy rolling reduction. Though the strength increases while the ductility decreases with increasing rolling thickness reduction, an excellent combination of strength and ductility expressed by the product of ultimate tensile strength (UTS) to total elongation (TEL) of ~ 50 GPa% was obtained through a wide process window of warm rolling. It is thus proposed that warm rolling is a promising way to simplify the traditional multi-stage rolling and annealing processes of Al-containing medium Mn steels, especially to overcome the cold rolling difficulty of medium Mn steels with high carbon content.

Mechanical properties of a newly developed microalloyed bainitic steel were investigated after the hot forging, air cooling and tempering process. The microstructure of the as-forged bainitic steel mainly consists of granular bainite and ∼20 vol. % martensite. The fraction of retained austenite remains unchanged until tempering at 200 °C, above which it decreases significantly. The increase of tempering temperature leads to decreases of both ultimate tensile strength and total elongation but decreases of both yield strength and reduction of area. The maximum and minimum values of impact toughness were observed after tempering at around 200 and 400 °C, respectively. These effects are mainly attributed to the decomposition of martensite/austenite constituents and the tempering effects in martensite. The tempering of the forged bainitic steel at around 200 °C results in an excellent combination of strength and toughness, which is comparable to that of the conventional quenched-and-tempered 40Cr steel. Therefore, low-tempering treatment coupled with post-forging residual stress relieving is a feasible method to further improve the mechanical properties of the bainitic forging steel.

The present study was attempted to evaluate the hydrogen embrittlement (HE) behavior of two high strength rail steels with pearlitic and bainitic microstructures by using slow strain rate test (SSRT) with notched round bar specimens. The results show that the bainitic rail steel with a mixed microstructure of bainite, tempered martensite and M/A constituents exhibits superior mechanical properties in comparison with the pearlitic rail steel. The results of SSRT revealed that the bainitic rail steel is more susceptible to HE than the pearlitic rail steel, which is ascribed mainly to the higher strength level and the microstructural characteristics of the bainitic rail steel. The attempt of re-tempering treatment of the as-received bainitic rail steel exhibits no notable improvement in the resistance to HE at no expense of strength. It is thus suggested that further efforts concerning obtaining low susceptibility to HE besides obtaining excellent combination of strength and toughness should be conducted to guarantee the safety service of the bainitic steel rails.

The thermal stability of retained austenite (RA) and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500 °C were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile testing. The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400 °C. This caused a slight increase in the ultimate tensile strength (UTS) and a slight decrease in the total elongation (TE); thus, the product of UTS to TE (UTS×TE) as high as 31 GPa • % was obtained and remained nearly unchanged. However, a portion of the RA began to decompose when tempered at 500 °C and thus caused a ∼35% decrease of the RA fraction and a ∼16% decrease of the value of UTS×TE. It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could be retained with tempering temperature up to 400 °C. Thus, thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400 °C.

Dynamic continuous cooling transformation (CCT) behavior of medium-carbon forging steels microalloyed with different V contents up to 0.29% was investigated by means of dilatometric measurement, microstructural observation and hardness measurement. The results showed that the dynamic CCT diagrams were similar and the main difference was that the fields of the transformation products were shifted to the right side of the diagrams with the increase of V content, and this effect was more noticeable with an addition of 0.29% V. The A c1 and A c3 temperatures showed increasing trends with increasing V content, while the critical cooling rates decreased with increasing V content. The increase of V content resulted in significant increase of hardness and this tendency was enhanced with increasing cooling rate until the formation of acicular ferrite (AF). A promising approach of remarkably improving the toughness of ferritic-pearlitic medium-carbon forging steels with suitable V addition and the introduction of AF without notable penalty on its strength level was suggested.

The present investigation was made to study the variations of microstructure and mechanical properties of hot-rolled 3%Al containing medium-Mn steel at different intercritical annealing temperatures. The results show suitable intercritical annealing could obtain a duplex microstructure of lathy retained austenite (RA) and bimodal size distributed lathy ferrite. The ultimate tensile strength (UTS) increases while the yield strength decreases with increasing annealing temperature, and the total elongation (TEL) initially increases, but then notably decreases with increasing annealing temperature after a maximum value at 730–750 °C. An excellent combination of UTS of 922 MPa, TEL of 61% and UTS×TEL of 56 GPa% could be obtained when annealed at 750 °C. These values are significantly superior to a number of previously studied medium-Mn steels with similar Mn content. The stability of RA decreases with increasing annealing temperature primarily due to the increase of the RA size. It is suggested that the RA with an intermediated size of lath width of ~0.51–0.70 µm has suitable stability from the standpoint of obtaining large TRIP effect during tensile testing.

High cycle fatigue behavior of two V-microalloyed medium carbon steels with ferritic-pearlitic and bainitic microstructures were studied by using rotating bending fatigue and fatigue crack growth (FCG) testing. The results show that fatigue strength σ w is a power function of K t with its coefficient equals to smooth fatigue strength. This function provides a simple method for predicting notch fatigue strength of components. Bainitic steel shows higher σ w , lower fatigue notch sensitivity and FCG rate than those of ferritic-pearlitic steel. It is concluded that bainitic steel is a promising candidate to manufacture automobile security components in terms of fatigue properties.

The present study was attempted to explore the influence of Ni up to 1% on hydrogen embrittlement (HE) behavior of medium-carbon high strength steel by using both constant load test (CLT) and slow strain rate test (SSRT). The results show that HE index (HEI) decreases notably with the addition of 0.55% Ni, and this decreasing tendency tends to slow down with the addition of 1.02% Ni. HEI of SSRT is comparatively higher than that of CLT. The faction of brittle crack initiation area decreases with increasing Ni content. There is a decreasing tendency of both absorbed hydrogen content in Walpole solution and pre-charged hydrogen content with the addition of Ni. Both corrosion rate and pit depth of specimens after immersed in Walpole solution tend to decrease with increasing Ni content. Further electrochemical polarization test reveals that adding Ni could enhance pitting corrosion resistance. It is concluded that the addition of Ni could further lower the HE susceptibility of the tested steels mainly owing to enhanced corrosion resistance and decreased hydrogen absorption.

This paper reports the effect of cooling rate on the microstructure and hardness of a kind of medium carbon steel microalloyed with two levels of V content (0.15% and 0.28%) after hot deformation by using single compression tests on a Gleeble-3800 thermal simulator. The results show that cooling rate has a significant effect on the microstructure and hardness of the tested steels. Both the fraction of pearlite and hardness increase with increasing cooling rate, whereas a further increase of the cooling rate above a critical value promotes the formation of acicular ferrite (AF), and thus leads to a decrease of hardness mainly owing to the decrease of pearlite fraction and replacing it by AF and the less effective precipitation strengthening. Increasing V content results in a significant increase of hardness, and this tendency enhances with increasing cooling rate until the formation of AF. Furthermore, increasing V content also significantly enhances the formation of AF structure at a lower cooling rate. The results also suggest that by controlling microstructure, especially the precipitation of fine V(C,N) particles through adjusting post-forging cooling, the strengthening and gradient function in one hot-forging part could be obtained.

The fracture splitting property of medium carbon steel 37MnSiS microalloyed with V up to 0.45% was investigated by using simulated fracture splitting test, for the development of new crackable medium carbon steel to manufacture high performance connecting rod. Conventional high carbon steel C70S6 was used for comparison. The results show that the volume fraction of both ferrite and V-rich M(C,N) particles increases, and the pearlite interlamellar spacing decreases with increasing V content, which in turn results in gradual increase of strength and decrease of ductility and impact energy. The fracture splitting property of the tested steel could be improved significantly due to the increase of V content mainly through the precipitation hardening mechanism of fine M(C,N) precipitates. The fraction of brittle cleavage fracture in the crack initiation area increases noticeably with increasing V content and full brittle cleavage fracture surface could be obtained when V content was increased to 0.45%. It is concluded that medium carbon steel with V content higher than about 0.28% possesses not only comparable or even higher mechanical properties with those of conventional steel C70S6, but also excellent fracture splitting property, and therefore, is more suitable to fabricate high performance fracture splitting connecting rod.

The influence of cold drawing and annealing on hydrogen embrittlement (HE) of newly developed cold hardening bainitic steel was investigated by using slow strain rate testing (SSRT) and thermal desorption spectrometry (TDS), for ensuring safety performance of 10.9 class high strength bolts made of this kind of steel against HE under service environments. Hydrogen was introduced into the specimen by electrochemical charging. TDS analysis shows that the hydrogen-charged cold drawn specimen exhibits an additional low-temperature hydrogen desorption peak besides the original high-temperature desorption peak of the as-rolled specimen, causing remarkable increase of absorbed hydrogen content. It is found that cold drawing significantly enhances the susceptibility to HE, which is mainly attributed to remarkable increase of diffusible hydrogen absorption, the occurrence of strain-induced martensite as well as the increase of strength level. Annealing after cold deformation is an effective way to improve HE resistance and this improvement strongly depends on annealing temperature, i.e. HE susceptibility decreases slightly with increasing annealing temperature up to 200 °C and then decreases significantly with further increasing annealing temperature. This phenomenon is explained by the release of hydrogen, the recovery of cold worked microstructure and the decrease of strength with increasing annealing temperature.

The very high cycle fatigue properties of spring steel 60SiCrV7 for automotive suspension system with different hydrogen contents were studied by using ultrasonic fatigue testing and fatigue crack growth testing. The results show that the S–N curves exhibit continuous drop of fatigue lives and no obvious horizontal line exists. Similar fracture surface features were observed for all the specimens that failed mainly from internal inclusions with surrounding granular bright facet (GBF). Fatigue strength decreases remarkably with increasing hydrogen content. The applied stress intensity factor range at the periphery of GBF ΔKGBF is approximately proportional to 1/3 power of the square of GBF area. The average values of ΔKGBF for uncharged specimens are close to crack growth threshold ΔKth, which indicates that ΔKGBF could be regarded as the threshold value governing the beginning of stable fatigue crack propagation. The increase of hydrogen content tends to reduce ΔKGBF.

The present study was attempted to investigate the very high cycle fatigue (VHCF) properties of Cr–Mo low alloy steel containing V-rich MC carbides by using ultrasonic fatigue testing machine. All the specimens were quenched and tempered at 500 °C and 600 °C and part of them were electrochemical hydrogen charged. Mainly owing to the precipitation of fine V-rich MC particles at 600 °C, the introduced hydrogen content of the 600 °C tempered specimen is much higher than that of the 500 °C tempered specimen. Both the hydrogen-charged and uncharged specimens revealed internal fractures in the long-life region and no notable fracture surface difference were observed. Both fatigue strength and fatigue strength ratio increase with increasing tempering temperature from 500 °C to 600 °C for both the hydrogen-charged and uncharged specimens, though the 600 °C tempered specimen possessed higher hydrogen content. Increasing hydrogen content was definitely detrimental to the fatigue properties. However, compared to other high strength steels investigated previously, the detrimental effect of hydrogen on fatigue properties of the tested steel was comparatively small, which was ascribed mainly to the presence of fine V-rich MC carbides acting as hydrogen trapping sites. In is concluded that appropriate controlling of fine V-rich MC carbides is a promising way to alleviate the detrimental effect of hydrogen on VHCF properties of high strength steels.

We elucidate here the effect of microalloying with niobium (Nb) on very high cycle fatigue (VHCF) behavior in high-strength C–Mn–Si–Cr bainite/martensite (B/M) multiphase steels studied through ultrasonic fatigue testing. The tensile strength (Rm) and fatigue limit strength after 109 cycles (σw9) and in the non-failure condition of the steel microalloyed with Nb were 1640 MPa and 900 MPa, respectively. Thus, the value of σw9/Rm exceeded in comparison to conventional steels and was approximate 0.55. Three types of failure modes were observed in Nb-bearing steels depending on the surface condition, inclusion, and the matrix microstructure, i.e., surface defect-induced failure mode (S-mode), inclusion-induced failure mode (I-mode), and non-inclusion induced failure mode (N-mode). Only two failure modes were observed in Nb-free steels, the S-mode and the N-mode. The study clearly suggests that Nb had a distinct effect on the VHCF properties of B/M steels. The VHCF limit of Nb-bearing steel was enhanced by 200 MPa because of refinement of the microstructure and pinning of dislocations by randomly distributed nanometer-sized Nb(C, N) precipitates. It is underscored that microalloying with Nb is a potential approach to enhance VHCF properties in advanced high-strength steels.

This investigation attempts to thoroughly explore the effect of microalloying element V on the microstructural characteristics and mechanical properties of 20Mn2SiCrS bainitic forging steel. The results show that the cooling rate range of bainitic transformation was broadened with a 0.13% V addition and mainly a granular bainite microstructure was obtained after post-forging continuous cooling. The addition of V could refine the martensite/austenite (M/A) constituent and inhibit the formation of pro-eutectoid ferrite. Physical-chemical phase analysis revealed that only ~8.5% of the added V was in the V(C,N) precipitate in the as-forged condition, which resulted in relatively small precipitation strengthening (~40 MPa) compared with that in ferritic-pearlitic medium carbon forging steel. The addition of V can enhance the overall mechanical properties of the steel and it is concluded that the primary role of V in the as-forged bainitic forging steel is mainly to promote the formation of bainite and to refine the microstructure especially the M/A constituents as well as to resist softening of the M/A during the post-forging continuous cooling or during subsequent tempering when tempering treatment is necessary.

The present investigation attempts to explore the influence of vacuum carburizing treatment on the high cycle fatigue behavior of a low carbon alloy steel 20Cr2Ni4 by using rotating bending fatigue test. The same heat steel subjected to conventional quenched & tempered (QT) treatment was used for comparison. It was found that the vacuum carburizing treatment significantly promoted the formation of a particular area of granular bright facet in the vicinity of non-metallic inclusion in the fatigue crack initiation region with fatigue life beyond ~106 cycles and thus caused mainly an interior inclusion-induced failure mode compared with the QT treatment. Both the high cycle fatigue strength and the threshold value of stress intensity factor range controlling macroscopic crack growth were significantly enhanced on condition that the crack initiation site was still within the carburized case layer. These beneficial enhancements are mainly ascribed to the introduction of a case layer with much higher hardness and compressive residual stress.

The effects of different cooling conditions after forging on the microstructural characteristics and mechanical properties of a kind of high-content V-microalloyed medium-carbon steel 37MnSiVS were investigated. The effects were studied by using optical microscopy, transmission electron microscopy and tensile tests. Increasing direct cooling rate after forging is found to increase strength while slightly decrease ductility. A significant increase of strength could be obtained after forced air cooling and then short time isothermal holding at 873 K, while strength decreases gradually with further increasing holding time. The variations of microstructural characteristics especially V(C,N) precipitation strengthening effects with cooling conditions are mainly responsible for these variations of tensile properties. For the investigated high V-containing MA steel, a direct cooling strategy after finish forging is proposed, which includes accelerated cooling with forced air in ferrite range, following by short ...

Single compression tests were carried out with a Gleeble-3800 thermal simulator to investigate hot deformation behavior of two vanadium-microalloyed medium-carbon steels for fracture splitting connecting rod. The tests were performed to a total true strain of 0.92 at true strain rates ranging from 10−2 to 10 s−1 and deformation temperature of 900–1150 °C. The results show that hot deformation behavior of the tested steels is similar to that of conventional medium-carbon microalloyed steels and dynamic recrystallization is easier to occur at higher deformation temperature and lower strain rate. The austenite deformation resistance and activation energy of deformation increase with increasing vanadium content from 0. 15% to 0. 28% and thus the starting time of dynamic recrystallization was delayed. Finer recrystallized austenite grain could be obtained at higher strain rate, lower deformation temperature and higher vanadium content. TEM observation of the specimens quenched just before and after deformation reveals that vanadium is mainly in dissolved solute condition in austenite and thus affects the dynamic recrystallization behavior of the tested steels mainly through solute-drag effect.

High-cycle fatigue properties of medium carbon microalloyed (MA) steel 38MnVS with different microstructural characteristics were studied by rotating bending fatigue test. Low alloy steel 40Cr with quenched and tempered (QT) microstructure was also used for comparison. The results show MA steel in the as-forged condition has the best fatigue property mainly owing to the significant precipitation strengthening of fine V(C,N) precipitates and much fine ferrite–pearlite microstructure, which is even better than that of the QT steel 40Cr. The formation of film-like ferrite along coarse prior austenite grain boundary and coarse bainitic type microstructure as well as the coarsening of the initial fine precipitates significantly deteriorated the MA steel's fatigue properties. SEM examination of fatigue fracture surfaces revealed that the fatigue fracture mechanism of MA steel is different from that of QT steel. It is concluded that microstructure of ferrite–pearlite type MA steels should be carefully controlled after rolling and/or forging to obtain improved fatigue properties.

The present investigation effort was made to study the effect of V up to 0.45% on the high-cycle fatigue properties of medium-carbon microalloyed (MA) steel 37MnSiVS, for the development of new crackable MA forging steel with excellent fatigue properties. The results show that the amount of V(C,N) precipitates increases with increasing V content and most of the precipitates are less than 5 nm. Owing to the significant precipitation strengthening effect of these nanosized particles, the hardness increase of ferrite with increasing V content is higher than that of pearlite and accordingly a decrease of pearlite/ferrite hardness ratio. Therefore, both fatigue strength and fatigue strength ratio increase with increasing V content and excellent fatigue properties could be obtained when V content is higher than about 0.28%. The fatigue crack growth (FCG) behavior is similar for all the three 37MnSiVS samples with an exponent m ≈ 3.5. It is concluded that V can improve the fatigue properties of ferrite–pearlite steel mainly through precipitation strengthening and therefore it is anticipated that MA steel’s fatigue property could be further improved as well as more fine V(C,N) particles be obtained.

The relationship of hydrogen, GBF (granular bright facet) and very high cycle fatigue properties of high strength spring steels 60Si2CrV with three different hydrogen contents were studied using hydrogen thermal desorption analysis and ultrasonic fatigue test. The results showed that the influence of hydrogen on the relationship between fatigue life and the ratio of GBF to inclusion size (AGBFAINC) was obvious, and the expression between fatigue life and AGBFAINC with different hydrogen contents can also be obtained. In addition, based on the research of hydrogen diffusion and GBF, it was explained why the GBF cannot form below 106 cycles. At last, the estimated critical fatigue life of GBF formation can be expressed accurately.

The effects of hot stamping (HS) and tempering on the hydrogen embrittlement (HE) behavior of a low-carbon boron-alloyed steel were studied by using slow strain rate tensile (SSRT) tests on notched sheet specimens. It was found that an additional significant hydrogen desorption peak at round 65&ndash, 80 &deg, C appeared after hydrogen-charging, the corresponding hydrogen concentration (CHr) of the HS specimen was higher than that of the directed quenched (DQ) specimen, and subsequent low-temperature tempering gave rise to a decrease of CHr. The DQ specimen exhibited a comparatively high HE susceptibility, while tempering treatment at 100 &deg, C could notably alleviate it by a relative decrease of ~24% at no expanse of strength and ductility. The HS specimen demonstrated much lower HE susceptibility compared with the DQ specimen, and tempering at 200 &deg, C could further alleviate its HE susceptibility. SEM analysis of fractured SSRT surfaces revealed that the DQ specimen showed a mixed transgranular-intergranular fracture, while the HS and low-temperature tempered specimens exhibited a predominant quasi-cleavage transgranular fracture. Based on the obtained results, we propose that a modified HS process coupled with low-temperature tempering treatment is a promising and feasible approach to ensure a low HE susceptibility for high-strength automobile parts made of this type of steel.

Fatigue properties of two Al-containing steels have been investigated by rotating bending fatigue tests. Results show that the fatigue limits (the fatigue strength at 107 cycles) were improved remarkably by plasma nitriding due to the high hardness of 1000 Hv and compressive stress of 400 MPa in the nitrided layer. Scanning electron microscopic observations show that after nitriding the fatigue crack initiation sites moved from the surface flaws or near-surface matrix into the AlN inclusions at around the case-core interface. Degassing treatment can increase the fatigue limit because it prevented fatigue crack initiation at AlN inclusions due to the reduced [N] contents and refined inclusion size.

The relationship between microstructures and mechanical properties of a high nitrogen stainless (HNS) steel (0.65N–1.8Ni, wt%) manufactured by argon oxygen decarburization and continuous casting was investigated in this article. The plates with different thicknesses were obtained by thermo-mechanical control process. The results revealed that the plates prepared by hot rolled and solution treated possessed good balanced mechanical properties, i.e., satisfactory strength and higher toughness. Compared with hot-rolled and solution-treated plates, the hot-rolled plate had much higher tensile strength, but its impact toughness was extremely low. Furthermore, with the increase of deformation strain, the plate with finer grains, more precipitates and higher strength was achieved, but its plasticity decreased obviously. The worm-like carbides formed along the grain boundary during the finish rollings at 850 °C, which are detrimental to the toughness of hot-rolled plates. In addition, small amount of ferrite precipitated in the steel due to the non-equilibrium solidification during continuous casting, and to ensure full austenitic structure, composition design method was recommended.

The effects of sulfur content and calcium addition on smooth axisymmetric tensile fracture ductility of case hardening steel DIN 18CrNiMo7–6 have been investigated. The quantitative metallographic analysis of sulfide inclusions and the correlations between sulfide inclusions and fracture ductility were examined. Sulfide inclusions were found to have deleterious effect on fracture ductility, whereas the effect can be offset to some extent by calcium-treatment due to less easily deforming of sulfides during hot-working. The product (AA · λAW) of sulfide inclusion area fraction (AA) and its area-weighted aspect ratio (λAW) can be used as a parameter to describe the effect of sulfide inclusions on fracture true strain.

Microstructure and mechanical properties of medium manganese steel (Fe–0.2C–5Mn) processed by annealing at 650 °C with annealing time up to 12 h after accelerated cooling were studied. It was found that the martensite structure was gradually transformed into a superfine ferrite and austenite duplex structure by austenite reverted transformation during annealing process. The annealing heat treatment results in a large volume fraction of austenite (34%) and an excellent combination of ultimate tensile strength (∼1000 MPa) and total elongation (>40%) at room temperature. The ultrahigh tensile strength and improved ductility of present steel were mainly attributed to the enhanced phase transformation induced plasticity due to the large fractioned austenite.