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The microstructure and mechanical properties of three kinds of low-carbon medium-manganese steels with different Si contents under an intercritical heat treatment process were studied. The results show that the microstructure of the test forged steel is mainly composed of ferrite and pearlite. After 900 °C complete austenitizing quenching + 720 °C intercritical quenching, the microstructure of the test steel is mainly composed of ferrite and martensite. With the increase in Si content, the microstructure becomes finer and more uniform. The microstructure of the test steel after 900 °C complete austenitizing quenching + 720 °C intercritical quenching + 680 °C intercritical tempering is dominated by ferrite and tempered martensite, with a small amount of retained austenite and cementite. As the Si content increases, the boundaries between ferrite and tempered martensite become more clear. The tensile strength and hardness of the test steel increase with the increase in Si content, while the elongation first increases and then decreases; the comprehensive performance of the test steel is the best when the Si content is 0.685 wt. %, with a tensile strength of 726 MPa, a yield ratio of only 0.65, the highest elongation of 30.5%, and the highest strong plastic product of 22,143 MPa.%. [ABSTRACT FROM AUTHOR]

• The H13 steel block were prepared by wire arc additive manufacturing. • The microstructure was studied in detail. • The tensile strength and elongation were studied. • The internal information related to micropores is studied. • The reason of the low elongation of printing block is deduced. The H13 steel block was successfully prepared by cold metal transfer(CMT) wire-arc additive manufacturing(WAAM) method. The microstructure characteristics and mechanical properties of the printed block were both studied by experiments. It was found that there were obvious difference in the microstructure and mechanical properties between the central part and overlapping parts of the printed block. It was shown from the microstructure observation that the printed block was mainly composed of martensite and residual austenite. There was an obvious primary austenite grain boundary observed in the base phase. In addition, compared with the overlapping part, the content of residual austenite in the central part was lower. It was shown that the average value of the microhardness in the central part was 596HV, while it was 534HV for the overlapping part due to the higher content of residual austenite. Tensile tests were conducted for different direction of the printed block. Micropores with a volume of 100–400 μm3 were observed in the printed block, with 90 % of the micropores in the range of 100–200 μm3 in volume. The micropores were uniformly distributed in the sample. The presence of micropores leads to the low elongation and tensile strength. [ABSTRACT FROM AUTHOR]