Study on modification and microalloying of cast steel parts in China

Modification and microalloying is one of the hotspots to improve the properties of steel casting parts for many years. Through modification and microalloying, the liquid steel can be purified, the cast structure of steel can be improved and the properties of steel can be improved. The common modifier for modification treatment is re element or composite modifier with re as the main element and other trace alloy elements. China is rich in rare earth resources. The application of rare earth in steel casting has involved most steel grades. Rare earth can deoxidize, desulfurize and fix hydrogen, refine the structure, reduce or eliminate columnar crystals, improve the uniformity of structure of steel casting parts and the morphology, size and distribution of inclusions in steel. In terms of modification treatment, after studying the effect of re + B modifier on the casting structure and properties of zg31crmn2si medium carbon low alloy steel, we found that re + B had priority to interact with s and O in the steel, forming high melting point rare earth oxide, rare earth sulfur oxide and boron carbon nitrogen composite inclusions, eliminating the enrichment of low melting point iron sulfide at the grain boundary and improving the morphology and distribution of inclusions, The inclusions changed from rod or chain distributed at the grain boundary at the modification front to spherical distributed in the crystal after modification, so that the tensile strength of the steel increased by 15.1%, reached 1772mpa, the yield strength increased by 18%, reached 1398mpa, the impact toughness of u-notch specimen at low temperature (- 60 ℃) increased by 74.8%, reached 61.2j/cm2, the hardness HRC remained at 50.5, and the wear resistance was significantly improved. After adding 0.033% lanthanum and cerium mixed rare earth modification to the casting of 20mncrni2mo low alloy wear-resistant steel, the microstructure of the steel is obviously refined and the properties are improved. Compared with the test steel without RE modification, the hardness is increased by 10.44% and the HRC is 33.97; The tensile strength increased by 5.95% to 1133.67mpa- At 40 ℃, the impact absorption energy, elongation and area reduction of steel increased by 36.87%, 40.49% and 39.42% respectively, reaching 15.37 J, 14.92% and 34.34%. RE modification also has an obvious effect on the microstructure and properties of cast carbon steel. After Y-Mg composite modification, the microstructure of SS400 steel is obviously refined, the pearlite sheet spacing is reduced from 335.4 nm to 13.3 nm, the number of grains per unit area is increased from 1330 to 4569, and the grain size grade is increased by 2 grades; After adding 0.03 ~ 0.035re modification to high carbon steel castings, the austenite grains are obviously refined, the pearlite sheet spacing is reduced, and the inclusion morphology changes from slender strip to spindle, ellipsoid and sphere. The tensile strength and yield strength of the steel increased by 11.8% and 28.1% respectively, reaching 760mpa and 410mpa, and the impact energy increased by 11j. After re + Ca composite modification, the grain of ultra-high carbon steel is obviously refined, and the eutectic carbide changes from coarse strip with continuous network distribution to fine strip and block with discontinuous network distribution. The impact toughness of the steel is nearly doubled and the hardness reaches 65hrc, but excessive dilute soil modification is not beneficial to improve the properties of the steel. After adding 0.084% Ce into special protective steel plate, the microstructure is obviously refined, the size and morphology of inclusions are obviously improved, the tensile strength and yield strength are increased by 5.24% and 3.41% respectively, reaching 1284.1mpa and 1082.2mpa, and the elongation is 13.9%.

Rare earth also has significant modification effect on cast high alloy steel. After 0.006% ~ 0.08% y modification treatment, 0Cr13 Stainless steel has significantly refined grains, reduced grain boundary inclusions, improved composition uniformity and mixed crystal phenomenon, reduced phase interface, and significantly improved corrosion resistance in 3.5% NaCl aqueous solution. When studying the modification effect of rare earth on 04cr13ni5mo hydraulic turbine stainless steel, it is found that after rare earth modification and 1030 ℃ air quenching + 650 ℃ tempering, the tensile strength, yield strength and area shrinkage of the stainless steel reach 880mpa, 775mpa and 69% respectively, which are 16.4%, 21.9% and 43% higher than those before modification. After adding 0.01-0.09% Ce to 409L Stainless steel, the microstructure is obviously refined, and the inclusions in the steel are transformed into fine spherical CE titanium nitrogen composite inclusions and ce-al-o-ti composite inclusions, which eliminates the long strip tin and tin-al2o3 composite inclusions and effectively improves the thermal fatigue resistance of the steel. However, when rare earth exceeds 0.05%, the thermal fatigue resistance of 409L Stainless steel decreases. After 0.25% rare earth modification + 960 ℃ normalizing treatment, the properties of high carbon chromium steel casting are the best. The impact toughness, relative toughness, bending strength and wear resistance are increased by 180%, 117%, 77% and 20% respectively. Grain boundary carbides change from continuous network carbides to discontinuous blocks, and dispersed granular carbides precipitate in the matrix. After the composite modification of renb and remgnb, the as cast microstructure of cr17ni2mo high carbon heat-resistant cast wear-resistant steel has been significantly refined, the columnar crystal has been eliminated, the microstructure uniformity has been improved, the grain boundary carbide has changed from coarse needle flake distributed in network to fine flake and block distributed discontinuously, and the as cast hardness and impact toughness of the steel have been increased by 4.8%, 33.8%, 4.6% and 38.9% respectively.

Microalloying can refine the structure, strengthen the matrix, improve the heat treatment characteristics of steel, and effectively improve the properties of steel casting parts. The commonly used alloying elements for microalloying are Ti, V, Nb, B, N and re, and the addition amount is generally no more than 0.3%. The effects of Ti, Nb and V Microalloying on the casting microstructure and properties of zg230-450 and zg345-570 steels were studied. The results showed that the tensile strength of the two steels increased by about 8% after adding 0.03% Ti alone; After adding 0.03% Nb alone, the tensile strength of the two steels increases by about 15%, and the tensile strength of the test steel increases by about 10%, but the elongation decreases to a certain extent. When two or three microalloyed elements are added, not only the tensile strength of the matrix steel is increased by about 10%, but also the average reduction of area is increased by about 22%. After 0.034% Nb microalloying treatment, the austenite grain of medium carbon low alloy wear resistant steel increases from 61.1 μ M reduced to 16.7 μ m. The hardness of the steel is increased by 9 Hb to 530 Hb, the Charpy impact energy at – 20 ℃ is increased from 29.4 J to 37.6 J, and the wear resistance is increased by 3.5%. Adding 0.1-0.3% Nb + 0.1-0.3% Ti to ferritic stainless steel can obviously refine the as cast structure, promote the equiaxed structure, effectively improve the equiaxed crystal rate of solidified structure and reduce the columnar crystal area.

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