A production method for 980MPa grade ultra-low carbon cold-rolled medium manganese steel

Abstract

The invention discloses a production method of 980MPa grade ultra-low carbon cold-rolled medium manganese steel. In order to ensure good welding performance, the composition design is based on ultra-low C, low Si and an appropriate amount of Mn elements. The good hardenability of the Mn element is used to ensure that the hot-rolled room temperature structure is full martensite. The cold hardened plate after acid rolling is annealed in two-phase region to obtain reverse transformation austenite, so as to obtain the cold-rolled medium manganese steel with good strength and plasticity. The continuous casting process scheme effectively improves the quality of the casting blank. The large-pressure rolling and uniform accelerated cooling effectively refine the grains of the steel plate. Through the process scheme of acid rolling front cover softening + acid rolling post-annealing, the high-strength and high-plasticity cold-rolled medium manganese steel plate can be produced on the existing industrial production line. The tensile strength is ≥980MPa, and the transverse and longitudinal elongation A is ≥20%. Complex safety parts can be formed to meet the demand for lightweight automobiles. The process is simple and can be mass-produced. Compared with the prior art, the production cost is reduced by about 15%.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of metal materials, and specifically relates to a production method of 980 MPa grade ultra-low carbon cold-rolled medium manganese steel. Background Art

[0002] Steel is the dominant material in automobile manufacturing. The development of high-strength steel for automobiles has been rapidly promoted with the requirements of lightweight automobiles, and the proportion of high-strength steel plates used in automobiles has increased year by year. The research and development of advanced high-strength steel for automobiles has gone through three stages. The strength-ductility product of the first-generation automobile steel (such as DP steel, CP steel, TRIP steel, etc.) is generally at the level of 15GPa·%, which is difficult to meet the lightweight and safety requirements of future automobiles. The second-generation automobile steel (such as TWIP steel and austenitic steel) has high plasticity and a high strength-ductility product of 50GPa·%; however, due to the high alloy content, its production process requirements are complex and the alloy cost of steel is high. Taking the third-generation automobile steel represented by medium manganese steel as an example, it cleverly combines the advantages of the first two generations of automobile steels, and is highly favored because of its performance advantages higher than the first-generation steel and its cost advantages lower than the second-generation steel. The main characteristics of medium manganese steel are low carbon medium alloy with a manganese mass fraction between 3% and 12%. Due to the low content of alloy elements, the cost is much lower than that of the second-generation advanced high-strength steel. Due to the TRIP effect of metastable austenite in medium manganese steel, the tensile strength can reach 750~2200MPa, the total elongation ranges from 15% to 85%, and the strength-plasticity product can reach 15~70GPa·%. The performance far exceeds that of the first-generation advanced high-strength steel. Therefore, the application scope of medium manganese steel is wider than that of general ultra-high-strength steel. Its application prospects include many body structural parts such as automobile A-pillars, B-pillars, anti-collision beams and door sill reinforcements. It has better market prospects and will have great demand in the future market.

[0003] In the prior art, for example, the patent with publication number CN106086640A provides a cold-rolled medium manganese steel with ultra-high strength and plasticity and a preparation method thereof. However, its high C content easily leads to unstable welding performance and cracks. In addition, a high Al content also leads to an increase in the viscosity of the protective slag, which easily causes blockage of the continuous casting nozzle and deterioration of the surface quality of the steel billet. The patent with publication number CN108330402A discloses a Nb-Mo alloy high-strength and high-plasticity cold-rolled medium manganese steel and a preparation method thereof. The technical process is relatively complex and currently cannot be industrialized. In addition, a high content of Nb element is added to the chemical composition of the patent, which increases the production cost. Summary of the invention

[0004] In order to overcome the defects of the above-mentioned prior art, the composition design of the present invention is based on ultra-low C, low Si and appropriate Mn elements, and the good hardenability of Mn elements is used to ensure that the hot-rolled room temperature structure is full martensite. The cold-hardened plate after acid rolling is annealed in the two-phase zone to obtain reverse transformation austenite, thereby obtaining cold-rolled medium manganese steel with good strength and plasticity. The medium manganese steel produced by this method has both good strength and plasticity and welding performance, and can be formed into complex safety parts to meet the needs of lightweight automobiles.

[0005] In order to achieve the above-mentioned invention object, the present invention provides a production method of 980MPa grade ultra-low carbon cold-rolled medium manganese steel, wherein the chemical composition mass percentage of the 980MPa grade ultra-low carbon cold-rolled medium manganese steel is C: 0.035%~0.045%, Si: 0.20%~0.30%, Mn: 5.5%~6.5%, Cr: 0.4%~0.6%, Ni: 0.4%~0.6%, Mo: 0.02%~0.03%, and the balance is iron and unavoidable impurities.

[0006] Reasons for designing the ingredients of the present invention:

[0007] C is a common strengthening element in steel materials, which can play a significant role in solid solution strengthening and precipitation strengthening. For advanced high-strength steel with high strength and plasticity, carbon is also an important factor in stabilizing austenite. However, since a large amount of manganese and other alloying elements are added to medium manganese steel, if a large amount of C element is added, it will directly affect the continuous casting quality of the steel billet and the welding performance of the final product. In order to achieve the purpose of the present invention, the inventor has found that it is more appropriate to limit the C element content to 0.035%~0.045% after a large amount of literature research and thermodynamic calculations.

[0008] Si is the main element that stabilizes ferrite, has a certain solid solution strengthening effect, and can improve the elastic limit of steel. For medium manganese steel, Si can also effectively inhibit the precipitation of carbides and accelerate the diffusion of carbon elements to reverse austenite, thereby improving the stability of austenite. However, too high a Si content will deteriorate the plasticity of steel and the surface quality of steel, and its suitable range is 0.20%~0.30%.

[0009] Mn is the most important alloying element of medium manganese steel, which can effectively improve the hardenability of steel, enable the steel plate to obtain full martensite structure in a wide cooling rate range, and form reverse transformation austenite in the subsequent intercritical annealing process. In order to achieve the strong plasticity of the present invention, it is more appropriate to limit the Mn element content to 5.5%~6.5%.

[0010] Cr can increase the hardenability of steel and have a secondary hardening effect, significantly increase the ductile-brittle transition temperature of steel, and improve the uniformity of the structure. The present invention believes that it is appropriate to control the Cr content to 0.4%-0.6%.

[0011] Ni can improve the hardenability of steel, increase the strength of steel without significantly reducing toughness, and can also improve the processing and welding properties of steel. The present invention believes that it is appropriate to control its content within the range of 0.4% to 0.6%.

[0012] Mo can improve the hardenability and heat strength of steel, and also has a certain solid solution strengthening effect on ferrite. For medium manganese steel, molybdenum can prevent the temper brittleness of martensite during tempering and improve low temperature impact toughness. In order to ensure the mechanical properties of the medium manganese steel of the present invention, the present invention believes that the Mo content is preferably controlled at 0.02%~0.03%.

[0013] A production method for the above-mentioned 980MPa grade ultra-low carbon cold-rolled medium manganese steel, the method includes molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, hot rolling, annealing, pickling, continuous annealing and other processes.

[0014] A method for producing the above-mentioned 980MPa grade ultra-low carbon cold-rolled medium manganese steel, further, in the smelting process, the ladle N is ≤40 ppm, alloying is carried out after the S removal is completed, and after the iron is added, argon is soft-blown for 15 to 20 minutes to strictly avoid exposure of the molten steel; the specific order of alloying is Si-Mn-Gr-Ni-Mo, and Al is added during the slag making process.

[0015] In the above technical scheme, further, the continuous casting process adopts independent pouring, and argon is used to purge the middle bag before pouring. No molten steel is exposed during the pouring process, the nitrogen absorption of the water nozzle is strictly controlled, the nitrogen increase is controlled to be ≤5 ppm, and the continuous casting superheat control target is not more than 25°C.

[0016] In the above technical scheme, further, in the hot rolling process, the continuous casting billet is loaded into the furnace after cleaning, and the regenerative heating furnace is heated to 1250~1270℃, the insulation time is 3~4h, and the furnace outlet temperature is 1230~1250℃; the starting rolling temperature of the rough rolling unit is 1120~1140℃; the finishing rolling inlet temperature is ≥980℃, and the finishing rolling outlet temperature is 860~920℃; after the strip leaves the finishing rolling unit, laminar water cooling is turned on to 650~550℃, and then coiling is carried out.

[0017] In the above technical solution, further, in the cover retreat process, the temperature is kept at 630~670℃ for 1~4h.

[0018] In the above technical solution, further, in the pickling process, the strip running speed is 100-130 m / min, the HCl pickling temperature is 75-85° C., and the cold rolling reduction rate is controlled at 60-70%.

[0019] In the above technical scheme, further, the process parameters of the continuous annealing are as follows: heating section (℃): 750±10; soaking section (℃): 750±10; slow cooling section (℃): 650±5; fast cooling section (℃): 310±10; over-aging section (℃): 280±5; the strip running speed is 85±5m / min; the edge insulation function is turned on to ensure that the lateral temperature difference of the strip is less than 2℃.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] ① The composition of the present invention is based on ultra-low C, low Si and an appropriate amount of Mn elements. The good hardenability of the Mn element is mainly used to ensure that the hot-rolled room temperature structure is full martensite, and the reverse transformation austenite is obtained by two-phase annealing, thereby obtaining a cold-rolled medium manganese steel with good strength and plasticity.

[0022] ② The continuous casting process scheme of the present invention effectively improves the quality of the ingot, thereby improving the performance of the final product.

[0023] ③ The manufacturing method of the present invention of large-pressure rolling and uniform accelerated cooling effectively refines the grains of the steel plate.

[0024] ④ The thickness of the 980MPa grade ultra-low carbon cold-rolled medium manganese steel plate of the present invention is ≥1.0mm, the longitudinal yield strength can reach 690~750MPa, the tensile strength reaches ≥980MPa, and the transverse and longitudinal elongation A is ≥20%. The composition design of the ultra-low carbon cold-rolled medium manganese steel of the present invention is reasonable, the process is simple, and it can be mass-produced on industrial equipment. Compared with the prior art, the production cost can be reduced by about 15%. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The engineering stress-strain curve of the medium manganese steel prepared in the example;

[0026] Figure 2 This is the microstructure of the medium manganese steel prepared in the example. DETAILED DESCRIPTION

[0027] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. To avoid redundant description, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0028] The implementation methods of the present invention are as follows: Table 1 is the chemical composition of 980MPa grade ultra-low carbon cold-rolled medium manganese steel in the embodiment; Table 2 is the continuous casting process of the corresponding embodiment; Table 3 is the slab heating process of the corresponding embodiment; Table 4 is the hot rolling process of the corresponding embodiment; Table 5 is the annealing process of the corresponding embodiment; Table 6 is the operating speed matrix of the continuous annealing unit in the corresponding embodiment; Table 7 is the mechanical properties of the products of the corresponding embodiment.

[0029] The engineering stress-strain curve of the medium manganese steel prepared in the embodiment is as follows: Figure 1 The microstructure of the medium manganese steel prepared in the embodiment is as shown in Figure 2 shown.

[0030] Table 1 Chemical composition (wt%) of 980MPa grade ultra-low carbon cold-rolled medium manganese steel in the example

[0031]

[0032] The chemical composition (wt%) of the above-mentioned 980MPa grade ultra-low carbon cold-rolled medium manganese steel meets the requirements of C: 0.035%~0.045%, Si: 0.20%~0.30%, Mn: 5.5%~6.5%, Cr: 0.4%~0.6%, Ni: 0.4%~0.6%, Mo: 0.02%~0.03%, and the balance is iron and unavoidable impurities.

[0033] Example

[0034] A method for producing 980MPa grade ultra-low carbon cold-rolled medium manganese steel, the method comprising molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, hot rolling, annealing, pickling, and continuous annealing; the specific process parameters are as follows:

[0035] Table 2 Continuous casting process of 980MPa grade ultra-low carbon cold-rolled medium manganese steel

[0036]

[0037] Table 3 Slab heating process of 980MPa grade ultra-low carbon cold-rolled medium manganese steel in Example

[0038]

[0039] Table 4 Example 980MPa grade ultra-low carbon cold-rolled medium manganese steel hot rolling process

[0040]

[0041] Table 5 Annealing process of 980MPa grade ultra-low carbon cold-rolled medium manganese steel

[0042]

[0043] Table 6 Operation speed matrix of continuous annealing unit for 980MPa grade ultra-low carbon cold-rolled medium manganese steel in Example

[0044]

[0045] Table 7 Mechanical properties of 980MPa grade ultra-low carbon cold-rolled medium manganese steel

[0046]

[0047] For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for producing 980 MPa grade ultra-low carbon cold-rolled medium manganese steel, characterized in that: The medium manganese steel has a thickness of ≥1.0 mm and a width of 1600 mm; a longitudinal yield strength of 690-750 MPa, a tensile strength of ≥980 MPa, and a transverse and longitudinal elongation A of ≥20%; The chemical composition mass percentage of the medium manganese steel is C: 0.035%~0.039%, Si: 0.20%~0.24%, Mn: 5.55%~6.5%, Cr: 0.46%~0.6%, Ni: 0.4%~0.6%, Mo: 0.021%~0.030%, and the balance is iron and unavoidable impurities; The production method of the 980 MPa grade ultra-low carbon cold-rolled medium manganese steel comprises molten iron pretreatment, converter smelting, refining outside the furnace, continuous casting, hot rolling, annealing, pickling and continuous annealing; wherein: The continuous casting process adopts independent pouring, and the tundish is purged with argon before pouring. No molten steel is exposed during the pouring process, the nozzle is controlled to absorb nitrogen, the increase in nitrogen is controlled to be ≤5 ppm, and the continuous casting superheat is controlled to be no more than 25℃; In the hot rolling process, the continuous casting billet is cleaned and then loaded into the furnace, and the regenerative heating furnace is heated to 1250℃~1270℃, the holding time is 3~4h, and the furnace temperature is 1230℃~1250℃; the starting temperature of the rough rolling unit is 1120℃~1140℃; the finishing inlet temperature is ≥980℃, and the finishing outlet temperature is 860℃~920℃; after the strip leaves the finishing mill, the laminar water is turned on to cool it to 650℃~550℃, and then it is coiled; In the hood-returning process, keep the temperature at 630℃~670℃ for 1~4 hours; In the pickling process, the strip running speed is 100~130 m / min, the HCl pickling temperature is 75℃~85℃, and the cold rolling reduction rate is 60%~70%; The continuous annealing process parameters are as follows: heating section: 750±10℃; soaking section: 750±10℃; slow cooling section: 650±5℃; fast cooling section: 310±10℃; aging section: 280±5℃; strip running speed: 85±5 m / min; turn on the edge insulation function to ensure that the lateral temperature difference of the strip is less than 2℃.

Images