High-Mn and high-Cr automobile steel and shallow intergranular oxide layer control method

Abstract

The invention discloses high-Mn and high-Cr automobile steel and a shallow intergranular oxide layer control method, the method comprises continuous casting, hot rolling, cooling coiling, cold rolling and annealing processes, and the annealing process is set before the cold rolling process; the steel for the automobile comprises the following chemical components in percentage by weight: 1.00-1.20% of Mn, 0.85-1.05% of Cr, less than or equal to 0.18% of C and less than or equal to 0.05% of Si. Through full-process control of continuous casting, hot rolling and cold rolling, an intergranular oxide layer of the high-Mn and high-Cr automobile steel is well controlled (the depth of the intergranular oxide layer is smaller than or equal to 4.5 microns); and on the basis that the spheroidization rate is larger than or equal to 95%, the depth of an intergranular oxide layer is reduced on the premise that follow-up punch forming is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel production, and particularly relates to a high-Mn high-Cr steel for automobiles and a method for controlling a shallow intergranular oxide layer. Background Art

[0002] The main application fields of special steel are the automobile, saw blade, chain, and cutting tool industries, etc., with a demand of about 1.5 to 1.6 million tons per year. Among them, each motor vehicle in the automobile industry requires about 150 precision stamping special parts. According to statistics, the production volume of motor vehicles in 2024 is about 30 million, and the total number of precision stamping special parts required for automobiles is 4.5 billion, and the market condition is good.

[0003] Except for the Cr-Mo series, the Mn and Cr contents of the other steel grades in alloy special steel for automobiles are relatively high, with a content of about 1%, accounting for more than 80%. During the hot rolling and cold rolling annealing processes of special steel strip, at a certain temperature and atmosphere, oxygen diffuses into the strip matrix and reacts with alloy elements enriched at the grain boundaries, especially with alloy elements such as Al, Si, Mn, and Cr that have a strong affinity for oxygen, and the reaction is particularly obvious. Intergranular oxidation causes a significant reduction in the surface strength, plasticity, and hardness of the strip. During the material stamping process, edge collapse or even tearing is likely to occur. Especially during the production of automotive parts, extremely high requirements are placed on intergranular oxidation, generally requiring it to be within 10 μm. Therefore, during the strip production process, especially the control of the shallow intergranular oxide layer of high-Mn high-Cr steel for automobiles is particularly important.

[0004] CN113106225B - A method for reducing the depth of intergranular oxidation of high-carbon tool steel - By controlling the air excess coefficient of different heating sections, the depth of intergranular oxidation caused by the hot rolling process is controlled, so that the depth of intergranular oxidation of high-carbon tool steel is 3.4 - 5.3 μm, the intergranular oxidation depth is shallow, there are no edge linear peeling defects, and the surface quality is good. The tool steel disclosed in this patent has a high carbon content and low Mn and Cr contents, and does not provide a solution to the intergranular oxidation problem of steel for automobiles with high Mn and Cr contents, nor does it provide a solution to the intergranular oxidation problem brought about by cold rolling and subsequent processes. Shougang CN114058949B - A hot stamping steel pickling sheet and a method for reducing its surface intergranular oxidation. By adding Sb and Nb, controlling the Si element content, and controlling the temperature and rolling speed during the finish rolling process, the surface intergranular oxidation of the hot stamping steel pickling sheet is solved. However, the Cr content of this hot stamping steel is relatively low. This patent does not provide a method for solving the intergranular oxidation problem of steel for automobiles with high Mn and Cr contents, nor does it disclose the actual effect of controlling the depth of intergranular oxidation.

[0005] CN116926416A - Low - cost non - intergranular oxidation wide - width tool steel and its manufacturing method. This patent uses combined deoxidation and desulfurization of Ba and Al to generate BaO to adsorb free O and H, avoiding intergranular oxidation; uses Si to form an FeSiO layer on the surface, which firmly adheres to the steel matrix and prevents the surface of the steel matrix from being oxidized, thus inhibiting the formation of intergranular oxidation; forms a large amount of AlN with N and Al to form small - angle grain boundaries with more than 70% less than 60 degrees, hindering intergranular oxidation. In this invention, the tundish superheat is below 25°C, the continuous casting speed is 1.0 - 1.4 m / min, controlling segregation and columnar crystal morphology to form more than 70% small - angle grain boundaries and avoid intergranular oxidation; continuous casting uses nitrogen cooling with a cooling rate of 5°C / min - 10°C / min instead of water spray cooling to avoid enrichment of O and H at the grain boundaries on the surface of the casting blank and prevent intergranular oxidation. For tool steel with a carbon content of 0.7% - 1.0%, when the surface of the continuous casting billet is cooled to 700°C - 750°C, SiO2 powder is sprayed with a spraying amount of 100 g - 200 g / m 2 , forming a surface coating to inhibit the formation of surface ferrite and effectively prevent surface decarburization and intergranular oxidation. In addition, this patent also controls the entire process of heating, rough rolling, finish rolling, final rolling, and pickling and cold rolling. The small - angle grain boundaries obtained from the cooling rate of the casting blank and after rolling inhibit surface intergranular oxidation and decarburization. The depth of intergranular oxidation of the hot - rolled coil is 0 mm, but it is only applicable to high - carbon tool steel, and relies on Ba / Al addition and nitrogen protection. The process control is complex and the cost is high, and it does not solve the problem of intergranular oxidation caused by spheroidizing annealing. Summary of the Invention

[0006] The purpose of the present invention is to solve the above - mentioned technical problems and provide a high - Mn high - Cr steel for automobiles and a method for controlling a shallow intergranular oxidation layer.

[0007] To achieve the above - mentioned purpose, the present invention provides a method for controlling a shallow intergranular oxidation layer of a high - Mn high - Cr steel for automobiles, including continuous casting, hot rolling, cooling and coiling, cold rolling, and annealing processes, and the annealing process is set before the cold rolling process; the chemical composition of the steel for automobiles by weight percentage includes Mn: 1.00 - 1.20%, Cr: 0.85 - 1.05%, C ≤ 0.18%, Si ≤ 0.05%.

[0008] Furthermore, in the continuous casting process, the target tundish temperature is controlled at 20 - 30 °C above the liquidus temperature. The superheat of the molten steel is crucial for the structure of the continuous casting billet. Reducing the superheat can effectively increase the equiaxed crystal ratio, reduce the thickness of columnar crystals, and disperse the central segregation. If the superheat is high, the temperature gradient at the solidification front of the billet is large, the time for directional heat transfer is long, which is conducive to the development of columnar crystals and inhibits the formation of equiaxed crystals, increasing the central segregation. In addition, dynamic soft reduction and electromagnetic stirring are applied, with the reduction amount being 6.5 - 6.8 mm, which is beneficial to improving the internal quality of the billet. By effectively controlling the superheat, applying dynamic soft reduction, and electromagnetic stirring, the dispersion distribution of Mn and Cr elements is ensured from the source.

[0009] Furthermore, in the hot rolling process, the continuous casting billet is heated in a heating furnace, and the tapping temperature is controlled at 1200 - 1220 °C, and the residence time in the furnace is 160 - 180 min. Reasonably setting the tapping temperature and the residence time in the furnace further promotes the uniform diffusion of Mn and Cr elements. In the rough rolling stage, it is rolled in the austenite recrystallization zone. The finishing rolling start temperature is controlled at 1050 - 1070 °C. In the finishing rolling stage, through cumulative large deformation, the deformation bands and dislocation density in the deformed austenite are increased, and the phase transformation nucleation points are increased to refine the grains. The finishing rolling temperature is controlled at 900 - 920 °C. Since the temperature in the finishing rolling stage is always higher than 900 °C, the oxygen adsorbed on the surface is easily combined with Fe on the strip surface to form scale, preventing oxygen from easily diffusing into the strip matrix and forming an intergranular oxide layer.

[0010] Furthermore, in the cooling and coiling process, rapid intensive cooling is adopted after finishing rolling. The cooling method is intensive cooling in the front section, and the cooling rate is controlled at 30 - 40 °C / s. The coiling temperature of the cooled steel plate is 500 - 520 °C. Mn and Cr diffuse relatively fast in ferrite. By rapid cooling, the diffusion driving force is reduced, preventing them from rapidly enriching at the grain boundaries and avoiding internal oxidation.

[0011] Furthermore, the annealing and cold rolling process: To facilitate the subsequent processing and forming of automotive steel, spheroidizing annealing treatment is carried out on the as-rolled structure. During the annealing process, hydrogen in the atmosphere reduces the oxygen in the scale on the surface of the hot coil. During the long-term holding process, it diffuses into the strip surface and forms an intergranular oxide layer. If the annealing temperature is too low or the holding time is too short, the spheroidization rate is too low or even no spheroidization occurs, which is not conducive to subsequent forming; if the annealing temperature is too high or the holding time is too long, although the spheroidization rate can be effectively guaranteed, the depth of the intergranular oxide layer increases after annealing. Therefore, in the annealing process, first heat up to 715 - 725 °C and hold for 10 - 11 h, then after 4 - 5 h, cool down to 400 - 500 °C, and then after another 4 - 5 h, cool down to below 100 °C, and finally cool in a hood to 70 - 80 °C and then take out of the furnace. And replace the atmosphere in the annealing furnace from the conventional all-hydrogen atmosphere with N2 and H2 (volume ratio 1:3) decomposed from NH3. By reducing the hydrogen content, the generation amount of free oxygen is reduced, thereby controlling the depth of the intergranular oxide layer after annealing. To meet the requirements of spheroidizing annealing (spheroidization rate ≥ 95%), a certain depth of intergranular oxide layer is generated after annealing. Through a certain ratio of cold rolling reduction, the intergranular oxide layer is compressed by a similar ratio, further reducing its depth. This process is completed on a single-stand rolling mill, and the reduction rate is controlled at 30 - 65%.

[0012] Through the full-process control of continuous casting, hot rolling and cold rolling, the present invention enables good control of the intergranular oxide layer of high-Mn high-Cr automotive steel (depth of intergranular oxide layer ≤ 4.5 μm); on the basis of a spheroidization rate ≥ 95%, that is, on the premise of meeting subsequent stamping forming requirements, the depth of the intergranular oxide layer is reduced.

[0013] Also provided is a high-Mn high-Cr automotive steel. The chemical composition of the steel by weight percentage includes C: 0.14 - 0.18%, Si: ≤ 0.05%, Mn: 1.00 - 1.20%, P: ≤ 0.015%, S: ≤ 0.005%, Alt: ≤ 0.020%, Cr: 0.85 - 1.05%, and the balance is iron and impurity elements; the steel is prepared by the above-mentioned method, the structure of the steel is globular pearlite, the spheroidization rate ≥ 95%, the yield strength Rp0.2: 295 - 335 MPa, Rm: 430 - 480 MPa, elongation: 33 - 35%, and the depth of the intergranular oxide layer ≤ 4.5 μm.

[0014] The closest prior art to the present invention is CN116926416A. The steel type involved in this patent is high-carbon tool steel (C: 0.38 - 1.0%), which is applied in tool fields such as cutting tools and dies, requiring high hardness (≥48HRC) and wear resistance. The core technical problem is to completely eliminate intergranular oxidation (depth 0mm) and avoid surface microcracks. The solutions to the technical problems start from two major directions. One is in the composition design, adding deoxidizing elements such as B, N, Al, Ba, etc. (such as Al: 0.01 - 0.1%, Ba: 0.005 - 0.05%), strictly restricting O≤0.003%, H≤0.00015%, Al / N≥2 (forming small-angle grain boundaries of AlN), Ba / O≥9 (BaO adsorbs free oxygen to achieve "non-oxidation"), and controlling intergranular oxidation through small-angle grain boundaries and non-oxidation. However, the composition is complex, the control is difficult, and the cost is relatively high. The other is starting from process control. This patent controls all processes of continuous casting, heating, rough rolling, finish rolling, final rolling, and pickling and cold rolling to obtain small-angle grain boundaries to inhibit surface intergranular oxidation and decarburization.

[0015] The steel type protected by the present invention is high-Mn high-Cr steel for automobiles (low-carbon steel, C: 0.14 - 0.18%), which is applied to precision stamping parts of automobiles, requiring high plasticity, good stamping formability (elongation 33 - 35%), and spheroidized structure (spheroidization rate ≥95%). The core technical problem is to control the shallow intergranular oxidation layer (≤10μm) and prevent stamping edge collapse / tearing.

[0016] The chemical composition design is simple, without special added elements, relying on process control for the dispersion distribution of Mn / Cr to avoid the enrichment of oxidation-affinity elements. In terms of process: soft reduction during continuous casting + electromagnetic stirring to ensure the dispersion of Mn / Cr and avoid the enrichment of oxidation-affinity elements, adopting the sequence of "annealing → cold rolling" + cold rolling to compress the oxidation layer. The annealing atmosphere is a mixture of N2:H2 (not pure hydrogen or pure nitrogen). The oxidation depth in the hot-rolled state is 0μm, and the oxidation layer depth after annealing is 6.5 - 9.2μm, which is compressed to the target value ≤4.5μm (such as from 6.5μm with a 65% reduction rate to 3.2μm) through a cold rolling reduction rate of 30 - 65%. The present invention innovatively integrates the entire process of "soft reduction during continuous casting + hot-rolled oxygen barrier layer + optimized annealing atmosphere + cold rolling to compress the oxidation layer" for the special requirements of automotive steel, solving the problem of "oxidation intensification caused by spheroidizing annealing" not covered by the prior art.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: The structure of the steel described in the present invention is spherical pearlite, and its mechanical properties are: yield strength Rp0.2: 295 - 335MPa, Rm: 430 - 480MPa, elongation: 33 - 35%, spheroidization rate ≥95%, and the depth of the intergranular oxidation layer ≤4.5μm. Detailed Embodiments

[0018] The following further illustrates the present invention in conjunction with specific embodiments.

[0019] The present invention provides a method for controlling the shallow intergranular oxidation layer of high-Mn and high-Cr steel for automobiles. The chemical composition of the steel includes, by weight percentage: C: 0.14 - 0.18%, Si: ≤0.05%, Mn: 1.00 - 1.20%, P: ≤0.015%, S: ≤0.005%, Alt: ≤0.020%, Cr: 0.85 - 1.05%, and the balance is Fe and unavoidable inclusions.

[0020] The chemical compositions of the examples and comparative examples of the present invention are shown in Table 1. The component detection is carried out according to GB / T 4336 "Spark source atomic emission spectrometry for carbon steel and medium and low alloy steel (conventional method)".

[0021] Table 1 Chemical compositions of the examples of the present invention

[0022] The process parameters of the steelmaking process for the examples and comparative examples of the present invention are shown in Table 2.

[0023] Table 2 Main process parameters of the steelmaking process for the examples of the present invention

[0024] The main rolling process parameters of the examples and comparative examples of the present invention are shown in Table 3.

[0025] Table 3 Rolling process parameters of the examples of the present invention

[0026] The hot-rolled state properties of the examples and comparative examples of the present invention are shown in Table 4.

[0027] Table 4 Hot-rolled state properties of the examples of the present invention

[0028] By effectively controlling the superheat, applying dynamic soft reduction, and electromagnetic stirring, the dispersion distribution of Mn and Cr elements is ensured from the source. Reasonably setting the tapping temperature and residence time in the furnace further promotes the uniform diffusion of Mn and Cr elements. Precisely controlling the finishing rolling start temperature and finishing rolling temperature, and the temperature in the finishing rolling stage is always higher than 900°C. The oxygen adsorbed on the surface is easy to combine with Fe on the strip surface to form scale, so that oxygen does not easily diffuse into the strip matrix to form an intergranular oxidation layer. And during the cooling process, through rapid and intensive cooling, the driving force for the diffusion of Mn and Cr in ferrite is reduced, so that they cannot quickly enrich at the grain boundaries, avoiding internal oxidation, and thus the depth of the intergranular oxidation layer of the hot-rolled coil is 0μm.

[0029] The annealing process parameters of the embodiments and comparative examples of the present invention and the depth of the intergranular oxide layer after annealing are shown in Table 5.

[0030] Table 5 Annealing process parameters of the embodiments of the present invention

[0031] After the annealing process described in Table 5, with different atmospheres, the average depth of the intergranular oxide layer of the steel plates in the embodiments increased by 6.5 - 9.2 μm, and that in the comparative examples increased by 23.8 - 25.4 mm. By reducing the hydrogen content, the generation amount of free oxygen is reduced, thereby controlling the depth of the intergranular oxide layer after annealing.

[0032] The properties after cold rolling of the embodiments and comparative examples of the present invention are shown in Table 6.

[0033] Table 6 Properties after cold rolling of the embodiments of the present invention

[0034] In summary, according to the weight percentages of chemical components given in the present invention for raw material selection, and then produced according to the manufacturing method given in the present invention, a method for controlling the shallow intergranular oxide layer of high-Mn high-Cr automotive steel is provided. The structure is spherical pearlite, and the mechanical properties are as follows: yield strength Rp0.2: 295 - 335 MPa, Rm: 430 - 480 MPa, elongation: 33 - 35%, spheroidization rate ≥ 95%, and the depth of the intergranular oxide layer ≤ 4.5 μm. The above embodiments are only the best examples and are not intended to limit the implementation manners of the present invention.

Claims

1. A method for controlling the shallow intergranular oxide layer of high-Mn high-Cr steel for automobiles, including continuous casting, hot rolling, cooling and coiling, cold rolling and annealing processes, characterized in that: The annealing process is set before the cold rolling process; the chemical composition of the automotive steel by weight percentage includes Mn: 1.00 - 1.20%, Cr: 0.85 - 1.05%, C ≤ 0.18%, Si ≤ 0.05%.

2. The method for controlling the shallow intergranular oxide layer of the high-Mn high-Cr steel for automobiles according to claim 1, wherein: The continuous casting process: the tundish target temperature is controlled at 20 - 30 °C above the liquidus temperature, and dynamic soft reduction and electromagnetic stirring are put into use, with the reduction amount being 6.5 - 6.8 mm.

3. The method for controlling the shallow intergranular oxide layer of the high-Mn and high-Cr steel for automobiles according to claim 1, wherein: The hot rolling process: the cast slab is heated in a heating furnace, the tapping temperature is controlled at 1200 - 1220 °C, the residence time in the furnace is 160 - 180 min, the finishing rolling starting temperature is controlled at 1050 - 1070 °C, and the finishing rolling temperature is controlled at 900 - 920 °C.

4. The method for controlling the shallow intergranular oxide layer of the high-Mn and high-Cr steel for automobiles according to claim 1, wherein: The cooling and coiling process: rapid intensive cooling is adopted after finishing rolling, the cooling method is intensive cooling in the front section, the cooling rate is controlled at 30 - 40 °C / s, and the coiling temperature of the cooled steel plate is 500 - 520 °C.

5. The method for controlling the shallow intergranular oxide layer of the high-Mn and high-Cr steel for automobiles according to claim 1, characterized in that: The annealing process: First, heat up to 715 - 725 °C and hold for 10 - 11 h, then after 4 - 5 h, cool down to 400 - 500 °C, and then after another 4 - 5 h, cool down to below 100 °C, and finally cool in a covered furnace to 70 - 80 °C and then take out of the furnace; The annealing process is carried out in a bell-type furnace, and the annealing atmosphere is N2 + H 2。 6. The method for controlling the shallow intergranular oxide layer of the high-Mn high-Cr steel for automobiles according to claim 5, characterized in that: The volume ratio of N2 to H2 in the annealing atmosphere is 1:

3.

7. The method for controlling the shallow intergranular oxide layer of the high-Mn and high-Cr steel for automobiles according to claim 1, wherein: The cold rolling process is completed by a single - stand rolling mill, and the reduction rate is controlled at 30 - 65%.

8. The method for controlling the shallow intergranular oxide layer of the high-Mn and high-Cr steel for automobiles according to any one of claims 1 to 7, characterized in that: The structure of the automotive steel produced by the method is spherical pearlite, the spheroidization rate ≥ 95%, and the depth of the intergranular oxidation layer ≤ 4.5 μm.

9. The method for controlling the shallow intergranular oxide layer of the high-Mn and high-Cr steel for automobiles according to any one of claims 1 to 7, characterized in that: The yield strength Rp0.2 of the automotive steel produced by the method is 295 - 335 MPa, Rm is 430 - 480 MPa, and the elongation is 33 - 35%.

10. A high-Mn and high-Cr steel for automobiles, characterized in that, The chemical composition of the steel by weight percentage includes C: 0.14 - 0.18%, Si: ≤ 0.05%, Mn: 1.00 - 1.20%, P: ≤ 0.015%, S: ≤ 0.005%, Alt: ≤ 0.020%, Cr: 0.85 - 1.05%, and the balance is iron and impurity elements; the steel is the steel prepared by the method according to any one of claims 1 - 7, the structure of the steel is spherical pearlite, the spheroidization rate ≥ 95%, the yield strength Rp0.2 is 295 - 335 MPa, Rm is 430 - 480 MPa, the elongation is 33 - 35%, and the depth of the intergranular oxidation layer ≤ 4.5 μm.

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