High Mn and High Cr Automobile Steel and Shallow Intergranular Oxide Layer Control Method

Abstract

This invention discloses a high-Mn, high-Cr automotive steel and a method for controlling a shallow intergranular oxide layer. The method comprises continuous casting, hot rolling, cold coiling, cold rolling, and annealing processes, with the annealing process performed before the cold rolling process. The chemical composition of the automotive steel, by weight, includes Mn: 1.00-1.20%, Cr: 0.85-1.05%, C ≤ 0.18%, and Si ≤ 0.05%. By fully controlling the continuous casting, hot rolling, and cold rolling processes, the method achieves excellent control of the intergranular oxide layer (intergranular oxide layer depth ≤ 4.5 μm) in the high-Mn, high-Cr automotive steel. The method also reduces the intergranular oxide layer depth while maintaining a spheroidization rate of ≥ 95%, thus ensuring that the steel meets the requirements for subsequent stamping and forming.

Description

Technical Field

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

[0002] Special steel is primarily used in the automotive, saw blade, chain, and tool industries, with annual demand estimated at 1.5 to 1.6 million tons. The automotive industry requires approximately 150 fine-blanked special parts per vehicle. With the production of approximately 30 million vehicles in 2024, the total demand for fine-blanked special parts will be 4.5 billion, representing a healthy market.

[0003] Except for the Cr-Mo series, alloy-based special steels for automotive applications have high Mn and Cr contents, around 1%, accounting for over 80%. During the hot rolling and cold rolling annealing processes of special steel strip, under certain temperatures and atmospheres, oxygen diffuses into the strip matrix and reacts with alloying elements concentrated at the grain boundaries. This reaction is particularly pronounced with Al, Si, Mn, and Cr, alloying elements with strong oxygen affinities. Intergranular oxidation significantly reduces the surface strength, plasticity, and hardness of the strip, making it prone to edge collapse and even tearing during the stamping and forming process. This is particularly true in the production of automotive parts, where intergranular oxidation requirements are extremely high, generally within 10μm. Therefore, controlling the shallow intergranular oxide layer in strip production, especially for high-Mn and high-Cr automotive steels, is particularly important.

[0004] CN113106225B - A method for reducing the intergranular oxidation depth of high-carbon tool steel - By controlling the excess air coefficient in different heating sections, the intergranular oxidation depth caused by the hot rolling process is controlled, resulting in an intergranular oxidation depth of 3.4-5.3 μm for high-carbon tool steel. The intergranular oxidation depth is shallow, there are no linear scaling defects on the edges, and the surface quality is good. This patent discloses a tool steel with a high carbon content and low Mn and Cr contents. It does not provide a solution to the intergranular oxidation problem in automotive steel with high Mn and Cr contents, nor does it provide a solution to the intergranular oxidation problem caused by cold rolling and subsequent processes. Shougang CN114058949B - A method for reducing intergranular oxidation on the surface of hot-formed steel pickling plate. This method solves intergranular oxidation on the surface of hot-formed steel pickling plate by adding Sb and Nb, controlling the Si content, and controlling the temperature and rolling speed during the finishing rolling process. However, this hot-formed steel has a low Cr content. This patent does not provide a method for solving the intergranular oxidation problem in automotive steel with high Mn and Cr contents, nor does it disclose the actual effect of controlling the intergranular oxidation depth.

[0005] CN116926416A - Low-cost, intergranular oxidation-free wide-width tool steel and its manufacturing method. This patent utilizes Ba and Al for combined deoxidation and desulfurization, generating BaO that absorbs free O and H, preventing intergranular oxidation. Si forms a FeSiO layer on the surface, firmly adhering to the steel substrate and preventing surface oxidation, thereby inhibiting intergranular oxidation. Nitrogen and Al form a large amount of AlN, resulting in over 70% low-angle grain boundaries less than 60 degrees, hindering intergranular oxidation. This invention includes maintaining a superheat below 25°C, a continuous casting speed of 1.0 to 1.4 m / min, and controlling segregation and columnar crystal morphology to form over 70% low-angle grain boundaries and prevent intergranular oxidation. Furthermore, continuous casting utilizes nitrogen cooling at a cooling rate of 5°C / min to 10°C / min, rather than water mist cooling, to prevent the accumulation of O and H at the grain boundaries on the ingot surface and prevent intergranular oxidation. For tool steel with carbon content of 0.7% to 1.0%, the surface of the continuous casting billet is cooled to 700℃ to 750℃ and sprayed with SiO2 powder, with a spraying amount of 100g to 200g / m 2 , forming a surface coating, inhibiting the formation of surface ferrite, and effectively preventing surface decarburization and intergranular oxidation. In addition, the patent also controls the entire process of heating, rough rolling, finishing rolling, final rolling, and pickling and cold rolling, and the cooling rate of the ingot and after rolling to obtain a small-angle grain boundary to inhibit surface intergranular oxidation and decarburization. The intergranular oxidation depth of the hot-rolled coil is 0mm, 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. It also 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 technical problems and provide a high-Mn and high-Cr automobile steel and a method for controlling shallow intergranular oxide layer.

[0007] To achieve the above objectives, the present invention provides a method for controlling the shallow intergranular oxide layer of high-Mn and high-Cr automotive steel, comprising continuous casting, hot rolling, cold coiling, cold rolling and annealing processes, wherein the annealing process is arranged before the cold rolling process; the chemical composition of the automotive steel comprises, by weight percentage, Mn: 1.00-1.20%, Cr: 0.85-1.05%, C ≤ 0.18%, and Si ≤ 0.05%.

[0008] Furthermore, the continuous casting process: The target temperature of the tundish is controlled at 20-30°C above the liquidus temperature. Molten steel superheat is crucial to the ingot structure. Reducing superheat can effectively increase the equiaxed crystal ratio and reduce the thickness of columnar crystals, leading to central segregation and dispersion. High superheat leads to a large temperature gradient at the solidification front of the ingot, maintaining directional heat transfer for a long time, which is beneficial to the development of columnar crystals, inhibits the formation of equiaxed crystals, and increases central segregation. Furthermore, dynamic soft reduction and electromagnetic stirring are employed, with a reduction of 6.5-6.8mm, to improve the internal quality of the ingot. By effectively controlling superheat, dynamic soft reduction, and electromagnetic stirring, the dispersed distribution of Mn and Cr elements is ensured from the source.

[0009] Furthermore, the hot rolling process is as follows: the ingot is heated in a heating furnace, the furnace exit temperature is controlled at 1200-1220°C, and the furnace time is 160-180 minutes. Reasonable setting of the furnace exit temperature and furnace time further promotes the uniform diffusion of Mn and Cr elements. The rough rolling stage is rolled in the austenite recrystallization zone, the finishing rolling start temperature is controlled at 1050-1070°C, and the finishing rolling stage accumulates large deformations to increase the deformation bands and dislocation density in the deformed austenite, increase the phase deformation nuclei and refine the grains, and the final rolling temperature is controlled at 900-920°C. The temperature in the finishing rolling stage is always higher than 900°C, and the oxygen adsorbed on the surface is easily combined with the Fe on the surface of the strip to form iron oxide scale, which prevents oxygen from easily diffusing into the strip matrix and forming an intergranular oxide layer.

[0010] Furthermore, the cooling and coiling process employs rapid intensive cooling after finish rolling, with a front-stage intensive cooling method and a cooling rate controlled at 30-40°C / s. The coiling temperature of the cooled steel plate is 500-520°C. Mn and Cr diffuse rapidly in ferrite, and rapid cooling reduces the driving force for diffusion, preventing their rapid accumulation at grain boundaries and thus preventing internal oxidation.

[0011] Furthermore, the annealing and cold rolling process: To facilitate subsequent processing and forming of automotive steel, the rolled structure undergoes spheroidizing annealing. During the annealing process, hydrogen in the atmosphere reduces the oxygen in the surface oxide scale of the hot coil. During the long holding period, this oxygen diffuses into the strip surface, forming an intergranular oxide layer. Annealing temperatures that are too low or holding times that are too short result in a low or even no spheroidization, hindering subsequent forming. Annealing temperatures that are too high or holding times that are too long effectively guarantee a spheroidization rate, but the depth of the intergranular oxide layer increases after annealing. Therefore, the annealing process involves first raising the temperature to 715-725°C and holding for 10-11 hours, then cooling to 400-500°C after 4-5 hours, and then cooling to below 100°C after another 4-5 hours. Finally, the steel is cooled to 70-80°C under a hood before being removed from the furnace. The annealing furnace atmosphere is replaced with a conventional all-hydrogen atmosphere of N2 and H2 (volume ratio of 1:3) derived from the decomposition of NH3. This reduces the amount of free oxygen generated by lowering the hydrogen content, 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. This depth is further reduced by applying a certain percentage of cold rolling reduction to compress the intergranular oxide layer. This process is completed in a single-stand rolling mill with a reduction ratio controlled between 30% and 65%.

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

[0013] Also provided is a high-Mn and high-Cr automotive steel, the chemical composition of which, 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, has a microstructure of spherical pearlite, a spheroidization rate ≥95%, a yield strength Rp0.2: 295-335 MPa, Rm: 430-480 MPa, an elongation: 33-35%, and an intergranular oxide layer depth ≤4.5 μm.

[0014] The closest prior art to the present invention is CN116926416A, which covers high-carbon tool steel (C: 0.38-1.0%) for use in cutting tools, molds, and other tooling applications. This patent requires high hardness (≥48 HRC) and wear resistance. The core technical challenge is to completely eliminate intergranular oxidation (to a depth of 0 mm) and prevent surface microcracks. The proposed solution addresses two key aspects: first, by designing the composition, adding deoxidizing elements such as B, N, Al, and Ba (e.g., Al: 0.01-0.1%, Ba: 0.005-0.05%), strictly limiting O to ≤ 0.003%, H to ≤ 0.00015%, Al / N ≥ 2 (forming AlN low-angle grain boundaries), and Ba / O ≥ 9 (BaO adsorbs free oxygen, achieving "oxidation-free"). This approach, combined with low-angle grain boundaries and the absence of oxidation, controls intergranular oxidation. However, this approach presents complex compositional challenges and high costs. The second is to start from process control. This patent obtains small-angle grain boundaries to inhibit surface intergranular oxidation and decarburization by controlling the entire process of continuous casting, heating, rough rolling, finishing rolling, final rolling and pickling and cold rolling.

[0015] The steel type protected by this invention is high-Mn and high-Cr automotive steel (low-carbon steel, C: 0.14~0.18%), which is used in automotive precision stamping parts and requires high plasticity, good stamping formability (elongation 33~35%) and spheroidized structure (spheroidization rate ≥95%). The core technical issue is to control the shallow intergranular oxide layer (≤10μm) to prevent stamping collapse / tearing.

[0016] The chemical composition design is simple, with no special added elements. It relies on process control to disperse the Mn / Cr distribution and avoid the enrichment of oxidation-affinity elements. Processing: Continuous casting with light reduction combined with electromagnetic stirring ensures the dispersion of Mn / Cr and prevents the enrichment of oxidation-affinity elements. The "annealing → cold rolling" sequence is followed by cold rolling to compress the oxide layer. The annealing atmosphere is a mixture of N2:H2 (not all hydrogen or all nitrogen). The hot-rolled oxide depth is 0μm, and the annealed oxide layer depth is 6.5-9.2μm. Cold rolling with a reduction ratio of 30-65% reduces the depth to a target value of ≤4.5μm (e.g., a 65% reduction ratio reduces it to 3.2μm). This invention addresses the special needs of automotive steel by innovatively integrating the entire process of "continuous casting with light reduction + hot rolling with an oxygen barrier layer + annealing atmosphere optimization + cold rolling to compress the oxide layer," solving the problem of "intensified oxidation caused by spheroidizing annealing" not addressed by existing technologies.

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

[0018] The present invention will be further described below with reference to specific embodiments.

[0019] The present invention provides a method for controlling a shallow intergranular oxide layer in high-Mn and 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 remainder is Fe and unavoidable inclusions.

[0020] The chemical compositions of the embodiments and comparative examples of the present invention are shown in Table 1. The composition test was carried out according to GB / T 4336 "Spark Source Atomic Emission Spectrometric Analysis of Carbon Steel and Medium and Low Alloy Steel (Conventional Method)".

[0021] Table 1 Chemical compositions of various embodiments of the present invention

[0022]

[0023] The steelmaking process parameters of the embodiments of the present invention and the comparative examples are shown in Table 2.

[0024] Table 2 Main process parameters of steelmaking process of various embodiments of the present invention

[0025]

[0026] The main rolling process parameters of the embodiments of the present invention and the comparative examples are shown in Table 3.

[0027] Table 3 Rolling process parameters of various embodiments of the present invention

[0028]

[0029] The hot-rolled properties of the embodiments of the present invention and the comparative examples are shown in Table 4.

[0030] Table 4 Hot-rolled properties of various embodiments of the present invention

[0031]

[0032] Through effective control of superheat, dynamic soft reduction, and electromagnetic stirring, the dispersed distribution of Mn and Cr elements is ensured from the source. Reasonable setting of the furnace exit temperature and the time in the furnace further promotes the uniform diffusion of Mn and Cr elements. The finishing rolling start and final rolling temperatures are precisely controlled. The temperature in the finishing stage is always above 900°C. The oxygen adsorbed on the surface easily combines with the Fe on the strip surface to form iron oxide scale, which prevents oxygen from easily diffusing into the strip matrix and forming an intergranular oxide layer. In addition, during the cooling process, through rapid and intensive cooling, the driving force for the diffusion of Mn and Cr in the ferrite is reduced, preventing them from quickly enriching at the grain boundaries and avoiding internal oxidation, thereby reducing the depth of the intergranular oxide layer of the hot-rolled coil to 0μm.

[0033] The annealing process parameters and the depth of the intergranular oxide layer after annealing of the embodiments of the present invention and the comparative example are shown in Table 5.

[0034] Table 5 Annealing process parameters of various embodiments of the present invention

[0035]

[0036] After the annealing process described in Table 5, the intergranular oxide layer depth of the example steel plates increased by an average of 6.5-9.2 μm under different atmospheres, while that of the comparative example increased by an average of 23.8-25.4 mm. The generation of free oxygen was reduced by reducing the hydrogen content, thereby controlling the intergranular oxide layer depth after annealing.

[0037] The cold-rolled properties of the embodiments of the present invention and the comparative examples are shown in Table 6.

[0038] Table 6 Cold-rolled properties of various embodiments of the present invention

[0039]

[0040] In summary, by selecting raw materials according to the chemical composition weight percentages provided in the present invention and then producing according to the manufacturing method provided in the present invention, a method for controlling the shallow intergranular oxide layer of high-Mn, high-Cr automotive steel is provided. The steel exhibits a spherical pearlite microstructure and mechanical properties, including yield strength Rp0.2 of 295-335 MPa, Rm of 430-480 MPa, elongation of 33-35%, spheroidization rate ≥95%, and an intergranular oxide layer depth of ≤4.5 μm. The above examples are merely preferred examples and are not intended to limit the scope of the present invention.

Claims

1. A method for controlling shallow intergranular oxide layer of high-Mn and high-Cr automotive steel, comprising continuous casting, hot rolling, cooling coiling, cold rolling and annealing processes, characterized in that: The annealing process is provided before the cold rolling process; the chemical composition of the automobile steel includes, by weight percentage, Mn: 1.00-1.20%, Cr: 0.85-1.05%, C≤0.18%, and Si≤0.05%; In the continuous casting process, the target temperature of the tundish is controlled at 20-30°C above the liquidus temperature, and dynamic soft reduction and electromagnetic stirring are used, with a reduction of 6.5-6.8 mm. In the hot rolling process, the cast slab is heated in a heating furnace, the furnace temperature is controlled at 1200-1220°C, the furnace time is 160-180 minutes, the finishing rolling start temperature is controlled at 1050-1070°C, and the final rolling temperature is controlled at 900-920°C. The annealing process is as follows: first, the temperature is raised to 715-725°C and kept for 10-11 hours, then the temperature is reduced to 400-500°C after 4-5 hours, and then the temperature is reduced to below 100°C after another 4-5 hours, and finally the steel is cooled to 70-80°C with a hood and taken out of the furnace; the annealing process is carried out in a hood furnace with an annealing atmosphere of N2+H2; the cold rolling process is completed by a single-stand rolling mill, and the reduction rate is controlled at 30-65%.

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

3. The method for controlling shallow intergranular oxide layer of high-Mn and high-Cr automobile steel according to claim 1, characterized in that: The volume ratio of N2 to H2 in the annealing atmosphere is 1:

3.

4. The method for controlling shallow intergranular oxide layer in high-Mn and high-Cr automotive steel according to any one of claims 1 to 3, characterized in that: The automobile steel produced by the method has a microstructure of spherical pearlite, a spheroidization rate of ≥95%, and a depth of an intergranular oxide layer of ≤4.5 μm.

5. The method for controlling shallow intergranular oxide layer in high-Mn and high-Cr automotive steel according to any one of claims 1 to 3, characterized in that: The automobile steel produced by the method has a yield strength Rp0.2 of 295-335 MPa, an Rm of 430-480 MPa, and an elongation of 33-35%.

6. A high-Mn and high-Cr automobile steel, characterized in that: 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 iron and impurity elements; the steel is prepared by the method according to any one of claims 1 to 3, the steel has a structure of spherical pearlite, a spheroidization rate ≥95%, a yield strength Rp0.2: 295-335 MPa, Rm: 430-480 MPa, an elongation: 33-35%, and an intergranular oxide layer depth ≤4.5 μm.

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