A method for manufacturing a high-oxidation-resistant stainless steel for brake discs

Abstract

The application provides a preparation method of high-oxidation-resistance stainless steel for brake disc, and the process flow of the preparation method is as follows: molten steel obtained by refining general carbon molten iron is continuously cast, hot-rolled, covered and retreated, and pickled; the composition of the molten steel is controlled as follows: Mn 0.3-1.0%, Ni 0.1-0.3%, and Cu 0.10-0.25%; meanwhile, four passes of dephosphorization are simultaneously conducted in the hot-rolling process; meanwhile, in the covering and retreating process, pure nitrogen is selected as the protective gas of the covering and retreating process, the heating of the covering and retreating is controlled to 640 DEG C, and after a certain holding time, the heating is continued, the time of the high-temperature holding section is shortened, and the temperature after cooling is reduced, so that after the pickling, a dense passivation film is left on the surface of the stainless steel product after the removal of the oxide scale, the oxidation resistance of the stainless steel product is improved, and the proportion of the yellow material after quenching in the process of processing the stainless steel product into brake disc is significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of stainless steel manufacturing, in particular to a preparation method of high-oxidation-resistance stainless steel for brake discs. BACKGROUND

[0002] The process path of the brake disc is cutting plate (stainless steel plate) → punching sheet → machine tool drilling → quenching → stamping → forming → electrophoresis → polishing. The quenching process in the process path of the brake disc refers to a heat treatment process of heating the stainless steel material to about 900 degrees for a period of time in the absence of a protective atmosphere. At present, the stainless steel used for machining and producing brake discs is mainly 12Cr13 martensitic stainless steel. After quenching, the surface of 12Cr13 martensitic stainless steel will be oxidized to present different colors such as yellow, blue and black, and the proportion of yellow is low (about 10%), and the rest is blue or black. The deeper the surface color, the thicker the oxidation film after quenching. And the too thick oxidation film will cause the paint film at the punching hole to easily fall off after electrophoresis, affecting the polishing efficiency of the disc surface. Therefore, in order to ensure that the paint film at the punching hole does not fall off after electrophoresis, it is necessary to strictly control the thickness of the surface oxidation film of 12Cr13 martensitic stainless steel after quenching, and improve the proportion of the surface showing yellow after quenching.

[0003] A Chinese patent with the application publication number CN115491609A discloses a manufacturing method of low-carbon martensitic stainless steel for brake discs, but this patent is a stainless steel manufacturing method developed to solve the problems of "hot-rolled strip steel edge cracking and brake disc finished product high-speed braking shaking", and the disclosed technical solution does not and cannot solve the problems of "easy over-thickness of surface oxidation film after quenching and easy over-deep color (presenting blue or black)" existing in the prior art.

[0004] Therefore, the existing manufacturing process of 12Cr13 martensitic stainless steel cannot meet the surface quality requirements after quenching when preparing brake discs, and special manufacturing processes need to be researched to achieve the surface quality requirements of the brake disc that the proportion of the material showing yellow after quenching. SUMMARY

[0005] The present application relates to the field of stainless steel manufacturing, in particular to a preparation method of high-oxidation-resistance stainless steel for brake discs.

[0006] The technical solution for achieving the purpose of the present application is: a preparation method of high-oxidation-resistance stainless steel for brake discs, the process flow of the preparation method is: molten steel prepared by refining of plain carbon molten iron → continuous casting → hot rolling → cover annealing → pickling;

[0007] The main component requirements of the molten steel are: C 0.09-0.13%; Si 0.4-0.8%; Mn 0.3-1.0%; Cr 12.0-13.0%; Ni 0.1-0.3%; Cu 0.10-0.25%; N 0.02-0.08%; Al 0.01-0.03%; P≤0.04%; S≤0.010%; and the rest is Fe;

[0008] The molten steel is obtained by continuous casting to get a slab;

[0009] The slab is dephosphorized in the process of hot rolling, the hot rolling comprises sequentially performed hot rolling heating, five-pass rough rolling and finish rolling and hot rolling coiling, the slab is dephosphorized for the first time after the hot rolling heating and before the first-pass rough rolling; the slab is dephosphorized for the second time simultaneously when the first-pass rough rolling is performed, and dephosphorized for the third time simultaneously when the third-pass rough rolling is performed, the slab is dephosphorized for the fourth time before the finish rolling entrance; the running speed of the slab in each pass of dephosphorization is controlled at 0.6-0.8 m / min; and the coiling temperature of the hot rolling coiling is controlled at 700-720 °C;

[0010] Pure nitrogen is selected as the protective gas for the cover annealing;

[0011] The cover annealing comprises sequentially arranged heating section, high-temperature holding section and cooling section; the specific operation of the heating section is: firstly continuously heating to 550-640 °C, and then continuously heating to 810-830 °C after holding for 2-3 hours; the temperature of the high-temperature holding section is controlled at 810-830 °C, and the holding time is 8-12 h; the cooling section comprises two stages of heating cover cooling and cooling cover cooling performed in sequence, and the cooling cover cooling is to a slab temperature ≤300 °C;

[0012] The stainless steel product is obtained after pickling.

[0013] The present application particularly controls the composition of the molten steel, and specifically controls: the Mn content of the molten steel is controlled at a low level (0.3-1.0%), manganese is an easily oxidized element, and a lower manganese content is beneficial to improving its oxidation resistance, avoiding the thickening of the oxide skin, and reducing the difficulty of subsequent pickling; the Ni content of the molten steel is 0.1-0.3%, and the Cu content is 0.10-0.25%, trace components are easily combined with oxygen at the grain boundary, thereby playing a protective role on the grain boundary, avoiding the exposure of the grain boundary to accelerate oxidation; the Al content of the molten steel is 0.01-0.03%, and aluminum oxide plays a role of a dense matrix passivation layer (i.e. a chromium-rich layer of the matrix in contact with the oxide skin), thereby inhibiting the outward diffusion of iron ions from the matrix.

[0014] Meanwhile, the application also adopts a special hot rolling process, specifically: first, through first-pass phosphorus removal (high-pressure phosphorus removal) after hot rolling heating and before rough rolling, the scale (also called: "first scale") generated in the hot rolling heating process of the slab is removed as much as possible; then, second and third pass phosphorus removal is respectively performed in the first and third pass rough rolling, and fourth pass phosphorus removal is performed before finish rolling, so as to remove the scale (also called: "second scale") generated in the rough rolling process as much as possible.

[0015] In addition, the application also optimizes the control of the bell annealing process, specifically: the bell annealing furnace generally uses nitrogen, nitrogen-hydrogen mixed gas or full hydrogen as the protective gas; in view of the fact that "hydrogen is a small molecule medium with strong penetration and can react with the scale to reduce the scale, thereby destroying the structure of the scale and making it loose", the effect of hydrogen is recognized to be superior to that of nitrogen in terms of reducing the probability of oxidation; however, the present inventors choose pure nitrogen as the protective gas of the bell annealing process, mainly based on the characteristics of nitrogen that it cannot penetrate the scale and does not react with the scale, so that nitrogen does not react with the matrix chromium-rich area inside the scale, avoiding the oxidation of chromium in the chromium-rich area, thereby making the matrix chromium-rich area larger and denser, so as to form a larger and denser passivation layer, improving the oxidation resistance of the product; at the same time, the present inventors also heat to 640°C, keep warm for a period of time, then heat to the required temperature of the high-temperature holding section, and keep warm for a period of time, in order to ensure that the temperature of the entire steel coil is uniform, the scale is uniform, and the pickling is facilitated; then, the time of the high-temperature holding section in the bell annealing process is shortened to 8h (conventionally more than 15h), and the temperature after cooling is reduced to below 300°C (conventionally below 350°C), so as to inhibit the generation of scale (also called: "third scale") in the bell annealing process as much as possible, so that after pickling, the surface of the stainless steel product without scale will have a dense passivation film (chromium-rich layer), improving the oxidation resistance of the stainless steel product, so that in the process of processing brake discs, the proportion of materials that present yellow after quenching can reach more than 68%, which is much larger than the 7.5-10% before improvement, significantly improving the yield rate of stainless steel products for making brake discs.

[0016] Further, the pickling includes sulfuric acid section, mixed acid 1 section and mixed acid 2 section in sequence, the sulfuric acid concentration of the sulfuric acid section is 140-220g / L, and the temperature is 65-80°C; the pickling solution of the mixed acid 1 section and the mixed acid 2 section both adopts a mixed solution of hydrofluoric acid and nitric acid, the hydrofluoric acid concentration of the mixed acid 1 section is 1-4g / L, the nitric acid concentration is 50-80g / L, and the temperature is 35-52°C, the hydrofluoric acid concentration of the mixed acid 2 section is 0-1g / L, the nitric acid concentration is 35-52g / L, and the temperature is 35-52°C. The application adopts weak acid cleaning, by controlling the acid concentration of each stage at a low level, the pickling degree is facilitated to be controlled, so as to avoid the corrosion of the crystal boundary due to over-pickling caused by the pickling time control. Attached Figure Description

[0017] Figure 1 This is a flowchart of the hot rolling process of the present invention;

[0018] Figure 2 Metallographic images of stainless steel material before and after pickling in Comparative Example 2 under an electron microscope;

[0019] Figure 3 Metallographic images of stainless steel material under an electron microscope after the cover was removed and before pickling in Example 1;

[0020] Figure 4 The image shows the metallographic image of the stainless steel product obtained after pickling in Comparative Example 2 under an electron microscope.

[0021] Figure 5 Metallographic image of the stainless steel product obtained after pickling in Example 1 under an electron microscope;

[0022] Figure 6 Photograph of the stainless steel product prepared in Comparative Example 2 after brake disc quenching.

[0023] Figure 7 This is a photograph of the stainless steel product obtained in Example 1 after quenching during the manufacturing process of the brake disc. Detailed Implementation

[0024] The preferred embodiment of the method for preparing the high oxidation-resistant stainless steel for brake discs of the present invention will be described in detail below with reference to the accompanying drawings.

[0025] A method for preparing high oxidation-resistant stainless steel for brake discs, the process flow of which is as follows: molten steel obtained by refining ordinary carbon iron → continuous casting → hot rolling → annealing → pickling; the main components of the molten steel are required to be: C 0.09~0.13%; Si 0.4~0.8%; Mn 0.3~1.0%; Cr 12.0~13.0%; Ni 0.1~0.3%; Cu 0.10~0.25%; N 0.02~0.08%; Al 0.01~0.03%; P≤0.04%; S≤0.010%;

[0026] The molten steel is continuously cast to obtain slabs;

[0027] like Figure 1As shown, the slab is desulfurized during hot rolling, the hot rolling comprises hot rolling heating, five rough rolling passes and finish rolling and hot rolling coiling in sequence, the first pass desulfurization is carried out after the hot rolling heating and before the first rough rolling pass; the second pass desulfurization is carried out during the first rough rolling pass, the third pass desulfurization is carried out during the third rough rolling pass, and the fourth pass desulfurization is carried out before the finish rolling entrance; the running speed of the slab in the desulfurization machine is controlled at 0.6-0.8 m / min; and the coiling temperature of the hot rolling coiling is controlled at 700-720 °C.

[0028] The cover annealing selects pure nitrogen as the protective gas;

[0029] The cover annealing comprises a heating section, a high-temperature holding section and a cooling section arranged in sequence; the specific operation of the heating section is: firstly continuously heating to 550-640 °C, then continuously heating to 810-830 °C after holding for 2-3 hours; the temperature of the high-temperature holding section is controlled at 810-830 °C, and the holding time is 8-12 h; the cooling section comprises two stages of heating cover cooling and cooling cover cooling in sequence, and the cooling cover cooling is to the slab temperature ≤300 °C;

[0030] After the pickling, the stainless steel product (the stainless steel with white surface) is obtained.

[0031] After the oxide skin is removed, the product obtained by the method has a dense passivation film on the surface.

[0032] After the stainless steel coil prepared by the above preparation method is cut into plates, punched into pieces, drilled by a machine tool, and quenched, the proportion of the material with yellow surface after quenching can reach more than 68%, which is much larger than 7.5-10% before the improvement, and the yield rate of the stainless steel product for manufacturing brake discs is greatly improved.

[0033] Table 1: Main component contents of each example and comparative example (weight percentage, %)

[0034]

[0035] Table 2: Key process parameters of each example and comparative example

[0036]

[0037] Table 3: Surface quality of products prepared in each example and comparative example

[0038]

[0039] According to the data in Table 1 and Table 2, and Figure 6 and Figure 7It can be seen that the present application greatly reduces the proportion of blue and black materials after quenching by combining the component system, hot rolling and cover annealing.

[0040] And, comparing Figure 2 and Figure 3 It can be seen that the surface oxide scale structure of the stainless steel material of the present application (Example 1) after cover annealing and before pickling is obviously different from that of Comparative Example 2. The oxide scale structure on the surface of the material after cover annealing of Comparative Example 2 is severely damaged. However, the present application has the original oxide scale morphology on the surface of the material after cover annealing.

[0041] At the same time, comparing Figure 4 and Figure 5 It can be seen that the surface morphology of the stainless steel material of the present application (Example 1) after pickling is obviously different from that of Comparative Example 2. Due to the enrichment of iron oxide on the surface of the material before pickling, the difficulty of pickling is increased, and the acid concentration has to be increased on site to avoid the occurrence of oxygen defects, often resulting in over-pickling and damage to the surface oxide layer. After pickling of the present application, only the surface oxide scale is removed, and the internal dense layer is retained, so that Figure 5 a honeycomb-like surface structure appears. It is found by dotting that the chromium content in the dense layer is high and the oxygen content is very low. The dense layer is a chromium-rich layer, which must have strong oxidation resistance.

[0042] Further, the pickling includes sulfuric acid section, mixed acid 1 section and mixed acid 2 section in sequence, the sulfuric acid concentration of the sulfuric acid section is 140-220 g / L, and the temperature is 65-80°C; the pickling solution of the mixed acid 1 section and the mixed acid 2 section both use a mixed solution of hydrofluoric acid and nitric acid, the hydrofluoric acid concentration of the mixed acid 1 section is 1-4 g / L, the nitric acid concentration is 50-80 g / L, and the temperature is 35-52°C, the hydrofluoric acid concentration of the mixed acid 2 section is 0-1 g / L, the nitric acid concentration is 35-52 g / L, and the temperature is 35-52°C. The present application uses weak acid cleaning to avoid over-pickling and wash off the substrate. The HF concentration of the conventional pickling process is relatively high (4-6 g / L), which can corrode the oxide scale deeply and possibly corrode the substrate, reducing the oxidation resistance of the substrate; at the same time, the HNO3 concentration of the conventional pickling process is relatively low (40-60 g / L), which is not conducive to improving the passivation ability. The present application divides the mixed acid into sections, controls the HF concentration at a relatively low level (1-4 g / L) in the mixed acid 1 section, and increases the HNO3 concentration (50-80 g / L) at the same time, avoiding over-corrosion of HF while improving the passivation ability of HNO3, increasing the thickness of the chromium-rich layer, and improving the oxidation resistance; the mixed acid 2 section is used for supplementary pickling, so that the oxide scale is completely removed, and the entire pickling process is more effective and controllable.

[0043] The heat rolling heating temperature, rough rolling pressure, and finishing rolling pressure in the preparation method of the high-oxidation-resistance stainless steel for brake discs are not limited parameters, and the existing conventional parameters of the stainless steel (12Cr13 stainless steel) for brake discs are adopted.

[0044] The specific process parameters of the different pass phosphorus removal steps of the present application are set to remove the oxide scale on the surface of the present slab as much as possible.

[0045] The above description is only an embodiment of the present application, and does not limit the patent scope of the present application, and any equivalent process transformation or direct or indirect application in other related technical fields using the content of the present application is also included in the patent protection scope of the present application.

Claims

1. A method for preparing high oxidation-resistant stainless steel for brake discs, wherein the process flow of the preparation method is as follows: molten steel obtained by refining ordinary carbon iron → continuous casting → hot rolling → annealing → pickling; characterized in that: The main components of the molten steel are required to be: C 0.09~0.13%; Si 0.4~0.8%; Mn 0.3~1.0%; Cr 12.0~13.0%; Ni 0.1~0.3%; Cu 0.10~0.25%; N 0.02~0.08%; Al 0.01~0.03%; P≤0.04%; S≤0.010%; the remainder being Fe; The molten steel is continuously cast to obtain slabs; During the hot rolling process, the slab is descaled. The hot rolling includes sequential hot rolling heating, five roughing passes, finishing rolling, and hot rolling coiling. The slab undergoes a first descaling pass after hot rolling heating and before the first roughing pass. The slab undergoes a second descaling pass during the first roughing pass and a third descaling pass during the third roughing pass. The slab undergoes a fourth descaling pass before entering the finishing roll. The slab running speed during each descaling pass is controlled at 0.6–0.8 m / min. The coiling temperature of the hot rolling coil is controlled at 700–720°C. The protective gas for the shield is pure nitrogen. The hood includes a heating section, a high-temperature holding section, and a cooling section arranged sequentially. The heating section operates as follows: first, the temperature is continuously raised to 550–640°C, held for 2–3 hours, and then continuously raised to 810–830°C. The temperature of the high-temperature holding section is controlled at 810–830°C, and the holding time is 8–12 hours. The cooling section includes two stages: cooling with a heating hood and cooling with a cooling hood, where the hood cools the slab to a temperature ≤300°C. After pickling, a stainless steel product is obtained.

2. The method for preparing a high oxidation-resistant stainless steel for brake discs according to claim 1, characterized in that: The pickling process includes a sulfuric acid stage, a mixed acid stage 1, and a mixed acid stage 2, performed sequentially. The sulfuric acid concentration in the sulfuric acid stage is 140–220 g / L, and the temperature is 65–80°C. The pickling solutions in both mixed acid stage 1 and mixed acid stage 2 are mixed solutions of hydrofluoric acid and nitric acid. In mixed acid stage 1, the hydrofluoric acid concentration is 1–4 g / L, the nitric acid concentration is 50–80 g / L, and the temperature is 35–52°C. In mixed acid stage 2, the hydrofluoric acid concentration is 0–1 g / L, the nitric acid concentration is 35–52 g / L, and the temperature is 35–52°C.

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