Production method for improving surface quality of cold-rolled high-strength steel

Abstract

The invention discloses a production method for improving the surface quality of cold-rolled high-strength steel, and belongs to the technical field of steel cold rolling. The method comprises the steps of smelting and casting, hot rolling, acid rolling and annealing. In the hot rolling process, the furnace entering temperature of a casting blank is controlled to be 500-700 DEG C, the rough rolling accumulated reduction rate is 80-90%, the time of a steel plate from a rough rolling R2 rolling mill to an inlet of a finish rolling F1 rolling mill is controlled to be 10-25 s, the coiling temperature is 350-500 DEG C, and after coiling, fans are placed at ventilation openings in a single-layer staggered mode for cooling; in the pickling rolling process, the total reduction rate of cold rolling is 35-55%; and in the annealing process, the dew point temperature in an annealing furnace is-15 to-35 DEG C, the volume fraction of H2 in a gas medium in a soaking and slow cooling section is 4-6%, and the volume fraction of H2 in a gas medium in a rapid cooling section is 15-20%. The surface color difference problem of the cold-rolled high-strength steel can be solved, and the surface quality and the coating property of the product are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of steel cold rolling, and in particular relates to a production method for improving the surface quality of cold-rolled high-strength steel. Background Art

[0002] At present, with the development of lightweight automobiles, the proportion of high-strength steel used in automobile manufacturing is increasing. As the strength of automobile steel increases, the amount of alloy added is also increasing accordingly. High-strength steel is generally based on a carbon-silicon-manganese composition system. Silicon is very easy to enrich on the surface to form iron olivine, which is difficult to remove; Mn is very easy to enrich on the surface, resulting in a decrease in the coatability of high-strength steel and deterioration of surface quality. At present, the method of reducing Si and Mn and adding alloying elements such as niobium and titanium is generally adopted to ensure the performance of plates and strips and improve the surface quality, but the production cost is high, it is not suitable for large-scale promotion and application, and it also increases resource consumption.

[0003] Chinese Patent 202110794705.5 "A Method for Improving the Surface Color Difference of GPa-Grade High-Strength Steel" discloses a method for improving the surface color difference of GPa-Grade high-strength steel by controlling descaling water, coiling temperature, straightening elongation, pickling temperature and time. However, the pickling speed is 50-60m / min, the production speed is low, and the limitations are large. The descaling water pressure is too small to achieve the purpose of secondary iron scale oxidation. At the same time, the coiling temperature is 500-550℃. For high-silicon and high-manganese GPa-grade cold-rolled high-strength steel, it is generally near the nose temperature of the iron scale eutectoid reaction, which increases the difficulty of pickling and descaling and the occurrence of surface defects.

[0004] Chinese patent 201910208062.4 "A method for producing high-strength steel" discloses a method for producing high-strength steel, and designs the composition of the steel grade. Si element is added for strengthening, and an anchor-like pinning morphology appears at the added interface. FeO wraps Fe2SiO4 and penetrates into the steel matrix to improve the adhesion of the inner iron sheet. Cr and Mo are added to reduce the formation of mesh iron sheet. Due to the addition of Mo element, there is a Mo element enrichment zone at the matrix interface, which plays a role in closing the oxidation interface. The removal force of iron sheet can be significantly improved by adding P element. This invention solves the technical problem of color difference defects, and can effectively reduce the incidence of color difference defects of cold-rolled sheets, thereby improving the surface quality of the product. However, the Mo content of this invention is relatively high, which increases the alloy cost. At the same time, the addition of P content deteriorates the toughness of the material during the cold working process and increases the risk of cracking during the material forming process. Therefore, effectively improving the surface quality of high-strength steel has become a top priority for improving the quality and quantity of advanced high-strength steel in the industry. Summary of the invention

[0005] The present invention provides a production method for improving the surface quality of cold-rolled high-strength steel, which can solve the problem of surface color difference of cold-rolled GPa-grade high-strength steel in the prior art and improve the surface quality of the product.

[0006] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is:

[0007] A production method for improving the surface quality of cold-rolled high-strength steel, comprising smelting and casting, hot rolling, acid rolling, and annealing processes;

[0008] In the hot rolling process, the temperature of the ingot entering the furnace is controlled at 500-700°C, the cumulative reduction rate of rough rolling is 80-90%, the time for the steel plate from the rough rolling R2 mill to the entrance of the finishing F1 mill is controlled at 10-25s, the coiling temperature is 350-500°C, and the steel coil is staggered in a single layer at the vent for fan cooling after being coiled off the line; the coiling temperature is represented by T, when 400≤T≤500°C, the average cooling rate is 10-20°C / h, cooled to 300°C and then naturally cooled; when 350≤T<400°C, the average cooling rate is 5-8°C / h, cooled to 300°C and then naturally cooled;

[0009] In the acid rolling process, the total cold rolling reduction rate is 35-55%;

[0010] In the annealing process, the dew point temperature in the annealing furnace is between -15 and -35°C, the H2 volume fraction in the gas medium in the soaking and slow cooling stages is between 4 and 6%, and the H2 volume fraction in the gas medium in the fast cooling stage is between 15 and 20%.

[0011] Furthermore, in the hot rolling process, the slab is heated to 1150-1220° C. in a heating furnace, and the slab is kept warm for 30-50 minutes, with a heating time of 120-150 minutes.

[0012] Furthermore, in the hot rolling process, the temperature of the rough rolling R1 mill is 1060-1100°C, and the temperature of the rough rolling R2 mill is 990-1090°C, wherein the time for the slab to be transferred from the heating furnace to the rough rolling R2 mill is controlled at 180-240s, and the descaling pressure is 19-22MPa.

[0013] Furthermore, in the hot rolling process, the finishing rolling inlet temperature is 960-1040° C., and the final rolling temperature is 840-890° C.; the steel plate after rolling is flattened with an elongation of 0.3-0.6%.

[0014] Furthermore, in the pickling process, the elongation of the straightening machine is 0.4-0.8%, and the insertion amount of the straightening roller is 25-35 mm; there are 3 pickling stages, the pickling time of the strip in the 3# pickling stage is 30-40 s, and the HCl concentration in the 3# pickling stage is 175-210 g / L.

[0015] Furthermore, in the acid rolling process, the cold rolling reduction rate of the F1 rolling mill is 10-20%, the reduction rate of the F2 rolling mill is 10-15%, the reduction rate of the F3 rolling mill is 6-15%, the reduction rate of the F4 rolling mill is 4-9%, and the reduction rate of the F5 rolling mill is 1-5%.

[0016] Furthermore, in the acid rolling process, the tension between the rolling mill stands is 200-400 KN, the emulsion flow rate of the rolling mill stands is 2000-4000 I / s, and the emulsion temperature is 55-62°C.

[0017] Furthermore, the composition and mass percentage of the high-strength steel are C: 0.07-0.12%, Si: 0.5-1.2%, Mn: 2.2-3.0%, P≤0.015%, S≤0.009%, Nb: 0.01-0.03%, Ti≤0.04%, Al: 0.03-0.2%, and the rest are Fe and unavoidable impurity elements.

[0018] Furthermore, the tensile strength of the high-strength steel is ≥800 MPa, the internal oxidation peak depth of silicon and manganese elements on the surface of the high-strength steel is between 0.5 μm and 1.5 μm, and there is no obvious external oxidation phenomenon.

[0019] The beneficial effects of the above technical solution are as follows: (1) The hot charging process + low temperature insulation is adopted to save the heating time, reduce the decarburization and coarsening of the surface chill layer of the ingot, and avoid the precipitation and ripening of microalloys, thereby strengthening the surface structure; (2) The large rough rolling reduction rate can increase the nucleation distortion energy of the steel billet at high temperature, promote the recrystallization of the subsurface structure of the high plate, and avoid the formation of mixed crystal structure on the surface; combined with high-pressure water descaling, it can prevent the occurrence of pickling defects of the plate and strip; (3) Optimize the finishing rolling time, reduce the high temperature residence time, and reduce secondary oxidation. (4) Low-temperature coiling and vent cooling are used to avoid the eutectoid reaction of the oxide scale, which increases the difficulty of removing the oxide layer, and at the same time avoid intergranular oxidation of the surface structure at high temperature, which deteriorates the surface quality; (5) A small reduction rate is used in acid rolling to reduce the deformation of the surface structure, reduce the breakage rate of the surface grains and the expansion of surface defects that deteriorate the surface quality; (6) The low dew point + high hydrogen mode is used in the annealing process, which is beneficial to inhibit the diffusion and enrichment of Si and Mn to the surface, thereby improving the surface quality and coatability of cold-rolled high-strength steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a cross-sectional microstructure diagram of the high-strength steel of Example 1;

[0021] Figure 2 This is the distribution diagram of silicon, oxygen and manganese elements with depth in the high-strength steel of Example 1. DETAILED DESCRIPTION

[0022] The present invention will be further described in detail below in conjunction with specific examples. These examples are only for describing the best implementation of the present invention and are not intended to limit the scope of the present invention.

[0023] Examples 1-6

[0024] A production method for improving the surface quality of cold-rolled high-strength steel includes smelting and casting, hot rolling, pickling and annealing processes; the specific steps and process parameter control are as follows:

[0025] (1) Smelting and casting to obtain slabs. The composition of the slabs in various embodiments is shown in Table 1.

[0026] Table 1 Chemical composition of slabs in various examples (wt / %)

[0027] project C Si Mn Al Ti Nb P S Example 1 0.08 1.2 2.2 0.03 0.01 0.01 0.015 0.005 Example 2 0.09 1.0 2.7 0.05 0.04 0.01 0.006 0.007 Example 3 0.12 0.9 3.0 0.17 —— 0.03 0.007 0.009 Example 4 0.10 0.8 2.3 0.20 0.02 0.02 0.012 0.009 Example 5 0.11 0.5 2.8 0.10 0.03 0.015 0.010 0.006 Example 6 0.10 1.0 2.6 0.09 0.02 0.02 0.013 0.008

[0028] (2) The slab heating and insulation, rough rolling and finish rolling processes are shown in Tables 2 and 3.

[0029] Table 2 Slab heating and insulation parameters of each embodiment

[0030]

[0031] Table 3 Rough rolling and finish rolling process parameters of each embodiment

[0032]

[0033] (3) The hot-rolled plate is pickled and cold-rolled to obtain a cold-hardened plate strip. The pickling and cold-rolling processes of each embodiment are shown in Table 4 and Table 5.

[0034] Table 4 Pickling and cold rolling process parameters of each embodiment

[0035]

[0036]

[0037] Table 5 Cold rolling process reduction ratio of each embodiment

[0038]

[0039] (4) The cold-hardened strip is annealed in an annealing furnace to obtain a cold-rolled high-strength steel strip. The annealing process and surface quality are shown in Table 6.

[0040] Table 6 Annealing process and surface quality of each embodiment

[0041]

[0042] Figure 1This is a cross-sectional microstructure diagram of the high-strength steel of Example 1. As can be seen from the figure, there is no obvious oxide film on the surface of the high-strength steel, and it has good coating and plating properties.

[0043] Figure 2 This is the distribution diagram of silicon, oxygen and manganese elements with depth in the high-strength steel of Example 1. It can be seen from the figure that there is almost no enrichment of silicon and manganese elements in the surface layer, and the average depth of the inner oxidation peak is 0.75μm, which further illustrates that the surface quality of the high-strength steel is good, which is conducive to the subsequent electrophoretic coating process.

Claims

1. A production method for improving the surface quality of cold-rolled high-strength steel, characterized in that: It includes smelting casting, hot rolling, pickling and annealing processes; In the hot rolling process, the temperature of the ingot entering the furnace is controlled at 500-700°C, the cumulative reduction rate of rough rolling is 80-90%, the time for the steel plate from the rough rolling R2 mill to the entrance of the finishing F1 mill is controlled at 10-25s, the coiling temperature is 350-500°C, and the steel coil is staggered in a single layer at the vent for fan cooling after being coiled off the line; the coiling temperature is represented by T, when 400≤T≤500°C, the average cooling rate is 10-20°C / h, cooled to 300°C and then naturally cooled; when 350≤T<400°C, the average cooling rate is 5-8°C / h, cooled to 300°C and then naturally cooled; In the acid rolling process, the total cold rolling reduction rate is 35-55%; In the annealing process, the dew point temperature in the annealing furnace is between -15 and -35°C, the volume fraction of H2 in the gas medium in the soaking and slow cooling stages is 4 to 6%, and the volume fraction of H2 in the gas medium in the fast cooling stage is 15 to 20%.

2. The production method for improving the surface quality of cold-rolled high-strength steel according to claim 1, characterized in that: In the hot rolling process, the slab is heated to 1150-1220° C. in a heating furnace, and the slab is kept warm for 30-50 minutes, and the heating time is 120-150 minutes.

3. The production method for improving the surface quality of cold-rolled high-strength steel according to claim 2, characterized in that: In the hot rolling process, the temperature of the rough rolling R1 mill is 1060-1100° C., and the temperature of the rough rolling R2 mill is 990-1090° C., wherein the time for the slab to be transferred from the heating furnace to the rough rolling R2 mill is controlled at 180-240s, and the descaling pressure is 19-22MPa.

4. The production method for improving the surface quality of cold-rolled high-strength steel according to claim 3, characterized in that: In the hot rolling process, the finishing rolling inlet temperature is 960-1040° C., and the final rolling temperature is 840-890° C.; the steel plate after rolling is flattened with an elongation of 0.3-0.6%.

5. The production method for improving the surface quality of cold-rolled high-strength steel according to claim 4, characterized in that: In the pickling process, the elongation of the straightening machine is 0.4-0.8%, and the insertion amount of the straightening roller is 25-35 mm. There are 3 pickling stages in total, and the pickling time of the strip in the 3# pickling stage is 30-40 seconds. The HCl concentration in the 3# pickling stage is 175-210 g / L.

6. The production method for improving the surface quality of cold-rolled high-strength steel according to claim 5, characterized in that: In the acid rolling process, the cold rolling F1 rolling mill has a reduction rate of 10-20%, the F2 rolling mill has a reduction rate of 10-15%, the F3 rolling mill has a reduction rate of 6-15%, the F4 rolling mill has a reduction rate of 4-9%, and the F5 rolling mill has a reduction rate of 1-5%.

7. The production method for improving the surface quality of cold-rolled high-strength steel according to claim 6, characterized in that: In the acid rolling process, the tension between the rolling mill frames is 200-400 KN, the emulsion flow rate of the rolling mill frames is 2000-4000 I / s, and the emulsion temperature is 55-62°C.

8. The production method for improving the surface quality of cold-rolled high-strength steel according to any one of claims 1 to 7, characterized in that: The components and mass percentage of the high-strength steel are C: 0.07-0.12%, Si: 0.5-1.2%, Mn: 2.2-3.0%, P≤0.015%, S≤0.009%, Nb: 0.01-0.03%, Ti≤0.04%, Al: 0.03-0.2%, and the rest are Fe and unavoidable impurity elements.

9. The production method for improving the surface quality of cold-rolled high-strength steel according to claim 8, characterized in that: The tensile strength of the high-strength steel is ≥800MPa, and the depth of the oxidation peak in the silicon and manganese elements on the surface of the high-strength steel is between 0.5μm and 1.5μm.

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