Method for reducing pickling difficulty of 12Cr13 stainless steel seed cover return product

Abstract

The application provides a method for reducing pickling difficulty of 12Cr13 stainless steel cover withdrawal products, which comprises the following steps: (1) refining and continuous casting; (2) hot rolling: four passes of phosphorus removal are carried out in the process of hot rolling, the slab running speed of each pass of phosphorus removal is controlled to be 0.6-0.8 m / min, and the head, middle and tail of the slab are subjected to phosphorus removal in each pass of phosphorus removal; the coiling temperature of the tail of the steel strip within 30 meters is 680 DEG C, the coiling temperature of the rest of the steel strip except the tail is 700-720 DEG C, and the steel strip is subjected to radial bundling during coiling; (3) cover withdrawal: pure nitrogen is selected as the protective gas for cover withdrawal; the burner structure in the cover withdrawal furnace is adjusted to be dispersed blowing to the steel strip; the cover withdrawal is heated to 550-640 DEG C, and then is kept for 2-3 hours, and then is continuously heated to the temperature of the high-temperature holding section; (4) cooling: the stack slow cooling is adopted during cooling.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of stainless steel manufacturing, in particular to a method for reducing pickling difficulty of 12Cr13 stainless steel cover annealed products. BACKGROUND

[0002] The chemical composition of 12Cr13 stainless steel mainly includes carbon (C): <0.15%; silicon (Si): <1.00%; manganese (Mn): <1.00%; sulfur (S): <0.030%; phosphorus (P): <0.035%; and chromium (Cr): 11.50%~13.50%. The main feature of 12Cr13 is high hardness, good wear resistance and corrosion resistance, and it is widely used in the fields of mechanical processing, tool manufacturing, bearing parts, etc.

[0003] The general processing flow of 12Cr13 stainless steel is: smelting→hot rolling→cover annealing (cover furnace annealing)→pickling. Under the same pickling conditions, the hot rolling tail (i.e. the pickling head) of the 12Cr13 strip steel has a 5~30 meter length difference in surface quality from the rest of the part, specifically: the 5~30 meter length of the hot rolling tail (i.e. the pickling head) has oxygen residues, which causes the surface to have "black line" defects, while the rest of the part does not have "black line" defects. In order to ensure uniformity of the material surface after pickling, the pickling speed has to be slowed down to ensure that the surface oxide scale of the entire batch of strip steel can be completely removed, which affects the hourly output and also increases the pickling difficulty. SUMMARY

[0004] The purpose of the present application is to provide a method for reducing pickling difficulty of 12Cr13 stainless steel cover annealed products.

[0005] The technical solution for achieving the purpose of the present application is: a method for reducing pickling difficulty of 12Cr13 stainless steel cover annealed products, comprising the following steps:

[0006] (1) refining and continuous casting: obtaining a slab by refining and continuous casting of plain carbon molten iron;

[0007] (2) Hot rolling: the hot rolling comprises heating, five rough rolling passes, and finish rolling and hot rolling coiling which are sequentially performed, descaling is performed on the slab before the first rough rolling pass after the slab is heated in the hot rolling, descaling is simultaneously performed on the slab in the first rough rolling pass, descaling is simultaneously performed on the slab in the third rough rolling pass after the second rough rolling pass, descaling is performed on the slab before the finish rolling inlet, the running speed of the slab in each descaling pass is controlled to be 0.6-0.8 m / min, and the head, middle and tail of the slab are all descaled in each descaling pass, the slab after descaling and rough rolling is finish rolled, and a steel strip is obtained after finish rolling; the coiling temperature of the tail of the steel strip within 30 meters is 680°C, the coiling temperature of the rest of the steel strip except the tail is 700-720°C, and the steel strip is radially bundled during coiling;

[0008] (3) Box annealing: pure nitrogen is selected as the protective gas; the burner structure in the box annealing furnace is adjusted to disperse blowing on the steel strip; the box annealing comprises a heating section, a high-temperature holding section and a cooling section which are sequentially arranged; 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 to be 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 which are sequentially performed, the cooling cover cooling is to a slab temperature ≤250°C, and a box annealing product is obtained;

[0009] The steel strip is cooled after the hot rolling step and before the box annealing step, and the steel strip is stack-cold-cooled during cooling.

[0010] The present application has the following technical effects:

[0011] (1) The present inventor finds that there is a difference in the structure of the surface oxide scale within 30 meters of the hot rolling tail and the rest of the material after box annealing through microscopic analysis of the box annealing product of 12Cr13 stainless steel by electron microscope (also known as "electron microscope"). Based on this, it is thought that the difficulty and efficiency of pickling can be reduced by improving the uniformity of the surface oxide scale of the box annealing product, and the problem of residual oxygen after pickling of only the hot rolling tail of the steel strip can be solved;

[0012] (2) The hot rolling process is adjusted, and the number of descaling passes is increased, so that not only the first and second descaling passes are performed at the inlet of the rough rolling to remove the oxide scale (also known as "primary oxide scale") generated during the heating process of the hot rolling, but also two descaling passes are performed during the entire rough rolling process to remove the oxide scale (also known as "secondary oxide scale") generated during the rough rolling process, and the running speed of the descaling machine is reduced, so that the primary oxide scale and the secondary oxide scale generated during the hot rolling process can be removed as much as possible;

[0013] (3) Further adjust the bell annealing process. Although "hydrogen is a small molecule medium, has strong penetration, can react with the oxide scale to reduce the oxide scale, thereby destroying the structure of the oxide scale, making it loose, is the best choice for the bell annealing protective gas". However, the inventor seeks a method to solve the surface oxidation defect from the "uniformity of the surface oxide scale structure" (not just the removal of the oxide scale effect), so the present application changes the bell annealing protective gas from hydrogen to nitrogen, and disperses the steel strip in the bell annealing furnace. Due to the characteristics of nitrogen that cannot penetrate the oxide scale and does not react with the oxide scale, the nitrogen does not react with the chromium-rich region of the matrix inside the oxide scale, avoiding the oxidation of chromium in the chromium-rich region, so that the chromium-rich region of the matrix becomes larger and denser, thereby forming a larger, denser and more uniform passivation layer on the surface of the steel strip inside the oxide scale. At the same time, the inventor also heats to 550-640°C, then keeps the temperature for a period of time, and then raises the temperature to the high temperature section, to ensure that the temperature is uniform at the head, middle and tail, the oxide scale and the passivation layer are uniform, which is beneficial to pickling, and also makes the surface oxidation resistance uniform;

[0014] (4) At the same time, the present application also shortens the time of the high temperature holding section in the bell annealing process to 8h (conventionally more than 15h) and reduces the temperature after cooling to below 250°C (conventionally below 350°C), thereby trying to inhibit the generation of oxide scale (also known as "third oxide scale") during the bell annealing process;

[0015] (5) In addition, the hot rolling coiling temperature of the steel strip within 30 meters of the head and the rest of the steel strip is differentially set, and the coiled steel strip is radially bunched, and after hot rolling, the steel strip is stacked and slowly cooled, so that the oxide scale formed on the surface of the steel strip is more uniform;

[0016] (6) The present application optimizes and adjusts a series of processes, so that the oxide scale structure formed on the surface of the 12Cr13 bell annealing product is uniform, the passivation layer (chromium-rich layer) inside the oxide scale is uniform and dense, the pickling difficulty and efficiency are reduced, and the steel strip parts are all oxygen-free (without "black line" defects) under the same pickling conditions, effectively improving the surface quality of the product.

[0017] The bell annealing furnace nozzle is an important component in the bell annealing furnace, which mainly sprays the gas generated after the fuel is burned and the protective gas into the bell annealing furnace, so that the bell annealing furnace can work efficiently and stably. Preferably, at least one group of nozzles is arranged on the wall of the bell annealing furnace along the axial direction, and each group of nozzles comprises a plurality of nozzles uniformly arranged along the circumferential direction of the bell annealing furnace. Each nozzle is arranged along the radial direction of the bell annealing furnace and inclined to one side by 5-10°, avoiding directly blowing the steel strip and causing local overburning of the hot-rolled tail of the steel strip.

[0018] Furthermore, the mask removal product from step (3) is subjected to pickling. The pickling 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. The hydrofluoric acid concentration in mixed acid stage 1 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 in mixed acid stage 2 is 0–1 g / L, the nitric acid concentration is 35–52 g / L, and the temperature is 35–52°C. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view of the oxide scale at the hot-rolled tail end of the coated product prepared in Comparative Example 1 (Prior Art) under an electron microscope;

[0020] Figure 2 The oxide scale cross section of the remaining part (hot-rolled head) of the coated product prepared in Comparative Example 1 (Prior Art) is shown under an electron microscope.

[0021] Figure 3 This is a cross-sectional view of the oxide scale at the hot-rolled tail end of the coated product prepared in any of Examples 1 to 3 under an electron microscope;

[0022] Figure 4 It is the cross-section of the oxide scale under an electron microscope of the remaining part (hot-rolled head) of the coated product obtained in any of Examples 1 to 3;

[0023] Figure 5 This is a comparison diagram of the mask removal product of Example 1 after pickling, showing the head within 30 meters and the rest of the product.

[0024] Figure 6 This is a comparison diagram of the mask removal product of Example 2 after pickling, showing the head within 30 meters and the rest of the product.

[0025] Figure 7 This is a comparison diagram of the mask removal product of Example 3 after pickling, showing the head within 30 meters and the rest of the product.

[0026] Figure 8 This is a comparison diagram of the mask removal product of Comparative Example 1 after pickling, showing the head within 30 meters and the rest of the product.

[0027] Figure 9 This is a partial axial cross-sectional view of the furnace structure of the present invention;

[0028] Figure 10 The present invention relates to the furnace edge of the hood. Figure 9 Schematic diagram of the cross-sectional structure of the middle BB line;

[0029] Figure 11 This is a flowchart of the hot rolling process of the present invention. Detailed Implementation

[0030] The preferred embodiment of the method for reducing the pickling difficulty of 12Cr13 stainless steel seed shell products of the present invention will be described in detail below with reference to the accompanying drawings.

[0031] A method for reducing the pickling difficulty of 12Cr13 stainless steel seed products includes the following steps:

[0032] (1) Refining and continuous casting: The ordinary carbon iron is refined and continuously cast to obtain slabs;

[0033] (2) Hot rolling: such as Figure 11 As shown, the hot rolling process includes sequential heating, five roughing passes, finishing rolling, and hot rolling coiling. The slab undergoes a first descaling pass (high-pressure descaling) after hot rolling heating and before the first roughing pass. The slab undergoes a second descaling pass during the first roughing pass, a third descaling pass during the third roughing pass, and a fourth descaling pass before the finishing rolling entrance. The slab running speed during each descaling pass is controlled at 0.6–0.8 m / min, and descaling is performed on the head, middle, and tail sections of the slab during each pass. The slab after roughing and descaling undergoes finishing rolling to obtain a steel strip. During steel strip coiling, the coiling temperature for the last 30 meters of the steel strip is 670–690°C, and the coiling temperature for the remaining portion of the steel strip excluding the last 30 meters is 700–720°C. Furthermore, the steel strip is radially bundled during the coiling process.

[0034] (3) Covering and annealing: Pure nitrogen is selected as the protective gas for the covering and annealing; the burner structure in the covering and annealing furnace is adjusted to disperse the blowing of the steel strip; the covering and annealing includes a heating section, a high-temperature holding section and a cooling section arranged in sequence; the specific operation of the heating section is as follows: first, the temperature is continuously raised to 550-640°C, and after holding for 2-3 hours, the temperature is 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 cover and cooling with a cooling cover, which are carried out in sequence. The cooling cover cools the slab temperature to ≤250°C to obtain the covered and annealed product;

[0035] The steel strip is cooled after the hot rolling step and before the sheathing step, and slow cooling is achieved by stacking.

[0036] Based on several embodiments (Examples 1 to 3) of the method for reducing the pickling difficulty of 12Cr13 stainless steel seed coating products of the present invention and the key process parameters of the existing manufacturing method of 12Cr13 stainless steel seed coating products (Comparative Example 1), the key process parameters are shown in Table 1.

[0037] The molten steel obtained by refining the molten iron in step (1) in each of the examples and the comparative example is continuously cast to obtain a slab, and the chemical composition of the molten steel satisfies: carbon (C): <0.15%; silicon (Si): <1.00%; manganese (Mn): <1.00%; sulfur (S): <0.030%; phosphorus (P): <0.035%; and chromium (Cr): 11.50% to 13.50%.

[0038] Further, in the existing manufacturing method of the 12Cr13 stainless steel cover annealed product (comparative example), the phosphorus removal by hot rolling is only three passes, two passes at the entrance of rough rolling and one pass of rough rolling, and the temperature is continuously increased to 810 to 830°C during the cover annealing (no treatment of 2 to 3 hours of holding at 550 to 640°C).

[0039] Table 1: Key process parameters of each example and the comparative example

[0040]

[0041] The cross-sectional images of the surface oxide scale of the cover annealed product manufactured by the existing manufacturing method of the 12Cr13 stainless steel cover annealed product (i.e., Comparative Example 1) within 30 meters of the tail end of hot rolling and the rest of the part (the head end of hot rolling) under the electron microscope are shown in Figure 1 , Figure 2 respectively. As can be seen from Figure 1 and Figure 2 , the thickness of the surface oxide scale at the tail end of hot rolling is thicker and more uneven than that at the head end of hot rolling.

[0042] The cross-sectional images of the surface oxide scale of the cover annealed product manufactured according to the method for reducing the pickling difficulty of the 12Cr13 stainless steel cover annealed product of the present application within 30 meters of the tail end of hot rolling and the rest of the part under the electron microscope are shown in Figure 3 , Figure 4 respectively. As can be seen from 3 and Figure 4 , the thickness of the oxide scale at the tail end of hot rolling and the head end of hot rolling is both thinner and more uniform.

[0043] The cover annealed products manufactured according to each example and Comparative Example 1 are subjected to pickling, which includes a sulfuric acid section, a mixed acid 1 section, and a mixed acid 2 section in sequence. The sulfuric acid section has a sulfuric acid concentration of 140 to 220 g / L and a temperature of 65 to 80°C. The pickling solution in the mixed acid 1 section and the mixed acid 2 section both use a mixed solution of hydrofluoric acid and nitric acid. The mixed acid 1 section has a hydrofluoric acid concentration of 1 to 4 g / L, a nitric acid concentration of 50 to 80 g / L, and a temperature of 35 to 52°C. The mixed acid 2 section has a hydrofluoric acid concentration of 0 to 1 g / L, a nitric acid concentration of 35 to 52 g / L, and a temperature of 35 to 52°C. The surfaces of the pickled products of each example and the comparative example are observed, and the results are shown in Table 2 below.

[0044] Table 2 Surface quality of the pickled products corresponding to each example and the comparative example

[0045]

[0046] The contrastive photos of the pickled head (i.e. hot-rolled tail, within 30 meters) and the rest part (pickled tail) of the steel strip of the cover-rolled product of Example 1-3 are shown in Figures 5 to 7 It can be seen that there is no oxygen residue in the head and tail of the pickled steel strip.

[0047] The contrastive photos of the pickled head (i.e. hot-rolled tail, within 30 meters) and the rest part (pickled tail) of the steel strip of the cover-rolled product of Comparative Example 1 are shown in Figure 8 It can be seen that there is oxygen residue in the pickled head (i.e. hot-rolled tail) of the pickled steel strip, but there is no oxygen residue in the pickled tail.

[0048] Therefore, according to the method for reducing the pickling difficulty of the cover-rolled product of 12Cr13 stainless steel, the surface of the pickled product (white skin) is clean, and there is no oxygen residue and "black line" defect in the pickled head (i.e. hot-rolled tail).

[0049] Preferably, at least one group of nozzles is arranged on the wall 201 of the cover-rolling furnace 200 along the axial direction thereof, each group of nozzles comprising a plurality of (for example, two groups) nozzles 300 arranged uniformly along the circumferential direction of the cover-rolling furnace 200. Each nozzle 300 is arranged along the radial direction of the cover-rolling furnace 200 and is inclined to one side by an angle A (5-10°). During cover-rolling, the coiled steel strip 100 is coaxially placed into the interior of the cover-rolling furnace 200. The nozzle structure design of the present application can avoid the gas sprayed by the nozzles 300 directly blowing on the steel strip 100, causing the hot-rolled tail of the steel strip 100 to be locally overburned.

[0050] The non-limited parameters in the method for reducing the pickling difficulty of the cover-rolled product of 12Cr13 stainless steel, such as the hot-rolled heating temperature and the rough-rolled pressure, are all the existing conventional parameters of the processing technology of 12Cr13 stainless steel.

[0051] The specific process parameters of the different pass phosphorus removal steps of the present application are all set to remove the oxide skin on the surface of the current slab as much as possible.

[0052] 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 reducing the pickling difficulty of 12Cr13 stainless steel seed products, characterized in that, Includes the following steps: (1) Refining and continuous casting: ordinary carbon iron is refined and continuously cast to obtain slabs; (2) Hot rolling: The hot rolling includes heating, five roughing passes, finishing and hot rolling coiling in sequence. The slab is dephosphorized in the first pass after hot rolling heating and before the first roughing pass. The slab is dephosphorized in the second pass during the first roughing pass and in the third pass during the third roughing pass. The slab is dephosphorized in the fourth pass before the finishing pass. The slab running speed during each dephosphorization pass is controlled at 0.6 to 0.8 m / min. The head, middle and tail of the slab are dephosphorized during each dephosphorization pass. The slab after dephosphorization and roughing is finished and steel strip is obtained. The coiling temperature of the steel strip within 30 meters of the tail is 680°C. The coiling temperature of the remaining part of the steel strip excluding the tail is 700 to 720°C. The steel strip is radially bundled during the coiling process. (3) Covering and annealing: Pure nitrogen is selected as the protective gas for the covering and annealing; the burner structure in the covering and annealing furnace is adjusted to disperse the blowing of the steel strip; the covering and annealing includes a heating section, a high-temperature holding section and a cooling section arranged in sequence; the specific operation of the heating section is as follows: first, the temperature is continuously raised to 550-640°C, and after holding for 2-3 hours, the temperature is 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 cover and cooling with a cooling cover, which are carried out in sequence. The cooling cover cools the slab temperature to ≤250°C to obtain the covered and annealed product; The steel strip is cooled after the hot rolling step and before the sheathing step, and slow cooling is achieved by stacking.

2. The method for reducing the pickling difficulty of 12Cr13 stainless steel seed shell products according to claim 1, characterized in that: In step (2), at least one set of nozzles is provided on the furnace wall along its axial direction, and each set of nozzles includes several nozzles evenly arranged along the circumference of the furnace.

3. The method for reducing the pickling difficulty of 12Cr13 stainless steel seed shell products according to claim 2, characterized in that: Each nozzle is set along the radial extension of the furnace and tilted to one side at 5-10°.

4. The method for reducing the pickling difficulty of 12Cr13 stainless steel seed shell products according to claim 1, characterized in that: The mask removal product from step (3) is pickled. 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. The hydrofluoric acid concentration in mixed acid stage 1 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 in mixed acid stage 2 is 0–1 g / L, the nitric acid concentration is 35–52 g / L, and the temperature is 35–52°C.

Images