Method for manufacturing a thin gauge hot formed pickled steel

Abstract

The application discloses a kind of manufacturing methods of thin gauge hot forming pickling steel, including the following steps: hot metal desulphurization, converter smelting, refining, thin slab continuous casting, rotary descaling, soaking furnace heating, finishing rolling, coiling, cooling, flattening, straightening, pickling, rust-proof coating.The thin gauge hot forming pickling steel manufactured by the application has good structure and performance, and also has good pickling surface quality, that is, the surface iron oxide scale indentation defect of the thin gauge hot forming pickling steel is ≤2 places / 1000 m; the surface plate width middle portion whiteness value of the thin gauge hot forming pickling steel is ≥70%, and the difference between surface plate width edge portion whiteness value and middle portion whiteness value is ≤5%; the surface black and white stripe color difference defect number of the thin gauge hot forming pickling steel is ≤5 strips / 1000 m, solving the unique surface quality problem in the thin gauge hot forming pickling steel thin slab continuous casting and rolling production process.

Description

Technical Field

[0001] This invention relates to a method for manufacturing steel, and more particularly to a method for manufacturing thin-gauge hot-formed pickled steel. Background Technology

[0002] Hot-formed steel is widely used in the production of automotive body-in-white due to its advantages such as ultra-high strength, high wear resistance, low springback of parts, high dimensional accuracy, machinability of complex parts, and lightweight. It can reduce the weight of the vehicle body by up to 30% and significantly improve the overall vehicle safety performance.

[0003] Existing automotive body-in-white technologies typically employ a hot-rolling + cold-rolling manufacturing process for hot-formed steel, which suffers from long processes, complex procedures, and high energy consumption. A solution is to directly produce thin-gauge automotive hot-formed steel using a thin-slab continuous casting and rolling process (short process), replacing traditional cold-rolled hot-formed steel—a process known as "hot instead of cold." However, in this short-process production, thin-gauge hot-formed pickled steel exhibits unique surface quality issues, including: oxide scale indentation defects, affecting yield; and a dull color on the pickled sheet surface, with noticeable edge color differences and black-and-white stripe color differences, impacting user perception and usability, and reducing the competitiveness of related products in the market and among users. Specifically, in areas with black-and-white stripe color difference defects, increased pearlite was detected on the surface. After pickling, these areas showed varying degrees of dissolution of grain boundaries and grains compared to normal areas. The interaction of light and the surface morphology resulted in black / white stripe differences visible to the human eye, with 200-500 black-and-white stripe differences per 1000m of strip steel. The increase in pearlite content indicates that carbon enrichment has occurred in this area. The carbon enrichment layer is 60-80μm deep and is inherited to the finished plate surface, causing defects. The carbon source comes from the powder slag layer in the crystallizer, the carbon-rich layer in the liquid slag layer, or the slag ring (a mixture of unreacted protective slag and molten slag).

[0004] Chinese invention patent ZL 201710822552.4 discloses a method for producing thin-gauge hot-formed steel based on the CSP process. The chemical composition and weight percentage of the thin-gauge hot-formed steel designed in this invention are as follows: C: 0.20-0.25%, Si: 0.15-0.30%, Mn: 1.15-1.5%, Cr: 0.20-0.40%, Ti: 0.015-0.05%, Nb: 0.018-0.03%, B: 0.002-0.005%, Als: 0.015-0.05%, P≤0.015%, S≤0.004%, N≤0.005%, with the remainder being Fe and unavoidable impurities. The CSP production method adopted includes the following steps: steelmaking, continuous casting, homogenization, descaling, finishing rolling, laminar flow cooling and coiling, leveling, and hot stamping. This production method can only guarantee the composition and basic properties of the strip steel, but cannot solve the surface defects unique to hot-formed pickled steel. Summary of the Invention

[0005] The purpose of this invention is to provide a method for manufacturing thin-gauge hot-formed pickled steel, which can effectively improve the pickling surface quality of the strip steel while ensuring that the strip steel structure and properties meet the standards and user requirements.

[0006] This invention is implemented as follows:

[0007] A method for manufacturing thin-gauge hot-formed pickled steel includes the following steps:

[0008] Step 1: Desulfurize the molten iron to ensure that the sulfur content in the molten iron is ≤0.002%;

[0009] Step 2: The desulfurized molten iron is smelted into molten steel through a converter steelmaking-ladle refining-vacuum refining process. The chemical composition of the molten steel includes, by mass percentage: C: 0.20-0.25%, Si: 0.15-0.35%, Mn: 1.10-1.50%, P≤0.008%, S≤0.006%, Cr: 0.15-0.30%, B: 0.002-0.005%, Ti: 0.02-0.05%, Als: 0.02-0.06%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0010] Step 3: Use thin slab continuous casting to continuously cast molten steel into a continuous casting billet; wherein, the protective slag used in continuous casting is made of ceramic material BN as the skeleton material; the casting speed is controlled at 3.6-4.2m / min, the liquid core reduction is controlled at the billet thickness range of 65-70mm, and the thickness of the liquid slag layer in the crystallizer is controlled at 15-18mm;

[0011] Step 4: Perform rotary descaling on the continuously cast billet;

[0012] Step 5: The continuously cast billet after rotary descaling is sent into a soaking furnace for heating. The billet exiting the furnace is at a temperature of 1190-1220℃ and the soaking time is 30-35 minutes.

[0013] Step 6: Descaling the homogenized continuous casting billet with high-pressure water at a pressure of 300-380 bar.

[0014] Step 7: Finish roll the continuously cast billet into strip steel. The reduction rate of the first pass is controlled at 55-65%, and the reduction rate of the second pass is controlled at 50-60%. Descaling is performed between the first and second passes, with a descaling water pressure of 180-200 bar. The final rolling temperature is controlled at 880-920℃.

[0015] Step 8: After cooling the strip, high-temperature coiling is performed at a temperature of 720-750℃.

[0016] Step 9: Cool the strip steel to room temperature and level it. The leveling force is 180-200 tons, the leveling uncoiling tension is 50-60KN, and the coiling tension is 100-120KN.

[0017] Step 10: Straighten the strip, with a straightening elongation of 1.0-1.5%;

[0018] Step 11: Pickling the steel strip. The pickling section consists of several pickling tanks connected in series. The pickling medium is a mixture of hydrochloric acid solution and pickling inhibitor. The steel strip passes through the pickling medium, and the flow direction of the pickling medium is opposite to the movement direction of the steel strip. The temperature of the pickling tank is 75-90℃, the pickling speed is 120-150m / min, and the total pickling time is 32-40s.

[0019] Step 12: Apply anti-rust oil to the surface of the strip steel to form thin-gauge hot-formed pickled steel.

[0020] The thickness range of the thin-gauge hot-formed pickled steel is 1.0-2.5 mm.

[0021] In step 3, the protective slag used in continuous casting, by mass percentage, comprises 67-70% SiO2+CaO+Al2O3 base material, 3.0-3.5% skeleton material BN, 4.0-4.5% Na2O, no more than 0.15% K2O, flux MgO, F, and Na2CO3, and the total amount of Na2O, K2O, flux MgO, F, and Na2CO3 accounts for 26.4-30.0% of the protective slag; free carbon C free The content is ≤0.03%, and the moisture content is ≤0.10%.

[0022] In step 4, the method of rotary descaling is as follows: a high-pressure water jet is continuously impacted on the surface of the continuously cast billet at different angles and directions through a rotating nozzle. The working pressure of the high-pressure water is 380-400 bar, the rotation speed of the nozzle is 500-700 rpm, and the impact pressure of the high-pressure water jet on the surface of the continuously cast billet is 7-8 MPa.

[0023] In step 5, the soaking furnace includes several furnace zones. According to the direction of continuous casting billet movement, the gas flow rate in the first few furnace zones is 15-20% higher than the reference flow rate, and the air excess coefficient in the first few furnace zones is 1.20-1.30. The gas flow rate in the remaining last few furnace zones is the reference flow rate, and the air excess coefficient in the remaining last few furnace zones is 1.15-1.25.

[0024] In step 11, according to the direction of strip movement, the concentration of hydrochloric acid solution in the first pickling tank is ≥50g / l, the concentration of hydrochloric acid solution in the last pickling tank is 180-200g / l, and the mass of the inhibitor stock solution added is 0.8-1.0% of the mass of the acid solution.

[0025] The inhibitor stock solution is composed of hexamethylenetetramine, propargyl alcohol and water, with a ratio of 25:5:70.

[0026] In step 12, the surface iron oxide scale indentation defect of thin-gauge hot-formed pickled steel is ≤2 locations / 1000m; the whiteness value of the middle part of the surface plate width of thin-gauge hot-formed pickled steel is ≥70%, and the difference between the whiteness value of the edge of the surface plate width and the whiteness value of the middle part is ≤5%; the number of black and white stripe color difference defects on the surface of thin-gauge hot-formed pickled steel is ≤5 stripes / 1000m.

[0027] The manufacturing method also includes a hot forming process for thin-gauge hot-formed pickled steel, wherein the hot forming process is as follows:

[0028] Step 13: After the strip steel is uncoiled and blanked, it is heated to austenitize it. The austenitizing temperature is 860-920℃, and it is held for 3-5 minutes.

[0029] Step 14: After stamping the strip into shape, hold the pressure for 15-20 seconds;

[0030] Step 15: Quench at a heating rate of 25-40℃ / s and cool to room temperature.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. This invention employs surface black and white stripe color difference control. Based on the principle of carbon increase mechanism of billet, it reduces or eliminates carbon increase during continuous casting, removes part of the carbon increase defect layer during rotary descaling, increases the oxidation loss of billet during heating, and performs high-reduction rolling during high-temperature rolling. By comprehensively optimizing the protective slag, continuous casting process, heating process, and rolling process, the number of black and white stripe color difference defects caused by carbon increase of billet in thin-gauge hot-formed pickled steel is ≤5 stripes / 1000m.

[0033] 2. This invention employs the control of iron oxide scale indentation defects. Based on the formation mechanism of iron oxide scale indentation defects, it reduces slag film adhesion during continuous casting, removes part of the slag film during rotary descaling, reduces the intrusion of anchor-shaped Fe2SiO4 into the strip steel substrate during the heating process, and descaling before finishing rolling. Through reasonable composition and comprehensive optimization of continuous casting, descaling, and heating processes, the iron oxide scale indentation defects on the surface of thin-gauge hot-formed pickled steel are ≤2 per 1000m.

[0034] 3. This invention employs blackening and edge color difference control. Based on the formation mechanism of blackening and edge color difference in hot-formed pickled steel, a large number of transverse cracks of iron oxide scale are generated during the flattening stage. During the pickling stage, high-concentration acid solution and inhibitor stock solution are used for rapid pickling, so that the whiteness value of the middle part of the pickled plate surface is ≥70%, and the whiteness value of the edge of the plate minus the whiteness value of the middle part is ≤5%.

[0035] The thin-gauge hot-formed pickled steel manufactured by this invention has good microstructure and properties, as well as good pickling surface quality. Specifically, it reduces the defects of iron oxide scale indentation, increases the whiteness value of the middle part of the strip width, reduces the whiteness difference between the edge and the middle of the strip width, and reduces the number of black and white stripe color difference defects. This solves the unique surface quality problem in the production process of thin-gauge hot-formed pickled steel. Attached Figure Description

[0036] Figure 1 This is a flowchart of the manufacturing method of thin-gauge hot-formed pickled steel according to the present invention. Detailed Implementation

[0037] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0038] Please see the appendix Figure 1 A method for manufacturing thin-gauge hot-formed pickled steel includes the following steps:

[0039] Step 1: Desulfurize the molten iron to ensure that the sulfur content is ≤0.002%. S is a harmful element in steel. Desulfurization limits the S content in the molten iron to below 0.002% to avoid affecting the purity and toughness of the steel.

[0040] Step 2: The desulfurized molten iron is smelted into molten steel through a converter steelmaking-ladle refining (LF)-vacuum refining (RH) process. The chemical composition of the molten steel includes, by mass percentage: C: 0.20-0.25%, Si: 0.15-0.35%, Mn: 1.10-1.50%, P≤0.008%, S≤0.006%, Cr: 0.15-0.30%, B: 0.002-0.005%, Ti: 0.02-0.05%, Als: 0.02-0.06%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0041] When designing the carbon content, strength is provided through interstitial solid solution strengthening, and the hardenability of the steel is improved. The composition design for the thin slab continuous casting and rolling process (short process) avoids the carbon equivalent range of 0.09% < C in semi-peritectic steel. eq <0.17% to avoid longitudinal cracks forming on the surface of the billet shell during solidification. The formula for calculating peritectic carbon equivalent is: C eq= 1.0 × %C + 0.014 × %Mn - 0.037 × %Si + 0.023 × %Ni - 0.222 × S% - 0.040 × %P. Furthermore, excessively high C content can reduce plasticity, making it difficult to cut materials; therefore, the C content range is limited to 0.20-0.25%.

[0042] When designing the Si content, the strength of the steel plate is improved by solid solution strengthening. However, a high Si content can easily cause difficulties in hot rolling descaling and increase the probability of iron oxide scale indentation defects. Therefore, the Si content range is limited to 0.15-0.35%.

[0043] When designing the Mn content, Mn can increase the hardenability of steel, delay the transformation of pearlite and bainite, and improve the strength of the matrix through solid solution strengthening. Mn content ≥1.6% will bring difficulties to the continuous casting of thin slabs because high Mn content will significantly reduce the thermal conductivity and result in a large temperature difference between the inside and outside of the slab shell. At the same time, due to its high coefficient of linear expansion, the volume shrinkage is large during the cooling process of the steel billet, thus forming large internal stress during the cooling process of the slab, which can cause surface cracks in the slab in severe cases. Therefore, the Mn content range is limited to 1.10-1.50%.

[0044] Phosphorus (P) tends to cause segregation at the center of the billet and easily agglomerates at grain boundaries, increasing the brittleness of the steel. Therefore, the P content is controlled below 0.008%. Sulfur (S) affects the purity and toughness of the steel, so the S content is controlled below 0.006%.

[0045] When designing the Cr content, Cr increases the hardenability of steel, so the Cr content range is limited to 0.15-0.30%. When designing the B content, B is an element that strongly improves the hardenability of steel; adding trace amounts of B significantly improves the hardenability of steel, so the B content range is limited to 0.002-0.005%.

[0046] When designing the content of Ti and N, adding Ti can fix the N in the steel and reduce the adverse effects of AlN precipitation on the high-temperature plasticity of the billet. However, excessive Ti will cause the loss of C, affecting the hardness and strength of quenched martensite. Therefore, the Ti content is limited to 0.02-0.05%, and the N content is controlled below 0.006%.

[0047] When designing the Al content, Al can play a deoxidizing role in steel, so the Al content range is limited to 0.02-0.06%.

[0048] The thickness range of the thin-gauge hot-formed pickled steel is 1.0-2.5mm. The production of thin-gauge hot-formed pickled steel can be realized through a short-process production line, achieving "replacing cold with hot".

[0049] Step 3: Use thin slab continuous casting to continuously cast molten steel through a continuous casting machine to form a continuous casting billet; control the free carbon in the protective slag, the thickness of the billet, and the thickness of the liquid slag layer in the crystallizer to reduce or eliminate carbon increase; optimize the K2O and Na2O content in the protective slag to reduce slag film adhesion.

[0050] The protective slag used in continuous casting, by mass percentage, comprises 67-70% SiO2+CaO+Al2O3 base material, 3.0-3.5% BN (ceramic material) as skeleton material, 4.0-4.5% Na2O, no more than 0.15% K2O, fluxes MgO, F, and Na2CO3, with the total amount of fluxes MgO, Na2O, K2O, F, and Na2CO3 being 26.4-30.0%; free carbon C free With a purity of ≤0.03% and moisture content of ≤0.10%, boron nuclei (BN) is used to replace the skeletal function of free carbon. Carbonaceous materials in the protective slag can lead to carbon enrichment on the surface of the billet during continuous casting, ultimately resulting in black and white stripe color differences on the surface of hot-formed pickled steel. BN, a ceramic material, has a crystalline structure and physical properties similar to graphite, and therefore can replace the skeletal function of carbonaceous materials. Using BN as the skeletal framework to replace free carbon can avoid surface defects in pickled steel caused by carbon enrichment in the billet.

[0051] The continuous casting speed is controlled at 3.6-4.2 m / min. The thickness range of the billet controlled by the liquid core reduction is increased from 55-60 mm to 65-70 mm. With the same finished product thickness, the reduction rate is larger, and the subsequent rolling process will reduce the depth of the carburized layer to a thinner level, making it more likely that the residual carburized layer will enter the iron oxide scale layer and be removed. The thickness of the liquid slag layer in the crystallizer is controlled at 15-18 mm, which reduces the probability of contact between the primary billet shell and the carbon-rich layer or slag ring.

[0052] The main functions of protective slag in continuous casting are: a. covering the molten steel for heat insulation; b. isolating the molten steel from air and preventing secondary oxidation; c. absorbing non-metallic inclusions floating to the steel-slag interface; d. reducing the resistance during billet pulling and lubricating the billet and the mold; e. forming a slag film in the gap between the billet shell and the mold, controlling the heat transfer rate and maintaining uniform billet shell growth. The melting rate of the protective slag is an indicator of how fast it melts. The melting rate is generally controlled by the type and amount of carbonaceous materials (such as carbon black, graphite, etc.) or carbonates added to the protective slag. However, the carbonaceous materials in the protective slag can cause carbonization on the surface of the billet during continuous casting, and the carbonized layer can reach more than 2 mm on the billet surface. There are two main carbonization mechanisms: I. Coating mechanism: the slag ring is the main source of carbon. When the slag ring moves up and down, the carbon in the slag ring is "coated" onto the billet shell. II. Standing Wave Mechanism: Standing waves are generated as molten steel flows in through the submerged entry nozzle (SEN), bringing the molten steel into contact with the carbon-rich layer of either the fine slag layer or the liquid slag layer. Because the slab thickness in short-process production lines is thinner than in traditional hot-rolling lines, and the slab heating temperature is lower, resulting in fewer descaling passes, thin-gauge hot-formed pickled steel is prone to black / white streaks due to the carbonization mechanism. Therefore, in the continuous casting stage, by controlling the continuous casting process and the protective slag, carbonization can be reduced or eliminated. With the same finished product thickness, the subsequent rolling process will reduce the depth of the carbonized layer to an even thinner depth.

[0053] The protective slag contains a high content of SiO2. When the slag layer adheres to the surface of the billet, the SiO2 can become a site for the formation of Fe2SiO4. Fe2SiO4 has a low melting temperature (1173℃) and easily penetrates the grain boundaries of the steel matrix, creating an anchoring effect that makes complete descaling difficult. The protective slag components K2O and Na2O, with their low surface tension, can improve the adhesion between the slag and the steel matrix. However, if the protective slag adheres too tightly to the surface of the continuously cast billet, the cooling shrinkage stress and bending stress in the secondary cooling zone and straightening zone are difficult to remove, potentially leading to subsequent iron oxide scale indentation defects. Therefore, the K2O and Na2O content in the protective slag should be controlled at a low level.

[0054] Step 4: Rotate the continuously cast billet to descale it, so that the defect layer is in a cyclical state of oxidation and descaling. Utilize the parabolic law of steel oxidation kinetics (i.e., when the iron oxide scale is thin, the iron oxide scale thickness increases faster) to accelerate the removal of the carbonized defect layer, and at the same time remove most of the firmly adhered slag film.

[0055] The rotary descaling method is as follows: a high-pressure water jet is continuously impacted on the surface of the continuously cast billet at different angles and directions through a rotating nozzle. The working pressure of the high-pressure water is 380-400 bar, the rotation speed of the nozzle is 500-700 rpm, and the impact pressure of the high-pressure water jet on the surface of the continuously cast billet is 7-8 MPa.

[0056] Step 5: The continuously cast billet after rotary descaling is sent into a soaking furnace for heating. The billet tapping temperature, soaking time, gas flow rate in different furnace zones, and excess air coefficient are controlled to allow the carbonized layer to continue to oxidize and burn off, while reducing the intrusion of anchor-shaped Fe2SiO4 into the strip matrix.

[0057] The furnace exit temperature of the continuously cast billet is 1190-1220℃, and the soaking time is 30-35 minutes.

[0058] The soaking furnace comprises several furnace zones. In the direction of billet movement, the gas flow rate in the first few furnace zones is 15-20% higher than the reference flow rate, and the excess air coefficient in these zones is 1.20-1.30. In the remaining later furnace zones, the gas flow rate is the reference flow rate, and the excess air coefficient is 1.15-1.25. By controlling the excess air coefficient and gas flow rate, short-term high-temperature heating is achieved, allowing the carbonized defect layer to continue oxidizing and burning away, while simultaneously reducing the intrusion of anchor-shaped Fe2SiO4 into the strip matrix. The reference flow rate for each furnace zone can be determined based on conventional production experience and production requirements.

[0059] The number of furnace zones and the length of each furnace zone can be determined according to the design of the furnace manufacturer, preferably 10. The gas flow rate in the first to fifth furnace zones is 15-20% higher than the reference flow rate, while the gas flow rate in the sixth to tenth furnace zones is the reference flow rate.

[0060] Step 6: Descaling the homogenized continuously cast billet with high-pressure water at a pressure of 300-380 bar. Since hot-rolled strip steel oxidizes upon contact with air during production, and after being heated in a homogenizing furnace, the billet surface undergoes severe oxidation, generating a thick layer of furnace-grown iron oxide scale. This descaling process reduces the occurrence of surface quality defects in the strip steel.

[0061] Step 7: Finish roll the continuously cast billet into strip. Control the reduction rate for the first pass at 55-65% and for the second pass at 50-60%. Perform inter-stand descaling between the first and second passes at a water pressure of 180-200 bar to remove the iron oxide scale layer from the strip surface, while also removing some of the carburized layer. Control the final rolling temperature at 880-920℃. In the finish rolling stage, increasing the total reduction of defects at the high-temperature front stand can further reduce the depth of the defect layer.

[0062] Step 8: After laminar flow cooling, the strip is coiled at high temperature. The temperature for high-temperature coiling is 720-750℃. Coiling the strip at high temperature can effectively prevent the flattening problem of thin strip.

[0063] Before strip steel is coiled, a three-layer iron oxide scale structure (FeO, Fe2O3, Fe3O4) forms on the surface of the strip steel from the inside out. After coiling, the strip steel coil is tightly rolled. Due to the influence of the strip steel coil geometry and strip crown, the innermost and outermost rings and the two end faces (edges) of the coil have sufficient oxygen supply, while the oxygen supply inside the coil is severely restricted. During the cooling process of the high-temperature coil, Fe ions in the strip steel matrix in the restricted oxygen supply area diffuse outward, causing the high-valence oxides (Fe2O3 and Fe3O4) to transform into low-valence oxides (FeO), and a FeO eutectoid decomposition reaction occurs to generate a Fe3O4 / Fe eutectoid structure. In addition, oxygen (O) in the iron oxide scale layer of the strip in the area where oxygen supply to the steel coil is restricted can continue to diffuse into the steel matrix. Furthermore, the grain boundary diffusion coefficient of O is tens of times greater than the bulk diffusion coefficient. When the oxygen partial pressure is insufficient to form iron oxides, O combines with elements in the hot-formed steel whose standard formation free energy is lower than that of Fe (Si, Mn, Cr elements) to form grain boundary oxides, resulting in grain boundary oxidation. Meanwhile, the strip at the end face of the steel coil has sufficient oxygen supply, and the three-layer iron oxide scale structure continues to grow and thicken. Therefore, along the width direction of the strip, an iron oxide scale structure will form on the surface of the steel matrix, including a first mixed structure layer (eutectoid Fe3O4 / Fe+Fe3O4 precipitation + residual FeO) formed by layered Fe2O3 and layered Fe3O4 at the edges, a second mixed structure layer (eutectoid Fe3O4 / Fe+Fe3O4 precipitation + residual FeO) in the middle, and a grain boundary oxide layer. This difference in the iron oxide scale structure along the width, if not controlled during the pickling process, will manifest as a color difference along the width of the steel plate after conventional pickling: the 40-100mm width at the edge of the strip appears bright white due to the absence of grain boundary oxidation, while the middle part, due to the presence of grain boundary oxidation, undergoes erosion after the iron oxide scale is removed by pickling, forming grain boundary trenches, which appear dark under macroscopic vision. The surface darkness is related to the depth and width of the grain boundary trenches; the deeper and narrower the trenches, the darker the color. Whiteness is an indicator of the degree of whiteness of a material's surface, expressed as a percentage of white content, in %. Measured using a photoelectric whiteness meter, the whiteness of the blackened area in the middle of thin-gauge hot-formed pickled steel is only about 55%.

[0064] Step 9: Cool the strip to room temperature and level it. The leveling force is 180-200 tons, the leveling uncoiling tension is 50-60 kN, and the coiling tension is 100-120 kN. Using a large leveling force, leveling uncoiling tension, and coiling tension can cause a large number of transverse cracks in the FeO-based iron oxide scale layer under tensile stress, providing a large number of artificial pickling nuclei for the pickling process.

[0065] Pickling of crack-free oxide scale is a nucleation and growth process, involving a slow incubation period called the "initial pickling-free time," which is the period of pickling nucleus formation. Scale breakage causes cracks in the oxide scale; the density and size of these cracks increase with the elongation of the oxide scale. These cracks act as artificial pickling nuclei during pickling. These cracks in the iron oxide scale allow acid to easily reach the steel substrate without the need for nucleation, significantly shortening the pickling incubation period (initial pickling-free time). In cases with high oxide scale elongation, the incubation period can even be completely eliminated.

[0066] Step 10: Straighten the strip steel using a tension leveler. The tension leveling elongation of the strip steel is 1.0-1.5%, which can further break down the iron oxide scale layer and facilitate its peeling.

[0067] Step 11: Pickling the strip steel to remove the surface iron oxide scale layer. The pickling section consists of several pickling tanks connected in series. The pickling medium is a mixture of hydrochloric acid solution and pickling inhibitor. The strip steel passes through the pickling medium, and the flow direction of the pickling medium is opposite to the movement direction of the strip steel. The temperature of the pickling tank is 75-90℃, the pickling speed is 120-150m / min, and the total pickling time is 32-40s. This allows the iron oxide scale layer on the surface of the strip steel to be quickly dissolved in the pickling tank. After pickling, only a very small amount of carbonized black and white stripe color difference defects remain in the iron oxide scale layer, and the carbonized layer is relatively shallow, achieving edge blackening and edge color difference control.

[0068] According to the direction of strip movement, the concentration of hydrochloric acid solution in the first pickling tank is ≥50g / l, and the concentration of hydrochloric acid solution in the last pickling tank is 180-200g / l. The mass of the inhibitor stock solution added is 0.8-1.0% of the mass of the acid solution.

[0069] The inhibitor stock solution is composed of hexamethylenetetramine, propargyl alcohol, and water in a ratio of 25:5:70. The molecules of the inhibitor stock solution adsorb onto the strip surface through their adsorption properties, forming an adsorption film. This adsorption film effectively inhibits the electrochemical reaction between the acid solution and the strip surface, greatly reducing corrosion at the grain boundaries of the substrate. Oxidized grain boundaries are also protected, preventing grain boundary erosion and the formation of pickling color differences. Other pickling inhibitors can also be used as needed to achieve the effect of forming an adsorption film on the strip surface.

[0070] Step 12: Apply anti-rust oil to the surface of the strip steel to form thin-gauge hot-formed pickled steel, which can prevent the thin-gauge hot-formed pickled steel from rusting.

[0071] The surface of the thin-gauge hot-formed pickled steel has ≤2 iron oxide scale indentation defects per 1000m; the whiteness value of the middle part of the surface plate width of the thin-gauge hot-formed pickled steel is ≥70%, and the difference between the whiteness value of the edge of the surface plate width and the whiteness value of the middle part is ≤5%; the number of black and white stripe color difference defects on the surface of the thin-gauge hot-formed pickled steel is ≤5 per 1000m.

[0072] The aforementioned thin-gauge hot-formed pickled steel can be used in the production of automotive body-in-white steel sheets. Its hot-forming process is as follows:

[0073] Step 13: After the strip steel is uncoiled and cut, it is heated to austenitize it. The austenitizing temperature is 860-920℃, and the temperature is held for 3-5 minutes.

[0074] Step 14: After stamping the strip steel into shape using a mold, hold the pressure in the mold for 15-20 seconds.

[0075] Step 15: Quench the part at a heating rate of 25-40℃ / s and cool it to room temperature to form a heat-treated part.

[0076] Examples 1-3:

[0077] Examples 1-3 describe the production of thin-gauge hot-formed pickled steel based on the thin slab continuous casting and rolling process (CSP short process), with a composition designed to be 1500MPa grade.

[0078] Step 1: Desulfurize the molten iron to ensure that the sulfur content in the molten iron is ≤0.002%.

[0079] Step 2: The desulfurized molten iron is smelted into molten steel through a converter steelmaking-ladle refining (LF)-vacuum refining (RH) process. The chemical composition of the molten steel includes, by mass percentage: C: 0.20-0.25%, Si: 0.15-0.35%, Mn: 1.10-1.50%, P≤0.008%, S≤0.006%, Cr: 0.15-0.30%, B: 0.002-0.005%, Ti: 0.02-0.05%, Als: 0.02-0.06%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0080] The chemical composition and content of the molten steel in Examples 1-3 and Comparative Examples 1-3 are shown in Table 1.

[0081] Table 1. Chemical composition and content of molten steel in Examples 1-3 and Comparative Examples 1-3.

[0082]

[0083] Step 3: Molten steel is continuously cast using a thin slab continuous casting machine to form a continuously cast billet; the continuous casting protective slag, by mass percentage, includes 67-70% SiO2+CaO+Al2O3 base material, 3.0-3.5% ceramic material BN as a skeleton material, 4.0-4.5% Na2O, no more than 0.15% K2O, and free carbon C. free The content of the solids is ≤0.03%, and the moisture content is ≤0.10%. The continuous casting speed is controlled at 3.6-4.2 m / min, the liquid core reduction is controlled at 65-70 mm for the billet thickness, and the slag layer thickness in the crystallizer is controlled at 15-18 mm.

[0084] The process parameters for continuous casting in Examples 1-3 and Comparative Examples 1-3 are shown in Table 2.

[0085] Table 2. Process parameters for continuous casting in Examples 1-3 and Comparative Examples 1-3

[0086]

[0087]

[0088] Step 4: Rotary descaling of the continuously cast billet. High-pressure water jets are continuously impacted on the surface of the continuously cast billet at different angles and directions through a rotating nozzle. The working pressure of the high-pressure water is 380-400 bar, the rotation speed of the nozzle is 500-700 rpm, and the impact pressure of the high-pressure water jet on the surface of the continuously cast billet is 7-8 MPa.

[0089] The process parameters for rotary descaling in Examples 1-3 and Comparative Examples 1-3 are shown in Table 3.

[0090] Table 3. Process parameters for rotary descaling in Examples 1-3 and Comparative Examples 1-3

[0091]

[0092] Step 5: The descaled continuous casting billet is sent to a soaking furnace for heating. The billet exiting the furnace is at a temperature of 1190-1220℃, and the soaking time is 30-35 minutes. The soaking furnace consists of 10 zones. In zones 1-5, the gas flow rate is 15-20% higher than the reference flow rate, and the excess air coefficient is 1.20-1.30. In zones 6-10, the excess air coefficient is 1.15-1.25.

[0093] The heating process parameters of the soaking furnace in Examples 1-3 and Comparative Examples 1-3 are shown in Table 4.

[0094] Table 4. Heating process parameters of the soaking furnace in Examples 1-3 and Comparative Examples 1-3

[0095]

[0096] Step 6: Descale the continuously cast billet after homogenization with high-pressure water at a pressure of 300-380 bar.

[0097] Step 7: Finish roll the continuously cast billet into strip steel. The reduction rate of the first pass is controlled at 55-65%, and the reduction rate of the second pass is controlled at 50-60%. Descaling is performed between the first and second passes, with a descaling water pressure of 180-200 bar. The final rolling temperature is controlled at 880-920℃.

[0098] Step 8: After laminar flow cooling, the strip is coiled at high temperature, which is 720-750℃.

[0099] The process parameters for finishing rolling and coiling in Examples 1-3 and Comparative Examples 1-3 are shown in Table 5.

[0100] Table 5. Process parameters for finishing rolling and coiling in Examples 1-3 and Comparative Examples 1-3

[0101]

[0102] Step 9: Cool the strip to room temperature and level it. The leveling force is 180-200 tons, the leveling uncoiling tension is 50-60 kN, and the coiling tension is 100-120 kN. The leveling process parameters in Examples 1-3 and Comparative Examples 1-3 are shown in Table 6.

[0103] Table 6. Process parameters for leveling in Examples 1-3 and Comparative Examples 1-3

[0104]

[0105] Step 10: Straighten the strip using a tension leveler. The tension leveling elongation of the strip is 1.0-1.5%.

[0106] Step 11: Pickling the steel strip. The pickling section consists of several pickling tanks connected in series. The pickling medium is a mixture of hydrochloric acid solution and pickling inhibitor. The steel strip passes through the pickling medium, and the flow direction of the pickling medium is opposite to the movement direction of the steel strip. The temperature of the pickling tank is 75-90℃, the pickling speed is 120-150m / min, and the total pickling time is 32-40s. According to the movement direction of the steel strip, the concentration of hydrochloric acid solution in the first pickling tank is ≥50g / l, and the concentration of hydrochloric acid solution in the last pickling tank is 180-200g / l. The mass of the inhibitor stock solution added is 0.8-1.0% of the acid solution mass. The inhibitor stock solution is a mixture of hexamethylenetetramine, propargyl alcohol, and water, with a ratio of 25:5:70.

[0107] The pickling process parameters in Examples 1-3 and Comparative Examples 1-3 are shown in Table 7.

[0108] Table 7. Pickling process parameters in Examples 1-3 and Comparative Examples 1-3

[0109]

[0110] Step 12: Apply anti-rust oil to the surface of the strip steel to form thin-gauge hot-formed pickled steel, which serves to prevent rust and corrosion.

[0111] Thin-gauge hot-formed pickled steel is used in the production of automotive body-in-white steel sheets. Its hot-forming process is as follows:

[0112] Step 13: After the strip steel is uncoiled and cut, it is heated to austenitize it. The austenitizing temperature is 860-920℃, and the temperature is held for 3-5 minutes.

[0113] Step 14: After stamping the strip steel into shape using a mold, hold the pressure in the mold for 15-20 seconds.

[0114] Step 15: Quench at a heating rate of 25-40℃ / s and cool to room temperature.

[0115] The microstructure of the thin-gauge hot-formed pickled steel in Examples 1-3 and Comparative Examples 1-3 before hot forming was ferrite (F) and pearlite (P), and the microstructure after hot forming was martensite (M). Their performance parameters are shown in Table 8.

[0116] Table 8. Microstructure and properties of thin-gauge hot-formed pickled steels from Examples 1-3 and Comparative Examples 1-3

[0117]

[0118]

[0119] The pickling surface quality of the thin-gauge hot-formed pickled steel produced in Examples 1-3 and Comparative Examples 1-3 is shown in Table 9.

[0120] Table 9. Pickling surface quality of thin-gauge hot-formed pickled steel produced in Examples 1-3 and Comparative Examples 1-3.

[0121]

[0122] As shown in Tables 8 and 9, the thin-gauge hot-formed pickled steel produced in Examples 1-3 of this invention has good microstructure and properties, meeting standards and user requirements, and also exhibits good pickling surface quality: surface oxide scale indentation defects ≤ 2 locations / 1000m, whiteness value in the middle of the surface plate width ≥ 70%, whiteness value at the edge of the surface plate width minus the whiteness value in the middle ≤ 5%, and number of black and white stripe color difference defects ≤ 5 lines / 1000m. The pickling surface quality of the thin-gauge hot-formed pickled steel produced in Examples 1-3 of this invention is significantly improved compared to the pickling surface quality of the thin-gauge hot-formed pickled steel produced in Comparative Examples 1-3.

[0123] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing thin-gauge hot-formed pickled steel, characterized in that: Includes the following steps: Step 1: Desulfurize the molten iron to ensure that the sulfur content in the molten iron is ≤0.002%; Step 2: The desulfurized molten iron is smelted into molten steel through a converter steelmaking-ladle refining-vacuum refining process. The chemical composition of the molten steel includes, by mass percentage: C: 0.20-0.25%, Si: 0.15-0.35%, Mn: 1.10-1.50%, P≤0.008%, S≤0.006%, Cr: 0.15-0.30%, B: 0.002-0.005%, Ti: 0.02-0.05%, Als: 0.02-0.06%, N≤0.006%, with the balance being Fe and unavoidable impurities. Step 3: Use thin slab continuous casting to continuously cast molten steel into a continuous casting billet; wherein, the protective slag used in continuous casting is ceramic material BN as the skeleton material, the continuous casting speed is controlled at 3.6-4.2m / min, the liquid core reduction is controlled at the billet thickness range of 65-70mm, and the thickness of the liquid slag layer in the crystallizer is controlled at 15-18mm; Step 4: Perform rotary descaling on the continuously cast billet; Step 5: The continuously cast billet after rotary descaling is sent into a soaking furnace for heating. The billet exiting the furnace is at a temperature of 1190-1220℃ and the soaking time is 30-35 minutes. The soaking furnace includes several furnace zones. According to the direction of continuous casting billet movement, the gas flow rate in the first few furnace zones is 15-20% higher than the reference flow rate, and the air excess coefficient in the first few furnace zones is 1.20-1.

30. The gas flow rate in the remaining last few furnace zones is the reference flow rate, and the air excess coefficient in the remaining last few furnace zones is 1.15-1.

25. Step 6: Descaling the homogenized continuous casting billet with high-pressure water at a pressure of 300-380 bar. Step 7: Finish roll the continuously cast billet into strip steel. The reduction rate of the first pass is controlled at 55-65%, and the reduction rate of the second pass is controlled at 50-60%. Descaling is performed between the first and second passes, with a descaling water pressure of 180-200 bar. The final rolling temperature is controlled at 880-920℃. Step 8: After cooling the strip, high-temperature coiling is performed at a temperature of 720-750℃. Step 9: Cool the strip steel to room temperature and level it. The leveling force is 180-200 tons, the leveling uncoiling tension is 50-60KN, and the coiling tension is 100-120KN. Step 10: Straighten the strip, with a straightening elongation of 1.0-1.5%; Step 11: Pickling the steel strip. The pickling section consists of several pickling tanks connected in series. The pickling medium is a mixture of hydrochloric acid solution and pickling inhibitor. The steel strip passes through the pickling medium, and the flow direction of the pickling medium is opposite to the movement direction of the steel strip. The temperature of the pickling tank is 75-90℃, the pickling speed is 120-150m / min, and the total pickling time is 32-40s. Step 12: Apply anti-rust oil to the surface of the strip steel to form thin-gauge hot-formed pickled steel.

2. The method for manufacturing thin-gauge hot-formed pickled steel according to claim 1, characterized in that: The thickness range of the thin-gauge hot-formed pickled steel is 1.0-2.5 mm.

3. The method for manufacturing thin-gauge hot-formed pickled steel according to claim 1, characterized in that: In step 3, the protective slag used in continuous casting, by mass percentage, comprises 67-70% SiO2+CaO+Al2O3 base material, 3.0-3.5% skeleton material BN, 4.0-4.5% Na2O, no more than 0.15% K2O, flux MgO, F, and Na2CO3, and the total amount of Na2O, K2O, flux MgO, F, and Na2CO3 accounts for 26.4-30.0% of the protective slag; free carbon C free ≤0.03%, moisture ≤0.10%.

4. The method for manufacturing thin-gauge hot-formed pickled steel according to claim 1, characterized in that: In step 4, the method of rotary descaling is as follows: a high-pressure water jet is continuously impacted on the surface of the continuously cast billet at different angles and directions through a rotating nozzle. The working pressure of the high-pressure water is 380-400 bar, the rotation speed of the nozzle is 500-700 rpm, and the impact pressure of the high-pressure water jet on the surface of the continuously cast billet is 7-8 MPa.

5. The method for manufacturing thin-gauge hot-formed pickled steel according to claim 1, characterized in that: In step 11, according to the direction of strip movement, the concentration of hydrochloric acid solution in the first pickling tank is ≥50g / l, the concentration of hydrochloric acid solution in the last pickling tank is 180-200g / l, and the mass of the inhibitor stock solution added is 0.8-1.0% of the mass of the acid solution.

6. The method for manufacturing thin-gauge hot-formed pickled steel according to claim 1 or 5, characterized in that: The inhibitor stock solution is composed of hexamethylenetetramine, propargyl alcohol and water, with a ratio of 25:5:

70.

7. The method for manufacturing thin-gauge hot-formed pickled steel according to claim 1, characterized in that: In step 12, the surface iron oxide scale indentation defect of thin-gauge hot-formed pickled steel is ≤2 locations / 1000m; the whiteness value of the middle part of the surface plate width of thin-gauge hot-formed pickled steel is ≥70%, and the difference between the whiteness value of the edge of the surface plate width and the whiteness value of the middle part is ≤5%; the number of black and white stripe color difference defects on the surface of thin-gauge hot-formed pickled steel is ≤5 stripes / 1000m.

8. The method for manufacturing thin-gauge hot-formed pickled steel according to claim 1 or 7, characterized in that: The manufacturing method also includes a hot forming process for thin-gauge hot-formed pickled steel, wherein the hot forming process is as follows: Step 13: After the strip steel is uncoiled and blanked, it is heated to austenitize it. The austenitizing temperature is 860-920℃, and it is held for 3-5 minutes. Step 14: After stamping the strip into shape, hold the pressure for 15-20 seconds; Step 15: Quench at a heating rate of 25-40℃ / s and cool to room temperature.

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