High-formability 430 stainless steel for eliminating black ribs and stripes and manufacturing method thereof

Abstract

The invention discloses high-formability 430 stainless steel for eliminating black ribs and stripes and a manufacturing method of the high-formability 430 stainless steel, the high-formability 430 stainless steel comprises the following components in percentage by mass: less than or equal to 0.050% of C, 0.20-0.65% of Si, 0.15-0.65% of Mn, less than or equal to 0.045% of P, less than or equal to 0.0040% of S, 16.0-17.5% of Cr, less than or equal to 0.20% of Ni, less than or equal to 0.15% of Mo, less than or equal to 0.10% of Cu, 0.05-0.15% of V, 0.050-0.090% of N and the balance of Fe and inevitable impurities, and the material strengthening index Fc is equal to 1.1-1.7, and Fc is equal to 28.84 * N + 2.09 * Si The pitting corrosion resistance equivalent PREN is greater than or equal to 17.0, and PREN = Cr + 3.3 Mo + 16N. According to the method, the problem of black rib and stripe defects unique to the SUS430 material is solved, and the requirement for surface attractiveness of downstream finished products is met.

Description

Technical Field

[0001] This invention relates to the field of stainless steel production technology, and in particular to a high-formability 430 stainless steel with black vein stripes and its manufacturing method. Background Technology

[0002] SUS430, as a typical grade of 400 series stainless steel, is highly cost-effective due to the absence of added Ni. Its material not only has good corrosion resistance but also excellent cold working performance, and is widely used in home appliances, kitchens, catering, and building decoration.

[0003] The study of ferritic stainless steel texture has long been a hot topic in this field, with research primarily focused on improving the wrinkling problem inherent in ferritic microstructures. It is generally believed that obtaining a higher proportion of γ-fiber textures in ferritic fibrous structures has an excellent effect on reducing wrinkling. Previous studies have extensively explored methods to achieve γ-fiber textures, such as controlling the equiaxed crystal ratio during continuous casting, continuous annealing after hot rolling, and the addition of Sn.

[0004] Wrinkling refers to the uneven, raised or recessed stripes that occur along the thickness direction of a material when it undergoes deformation due to differences in its microstructure. The wrinkling effect of ferrite leads to many other defects, one of which is the "black streaks" defect.

[0005] "Black streaks" are mainly produced during the cold rolling process. During cold rolling deformation, the material exhibits length extension and thickness reduction, resulting in uneven thickness changes in ferritic stainless steel and inevitably causing wrinkling. However, due to the rolling action during cold rolling, the "protrusions" caused by wrinkling are either directly pressed into the material matrix surface or pressed along the rolling direction, depending on their hardness. Generally, if the "protrusions" undergo severe structural deformation, the material's roughness and reflectivity will be significantly reduced, thus macroscopically manifesting as black parallel lines along the rolling direction, i.e., "black streaks."

[0006] In the stainless steel industry, downstream cold-working customers, drawing inspiration from similar features in everyday life, refer to the parallel black lines distributed along the rolling direction on the surface of 430 cold-rolled steel sheets as "black veins." No research on this type of defect has been found in the 400 series literature. This defect severely affects the surface aesthetics and smoothness of downstream products. Although surface grinding and polishing can eliminate the defect, it adds extra costs to downstream processing and is extremely inconvenient.

[0007] Ferritic stainless steel has significant advantages in cold forming. To maintain good cold forming performance, 430 stainless steel often requires a annealing process to soften the material and facilitate subsequent processing. However, existing research shows that annealed products exhibit severe "black veins" (striking streaks), while unannealed products lack these streaks but have poorer formability. This makes it difficult to simultaneously achieve both surface quality and forming performance, a pressing technical challenge currently faced by downstream cold rolling processes for 430 stainless steel. Therefore, the ultimate goal and subject of this invention is to focus on addressing the industry's key challenges and effectively improve the "black vein" effect in 430 stainless steel products while maintaining its unique ferritic stainless steel formability.

[0008] Chinese patent CN119839096A discloses "a method for preventing surface defects in 430 stainless steel". This method eliminates sparks between the side guide and the strip by adjusting the pressure at the side guide head and adding auxiliary rollers to adjust the friction mode, thereby avoiding scratches. Furthermore, by adjusting the thermal conductivity coefficient, winding temperature accuracy, and roll gap, slippage between the strip and the auxiliary winding rollers during head winding is prevented, reducing roll marks or adhesion.

[0009] Chinese patent CN119259707A discloses a "hot rolling control method for improving surface color difference defects in 430 stainless steel thick cold-rolled sheets." By controlling the coiling temperature, laminar flow cooling method, placement time after rolling, and the annealing process, the color difference defects after cold rolling of 430 stainless steel are effectively improved. This patent research indicates that the color difference is mainly caused by differences in the degree of material recrystallization.

[0010] Chinese patent CN115747441A discloses "a method for improving gold dust defects in 430 series stainless steel". The method improves surface gold dust defects by limiting C and N content, optimizing the annealing process, limiting the concentration of pickling acid, and controlling the surface roughness of the material.

[0011] All of the above patents relate to surface defects in 430 stainless steel. Among them, scratch marks are a type of abrasion damage, exhibiting obvious localized macroscopic features of foreign object contact. While color difference is also a visual "black and white" variation, it is significantly different from "black streaks." In fact, after cold rolling and annealing, the appearance of such a wide macroscopic "black and white" feature and distinct "black and white" boundaries in 430 stainless steel is largely related to the roughness of the cold rolling rolls and the contact friction between the rolls and the material. Gold dust defects, commonly known as "gold powder" in the 400 series stainless steel industry, refer to the characteristic of shiny particles appearing on the surface of 430 cold-rolled annealed plates after degumming. In reality, "gold powder" manifests as tiny peeling at the 0.01mm level, mainly due to high carbide aggregation or low ferrite recrystallization, resulting in low plasticity and toughness in localized areas of the material. When the material surface degummes, tiny peeling occurs, presenting a granular, shiny characteristic. Generally, improving defects like "gold powder" requires effective annealing treatment of the material to allow for sufficient recrystallization and softening, and to disperse the carbides. "Gold powder" defects are commonly found in 430 cold-rolled annealed sheets made from hot-rolled raw materials or raw materials with insufficient annealing.

[0012] While these patents have made many optimizations and improvements to the process for solving surface defects, they do not address improvements to the "black stripes" of the 430 material. Moreover, these patents generally focus on adjusting process parameters and do not analyze or study the defect characteristics, formation mechanism, and causes, so the theoretical basis for improving the defects is somewhat insufficient. Summary of the Invention

[0013] The purpose of this invention is to provide a high formability 430 stainless steel with black vein stripes and its manufacturing method, which solves the "black vein stripe" defect unique to SUS430 material and meets the surface aesthetic requirements of downstream finished products.

[0014] To achieve the above objectives, the technical solution of the present invention is as follows: A high-formability 430 stainless steel with eliminated black streaks, comprising the following composition by mass percentage: C≤0.050%, Si: 0.20~0.65%, Mn: 0.15~0.65%, P≤0.045%, S≤0.0040%, Cr: 16.0~17.5%, Ni≤0.20%, Mo≤0.15%, Cu≤0.10%, V: 0.05~0.15%, N: 0.050~0.090%, with the balance including Fe and unavoidable impurities, and simultaneously satisfying the following: The material strengthening index Fc = 1.1~1.7. Fc=28.84×N+2.09×Si+5.18×Ni+3.79×Cu-2.15; The pitting corrosion resistance equivalent PREN ≥ 17.0, PREN = Cr + 3.3Mo + 16N.

[0015] Furthermore, its composition balance is Fe and unavoidable impurities.

[0016] The 430 stainless steel described in this invention has a Rockwell hardness of HRB 90~95 and a tensile strength R m The yield strength is 600~850MPa, and the yield strength R is... p0.2 The strength is 450~600MPa, and the elongation after fracture (A) is 12~20%.

[0017] In the composition design of the 430 stainless steel described in this invention: C: As an austenite forming element, its content should not be too high; moreover, since C easily combines with Fe and Cr to form carbides, its large precipitation can easily lead to "depleted" Cr in the surrounding area, so the content of C element needs to be controlled.

[0018] Si: As a ferrite-forming element, it can improve the strength of materials to a certain extent and improve the passivation film structure to enhance corrosion resistance. However, excessive content can easily lead to a decrease in the plasticity of materials.

[0019] Mn, Ni, and Cu: These elements are beneficial for austenitization and stability of the microstructure. However, in ferritic materials like SUS430, their content should not be too high. Especially for high-Cr materials like SUS430, a high content of austenite-forming elements makes it easier to obtain an austenitic structure at high temperatures. Furthermore, rapid cooling to room temperature fails to maintain the austenitic morphology, leading to martensite and drastically reducing the material's plasticity. Additionally, the addition of Cu easily causes brittle cracking during hot working; therefore, the Cu content should not be too high.

[0020] Mo: As a ferrite-forming element, although increasing its content can improve corrosion resistance, it is easy to precipitate hard phases, which leads to a decrease in the plasticity and toughness of the material. Moreover, Mo alloys are expensive, so the content should not be too high.

[0021] V: As a microalloying element, it readily forms precipitates with C and N, thereby strengthening the microstructure. In SUS430 material, the presence of V plays a role in refining carbide size.

[0022] Nitrogen (N): As an austenite-forming element, its content should generally not be too high; moreover, nitrogen dissolved in the matrix easily produces large lattice distortions, which significantly improves the material's strength. In this invention, to improve the "black vein stripes," it is necessary to appropriately increase the material's strength to resist external deformation; at the same time, increasing the nitrogen content can promote the recrystallization process of the material during hot rolling, which can refine the grains and also help improve the "black vein stripes."

[0023] In particular, the components of this invention are required to meet the following requirements: The material strengthening index Fc = 1.1~1.7, Fc = 28.84×N + 2.09×Si + 5.18×Ni + 3.79×Cu - 2.15.

[0024] This invention requires that the composition of the 430 stainless steel meet the material strengthening index Fc=1.1~1.7, so that the material achieves the expected hot rolling performance strengthening requirements, thereby improving the overall microstructure's ability to resist external deformation during subsequent cold rolling. After obtaining the expected composition, the material can be thinned relatively uniformly in the thickness direction during cold rolling, effectively suppressing large local deformations along the rolling direction that occur during cold rolling deformation, thereby suppressing the "black vein stripe" defect.

[0025] To improve material strength while maintaining a certain level of plasticity, the material composition can be optimized to achieve a certain level of strengthening. Under the same processing conditions, hot-rolled properties can better reflect the influence of chemical elements on strength and hardness; however, in the annealed state, the changes in parameters such as strength or elongation are relatively small. In this state, it is not possible to accurately assess the influence trend of elements on the properties of the base material. Annealing, to some extent, "covers up" the influence of chemical elements on properties.

[0026] In order to facilitate a comprehensive evaluation of multiple indicators (a single parameter cannot accurately evaluate material properties; listing multiple parameters cannot take into account and balance them), this invention normalizes the above parameters (the strengthening level is introduced by elongation to take into account the plasticity of the material).

[0027] Parameter P1: Hardness / 90 (The index is directly proportional to the strength effect; the denominator is the average of the performance parameters, rounded down). Parameter P2: Tensile strength / 600 (The index is directly proportional to the strength effect, and the denominator is the average of the performance parameters, rounded down). Parameter P3: Yield strength / 450 (The index is directly proportional to the strength effect; the denominator is the average of the performance parameters, rounded down). Parameter P4: 20 / elongation after fracture (the index is inversely proportional to the strength effect, the numerator is the average of the performance parameters, rounded down), then perform pairwise multiplication and average calculation, the formula is as follows: parameter , parameter , parameter , parameter , Performance comprehensive influencing factor parameters , In the formula: HRB, Rockwell hardness; Rm, tensile strength; Rp0.2, yield strength; A, elongation after fracture.

[0028] Through P IF By fitting the linear trend relationship with the relevant components, an approximate empirical formula for element calculation is derived: Fc = 28.84 × N - 9.99 × C + 2.09 × Si + 0.03 × Mn + 33.86 × P + 139.53 × S - 0.11 × Cr + 5.18 × Ni - 37.90 × Mo + 3.79 × Cu - 3.88 × V + constant; Within the composition range of 430 stainless steel, the influence of P and S inclusions is eliminated; simultaneously, parameters with coefficients <0.2 (low influence within the composition system) and parameters with negative effects are also eliminated (generally, for strength-related parameters, if the process conditions and phase transformation degree are the same or similar, the higher the content of added alloying elements, the higher the material strength). The corrected empirical formula is as follows: Fc=28.84×N+2.09×Si+5.18×Ni+3.79×Cu-2.15; Based on the statistical distribution of tensile properties and hardness data obtained during actual production, the control range of Fc is 1.1~1.7.

[0029] The empirical formula for the material strengthening index Fc, due to the limitations of the data itself, is currently only applicable to 430 stainless steel.

[0030] The material strengthening index Fc, as a parameter, can roughly determine the influence of chemical elements on the degree of material strengthening. In the field of metallic materials, the strength of a material can generally be predicted by its chemical composition, thereby evaluating whether the material performance meets the expected ideal. However, this empirical formula is only applicable to specific material systems or material states (solution or hot rolling, etc.). This evaluation method is similar to the methods used to evaluate the basic properties of materials, such as pitting corrosion equivalent PREN, martensitic transformation point Ms, and stacking fault energy SFE. Although these parameters are affected by material state and environmental factors, they do not affect the research on material performance. For example, in evaluating the corrosion resistance of materials, the material composition can generally be optimized by using the PREN value. However, this design method cannot completely predict the corrosion resistance of materials; it can only predict the trend of corrosion resistance changes. After the material is designed and manufactured, it needs to be evaluated by parameters such as the pitting corrosion rate obtained experimentally. Moreover, the pitting corrosion rate and the PREN value do not show a one-to-one direct proportional relationship; sometimes, even an abnormal trend appears. However, this does not affect or even negate the value of PREN in evaluating the corrosion resistance of materials. Strictly speaking, the introduction of any element will affect corrosion resistance, but PREN has selected parameters with obvious trends and significant impacts in its design.

[0031] The empirical formula for the material strengthening index Fc designed in this invention is similar to the parameters mentioned above, but the difference lies in that the empirical formula for the material strengthening index Fc is derived from the comprehensive performance influencing factor P. IF The formula was derived, selecting only chemical elements with significant influence, and this empirical formula is only applicable to the composition system of this invention. The empirical formula for the material strengthening index Fc is used for composition design and to evaluate the influence trend of composition on the strengthening degree of 430 stainless steel. The comprehensive performance influence factor P... IF It can provide a relatively intuitive assessment of the strengthening level of 430 stainless steel in its hot-rolled state and affect the subsequent "black rib stripe" defect; both are indispensable.

[0032] Using the empirical formula for the material strengthening index Fc, this invention preliminarily determines the material strengthening level of the 430 material system through chemical composition. Within the required range of 1.1 to 1.7, a higher index value indicates a higher material strengthening level. Under the condition of ensuring a certain degree of plasticity, compositional optimization introduces greater distortion energy into the material. This distortion energy can act as a driving force during hot rolling to promote new grain nucleation and recrystallization, thereby contributing to grain size refinement and improving the material strengthening effect. Figure 1 , Figure 2 The figure shows the EBSD morphology of the hot-rolled microstructure of 430 stainless steel before and after composition optimization. It can be seen that there are more recrystallized fine grains in the microstructure after composition optimization.

[0033] According to the empirical formula for the material strengthening index Fc, although carbon (C) is a relatively strong austenite-forming element in 430 stainless steel, its strengthening effect is limited because C readily forms carbides with Cr and Fe. Furthermore, these carbides have a much lower second-phase strengthening effect compared to carbides formed by microalloying elements. Therefore, C does not have a significant strengthening effect within the composition range of 430 stainless steel; strength is only improved by obtaining martensite through phase transformation. Mn, also an austenite-forming element, has a limited strengthening effect. While v is a microalloying element, its microalloying effect is still less effective than that of Nb and Ti. In 430 stainless steel, its strength is not significantly enhanced, possibly due to its low content. Among the components, nitrogen (N) has the most significant strengthening effect, and its solid solution in ferrite also promotes texture optimization. However, N has a significant negative impact on material forming and processing, easily leading to processing cracks. Therefore, phosphorus (P) is needed. IF While improving the strengthening level of materials, certain limitations must be imposed. Ni and Cu, as austenitizing elements, are present in relatively low amounts in stainless steel and can be added as part of a balanced strategy for controlling nitrogen content. Si, as a ferrite-forming element, can also be added using a balanced strategy for controlling nitrogen content; however, as a non-metallic element, Si can negatively impact material plasticity. Considering all factors, these four main influencing elements cannot be added indefinitely to achieve material strengthening; their contents are all subject to certain limitations.

[0034] This invention optimizes the composition using the empirical formula of the material strengthening index Fc, maximizing the strengthening effect of the chemical composition and enabling the designed material to have the potential to achieve the expected strengthening level, thereby helping to improve the "black vein stripe".

[0035] Regarding the PREN value, the general formula for stainless steel is PREN = Cr + 3.3Mo. However, in our research on corrosion resistance, we discovered that nitrogen (N) has a certain influence on the passivation film and potential of 430 stainless steel. Therefore, this invention optimizes the formula to: PREN = Cr + 3.3Mo + 16N. Here, Cr improves the stability of the passivation film, Mo significantly enhances the material's local repassivation ability and strengthens the repair of local pitting corrosion, and N increases the pitting potential, strengthens the passivation film, and improves the morphology of carbides.

[0036] The method for manufacturing high-formability 430 stainless steel with black vein stripes according to the present invention includes the following steps: 1) Smelting and casting The stainless steel is smelted and continuously cast into slabs according to the aforementioned composition; the superheat is 20~30℃, the casting speed is 1.0~1.2m / min, electromagnetic stirring is used, and the equiaxed crystal ratio of the continuously cast slab is controlled to be ≥60%; 2) Heating of the billet The billet heating temperature is 1150~1200℃, the furnace time is 120~180min, and the furnace entry temperature is ≤500℃; 3) Hot rolling Total reduction rate in rough rolling ≥ 80%; Finishing mill inlet temperature: 950~1050℃, finishing mill outlet temperature: 850~1000℃; the reduction rate of each of the first two or three passes of finishing milling is 25~60%, and the strain rate of each pass is >10s. -1 The reduction rate of each of the last two or three passes of the finishing rolling process is controlled at 5-60%, and the strain rate of each pass is >90s. -1 ; 4) Heat treatment After hot rolling, heat treatment is carried out by a semi-enclosed annealing method, with an annealing temperature of 840~860℃ and a holding time of 10~15h. 5) Cold rolling Control the cold rolling deformation to ≤85%.

[0037] Preferably, in step 3), the hot rolling roughing is performed in 5 or 7 passes.

[0038] Preferably, in step 3), the finishing rolling process involves 5 to 7 passes, wherein... The rolling process consists of 5 passes, with reduction rates of 25-60% and 5-60% for passes 1, 2, and 3, and for passes 4 and 5, respectively, and strain rates >10 s⁻¹ for each pass. -1 and >90s -1 ; The rolling process consists of 6 passes, with reduction rates of 25-60% and 5-60% for passes 1, 2, and 3, and for passes 4, 5, and 6, respectively, and strain rates >10 s⁻¹ for each pass. -1 and >90s -1 ; The rolling process consists of 7 passes, with reduction rates of 25-60% and 5-60% for passes 1, 2, 3, and 4, and 5, 6, and 7, respectively, and strain rates of >10 s⁻¹ for each pass. -1 and >90s -1 .

[0039] Preferably, step 5) cold rolling uses a 20-roll mill with a work roll diameter of 65-75 mm. When rolling with a small-diameter mill, the contact arc area between the material and the rolls is smaller, and the degree of deformation of the material surface along the rolling direction is relatively smaller, making it less likely for surface rolling coverage to occur along the rolling direction. Therefore, small-diameter mill rolling is superior in improving "black ridges and streaks".

[0040] In the manufacturing method of high formability 430 stainless steel for eliminating black vein stripes as described in this invention: Step 1) In the smelting and casting process, in order to optimize the fiber texture in the cold-rolled annealed sheet, it is necessary to appropriately control the superheat of the molten steel and the continuous casting speed, and to perform electromagnetic stirring to increase the equiaxed crystal ratio in the continuous casting structure. Therefore, the superheat is controlled at 20~30℃, the casting speed at 1.0~1.2m / min, and electromagnetic stirring is used to obtain a 430 continuous casting billet with an equiaxed crystal ratio ≥60%.

[0041] By increasing the proportion of equiaxed crystals, the subsequent microstructure can obtain more γ-fiber texture due to the genetic characteristics of ferrite structure. This has a promoting effect on improving wrinkling in the subsequent cold rolling process, thereby reducing the degree of local deformation on the material surface during cold rolling.

[0042] Furthermore, the 60% equiaxed crystal ratio is a comprehensive consideration of various factors on-site. On one hand, controlling superheat and continuous casting speed is necessary to balance production efficiency and casting quality. On the other hand, while stronger electromagnetic stirring is more conducive to obtaining equiaxed crystals, increasing the intensity of electromagnetic stirring can easily lead to unstable conditions such as surface fluctuations and white bright bands in the continuous casting liquid, affecting casting quality. Therefore, under the existing process equipment conditions, controlling the equiaxed crystal ratio to ≥60% is sufficient. The preferred electromagnetic stirring control is: current 300~600A, frequency 3~9Hz.

[0043] Step 2) During the billet heating stage, control the heating temperature to 1150~1200℃ and the furnace time to 120~180min. Ensure the material can be rolled smoothly while avoiding excessively high temperatures or prolonged holding times that could lead to material deformation. Control the furnace entry temperature to ≤500℃ to prevent the surface oxide scale from being difficult to remove due to excessively high entry temperature, which would affect the subsequent surface quality.

[0044] Step 3) In the hot rolling roughing stage, the total reduction rate is ≥80%, and the preferred target thickness of the steel plate is 35~45mm. At high temperatures, the material strength decreases. Roughing deformation helps the material achieve a suitable thickness before finish rolling. After roughing, as the temperature decreases and time increases, the material temperature approaches the temperature corresponding to a high proportion of austenite, and recrystallization begins, although the degree of recrystallization is low at this stage. In fact, the roughing process for this type of material, besides dimensional control, is a pre-deformation process before finish rolling. The control of deformation amount and dimensions aims to provide sufficient distortion energy for the dynamic recrystallization process during hot rolling, while ensuring the stability of the rolled dimensions and the surface quality of the subsequent finish rolling.

[0045] Step 3) In the hot rolling finishing stage, the overall strain rate of 430 in the finishing stage is increased, which can promote material recrystallization to a certain extent. By refining the grains and improving the material strengthening level, the "black stripe" defect that appears in subsequent processes can be effectively improved.

[0046] Optimizing the hot rolling process promotes the recrystallization process during the hot rolling stage, thereby further enhancing the material's strength and which is beneficial for improving the "black stripe" defect in the future.

[0047] According to the formula It is known that the higher the strain rate, the higher the degree of material recrystallization, the finer the grains, and the greater the strength. By controlling parameters such as rolling speed and reduction rate, the strain rate comparison results before and after optimization are obtained as follows: Figure 3 As shown. After optimization, the overall strain rate of each pass is higher than that before optimization. As expected, the degree of recrystallization of the microstructure after hot rolling will be higher, and the grain refinement will be higher, thus effectively improving the "black stripe" defect.

[0048] Regarding the effect of temperature on the degree of recrystallization, generally as the temperature increases, the material structure gradually transitions from recovery to recrystallization, and the grains gradually show a trend of refinement; however, after the temperature rises to a certain level, the effect of grain growth also becomes increasingly obvious, and the grain refinement effect manifested by recrystallization will be "covered" by grain growth or even secondary recrystallization.

[0049] The finishing rolling temperature is chosen to be close to the "peak" of the austenite phase region at 430°C. The presence of austenite significantly promotes material recrystallization (in the roughing stage, the higher temperature has a relatively weaker effect on recrystallization). Furthermore, the presence of both austenite and ferrite phases during rolling affects the setting of rolling parameters due to factors such as the uniformity of deformation of the two phases, roll stress, surface adhesion between the rolls and the material, and changes in the roll gap caused by temperature increases. In the finishing rolling stage, the temperature is maintained at 850–1050°C, within the high-temperature region where the austenite phase exists. Controlling the overall finishing rolling temperature within the austenite-rich region promotes dynamic recrystallization in these areas through the high-temperature deformation of austenite, thus increasing the overall ferrite recrystallization degree. This trend is clearly reflected in the EBSD results of the hot-rolled microstructure. If the rolling temperature continues to rise during the finishing rolling stage, in addition to affecting the austenite content, the matrix ferrite microstructure will also undergo abnormal changes and exhibit a trend of decreased plasticity. Therefore, the finishing rolling temperature should not be too high. Furthermore, finishing rolling is performed at a lower temperature (below 850°C). While this increases the recrystallization driving force by increasing distortion energy, the drastically increased rolling load makes the surface of 430 stainless steel prone to rolling rust and even cracking. Therefore, this invention also limits the lower limit of the finishing rolling temperature. Generally, after rolling, laminar flow cooling is used to reduce the coil temperature to prevent excessively high temperatures during material coiling from affecting the service life of the coiling equipment. Although increasing laminar flow cooling yields more martensite and produces a strengthening effect, martensite itself has a high stress concentration, and its increased content can easily lead to poor coil shape during the final coiling process. The large presence of martensite in the microstructure places stricter requirements on the final insulation of the material and cannot effectively guarantee the material's ductility and toughness. Considering all factors, under the condition that the hot-rolled performance of the material meets the expected requirements and production control is stable, the intensity and control method of post-rolling laminar flow cooling are not limited.

[0050] Regarding the control of deformation during finishing rolling, in the early stage of finishing rolling, the steel plate size is relatively large, the rolling speed is relatively low, and the material reduction rate is relatively high within the capacity of the rolls. Therefore, this invention controls the reduction rate of each of the first two or three passes in finishing rolling to be 25-60%, with the overall strain rate of the material at a low level and the strain rate of each pass >10s. -1 By subjecting the material to larger deformation, it is possible to bring the material closer to the target size, while allowing the dynamic recrystallization of the material to occur and proceed gradually, so that the dynamic recrystallization process can "adapt" to the temperature conditions at this time.

[0051] In the later stages of finishing rolling, the rolling speed continuously increases, and the rolling reduction rate decreases accordingly. The reduction rate of each of the last two or three passes in finishing rolling is controlled at 5-60% to avoid excessive temperature rise leading to roll gap differences that affect dimensional accuracy. This also reduces contact friction between the steel material and the rolls, minimizing or eliminating defects such as surface peeling and rust. Simultaneously, the strain rate is increased, with each pass exceeding 90 s⁻¹. -1 This further promotes the dynamic recrystallization process of the material. In terms of parameter selection, only two main factors were chosen: deformation amount and strain rate. Deformation amount is easier to control in terms of operation; strain rate is not only related to parameters such as rolling speed, but is also an important indicator of the degree of recrystallization.

[0052] After finishing rolling, with dynamic recrystallization maintained at a high level, the material microstructure continues its static recrystallization process thanks to the high temperature and retained deformation energy. This sustained recrystallization process leads to more grain nucleation and growth, promoting grain refinement and enhancing material strength. Generally, in stainless steel sheet rolling, the dimensions decrease as the finishing rolling stage progresses, and the rolling speed increases. Due to differences in the stress on the rolls in each pass, the reduction in each pass decreases sequentially. This is especially true for the last few passes, which typically use ordinary rolls with relatively weaker stress, requiring a lower reduction rate. Furthermore, the material heats up instantaneously during rolling. Controlling the reduction rate sequentially with increasing rolling speed prevents excessively high rolling temperatures from affecting the roll gap parameters, thus ensuring the target material dimensions remain within acceptable limits and effectively guaranteeing dimensional accuracy. The "black band" structure in the hot-rolled metallographic structure, in addition to small ferrite grains, also contains a small amount of martensite. After fine rolling optimization of conventional composition, the "black band" structure in the metallographic structure is more abundant, the spacing between the bands is smaller, and the degree of recrystallization is also higher. Figure 4 , Figure 5 As shown.

[0053] As a ferritic stainless steel, 430 is more prone to surface quality problems due to the Cr-Fe material system reacting more readily with the rolls during hot rolling. At higher rolling temperatures, the reaction with the rolls intensifies with increased deformation, leading to defects such as surface peeling, oxide scale indentation, and rolling rust. In terms of finishing rolling passes, a 7-pass or fewer pass configuration results in a higher overall strain rate (the conventional 8-pass finishing rolling process for 430 hot rolling) which is more conducive to recrystallization, thus refining the subsequent grain structure and improving the material's strength. However, considering the roll's rolling force limit (rolling load), surface defect control, and the need for better speed coordination between rolling passes, this invention selects 5-7 passes for finishing rolling. In principle, provided the billet can be rolled smoothly and the plate surface quality is good, a higher strain rate is more conducive to recrystallization, effectively improving the "black streaks" defect.

[0054] While ensuring that hot rolling does not result in surface quality issues such as peeling or rust, and that the thickness dimensions remain normal, the degree of recrystallization of the material is increased, thereby refining the grain size of the material after cold rolling and annealing. On one hand, by refining the grain size and promoting recrystallization, the negative effects of the α-fiber texture are mitigated (the larger the grain size, the more pronounced the size effect of deformation on the grains), effectively suppressing the wrinkling effect of the ferrite structure and reducing the degree of deformation on the material surface caused by the cold rolling rolls. On the other hand, refining the grain strengthens the material, thereby improving the overall resistance of the material to external deformation and preventing excessive softness in certain areas, which could lead to deformation along the rolling direction. Therefore, the "black rib stripes" are improved by suppressing the wrinkling effect and increasing the degree of material strengthening.

[0055] Step 4) Heat treatment stage: Heat treatment is carried out by half-cover annealing, with an annealing temperature of 840~860℃ and a holding time of 10~15h.

[0056] For 430 ferritic stainless steel, the annealing process typically involves prolonged holding (20-30 hours) at temperatures below but close to the phase transformation temperature. After annealing, the pickled sludge undergoes cold rolling, followed by continuous annealing to obtain materials with different surface finishes, such as 2B and BA plates. During cold rolling, the ferrite does not undergo a phase transformation; instead, dislocations accumulate within the ferrite through deformation. Due to the equiaxed and columnar crystal distribution characteristics of continuously cast ferritic stainless steel, the material acquires fibrous textures with varying orientations after annealing. When these fibrous textures deform, the structural slip or grain size deformation exhibits different characteristics depending on the ease of slippage. For example, α-fiber textures have a single slip system and a single grain deformation direction; γ-fiber textures have more slip systems, allowing grains to deform in relatively more directions. Therefore, during the cold rolling process, certain specific areas (such as the wrinkled "protrusions") undergo greater deformation during the deformation process, resulting not only in stress concentration but also in the formation of rolled coatings on the material surface (such as...). Figure 6 As shown in the image, these areas become non-reflective and appear darker, resulting in defects such as "black streaks." In the microstructure of normal areas, there are no obvious calendering overlay characteristics (such as...). Figure 7 As shown). Figure 8 As shown, this is the macroscopic morphology of the "black stripes," which exhibit black lines along the rolling direction.

[0057] Defects like "black streaks" can be improved by a two-pass cold rolling and annealing process (effectively reducing deformation wrinkling and thus suppressing the occurrence of "black streaks"). However, this method requires two cold rolling and annealing processes, resulting in extremely high production costs, which is not conducive to the efficient and high-quality production of 430 ferritic stainless steel.

[0058] Experimental studies revealed that the nuclear average orientation difference (KAM) value is more significant in the "black stripe" region, and the "green" distribution area is more abundant in the KAM diagram. Since the KAM value is directly related to the dislocation density in the local area, the "green" area with a larger nuclear average orientation difference indicates that there are obvious stress-strain distribution characteristics in that region, that is, there is relatively obvious stress concentration in the "black stripe" region (e.g., Figure 9 As shown), the stress-strain distribution in the normal region is relatively weak (e.g. Figure 10 As shown); combined with the tissue texture characteristics of the corresponding area, it can be seen that the occurrence of "black veins" is related to the {110} and {100} textures in the tissue, while the normal area contains a considerable amount of {111} texture (such as...). Figure 11 , Figure 12(As shown). This indicates that textures like {110} and {100} are prone to deformation during cold rolling. In fact, these textures are prone to dimensional changes in the thickness direction during deformation; during further cold rolling, these textured areas are also prone to deformation along the rolling direction and cover the material surface. 430 ferritic stainless steel, due to differences in ferritic texture, is prone to wrinkling during drawing or forming, resulting in uneven stripes on the material surface. The degree of unevenness directly affects the degree of surface deformation after rolling. For example... Figure 13 As shown, the higher the wrinkle height, the more severe the deformation of the material surface along the rolling direction after roll forming. Figure 14 As shown; conversely, as Figure 15 , Figure 16 As shown, a small wrinkling height results in minimal deformation of the material surface along the rolling direction after roll forming. During cold rolling, the higher the proportion of {110} and {100} textures in the microstructure, the greater the dimensional change in the thickness direction, and the more severe the wrinkling effect. This leads to a greater degree of surface deformation after roll forming (especially along the rolling direction), making it more prone to the appearance of "black veins and streaks".

[0059] Current research on ferrite texture, while achieving γ-fiber texture can effectively improve wrinkling issues during forming, even the hot-rolling followed by continuous annealing treatment mentioned in the literature has not shown significant improvement in actual production. For wrinkling, compositional optimization is far more effective than process optimization. Early research in Japan involved the addition of the ferrite element Sn; however, considering Sn's status as a harmful element and its low melting point, it is prone to rolling cracking during hot deformation, thus compromising surface quality. In recent years, EBSD has made significant progress in analyzing texture characteristics, but in the actual production of ferritic stainless steel, apart from the "two-pass" method, no manufacturer has been able to effectively improve wrinkling issues through a single-pass cold rolling continuous annealing process. Increasing the equiaxed grain ratio only optimizes the microstructure to a certain extent and cannot decisively improve wrinkling. Therefore, improving the wrinkling process to improve the "black stripe" is a difficult and costly method. Although the wrinkling effect of ferritic stainless steel is the source of the "black stripe" defect, improving the "black stripe" does not necessarily require improving the wrinkling process.

[0060] In the production of 430 ferritic stainless steel, a annealing process is required to improve the material's subsequent plastic processing capabilities. However, the longer the annealing time, the more pronounced the texture inheritance characteristics in the microstructure become, which is detrimental to improving the "black vein stripe" defect by optimizing wrinkling. In fact, hot-rolled 430 ferritic stainless steel rarely exhibits "black vein stripes" on its surface after cold rolling annealing; however, annealed 430 ferritic stainless steel shows severe "black vein stripe" defects on its surface after cold rolling annealing. However, due to plasticity deviations, cold-rolled annealed sheets obtained from hot-rolled 430 ferritic stainless steel are prone to forming cracks during downstream processing, and their corrosion resistance is also poor due to uneven carbide distribution. Therefore, this invention selects a semi-annealing method in the annealing process, balancing material plasticity and the "black vein stripe" defect problem. In actual production, semi-annealing is also beneficial for controlling the fine and dispersed distribution of carbides, such as... Figures 17-19 As shown, the metallographic morphology of 430 ferritic stainless steel in different states shows that the carbide size is the smallest in the semi-annealed state. Carbide growth requires the consumption of Cr elements in the surrounding matrix, so controlling the carbide size is beneficial for improving the corrosion resistance of the material. By comparing the defect differences shown by whether or not the annealing process is used, it is helpful to understand the manifestation characteristics of the "black streaks" defect. Taking this detail as a "starting point," this invention provides ideas and directions for improving the "black streaks" from the aspects of composition and process.

[0061] As a ferritic stainless steel, 430 has relatively low strength. Although the semi-annealed 430 cold-rolled sheet with conventional composition can improve the "black vein stripe" defect to some extent, the incidence of this defect is still very high during downstream customer processing. Based on the above analysis and experimental results regarding the "black vein stripe" defect in 430 ferritic stainless steel and its characteristics, it can be seen that the "black vein stripe" corresponds to regions with a high proportion of {110} and {100} morphologies in the ferrite structure, and the KAM stress-strain degree is high. Furthermore, the microstructure of the "black vein stripe" shows significant rolling deformation along the rolling direction. Additionally, the surface of the cold-rolled annealed sheet corresponding to hot-rolled raw materials exhibits no "black vein stripe." Based on these results and characteristics, this invention proposes a concept: the "black vein stripe" defect can be improved by increasing the material strength and the degree of recrystallization. Because increased material strength helps the microstructure effectively resist external deformation, this area is pressed into the material matrix surface, thus suppressing large local deformation of the "protruding" parts (such as deformation along the rolling direction pressing into the material matrix surface). Conversely, if the material is softer, during cold rolling deformation, local "protruding" areas are more likely to be covered by rolling along the rolling direction, leading to defects such as "black streaks". To balance the subsequent forming and processing of cold-rolled materials and the corrosion resistance of the material, this invention effectively solves the surface quality problem of "black streaks" under the basic conditions of the semi-covering process. In addition to improving the distribution of texture characteristics, this invention improves material strength and promotes recrystallization through the above-mentioned component optimization, hot rolling process, and covering process adjustments and improvements. Among these, the semi-covering process plays a role in balancing the subsequent processing and forming of the material, corrosion resistance, and the control of "black streaks" defects.

[0062] Step 5) In the cold rolling stage, the diameter of the cold rolling work rolls is 65~75mm, the relative reduction rate per cold rolling pass is <20%, and the total reduction rate is ≤85%. Specifically, the relative reduction rate per pass = (dimension before reduction - dimension after reduction) / dimension before reduction × 100%, and the total reduction rate = (raw material size - finished cold-rolled product size) / raw material size × 100%.

[0063] The working roll diameter can be selected according to the rolling pressure characteristics and equipment operating speed on site. To improve the "black stripe" effect, it is recommended to use a smaller working roll diameter. In the rolling process, the strength of the material is higher as it approaches the last pass, and the rolls are subjected to greater forces. Therefore, the roll diameter should not be too small, because from the perspective of torque force, a rolling roll diameter that is too small is not conducive to the efficient utilization of motor power and mill pressure.

[0064] During the cold rolling stage, controlling the corresponding cold rolling parameters can improve the appearance of "black veins and streaks." Regarding controlling the cold rolling reduction rate, generally, the higher the reduction rate, the greater the degree of material deformation, and the more likely "black veins and streaks" will appear. In actual production, the larger the roll diameter, the greater the rolling force and the larger the contact area (e.g., ...). Figure 20 As shown in the diagram, the material surface is more prone to large-scale deformation, therefore a work roll diameter of 65-75mm is recommended. Based on field experience, for standard 430 stainless steel, the incidence of "black vein stripes" defects is high when the cold rolling deformation exceeds 85%. Therefore, the deformation amount can be appropriately controlled during the cold rolling process. In the initial stage of cold rolling, the material is relatively soft, so a certain reduction rate needs to be controlled to avoid excessive deformation leading to obvious rolling coverage on the material surface. In the later stage of cold rolling, the material is generally harder, making "black vein stripes" less likely to appear. Therefore, in cold rolling control, for 430 stainless steel with conventional composition and process, the reduction rate of each pass needs to be reasonably matched according to the actual situation in the initial stage of cold rolling; for 430 stainless steel with optimized composition and process, there are no particularly strict restrictions on this cold rolling process. This cold rolling control is only a supplementary and preferred step to improve the "black vein stripe" defect in 430 stainless steel.

[0065] The cold rolling process directly affects the appearance of "black streaks" in 430 stainless steel, being a key characteristic of this phenomenon. Therefore, proper control of the work roll diameter and deformation amount can effectively improve the appearance of "black streaks." When downstream cold rolling processes utilize large-diameter (approximately 210mm) mills with eight-roll eccentric mills, the contact area for material deformation is larger, making it easier for surface deformation along the rolling direction to occur. Therefore, under large-roll rolling conditions, the cold rolling reduction rate should be appropriately controlled according to the actual situation to avoid the appearance of "black streaks" due to excessive deformation or increased susceptibility to deformation along the rolling direction.

[0066] In actual production, the 430 stainless steel described in this invention, after undergoing optimized control of hot rolling, semi-annealing, and cold rolling annealing to obtain 2B plates, shows a significant reduction in surface wrinkling after 20% drawing. Figure 21 , Figure 22 As shown. Compare with EBSD orientation maps (e.g.) Figures 23-26 As shown in the figure, the optimized ferrite microstructure not only has smaller grain size but also a higher proportion of {111} texture. This indicates that the optimized 430 stainless steel not only has an optimized fiber texture and smaller variations in thickness, resulting in less wrinkling, but also, due to the increased material strength, its microstructure's ability to resist external deformation is enhanced, making it less prone to localized rolling deformation along the rolling direction during cold rolling. In the optimized 430-BA plate (after brightening treatment, surface defects are easier to observe), the "black veins" on the material surface are significantly improved, and the black rolling marks along the rolling direction are completely eliminated, as shown in the figure.Figure 27 , Figure 28 The image shows a comparison of the surface before and after optimization.

[0067] The optimized 430 stainless steel 2B plate meets the downstream customer's requirements for mechanical and formability properties (as shown in Table 1). Regarding corrosion resistance (as shown in Table 2), the improved pitting corrosion rate and potential level result in significant salt spray corrosion resistance. Figure 29 , Figure 30 As shown, the optimized material did not show significant rusting in the alternating wet and dry salt spray test. Corrosion resistance is a fundamental characteristic of stainless steel, and its quality is crucial for the material's service life. In other words, the optimized composition and processing of 430 stainless steel not only effectively improves the "black streaks" defect but also maintains good mechanical and formability properties (such as...). Figure 31 , Figure 32 As shown, the optimized material not only did not crack in LDR2.1, but also showed very little wrinkling after cupping, as well as good corrosion resistance.

[0068]

[0069]

[0070] Compared with the prior art, the advantages of the present invention are as follows: This invention utilizes EBSD to analyze the distribution characteristics of the "black streaks" defect, revealing a strong correlation between the defect and ferrite deformation wrinkling. Macroscopically, it exhibits localized plastic deformation along the rolling direction, while microscopically, it displays a high distribution of {110} and {100} texture types. Simultaneously, the stress-strain concentration is high, consistent with the degree of macroscopic plastic deformation.

[0071] Based on the performance characteristics of SUS430 material, this invention provides a simple calculation method for determining the strengthening degree of SUS430 material, namely, the comprehensive performance influence factor P. IF Based on an understanding of the formation mechanism and characteristics of the "black streaks" defect, through P... IFThe derived material strengthening index Fc and its corresponding control range were used to optimize and adjust the composition, ensuring that the properties of SUS430 after hot rolling met the expected requirements for improving the "black rib stripe" defect. Simultaneously, by optimizing the ferrite texture distribution, recrystallization degree, and improving material strength, the "black rib stripe" defect was further improved through controlling the equiaxed crystal ratio in continuous casting, hot rolling finishing control, and semi-enclosed annealing treatment. After cold rolling and continuous annealing, the material not only effectively improved the "black rib stripe" defect but also maintained good formability and mechanical properties, and its corrosion resistance was further enhanced. Without increasing additional alloy costs or production costs, the related defect problems were solved relatively efficiently without significantly reducing the material's basic properties. Attached Figure Description

[0072] Figure 1 Electron backscatter diffraction (EBSD) orientation diagram of the longitudinal section of SUS430 hot-rolled material (composition before optimization). Figure 2 Electron backscatter diffraction (EBSD) orientation pattern of the longitudinal section of SUS430 hot-rolled material (optimized composition). Figure 3 A comparison chart of strain rates for finishing passes of SUS430 hot rolling; Figure 4 Metallographic images of hot-rolled SUS430 with standard composition (before fine rolling optimization); Figure 5 Metallographic images of hot-rolled SUS430 with standard composition (after fine rolling optimization). Figure 6 Metallographic image of the surface of SUS430-BA plate ("black streaks" defect area). Figure 7 Metallographic morphology of the surface of SUS430-BA plate (normal area); Figure 8 Macroscopic morphological photograph of the "black streaks" defect on the SUS430-BA board; Figure 9 KAM diagram of the average orientation difference of the nuclei on the surface of the SUS430-BA plate ("black stripe" defect area). Figure 10 KAM diagram of average orientation difference of nuclei on the surface of SUS430-BA plate (normal area); Figure 11 Electron backscatter diffraction (EBSD) orientation pattern of the SUS430-BA plate surface ("black stripe" defect area). Figure 12 Electron backscatter diffraction (EBSD) orientation pattern of the SUS430-BA plate surface (normal region). Figure 13A schematic diagram illustrating the formation of "black ridges" on the surface of a SUS430-BA board (due to the large initial surface wrinkling height); Figure 14 A top view diagram illustrating the severe "black streaks" defect on the surface of the SUS430-BA board; Figure 15 A schematic diagram illustrating the formation of "black ridges" defects on the surface of SUS430-BA board (with small initial surface wrinkling height); Figure 16 A top view diagram illustrating the formation of a slight "black streaks" defect on the surface of a SUS430-BA board; Figure 17 Metallographic morphology photograph of SUS430 cross section (hot rolled state); Figure 18 Metallographic image of a cross section of SUS430 (half-coverage). Figure 19 Metallographic image of a cross section of SUS430 (full cover). Figure 20 This is a schematic diagram illustrating the effect of roll diameter on cold rolling deformation; (R1 is the large roll diameter, R2 is the small roll diameter). Figure 21 Photographs of the wrinkling morphology of a SUS430-2B board after 20% pull-out (before optimization, from left to right: 0°, 45°, 90°); Figure 22 Photographs of the wrinkling morphology of a SUS430-2B plate after 20% pull-out (optimized, from left to right: 0°, 45°, 90°); Figure 23 Electron backscattering diffraction (EBSD) orientation pattern at the thickness edge of the longitudinal section of plate 430-2B (before optimization); Figure 24 Electron backscattering diffraction (EBSD) orientation pattern at the thickness edge of the longitudinal section of plate 430-2B (after optimization). Figure 25 Electron backscattering diffraction (EBSD) orientation diagram of the thickness center of the longitudinal section of plate 430-2B (before optimization); Figure 26 Electron backscattering diffraction (EBSD) orientation pattern at the thickness center of the longitudinal section of plate 430-2B (after optimization). Figure 27 Comparison of the surface macromorphology of the SUS430-BA board (before optimization); Figure 28 Comparison of the surface macromorphology of the SUS430-BA board (after optimization); Figure 29 Comparison of SUS430-2B plate wet and dry alternating / 24h salt spray test (before optimization); Figure 30Comparison chart of SUS430-2B plate dry and wet alternation / 24h salt spray test (optimized); Figure 31 Comparison chart of LDR2.1 results for SUS430-2B plate punch cup (before optimization); Figure 32 Comparison chart of LDR2.1 results for SUS430-2B plate punch cup (after optimization); Figure 33 Metallographic corrosion morphology of SUS430-2B plate surface at 500x magnification (hot-rolled raw material); Figure 34 Metallographic corrosion morphology of SUS430-2B plate surface at 500x magnification (half-covered raw material). Figure 35 Electron probe corrosion morphology of SUS430-2B plate surface (hot rolled raw material). Figure 36 Electron probe etching morphology of SUS430-2B board surface (half-covered raw material removal). Detailed Implementation

[0073] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0074] The composition of the 430 stainless steel embodiments and comparative examples of this invention is shown in Table 3, with the remainder including Fe and unavoidable impurities. The manufacturing process parameters of the embodiments and comparative examples of this invention are shown in Table 4, and Table 5 shows the performance parameters of the embodiments and comparative examples of this invention.

[0075] As shown in Table 3, the material strengthening index Fc estimated by the composition of Examples 1-10 of the present invention is greater than 1.1, corresponding to the calculated P of the actual hot-rolled performance. IF It is also greater than 1.1. According to historical data from SUS430, P is generally... IF If the value is less than 1.7, it usually indicates an elongation rate of less than 10%, which is generally considered a failure to meet performance standards. No such data has been found in the current data.

[0076] Referring to Tables 4 and 5, the "black streaks" defect in the 430 stainless steel products obtained by the manufacturing method described in this invention was basically eliminated after cold rolling and continuous annealing; the LDR2.1 cup test was good, with no cracking; and no obvious signs of rust were observed in the 24-hour alternating wet and dry salt spray test. The components in the examples all had an overall PREN value greater than 17.0, and their actual salt spray corrosion resistance was superior to that of the comparative examples.

[0077] Comparative Example 1, with a annealing temperature and time of 0, represents hot-rolled SUS430. Compared to annealed raw materials, hot-rolled raw materials yield finished products with higher strength and stronger resistance to external deformation, making them less prone to "black streaks." The LDR2.1 cup test resulted in cracking because the lack of annealing treatment prevented sufficient carbide dispersion, leading to heavier salt spray rusting. Figures 33-36 The image shows the corrosion metallographic and probe morphology of 2B plates corresponding to hot-rolled SUS430 and semi-removed SUS430, respectively. Without the removal treatment, not only is the carbide distribution uneven, affecting corrosion resistance, but the degree of ferrite recrystallization is also low, affecting formability.

[0078] In Comparative Example 2, although a half-coverage process was adopted, the "black stripe" defect appeared due to the lack of composition optimization and the absence of pass optimization in the finishing rolling. However, the distribution ratio was relatively low.

[0079] Comparative Example 3, which adopted a half-coverage process, still exhibited "black stripe" defects due to the lack of composition optimization, although the degree of distribution was lower than that of Comparative Example 2.

[0080] Comparative Examples 4 and 6, which were subjected to full-coverage processing, had no optimized composition or rolling process, resulting in severe "black rib stripe" defects.

[0081] Comparative Example 5, which adopted full-coverage backing treatment, did not optimize the composition or rolling process, and after increasing the amount of cold rolling deformation, the "black rib stripes" were severely present throughout the roll.

[0082] Comparative Example 7, a new steel grade tested in continuous annealing, has an estimated material strengthening index (Fc) of -0.184 based on its composition, and its calculated P... IF The value is 1.015, indicating that this steel grade is not suitable for the SUS430 system of this invention in terms of composition. Even so, it can still achieve the corresponding material strengthening level through actual performance. Due to the continuous annealing process, the shorter annealing time results in a higher material strength level, and no "black streaks" appeared during subsequent cold rolling. However, cracks appeared in the stamping cup, and severe rusting occurred.

[0083] In Comparative Example 8, the equiaxed crystal ratio was controlled at 52% during continuous casting. Compared with Comparative Example 2, the ratio of "black veins and stripes" increased after the equiaxed crystal ratio was reduced.

[0084] The results from the examples and comparative examples show that the material strengthening index Fc obtained from the composition estimation is consistent with the P calculated from the actual performance. IF The results differ somewhat. The results from component estimation can only predict the enhancing effect caused by the components. After all, Fc originates from P. IF Empirical formulas can only provide a preliminary assessment of the influence of composition on the strengthening effect; ultimately, the degree of material strengthening needs to be reflected through actual hot-rolled performance.

[0085] The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Any changes, modifications, and application of formulas and algorithms to the above embodiments within the essential spirit of the present invention will fall within the scope of the claims of the present invention.

[0086]

[0087]

[0088]

Claims

1. High-formability 430 stainless steel with eliminated black veins and streaks, its composition by mass percentage is: C：≤0.050%， Si: 0.20~0.65%, Mn: 0.15~0.65%, P：≤0.045%， S：≤0.0040%， Cr：16.0~17.5%， Ni: ≤0.20%, Mo: ≤0.15%, Cu: ≤0.10%, V：0.05~0.15%， N：0.050~0.090%， The balance includes Fe and unavoidable impurities, and simultaneously satisfies: The material strengthening index Fc = 1.1~1.

7. Fc=28.84×N+2.09×Si+5.18×Ni+3.79×Cu-2.15; The pitting corrosion resistance equivalent PREN ≥ 17.0, PREN = Cr + 3.3Mo + 16N.

2. The high-formability 430 stainless steel with black vein stripe elimination as described in claim 1, characterized in that, Its composition balance is Fe and unavoidable impurities.

3. The high-formability 430 stainless steel with eliminated black vein stripes as described in claim 1 or 2, characterized in that, The Rockwell hardness HRB of the hot-rolled 430 stainless steel is 90~95, and the tensile strength R... m The yield strength is 600~850MPa, and the yield strength R is... p0.2 The strength is 450~600MPa, and the elongation after fracture (A) is 12~20%.

4. The method for manufacturing high-formability 430 stainless steel with eliminated black vein stripes as described in claim 1, 2, or 3, characterized in that, Includes the following steps: 1) Smelting and casting Smelting and continuously casting into slabs according to the composition of claim 1 or 2; superheating degree 20~30℃, casting speed 1.0~1.2m / min, electromagnetic stirring, and controlling the equiaxed crystal ratio of the continuously cast slab to be ≥60%; 2) Heating of the billet The billet heating temperature is 1150~1200℃, the furnace time is 120~180min, and the furnace entry temperature is ≤500℃; 3) Hot rolling Total reduction rate in rough rolling ≥ 80%; Finishing mill inlet temperature: 950~1050℃, finishing mill outlet temperature: 850~1000℃; the reduction rate of each of the first two or three passes of finishing milling is 25~60%, and the strain rate of each pass is >10s. -1 The reduction rate of each of the last two or three passes of the finishing rolling process is controlled at 5-60%, and the strain rate of each pass is >90s. -1 ; 4) Heat treatment After hot rolling, heat treatment is carried out by a semi-enclosed annealing method, with an annealing temperature of 840~860℃ and a holding time of 10~15h. 5) Cold rolling Control the cold rolling deformation to ≤85%.

5. The method for manufacturing high-formability 430 stainless steel with eliminated black vein stripes as described in claim 4, characterized in that, In step 3), the hot rolling roughing process uses 5 or 7 passes.

6. The method for manufacturing high-formability 430 stainless steel with eliminated black vein stripes as described in claim 4 or 5, characterized in that, In step 3), Finishing rolling involves 5 to 7 passes, among which... The rolling process consists of 5 passes, with reduction rates of 25-60% and 5-60% for passes 1, 2, and 3, and for passes 4 and 5, respectively, and strain rates >10 s⁻¹ for each pass. -1 and >90s -1 ; The rolling process consists of 6 passes, with reduction rates of 25-60% and 5-60% for passes 1, 2, and 3, and for passes 4, 5, and 6, respectively, and strain rates >10 s⁻¹ for each pass. -1 and >90s -1 ; The rolling process consists of 7 passes, with reduction rates of 25-60% and 5-60% for passes 1, 2, 3, and 4, and 5, 6, and 7, respectively, and strain rates >10 s⁻¹ for each pass. -1 and >90s -1 .

7. The method for manufacturing high-formability 430 stainless steel with eliminated black vein stripes as described in claim 4, characterized in that, Step 5) Cold rolling is performed using a 20-roll mill with a work roll diameter of 65~75mm.

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