Method for inhibiting titanium stripes on cold-rolled surface of titaniferous ferritic stainless steel

Abstract

The invention discloses a method for inhibiting titanium stripes on a titanium-containing ferritic stainless steel cold-rolled surface, which comprises the working procedures of smelting, casting, hot rolling and the like which are performed in sequence: in the casting process, stainless steel molten iron is continuously injected into a water-cooling crystallizer through tundish drainage; the initial blank shell continuously pulled out from the outlet of the water-cooling crystallizer is subjected to secondary cooling and withdrawal and straightening to be solidified and formed, and a continuous casting blank is obtained; wherein the temperature of molten iron in a tundish is controlled to be 1545-1565 DEG C, and the temperature of a steel billet obtained through the secondary cooling procedure is controlled to be 600-700 DEG C by improving the cooling strength of a water cooling crystallizer and the secondary cooling procedure; in the hot rolling process, the steel drawing temperature of a hot rolling heating furnace is controlled to be 1140 DEG C + / -30 DEG C; the titanium stripe defect retention ratio of the finally prepared cold-rolled finished coil can be reduced to be within 2%.

Description

Technical Field

[0001] This invention relates to steel smelting processes, and more particularly to a method for suppressing titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel. Background Technology

[0002] 409L stainless steel, as an ultra-pure ferritic stainless steel, boasts excellent corrosion resistance and an affordable price, making it a highly cost-effective material. It has been widely and extensively used in the automotive exhaust pipe manufacturing industry, becoming one of the preferred stainless steel materials in this field. Its composition includes a certain amount of titanium, which plays a role in refining grain size, improving the material's mechanical properties, and enhancing corrosion resistance during the stainless steel production process. Titanium is a crucial component ensuring the core performance of 409L stainless steel.

[0003] However, under current conventional production processes, 409L stainless steel cold-rolled finished products with this composition are highly susceptible to titanium streaks on their surface, with production statistics showing an occurrence rate of approximately 10%. These titanium streaks refer to the titanium nitride (TiN) phase, formed by the combination of titanium and nitrogen elements within the material, which locally accumulates on the finished product surface during cold rolling. This results in streak-like defects distributed along the rolling direction and differing in color from the base material, a typical surface compositional clustering defect. It should be noted that 409L stainless steel is primarily used in automotive exhaust pipes, which are typically located under the vehicle and are concealed components. Therefore, in the past, customers generally did not pay much attention to these surface titanium streaks, and they did not significantly impact product delivery. However, in recent years, with increasing competition in the automotive parts industry, downstream customers have begun to use these surface defects as bargaining chips to lower purchase prices, making titanium streaks a significant factor restricting product profitability and market promotion. Meanwhile, due to previous low industry attention to this defect, there is a lack of specific research on titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel, and a lack of mature and economical solutions. These defects not only disrupt the uniformity and smoothness of the stainless steel surface but are also used by customers in price negotiations, significantly impacting product market acceptance, delivery qualification rates, and profit margins.

[0004] To address the aforementioned titanium stripe defect problem, the industry has attempted various improvement measures, but all have significant limitations, making it difficult to maintain product cost competitiveness while ensuring defect suppression. Specifically, existing improvement methods mainly include the following categories: Firstly, by adding shot blasting, sulfuric acid chemical pretreatment and mixed acid pickling after tension straightening, the hot-rolled white strip of titanium-containing ultrapure ferritic stainless steel is pickled twice to remove granular TiN inclusions (such as CN115430717A) from the surface of the strip. Secondly, by adding a slab surface grinding process, the TiN enriched layer formed on the slab surface can be removed in a targeted manner; however, both of these processes will prolong the overall production process, significantly reduce production efficiency, and the degree of pickling and grinding precision are difficult to control precisely, which can easily lead to secondary problems such as uneven slab thickness and excessive surface roughness. At the same time, it cannot eliminate the stripe defects caused by the enrichment of TiN phase inside the material from the root, and the overall treatment effect is limited. Third, by reducing the nitrogen (N) content in the material, the reactants that combine with titanium to form the TiN phase can be reduced from the source. However, practice has shown that there are technical bottlenecks in reducing nitrogen content. Under conventional processes, the nitrogen content can only be controlled within the range of 0.005%-0.01%. Further reduction of nitrogen requires breaking through the limitations of existing smelting technology. The process is extremely difficult to implement and cannot be applied on a large scale.

[0005] In summary, existing methods for suppressing titanium stripe defects on the cold-rolled surface of titanium-containing ferritic stainless steel (such as 409L stainless steel) are limited in their effectiveness, technical bottlenecks, and increased costs. They cannot simultaneously achieve both defect suppression and the needs of large-scale production, and thus cannot meet the dual requirements of appearance quality and production stability for stainless steel used in automotive exhaust pipes. Summary of the Invention

[0006] The purpose of this invention is to provide a method for suppressing titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel.

[0007] The technical solution to achieve the objective of this invention is: a method for suppressing titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel, comprising the following steps: (1) Smelting and casting: The titanium-ferritic stainless steel feedstock is smelted in a converter to obtain stainless steel molten iron. The stainless steel molten iron is continuously injected into a water-cooled crystallizer through a tundish. The initial billet shell continuously pulled out from the outlet of the water-cooled crystallizer is then cooled and straightened to solidify and form a continuously cast billet. The temperature of the molten iron in the tundish is controlled at 1545-1565℃. By increasing the cooling intensity of the water-cooled crystallizer and the secondary cooling process, the temperature of the billet after the secondary cooling process is controlled at 600-700℃. (2) Hot rolling: The continuous casting billet obtained in step (1) is sent into a hot rolling furnace for heating. The steel extraction temperature is 1140℃±30℃. It is then rolled to the required thickness by a hot rolling mill to obtain a black coil. (3) Solution annealing, scale breaking, shot blasting and pickling: The black leather rolls obtained in step (2) are subjected to conventional solution annealing, scale breaking, shot blasting and pickling treatments in sequence to obtain white leather rolls; (4) Cold rolling, solution annealing and pickling: The white paper roll obtained in step (3) is subjected to conventional cold rolling, annealing and pickling to obtain the cold-rolled finished roll.

[0008] By performing electron microscopy on samples from the surface and center of the continuously cast billet, the inventors found that the TiN particles in the surface sample were coarse and unevenly distributed, while the TiN particles in the center sample were finer and more evenly distributed. Therefore, they hypothesize that the formation mechanism of TiN streaks is as follows: During continuous casting, precipitation and enrichment occur due to temperature drop. Under the cooling effect of the crystallizer, the surface layer of the molten steel in contact with the crystallizer wall rapidly cools and solidifies, thus forming the primary billet shell. After TiN precipitation and enrichment, it is trapped near the surface of the primary billet shell and cannot move, easily forming titanium streak defects. However, near the center, due to the higher temperature, it can continue to diffuse, and the diffused TiN is finer and more evenly distributed, thus less prone to forming titanium streak defects. Based on the above findings and analysis, the inventors have increased the tundish molten iron temperature during continuous casting to 1545–1565°C, breaking with conventional methods (currently, the tundish molten iron temperature in titanium-ferritic stainless steel production processes is generally controlled below 1540°C). By increasing the superheat of the molten steel before solidification, the precipitation of TiN is delayed, reducing the number of nuclei formed at the solidification front. Simultaneously, the higher molten iron temperature also facilitates the re-dissolution of TiN particles and inhibits their growth, while enhancing atomic diffusion capabilities, resulting in a more uniform composition and reducing TiN aggregation in localized areas. More importantly, while increasing the tundish molten iron temperature, the cooling intensity of the water-cooled crystallizer and the secondary cooling process is also increased, allowing for improvements without altering existing conventional water-cooled crystallizer equipment and secondary cooling processes. In this case, the temperature of the billet obtained after secondary cooling can be reduced to the conventional temperature of 600-700℃. This avoids the leakage alarm caused by excessively wide bandwidth after hot rolling due to increased tundish molten iron temperature. At the same time, the molten steel will solidify more quickly during the process of flowing through the water-cooled crystallizer (increased cooling rate), which shortens the distance that TiN precipitates and moves. This reduces the number of TiN particles that reach the surface and are captured by the surface, reducing the aggregation of TiN particles on the surface and thus mitigating the appearance of titanium streaks. Furthermore, by increasing the hot rolling temperature (1140℃±30℃) during hot rolling, the enriched TiN continues to expand, changing from large TiN particles to small TiN particles. Ultimately, most TiN particles still maintain a fine and dispersed distribution, effectively suppressing the generation of titanium streak defects.

[0009] Therefore, the inventors have optimized and increased the molten iron temperature in the tundish of the continuous casting process and the steel extraction temperature in the heating furnace of the hot rolling process to reduce the incidence of titanium streaks in cold-rolled finished products. In terms of performance, the titanium streak defect retention rate of the cold-rolled finished coils produced by this invention can be reduced to less than 2%, a significant improvement compared to the approximately 10% titanium streak defect retention rate before optimization.

[0010] Further, the titanium-containing ferritic stainless steel feedstock (wt%) in step (1) is: C 0.005-0.020%, Mn 0.05-0.25%, Si 0.2-0.5%, Cr 11.0-11.7%, N 0.005-0.015%, Ti 0.12-0.2%, with the remainder being iron and unavoidable impurities. This application, while increasing the temperature of the molten iron in the tundish and the steel extraction temperature of the heating furnace in the hot rolling process, also strictly controls the Ti and N elements at low levels, specifically controlling the Ti element in the stainless steel at 0.12-0.20% and the N element at 0.005-0.015%. This does not affect the corrosion resistance and other properties of the stainless steel product, while further reducing the generation of TiN particles, ultimately reducing the proportion of titanium stripe defects to less than 1%.

[0011] Furthermore, in the casting process of step (1), the cooling water volume of the wide and narrow sides of the water-cooled crystallizer is controlled at 2800-3000 L / min (2400-2700 L / min before improvement) and 390-420 L / min (340-380 L / min before improvement), respectively; the cooling water volume of the secondary cooling process is controlled at 3100-3500 L / min (2500-3000 L / min before improvement) to increase the cooling rate, thereby achieving the purpose of increasing the cooling intensity. Ultimately, without changing the existing conventional water-cooled crystallization equipment and secondary cooling process, the temperature of the steel billet obtained after secondary cooling can be reduced to the conventional temperature of 600-700℃. Attached Figure Description

[0012] Figure 1 Electron micrograph of a surface sample of an existing 409L cold-rolled finished coil; Figure 2 Electron micrograph of the center sample of an existing 409L cold-rolled finished coil; Figure 3 A photograph of a cold-rolled finished coil with "slight" titanium stripes produced by the present invention; Figure 4 This is a photograph of a cold-rolled finished coil with "medium" titanium stripe intensity produced by the present invention. Detailed Implementation

[0013] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0014] Take 409L stainless steel as an example.

[0015] A method for suppressing titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel (409L stainless steel) includes the following steps: (1) Smelting and casting: The titanium-ferritic stainless steel feedstock is smelted in a converter to obtain stainless steel molten iron. The stainless steel molten iron is continuously injected into a water-cooled crystallizer through a tundish. The initial billet shell continuously pulled out from the outlet of the water-cooled crystallizer is then cooled and straightened to solidify and form a continuously cast billet. The temperature of the molten iron in the tundish is controlled at 1545-1565℃. By increasing the cooling intensity of the water-cooled crystallizer and the secondary cooling process, the temperature of the billet after the secondary cooling process is controlled at 600-700℃. (2) Hot rolling: The continuous casting billet obtained in step (1) is sent into a hot rolling furnace for heating. The steel extraction temperature is 1140℃±30℃. It is then rolled to the required thickness by a hot rolling mill to obtain a black coil. (3) Solution annealing, scale breaking, shot blasting and pickling: The black leather rolls obtained in step (2) are subjected to conventional solution annealing, scale breaking, shot blasting and pickling treatments in sequence to obtain white leather rolls; (4) Cold rolling, solution annealing and pickling: The white paper roll obtained in step (3) is subjected to conventional cold rolling, annealing and pickling to obtain the cold-rolled finished roll.

[0016] The embodiments of the method for suppressing titanium streaks on the cold-rolled surface of titanium-ferritic stainless steel of the present invention, the weight percentage of chemical components in the comparative batches (unit: %), the balance being Fe and unavoidable impurity elements are shown in Table 1, and the key process parameters such as the molten iron temperature in the tundish, the billet temperature after the secondary cooling process, and the steel extraction temperature in the hot rolling furnace are shown in Table 2.

[0017] Table 1

[0018] Table 2

[0019] All cold-rolled finished coils from the same batch as the examples and comparative examples were uniformly classified into three levels according to the degree of surface titanium stripe defects: "slight", "slight-moderate", and "moderate". Minor—qualified; Medium - Disqualified; Minor to moderate—Customers usually accept the materials as acceptable first, but at the same time remind the raw material supplier to make improvements.

[0020] Table 3 lists the total number of rolls, the number of non-conforming rolls, and the percentage of non-conforming rolls with titanium stripe defects for each group (number of non-conforming rolls / total rolls, %).

[0021] The criteria for classifying titanium streaks as "slight," "slight-moderate," and "moderate" are as follows: "Slight": 0-2 distinct titanium streaks are visible across the entire width; "Slight to Medium": Three or four distinct titanium stripes are visible across the entire width; "Medium": 5-10 distinct titanium stripes are visible across the entire width.

[0022] To more intuitively illustrate the "slight" and "moderate" severity levels of titanium streaks in Table 3, the following are provided: Figure 3 (Mild) and Figure 4 (Medium) The actual photograph shown, in which Figure 3 No obvious titanium stripes were observed across the entire width. Figure 4 There are about seven distinct titanium stripes clearly visible across the entire width.

[0023] Table 3

[0024] As can be seen from Tables 1-3, by adjusting the key process steps of this invention, the percentage of defective coils in the resulting 409L cold-rolled finished coils is less than 2% (Examples 1-12). Furthermore, by controlling the weight percentage (wt%) of the titanium-ferritic stainless steel ingredients as follows: C 0.005-0.020%, Mn 0.05-0.25%, Si 0.2-0.5%, Cr 11.0-11.7%, N 0.005-0.015%, Ti 0.12-0.2%, with the remainder being iron and unavoidable impurities. Importantly, the Ti and N elements are controlled at low levels (Ti 0.12-0.2%, N...). (0.005~0.015%), further reducing the generation of TiN particles, and finally reducing the proportion of titanium stripe defects to less than 1%, that is, the pass rate can reach more than 99% (as in Examples 1 to 10); while the failure rate of the same batch of 409L cold-rolled finished coils before optimization and improvement was as high as 12.28% (as in Comparative Example 6). When the composition system meets the requirements of "Ti 0.12~0.2%, N 0.005~0.015%", and the molten iron temperature in the tundish is below 1545℃ or / and the steel drawing temperature in the hot rolling furnace is below 1100℃ (as in Comparative Examples 1-4), the improvement of titanium stripe defects in the resulting cold-rolled coils is significantly worse. Simultaneously, when the molten iron temperature in the tundish exceeds 1565℃, the excessively high temperature leads to a significant decrease in drawing speed, easily forming vibration marks and increasing the risk of surface defects. When the steel drawing temperature in the hot rolling furnace exceeds 1170℃, the excessively high temperature causes the slab to easily collapse within the furnace, which scratches the padding blocks, increasing the risk of surface scratches. Therefore, this invention controls the molten iron temperature in the tundish at 1545℃~1565℃ and the steel drawing temperature in the hot rolling furnace at 1110℃~1170℃ (i.e., 1140℃±30℃). Furthermore, as can be seen from Comparative Example 5, simply adjusting the composition design cannot prevent the occurrence of titanium streaks.

[0025] In the specific implementation process, the cooling water volume of the wide and narrow sides of the water-cooled crystallizer in the casting process of step (1) is controlled at 2800-3000 L / min (2400-2700 before improvement) and 390-420 L / min (340-380 before improvement), respectively; the cooling water volume of the secondary cooling process is controlled at 3100-3500 L / min (2500-3000 before improvement) to increase the cooling rate, thereby achieving the purpose of increasing the cooling intensity. Ultimately, without changing the existing conventional water-cooled crystallization equipment and secondary cooling process, the temperature of the steel billet obtained after secondary cooling can be reduced to the conventional temperature of 600-700°C. Of course, the present invention can also achieve the purpose of increasing the cooling intensity by appropriately reducing the water temperature.

[0026] The method of the present invention for suppressing titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel is applicable not only to the production of 409L stainless steel, but also to other titanium-containing pure ferritic stainless steels, such as SUS436L, 439M, 441, 443, 444, etc.

[0027] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent process transformations made using the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for suppressing titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel, characterized in that, Includes the following steps: (1) Smelting and casting: The titanium-ferritic stainless steel feedstock is smelted in a converter to obtain stainless steel molten iron. The stainless steel molten iron is continuously injected into a water-cooled crystallizer through a tundish. The initial billet shell continuously pulled out from the outlet of the water-cooled crystallizer is then cooled and straightened to solidify and form a continuously cast billet. The temperature of the molten iron in the tundish is controlled at 1545-1565℃. By increasing the cooling intensity of the water-cooled crystallizer and the secondary cooling process, the temperature of the billet after the secondary cooling process is controlled at 600-700℃. (2) Hot rolling: The continuous casting billet obtained in step (1) is sent into a hot rolling furnace for heating. The steel extraction temperature is 1140℃±30℃. It is then rolled to the required thickness by a hot rolling mill to obtain a black coil. (3) Solution annealing, scale breaking, shot blasting and pickling: The black leather rolls obtained in step (2) are subjected to conventional solution annealing, scale breaking, shot blasting and pickling treatments in sequence to obtain white leather rolls; (4) Cold rolling, solution annealing and pickling: The white paper roll obtained in step (3) is subjected to conventional cold rolling, annealing and pickling to obtain the cold-rolled finished roll.

2. The method for suppressing titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel according to claim 1, characterized in that: The titanium-ferritic stainless steel ingredients in step (1) are as follows (wt%): C 0.005~0.020%, Mn 0.05~0.25%, Si 0.2~0.5%, Cr 11.0~11.7%, N 0.005~0.015%, Ti 0.12~0.2%, with the remainder being iron and unavoidable impurities.

3. The method for suppressing titanium streaks on the cold-rolled surface of titanium-containing ferritic stainless steel according to claim 1, characterized in that: In step (1), the cooling water volume of the wide and narrow sides of the water-cooled crystallizer in the casting process is controlled at 2800-3000 L / min and 390-420 L / min, respectively; the cooling water volume of the secondary cooling process is controlled at 3100-3500 L / min.

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