Method for improving water beam printing defects and ferritic stainless steel

Abstract

The present application relates to a method for improving water beam printing defects and a ferritic stainless steel, the method comprising: calculating a relative temperature of furnace temperature according to the mass fraction of chromium, niobium and titanium elements in the slab, and determining the heating temperature of each section of the heating furnace when the slab is in the heating furnace according to the relative temperature; obtaining the sum of the residence time of the slab in the second heating section and the soaking section of the heating furnace; determining a dephosphorization scheme according to the sum of the residence time, and performing dephosphorization treatment on the slab heated by the heating furnace by using the dephosphorization scheme; and determining the deformation rate of each pass of rough rolling according to the sum of the residence time, and performing rough rolling treatment on the slab after dephosphorization treatment according to the determined deformation rate. Through the synergistic innovation of charging, temperature control, scale removal and rough rolling, the present application completely changes the passive situation of the traditional process of "patching up", significantly reduces the occurrence rate of water beam printing and derived defects of 439 super-pure ferritic stainless steel, and greatly improves the surface quality of the rolled steel plate.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel hot rolling process technology in the metallurgical industry, specifically relating to a method for improving watermark defects and ferritic stainless steel. Background Technology

[0002] The equipment layout diagram of the hot strip rolling line is as follows: Figure 1 As shown, the specific production process of hot continuous rolling is as follows: The slab is first heated in the heating furnace 1 at the process temperature. After exiting the furnace, it is descaled by high-pressure water in the high-pressure water descaling box 2 to remove the iron oxide scale on the surface. Then it enters the roughing rolling zone for reversible rolling, which usually requires 5-7 passes. The rolling is carried out by the cooperation of vertical roll mill 3 and horizontal roll mill 4 to control the width and thickness of the strip after rolling. After roughing, the strip is treated by the heat insulation cover 5 and the rotary drum cutting head flying shear 6 and then enters the finishing mill 7 for continuous finishing rolling. The finishing mill 7 includes 7 finishing mill stands. The purpose of continuous finishing rolling is to make the strip reach the target thickness and temperature. After that, it is inspected by the crown gauge 8, width gauge 9, thickness gauge 10 and straightness gauge 11. The qualified steel strip is cooled by the laminar flow cooling device 12. Finally, the cooled steel strip is coiled into a steel coil by the coiler 13.

[0003] 439 series ultra-pure ferritic stainless steel, with a Cr content of approximately 17% and a low C / N ratio, possesses advantages such as excellent corrosion resistance and lower cost than austenitic stainless steel (no need to add high-priced Ni). It is widely used in automotive, construction, and home appliance industries where stringent surface quality requirements exist. However, this type of stainless steel exhibits significant surface quality issues in actual hot rolling production lines, with watermark defects being the core problem hindering quality improvement.

[0004] The water beam mark defect manifests as a dark mark on the strip surface corresponding to the location of the water beam in the heating furnace, such as... Figure 2 The dark marks circled in red are closely related to the way the slab is supported in the heating furnace and its high-temperature deformation characteristics. Specifically, the slab "collapses" (creep deformation) between the support points of the water beams due to its own weight, resulting in indentations in the area in contact with the water beams. The iron oxide scale in this area is difficult to remove and is pressed into the matrix during subsequent rolling, eventually forming irreversible water beam mark surface defects.

[0005] To address the surface defect of water beam imprints, in walking beam furnaces, direct contact between the slab and the heat-resistant pads on the water beams is a fundamental condition for the formation of water beam imprints. The continuous flow of cooling water inside the water beams causes the temperature of the top surface of the pads to be significantly lower than the furnace temperature (typically 150-250°C lower). Simultaneously, the water beam structure creates a shielding effect on the slab, hindering the uniform transfer of radiant heat. This dual effect of "low-temperature contact and heat transfer shielding" results in a distinct low-temperature zone in the contact area between the slab and the pads, known as the "water beam imprint." This area exhibits a faint imprint due to the temperature difference, significantly different from the normally heated area.

[0006] Statistics show that the defect rate of watermarks in 439 series ultra-pure ferritic stainless steel is as high as 10%, and more than 5 slabs are scrapped annually due to severe "waist collapse" that cannot be extracted. The defective coils obtained in the end need to be downgraded or salvaged by cold rolling, but downgrading and cold rolling will affect contract delivery and lead to a significant increase in production costs.

[0007] 439 series ultra-pure ferritic stainless steel (low C and N content) is soft and has low strength at high temperatures. During prolonged heating, the slab will creep and deform under its own weight between the water beam support points (commonly known as "waist collapse"). This results in indentations, or "water beam marks," at the contact points between the slab and the water beams. Iron oxide scale in these indentation areas is difficult to completely remove and will be pressed into the matrix during subsequent rolling, forming a derivative defect—iron oxide scale indentation defect. Both water beam marks and iron oxide scale indentation defects severely affect the surface quality after rolling. Based on production practice, the specific causes can be divided into the following four categories: I. Insufficient adaptability of slab loading methods: The traditional slab loading method is "alternating alignment at both ends," meaning that the previous slab is aligned with one end and the next with the other. This loading method leads to inconsistent overhang lengths of the slabs. Due to insufficient support, the overhanging parts experience increased creep deformation, and temperature dead zones easily form at the junction of the head and tail of the slab. Ultimately, this results in fixed water beam marks, which mostly appear at the head and tail. The non-standard length of traditional slabs makes it impossible to meet the maximum cantilever beam deflection control requirements. This problem further aggravates the deformation of the overhanging parts and exacerbates the water beam mark defect. II. Lack of Composition-Specific Furnace Temperature Control: The Nb / Ti content varies significantly among different grades of 439 series stainless steel (such as 439M, 439, and 430LX). Nb is a strong carbide-nitride forming element and a ferrite-forming element. Solid-solution Nb can significantly improve high-temperature strength. Solid-solution Nb has the greatest impact on the high-temperature yield strength of ferritic stainless steel. In Nb / Ti bistabilized slabs, fine precipitates such as Nb(CN) improve strength by precipitation strengthening and grain refinement through grain pinning. Under the same heating conditions, single-Ti stabilized slabs have significantly lower high-temperature strength than Nb / Ti bistabilized slabs due to the lack of Nb strengthening effect. The current heating process does not dynamically adjust the temperature according to the Nb / Ti content, making single-Ti slabs more prone to water beam marks due to high-temperature waist collapse. Although low-temperature production can improve waist collapse, it will inhibit the formation of continuous oxide film, increasing the risk of roll sticking (i.e., roll mark defects) caused by direct contact between the exposed substrate and the rolls. 3. Insufficient adaptability of descaling process: Due to the high coefficient of friction between 439 series ultrapure ferritic stainless steel and the rolls, the traditional process requires "no descaling throughout the process" to avoid the exposed matrix sticking to the rolls and forming roll mark defects after descaling. However, production data shows that when the rolling line is abnormal (such as equipment failure or scrap steel) and the slab stays in the furnace for too long, the occurrence rate of water beam marks increases significantly. That is, the long-term high temperature environment will aggravate the risk of residual iron oxide scale in the water beam mark area, which is easily pressed into the matrix during subsequent rolling to form iron oxide scale indentation defects. IV. Insufficient adaptability of rough rolling process: Traditional rough rolling adopts a fixed 5-pass rolling process. If the iron oxide scale is thick, the large deformation will press it into the matrix, forming an iron oxide scale indentation defect.

[0008] Regarding the four causes of the aforementioned water beam mark defect, the existing process lacks a systematic approach to controlling the water beam mark. This is manifested in the following ways: the use of an alternating alignment method at both ends leads to uneven slab overhang length, exacerbating the "waist collapse" deformation and water beam mark defect; the heating temperature does not take into account the compositional differences of different grades of 439 steel (single Ti type and Nb / Ti bistable type), making it difficult to balance the contradiction between water beam mark defect and roll mark defect; although the "no descaling throughout the process" operation avoids or reduces roll mark defect, it leads to excessive furnace dwell time, which in turn increases the risk of residual iron oxide scale, resulting in iron oxide scale indentation defect; and the fixed deformation rate of each pass in rough rolling makes it easy to press residual iron oxide scale into the matrix, forming iron oxide scale indentation defect.

[0009] In summary, the limitations of individually optimizing the furnace charging method, furnace temperature control, descaling process, and rough rolling process have led to a passive situation where "solving one defect causes another." Specifically, solving the water beam mark defect increases the roll track defect, and controlling the roll track defect exacerbates the water beam mark defect. Therefore, developing a process method that can coordinately control the water beam mark defect throughout the entire process from furnace charging, heating, descaling to rough rolling is of great significance for improving product quality and reducing production costs. Summary of the Invention

[0010] In order to solve all or some of the above problems, the present invention aims to provide a method for improving watermark defects and ferritic stainless steel. According to one aspect of the present invention, a method for improving watermark defects is provided, comprising: The relative temperature of the furnace is calculated based on the mass fractions of chromium, niobium and titanium in the slab, and the heating temperature of each section of the furnace is determined based on the relative temperature of the slab in the furnace. The sum of the dwell time of the slab in the second heating section and the soaking section of the heating furnace is obtained; Based on the sum of the furnace dwell times, a dephosphorization scheme is determined, and the dephosphorization scheme is used to dephosphorize the slabs heated in the furnace; and Based on the sum of the furnace dwell times, the deformation rate of each rough rolling pass is determined, and the descaling slab is rough rolled according to the determined deformation rate.

[0011] Furthermore, before calculating the relative temperature of the furnace based on the mass fractions of chromium, niobium, and titanium in the slab, and determining the heating temperature of each section of the furnace when the slab is in the furnace based on the relative temperature, the method further includes: loading the slab into the furnace using a center-aligned steel loading method where the center line of the slab coincides with the center line of the furnace chamber.

[0012] Furthermore, before calculating the relative temperature of the furnace based on the mass fractions of chromium, niobium, and titanium in the slab, and determining the heating temperature of each section of the furnace when the slab is in the furnace based on the relative temperature, the method further includes: controlling the length of the slab to be 9-9.8 meters, and loading the slab into the furnace.

[0013] Furthermore, the step of calculating the relative furnace temperature based on the mass fractions of chromium, niobium, and titanium in the slab, and determining the heating temperature of each section of the heating furnace based on the relative temperature of the slab inside the furnace, further includes: The relative upper limit of furnace temperature is calculated based on the mass fractions of chromium, niobium and titanium in the slab. The lower relative temperature limit of the furnace is calculated based on the upper relative temperature limit; and The heating temperature of each section of the heating furnace is determined based on the upper limit of the relative temperature and the lower limit of the relative temperature when the slab is in the heating furnace.

[0014] Furthermore, the calculation of the relative upper limit of furnace temperature based on the mass fractions of chromium, niobium, and titanium in the slab is specifically as follows: the value of the relative upper limit of furnace temperature is equal to the sum of 700, 100 times the mass fraction of chromium multiplied by a first coefficient, 100 times the mass fraction of niobium multiplied by a second coefficient, and 100 times the mass fraction of titanium multiplied by a third coefficient, wherein the unit of the relative upper limit of temperature is degrees Celsius; the first coefficient is equal to 30, the second coefficient is equal to 70, and the third coefficient is equal to 10.

[0015] Furthermore, the calculation of the lower relative temperature of the furnace temperature based on the upper relative temperature specifically means that the value of the lower relative temperature of the furnace temperature is equal to the product of the upper relative temperature of the furnace temperature and 0.97, and the unit of the lower relative temperature is degrees Celsius. Furthermore, when determining the slab's position in the heating furnace based on the upper and lower relative temperatures, the heating temperatures of each section of the heating furnace are as follows: When the slab is in the heating furnace, the heating temperature range of the first heating section is from the lower relative temperature minus 70 degrees Celsius to the lower relative temperature minus 50 degrees Celsius; the heating temperature range of the second heating section is from the upper relative temperature minus 40 degrees Celsius to the upper relative temperature minus 20 degrees Celsius; and the heating temperature range of the soaking section is from the upper relative temperature minus 30 degrees Celsius to the upper relative temperature minus 10 degrees Celsius.

[0016] Furthermore, based on the sum of the furnace dwell times, a dephosphorization scheme is determined, and the dephosphorization treatment of the slab heated in the furnace using the dephosphorization scheme further includes: If the sum of the dwell times of the slab in the second heating section and the soaking section of the heating furnace is greater than 200 minutes, the dephosphorization scheme is determined to be two post-furnace dephosphorization processes and one rough rolling dephosphorization process; if the sum of the dwell times is greater than 150 minutes and less than or equal to 200 minutes, the dephosphorization scheme is determined to be one post-furnace dephosphorization process and one rough rolling dephosphorization process; if the sum of the dwell times is greater than 130 minutes and less than or equal to 150 minutes, the dephosphorization scheme is determined to be one rough rolling dephosphorization process; if the sum of the dwell times is less than or equal to 130 minutes, a zero dephosphorization scheme is adopted. The phosphorus removal scheme is used to remove phosphorus from slabs heated in a heating furnace.

[0017] Furthermore, the step of determining the deformation rate of each roughing pass based on the sum of the furnace dwell times, and performing roughing treatment on the descaling slab according to the determined deformation rate, further includes: If the sum of the furnace dwell times is greater than 200 minutes, the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 15%; if the sum of the furnace dwell times is greater than 150 minutes and less than or equal to 200 minutes, the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 20%; if the sum of the furnace dwell times is greater than 130 minutes and less than or equal to 150 minutes, the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 25%; if the sum of the furnace dwell times is less than or equal to 130 minutes, the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 30%. The descaling slab is rough rolled according to the determined deformation rate.

[0018] This invention also provides a ferritic stainless steel, which is prepared using any of the methods described above for improving watermark defects.

[0019] As can be seen from the above technical solution, the method for improving watermark defects and the ferritic stainless steel provided by the present invention have the following beneficial effects: This invention, through the synergistic innovation of furnace loading, temperature control, descaling, and rough rolling, completely changes the passive situation of the traditional process of "robbing Peter to pay Paul," significantly reduces the occurrence rate of watermarks and derivative defects in 439 ultra-pure ferritic stainless steel, and greatly improves the surface quality of the rolled steel plate. The implementation of this invention can reduce the cost of quality remediation in the cold rolling process, ensure timely delivery of contracts, and improve product market competitiveness, thus having significant economic benefits and technical value. Attached Figure Description

[0020] Figure 1 This is a layout diagram of the hot strip mill line equipment. Figure 2 Photographs of the water beam printing form; Figure 3 This is a flowchart of a method for improving watermark defects according to an embodiment of the present invention. Detailed Implementation

[0021] To fully understand the purpose, features, and effects of this invention, the following detailed embodiments are provided. Except as described below, the process methods of this invention employ conventional methods or apparatus in the art. Unless otherwise specified, the terms and expressions used below have the meanings commonly understood by those skilled in the art.

[0022] When a range of values ​​is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0023] The purpose of this invention is to develop an optimized hot rolling process method for 439 series ultra-pure ferritic stainless steel. This method addresses the root cause of watermark defects in this steel grade caused by its softness and low high-temperature strength during hot rolling by systematically improving key process parameters in slab heating, descaling, and rough rolling. Specifically, it includes the following objectives: I. Elimination of water beam marks and derivative defects: For surface defects such as low-temperature marks and iron oxide scale indentation formed by the contact between the "waist" deformation and water beam of 439 series stainless steel, the water beam mark defect rate is significantly reduced through process optimization, while avoiding derivative problems such as roll marks and iron scale covering that may occur during descaling and rolling. II. Ensure production stability: Reduce the situation where slabs cannot be extracted from the heating furnace due to severe "waist collapse" and are forced to be scrapped, reduce the risk of production interruption, and ensure continuous and stable rolling of 439 series stainless steel on the hot continuous rolling production line. III. Reduce quality losses and costs: By improving the surface quality of products, reduce quality losses such as downgrading due to watermark defects and additional salvage in the cold rolling process, improve contract fulfillment rate, and ultimately improve economic benefits.

[0024] Based on the above objectives, this invention provides a solution that can be directly applied to industrial production for key process aspects such as slab charging specifications for 439 ultra-pure ferritic stainless steel, dynamic heating control based on composition, intelligent descaling linkage based on furnace dwell time, and adjustment of deformation rate in roughing rolling passes.

[0025] Specifically, Figure 3 An embodiment of the present invention illustrates a method for improving watermark defects, such as... Figure 3 As shown, the method of this embodiment of the invention includes the following steps: Step S001: Calculate the relative temperature of the furnace based on the mass fraction of chromium, niobium and titanium in the slab, and determine the heating temperature of each section of the furnace when the slab is in the furnace based on the relative temperature. Step S002: Obtain the sum of the dwell time of the slab in the second heating section and the soaking section of the heating furnace; Step S003: Based on the sum of the furnace dwell times, determine the dephosphorization scheme, and use the dephosphorization scheme to dephosphorize the slabs heated in the furnace; and Step S004: Determine the deformation rate of each pass of rough rolling based on the sum of the furnace dwell time, and perform rough rolling on the descaling slab according to the determined deformation rate.

[0026] The embodiments of this invention determine the heating temperature of each section of the heating furnace based on the mass fraction of chromium, niobium and titanium in the slab, thereby balancing the temperature contradiction between "reducing water beam marks" and "avoiding roll marks" in temperature control; the invention determines the dephosphorization scheme by summing the furnace dwell time, thereby taking into account both "removing iron oxide scale" and "avoiding roll marks"; the embodiments of this invention optimize the deformation rate of rough rolling passes according to the furnace dwell time, thereby avoiding or reducing the situation where iron oxide scale is pressed into the matrix.

[0027] In step S001, before calculating the relative temperature of the furnace based on the mass fractions of chromium, niobium and titanium in the slab, and determining the heating temperature of each section of the furnace when the slab is in the furnace based on the relative temperature, the method of this embodiment of the invention further includes: loading the slab into the furnace by using a center-aligned steel loading method in which the center line of the slab coincides with the center line of the furnace.

[0028] Specifically, in this embodiment of the invention, the traditional "alternating alignment at both ends" steel loading method is changed to a "centering" steel loading method in which the center lines of all slabs coincide with the center line of the furnace, thereby ensuring that the overhang lengths at both ends of the slab are consistent, reducing creep deformation of the overhanging parts, and avoiding excessive deformation caused by insufficient local support.

[0029] Furthermore, before calculating the relative temperature of the furnace based on the mass fractions of chromium, niobium, and titanium in the slab in step S001, and determining the heating temperature of each section of the heating furnace when the slab is in the heating furnace based on the relative temperature, the method of this embodiment of the invention further includes: controlling the length of the slab to be 9-9.8 meters, and loading the slab into the heating furnace.

[0030] The purpose of uniformly controlling the billet length to 9-9.8 meters in this embodiment of the invention is to meet the deflection control requirements and reduce deformation differences caused by non-standard lengths. In specific implementation, for example, the length of the slab can be controlled to be 9 meters, 9.1 meters, 9.2 meters, 9.3 meters, 9.4 meters, 9.5 meters, 9.6 meters, 9.7 meters, or 9.8 meters.

[0031] The present invention addresses the problem of insufficient adaptability of slab loading methods in the prior art, which leads to water beam defects. The present invention reduces creep deformation of overhanging parts by standardizing the loading method and unifying the billet length.

[0032] This invention, by standardizing the slab loading method (using the "center-to-center" steel loading method) and unifying the billet length (9-9.8 meters), ensures that the overhang lengths at both ends of the slab are consistent and meet the maximum cantilever beam deflection control requirements (≤0.3m). This reduces the "waist collapse" phenomenon caused by excessive or non-standard overhangs from the perspective of structural support, fundamentally reduces the probability of water beam imprint formation, and solves the problem of local low temperature zones and indentations caused by overhang differences under the traditional alternating steel loading method.

[0033] Step S001, which calculates the relative furnace temperature based on the mass fractions of chromium, niobium, and titanium in the slab, and determines the heating temperature of each section of the heating furnace based on the relative temperature, further includes: Step S0011: Calculate the relative upper limit of furnace temperature based on the mass fractions of chromium, niobium and titanium in the slab; Step S0012: Calculate the lower relative temperature limit of the furnace based on the upper relative temperature limit; and Step S0013: Determine the heating temperature of each section of the heating furnace when the slab is in the heating furnace based on the upper limit of the relative temperature and the lower limit of the relative temperature.

[0034] Specifically, the determination of the relative upper and lower limits of the furnace temperature takes into account the mass fractions of chromium, niobium and titanium in the slab, thus solving the problem in the existing technology that the heating temperature does not take into account the compositional differences of different grades of 439 steel, making it difficult to balance the contradiction between water beam defects and roll defects. This achieves a balance between water beam defects caused by high-temperature collapse and roll defects caused by low temperature.

[0035] In step S0011, the relative upper limit of furnace temperature is calculated based on the mass fractions of chromium, niobium, and titanium in the slab. Specifically, the value of the relative upper limit of furnace temperature is equal to the sum of the products of 700 and 100 times the mass fraction of chromium with the first coefficient, the products of 100 times the mass fraction of niobium with the second coefficient, and the products of 100 times the mass fraction of titanium with the third coefficient. The unit of the relative upper limit of temperature is degrees Celsius.

[0036] Set T max This indicates the relative upper limit of furnace temperature, with the unit being degrees Celsius (°C). The relative upper limit of furnace temperature is represented by the following formula: [Cr%], [Nb%], and [Ti%] represent the mass fractions of chromium, niobium, and titanium, respectively; K1, K2, and K3 represent the first, second, and third coefficients, respectively. = 700 + 100K1×[Cr%] + 100K2×[Nb%] + 100K3×[Ti%] In this formula, the first coefficient K1 equals 30, the second coefficient K2 equals 70, and the third coefficient K3 equals 10. Substituting the first coefficient K1, the second coefficient K2, and the third coefficient K3 into the above formula will give the value of the relative upper limit of the furnace temperature, and thus the relative upper limit of the furnace temperature.

[0037] Specifically, step S0012 calculates the lower limit of the relative temperature of the furnace based on the upper limit of the relative temperature as follows: the value of the lower limit of the relative temperature of the furnace is equal to the product of the upper limit of the relative temperature of the furnace and 0.97, and the unit of the lower limit of the relative temperature is degrees Celsius.

[0038] Set T min This indicates the lower relative lower limit of the furnace temperature, expressed in degrees Celsius (°C). The value representing the lower limit of the relative temperature of the furnace can be calculated using the following formula: =0.97×

[0039] Based on this, the relative lower limit of the furnace temperature can be calculated, and thus the relative lower limit of the furnace temperature can be obtained.

[0040] Step S0013 determines the heating temperature of each section of the heating furnace when the slab is in the heating furnace based on the upper limit and lower limit of the relative temperature. Specifically, when the slab is in the heating furnace, the heating temperature range of the first heating section of the heating furnace is from the lower limit of the relative temperature minus 70 degrees Celsius to the lower limit of the relative temperature minus 50 degrees Celsius; the heating temperature range of the second heating section of the heating furnace is from the upper limit of the relative temperature minus 40 degrees Celsius to the upper limit of the relative temperature minus 20 degrees Celsius; and the heating temperature range of the soaking section of the heating furnace is from the upper limit of the relative temperature minus 30 degrees Celsius to the upper limit of the relative temperature minus 10 degrees Celsius.

[0041] Specifically, the heating furnace in this embodiment is divided into a first heating section, a second heating section, and a soaking section. The temperature of each section is determined based on the upper limit of the relative temperature and the lower limit of the relative temperature. In specific implementation, for example, the heating temperature of the first heating section of the heating furnace is T1, T... min -70℃≤T1≤T min -50℃, that is, the range of heating temperature T1 in the heating section of the heating furnace is T min -70℃ to T min -50℃. In practical implementation, for example, T1 equals T... min -70℃, T min -60℃ or T min -50℃.

[0042] The heating temperature of the second heating section of the heating furnace is T2, T max -30℃ -10℃ ≤ T2 ≤ T max -30℃ to +10℃, that is, the heating temperature T2 of the second heating section of the heating furnace is in the range of T. max -40℃ to T max -20℃. In practical implementation, for example, T2 equals T. max -40℃, T max -30℃ or T max -20℃.

[0043] The heating temperature of the soaking zone in the heating furnace is T3, T max -30℃≤T3≤T max -10℃, that is, the heating temperature T3 of the soaking zone of the heating furnace is in the range of T. max -30℃ to T max -10℃. In practical implementation, for example, T3 equals T. max -30℃, T max -20℃ or T max -10℃. Regarding step S001, in this embodiment of the invention, the heating temperature range is dynamically adjusted according to the mass fractions of chromium, niobium and titanium in different grades of 439 stainless steel. This ensures that single Ti steel (low high-temperature strength) reduces deformation through appropriate temperature control, and Nb and Ti dual-stable steel avoids roll marks through precise temperature control, thus achieving high-temperature strength matching and surface quality balance.

[0044] Regarding step S002, which involves obtaining the sum of the dwell times of the slab in the second heating section and the soaking section of the heating furnace, this embodiment of the invention uses a temperature sensor inside the heating furnace and a slab position tracking system to record the time of the slab in the second heating section and the time in the soaking section in real time, thereby obtaining the sum of the dwell times of the slab in the second heating section and the soaking section of the heating furnace.

[0045] Step S003, based on the sum of the furnace dwell times, determines the dephosphorization scheme, and uses the dephosphorization scheme to perform dephosphorization treatment on the slab heated in the heating furnace, further including: Step S0031: If the sum of the dwell times of the slab in the second heating section and the soaking section of the heating furnace is greater than 200 minutes, the dephosphorization scheme is determined to be two post-furnace dephosphorization and one rough rolling dephosphorization; if the sum of the dwell times is greater than 150 minutes and less than or equal to 200 minutes, the dephosphorization scheme is determined to be one post-furnace dephosphorization and one rough rolling dephosphorization; if the sum of the dwell times is greater than 130 minutes and less than or equal to 150 minutes, the dephosphorization scheme is determined to be one rough rolling dephosphorization; if the sum of the dwell times is less than or equal to 130 minutes, a zero dephosphorization scheme is adopted. Step S0032: Dephosphorize the slab heated in the heating furnace using the determined dephosphorization scheme.

[0046] The embodiments of the present invention establish a linkage model between the sum of furnace dwell time and the descaling scheme based on the sum of furnace dwell time, thereby achieving the purpose of descaling only during high-risk periods based on the sum of furnace dwell time, solving the problem of insufficient adaptability of existing descaling processes. Thus, the embodiments of the present invention take into account the contradiction between the tendency of high-temperature collapse to form watermarks and the inhibition of oxide film formation at low temperatures.

[0047] Specifically, the phosphorus removal scheme of this invention is as follows: When the sum of the dwell time of the slab in the second heating section and the soaking section of the heating furnace is greater than 200 min: start the main descaling of the furnace after the furnace and the roughing descaling of the roughing mill after the furnace. The main descaling refers to the descaling of the slab by using the high-pressure water in the high-pressure water descaling box after the heating furnace. The pressure of the high-pressure water is 18 MPa. The roughing descaling refers to the descaling of the slab by using the high-pressure water at the entrance of the roughing mill. The pressure of the high-pressure water is 18 MPa. When the sum of the dwell time of the slab in the second heating section and the soaking section of the heating furnace is >150min and ≤200min, then start the main descaling after furnace and the rough rolling descaling. The pressure of the high-pressure water during the main descaling process is 18MPa, and the pressure of the high-pressure water during the rough rolling descaling process is 18MPa. When the sum of the dwell time of the slab in the second heating section and the soaking section of the heating furnace is >130 min and ≤150 min, then the first rough rolling descaling is started. The pressure of the high-pressure water during the rough rolling descaling process is 18 MPa. When the sum of the dwell time of the slab in the second heating section and the soaking section of the heating furnace is ≤ 130 min, a zero descaling scheme is adopted, and the contact between the exposed substrate and the rolls is reduced.

[0048] The embodiments of the present invention determine the descaling scheme based on the sum of the furnace dwell time, thereby achieving precise removal of iron oxide scale in the water beam area when the furnace dwell time is too long and the risk of iron oxide scale residue is high. At the same time, during low-risk periods, such as when the sum of the furnace dwell time is ≤130 min, the zero descaling scheme is strictly maintained, thereby minimizing the problem of roller marks on the exposed substrate caused by conventional descaling.

[0049] Step S004, based on the sum of furnace dwell times, determines the deformation rate for each pass of rough rolling, and further includes rough rolling the descaled slab according to the determined deformation rate: Step S0041: If the sum of the furnace dwell times is greater than 200 minutes, then the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 15%; if the sum of the furnace dwell times is greater than 150 minutes and less than or equal to 200 minutes, then the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 20%; if the sum of the furnace dwell times is greater than 130 minutes and less than or equal to 150 minutes, then the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 25%; if the sum of the furnace dwell times is less than or equal to 130 minutes, then the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 30%. Step S0042: Perform rough rolling on the descaling slab according to the determined deformation rate.

[0050] The embodiments of the present invention also achieve dynamic adjustment of the deformation rate of roughing passes based on the sum of the aforementioned furnace dwell times, thereby solving the problem of insufficient adaptability of the existing roughing process.

[0051] Specifically, the roughing scheme of this invention is as follows: When the sum of the furnace dwell time of the slab is ≤130min, the deformation rate of the first two passes of rough rolling is determined to be ≤30%, for example, 28%, 25%, 22% or 15%, and the subsequent passes are rolled normally with the traditional deformation rate of 40%. When the sum of the furnace dwell time of the slab is greater than 200 min, the deformation rate of the first two passes of rough rolling is determined to be ≤15%, for example, 12%, 10% or 8%, and the subsequent passes are rolled normally with the traditional deformation rate of 40%. When the sum of the furnace dwell time of the slab is greater than 150 min and less than or equal to 200 min, the deformation rate of the first two passes of rough rolling is determined to be less than or equal to 20%, for example, 18%, 15% or 12%, and subsequent passes are rolled normally with a deformation rate of 40%. When the sum of the furnace dwell time of the slab is greater than 130 min and less than or equal to 150 min, the deformation rate of the first two rough rolling passes is determined to be less than or equal to 25%, for example, 22%, 20% or 18%, and subsequent passes are rolled normally with a deformation rate of 40%.

[0052] This invention achieves a balance between reducing iron oxide scale indentation defects and improving production efficiency by reducing the deformation rate of the first two passes, breaking up residual iron oxide scale, and preventing iron oxide scale from being pressed into the matrix, while maintaining a normal deformation rate in subsequent passes to ensure rolling efficiency.

[0053] The above-described method of this invention significantly reduces water beam marks caused by contact between 439 series ultrapure ferritic stainless steel and water beams; by precisely controlling the heating temperature in the heating furnace, it suppresses waist collapse and prevents low-temperature sticking to the rolls; by intelligently applying descaling, it removes residual iron oxide scale without inducing roll sticking; and by optimizing rough rolling deformation, it avoids iron oxide scale being pressed into the matrix.

[0054] The embodiments of the present invention have achieved a systematic improvement in the surface quality of 439 series ultrapure ferritic stainless steel, ensuring timely delivery of contracts, reducing quality losses caused by cold rolling salvage, and achieving significant economic benefits.

[0055] Specifically, this invention, through the construction of a synergistic optimization model of composition, process, and quality, achieves dynamic adjustment of key process parameters throughout the entire production process of 439 series ultra-pure ferritic stainless steel. This fundamentally solves the process contradiction of 439 series ultra-pure ferritic stainless steel being prone to watermarks due to its low strength at high temperatures, while simultaneously being susceptible to low-temperature roller adhesion due to its steel properties. Specifically, this invention has the following beneficial effects: I. Reduced waist collapse deformation from the source, lowering the basis for water beam mark defects: By standardizing the slab loading method (using the center-to-center steel loading method) and unifying the billet length (9-9.8 meters), it was ensured that the overhang lengths at both ends of the slab were consistent and met the maximum cantilever beam deflection control requirements (≤0.3m). This reduced the "waist collapse" phenomenon caused by excessive or non-standard overhang from the perspective of structural support, fundamentally reducing the probability of water beam mark formation and solving the problem of local low temperature zones and indentations caused by overhang differences under the traditional alternating steel loading method.

[0056] Second, the use of precise temperature control with appropriate composition balances the contradiction between water beam printing and roller marks: an innovative dynamic furnace temperature control based on the mass fraction of chromium, niobium and titanium elements has been established, realizing precise and personalized setting of heating temperature. Moreover, the optimal temperature window is dynamically calculated according to the actual alloy content of the slab, which effectively improves the high-temperature strength of 439 ultra-pure ferritic stainless steel slab to suppress waist collapse, while ensuring sufficient temperature to form a protective oxide film, avoiding the defect of low-temperature roller sticking, and breaking through the process bottleneck that traditional low-temperature printing reduction inevitably increases roller marks.

[0057] III. Intelligent Descaling for Precise Removal of Iron Oxide Scale Residue: An intelligent descaling scheme based on high-temperature furnace dwell time has been established. This scheme only initiates descaling (roughing descaling and / or main descaling) in a gradient during high-risk periods when the sum of furnace dwell time exceeds a specific time, thereby specifically and thoroughly removing the stubborn iron oxide scale remaining in the water beam imprint area. During low-risk periods when the furnace dwell time is ≤130 min, zero descaling is strictly maintained, thereby minimizing the problem of exposed substrate sticking to the rolls and causing roll trace defects caused by conventional descaling. Thus, this application not only thoroughly removes the stubborn iron oxide scale generated in the water beam imprint area due to long-term furnace dwell, but also avoids the roll adhesion problem caused by conventional descaling, achieving the dual goals of "iron oxide scale removal" and "roll surface protection".

[0058] IV. Dynamically Adjusting the Rough Rolling Deformation Rate to Prevent Iron Oxide Scale from Being Pressed into the Matrix: An innovative strategy for dynamically adjusting the deformation rate of the first rough rolling pass based on the sum of the furnace dwell time was implemented. For different iron oxide scale residual risk levels, corresponding to different furnace dwell time ranges, the deformation rate of the first two passes was significantly reduced. This small deformation rate initial rolling can effectively break up the residual iron oxide scale in the water beam mark area without pressing it into the matrix. Subsequent passes resume normal rolling to ensure efficiency, significantly reducing the occurrence rate of "iron scale covering" defects. This measure further blocks the transmission of water beam mark defects to the finished product from the rolling stage.

[0059] V. Systematically improve product quality and production efficiency: Through collaborative innovation in furnace loading, temperature control, descaling, and rough rolling, the passive situation of "robbing Peter to pay Paul" in traditional processes has been completely changed, significantly reducing the occurrence rate of watermarks and derivative defects in 439 ultra-pure ferritic stainless steel, and greatly improving the surface quality of rolled steel plates.

[0060] The implementation of this invention can reduce the cost of quality remediation in the cold rolling process, ensure timely contract delivery, and improve product market competitiveness, thus having significant economic benefits and technical value.

[0061] The following describes in detail the implementation of the method for improving watermark defects according to an embodiment of the present invention, taking into account specific production processes and parameters.

[0062] Example 1: The steel used in this embodiment is 439M stainless steel of the single titanium type, with a composition mass fraction of Cr=17.5%, Nb=0%, and Ti=0.45%. The steel is soft and has low strength at high temperatures. The sum of the furnace dwell time of the slab in the second heating section and the soaking section of the heating furnace is 120 minutes (normal operating conditions).

[0063] Slab loading method: The slab centerline is aligned with the furnace centerline, and a laser positioning system is used to ensure that the slab centerline and the furnace centerline are aligned.

[0064] Billet length: 9.5m long slabs are selected to ensure that the cantilever at both ends does not exceed the maximum control requirements.

[0065] Heating temperature control implementation: Temperature calculation: Substituting the values ​​of the upper and lower relative temperature limits into the calculation formula, we get: = 700 + 100×30×17.5%+0 + 100×10×0.45%= 1229.5≈1230, That is, the upper limit of relative temperature is 1230℃, and the lower limit of relative temperature is 1192.6 degrees Celsius; The heating process is determined as follows: the temperature of the first heating section is 1140℃, the temperature of the second heating section is 1200℃-10℃ to 1200℃+10℃, and the temperature of the soaking section is 1210℃.

[0066] Descaling scheme implementation: If the sum of the furnace dwell times is less than 130 minutes (120 minutes), a zero descaling scheme is implemented, and the main descaling and roughing mill inlet descaling valves are closed to reduce the contact between the rolls and the exposed substrate.

[0067] Implementation of roughing rolling pass deformation rate: The roughing rolling process consists of 5 passes with a target exit thickness of 35mm. In the first pass, the slab is rolled from 200mm thick to 145mm thick with a deformation rate of 27.5%. In the second pass, the slab is rolled from 145mm thick to 102mm thick with a deformation rate of 29.7%. Passes 3-5 are rolled with conventional deformation rates (35-40%), resulting in a final thickness of 35mm.

[0068] Example 2: The steel used in this embodiment is 439M stainless steel of the single titanium type, with a composition mass fraction of Cr=17.5%, Nb=0%, and Ti=0.45%. The steel is soft and has low strength at high temperatures. Due to a brief failure of the rolling line, the sum of the time the slab spends in the second heating section and the soaking section of the heating furnace is extended to 130 minutes (critical high-risk value).

[0069] Slab loading method: Same as in Example 1.

[0070] Blank length: Same as in Example 1.

[0071] Heating temperature control implementation: Same as in Example 1.

[0072] Descaling scheme implementation: Start the first roughing mill descaling, start the first descaling at the entrance of the roughing mill, and use high-pressure water at 18MPa to remove the initial iron oxide scale in the water beam area.

[0073] Deformation rate implementation for roughing passes: In the first pass, the slab is rolled from 200mm thick to 150mm thick with a deformation rate of 25%; in the second pass, the slab is rolled from 150mm thick to 114mm thick with a deformation rate of 24%≤25%; in the third to fifth passes, the slab is rolled with a conventional deformation rate (35-40%), and the final thickness is 35mm.

[0074] Example 3: The steel used in this embodiment is 439M stainless steel of the single titanium type, with a composition mass fraction of Cr=17.5%, Nb=0%, and Ti=0.45%. The steel is soft and has low strength at high temperatures. Due to a brief failure of the rolling line, the sum of the time the slab spends in the second heating section and the soaking section of the heating furnace is extended to 200 minutes (high-risk condition), and the iron oxide scale in the water beam imprint area thickens.

[0075] Slab loading method: Same as in Example 1.

[0076] Blank length: Same as in Example 1.

[0077] Heating temperature control implementation: Same as in Example 1, the actual temperature inside the heating furnace is controlled at 1220-1240 degrees Celsius.

[0078] Descaling scheme implementation: The sum of furnace dwell time = 200 min, start 2 main descaling and 1 roughing mill descaling, the main descaling machine adopts 2 passes of high-pressure water descaling, the high-pressure water pressure is 18MPa, the roughing mill inlet adopts 1 pass of descaling, the high-pressure water pressure is 18MPa, in order to remove residual iron oxide scale.

[0079] The deformation rate for roughing passes is as follows: in the first pass, the slab is rolled from 200mm thick to 170mm thick with a deformation rate of 15%; in the second pass, the slab is rolled from 170mm thick to 144.5mm thick with a deformation rate of 15%; in the third to fifth passes, the slab is rolled with a conventional deformation rate (35-40%), resulting in a final thickness of 35mm.

[0080] Example 4: The steel used in this embodiment is Nb and Ti bistabilized 439 stainless steel, with a composition mass fraction of Cr=17.5%, Nb=0.35%, and Ti=0.15%. The sum of the furnace dwell time of the slab in the second heating section and the soaking section of the heating furnace is 110 min.

[0081] Slab loading method: Same as in Example 1.

[0082] Blank length: Same as in Example 1.

[0083] Heating temperature control implementation: Temperature calculation: Substituting the values ​​of the upper and lower relative temperature limits into the calculation formula, we get: = 700 + 100×30×17.5% + 100×70×0.35% + 100×10×0.15% = 1250, ≈1213, which means the upper limit of relative temperature is 1250℃ and the lower limit of relative temperature is 1213℃; The heating process is determined as follows: the temperature of the first heating section is 1160℃, the temperature of the second heating section is 1220℃-10℃ to 1220℃+10℃, and the temperature of the soaking section is 1230℃.

[0084] Descaling scheme implementation: If the sum of the furnace dwell times is less than 130 minutes (120 minutes), a zero descaling scheme is implemented, and the main descaling and roughing mill inlet descaling valves are closed to reduce the contact between the rolls and the exposed substrate.

[0085] The deformation rate of the roughing passes is implemented as follows: in the first pass, the slab is rolled from 200mm thick to 144mm thick with a deformation rate of 28%; in the second pass, the slab is rolled from 144mm thick to 102mm thick with a deformation rate of 29%; in the third to fifth passes, the slab is rolled with a conventional deformation rate (35-40%), and the final thickness is 35mm.

[0086] Example 5: The steel used in this embodiment is Nb and Ti bistabilized 439 stainless steel, with a composition mass fraction of Cr=17.5%, Nb=0.35%, and Ti=0.15%. The sum of the furnace dwell time of the slab in the second heating section and the soaking section of the heating furnace is 130 min.

[0087] Slab loading method: Same as in Example 4.

[0088] Blank length: Same as in Example 4.

[0089] Heating temperature control implementation: Same as in Example 4.

[0090] Descaling scheme implementation: Start the first roughing mill descaling, start the first descaling at the entrance of the roughing mill, and use high-pressure water at 18MPa to remove the initial iron oxide scale in the water beam area.

[0091] Implementation of roughing rolling pass deformation rate: The roughing rolling process consists of 5 passes with a target exit thickness of 35mm. In the first pass, the slab is rolled from 200mm thick to 150mm thick with a deformation rate of 25%. In the second pass, the slab is rolled from 150mm thick to 114mm thick with a deformation rate of 24%. Passes 3-5 are rolled with conventional deformation rates (35-40%), resulting in a final thickness of 35mm.

[0092] This invention also provides a ferritic stainless steel, which is prepared using any of the methods described above for improving watermark defects.

[0093] The preferred embodiments have been disclosed above; however, those skilled in the art should understand that these embodiments are merely illustrative of the invention and should not be construed as limiting its scope. It should be noted that any variations and substitutions equivalent to these embodiments should be considered to be covered within the scope of the claims. Therefore, the scope of protection of this invention should be determined by the scope defined in the claims.

Claims

1. A method for improving watermark defects, characterized in that, include The slab is loaded into the heating furnace using a center-aligned steel loading method where the center line of the slab coincides with the center line of the furnace. The heating furnace includes a first heating section, a second heating section, and a soaking section. The slab is a 439 series stainless steel slab. The relative temperature of the furnace is calculated based on the mass fractions of chromium, niobium and titanium in the slab, and the heating temperature of each section of the furnace is determined based on the relative temperature of the slab in the furnace. The sum of the dwell time of the slab in the second heating section and the soaking section of the heating furnace is obtained; Based on the sum of the furnace dwell times, a dephosphorization scheme is determined. This scheme is then used to dephosphorize the slabs heated in the furnace. The dephosphorization process is as follows: if the sum of the furnace dwell times in the second heating section and the soaking section of the furnace is greater than 200 minutes, the dephosphorization scheme is determined to be two post-furnace dephosphorization passes and one rough rolling dephosphorization pass; if the sum of the furnace dwell times is greater than 150 minutes and less than or equal to 200 minutes, the dephosphorization scheme is determined to be one post-furnace dephosphorization pass and one rough rolling dephosphorization pass; if the sum of the furnace dwell times is greater than 130 minutes and less than or equal to 150 minutes, the dephosphorization scheme is determined to be one rough rolling dephosphorization pass; if the sum of the furnace dwell times is less than or equal to 130 minutes, a zero-dephosphorization scheme is adopted. The determined dephosphorization scheme is then used to dephosphorize the slabs heated in the furnace. as well as Based on the sum of the furnace dwell times, the deformation rate of each pass in the rough rolling is determined, and the descaling slab is then rough rolled according to the determined deformation rates. The rough rolling process is as follows: if the sum of the furnace dwell times is greater than 200 minutes, the deformation rate of the first two passes in the rough rolling is determined to be less than or equal to 15%; if the sum of the furnace dwell times is greater than 150 minutes and less than or equal to 200 minutes, the deformation rate of the first two passes in the rough rolling is determined to be less than or equal to 20%; if the sum of the furnace dwell times is greater than 130 minutes and less than or equal to 150 minutes, the deformation rate of the first two passes in the rough rolling is determined to be less than or equal to 25%; if the sum of the furnace dwell times is less than or equal to 130 minutes, the deformation rate of the first two passes in the rough rolling is determined to be less than or equal to 30%. The descaling slab is then rough rolled according to the determined deformation rates.

2. The method according to claim 1, characterized in that, Before calculating the relative temperature of the furnace based on the mass fractions of chromium, niobium and titanium in the slab, and determining the heating temperature of each section of the furnace when the slab is in the furnace based on the relative temperature, the method further includes: controlling the length of the slab to be 9-9.8 meters and loading the slab into the furnace.

3. The method according to claim 1, characterized in that, The step of calculating the relative furnace temperature based on the mass fractions of chromium, niobium, and titanium in the slab, and determining the heating temperature of each section of the heating furnace based on the relative temperature of the slab inside the furnace, further includes: The relative upper limit of furnace temperature is calculated based on the mass fractions of chromium, niobium and titanium in the slab. The lower relative temperature limit of the furnace is calculated based on the upper relative temperature limit; and The heating temperature of each section of the heating furnace is determined based on the upper limit of the relative temperature and the lower limit of the relative temperature when the slab is in the heating furnace.

4. The method according to claim 3, characterized in that, The relative upper limit of furnace temperature calculated based on the mass fractions of chromium, niobium, and titanium in the slab is specifically as follows: the value of the relative upper limit of furnace temperature is equal to the sum of the products of 700 and 100 times the mass fraction of chromium with the first coefficient, the products of 100 times the mass fraction of niobium with the second coefficient, and the products of 100 times the mass fraction of titanium with the third coefficient. The unit of the relative upper limit of temperature is degrees Celsius. The first coefficient is equal to 30, the second coefficient is equal to 70, and the third coefficient is equal to 10.

5. The method according to claim 3, characterized in that, The calculation of the lower relative temperature limit of the furnace temperature based on the upper relative temperature limit specifically means that the value of the lower relative temperature limit of the furnace temperature is equal to the product of the upper relative temperature limit of the furnace temperature and 0.97, and the unit of the lower relative temperature limit is degrees Celsius.

6. The method according to claim 3, characterized in that, When determining the slab's position in the heating furnace based on the upper and lower relative temperatures, the specific heating temperatures of each section of the heating furnace are as follows: When the slab is in the heating furnace, the heating temperature range of the first heating section is from the lower relative temperature minus 70 degrees Celsius to the lower relative temperature minus 50 degrees Celsius; the heating temperature range of the second heating section is from the upper relative temperature minus 40 degrees Celsius to the upper relative temperature minus 20 degrees Celsius; and the heating temperature range of the soaking section is from the upper relative temperature minus 30 degrees Celsius to the upper relative temperature minus 10 degrees Celsius.

7. A ferritic stainless steel, characterized in that, It was prepared using the method for improving watermark defects as described in any one of claims 1-6.

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