Hot rolling control method for improving surface roller mark defects of 443 ultra-pure stainless steel

Abstract

The invention provides a hot rolling process control technology for improving the roller mark defect of 443 ultra-pure stainless steel in a hot rolling process. By innovating the heating temperature and the high-temperature furnace staying time of the 443 ultrapure stainless steel and innovating 443 finish rolling temperature control, the friction coefficient of the 443 and a roller is reduced, the working environment of the roller in the 443 production process is improved, and the roller is prevented from being pulled; the carbon steel and stainless steel mixed rolling mode is innovated for production, the carbon steel makes contact with the roller for roller polishing, and therefore the roller is prevented from being strained. By implementing the technology of the patent and combining the two measures, the roller strain can be effectively avoided, the 443 roller mark defect is obviously improved, and the failure rate is reduced from 15% to 3% or below, so that the overall production efficiency is improved, and good benefits can be obtained.

Description

Technical Field

[0001] The invention relates to the field of metallurgical science and technology steel rolling technology, and in particular to a hot rolling control method for improving roller mark defects on the surface of 443 ultra-pure stainless steel. Background Art

[0002] The present invention is a targeted hot rolling control technology invented to solve the problem that the surface roller marks of the hot plate after pickling are serious when the 443 ultra-pure stainless steel is produced by the 1549 hot rolling production line of Taigang.

[0003] The equipment layout of the hot rolling line is divided into furnace area, rough rolling area, finishing rolling area, laminar cooling area and coiling area.

[0004] The main production process of the hot rolling production line is that the slab is first heated in the heating furnace according to the temperature specified by the process. There are four heating furnaces, each of which is divided into four soaking sections: preheating, first heating, second heating, and soaking. After heating to the target temperature and the residence time in the furnace reaches the requirement, it is taken out of the furnace, first descaled, and then enters the rough rolling mill for rolling. The rough rolling vertical roll controls the width and the flat roll controls the thickness. Reversible rolling is performed in the rough rolling unit, generally 5 to 7 times. After rolling in the rough rolling unit, the strip reaches the preset target thickness, width and temperature. Then it enters the finishing mill for seven-stand flat roll continuous rolling to make the strip reach the preset target thickness and temperature. After that, the strip is controlled by laminar cooling to make the strip reach the target coiling temperature. Finally, the strip is formed into a steel coil by the coiler.

[0005] 443 ultra-pure ferritic stainless steel is generally used to replace 304 in household appliances, elevators and other industries due to its excellent formability and corrosion resistance and relatively low price compared to 304. However, after pickling, this type of steel often has roller mark defects on the surface, which seriously affects the surface quality. It must be pickled again or the grinding process must be added for treatment, resulting in great quality loss.

[0006] After analysis, it was found that the reason why 443 stainless steel had roller mark defects was that the chromium content of 443 stainless steel was as high as 21%, the high temperature strength was low, and the steel was relatively soft. During the hot rolling production process, the roller sticking problem often occurred, which damaged the roller and caused roller mark defects in the subsequent rolling process. Since the hot rolling surface detector could not detect such defects, it was analyzed that they were caused by the front-end rack.

[0007] Tracking the rolling mill roll surface, it was found that the F1-4 roll surface had traces of tearing, while the F5-7 roll surface was normal, which further confirmed the correctness of the above analysis.

[0008] The non-conformity rate of such defects is as high as 15%, which not only affects the surface appearance quality of the material, but also causes great complaints from users, who do not accept such defects. After such defects occur, the cold rolling must be repaired and saved by a grinding unit. Severe rolls may require two to three repeated grindings, resulting in great quality losses.

[0009] Patent CN101556267B starts from the perspective of ensuring the original oxide layer and avoiding matrix oxidation. It does not remove scale throughout the process, performs seven-pass rolling, and controls the deformation rate of the first four passes of finishing rolling. However, it does not take into account the uneven load distribution of thin gauges below 3.0 mm caused by the unit capacity limitation and the large temperature drop at the edge caused by the seven-pass rough rolling. It is not suitable for improving the roller mark defects of 443 ultra-pure stainless steel on the 1549 production line.

[0010] Therefore, it is particularly important to study a method to improve the 443 roller track based on the existing equipment and technology.

[0011] The purpose of the present invention is to provide a hot rolling process for improving the roller trace defects of 443 ultra-pure stainless steel, thereby significantly reducing the roller trace defects of 443 stainless steel, reducing the rescue cost of the cold rolling process, and achieving good economic benefits. Summary of the invention

[0012] The purpose of the present invention is to provide a hot rolling control method for improving the roller mark defects on the surface of 443 ultra-pure stainless steel in view of the above problems.

[0013] The object of the present invention is achieved as follows: A hot rolling control method for improving the roller mark defects on the surface of 443 ultra-pure stainless steel, comprising the following aspects: (1) Heating process: The heating process is produced according to a high temperature process of 1120±20℃ in the first heating section, 1240±20℃ in the second heating section and the soaking section, and the residence time of the high temperature section of the second heating section and the soaking section is 120-200 minutes; (2) Matching the threading speed and the final rolling temperature according to the thickness specification: 1) The final rolling temperature of 443 ultra-pure stainless steel with a thickness of more than 3.5mm is controlled at 880±20℃, and the threading speed is controlled at 880±20℃. The belt speed is 4.5-6m / s; 2) The final rolling temperature of thin-gauge 443 ultra-pure stainless steel ≤3.5mm is controlled at 920±20℃, and the belt threading speed is 6.5-8m / s; (3) Carbon steel and 443 stainless steel are arranged in a mixed rolling mode: Carbon steel and stainless steel are organized in a mixed rolling mode: 4 heating furnaces are produced in the following manner: Thick-gauge 443 ultra-pure stainless steel >3.5mm is produced in a carbon steel: stainless steel = 1:3 mode, while thin-gauge 443 ultra-pure stainless steel ≤3.5mm is produced in a carbon steel: stainless steel = 1:1 mode.

[0014] The production mode of carbon steel: stainless steel = 1:3 is that three heating furnaces are loaded with stainless steel and the other heating furnace is loaded with carbon steel; the steel is continuously drawn in the manner of drawing 3 pieces of stainless steel and then drawing 1 piece of carbon steel. The production mode of carbon steel: stainless steel = 1:1 is that two heating furnaces are loaded with stainless steel and two heating furnaces are loaded with carbon steel; the steel is continuously drawn in the manner of drawing 1 piece of stainless steel and then drawing 1 piece of carbon steel.

[0015] The beneficial effects of the present invention are: by adjusting the heating temperature of 443 ultra-pure stainless steel, limiting the high-temperature furnace time, adjusting the finishing rolling speed and final rolling temperature according to thickness and other process parameters, the friction coefficient between 443 and the roller is reduced, the working environment of the roller in the 443 production process is improved, and the roller is prevented from being damaged; in addition, the mixed rolling mode of carbon steel and stainless steel is innovatively proposed, so that the contact between carbon steel and the roller is used to grind the roller, thereby preventing the roller from being damaged. By implementing the above patent technology, the two measures are combined to effectively avoid roller damage, the 443 roller trace defect is significantly improved, and the non-conformity rate is reduced from 15% to less than 3%, improving product quality, thereby improving overall production efficiency, and achieving good benefits. DETAILED DESCRIPTION

[0016] The main innovations of the present invention include: 1) Innovative heating process, proposed effective furnace dwelling time in high temperature section, changing the previous concept of only guaranteeing total furnace dwelling time; 2) Innovative 443 final rolling temperature control, achieving matching of final rolling temperature and threading speed according to thickness specifications. 3) Innovative 443 production mode. Thick specifications 443 with a thickness of >3.5mm are produced according to carbon steel: stainless steel = 1:3, while thin specifications 443 with a thickness of ≤3.5mm are produced according to carbon steel: stainless steel = 1:1.

[0017] Compared with the existing technology, this technical improvement has significantly improved the surface roller mark defects of 443 stainless steel. After the implementation of this patented technology, the non-conformity rate of 443 roller mark defects has been reduced from 15% to less than 3%, the cold rolling process contract fulfillment rate has been greatly improved, and the cost of cold rolling rework and repair has been reduced, with significant economic benefits.

[0018] Roller strain is the main cause of roller marks on the surface of 443 stainless steel, and the unstable friction coefficient between 443 strip and roller is the main cause of roller strain. Furthermore, how to improve the friction relationship between 443 stainless steel and roller and avoid roller marks caused by roller strain during contact rolling friction is an effective way to solve 443 roller marks.

[0019] Data show that the friction coefficient is negatively correlated with the strip temperature and rolling speed, that is, the higher the temperature and the faster the speed, the smaller the friction coefficient.

[0020] u=1.05-0.0005t-0.056v, where t: strip temperature, v: rolling speed.

[0021] This plan conducts relevant research around improving the friction coefficient between 443 stainless steel and the roll and improving the roll condition. By optimizing and strictly controlling the process parameters that affect the friction coefficient, such as heating temperature, high-temperature furnace time, and finishing speed, the friction coefficient between the strip and the roll is reduced to avoid roll strain. In addition, an innovative carbon steel and stainless steel mixed rolling production mode is proposed, so that the contact between carbon steel and the roll is used to grind the roll to prevent roll strain.

[0022] The technical solution of the present invention mainly includes the following aspects: 1. Innovative heating process: The strip temperature is most affected by the strip temperature. On-site tracking found that the roll mismatch rate is very high for coils with an average heating temperature lower than 1240°C. However, due to the softness of 443 steel, excessively high temperature can easily cause the furnace to collapse and cause production accidents. Comprehensive tests show that the furnace temperature is best controlled at 1240±20°C.

[0023] The previous process required a residence time of 8-12min / 10mm, which was mainly calculated based on the combustion calorific value of the heating furnace and the thermal conductivity coefficient of each slab. Statistics show that the roller trace of 443 stainless steel has no obvious correlation with the total residence time, but is negatively correlated with the residence time of the high-temperature section (two heating + soaking section), that is, the shorter the residence time of the high-temperature section, the higher the roller trace mismatch rate. Using the Six Sigma tool to trace the data, it was found that the watershed was 120 minutes. For this reason, the heating process of 443 was re-planned as follows: the heating process is produced according to the high-temperature process of 1120±20℃ for the first heating, and the second heating and soaking are produced according to the high-temperature process of 1240±20℃. At the same time, the residence time of the high-temperature section of the second heating and soaking is guaranteed to be 120-200 minutes to ensure high temperature uniformity.

[0024] 2. Match the threading speed and final rolling temperature according to thickness specifications: The finishing rolling speed actually depends on the final rolling temperature and the deformation rate of each pass. Data show that the higher the final rolling temperature, the faster the threading speed; the greater the pass load deformation rate, the faster the rolling speed.

[0025] In the past, when producing 443, the final rolling temperature was required to be controlled at a low temperature of no more than 860℃. This may not have much impact on thick-gauge 443, but for thin-gauge 443, low-temperature control will inevitably lead to low-speed rolling, thereby aggravating friction. At the same time, the deformation rate of each frame of thin-gauge 443 is relatively larger than that of thick-gauge. Low-speed rolling under large deformation rate further deteriorates the rolling environment, causing the roller track mismatch rate of thin-gauge 443 to be much higher than that of thick-gauge 443.

[0026] In response to the above problems, this plan is based on the results of field practice, combined with the improvement of roller tracks and the performance of the actual finished product, to control the threading speed and final rolling temperature by specification, as shown below: 1) For thick specifications 443 with a thickness of more than 3.5 mm, the final rolling temperature is controlled at 880±20℃, and the threading speed is 4.5-6m / s. 2) For thin specifications 443 with a thickness of ≤3.5 mm, the final rolling temperature is controlled at 920±20℃, and the threading speed is 6.5-8m / s.

[0027] 3. Innovative production scheduling mode - carbon steel and 443 stainless steel are scheduled in mixed rolling mode: In order to prevent roller strain during the production of 443, the production mode is optimized and adjusted, that is, the production is changed to a mixed rolling mode of carbon steel and stainless steel: thick specifications of 443 >3.5mm are produced according to the ratio of carbon steel: stainless steel = 1:3, and thin specifications of 443 ≤3.5mm are scheduled according to the ratio of carbon steel: stainless steel = 1:1.

[0028] The implementation of the present invention can reduce the friction coefficient between 443 and the roller, thereby effectively improving the working environment of the roller in the production process of 443; at the same time, by innovating the mixed rolling mode of carbon steel and stainless steel, the roller is polished by carbon steel to further ensure the surface of the roller. The above two measures are combined to ensure that there are no roller mark defects on the surface of 443 after rolling.

[0029] Combined with the actual application of roller trace improvement of 443 stainless steel on line 1549 of TISCO hot rolling mill, the improvement effect of this patent on 443 roller trace defects is explained in detail through examples. Example 1

[0030] The process parameters for producing 5.0mm thick 443 ultra-pure stainless steel are as follows: 1. Heating process: first heating at 1120±20℃, second heating and equalizing are produced according to the high-temperature process of 1240±20℃. At the same time, the residence time of the second heating and equalizing high-temperature sections is ≥120 minutes to ensure high-temperature uniformity.

[0031] 2. Final rolling temperature: 880±20℃, finishing rolling speed: 5m / s.

[0032] 3. Carbon steel and 443 stainless steel are scheduled for production in a 1:3 mixed rolling mode. Example 2

[0033] The process parameters for producing 4.0mm thick 443 ultra-pure stainless steel are as follows: 1. Heating process: first heating at 1120±20℃, second heating and equalizing are produced according to the high-temperature process of 1240±20℃. At the same time, the residence time of the second heating and equalizing high-temperature sections is ≥120 minutes to ensure high-temperature uniformity.

[0034] 2. Final rolling temperature: 880±20℃, finishing rolling threading speed 5.5m / s.

[0035] 3. Carbon steel and 443 stainless steel are scheduled for production in a 1:3 mixed rolling mode. Example 3

[0036] The process parameters for producing 3.5mm thick 443 ultra-pure stainless steel are as follows: 1. Heating process: first heating at 1120±20℃, second heating and equalizing are produced according to the high-temperature process of 1240±20℃. At the same time, the residence time of the second heating and equalizing high-temperature sections is ≥120 minutes to ensure high-temperature uniformity.

[0037] 2. Final rolling temperature: 920±20℃, finishing rolling threading speed 6.8m / s.

[0038] 3. Carbon steel and 443 stainless steel are scheduled for production in a 1:1 mixed rolling mode. Example 4

[0039] The process parameters for producing 3.0mm thick 443 ultra-pure stainless steel are as follows: 1. Heating process: first heating at 1120±20℃, second heating and equalizing are produced according to the high-temperature process of 1240±20℃. At the same time, the residence time of the second heating and equalizing high-temperature sections is ≥120 minutes to ensure high-temperature uniformity.

[0040] 2. Final rolling temperature: 920±20℃, finishing rolling threading speed 7.5m / s.

[0041] 3. Carbon steel and 443 stainless steel are scheduled for production in a 1:1 mixed rolling mode.

[0042] The above description is only a specific embodiment of the present invention, but the structural features of the protection scope of the present invention are not limited thereto. Any changes or modifications made by any technician in the field of the present invention are all covered by the patent scope of the present invention.

Claims

1. A hot rolling control method for improving the roller mark defect on the surface of 443 ultra-pure stainless steel, characterized in that: Including the following aspects: (1) Heating process: The heating process is produced according to the high temperature process of 1120±20℃ in the first heating section, 1240±20℃ in the second heating section and the soaking section, and the residence time of the high temperature section of the second heating section and the soaking section is 120-200 minutes; (2) Match the threading speed and final rolling temperature according to the thickness: 1) The final rolling temperature of thick 443 ultra-pure stainless steel with a thickness of >3.5mm is controlled at 880±20℃, and the threading speed is 4.5-6m / s; 2) The final rolling temperature of thin 443 ultra-pure stainless steel with a thickness of ≤3.5mm is controlled at 920±20℃, and the threading speed is 6.5-8m / s; (3) Carbon steel and 443 stainless steel are arranged in a mixed rolling mode: Carbon steel and stainless steel are organized in a mixed rolling mode: The four heating furnaces are produced in the following manner: Thick 443 ultra-pure stainless steel with a thickness of >3.5mm is produced in a carbon steel: stainless steel = 1:3 mode, while thin 443 ultra-pure stainless steel with a thickness of ≤3.5mm is produced in a carbon steel: stainless steel = 1:1 mode.

2. A hot rolling control method for improving roller mark defects on the surface of 443 ultra-pure stainless steel according to claim 1, characterized in that: Carbon steel: stainless steel = 1:3 mode production, that is, three heating furnaces are loaded with stainless steel, and another heating furnace is loaded with carbon steel; steel drawing is continuously produced by drawing 3 pieces of stainless steel and then drawing 1 piece of carbon steel, carbon steel: stainless steel = 1:1 mode production, that is, two heating furnaces are loaded with stainless steel and two heating furnaces are loaded with carbon steel; Steel is produced continuously by drawing one piece of stainless steel and then one piece of carbon steel.