JPH0582453B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing thick steel plates of austenitic stainless steel and duplex stainless steel.
In particular, the present invention relates to a method for manufacturing thick stainless steel plates that can simplify the manufacturing process. (Conventional technology) Conventionally, Cr as represented by 18Cr-8Ni stainless steel
- After hot rolling, austenitic stainless steels and duplex stainless steels, which are mainly Ni-based and Cr-Ni-Mo-based, undergo solid solution treatment, which involves reheating from room temperature to a temperature of 1000°C or higher to maintain the temperature. Thus, it has been manufactured by a method of recrystallizing the hot-worked structure to adjust the grain size and redissolving the carbide into solid solution to restore intergranular corrosion resistance. The purpose of solution heat treatment using this method is to recrystallize and adjust grain size, redissolve carbides, and diffuse and eliminate residual solidification segregation to ensure uniform material quality and corrosion resistance throughout the entire length, width, and thickness of the plate. The aim is to However, in order to achieve this goal, it is necessary to reheat the plate to over 1000°C, uniformly heat the entire plate, and then take a longer holding time.Currently, the total furnace time is 20 to 30 minutes. This has become a major problem in terms of energy and productivity. Therefore, there has been a strong desire to simplify this process. Japanese Patent Publication No. 57-38654 has already disclosed a method for producing hot coils for the same purpose as described above, in which hot strip rolling is performed, followed by air cooling for 3 to 10 seconds, rapid cooling, and coiling at 400 to 600°C. , and in Special Publication No. 59-46287,
After hot rolling at 850 to 1150°C with a cumulative reduction rate of 50% or more and a finishing temperature of 850°C to 1150°C, the temperature range of 850 to 550°C is V = C ² ×
1000 (where V: average cooling rate (° C./sec), C: carbon content (%) of the target steel) A method of omitting the solution treatment is disclosed. Furthermore, in Japanese Patent Application Laid-open No. 52-131959, a method of rolling hot strips after finish rolling is disclosed.
A method of reheating to 1000 to 1150°C, cooling to 500°C, and winding it up is also disclosed. However, since all of the above methods are intended for hot coils, the finish rolling is unidirectional rolling at extremely high speed and short interpass time, and is also premised on winding after rolling. Thus, no sufficient studies have been made in the past on simplifying the solution treatment for rolling thick plates of various sizes by reverse rolling. (Problems to be Solved by the Invention) The present inventors have particularly focused on stainless steel plates.
As a result of examining these conventional methods, we found that due to the characteristics of thick steel plates, there are many types of plate thickness, plate width, and plate length, and depending on these sizes, hot rolling methods may vary, such as unidirectional rolling, cross rolling, etc. It also differs from hot strip rolling in that the number of passes and rolling reduction ratios vary. Therefore, the temperature and rolling time of each plate are different, and the temperature also varies depending on the part of the plate. In order to simplify the solution heat treatment of thick plates and obtain uniform material quality over the entire length, width, and thickness of the thick plates, in addition to conventional techniques,
Furthermore, it was found that improvements were needed in omitting the components, hot rolling method, and solution treatment method. In other words, in order to omit solution heat treatment, which is the final heat treatment for stainless steel sheets, recrystallization and grain size adjustment must be made uniform throughout the sheet thickness to prevent the generation of mixed grains, and the re-solution of carbides must be made uniform. At the same time, it was found that it was necessary to eliminate δ ferrite caused by solidification segregation and to uniformize Ni microsegregation. (Means for Solving the Problems) In order to solve the problems of the present invention,
It is necessary to control the starting steel composition and each manufacturing process from heating to hot rolling. The starting steel components include δcal (%) = 3 (Cr + Mo +
1.5Si)−2.8(Ni+0.5Mn+0.5Cu)−84(C+N)−
It is desirable that δcal (%) determined by 19.8 be -3% or more. Figure 1 shows the 18Cr-8Ni product plate.
This figure shows the influence of δcal (%) on Ni microsegregation, and segregation is reduced when δcal (%) is -3% or more. In other words, for the purpose of omitting solution heat treatment.
Test material of SUS304 with significantly changed composition was heated to 1250℃.
This is the result of an EPMA investigation of Ni microsegregation in steel sheets for materials that were heated for 20 minutes, then hot-rolled from 50mm to 8mm, and immediately water-cooled to a rolling finish temperature of 950°C to allow sufficient recrystallization. be. Ni
If the micro-segregation is large, corrosion patterns are likely to occur and the surface after electrolytic polishing will be seriously damaged. In this way, the influence of ingredients on micro-segregation is large;
Determined by δcal (%), if δcal (%) is less than -3% and austenite is stable in the early stage of solidification, micro-segregation is poor, and if δcal (%) is -3% or more, it passes through δ ferrite in the early stage of solidification. The micro-segregation is reduced even after the solution treatment is omitted.
Thus, it was found that passing through the δ-ferrite phase at the early stage of solidification has a great effect on reducing microsegregation. In a heating furnace, it is necessary to eliminate these δ ferrites by heating to 1100 to 1300°C for 10 minutes or more. Below 1100°C, δ does not disappear even over a long period of time, and it progresses fastest at 1250°C, disappearing after 10 minutes of heating, and when the temperature exceeds 1300°C, δ ferrite increases again. The main components of the austenitic stainless steel in the present invention are usually Cr: 18.0 to 22.0%, Ni:
6.0-15.0%, Mo: 0-4.0%, Si: 1.0% or less,
Cu: 0 to 2.0%, C: 0.08% or less, N: 0.4% or less, and the main components of duplex stainless steel are:
Cr: 19-27%, Ni: 4-7%, Mo: 1.0-3.5%,
Cu: 0 to 2%, Si: 1.0% or less, C: 0.08% or less,
N: 0.4% or less. Unlike hot rolling, which is tandem rolling such as hot strip rolling, in reverse rolling such as thick plate rolling, it is possible to appropriately control the reduction rate for each pass and the time between passes. For recrystallization, rolling with large reductions at as high a temperature as possible is effective, but recrystallization is performed over the entire thickness of the plate to prevent the generation of mixed grains, adjust the grain size, and remove residual solidification segregation. In order to make δ ferrite and Ni segregation uniform, it is necessary to control the hot rolling temperature, reduction rate, and interpass time, and to proceed with rolling while recuperating heat on the steel sheet surface. The detailed results of these studies are described below. Figure 2 shows SUS304 (Cr18.2%, Ni8.6%, C0.04
%, N0.04%) CC slab (thickness 130mm) at 1250℃
Heated for 20 minutes, hot rolled to a thickness of 50mm, cooled to room temperature, heated again to 1200℃, cooled,
The recrystallized structure is shown when water cooling is started after a 50% reduction is applied in one pass from 1050°C, 1000°C, and 950°C, and after a certain period of air cooling time. As already stated in Japanese Patent Publication No. 57-38654, recrystallization progresses by increasing the air cooling time, and becomes almost uniform in 3.2 seconds at 1050℃ and 18 seconds at 1000℃. I understand that. In this way, it was found that how to determine the air cooling time is important for uniform recrystallization. Figure 3 shows SUS304CC slab (130mm) heated to 1250℃.
After heating for 30 minutes, reverse rolling to 22 mm, air cooling for 30 seconds, and uniformly recrystallizing, the material was tandem rolled in 5 passes from 1050°C with almost no time between passes, and a cumulative reduction of 86% was applied. This is the result of finishing rolling at 988°C and then quenching, showing the recrystallized structure at the surface and center of the thickness. As described above, in hot coil rolling which does not take time between passes, there is a difference in the behavior of recrystallization and δ ferrite between the surface area and the thickness center area, resulting in non-uniformity. On the other hand, Figure 4 shows the SUS304CC slab at 1250°C.
After heating for 30 minutes, reverse rolling to 1050
℃ to 7 passes with an inter-pass time of 7 to 15 seconds for a cumulative 86% rolling, and the structure was water-cooled immediately after finishing at 922℃, and the surface part of the plate thickness was 1/4
Uniform recrystallized grains were obtained in the thick part and center. In this way, it was found that the combination of temperature, rolling reduction, and interpass time is important for uniform recrystallization of the plate thickness cross section. That is, in hot rolling and recrystallization of CC slabs, it is necessary to apply a large reduction or cumulative reduction from the initial stage of rolling, and to advance the rolling by taking the time between passes or the cumulative interpass time. If the time between passes is insufficient, recrystallization becomes non-uniform at the beginning,
It causes mixed grains. Also, by taking the time between passes,
The surface of the steel sheet recuperates, a uniform recrystallized structure can be obtained in the cross section of the steel sheet, and further, the disappearance of δ ferrite and the diffusion disappearance of segregation proceed. In plate rolling, which is reverse rolling, it is advantageous to be able to select the rolling reduction ratio of each pass and the time between passes.In order to obtain a uniform recrystallized structure and a rolled structure with less segregation, it is necessary to The time between passes for at least half of the number of passes is 3 seconds or more.
It was found that hot rolling for 40 seconds was a necessary requirement to achieve the above objective. Also, time between passes is necessary for reheating the surface of the steel plate. If it is less than 3 seconds, the effect will be small, and the longer the time, the better, but the adverse effect of temperature drop will occur, so the upper limit is 40
It was up to seconds. Regarding the rolling reduction rate, it is effective to preferably roll at least half of the total number of rolling passes at a rolling reduction rate of 15% or more. When hot rolling is carried out as described above, the final hot rolling temperature must be 900°C or higher for recrystallization and grain size adjustment. It is also necessary to finish the process at 900°C or higher in each part. After hot rolling, cool down as quickly as possible to 700~800
It is necessary to rapidly cool the carbide precipitation region at ℃ to prevent carbide precipitation and growth. According to the research conducted by the present inventors, as shown in Figure 5, after hot rolling at a temperature of 900°C or higher, water cooling is started when the surface temperature of the steel sheet is 800°C or higher, and at a cooling rate of 3°C/sec or higher, water cooling is lower than 500°C. No carbide precipitates when cooled to Therefore, solution heat treatment is not necessary. Note that it is more effective if the water cooling start temperature after hot rolling is high, and the solution treatment can be omitted when the entire plate is cooled from 800°C or higher. Rapid cooling after hot rolling can be done up to 500℃ or less, and the cooling rate is 3℃/sec at the average cooling rate between 800 and 300℃.
The above is sufficient. Of course, as mentioned above, these are valid for steel sheets that use component control and hot rolling methods to prevent segregation, and even in this case, adding simple heat treatment after hot rolling and water cooling can further reduce these micro-segregations. It is effective for The heat treatment time to be added at this time may be short, and 5 minutes at most is sufficient. The effect is saturated for more than 5 minutes. The stainless steel plate manufactured by the method of omitting the solution heat treatment described above has the following additional advantages. In other words, 1100℃ like conventional
Compared to the above method of leaving the reactor in the furnace for 20 minutes or more, by omitting the solution heat treatment according to the present invention, scale growth during this period is suppressed. Therefore, the Cr-free layer on the surface of the steel sheet becomes thinner, which has an advantageous effect on the corrosion resistance of the product. There is also the advantage that the scale thickness is reduced and therefore the descaling time is shortened. The present invention applies not only to austenitic stainless steels of which 18Cr-8Ni is a typical example, but also to 20~25Cr-4~
It can also be applied to duplex stainless steels whose main components are 7Ni-1.0 to 4Mo. Hereinafter, the present invention will be explained based on examples. (Example) Example 1 A CC slab (140t) of SUS304 with the chemical composition shown in Table 1 was heated to 1250℃ for 30 minutes, and after extraction, reverse rolling was started at 1100℃, and the plate thickness was reached to 15mm in 15 passes. Rolled. The time between nine of the 15 passes during this period was 8 seconds at the shortest and 34 seconds at the longest.
After 13 passes and 15 passes (final pass) during hot rolling,
The surface was reheated using a heating device installed on the line to achieve a finishing rolling temperature of 980°C. 59 seconds after the end of hot rolling, the surface temperature was 880°C and then water-cooled. No carbides were precipitated, the steel plate uniformly recrystallized throughout the thickness section, no micro-segregation of δ ferrite or Ni was observed, and the mechanical properties were good as shown in Table 2 below, making it possible to omit solution heat treatment. I can do it.

【table】

[Table] (Effects of the Invention) The present invention relates to a method for manufacturing stainless steel plates, and is aimed at omitting solution treatment, particularly in relation to hot rolling. According to the present invention, the conventional heating method of 20 minutes or more at 1100° C. or higher can be shortened to 5 minutes or less, so it has great advantages not only in terms of energy cost but also in terms of productivity.

[Brief explanation of the drawing]

Figure 1 shows the influence of δcal (%) on Ni microsegregation of products of the present invention, and Figure 2 shows the effect of SUS304.
(Cr: 18.2%, Ni: 8.6%, C: 0.04%, N: 0.04
%) CC slab (thickness 130mm) was heated to 1250℃ for 20 minutes, hot rolled to 50mm thickness, cooled to room temperature, heated again to 1200℃ and then cooled to 1050℃,
After applying 50% pressure reduction in one pass from 1000℃ and 950℃,
Metallic microscopic structure photograph showing the recrystallized structure when water cooling is started after a certain period of air cooling time, No. 3
The figure is a micrograph showing the metallographic structure of the surface and center of a tandem-rolled material obtained by the conventional method. Figure 4 is a microscopic photo showing the metallographic structure of the surface and center of a reverse-rolled material obtained by the method of the present invention. Figure 5 Figure 6 shows an example of water cooling start temperature after hot rolling of SUS304 steel.
The figure is a diagram showing examples of precipitation, growth, and dissolution behavior of carbides during temperature rise corresponding to each water-cooling start temperature in FIG. 5.

Claims (1)

[Claims] 1. In austenitic and duplex stainless steels, δcal (%) = 3 (Cr + Mo + 1.5Si) - 2.8 (Ni +
δcal determined by 0.5Mn+0.5Cu)-84(C+N)-19.8
Continuously cast slabs (hereinafter referred to as CC slabs) or slabs that have undergone blooming rolling are heated to a heating temperature of 1100 to 1300°C for 10 minutes or more, In hot rolling, at least half of the total number of rolling passes are rolled with an interpass time of 3 to 40 seconds, and the finishing temperature of rolling is 900 ° C. or higher,
The temperature is raised on the rolling line during or after rolling, and after hot rolling, water cooling is started when the steel plate temperature is 800℃ or higher, and the average cooling rate between 800 and 300℃ is 500℃ at 3℃/sec or higher. A method for manufacturing thick stainless steel plates characterized by rapid cooling to a temperature below ℃.