JPH0366368B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) 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 mainly composed of Ni and Cr-Ni-Mo systems undergo solid solution treatment, which involves reheating and holding the steel from room temperature to a temperature of 1000℃ or higher. Therefore, the grain boundary corrosion resistance has been restored by recrystallizing the hot-worked structure, adjusting the grain size, and re-solidifying the carbide. The purpose of solution heat treatment using this method is to recrystallize, adjust particle size, re-dissolve carbides,
Furthermore, we aim to diffuse and eliminate the remaining parts of solidification segregation,
The aim is to make the material and corrosion resistance uniform across the entire length, width, and thickness of the plate. 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. In addition, in Japanese Patent Publication No. 59-46287, hot rolling is carried out at 850 to 1150°C with a cumulative reduction rate of 50% or more and a finishing temperature of 850°C to 1150°C, followed by rolling at 850°C. ~550℃ temperature range V
= C ² × 1000 (V: average cooling rate (°C/sec),
C: A method is disclosed in which the solution treatment is omitted by rapid cooling at a rate higher than the average cooling rate indicated by the carbon content (%) of the target steel. (Problems to be solved by the invention) In particular, the inventors have solved the problem in 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 include unidirectional rolling, cross rolling, etc. It also differs from hot strip rolling in that the number of passes and rolling reduction ratio 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 such thick plates and obtain uniform material properties over the entire length, width, and thickness of the plate, in addition to conventional techniques, we must further improve the composition, hot rolling method, and It was found that improvements were needed to simplify the solution treatment method. In other words, in order to simplify and omit solution heat treatment, which is the final heat treatment for stainless steel plates, it is necessary to uniformize recrystallization and grain size adjustment over the entire thickness of the plate, prevent the generation of mixed grains, and make the solution of carbides 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℃.
After heating for 20 minutes, the material was hot-rolled from 50mm to 8mm, the rolling end temperature was set at 950℃, and immediately water-cooled to allow sufficient recrystallization.The results of an EPMA investigation of Ni microsegregation in steel sheets. be. If the micro-segregation of Ni 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. It is necessary to eliminate these δ ferrites in a heating furnace, which is heated 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:
Cr19~27%, Ni4~7%, Mo1.0~3.5%, Cu:
0 to 2%, Si: 1.0% or less, C: 0.08% or less, N:
It is 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 S304 (Cr18.2%, Ni8.6%, C0.04%,
N0.04%) 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℃.
The recrystallized structure is shown when water cooling is started after a certain period of air cooling time after applying a 50% reduction in one pass from °C, 1000 °C, and 950 °C. As already stated in Special Publication No. 57-38654,
It can be seen that recrystallization progresses by increasing the air cooling time, and becomes almost uniform in 3.2 seconds at 1050°C and 18 seconds at 1000°C. It was thus revealed that how to take the air cooling time is important for uniform recrystallization. Figure 3 shows SUS304CC slab (thickness 130mm) of 1250mm.
Heat to 30°C for 30 minutes and reverse roll to 22mm.
After being air-cooled for a few seconds and recrystallized uniformly, the result was tandem rolling from 1050°C in 5 passes with a cumulative reduction of 86% with little time between passes, finish rolling at 988°C, and immediately quenching. , shows the recrystallized structure at the surface of the plate and at the center of the plate. If the time between passes is not taken in this way, there will be 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, and the structure was water-cooled immediately after finishing at 922℃.
Uniform recrystallized grains were obtained in the center of the 4th thick part. 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. For recrystallization and grain growth, it is necessary to apply a large pressure reduction in a high temperature range and take time between passes. If the time between passes is insufficient, recrystallization becomes non-uniform in the initial stage, causing mixed grains. Furthermore, 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 furthermore, 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 It has been found that rolling with an inter-pass time of 3 seconds to 40 seconds for at least half of the number of passes is a necessary requirement to achieve the above objective. If the interpass time is less than 3 seconds, the effect will be small, and a longer time is preferable, but since the adverse effect of temperature drop occurs, the upper limit is set to 40 seconds. Furthermore, the rolling reduction ratio of each pass is preferably large, and preferably 15% or more. When hot rolling is carried out as described above, the final hot rolling temperature is preferably 900°C or higher for recrystallization and grain size adjustment, and in particular, in order to omit solution heat treatment, it is desirable to It is necessary to finish at 900℃ or higher. However, in the case of thin plate rolling, the hot rolling temperature is often around 800°C. In any case, after hot rolling, cool it down as quickly as possible.
It is necessary to rapidly cool the carbide precipitation region at 700 to 800°C to prevent carbide precipitation and growth. The present inventors investigated the precipitation and growth of carbides during cooling after hot rolling, and the precipitation and growth during reheating of SUS304.
As a result of a detailed study of the growth and extinction processes, we found the following. 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 if the cooling rate is 3°C/sec or higher to 500°C or lower, no carbides will be present. Does not precipitate. Therefore, solution heat treatment is not necessary. If water cooling is started at 800℃ or less after hot rolling,
If the water cooling start temperature is low (for example, 700°C), carbide precipitation is observed, but if the water cooling is rapidly cooled to 500°C, the growth can be suppressed. If water-cooled at a temperature below 650°C, carbides will grow and the effect will be small. If the hot rolled steel sheet is air cooled as usual without water cooling, carbides precipitate and grow during cooling, and are continuously deposited at the grain boundaries. We investigated the behavior of materials with various degrees of carbide precipitation when the temperature was increased during reheating. The temperature increase speed was 400 to 600°C/min. When the temperature reaches 800°C, new carbides begin to precipitate, and carbides that have already precipitated begin to grow. As shown in Figure 6, the growth is noticeable at 900°C, and when the temperature reaches 950°C, the carbide begins to disappear, and when it reaches 1000°C, it almost disappears. However, in the case where carbides have grown significantly due to air cooling after hot rolling, complete disappearance of carbides cannot be achieved by heating alone, and a holding time is required. Typical carbide behavior is shown in Figure 7. Considering the results in Figures 5 and 6, the growth of carbides is suppressed at least when the steel plate surface temperature is 650°C or higher and water cooling is performed, so solution heat treatment is effective at heating the steel plate to 950°C or higher. This is achieved by increasing the temperature. Therefore, the retention time may be any time necessary to homogenize the tissue, and 5 minutes or less is sufficient. 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. Cooling after hot rolling may be up to 500°C, and an average cooling rate of 3°C/sec or more between 800 and 300°C is sufficient. Of course, these are valid for steel sheets that use component control and hot rolling methods to prevent segregation as described above, and even in this case, adding simple heat treatment after hot rolling and water cooling will further prevent these micro-segregation. Effective for mitigation. 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 above-described simple solution treatment method or the method omitting the solution treatment has the following additional advantages.
In other words, compared to the conventional method of leaving the product in a furnace at 1000°C or higher for 20 minutes or more, according to the present invention, by omitting the solution heat treatment or by shortening it to 5 minutes or less, scale growth during this period is suppressed. Ru. 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. Also, the scale thickness becomes thinner,
Therefore, there is an advantage that 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 steel whose main component is 7Ni-1.0 to 4Mo. In the production of these thick steel plates, it has been confirmed that the criteria for determining the simplification of solution heat treatment are the finishing temperature of rolling and the starting temperature of water cooling, and by knowing these values, it is possible to It has become clear that the temperature and time of solution heat treatment can be determined, and furthermore, that it can be omitted in production. Hereinafter, the present invention will be explained based on examples. (Example of the present invention) Example 1 A normal SUB304 CC slab (140t) was heated to 1250℃ for 30 minutes.
After extraction, reverse rolling was started at 1100°C and rolled to a thickness of 20 mm in 12 passes. The finishing temperature of rolling was 970°C. During this period, the rolling reduction ratio in 7 out of 12 passes was 15% or more. The time between these 7 passes is as short as 8 seconds and as long as 34 seconds.
It was hot in seconds. Surface temperature after 59 seconds after hot rolling,
It was water cooled from 880℃. The steel plate is uniformly recrystallized throughout the thickness section, with no carbides observed, and δ ferrite and
No Ni microsegregation was observed, and the mechanical properties were good as shown below.

【table】

[Table] Example 2 A normal SUS304 CC slab (140t) was heated to 1200℃ for 20 minutes.
After heating for more than a minute and extracting, reverse rolling was started at 1070°C to produce a 40 mm thick plate in 10 passes and a 10 mm thick plate in 12 passes. The respective finishing temperatures are 980℃ and 860℃
It was warm at ℃. During this period, 7 passes and 8 passes were made respectively.
Rolling at a reduction rate of 15% or more, 8 seconds for short rolls,
The long one took 32 seconds between passes to complete rolling. Water cooling starts at 890℃ for 40mm thick plates, and for 10mm thick plates.
It was 730℃. Thereafter, the steel plate was heated in a heat treatment furnace at a heating rate of 500°C/min, and the 40mm thick plate was cooled with water one minute after reaching 1040°C. The 10 mm thick plate was water cooled 1 minute after reaching 1100°C. The test results of these thick plates showed that the recrystallized grain size was uniform in the cross section of the plate, no carbides were observed, no micro-segregation of Ni was observed, and the mechanical properties were good.

【table】

[Table] (Effects of the Invention) The present invention relates to a method for manufacturing stainless steel plates, and is aimed at simplifying solution treatment, particularly in relation to hot rolling. The present invention shortens the conventional heating method of 20 minutes or more to 1000° C. or higher to 5 minutes or less, which has great advantages not only in terms of energy cost but also in terms of productivity.

[Brief explanation of drawings]

Figure 1 shows the influence of δcal (%) on Ni microsegregation of the product of the present invention, and Figure 2 shows the influence of δcal (%) on the Ni microsegregation of the product of the present invention.
(Cr18.2%, Ni8.6%, C0.04%, N0.04%) CC slab (thickness 130mm) was heated to 1250℃ for 20 minutes, hot rolled to 50mm thickness, and once cooled to room temperature. 1050
℃, 1000℃, and 950℃ after applying a 50% reduction in one pass, allowing air cooling for a certain period of time, and then starting water cooling. Microscopic structure photograph showing the metal structure of the surface and center of the tandem rolled material by the method, No. 4
The figure is a metallurgical microscopic photograph showing the metallographic structure of the surface and center of a reverse-rolled material produced by the method of the present invention. Figure 5 is a diagram showing an example of the water cooling start temperature after hot rolling of SUS304 steel. Figure 5 shows examples of carbide precipitation, growth, and dissolution behavior during temperature rise corresponding to each water cooling start temperature in Figure 5. Figure 7 shows the relationship between cooling start temperature and carbide behavior and recrystallization behavior during temperature rise. It is a metal microscopic structure photograph.

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, a reduction rate of 15% or more is applied to at least half of the total number of rolling passes, rolling is performed with a time between passes of 3 to 40 seconds, and the finishing temperature of rolling is 900°C or higher. After rolling,
Water cooling starts when the steel plate temperature is over 800℃, and
A method for manufacturing a thick stainless steel plate, characterized by rapidly cooling to a desired temperature at an average cooling rate of 3°C/sec or more over a period of 300°C. 2 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, During hot rolling, a reduction rate of 15% or more is applied to at least half of the total number of reduction passes, and rolling is performed with an interpass time of 3 to 40 seconds, and after this hot rolling, the steel plate temperature is 650℃ or higher. Start water cooling from , and increase the average cooling rate between 800 and 300℃ to 3℃/sec.
A method for producing a thick stainless steel plate, characterized by rapidly cooling the above to a desired temperature, then heating to 950°C or higher for a holding time of 5 minutes or less, and then rapidly cooling.