JPS63309307A - Method for hot rolling to control crystal grains of austenitic stainless steel - Google Patents

Abstract

PURPOSE:To control a dimension of crystal grains of an austenitic stainless steel to be uniform and their size to be small by specifying a relation between the total draft and a draft condition and a cooling condition, amount in final two passes in hot rolling and a draft temp. CONSTITUTION:A relation of the total amount of drafts A1 and A2 in final two passes and a draft temps. is positioned in the hatched region shown by points of A, B, C, D for hot rolling of an austenitic stainless steel. Rolling is performed under conditions in which a relation between the A1 and A2 meets the condition shown by the inequality I and an interval between A1 draft and A2 draft is set to a time of >=10sec. Then, cooling is started from a temp. of >=T( deg.C) shown by the equation II. In another way, a relation of the total amount of drafts in final two passes and draft temp. is positioned in the hatched region shown by points of E, F, G, H, I. After rolling under the same conditions as the above conditions, solution heat treatment, which heats a stock at a temp. of >=950 deg.C and between a temp. shown by the equation II and 1150 deg.C and water cools the stock, is performed. Thus, a homogeneous thick steel plate having high strength and good ductility is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a hot rolling method for controlling the crystal grains of austenitic stainless steel to be uniform and fine.

[Prior art] Austenitic stainless steel has excellent high-temperature properties,
Due to its workability and weldability, it is used, for example, as a primary structural material for fast breeder reactors. Since fast breeder reactors are large welded structures, thick steel plates are used. As fast breeder reactors become larger, the thickness of the austenitic stainless steel plates used tends to further increase.

Generally, as the plate thickness increases, the crystal grains become coarser. On the other hand, the properties of materials are closely related to the grain size; for example, the strength decreases as the grains become coarser, and the ultrasonic penetration ability decreases as the grains coarsen, so ultrasonic flaw detection is not sufficient. There are also problems such as not being able to do so. As described above, coarsening of the crystal grains as the plate thickness increases significantly reduces the usability and workability of the material, and therefore it is necessary to take measures to make the grains finer. Since phase transformation does not occur in austenitic stainless steel in the industrial hot working temperature range, crystal grain control relies exclusively on recrystallization during solution treatment. Therefore, the conventional grain refinement method was to accumulate strain by hot working at the lowest possible temperature, and then refine the recrystallized grains during the subsequent solution treatment. . Or “I'i, Nb
It has also been attempted to form stable carbides and nitrides by adding such substances to refine crystal grains. However, each of these methods has the following problems. That is, regarding grain refinement due to strain accumulation, it becomes difficult to uniformly introduce rolling strain as the plate thickness increases. In other words, in order to uniformly introduce strain into a thick material, a rolling mill with a very strong rolling force that can apply a large reduction at low temperature is required. The latter method using T1, Nb, etc. has problems such as a lack of stability due to changes in the material properties due to these additive elements or difficulty in controlling the precipitation of carbides and nitrides.

[Problem to be solved by the invention 1 As mentioned above, in the conventional method of refining the crystal grains of austenitic stainless steel, the rolling strain accumulation method requires a huge rolling mill as the plate thickness increases. Or N
The b addition method has problems such as being susceptible to changes in material properties and non-uniformity. The present invention does not require increasing the size of the rolling mill.
In addition, the present invention provides a rolling method for refining the crystal grains of a thick steel sheet without using any additive elements.

[Means for Solving the Problems] The present invention provides the following features: (1) In hot rolling of austenitic stainless steel, the rolling ratio A in the final two baths, A, (in this specification, the final rolling The total amount of the rolling reduction rate (%) of the pass is called A2, and the rolling reduction rate (%) of the pass immediately before the final pass is called A1) and the rolling temperature are in the diagonally shaded areas in Figure 1 (^, B, C).

D), and the eight relationships with A satisfy the following formula (1), further rolling is carried out under conditions that ensure the interval of A This is a grain control rolling method characterized by water cooling from a temperature T (''C) below, 0.6<A/A, <1.4...・
... (1) T = 2000 The total amount of A2 and the rolling temperature are in the shaded area in Figure 2 ([E, F, G, It,
I), and the relationship between A From the temperature 'r(''c) shown in (2) to 1150℃
This is a grain control hot rolling method characterized by performing a solution treatment of heating and water cooling during the process.

Note that the austenitic stainless steel in the present invention is Ni: ~17'%, Cr: 15~22%, MO:
The basic component is 0 to 5%. Furthermore, water cooling refers to cooling treatment by immersion in water or spraying.

[Operation] The basic idea of the present invention is to utilize the recrystallization phenomenon that progresses immediately after hot rolling. Furthermore, since the recrystallized grains become finer as the initial crystal grain size becomes finer, details of the invention will be explained in which a fine grain structure can be obtained by repeated rolling under conditions where such a phenomenon occurs. .

Figures 1 and 2 show the results of investigating the influence of rolling conditions on the grain size of as-rolled materials for 5US304 series stainless steel having the chemical composition shown in Table 1. From this figure, it can be seen that recrystallization does not proceed in the low temperature or low rolling area, and that it is necessary to ensure a certain level of reduction and rolling temperature for recrystallization. Furthermore, as the temperature rises further, the growth rate of crystal grains increases, and the tendency for crystal grains to become coarse is cursed120.
At temperatures above 0°C, the structure becomes quite coarse. Figure 4 shows the change in recrystallization behavior over time after hot rolling, and it can be seen that the recrystallization rate increases with time and recrystallization is completed in about 10 seconds. ) was made based on such knowledge, and is based on the range of rolling conditions that cause recrystallization shown in FIG. Regarding the temperature in this range of rolling conditions, the upper limit was set at 1200° C. in order to prevent grain coarsening.

Further, the rolling reduction ratio was set to 40% or less in total for two passes, since excessive rolling requires a significant increase in the capacity of the conventional rolling mill and it becomes extremely difficult to secure the rolled shape. Therefore, the claimed range of rolling conditions falls within the shaded area shown in FIG. Furthermore, in order to achieve grain refinement through repeated rolling, recrystallization from the previous rolling must be completed, and therefore intervals of 10 seconds or more are ensured. Furthermore, as a necessary condition for recrystallization in each of the final two passes, it is necessary to ensure a certain degree of reduction in 80 and 80, respectively. This relationship is expressed by the above formula (1
), it is necessary to satisfy this formula in order to utilize the cumulative rolling effect.

By performing hot rolling under such conditions, a homogeneous structure with relatively fine grains can be ensured as rolled. Therefore, if carbide precipitation during cooling after rolling can be prevented, solution heat treatment can be omitted. Figure 5 shows the relationship between the water cooling start temperature after rolling, the carbon content in the steel, and carbide precipitation, and shows that as the carbon content increases, it is necessary to increase the cooling start temperature to ensure a solid solution state. I understand.

Since the limit temperature is given by equation (2), it is necessary to set the water cooling start temperature after rolling to "1' (° C.) or higher" expressed by equation (2).

Figure 6 shows the material shown in Figure 3 with an additional 1050
This figure shows the recrystallization behavior and crystal grain size of a material subjected to solution treatment at ℃ for 1 hour. By performing solution treatment, the recrystallized region is greatly expanded. In addition, in the area outside the shaded area in the figure, i.e. fl (reduction area), recrystallization proceeds by solution treatment, but it becomes a so-called mixed grain structure with large fluctuations in crystal grain size, and from the point of view of material homogeneity. The invention (2), which is in an unfavorable region, was made based on this knowledge and is based on the range of rolling conditions in which the grain size structure shown in Fig. 6 can be obtained.For this range of rolling conditions, Regarding the temperature, the first limit was set at 1200°C in order to prevent crystal grain coarsening.As for the rolling reduction rate, it is important to note that excessive rolling requires a significant increase in the capacity of conventional rolling mills, and that Since it would be extremely difficult to ensure this, the total amount of the two passes was set at 40% or less.According to FIG. 6, grain refinement can be achieved even at a rolling temperature of 900°C or less.

However, due to the increase in deformation resistance associated with a decrease in temperature, it is necessary to significantly increase the rolling force of conventional rolling mills in order to secure the rolling reduction rate for thick steel plates, so temperatures below 950℃ are realistic. Therefore, the lower limit temperature for rolling was set at 950°C. For the above reasons, the shaded area shown in FIG. 2 is defined as the claimed range of rolling conditions. Furthermore, in order to achieve grain refinement through repeated rolling, recrystallization from the previous rolling must be completed, and therefore intervals of 10 seconds or more are ensured. Regarding the solution treatment conditions, from the viewpoint of solid solution of carbides, the lower limit is the temperature 71' (
℃) or higher, but heating to 950°C or higher is necessary to complete recrystallization. Therefore, the lower limit temperature for solution treatment is 950°C or according to formula (2)
When 'r (℃) shown in is 950"C or more, 'r (
℃). On the other hand, since treatment at 1150° C. or higher causes coarsening of crystal grains, this temperature was set as the upper limit.

[Example 2] Steel plates were manufactured using the four types of austenitic stainless steels shown in Table 2 under the rolling conditions according to the grain control rolling method according to the present invention and the rolling conditions according to the conventional rolling method shown in Table 3. Table 4 shows the grain size and mechanical properties of these steel plates. As is clear from the results and mechanical properties of these microstructures, it can be seen that the rolling and rolling-solution treatment methods of the present invention have finer grains than conventional rolling methods, resulting in a well-ordered structure with <1 .

[Effect of the invention] As stated above, rolling or rolling by the method of the present invention
The solution-treated material has fine crystal grains and an evenly grained structure, making it a material with excellent strength and ductility [L] and excellent homogeneity, making it suitable for industrial use as thick steel plates for large structures. It is extremely effective.

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams showing claims regarding rolling conditions, and FIG. 3 is a diagram showing recrystallization behavior as rolled and the relationship between grain size and rolling conditions. Figure 4 is a diagram showing the influence of holding time on the recrystallization rate after rolling, Figure 5 is a diagram showing the influence of cooling start temperature and carbon on carbide precipitation after rolling, and Figure 6I32I is after solution treatment. FIG. 3 is a diagram showing the relationship between grain structure and rolling conditions.

Claims (1)

[Claims] (1) In hot rolling of austenitic stainless steel, the total amount of rolling reduction ratios A_1 and A_2 and rolling temperature in the final two passes are in the shaded area (A, B, C, D) in Figure 1, and Rolling is carried out under the conditions that the relationship between A_1 and A_2 satisfies the following formula (1), and the interval between A_1 rolling and A_2 rolling is secured for 10 seconds or more, and then the temperature T (℃) shown in the following formula (2) or higher is applied. A grain control hot rolling method characterized by water cooling. 0.6<A_1/A_2<1.4...(1) T=20
00×(carbon content in steel: weight%)+810...(2)
(2) In hot rolling of austenitic stainless steel, the total amount of reduction ratios A_1 and A_2 and the reduction temperature in the final two passes are in the shaded area (E, F, G, H, I) in Figure 2. , and the relationship between A_1 and A_2 satisfies the above formula (1), further rolling is performed under conditions that ensure an interval of 10 seconds or more between A_1 rolling and A_2 rolling, and then rolling is performed at a temperature of 950°C or higher and the temperature shown in the above formula (2). A grain control hot rolling method characterized by performing a solution heat treatment of heating and water cooling between T (°C) or higher and 1150°C.