JPH03264618A - Rolling method for controlling crystal grain in austenitic stainless steel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a hot rolling method using oil to control the crystal grains of a thick austenitic stainless steel plate into regular and fine grains. be.

(Conventional technology) 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 a material are closely related to the grain size; for example, strength decreases as the grain size becomes coarser. Furthermore, as the crystal grains become coarser, the ultrasonic penetration ability decreases, resulting in problems such as insufficient ultrasonic flaw detection.

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 heat 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 heat treatment. . Alternatively, attempts have been made to form stable carbides and nitrides by adding Ti, Nb, etc. 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. In addition, the latter Ti or N
The method using b and the like 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.

(Problems to be Solved by the Invention) As described above, the rolling strain accumulation method, which is a conventional method for refining the crystal grains of austenitic stainless steel, requires a huge rolling mill as the plate thickness increases. N
The b addition method has problems such as being susceptible to changes in material properties and non-uniformity.

The present invention provides a rolling method for refining the crystal grains of a thick steel sheet without increasing the size of a rolling mill and without using additional elements.

(Means and effects for solving the problems) The present invention has the following features:1. In the hot rolling of austenitic stainless steel, the cooling water in the final two baths of rolling is 0.
005 to 0.15% by weight of oil was mixed, and the rolling reduction rate A1. A2 (In this specification, the rolling reduction ratio (%) of the final pass of hot rolling is referred to as A2, and the rolling reduction ratio (%) of the pass immediately before the final pass is referred to as A2.
) is defined as A1) and the rolling temperature in the diagonally shaded area (A, B) in Figure 1.

C, D, E), and the relationship between A1 and A2 is the following formula (
A crystal that satisfies 1) and is further rolled under conditions that ensure an interval of 10 seconds or more between A rolling and A2 rolling, and then water cooling from a temperature T (0C) or more shown in the following formula (2). This is a grain control hot rolling method.

0, [i <A, /A2<1.4...
・・・・・・・・・ (1) T=2000X (double amount of carbon in steel%) +810・・・・・・・・・・・・(
2> Also, 2. In the hot rolling of austenitic stainless steel, 0.00% is added to the cooling water in the final two passes of rolling.
5 to 0.15% by weight of oil was mixed, and the rolling reduction rate of the two baths was A1. The total amount of A2 and the rolling temperature are in the shaded area in Figure 2 (
F, G, H, I, J, K), and the relationship between A1 and A2 satisfies the above formula (1), and further rolling is performed under conditions that ensure an interval of 10 seconds or more between A rolling and A2 rolling, This is a grain control hot rolling method characterized in that a solution heat treatment is then performed in which heating is performed at 950° C. or higher and from a temperature T (° C.) shown in the above formula (2) to 1150° C. followed by water cooling.

In addition, the austenitic stainless steel in the present invention is Nj near ~17%, Cr:I5~22%, Mo:0
~5% is the basic component. Furthermore, water cooling refers to cooling treatment by immersion in water or spraying.

The basic idea of the present invention is to utilize the recrystallization phenomenon that occurs immediately after hot rolling. Furthermore, since the recrystallized grains become finer as the initial crystal grain size becomes finer, a fine grain structure can be obtained by repeatedly rolling under conditions where such a phenomenon occurs.

In addition, by mixing an appropriate amount of oil into the cooling water during hot rolling, we have succeeded in significantly reducing the rolling reaction force by taking advantage of the phenomenon that the friction coefficient between the roll and the material is significantly reduced. This makes it possible to refine grains down to the size of steel sheets.

Next, details of the invention will be explained.

FIG. 3 shows the results of an investigation into the influence of rolling conditions on the grain size of as-rolled materials for SUS 304 series stainless steel having the chemical composition shown in Table 1.

Table 1 Steel type CSlMn P S NI CrMo N
5US3040.04 0.6 +, 5 0.0
30 0.004 g, 9 18,3 0
0.05 From this figure, it can be seen that recrystallization does not proceed in the low temperature or low pressure region, and that it is necessary to secure a certain amount of reduction and rolling temperature for recrystallization.

Furthermore, as the temperature rises further, the growth rate of crystal grains increases, and the crystal grains tend to become coarser, resulting in a considerably coarser grain structure at temperatures above 1200°C.

FIG. 4 shows changes 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.

The present invention (1) was developed based on this knowledge, and is based on the range of rolling conditions that cause recrystallization as shown in FIG. Regarding the temperature in this range of rolling conditions, the upper limit was set to 1200° C. in order to prevent crystal grain coarsening by 1F.

In addition, regarding the rolling reduction ratio, in the normal rolling method, a large reduction requires a significant increase in the capacity of the rolling mill, and it is extremely difficult to secure the rolled shape, so the limit is 40% or less for the total of two passes. Met.

However, as shown in Figure 5, by mixing oil into the roll cooling water during rolling, the rolling reaction force can be significantly reduced. It becomes possible.

Therefore, the total reduction amount of the two passes was set to 50%.

The figure shows the rolling reaction force during hot rolling from a plate thickness of 50mm to 44mm.

In other words, Figure 5 shows the influence of the oil mixture rate in the roll cooling water on the rolling reaction force.The rolling reaction force decreases as the oil mixture rate in the cooling water increases, but the effect is approximately Since it becomes noticeable from 0.005%, the lower limit of the mixing rate was set to 0.005%. On the other hand, when the mixing rate of oil reaches about 0.15%, it tends to be saturated, and further mixing causes slippage between the work roll and the backup roll, so the upper limit of the oil mixing rate is set to 0.15%.

Therefore, the range of rolling conditions to which such a hydraulic rolling method is applied is the shaded area shown in FIG. Furthermore, in order to achieve grain refinement through repeated rolling, recrystallization from the previous rolling must be completed, so 1
An interval of 0 seconds or more is ensured. Furthermore, as a necessary condition for recrystallization in each of the final two passes, it is necessary to ensure a certain reduction amount for each of A and A2.

Since this relationship is expressed as the above equation (1), it is necessary to satisfy this equation 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 in the as-rolled state. Therefore, if carbide precipitation can be prevented during cooling after rolling,
This means that solution heat treatment can be omitted.

Figure 6 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 expressed by equation (2), it is necessary to set the water cooling start temperature after rolling to be equal to or higher than T (° C.) expressed by equation (2).

FIG. 7 shows the material shown in FIG. 3 with an additional 1050
This figure shows the recrystallization behavior and crystal grain size of a material that was held at ℃ for 1 hour and then subjected to water-cooling solution treatment.

By performing the solution treatment, the recrystallized region becomes thicker and expanded. Recrystallization also progresses through solution treatment in the area outside the shaded area in the figure, that is, the area under low pressure, but this results in a so-called mixed grain structure with large fluctuations in grain size, which is considered an unfavorable area from the viewpoint of material homogeneity. Become.

The present invention (2) was developed 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. Regarding the temperature in this range of rolling conditions, the upper limit was set at 1200° C. in order to prevent crystal grain coarsening by 11 U.

Regarding the rolling reduction rate, in the normal rolling method, excessive rolling requires a significant increase in the capacity of the conventional rolling mill, and it is extremely difficult to secure the rolled shape.
The total limit for the two buses is 40%.

However, as shown in Figure 5, by mixing oil into the roll cooling water during rolling, the rolling reaction force can be significantly reduced, so by applying the hydraulic rolling method, it is possible to achieve a rolling rate of over 40% in two passes in total. Become.

Therefore, the total reduction amount of the two passes was set to 50%.

Further, according to FIG. 7, grain refinement can be achieved even at a rolling temperature of 900° C. or lower. 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 900℃ are realistic. Not on target.

However, as shown in FIG. 5, since the rolling reaction force can be significantly reduced by hydraulic rolling, a predetermined rolling reduction amount can be secured even at low temperatures by applying the hydraulic rolling method. Therefore, the lower limit temperature of rolling was set to 800°C.

For the above reasons, the shaded area shown in FIG. 2 was defined as the 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, the lower limit must be at least the temperature T (°C) shown in equation (2) from the viewpoint of solid solution of carbides, but heating to 950°C or higher is necessary to complete recrystallization. is necessary. Therefore, the lower limit temperature for the solution 1 treatment is 950°C or according to formula (2)
When T (°C) shown in is 950°C or higher, it becomes T (°C). 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) Regarding the four types of austenitic stainless steels shown in Table 2, rolling conditions according to the grain control rolling method according to the present invention in which oil is mixed in roll cooling water and the conventional rolling method shown in Table 3 A steel plate was manufactured by.

Table 4 shows the grain size and mechanical properties of these steel plates.

As is clear from these microstructure observation results and mechanical properties, the rolling and rolling-solution treatment methods of the present invention have finer grains and a more regular grain structure than the conventional rolling methods. At the same time, it can be seen that the crystal grains of thick-walled steel sheets can be made finer by increasing the reduction amount by hydraulic rolling.

2 3 4 (Effects of the Invention) As described above, the thick steel plate rolled or rolled-solution treated by the method of the present invention has fine crystal grains and a well-organized structure, so it has excellent strength and ductility and is highly homogeneous. It is also an excellent feed, and is extremely effective industrially as a thick steel plate for large structures.

[Brief explanation of drawings]

Figures 1 and 2 are charts showing the scope of the present invention regarding rolling conditions, Figure 3 is a chart showing recrystallization behavior as rolled and the relationship between grain size and rolling conditions, and Figure 4 is a chart showing the relationship between rolling conditions and rolling conditions. A chart showing the influence of holding time on the recrystallization rate, Fig. 5 a chart showing the relationship between rolling reaction force and oil contamination rate in cooling water, and Fig. 6 a chart showing the effect of cooling start temperature and carbon content on carbide precipitation after rolling. FIG. 7 is a chart showing the relationship between grain structure after solution treatment and rolling conditions. Agent Patent attorney Tatsuo Chanoki 7 (%) Talented person / 2V + / 17' w,) ヰ' tongue / 4. uunωhi/OtsuzV1000/V

Claims (1)

[Claims] 1. In hot rolling of austenitic stainless steel, 0.00% of the roll cooling water in the final two passes of rolling is added.
5 to 0.15% by weight of oil is mixed, and the total amount of rolling reduction ratios A_1 and A_2 of the two passes and the rolling temperature are in the shaded area (A, B, C, D, E) in Fig. 1. , and A_1 and A_
2 satisfies the following formula (1), and furthermore, A_1 pressure and A
_2 A grain control hot rolling method, characterized in that rolling is carried out under conditions that ensure a rolling interval of 10 seconds or more, and then water cooling is carried out from a temperature T (° C.) or more shown in the following formula (2). 0.6<A_1/A_2<1.4…………(1)T
=2000×(Carbon content in steel: weight%)+810……
...(2) 2. In the hot rolling of austenitic stainless steel, 0.00% is added to the roll cooling water in the final two passes of rolling.
5 to 0.15% by weight of oil is mixed, and the total amount of the rolling reduction ratios A_1 and A_2 of the two passes and the rolling temperature are in the shaded areas (F, G, H, I, J, K) in Fig. 2. , and the relationship between A_1 and A_2 satisfies the following formula (1), and further rolling is performed under conditions that ensure an interval of 10 seconds or more between A_1 rolling and A_2 rolling. A grain control hot rolling method characterized in that a solution heat treatment is performed by heating and cooling from a temperature T (°C) shown above to 1150°C. 0.6<A_1/A_2<1.4…………(1)T
=2000×(Carbon content in steel: weight%)+810……
...(2)