JPH04253506A - Method for hot-rolling boron containing austenite stainless steel material - Google Patents

Abstract

PURPOSE:To establish a method for hot rolling free from generation of cracks to austenite stainless steel material containing 0.3-2.0wt.% boron by a simple means. CONSTITUTION:This stainless steel material is used as a core material 11, the side part 11a of the core material 11 is covered with steel material smaller in deforming resistance (average) to the rolling temperature than the steel material of the core material 11 as a frame material 12 to obtain a base material 10, and the base material 10 obtained is heated up to 1100-1200 deg.C, then, it is finish-rolled at a temperature exceeding a temperature T shown in equation (1). T=53B+700...(1) Where B: content of boron (wt.%).

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention uses boron 0.3 to 2.0w.
The present invention relates to a hot rolling method for austenitic stainless steel containing t%.

0002

[Prior Art] Austenitic stainless steel materials containing 0.3 to 2.0 wt% boron have excellent thermal neutron absorption properties, and are used in containers for transporting spent nuclear fuel (generally called casks), etc. Usually, medium-thick plates with a thickness of 10 mm or less, or thin plates with a thickness of 3 mm or less are often used as materials for fuel baskets.

[0003] However, due to the high boron content, hard and brittle borides are likely to be formed in stainless steel materials, which significantly reduces hot workability.

That is, when a normal rolling method is applied, it becomes difficult to secure a rolling temperature that maintains hot workability, and cracks occur due to deterioration of hot workability. In this case, it is possible to raise the rolling temperature by increasing the heating temperature, but since the aforementioned boride melts at 1200°C or higher, the heating temperature cannot be raised higher than 1200°C. Therefore, various measures have been proposed to improve hot workability.

[0005] In JP-A No. 63-96221, an austenitic stainless steel material containing 0.2 to 1.2 wt% boron is heated to a temperature of 1150°C or more and 1200°C or less, and then heated using a rough rolling mill and a finishing rolling mill. Therefore, a method has been proposed in which hot rolling is carried out so that the rolling end temperature is 1000° C. or higher. However, with this method, it is necessary to repeat heating and rolling many times, and when working with plates of the above-mentioned thickness, it is difficult to maintain a constant temperature, especially on the side parts of the plate, resulting in so-called edge cracking. arise.

[0006] Furthermore, to improve hot workability, it has been proposed to add Al or V to boron-containing austenitic stainless steel (Japanese Patent Publication No. 57-45464, Japanese Patent Application Laid-Open No. 55-89459). Public bulletin). However, it is difficult to manufacture it using a normal hot rolling mill or the like.

On the other hand, JP-A-63-293139 proposes a method for producing boron-containing austenitic stainless steel using a powder metallurgy method. However, with this method, it is difficult to meet the demand for mass supply, and the manufacturing cost becomes extremely high.

[0008] Therefore, currently, industrially, JP-A-63-
Pack rolling methods such as those disclosed in Japanese Patent No. 220904 and the like are often employed. FIG. 9 is a diagram showing the relationship between the amount of boron added and the finished rolling temperature. ○ mark indicates no crack, and ● mark indicates crack. As shown in Figure 9, regarding the hot rolling workability of boron-containing stainless steel materials, the higher the boron content, the more borides, and the higher the critical temperature at which cracks occur. ) There is a temperature range in which hot rolling is possible. In this case, the relationship between the rolling finishing temperature and the limit rolling depending on the amount of boron contained is shown by equation (2).

[0009]T=53B+870 ---(2) However
T: Finish rolling limit temperature (°C) B: Boron content (
wt%)

[0010] Here, a pack rolling method is proposed in order to maintain the temperature at or above the temperature shown by the above equation (2), that is, to prevent the temperature of the material from dropping.

FIG. 10 is a sectional view showing an example of a pack-rolled material used in the pack-rolling method. In the figure, 1 is a core material, which is a boron-containing austenitic stainless steel ingot or continuous cast slab. The pack rolled material is assembled by covering the upper and lower surfaces of three laminated core materials 1 with a cover material via a release material 4, surrounding them with a spacer 3, and welding the cover material 2 and spacer 3 together. be. 5 indicates a welded part. By this rolling method, a sound hot-rolled plate with no edge cracks can be obtained.

0012

However, the pack rolling method disclosed in Japanese Unexamined Patent Publication No. 63-220904 mentioned above also has the following problems. ■Assembling the pack material, taking out the product, etc. is complicated and costly. ■Since the core material of boron-containing austenitic stainless steel is surrounded by packs, there is no means to measure the actual thickness, making it difficult to ensure thickness accuracy during hot rolling.

The present invention has been made to solve the problems of the prior art as described above, and uses simple means to produce boron-containing austenitic stainless steel that does not generate cracks while ensuring plate thickness accuracy. The object of the present invention is to provide a hot rolling method.

[0014]

[Means for Solving the Problems and Effects] In order to achieve the above object, the present invention provides a structure in which the side part of an austenitic stainless steel material containing 0.3 to 2.0 wt% boron has a higher resistance to rolling temperature than the stainless steel material. Boron is made by heating a material made by contacting and coating a steel material with low deformation resistance (on average) to a temperature of 1100°C or more and 1200°C or less, and then finishing rolling at a temperature not less than the temperature shown by the following formula (1). This is a method for hot rolling an austenitic stainless steel material.

[0015] T=53B+700 ---(1) However
T: Finish rolling limit temperature (°C) B: Boron content (
wt%)

[0016] The material used in the present invention has a weight of 0.3 to 2.0wt.
% boron-containing austenitic stainless steel material, a steel material having a coefficient of linear expansion equal to or lower than that of the stainless steel material and having a small deformation resistance (on average) against rolling temperature is adhered in a frame shape to cover the side surfaces. or an austenitic stainless steel material containing 0.3 to 2.0 wt% boron, a steel material having a lower deformation resistance (on average) at rolling temperature than the stainless steel material is bonded and coated by welding means. It can be made into a material.

The present invention is directed to an austenitic stainless steel material containing 0.3 to 2.0 wt% boron. When the boron content is less than 0.3 wt%, the generation of borides is small and the effect on hot rolling is small. Boron is 2
．． If it exceeds 0 wt%, the ductility and toughness of the steel at room temperature tend to deteriorate, making it impractical.

[0018] Prior to the present invention, the present inventors had determined that
．． The present invention was arrived at after conducting various studies on rolling manufacturing techniques for austenitic stainless steel materials containing 0 wt% boron.

As mentioned above, regarding the hot rolling workability of stainless steel materials, the higher the boron content, the higher the boride content and the higher the critical temperature at which cracks occur. However, there is a temperature range in which hot rolling is possible depending on the boron content (wt%). In this case, the relationship between the rolling finishing temperature and the limit rolling depending on the amount of boron contained is known as shown in equation (2).

[0020] T=53B+870 ---(2) However
T: Finish rolling limit temperature (°C) B: Boron content (
wt%)

1) Detailed study of cracks: The inventors of the present invention have investigated the cracks in detail and discovered the following fact. That is, (1) cracks below the above-mentioned critical rolling temperature occur from the edges. (2) However, no cracks occurred on the surface even below the above-mentioned limit rolling temperature.

The cause of edge cracking considered from this observed fact will be explained with reference to FIG. (a) As shown in the figure, when examining the temperature drop of the target boron-containing austenitic stainless steel slab 6 after heating to a temperature of 1100°C or more and 1200°C or less,
The temperature of the edge portion 6b is lower than that of the center portion 6a, so the workability of the edge portion 6b is lowered and cracks occur in that portion. The direction of heat dissipation from the slab 6 is indicated by an arrow.

On the other hand, in Figure (b), when the target boron-containing austenitic stainless steel slab 6 is hot-rolled with rolls 7, the central part 8a of the rolled slab 8 is as shown by the thick (long) arrow. , the elongation is large and causes plane strain deformation. However, since the edge portion 8b expands in the width direction as shown by the thin arrow, the elongation is small as shown by the thick (short) arrow, resulting in three-dimensional deformation. The slab 8 is being transferred from left to right.

Figure (c) shows the relationship between compressive stress and tensile stress in that case. The rolled slab 8 is being transferred from left to right as shown in Figure (b). Here, the diagonal line A indicates compressive stress (-), and the diagonal line B indicates tensile stress (+). With respect to a reference line C shown by a two-dot chain line in the width direction of the rolled slab 8, the center part 8a produces a compressive stress (-) effect, and the edge part 8b produces a tensile stress (+) effect. arise.

From the above, the causes of edge cracking are as follows. ■Workability decreases at the edge due to temperature drop■
A tensile stress effect occurs at the edge. A synergistic effect of the above ■ and ■ occurs at the edge.

Therefore, a test material having a narrow width (uniform temperature distribution in the rolling width direction) was further rolled to examine its workability. As shown in FIG. 4, the uniform temperature lowers the processing limit temperature. In this case, the relationship between the rolling finish temperature and the limit rolling depending on the amount of boron contained is shown in equation (3).

[0027] T=53B+800 ---(3) However
T: Finish rolling limit temperature (°C) B: Boron content (
wt%) The relationship (2) for limit rolling is shown for comparison.

However, even in this state, cracks were observed to spread from the edges. Therefore, although the cause of cracking in boron-containing austenitic stainless steel due to rolling is the above-mentioned item (2), it is thought that the effect of item (2) is also quite large.

As mentioned above, the inventors of the present invention have elucidated the cause of the cracks and have further studied ways to prevent the temperature drop on the side of the plate and change the stress state, and have arrived at the present invention. .

[0030] In the present invention, hot rolling is performed using a material whose side portions of the steel material are coated as shown in FIG. 1 or 2, which will be described later. In the present invention, the material for covering the side portion of the steel material is limited to a steel material for covering, which has a lower deformation resistance (on average) at rolling temperature than the target stainless steel material, as it is a material with high workability. This is because the coating steel material, which has a low deformation resistance (on average), restrains the deformation of the edge portion of the target stainless steel material during hot rolling, and can deform in a state close to a plane strain state. Further, if necessary, a steel material for coating may be used which has a coefficient of linear expansion with respect to temperature that is the same or lower. If the steel material for coating is placed in close contact with the stainless steel material to be continuously cast slab or ingot at room temperature, the difference in coefficient of linear expansion will cause
This is because the adhesion is further enhanced, and edge restraint and heat retention can be improved.

The material shown in FIG. 1 has a core material made of austenitic stainless steel and a core material that is coated in contact with the sides and has a lower deformation resistance (on average) at rolling temperature than the core material, and a coefficient of linear expansion that is equal to or lower than that of the core material. Since it is constructed from a frame material made of steel, during hot rolling, the edge portions of the core material are restrained from deformation by the frame material, deforming in a state close to plane strain, and the temperature of the side portions of the core material also varies with the frame material. Retained by the heat retention effect of the material.

[0032] If the coefficient of linear expansion of the frame material is smaller than that of the core material, the frame material is placed in close contact with a continuously cast slab or hot rolled ingot at room temperature and heated in a heating furnace. Due to the difference in coefficient of linear expansion, it is possible to further enhance adhesion and improve edge restraint and heat retention. However, even if the coefficient of linear expansion of the frame material is equivalent to that of the core material, the object of the present invention can be achieved because the deformation resistance (average) against rolling temperature is small.

Furthermore, when removing the frame material after rolling, the core material contracts due to this difference in coefficient of linear expansion, making the work very easy.

Further, if edging rolling is possible, edging rolling can be performed before finish rolling in order to improve the adhesion between the core material and the frame material.

In the present invention, the rim material is made of steel with low deformation resistance (on average) against rolling temperature, as shown in FIG. , and the welded part is deformed in a plane strain state,
It can also maintain temperature. In this case, one of the characteristics of the rim material is that it must be able to be welded to a boron-containing austenitic stainless steel plate.

[0036] Concerning the steel material for the coating used in the present invention, a suitable steel material was specifically selected based on the following study. That is, the deformation resistance of an austenitic stainless steel plate containing 1.00 wt% boron was investigated, and an appropriate frame material was studied. This is shown in Figure 3. ○ mark is boron-containing austenitic stainless steel plate, ● mark is austenitic stainless steel plate (SUS304), × mark is carbon steel (carbon equivalent = 0
．． 21%). In this case, the strain is 0.2 and the strain rate is 1.
It was set to 0/second.

As is clear from the figure, 1.00wt%
Boron-containing austenitic stainless steel sheet (SUS
304) has an average deformation resistance of 30 to 30% compared to SUS304 material.
About 40% larger. Also, carbon steel (carbon equivalent = 0.21%
), it is about 1.7 to 2 times. In addition, FIG. 4 shows the respective linear expansion coefficients. ○, ●, and × marks are in Figure 3.
is the same as As is clear from the figure, 1.00wt%
Boron-containing austenitic stainless steel plate (SUS3
The coefficient of linear expansion of 04) is equivalent to that of SUS304 material. In addition, for carbon steel (carbon equivalent = 0.21%), 30~
About 40% higher. Therefore, both SUS304 material and carbon steel can be stably rolled as a frame material.

In this method, the temperature at which hot rolling is possible according to the boron content has the relationship shown in FIG. It can be guided as shown.

[0039] T=53B+700 ---(1) However
T: Finish rolling limit temperature (°C) B: Boron content (
wt%)

The conventional limit rolling relationship (2) is shown for comparison. As is clear from the figure, according to the method of the present invention, the range in which workability can be maintained is expanded compared to the conventional method.

FIG. 6 shows the rolling rate (indicated by rolling reduction rate) at each temperature of the austenitic stainless steel containing 1.0 wt % boron according to the method of the present invention. This has greatly improved the rolling reduction per pass, so even when rolling a thick plate, for example, fewer passes are required and a drop in temperature can be prevented. A conventional material produced by the conventional method was used for comparison.

Next, the reason for limiting the heating temperature according to the method of the present invention will be described. Heating temperature 1100℃ or higher 1200℃
The reason for setting the temperature below 1100°C is that heating below 1100°C is unfavorable because the finished temperature will drop significantly and the temperature range that can be rolled is narrowed, and heating above 1200°C will cause the boride to melt. In order to avoid cracking during rolling, the heating temperature was set at 1100°C to 1200°C.

[0043] Boron 0.3 to 2, which is the object of the present invention,
．． The components of austenitic stainless steel containing 0 wt% are generally as follows. B: 0.3 to 2.0wt% C: 0.08wt% or less Si: 2.0wt% or less Mn: 2.0wt% or less P: 0.050wt% or less S: 0.03wt% or less Cr: 16.0 ~20.0wt% Ni: 8.0~15.0wt% Mo: 3.0wt% or less N: 0.15wt% or less The balance refers to steel consisting of Fe and unavoidable impurities.

[0044]

EXAMPLE FIG. 1 is a diagram showing an example of the material used in the present invention. The figure (a) is a perspective view, and the figure (b) is a perspective view.
It is a figure which shows the cross section of the II line of a figure. The direction of material transfer is indicated by a thick arrow. In these figures, a material 10 is a side portion 11a and an end portion 1 of an austenitic stainless steel plate (core material here) containing 0.3 to 2.0 wt% boron.
A frame material 12 made of a steel material having a lower deformation temperature (on average) with respect to rolling temperature than the core material 11 is arranged in a frame shape around the core material 1b.

In the method of the present invention, after heating this material in a heating furnace to a temperature of 1100°C or more and 1200°C or less, the following (
1) Finish rolling is performed at a temperature equal to or higher than the temperature shown by the formula.

[0046] T=53B+700 ---(1) However
T: Finish rolling limit temperature (°C) B: Boron content (
wt%)

FIG. 2 is a diagram showing another embodiment of the material used in the present invention. The direction of material transfer is indicated by a thick arrow.

In the figure, the material 13 is boron 0.3 to 2
．． This material is made by bonding and covering the side portion 11a of an austenitic stainless steel plate 11 containing 0 wt% boron with a rim material 14 made of a steel plate that has a smaller (average) deformation resistance at rolling temperature than the stainless steel plate 11 by welding. 15 indicates a welded portion. As the welding means, MIG welding, electron beam welding, etc. can be used. in this case,
It is necessary that the rim material 14 can be welded to the stainless steel plate 11.

In the method of the present invention, after heating this material in a heating furnace to a temperature of 1100°C or more and 1200°C or less, the following (
1) Finish rolling is performed at a temperature equal to or higher than the temperature shown by the formula.

[0050] T=53B+700 ---(1) However
T: Finish rolling limit temperature (°C) B: Boron content (
wt%)

1 and 2 during hot rolling in the method of the present invention.
As shown in the figure, boron 0.3 to 2.
Since the upper and lower surfaces of the austenitic stainless steel plate 11 containing 0 wt% boron are exposed, the thickness of the stainless steel plate 11 can be measured accurately.

Next, examples of the method of the present invention will be explained in detail. Example 1 Continuously cast slabs and steel ingots of boron-containing austenitic stainless steel having the chemical components shown in Table 1 were manufactured.

[0053]

[Table 1]

After manufacturing these slabs or steel ingots according to the blooming slab manufacturing conditions shown in Table 2, the same thickness is applied around the sides and ends of the slabs, etc., in order to form a material as shown in FIG. Carbon steel (mild steel) with a width of 100 mm was placed closely together as a frame material, hot rolled into a steel strip under the hot rolling conditions shown in Table 2, and the frame material was rolled while unwinding the coil in a cold state. It was removed and checked for edge cracks. For comparison, the above-mentioned bloomed slabs and continuously cast slabs were hot-rolled as they were without employing this method, and rolled into steel strips as examples.

[0055]

[Table 2]

As can be seen from the results in Table 2, the method of the present invention produced better results with no cracks than the conventional hot rolling method. In this case, the plate thickness accuracy was also good.

Example 2 Steel ingots of boron-containing austenitic stainless steel having the chemical components shown in Table 3 were produced.

[0058]

[Table 3]

[0059] After manufacturing this steel ingot according to the blooming slab manufacturing conditions shown in Table 4, a plate with the same thickness and a width of 100 mm was placed on the side of the plate.
Carbon steel (mild steel) was welded by electron beam welding and hot rolled under the plate rolling conditions shown in Table 4. Conventionally, in order to finish to this thickness, about 12 passes were required due to the limit of the amount of reduction per pass, but with this method, it was completed in 6 passes. No cracking was observed in the boron-containing austenitic stainless steel material. In this case, the plate thickness accuracy was also good.

[0060]

[Table 4]

[0061]

[Effects of the Invention] According to the hot rolling method of the present invention, crack-free hot rolling can be performed at low cost while ensuring plate thickness accuracy using simple means.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a rolled material used in the present invention.

FIG. 2 is a diagram showing another example of the rolled material used in the present invention.

FIG. 3 is a diagram showing the relationship between rolling temperature and deformation resistance (average) of frame materials used in the present invention.

FIG. 4 is a diagram showing the relationship between temperature and coefficient of linear expansion for selecting a frame material used in the present invention.

FIG. 5 is a diagram showing the relationship between rolling finish temperature and limit rolling depending on boron content according to the present invention.

FIG. 6 is a diagram showing the relationship between processing rate per pass and rolling finish temperature according to the present invention.

FIG. 7 is an explanatory diagram showing the cause of edge cracking in a boron-containing austenitic stainless steel material.

FIG. 8 is a diagram showing an example of the relationship between the rolling finishing temperature and boron content of a material that is uniform in the width direction.

FIG. 9 is a diagram showing an example of the relationship between conventional rolling finishing temperature and limit rolling depending on boron content.

FIG. 10 is a diagram showing an example of a pack material used in a conventional pack rolling method.

[Explanation of symbols]

10, 13 Material 11 Boron-containing austenitic stainless steel material 11a Side portion 11b End portion 12 Frame material 14 Ream material 15 Welded portion

Claims (3)

[Claims] Claim 1: A material made by coating the sides of an austenitic stainless steel material containing 0.3 to 2.0 wt% boron with a steel material that has lower deformation resistance (on average) at rolling temperatures than the stainless steel material. , 1100℃ or higher 120
A method for hot rolling a boron-containing austenitic stainless steel material, which comprises heating to a temperature of 0° C. or lower and then finishing rolling at a temperature equal to or higher than the temperature expressed by the following equation (1). T=53B+700 ---(1) However, T: Finish rolling limit temperature (℃) B: Boron content (wt%) 2. The side and end portions of an austenitic stainless steel material containing 0.3 to 2.0 wt% boron have a linear expansion coefficient equal to or lower than that of the stainless steel material, and have deformation resistance against rolling temperature. 2. The method of hot rolling a boron-containing austenitic stainless steel material according to claim 1, wherein the material is formed by closely adhering steel materials with a small average) in a frame shape and covering the sides thereof. 3. A steel material having a lower deformation resistance (on average) at rolling temperature than the stainless steel material is bonded by welding to the side of the austenitic stainless steel material containing 0.3 to 2.0 wt% boron. 2. The method of hot rolling a boron-containing austenitic stainless steel material according to claim 1, wherein the material is a coated material.