JPS6396221A - Production of boron-containing austenitic stainless steel strip - Google Patents

Abstract

PURPOSE:To permit satisfactory mass production of hot rolled steel sheets without cracking flaws, by subjecting a slab or ingot of the titled steel contg. a specific ratio of B to hot rolling with a roughing mill and finishing mill at a specified heating temp. and rolling end temp. CONSTITUTION:The slab or ingot of the austenitic stainless steel contg. 0.2-1.2wt% B is heated to 1,150-1,200 deg.C and is subjected to the hot rolling by using the roughing mill and finishing mill in such a manner that the rolling end temp. attains >=1,000 deg.C. The hot rolled steel strip produced under the above- mentioned conditions is otherwise subjected to one pass of cold rolling or plural passes of cold rolling including intermediate annealing at <=50% cold rolling daft. The steel contg. B in the above-mentioned range is used in this production process because the workability and toughness decreased to the level not permissible in this production process if the content is >1.2% B. >=0.2% B is required in order to impart the effect of attenuating thermal neutrons to the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing a boron-containing austenitic stainless steel strip.

[Conventional technology]

Boron-containing austenitic stainless steel is suitable as a thermal neutron shielding material because it has a large thermal neutron absorption cross section and has excellent corrosion resistance. However, it is difficult to produce a steel strip because the precipitation of intermetallic compounds, mainly boride, which is a combination of B in the steel and Fe and Cr, significantly impairs hot workability and toughness.

Japanese Patent Publication No. 36-8411 teaches that the toughness of such B-containing stainless steels can be improved by adding an appropriate amount of Ti and further by supplementary addition of deoxidizing elements.

Japanese Patent Publication No. 57-45464 teaches that the calorific properties of B-containing stainless steel can be improved by adding an appropriate amount of AA.

Japanese Patent Application Laid-Open No. 55-89459 discloses ■ B-containing stainless steel.
It is taught that the addition of boride improves hot working by improving the morphology of the boride.

[Purpose of the invention]

Despite the teachings that the addition of alloying elements improves the toughness of the material itself and improves hot working, it is difficult to produce a boron-containing austenitic stainless steel strip using a normal hot rolling mill or cold rolling mill. be. In fact, none of the above-mentioned publications describes a method for producing copper strips of B-containing stainless steel. Furthermore, according to experiments conducted by the present inventors, it has been found that the addition of special elements as taught in the publication does not sufficiently improve hot workability, and may conversely lead to a decrease in toughness. Ta.

Therefore, it is desired to develop a technology that can produce a boron-containing austenitic stainless steel strip without the addition of special elements, which have both merits and demerits.

The object of the present invention is to establish mass production technology for manufacturing copper strips of boron-containing austenitic stainless steel, which has extremely poor toughness and hot workability.

[Summary of the invention]

The gist of the present invention, which aims to achieve the above object, is to produce a hot rolled steel strip of austenitic stainless steel containing 0.2 to 1.2 weight percent of B. Heating the ingot to a temperature of 1150°C or higher and 1200°C or lower, and then hot rolling it using a rough rolling mill and a finishing rolling mill so that the rolling end temperature is 1000°C or higher, and further manufacturing a cold rolled steel strip. In doing so, the hot rolled steel strip produced under the above conditions is cold rolled once or multiple times with intermediate annealing at a cold rolling rate of 50% or less each time.

[Details of the invention]

It is possible to improve the toughness and hot workability of B-containing stainless steel without necessarily adding special elements such as those taught in the above publication.
．． In the case of austenitic stainless steel containing 2% by weight of boron, as will be demonstrated in Examples below, a slab or steel ingot of the steel is heated to a temperature of 1150°C or higher and 1200°C or lower, and then subjected to rough rolling and finishing rolling. It has been found that when hot rolling is carried out using a machine such that the rolling end temperature is 1000° C. or higher, a hot rolled steel strip can be produced satisfactorily without cracking. In addition, when manufacturing cold rolled steel strip from this hot rolled steel strip, it was found that if the cold rolling rate per round is set to 50% or less, the cold rolled steel strip can be manufactured satisfactorily without cracking. . In addition, when cold rolling is performed after trimming the hot rolled steel strip, it is necessary to perform cold rolling after the trimming treatment and after annealing the trimming surface.

In the method for manufacturing a steel strip of the present invention, the content of B is 0.2 to
The target is 1.2% austenitic stainless steel. The reason for this is that the workability and toughness of steel with a B content of more than 1.2 weight percent are unacceptably reduced by the steel strip manufacturing method of the present invention. Further, in order to impart a thermal neutron attenuation effect, it is necessary to contain at least 0.2% by weight of B.

Figure 1 shows round bar tensile test pieces taken from the as-cast state of boron-containing austenitic stainless steel having the chemical composition values shown in Table 1 below, and each test piece heated to the indicated temperature using a high-frequency heating device. The graph shows the results of examining the cross-sectional shrinkage rate when a tensile test was conducted at a strain rate of 15 mm/sec while the sample was maintained at that temperature. From Figure 1.

It can be seen that when the temperature exceeds 1200° C., the cross-sectional shrinkage rate of all boron-containing austenitic stainless steels rapidly decreases to zero. This is considered to be due to the melting of the boride. When hot rolling steel using a rough rolling mill and a finishing rolling mill, the cross-sectional shrinkage rate of the steel that can be hot rolled must be 40% or more.

FIG. 1 shows that when the heating temperature exceeds 1000° C., the cross-sectional shrinkage rate becomes 40% or more. Therefore, when hot rolling boron-containing austenitic stainless steel, the hot rolling must be completed at 1000°C or higher.

According to the results shown in Figure 1, the temperature should not exceed 1200°C. That is, austenitic stainless steel containing 0.2 to 1.2% by weight of B has a temperature of 1200°C or less and 100% by weight.
Hot rolling can be achieved by performing rolling at a temperature of 0° C. or higher. To this end, the heating temperature of the steel slab or steel ingot must be 1150°C or higher, taking into account the temperature drop during operation, and 120°C or higher.
It is necessary to keep the temperature below 0°C.

In addition, if the temperature of the edge of the steel strip falls preferentially than the center of the steel strip during hot rolling, and only the edge drops below 1000°C, cracks are likely to occur in the edge. Become. In order to prevent this, it is preferable to provide an edge heater between the rough rolling mill and the finishing mill to maintain the temperature of the edge of the steel strip at a temperature of 1000 to 1200°C.

Figure 2 shows 1 for the test steel floors 1 and N14 in Table 1 below.
This figure shows the relationship between the cold rolling rate, Charpy impact value, and Vickers hardness when hot rolling was performed under the hot rolling conditions according to the present invention, and then cold rolling was performed by changing the cold rolling rate.

As shown in FIG. 2, the Charpy impact value of boron-containing austenitic stainless steel sharply decreases as the cold rolling rate increases, the toughness decreases, and the steel hardens. and.

As shown in Examples below, when the cold rolling rate exceeds 50%, edge breakage occurs in the steel strip. Therefore, a cold rolled steel strip of boron-containing austenitic stainless steel can be produced by cold rolling a hot rolled steel strip produced according to the above conditions at a cold rolling rate of 50% or less each time. If the reduction rate exceeds 50% to reach the target finished plate thickness, cold rolling may be performed several times at a cold rolling rate of 50% or less with intermediate annealing in between.

Furthermore, when trimming is performed in the production of cold-rolled steel strips, it is necessary to anneal the trimming surface before performing cold rolling. This is because when trimming is performed on boron-containing austenitic stainless steel, the edges are work hardened and the toughness is significantly reduced, causing cold rolling defects. FIG. 3 shows typical examples of changes in hardness of the trimmed surface of a boron-containing austenitic stainless steel strip in the as-trimmed state and in the case of annealing. As seen in this figure, annealing softens the band trimming part and restores toughness, making cold rolling possible.

An example in which a hot-rolled steel strip and a cold-rolled steel strip of boron-containing austenitic stainless steel were produced in accordance with the present invention will be listed below.

Example 1 Continuous Vt casting slabs and steel ingots of boron-containing austenitic stainless steel having the chemical components shown in Table 1 were manufactured.

These slabs or steel ingots were heated in a heating furnace at different heating temperatures and subjected to one-segment rolling or hot rolling. The hot rolling conditions (heating temperature, rolling end temperature, original thickness/finished thickness) and rolling results are summarized in Table 2.

As can be seen from the results in Table 2, when hot-rolled under the hot-rolling conditions according to the present invention, even boron-containing austenitic stainless steel can be produced into a good hot-rolled steel strip without cracking. . On the other hand, as seen in the comparative example, when the heating temperature exceeds 1200°C, cracks occur and hot rolling is not possible (Comparative Example H), and 1 surface scale occurs, resulting in a defective product. (Comparative Example I). Also, the rolling end temperature is 10
When the temperature was lower than 00°C, the edge cracks were large and the rolling was defective (Comparative Examples J-K).

Example 2 Cold rolled steel strips were manufactured from several hot rolled steel strips obtained in Example 1 by changing the cold rolling rate. The manufacturing conditions and results are summarized in Table 3.

As shown in Table 3, when cold rolling is performed at a cold rolling reduction of 50% or less, or when the cumulative rolling reduction to the target thickness exceeds 50%, intermediate annealing is performed before cold rolling. When cold rolling was repeated at a rate of 50% or less, a cold rolled steel strip could be produced without edge breakage. On the other hand,
When the cold rolling rate exceeded 50%, edge breakage occurred and the product was defective.

Table 4 shows the rolling results when the hot rolled steel strip shown in 1 was trimmed and cold rolled after annealing the trimmed surface, and when cold rolling was performed after trimming and without annealing. It is something that

As seen in the results, when the trimmed surface was softened and annealed, it was possible to cold-roll to the target gauge without edge breakage, but when it was not annealed, edge breakage occurred.

[Brief explanation of the drawing]

Figure 1 shows the relationship between test temperature and cross-sectional shrinkage when boron-containing austenitic stainless steel is subjected to a high-temperature tensile test, and Figure 2 shows the cold rolling reduction, Charpy impact value, and Vickers hardness of boron-containing austenitic stainless steel. FIG. 3 is a diagram showing changes in hardness depending on whether or not the trimmed surface is annealed when a boron-containing austenitic stainless steel strip is trimmed.

Claims (3)

[Claims] (1) A slab or steel ingot of austenitic stainless steel containing 0.2 to 1.2% by weight of B is heated to a temperature of 1150°C or higher and 1200°C or lower, and then rolled using a rough rolling mill and a finishing rolling mill. A method for producing a boron-containing austenitic stainless steel strip by hot rolling at a temperature of 1000°C or higher. (2) A slab or steel ingot of austenitic stainless steel containing 0.2 to 1.2% by weight of B is heated to a temperature of 1150°C or higher and 1200°C or lower, and then rolled using a rough rolling mill and a finishing rolling mill. Hot rolling is carried out at a temperature of 1000°C or higher, and the resulting hot rolled steel strip is cold rolled once or multiple times with intermediate annealing at a cold rolling rate of 50% or less each time. A method for producing boron-containing austenitic stainless steel strips by repeated cold rolling. (3) The method for producing a boron-containing austenitic stainless steel strip according to claim 2, wherein the hot-rolled steel strip is trimmed, the trimmed surface is annealed, and then cold-rolled.