JPS6155571B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a steel billet for obtaining a sound hot coil surface quality of high-alloy austenitic stainless steel. Conventionally, high-alloy austenitic stainless steel containing 16-26% Cr and 12-22% Ni
When producing steel slabs by blooming a steel ingot with the above components using the ingot-forming method, which is soaked at about 1280℃, cracks occur on the surface of the steel slab, resulting in a significant decrease in the yield of good products. Ta. Continuously cast slabs (hereinafter referred to as continuous cast slabs)
Even when sizing is performed using a blooming mill, the conventional method causes large cracks on the surface of the slab, making it impossible to feed the slab to the hot rolling process. Even if steel slabs could be produced, the yield was extremely low and this method was not satisfactory from an industrial standpoint. Furthermore, in the conventional method, REM,
There is a method of adding components such as Ca to prevent cracking, but satisfactory results have not been obtained. In order to prevent the above-mentioned cracks, the present inventors conducted various investigations and found that in the conventional rolling method, the surface temperature of the steel ingot or slab decreases as rolling progresses, and the above-mentioned high temperature 1000, which is the region where the hot deformability of alloy austenitic stainless steel decreases.
It was found that the rolling process had to be carried out at a surface temperature of around °C, which caused cracks to occur. The present invention aims to prevent cracks that occur during rolling such as blooming in a method for producing high-alloy austenitic stainless steel slabs containing 16 to 26% Cr and 12 to 22% Ni. REM for,
It is an object of the present invention to provide a method for manufacturing the steel billet, which does not require additives such as Ca, and can prevent cracking while leaving equipment as is. That is, the gist of the present invention is as follows. In a method of producing a steel slab by soaking a steel ingot or continuously cast slab of high-alloy austenitic stainless steel containing Cr: 16 to 26% and Ni: 12 to 22%, the steel ingot is Or slabs
A method for producing a high-alloy austenitic stainless steel billet, which comprises soaking at 1100°C to 1300°C and then starting rolling at a surface temperature of 950°C or lower and 800°C or higher. The present invention will be explained in detail below. In the case of steel ingots, the cracks that occur during conventional rolling are transverse cracks at the center of the steel ingot surface, as shown in Figure 1a, and during blooming rolling (during sizing processing) of continuous cast slabs. This crack is also a horizontal crack in the center of the slab surface, as shown in Figure 1b. That is, in both steel ingots and continuously cast slabs, cracks are concentrated at the center of the slab surface, and are relatively less likely to occur at the corners and edges. High-alloy austenitic stainless steel has significantly lower deformability in hot water than carbon steel.
Furthermore, as the hot working temperature decreases, the deformation resistance increases rapidly, resulting in poor rolling properties. So usually,
Steel ingots or continuously cast slabs are heated to 1,100 to 1,300°C, soaked, and rolled, but the surface temperature drops during transportation from being extracted from the heating furnace or soaking furnace to being bitten by rolling rolls. do. According to actual measurements, the surface temperature of the central part of the steel slab or continuously cast slab at this time had fallen to around 1000℃, which is the region where the hot deformability decreases as described above, and therefore cracks did not occur after rolling. It has occurred. However, the surface temperature of the corners and edges of the steel ingot or continuously cast slab is lower than that of the center, and
Even though the temperature is ~900℃ or lower, there are few cracks. Focusing on this seemingly contradictory phenomenon, we repeated experiments to investigate the cause, and found that high-alloy austenitic stainless steel containing 16% to 26% Cr and 12% to 22% Ni
The present inventors have found that the deformability decreases significantly between 1100°C and 950°C, but when the temperature becomes lower than 950°C, the deformability shows signs of recovery, and becomes considerably large between 900°C and 850°C. FIG. 2 shows the relationship between the deformability and temperature of the stainless steel. Based on this knowledge, only the surface of the stainless steel ingot or continuously cast slab that has been heated and soaked to 1100℃ to 1300℃ is cooled by air or water cooling, and the surface temperature is lowered to 950℃.
The creation of the present invention was based on the idea that cracking can be prevented by rolling at temperatures below .degree. At this time, the center of the steel ingot or continuously cast slab is 1100
Uniformly heated between ℃ and 1300℃, surface temperature is 950℃
It is natural that there is a temperature range of 1000℃ to 1100℃ where the deformability is extremely small, and it is assumed that internal cracks will inevitably occur during rolling. Even if it were to occur, it was found that because it did not come into direct contact with the surrounding atmosphere, it would become firmly adhered again by the huge rolling force and would not become a defect. In the present invention, the upper limit of the surface temperature of the steel ingot or continuously cast slab is set at 950°C before the deformability has been recovered, because when it is rolled, it comes into contact with rolling rolls cooled by roll coolant. This is because the surface temperature drops slightly instantaneously and enters the deformability recovery temperature range, and this was also proven by the examples described below. The reason why we set the lower limit surface temperature to 800℃ or higher is because if the temperature is lower than this, even if the internal temperature is 800℃.
This is because, even if the temperature exceeds .degree. C., the deformation resistance increases significantly and rolling becomes impossible. In addition, if the steel ingot or continuous cast slab is heated to less than 1100℃ and rolled after being soaked, taking into account the temperature drop during transportation, it is expected that surface cracks will not occur, but the steel ingot or continuously cast slab is Deformation resistance increases overall,
Since it requires a large rolling force, it is undesirable from an industrial standpoint. The reason why cracks occur when the deformability decreases is thought to be because there is a large difference in strength between grain boundaries and within grains in the steel crystal structure, and the grain boundaries slip during rolling, leading to cracks. Therefore, as mentioned above, cracks occur when a high-alloy austenitic stainless steel ingot that still has a cast structure is heated and rolled (when a steel ingot is bloomed, or when a continuously cast slab is sized) )
However, when a steel piece that has been rolled and has changed to a rolled structure is reheated to 1100°C to 1300°C and rolled again, no noticeable cracks occur and the cracks are only cracked by finishing the surface with a grinder. It fits. As mentioned above, the continuously cast slab is charged as it is, that is, with the cast structure still in place, into a heating furnace of a hot strip rolling line or a plate rolling line, and heated to 1100°C or higher.
When heated and rolled to 1300°C, cracks occur significantly, so according to the present invention, the continuously cast slab may be air-cooled or water-cooled to bring the surface temperature to 950°C or lower and 800°C or higher, and then rolled. If the finishing rolling temperature becomes too low and rolling becomes impossible, roll the continuously cast slab at a surface temperature of 950°C or lower or 800°C or higher to change the structure of the continuously cast slab to a rolled structure, and then reheat it. Finish rolling may be performed to the final thickness. Next, the Ni of the steel ingot or continuously cast slab of the high-alloy austenitic stainless steel targeted in the present invention
Table 1 shows the reasons why the Cr content, the heating temperature of the steel ingot or continuous cast slab, and the surface temperature at the start of rolling were limited as described above.

[Table] Examples of the present invention will be described below in comparison with comparative examples using conventional methods. Example 1 A steel billet with a thickness of 130 mm and a width of 1065 mm was manufactured from a SUS310S steel ingot with an upper section of 1232 mm x 555 mm, a lower section of 1160 mm x 440 mm, and an ingot height of 1850 mm.
The chemical composition of the seven steel ingots at the same charge is shown in Table 2.

[Table] After soaking the above seven steel ingots at 1280℃ for 7 hours, in Comparative Example 1, steel ingots were soaked from four steel ingots A, B, C, and D by the conventional method, that is, from a soaking furnace. After extraction, immediately feed the steel ingot to a blooming mill and carry out normal blooming rolling without temperature regulation to obtain a size of 130mm x 1065mm.
Finished in a piece of steel. At this time, in Comparative Example 1, the surface temperature at the start of rolling was 1150°C, and the surface temperature after rolling was 1010°C. In Example 1 of the present invention, three steel ingots E, F, and G were extracted from the soaking furnace and then placed on the table line for 10 minutes.
After cooling for a minute and the surface temperature reached 980℃, normal blooming rolling was carried out in the same manner as in Comparative Example 1.
Finished in a 1065mm steel piece. As a result of the above double-sided implementation, the surface of the steel billet manufactured in Comparative Example 1 had cracks in the direction transverse to the rolling direction on almost the entire surface, but the surface of the steel billet manufactured in Example 1 had no cracking. There were no outbreaks. Then, for all the billets produced by both sides,
After shearing the crops at both ends of the billet and cooling it,
The steel pieces were ground by a self-propelled grinder in the cold. Comparing these single steel handling losses in Example 1 and Comparative Example 1, the average value of steel single handling loss (%) in Example 1 was 6.8% higher than that in Comparative Example 1.
It was also recognized that there were few As a result, it is clear that according to the present invention, it is possible to improve the steel handling loss. Examples 2 to 5 Experiments No. 1 to No. 3 were conducted to compare the effects of the present invention and the conventional method in the case of producing steel slabs by sizing continuous cast slabs by blooming rolling. I did it. The dimensions of the continuously cast slab in each experiment were 200 mm x 1140 mm, which were bloomed into steel slabs of 135 mm x 1085 mm. Comparative experiments were conducted for each casting unit. Third
In the table, the content and results of the experiment are shown by comparing the example and the comparative example for each experiment number.

[Table] In Experiment No. 1 in Table 3, after soaking the slab at 1250°C for 7 hours to produce steel slabs from the slab, after extracting the slab from the soaking furnace, Comparative Example 2 Then, the pieces were immediately fed to a blooming mill, and four pieces each were rolled in a normal manner. At this time, the average slab surface temperature at the start of rolling is
Although the temperature was 1080°C, cracks were generated on the entire surface in the direction transverse to the rolling direction, and not a single good product could be collected. In Example 2, the slab surface temperature at the start of rolling was set to 950.
The same rolling as in Comparative Example 2 was carried out except that the temperature was 0.degree. As a result, there were no cracks on the surface of the steel slab, and the entire surface was ground with a self-propelled grinder.
The average value was 3.0%, which was good. In Experiment No. 2, in Comparative Example 3, the slab was soaked at 1250°C for 5 hours, further soaked at 1300°C for 3 hours, and the slab was immediately fed to the blooming mill and rolling started at a high surface temperature of 1104°C. A slab of the above dimensions was rolled into a steel slab of the above dimensions using normal rolling. As a result, as shown in FIG. 1b, cracks occurred over almost the entire length of the center in the width direction, making it impossible to collect non-defective products. On the other hand, in Example 3, the rolling start temperature was
The temperature was lowered to 862°C, and rolling was carried out in the same manner as in Comparative Example 2. As a result, there was no cracking, and the steel piece 6
The average maintenance loss during full-surface grinding using a self-propelled grinder was 3.2%, which was good. In addition, in Comparative Example 3', heating and soaking conditions exceeding the upper limit temperature of 1300°C for soaking in the present invention, i.e.
After heating at 1250°C for 4 hours and then at 1320°C for 2 hours, the rolling start temperature was set at 950°C, resulting in surface cracking and a 6.2% maintenance loss. Experiment No. 3 was carried out to compare the conventional method and the present invention, both of which perform low-temperature soaking. In Comparative Example 4, after soaking at 1150° C. for 15 hours, the slabs were immediately fed to a blooming mill and two slabs were rolled normally. The surface temperature conditions at the start of rolling were as listed in Table 3, but it was not possible to prevent surface cracks. The average maintenance loss for full-surface grinding using a self-propelled grinder was 6.7%. In Example 4, under the same soaking and rolling conditions as in Comparative Example 4, only the surface temperature at the start of rolling was adjusted, and as a result, a steel billet without cracking could be manufactured, and the results were 3.1
A good product was obtained with a grinding loss of %. Furthermore, Table 4 shows the cases in which steel slabs are manufactured by sizing a continuous cast slab having a composition other than the composition range of the continuously cast slab in the present invention by blooming rolling, and the results are shown in Table 4 for the case where a steel slab is manufactured by sizing by blooming rolling. A comparison is shown between Comparative Example 5 and Example 5 of the present invention. The dimensions of the continuously cast slabs in Table 4 were 200 mm x 1310 mm, which were bloomed into slabs of 140 mm x 1270 mm.

[Table] In Comparative Example 5, when producing steel slabs from the slabs, the slabs were soaked at 1250°C for 7 hours, and then the slabs were extracted from the soaking furnace and immediately fed to a blooming mill. Then, normal rolling was performed on four sheets each. At this time, the average surface temperature of the slab at the start of rolling was 1030°C, and cracks appeared on the entire surface in the direction transverse to the rolling direction.
The depth of the cracks was deep, and the average loss (%) for full-surface grinding using a self-propelled grinder was 7.9%. In Example 5, the slab surface temperature at the start of rolling was set to 870.
The same rolling as in Comparative Example 5 was carried out except that the temperature was 0.degree. As a result, no cracking occurred on the surface of the steel slab, and the average maintenance loss (%) of the entire surface grinding using the self-propelled grinder was 2.7%, which was good. Note that the surface temperature was actually measured using a radiation thermometer. As described above, the details of the present invention have been described in comparison with the conventional method based on Examples and Comparative Examples, and it has become clear that the present invention can prevent cracking during rolling that occurs in the conventional method. Therefore, according to the present invention, since cracks do not occur during rolling, it is possible to stably produce good steel slabs and supply them to the hot rolling process, making it possible to significantly improve the yield of good products. Ta.

[Brief explanation of the drawing]

Fig. 1 is a perspective view schematically showing the state of occurrence of cracks in steel slabs produced by the conventional method, in which a shows a steel billet produced from a steel ingot, and b shows a steel billet produced from a continuously cast slab. It's a piece of steel. Figure 2 shows Cr: 16~
26% and a chart showing the relationship between deformability and temperature of high-alloy austenitic stainless steel containing 12 to 22% Ni.

Claims (1)

[Claims] 1. A method of producing a steel billet by soaking and rolling a high alloy austenitic stainless steel ingot or continuous casting slab containing 16 to 26% Cr and 12 to 22% Ni. After soaking the lump or slab to 1100℃~1300℃, the temperature is 800℃ below 950℃.
A method for producing a high-alloy austenitic stainless steel billet, the method comprising starting rolling at a surface temperature of ℃ or higher.