JPH0347921B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial application field) Direct rolling (hot rolling of a steel billet immediately after ingot making, especially continuous casting) or hot charge rolling (ingot making, especially hot rolling of a steel billet immediately after continuous casting) (charging and then hot rolling), it is possible to improve productivity and reduce thermal energy consumption by effectively preventing the occurrence of surface cracks that often occur during hot rolling. A hot rolling method including useful steps is proposed here. When manufacturing steel materials, the molten steel is made in a converter or electric furnace, turned into slabs by the ingot-forming method or continuous casting method, and then left to cool, and then subjected to maintenance such as removing surface defects in the cold. In this example, the material is heated, charged into a heating furnace, and then subjected to hot rolling. On the other hand, in recent years, from the perspective of energy conservation, hot slabs after ingot making or continuous casting are directly subjected to hot rolling without being allowed to cool, or are rolled through a heat retention furnace, and directly manufactured into products. Or a process was developed to make it into steel billets. However, in this case, due to the cold surface treatment process and the structure improvement accompanying the passing of the transformation point during the cooling-reheating process, the generation of surface defects due to hot rolling does not pose a particular problem in the conventional method. On the other hand, in direct rolling or hot charge rolling, it is difficult to ensure perfect hot treatment and surface defects often occur. (Prior art) Various methods have been devised to solve this problem.
Rolling is performed at least twice with a reduction rate of 15% or more per pass in the temperature range of 1300 to 1150℃, and then
Although it is disclosed that the second rolling is performed in a temperature range below 1150°C, the following drawbacks remain. That is, the above conditions are intended to prevent the occurrence of hot cracking in anticipation of changes in the grain boundary precipitation form of elements originally harmful to hot workability and refinement of austenite grains. However, the surface of the steel slab immediately after ingot formation is usually rough with hot water wrinkles, etc., and continuously cast slabs have so-called oscillation marks due to the vertical vibration of the mold, and both have many unevenness. 1300 without pre-maintaining the slab in this condition.
If a reduction is applied at a temperature of ~1150°C and a reduction rate of 15% or more, the tensile stress at the above-mentioned irregularities, especially at the recesses, becomes excessive and becomes the starting point of cracks, and minute defects also grow and develop into cracks. 15% in this state
If the steel is continuously rolled at the above rolling reduction ratio, the cracks will only continue to expand. Cracks that occur during hot working are mainly caused by precipitates of harmful elements, such as (Fe, Mn) O, (Fe, Mn) S, (Fe, Mn), at austenite grain boundaries.
It is widely known that P, BN, Nb (C, N), AlN, etc. precipitate, significantly reducing grain boundary strength and becoming the main path for cracking. Therefore, even if the austenite grains are made finer by recrystallization, harmful precipitates still exist at the grain boundaries, and this is not a fundamental countermeasure to prevent intergranular cracking in the surface layer. Therefore, it is difficult to completely eliminate hot cracking. (Problems to be Solved by the Invention) The earliest stage of rolling is prone to surface cracks due to the solidified structure, and the presence of minute flaws can lead to surface defects in the final product, causing many problems. . From this point of view, the inventors conducted detailed research on the mechanism and preventive measures for the occurrence of surface cracks in hot slabs during direct rolling and hot charge rolling, and found that the occurrence of surface cracks during hot rolling under specific rolling conditions. It has been discovered that it can be completely prevented. (Means for Solving the Problems) This invention provides at least one light reduction rolling at 1300 to 1000°C as an initial stage of rolling, with a reduction rate of 3 per pass during direct rolling or hot charge rolling of steel materials.
% or more and less than 15% and a total reduction of less than 15% over two or more times, and then proceeding to normal rolling. It is. (Function) As mentioned above, the surface layer of hot pieces subjected to direct rolling or hot charge rolling has unevenness, minute cracks,
Cracks and blowholes just below the surface are often present. A steel billet in this state is heated to 1300 to 1000℃ for 3 passes per pass.
% or more and less than 15%, or if the total rolling reduction is limited to less than 15% even when performing two or more times, the tensile stress affecting surface irregularities and minute defects is slight. Therefore, the degree of stress concentration is low, and cracks do not grow. On the contrary, minute defects are compressed without opening in the surface layer, and the defects are eliminated. Furthermore, as an advantage of light reduction, in many steel materials, in the temperature range of 1300 to 1000℃, the reduction is 15% per pass.
%, the austenite grains elongate without recrystallizing, and processing strain accumulates within the grains. In other words, precipitates generally precipitate along grain boundaries, subgrain boundaries, transformation zones, and dislocation lines, so for austenite grains with working strain, so-called strain-induced precipitates occur in large numbers within the grains in addition to the grain boundaries. This is because it causes precipitation. Hot embrittlement mainly increases cracking susceptibility due to the precipitation of harmful elemental precipitates at austenite grain boundaries, whereas the precipitates that precipitate within the grains are fine and are consistent with the austenite base. Because of the high
Does not induce hot cracking. In steel that has alloying elements that precipitate at grain boundaries, it is difficult to avoid the risk of cracking during hot working even if recrystallization progresses, but in the case of intragranular precipitation due to strain induction as described above. In this case, it is possible to arbitrarily perform hot working in the precipitation temperature range. Since strained precipitation progresses in the unrecrystallized state of austenite, in order to use the above method advantageously, it is necessary to control the rolling reduction rate so that recrystallization does not proceed until the harmful precipitation ends. be. The critical pressure reduction required to cause recrystallization varies significantly depending on chemical composition, processing temperature, etc.
In the temperature range of 1,000 to 1,300°C, recrystallization does not occur at a total rolling reduction of less than 15%, and the product is in a completely unrecrystallized state. It was set at less than 15%. Next, the reason why the lower limit of the light rolling reduction rate is set at 3% is 3%.
Under a light pressure of less than 1, the processing strain does not accumulate within the grains but concentrates at the grain boundaries, and the strain is released in the form of grain boundary movement, so-called strain-induced grain boundary movement. As a result, hot embrittlement is induced due to coarsening of austenite grains and grain boundary precipitates. In this respect, it is necessary to exclude the application of a reduction ratio of less than 3%. Here, this light reduction shows a remarkable effect in reducing hot embrittlement by performing it once, but there is no problem even if the light reduction rate rolling is applied in several times, and in fact, it can be introduced. This is more desirable in terms of uniformity of strain. However, at this time, the rolling reduction rate per roll should be 3% or more,
In addition, as mentioned above, it is necessary to keep the total rolling reduction ratio when dividing into multiple steps to less than 15%. Next, in the method of this invention, the temperature range for applying light pressure was limited to 1300 to 1000°C, but the upper temperature limit was set to 1300°C.
℃ is used for steels containing alloying elements with low melting points.
This is because at temperatures above 1300℃, some of the grain boundaries melt and the grain boundary strength decreases significantly, which often causes slab cracking when hot working is performed.The lower limit was set at 1000℃. The reason for this is that when controlling the shape of large slabs, the deformation resistance becomes too high at temperatures below 1000℃, and it becomes difficult to secure the number of rolling passes required to achieve the specified dimensions, as well as operational reasons. , harmful precipitates that precipitate at relatively high temperatures below 1000℃, such as (Fe, Mn) O, (Fe, Mn) S,
This is because (Fe, Mn) P, BN, Nb (C, N), etc. are mostly precipitated and the feature of the present invention due to the formation of intragranular precipitates can no longer be fully exhibited. (Example) Steels A and B having the chemical compositions shown in Table 1 were melted in a converter (300 tons), and the cross-sectional size was melted in a continuous casting machine.
After being cast into a bloom of 400 x 560 mm, it was immediately cut and hot-rolled under the heating and rolling conditions shown in the pattern shown in Figure 1 to obtain a steel billet.

[Table] Table 2 summarizes the relationship between the frequency of cracking on the surface of the steel slab and the initial rolling conditions. The frequency of occurrence of cracks on the surface of the steel slab was calculated by visually observing the entire surface of the steel slab after hot rolling and counting. In Table 2, samples No. 1 to No. 12 were hot-rolled by direct rolling from blooms cut after continuous casting, contrary to the heat pattern shown in Figure 1 for hot charge. Same No.13～32
The cut bloom is once cooled to 900 to 800°C, charged into a heating furnace, heated to rolling temperature, and rolled, which is called hot-rolling.

【table】

[Table] No.1, No.5-6, No.8-No.13, No.17-21, No.25
-26 and Nos. 28-32 are all comparative examples. Although there are some differences depending on the manufacturing conditions, the frequency of cracking on the surface of the steel billet is at a high level in all cases. On the other hand, according to this invention method, No. 2 to 4, No.
7. As in the examples of Nos. 14 to 16, Nos. 22 to 24, and No. 27, the occurrence of cracks on the surface of the steel piece is extremely small.
Note that these effects apply to steels A and B, which have different chemical compositions.
It is clear that there is no distinction between (Effects of the Invention) The present invention is useful for effectively avoiding hot cracking, which has hitherto been unavoidable in direct or hot charge rolling of steel materials and for which no effective prevention measures have been taken.

[Brief explanation of drawings]

FIG. 1 is a diagram of the manufacturing process of steel materials.

Claims (1)

[Claims] 1. During direct rolling or hot charge rolling of steel materials, as an initial stage of rolling, at least one light reduction rolling is carried out at a temperature range of 1300 to 1000°C, and the reduction per pass is 3% or more to 15%. 1. A method for hot rolling steel materials with less occurrence of surface cracks, characterized in that the total rolling reduction is less than 15% even in two or more times, and then normal rolling is carried out.