JPH0369967B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a thick steel plate that has excellent low-temperature toughness and has few internal defects. (Prior art) Conventional controlled rolling methods aimed at improving toughness involve suspending rolling after completing rough rolling in a high temperature range and waiting for the temperature of the entire rough rolled material to drop to the austenite non-recrystallization temperature range. A method of finishing rolling is used, and the temperature drop during this process is due to natural cooling, so the waiting time for the temperature to drop is long. During this cooling, the austenite crystal grains grow and become coarser, resulting in the formation of mixed grains after finishing rolling. In some cases, the toughness was impaired. In addition, cooling by cooling causes only a slight temperature distribution in the thickness direction of the rough-rolled material, which has the disadvantage that internal defects at the center of the thickness cannot be reduced during finish rolling. In addition, as a method of interrupting rolling and performing forced cooling, a method described in Japanese Patent Publication No. 49-7293 has been proposed, but in this method, the roughly rolled material is forcedly cooled to near the transformation end temperature, and then reheated. This is a method of finishing rolling after making the temperature of the rough rolled material uniform, and is intended to refine the austenite grains through transformation through cooling and reheating. This method can be expected to improve toughness to some extent through grain refinement, but it cannot reduce internal defects in the center of the plate thickness because the rough rolled material is reheated to achieve a uniform temperature distribution and then finished rolled. . Furthermore, since reheating is required, there is a problem in that the energy required for manufacturing increases. (Problems to be Solved by the Invention) The present invention provides a method for producing a thick steel plate with excellent low-temperature toughness that can eliminate the drawbacks of the conventional methods as described above. It is an object of the present invention to provide a method for manufacturing a thick steel plate that can prevent coarsening and reduce internal defects in the center of the steel plate. Further, the present invention provides a method of actively creating a temperature difference between the surface layer and the center in the thickness direction of the rough rolled material after rough hot rolling a steel billet or steel ingot and before entering the finish rolling mill. It is something. (Means for Solving the Problems) The gist of the present invention is to heat a steel billet or steel ingot to a temperature range of Ac ₃ or higher and perform rough hot rolling, and then to process the resulting rough hot rolled Immediately Ar ₃ points +
By forced cooling to a temperature range of 100°C or lower and Ar ₃ points or higher, the surface layer of the rough hot rolled material is cooled and hardened, and a temperature difference is applied in the thickness direction to form an austenite single phase. The present invention provides a method for producing a thick steel plate with excellent low-temperature toughness, characterized in that finishing hot rolling is started in a state in which the steel plate is finished hot rolled. The present invention will be explained in detail below. In carrying out the present invention, first, a steel billet or steel ingot is heated to Ac ₃ or higher to austenite, and then rough rolled.
The amount of reduction in rough rolling is determined by the amount of reduction required in the subsequent finish rolling, and is usually 0 to 80%. After rough rolling, the rough rolled material is forced to cool down to Ar ₃ points +
It is cooled to a temperature range below 100℃ and above ₃ points of Ar. Here, forced cooling means cooling at a higher cooling rate than natural cooling. This cooling method may be any method such as jet cooling or immersion cooling, and any refrigerant such as water, gas, or molten salt may be selected. If the above forced cooling end temperature exceeds Ar ₃ +100°C, the high finish rolling temperature causes partial recrystallization of austenite crystal grains and generates mixed grains, which causes deterioration of toughness. The lower limit of forced cooling end temperature is
Ar: ₃ points. In this way, the structure of the steel is maintained as a single austenite phase. Rough-rolled material that has undergone forced cooling has a larger temperature distribution in the thickness direction than when it is left to cool, and the surface layer of the rough-rolled material is a low-temperature area and is hard, while the center is a high-temperature area. Because of this, it has become softer. If finish rolling is started with a large temperature difference without reheating such austenitic single-phase steel, the rolling deformation resistance will be large at the surface layer and small at the center, so rolling stress during finish rolling will be concentrated in the center. However, defects specific to the center of the plate thickness, such as dents, are crimped and the internal quality of the center is improved. In addition, the concentration of the effect of rolling at the center of the plate thickness also leads to refinement of the structure after transformation from a metallurgical perspective. From the viewpoint of increasing the temperature difference between the surface layer and the center and preventing coarsening of austenite crystal grains, which are the objects of the present invention, the higher the forced cooling rate, the better. When the thickness of rough rolled material is up to 50mm, the temperature is 3℃/
sec or more, preferably 6℃/sec or more, 50mm or more
If the diameter is less than 100 mm, a cooling rate of 2°C/sec or more is suitable for actual operation, and if it is 100mm or more, a cooling rate of 0.6°C/sec or more is suitable for actual operation. Regarding the distribution of rolling reductions for rough rolling and finishing rolling,
As mentioned above, the thicker the rough rolled material during forced cooling, the more likely it is that temperature distribution will occur due to forced cooling, so it is desirable that the reduction ratio in finish rolling be 40% or more. A more preferable finish rolling reduction is 60% or more, and if the rolling capacity permits, the rough reduction may be 0%, that is, the entire rolling may be performed after forced cooling. As mentioned above, in the present invention, it is necessary to perform finish rolling while maintaining the temperature distribution between the surface layer and the center, so no reheating is performed after the start of rolling, and after forced cooling, Finish rolling will continue. Further, in the present invention, any conventional treatment such as standing cooling, controlled cooling, quenching, and tempering may be performed after finish rolling, and may be appropriately selected depending on the type of steel to be obtained. Note that since the present invention aims at preventing grain coarsening in the austenitic state and improving internal quality through finish rolling, there is no particular restriction on the composition range of the target steel to which the present invention is applied. That is, in the present invention, C: 0.3 wt% or less, Si: 0.5
%wt% or less, Mn: 3.0wt% or less, Al: 0.005wt%
It is effective for steels with chemical compositions of thick steel plates that are normally manufactured and contain less than 0.2 wt% of Fe, with the balance consisting of Fe and unavoidable impurities. In addition, Cu, Ni, Cr, Mo,
Of course, Nb, Ti, V, etc. may be added. (Example) Next, the present invention will be described based on an example. First, the case where the present invention and the comparative method shown in Table 2 are applied to test steels having typical components shown in Table 1 will be described. Comparative examples in Table 2, namely test steels A, D, F,
H indicates that forced cooling is not applied.

【table】

[Table] The _three Ar points for each sample steel in Table 2 are as follows. Test steel A: 750°C, Test steel B: 712°C, Test steel C: 685°C, Test steel D: 765°C, Test steel E: 718°C, Test steel F: 755°C, Test steel Steel G: 712°C, Test Steel H: 755°C, Test Steel I: 728°C, Test Steel J: 712°C Table 3 shows the mechanical properties of the steel manufactured by the manufacturing method shown in Table 2. show.

【table】

[Table] Next, test steels having various components shown in Table 4 were rolled under the rolling conditions of the present invention and comparative method shown in Table 5. In the present invention, immediately after the rough rolling was completed, forced cooling was performed, and then finish rolling was started in a single austenite phase state. On the other hand, in the comparative method, after rough rolling was completed, finish rolling was started after natural cooling.

【table】

[Table] By cooling Table 6 shows the mechanical properties of the steels shown in Table 4 produced by the manufacturing methods shown in Table 5.

【table】

[Table] (Effects of the Invention) As is clear from the above examples, the steel whose rough rolled material was subjected to forced cooling exhibits better elongation and toughness than the comparative steel, and also has a higher yield stress. As a result, the effect of preventing coarsening of austenite crystal grains and reducing internal defects due to forced cooling during rough rolling is clearly recognized. Further, according to the present invention, the waiting time for temperature reduction between rough rolling and finish rolling during controlled rolling is significantly shortened, and productivity is also dramatically improved.

Claims (1)

[Claims] 1. After heating a steel billet or steel ingot to a temperature range of Ac ₃ points or higher and rough hot rolling, the obtained rough hot rolled material is immediately heated to a temperature of Ar ₃ points + 100℃ or lower and at a temperature of Ar ₃ points or higher. By forcing the surface layer of the rough hot-rolled material to cool and harden until the material reaches A manufacturing method for thick steel plates with excellent low-temperature toughness.