JPS636612B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing non-thermal treated steel, and more specifically, the present invention relates to a method for producing non-thermal treated steel, which has high strength and toughness by appropriately combining hot rolling conditions and accelerated cooling conditions after rolling and dehydrogenation treatment. It seeks to provide law. In recent years, as steel structures have become larger and their operating conditions have become more severe, demands on steel materials used in terms of weldability, low-temperature toughness, strength, etc. have become far more advanced than in the past. As a way to simultaneously satisfy these demands,
Proposed methods include lowering the carbon equivalent by adding alloying elements such as Cu and Ni, controlled rolling mainly in two-phase region rolling, and accelerated cooling after grain refinement. However, alloy addition and controlled rolling have intolerable drawbacks in terms of economy and productivity, while accelerated cooling has the drawback of impairing the internal quality of the steel. The present invention was created through repeated various experiments and considerations in order to solve the above problems.
The gist of this is that after heating the steel to Ac ₃ points or higher, it is hot-rolled to form unrecrystallized austenite grains, then accelerated cooling is performed at a cooling rate of 1°C/sec or higher, and the center of the plate thickness is Hydrogen Hc in steel
The present invention relates to a method for producing non-tempered steel, which is characterized by performing dehydrogenation treatment such that Hc (ppm) becomes Hc (ppm) 1.10-0.005σy, thereby maintaining high strength and excellent toughness. [However, σy: Yield stress of steel plate at room temperature (Kg/mm ² )]. In the final ultrasonic flaw detection test (UST) of products manufactured by accelerated cooling, there are rare cases where products fail to pass, and we investigated the cause of this in detail and found that products of various strength levels In relation to the amount of hydrogen in the steel at the center of the plate thickness,
The present invention was completed by discovering that UST defects can be sorted out. In other words, the relationship is shown in Figure 1, and the hydrogen in the steel at the center of the plate thickness is the first in the relationship with the yield stress σy (Kg/mm ² ) of the steel plate at room temperature.
As shown in the figure, it was found that by satisfying the condition of Hc (ppm) 1.10-0.005σy, it was possible to prevent the formation of minute internal cracks caused by hydrogen in the steel, and as a result, it was possible to prevent ultrasonic flaw detection defects [in the figure, 〇 means the product passed (this invention), ● means the product failed (conventional method)].
When a steel plate is acceleratedly cooled immediately after hot rolling in order to simultaneously achieve high strength and high toughness as in the present invention, hydrogen in the steel is hardly released. Therefore, if the hydrogen content in the steel is high before hot rolling, the internal quality of the steel sheet will be damaged by defects caused by hydrogen, which will be detected as ultrasonic defects. Next, the reason why the hot rolling conditions are limited in the present invention will be explained. The heating temperature is set to Ac ₃ or above for solutionization.
In the present invention, accelerated cooling is performed to increase the strength. However, if the austenite grains during transformation are coarse, a transformed structure with deteriorated toughness will occur. Therefore, controlled rolling is performed to make the austenite grains finer before transformation. Toughness is improved by rolling in a non-recrystallized temperature range. In addition, since sufficient strength and toughness cannot be obtained with accelerated cooling from the two-phase region, it is necessary to finish rolling at the Ar point of ₃ or more and accelerate cooling without delay before the unrecrystallized austenite recrystallizes. Next, if the accelerated cooling rate between Ac ₃ points and 500℃ is less than 1℃/sec, it will be the same as air cooling depending on the plate thickness, and there is no meaning in accelerated cooling. 5-30℃/ to obtain toughness
sec is preferred. On the other hand, the reason why the permissible limit of hydrogen Hc (ppm) in the steel at the center of the plate thickness depends on the yield stress σy of the steel plate is that the hydrogen sensitivity is a function of σy through the components, and furthermore, the micro hydrogen concentration due to residual stress etc. This is thought to be because σy is a function of σy. However, the hydrogen Hc (ppm) in the steel at the center of the plate thickness is
This suppresses Hc (ppm) to 1.10−0.005σy. The hydrogen in the steel can be removed by conventionally known methods, such as (a) vacuum degassing of molten steel, (b) slow cooling of steel slabs,
(c) Soaking and diffusion of the steel billet, (d) Thickness reduction (breakdown, sizing) of the steel billet and subsequent dehydrogenation slow cooling, (e)
One or more of these methods, such as dehydrogenation during rolling of a thick plate and (f) hydrogen release during cooling of a steel plate, may be carried out in combination. Next, examples will be given. Continuously cast slabs having the components shown in Table 1 were subjected to controlled rolling and accelerated cooling shown in Table 2 to produce thick steel plates with a thickness of 32 mm. The mechanical properties and UST results after cooling are also shown in Table 2.

【table】

[Table] As is clear from Tables 1 and 2, A and B obtained by the method of the present invention have high strength without losing toughness even after accelerated cooling, and are internally sound due to the limited hydrogen content in the steel. We were able to produce a steel plate with excellent properties.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the relationship between yield stress of a steel plate, hydrogen in the steel at the center of the plate thickness, and ultrasonic flaw detection results.

Claims (1)

[Claims] 1 Steel heated to Ac ₃ points or higher and then hot rolled to form unrecrystallized austenite grains 1
With accelerated cooling at a cooling rate of ℃/sec or more, the hydrogen Hc (ppm) in the steel at the center of the plate thickness decreases to Hc (ppm).
A method for manufacturing non-tempered steel characterized by dehydrogenating the steel to a temperature of 1.10−0.005σy to maintain high strength and excellent toughness. However, σy: Yield stress of steel plate at room temperature (Kg/mm ² )