JPH01111815A - Production of tough direct rolled thick steel plate - Google Patents

Abstract

PURPOSE:To produce a tough direct rolled thick steel plate at a low cost by subjecting a hot steel ingot having a specific compsn. consisting of C, Si, Mn, Al and Fe directly to specific rolling and ending the hot rolling at the temp. above the Ar3 point. CONSTITUTION:The steel having the compsn. which contains 0.05-0.18wt.% C, 0.05-0.40% Si, 0.3-1.80% Mn, and 0.005-0.05% Al, contains further >=1 kinds among <=0.05% Nb, <=0.05% Ti, <=1.0% Cu, <=1.5% Ni, <=0.5% Mo, <=1.0% Cr, <=0.10% V, and <=0.0025% B at need, consists of the balance Fe and unavoidable impurities, and in which the carbon equiv. of C+Si/24+Mn/6 is <=0.35 is continuously cast. The resulted hot ingot is directly rolled or the rolling thereof is started after the ingot is subjected to holding and heating to about the degree at which the surface temp. is increased to the same temp. as the central temp. The total reduction ratio (slab thickness/finish sheet thickness) is set at >=4 and the rolling rate in the recrystallization region at >=50%. The hot rolling is ended at the temp. above the Ar3 point and thereafter, the sheet is cooled with water down to <=600 deg.C at need, by which the direct rolled thick steel plate is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing direct rolled W and steel sheets having excellent toughness.

(Prior Art) Generally, methods for strengthening steel include methods such as solid solution strengthening, precipitation strengthening, and grain refinement. Among these, grain refinement is the only method that can strengthen steel without sacrificing toughness, and is a very effective means for toughening steel.

On the other hand, as part of energy-saving measures in the hot rolling process, the so-called direct rolling method involves starting hot rolling directly or after heating the cast hot slab directly or after heating the slab to a level that makes the surface temperature the same as the center temperature. is being developed.

The material-related issue when producing strong steel using the direct rolling method is how to obtain a fine transformed structure from the coarse austenite grains before rolling. In hot strip rolling, the reduction ratio of each bath is large, and austenite recrystallization is said to be mainly controlled by dynamic recrystallization behavior.

Furthermore, since the total rolling reduction ratio is as large as several 10 to 100, it is presumed that the coarse austenite grains before rolling, which are a characteristic of the direct rolling method, become similar to those of the conventional reheat rolling method due to rolling.

However, in the rolling of thick steel plates, the reduction ratio of each bath is small, and austenite recrystallization is thought to be mainly controlled by static recrystallization behavior. From the viewpoint of grain refinement, it is predicted that the direct rolling method is more disadvantageous than the conventional reheat rolling method.

Many direct rolling techniques are known.

For example, Japanese Patent Publication No. 56-24018, Japanese Patent Publication No. 5B-9
There are methods described in JP-A No. 812, JP-A-57-123927, and JP-A-60-56024.

However, these methods mainly apply direct rolling to hot strip rolling, and there are no detailed rolling conditions, and it is difficult to produce strong thick steel plates by direct rolling using only known techniques.

(Problems to be Solved by the Invention) The present invention eliminates these drawbacks and provides an r! i. The present invention aims to provide a method for manufacturing steel sheets.

(Means for Solving the Problems) The biggest metallurgical problem with the direct rolling method is how to make coarse austenite grains as solidified finer. Particularly in the case of manufacturing thick steel plates, it is essential to set appropriate rolling conditions because the total rolling reduction ratio and the rolling reduction ratio of each bath are small.

The present invention is characterized in that the rolling conditions are a total reduction ratio of 4 or more, a recrystallization zone rolling ratio of 50% or more, and hot rolling is completed at a temperature of Ar= point or higher. By performing such rolling, coarse austenite grains that remain solidified can be made as fine as those in conventional reheat rolling by repeating recrystallization at the end of recrystallization zone rolling. After rolling, either air cooling or water cooling can be used depending on the purpose.

The present invention will be explained in detail below.

First, the reasons for limiting the components of the present invention will be described.

C is effective for strengthening steel, and if it is less than 0.05%, effective strength cannot be obtained, while if it exceeds 0.18%, it deteriorates toughness and even weldability, so it should not be more than 0.05%. .1
8% or less.

St is effective as a deoxidizing element and a steel strengthening element, but if it is less than 0.05%, it has no effect, and if it exceeds 0.40%, it causes deterioration in workability and impairs the surface quality of the steel sheet.

Mn is effective in strengthening steel, but if it is less than 0.30%, it has no effect and if it exceeds 1.80%, workability deteriorates.

For the above three elements C+ Si / 24 soil Mn
If the carbon equivalent of /6 exceeds 0.35, weldability deteriorates, so it is limited to 0.35 or less.

Al is added as a deoxidizing element, but if it is less than 0.005%, it has no effect, and if it exceeds 0.05%, the effect is saturated.

Since both Nb and Ti are advantageous for grain refinement and precipitation hardening when added in small amounts, they may be added to an extent that does not deteriorate the weld toughness. Therefore, the upper limit of the amount of Nb and Ti added is
Both are 0.05%.

Cu, Ni, and CrSMo are all known as elements that improve hardenability, and when added to the steel of the present invention,
Although it can increase the strength of steel, excessive addition impairs weldability, so Cu is 1.0% or less and Ni is 1.5%.
% or less, Cr is limited to 1.0% or less, and Mo is limited to 0.5% or less.

(2) is effective in increasing the upper limit of strength through precipitation hardening, but excessive addition impairs toughness, so the upper limit is set at 0.10%.

B is known as an element that improves hardenability, and when added to the steel of the present invention, it can increase the strength of the steel, but excessive addition increases B precipitates and impairs toughness, so there is an upper limit. is set to 0.0025%.

Next, we will discuss the manufacturing method.

In the present invention, the direct rolling method described above is used. The direct rolling method referred to here is not only a method of directly rolling hot slabs obtained by continuous casting without passing through a heating furnace, but also heat retention or heating to the extent that the surface temperature of the slab is the same as the center temperature. It also includes a method of starting rolling after performing.

As mentioned above, rolling conditions are very important in this direct rolling. The reason why the total reduction ratio in rolling is set to 4 or more is because if it is less than 4, it is difficult to make the coarse austenite grains before rolling into fine ones by rolling.

Recrystallization zone rolling is essential for converting coarse austenite grains into fine recrystallized austenite grains.

The finer the recrystallized austenite grains are, the better, in order to make the subsequent rolling in the non-recrystallized region more effective, so the rolling ratio in the recrystallized region was set to 50% or more.

If it is less than 50%, rolling is performed in the non-recrystallized area without sufficient refinement in the recrystallized area, which tends to result in coarse elongated austenite, resulting in transformation and Il$41 exhibiting significant mixed grains, resulting in deterioration of properties. cause

The reason why hot rolling is finished at a temperature above the Ars point is that rolling at a temperature lower than that will process the ferrite and deteriorate the toughness.

After rolling, water cooling is performed when it is desired to further increase the strength. In this case, the reason why the temperature is lower than 600° C. is because there is almost no cooling effect above that temperature.

(Example) Table 2 shows the results of experiments conducted on the inventive steel and comparative steel with the components shown in Table 1. Items underlined in the table do not meet the conditions of the present invention. .

Tests 1 to 9 in Table 2 are the methods of the present invention, and exhibit excellent strength and toughness. These properties are equal to or better than those of steel sheets produced by conventional reheating methods. In particular, in steels containing trace elements such as Nb and Ti, these elements remain in solid solution before rolling, and precipitate finely during and after rolling, suppressing austenite recrystallization and grain growth, and causing precipitation in ferrite. Phenomena such as reinforcement show excellent properties.

10-17 is a component within the scope of the present invention, but the toughness deteriorates because l or 2 of the manufacturing method, that is, the total rolling reduction ratio, recrystallization zone rolling rate, and finishing rolling temperature do not match the method of the present invention. There is. In No. 18 and No. 19, the toughness was significantly deteriorated due to excessive amounts of C and Si, respectively.

(The following margins continue on the next page) (Effects of the invention) As described above, according to the present invention, the extremely low-cost direct rolling method achieves properties equivalent to or equal to those of thick steel plates produced by the conventional reheating method. Since it is possible to produce an RG-steel sheet with better properties than this, it is extremely advantageous industrially.

Agent: Patent Attorney Tatsuo Cha Noki

Claims (4)

[Claims] (1) C: 0.05-0.18%, Si: 0.05-0.40%, Mn: 0.3-1.80%, Al: 0.005-0.05%, in weight% The balance consists of iron and unavoidable impurities, and C+Si/2
A hot slab obtained by continuous casting of steel with a carbon equivalent of 4+Mn/6 of 0.35 or less is directly or after heat retention and heating to the extent that the surface temperature is the same as the center temperature. Start rolling and increase the total reduction ratio (slab thickness/finished plate thickness) to 4.
As described above, the method for producing a strong direct-rolled thick steel plate is characterized in that the rolling ratio in the recrystallization zone is 50% or more and the hot rolling is finished at a temperature of Ar_3 or higher. (2) The method for producing a strong direct-rolled thick steel plate according to claim 1, characterized in that the hot rolling step is water-cooled to an arbitrary temperature of 600° C. or lower. (3) C: 0.05-0.18%, Si: 0.05-0.40%, Mn: 0.3-1.80%, Al: 0.005-0.05%, in weight% Furthermore, Nb:≦0.05%, Ti:≦0.05%, Cu:≦1.0%, Ni:≦1.5%, Mo:≦0.5%, Cr:≦1.0%, V :≦0.10%, B:≦0.0025%, contains one or more of the following, the balance consists of iron and inevitable impurities, and the carbon equivalent of C + Si / 24 + Mn / 6 is 0.35 or less 2. The method for manufacturing a strong direct-rolled thick steel plate according to claim 1, characterized in that a hot slab obtained by continuous casting of component steel is used. (4) The method for producing a strong direct-rolled thick steel plate according to claim 3, characterized in that the hot rolling step is water-cooled to an arbitrary temperature of 600° C. or lower.