JPH04141517A - Production of steel plate excellent in brittle crack propagation arresting property and toughness at low temperature - Google Patents

Abstract

PURPOSE:To form superior brittle crack propagation arresting property over the whole steel plate and to produce a thick steel plate for structural use particularly excellent in toughness in the central part of plate thickness as well as in the upper and the lower surface layer part by exerting rolling in parallel with ferrite recrystallization in the course of temp. rise after inverse transformation or exerting rolling in the course of inverse transformation from ferrite to austenite. CONSTITUTION:This invention relates to a method for producing a steel plate excellent in brittle crack propagation arresting property and toughness at low temp. by using a stock of a structural steel which has a composition consisting of, by weight, 0.01-0.30% C, <=0.5% Si, <=2.0% Mn, <=0.1% Al, 0.001-0.01% N, and Fe with inevitable components and further containing, if necessary, one or >=2 kinds among <=0.5% Cr, <=0.1% Ti, <=1.0% Ni, <=0.05% Nb, <=0.5% Mo, <=0.0015% B, <=0.1% V, and <=0.9% Cu. In this method, in the course where the above steel stock is subjected, once or more, to a repetition of starting the application of cooling from a temp. not lower than the Ar3 point at >=2 deg.C/sec cooling rate to the region of the upper and the lower surface layer part corresponding to 2-33% of the thickness of this steel stock, exerting cooling down to the Ar3 point or below, stopping this cooling, and performing recuperation, finish rolling is exerted in the course between the stoppage of cooling for the surface layer parts of this steel stock and the completion of recuperation and the upper and the lower surface layer part of the resulting steel plate after finish rolling is recuperated to a temp. lower than the Ac3 point or to a temp. not lower than the Ac3 point or to a temp. of the Ac3 point and in its vicinity.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention C has a temperature of 4.
The present invention relates to a method for manufacturing a structural steel plate having a brittle crack propagation arresting characteristic of 00 kg f-mm-''2 or more and a toughness of more than 1100° C. in the center.

(Prior Art) A brittle crack that occurs in a steel plate usually propagates with the main crack connected to a preceding crack.

This connection is formed by a ductile fracture called a tear at the grain boundary, and at this time, the crack propagation energy is absorbed by the ductile fracture of the tear.

This absorptive capacity is enhanced by increasing the chances of the above-mentioned ductile fracture due to the refinement of the crystal grain size, thereby improving the crack slowing effect and improving the above-mentioned brittle crack propagation arresting property.

We have actually come a long way towards improving brittle crack propagation arresting properties! This is due to a plastic deformation region called a shear lip that occurs in the surface layer of a steel sheet when a brittle crack propagates, and the ability of this shear lip to absorb the propagation energy of the brittle crack as it propagates increases as the crystal grains become finer.

Therefore, various attempts have been made to effectively refine crystal grains.

In order to realize this, for example, Japanese Patent Application Laid-Open No. 131-23553
Publication No. 4, as shown in Figures 1 (a) and (d), covers a distance of 1/8 or more of the thickness of the slab from the surface of the slab with a temperature of 3 points or higher to the center of the slab, as shown in Figures 1 (a) and (d). After cooling 17, rolling is started with a temperature difference in the thickness direction of the slab, and during or after rolling, the entire thickness of the slab is reheated to Ac3 point or higher, so that the slab is cooled by the ESSO test. , Kea, which represents brittle crack propagation arresting characteristics at 20°C, is 460 to 9.
We are proposing a method for manufacturing 60kg f-up-"2f1 degree thick steel plate.

17 Kca of the above-mentioned Japanese Patent Application Laid-open No. 61-235534
The value is the value at -20℃, and this is the condition not affected by the plate thickness effect and 1. 1. Keep the plate thickness constant. When converted to the Kca value at -50℃, the Kca value at -50℃ is -20℃.
It is about 1/2.5-1/2.7 of the Kca value of 50℃
The Kca value is approximately 371) to 380kg f −+u−
The Kea value is about 40, which is required in this field.
The reality is that it is not possible to satisfy the requirement of 0 kg f - mm "" or more.

In addition, the proposal in Japanese Patent Application Laid-open No. 81-235534 reheats the entire area of the slab to the A c 3 point or more, which increases the reheating time and reduces productivity, as well as lowering the A r s point. The thickness range to be cooled is small, and therefore the region where reverse transformation from austenite to ferrite and recrystallization can be utilized is narrow, grain refinement and structure refinement are insufficient, and the shear lip effect is not fully exerted. is a practical hindrance.

Further, as a method for producing a thick steel plate having excellent weldability and good strength and toughness, there is a proposal described in, for example, Japanese Patent Publication No. 49-7291.

This proposal involves repeated cooling and heating.1. The method is to increase the number of transformations to reach the final structure and refine the grains, or simply increasing the number of transformations has a limit to grain refinement, and it is not possible to effectively stop brittle crack propagation. Manufacturing methods that do not provide good properties and toughness, and that involve repeating only such temperature control have poor economic efficiency and productivity.

(Problems to be Solved by the Invention) The present invention is directed to the above-mentioned 1. without the problems of the conventional technology.
Forms excellent brittle crack propagation arresting properties throughout the steel plate1
-1 Furthermore, it is an object of the present invention to provide a method for manufacturing thick structural steel plates with good productivity and economically, which have particularly excellent toughness not only in the surface layer but also in the center of the plate thickness.

(Means for Solving the Problems) In order to achieve the above-mentioned problems, the present invention provides C: 0 in weight%.
．． 0] to 0.30%, Si: 50.5%, Mn: 52.
0%, A1.50.1%, N: 0.001-0.01
% further as necessary Cr: 50.5%, Ti: ≦0.
1%, Ni≦1.0%, Nh:20.05%,
MO: ≦0.5%, B: 50.0015%, ■, ≦0.
1%, Cu: 60.9% of one or more types,
In addition, cooling of the upper and lower surface areas of the structural steel material consisting of Fe and other unavoidable components corresponding to 2 to 83% of the steel material thickness is started from a temperature of Ar 3 or higher at a cooling rate of 2°C/see or higher. 12. Finishing after cooling the surface layer of the steel material to 3 points or less, stopping the cooling, and reheating the steel material at least once until the reheating is completed. Rolling is performed, and the upper and lower surface parts of the steel plate after finish rolling are below the Ac point, on the Ac point, or on the A ca point.
The purpose of this invention is to produce a steel plate with excellent low-temperature toughness and brittle crack propagation arresting properties that regenerate heat in the temperature range above and below the point.

The reasons for limiting the above-mentioned components will be explained below.

C is added as a reinforcing component to steel materials, and an upper limit is set to prevent deterioration of the toughness of the welded part.

Si is added for the purpose of deoxidizing and maintaining strength, and the upper limit is set to prevent deterioration of weldability.

Mn is added for the purpose of improving low-temperature toughness, and the upper limit is set to prevent weld cracking.

N is added together with Ai) for the purpose of refining crystal grains by forming nitrides, and the upper limit is set to prevent deterioration of the toughness of the weld zone.

Cr, Nl, Mo, B, and Cu are all added to improve hardenability and effectively increase strength, and the upper limit is set to suppress the formation of low-temperature transformation products and prevent a decrease in the ferrite area ratio. It has established.

Ti and Nb are added for the purpose of refining crystal grains, and the upper limit is set to prevent reduction in toughness of the weld zone.

(2) is added for the purpose of precipitation strengthening, and the upper limit is set for economic reasons.

Moreover, the structural steel materials targeted by the present invention have a thickness of 50 to 40
0 mm High-temperature steel billets as they are continuously cast, or steel billets that have been cooled and then reheated, and further rolled by continuous casting, shape adjusting rolling, rough rolling, etc., with a thickness of 10 to 380 mm.
refers to high-temperature steel plates, etc.

(Function) In weight%, the present inventors set C: 0.01 to 0.30%,
Si: 50.5%, Mn: ≦2.0%, AII: 50.
1%, N: 0.001-0.01% Furthermore, if necessary, Cr: 50.5%, Ti: 5061%, Ni: ≦1,0
%, Nb:≦0.05%, Mo; 50.5%, B: 50
．． 0015%, V: 50.1%, Cu: 50.9% 1
Various finishing rolling experiments were repeated using structural steel materials containing one or more types of iron, Fe, and other unavoidable components, and having a thickness of 10 to 360 mm.

As a result, as shown in Fig. 1(a) and (b), the upper and lower surface areas corresponding to 2 to 33% of the steel thickness of the steel material were
At a cooling rate of ℃/sec or more, advance cooling is carried out at three points or more, and finish rolling is started with a temperature difference between the surface layer and the center of the steel material. After finishing the finish rolling, the thickness of the steel material is reduced. 2-33
When the upper and lower surface regions corresponding to % are reheated to below the A c a point, the ferrite is sufficiently recrystallized and the crystal grains are refined, resulting in high toughness because the rolling is performed during the heating process. It was also found that the brittle crack propagation arresting properties were further improved.

In addition, as shown in FIGS. 1(a) and (C), the upper and lower surface areas corresponding to 2 to 38% of the steel thickness of the steel material are A c a
When reheating is performed on the surface, the rolling is performed during the reverse transformation process, so the crystal grains become even finer due to the reverse transformation from ferrite to austenite, resulting in not only higher toughness but also improved brittle crack propagation arresting properties. I learned that.

At this time, the reverse transformation by cooling to below 3 points of A r and recuperation to above A c s is carried out by cooling conditions, plate thickness,
It has been found that when heating is performed one or more times at a constant heating temperature, the brittle crack propagation arresting characteristics and VTrS are further improved as shown in FIGS. 4(a) and 4(b).

In addition, the rolling at this time follows the pattern shown in Fig. 2 in any case, and the material to be rolled at that time has upper and lower surface layer parts corresponding to 2 to 33% of the steel material thickness, as shown in Fig. 3. There is a temperature difference between the region and the center, and there is also a difference in crystal grain size between the upper and lower surface regions corresponding to 2 to 33% of the steel material thickness and the center.

If this material is rolled as it is, the rolled material will become a plate-like resistor in which the upper and lower surface regions corresponding to 2 to 33% of the steel material thickness have large deformation resistance, and the center region with low deformation resistance will be formed. It has been found that as a result of the plate-shaped resistor being strongly compressed, an extremely large strain is accumulated in the center, and the ferrite after transformation becomes even finer, improving the toughness of the center and compressing the center porosity.

In addition, the thickness range in which the surface layer becomes finer is 2% or more, which has no practical effect, and if it becomes 33% or more, the temperature at the center of the plate thickness decreases too much because the sensible heat of the steel material itself disappears, and the toughness deteriorates. I discovered that I could do it.

In addition, at this time, in the steel sheet containing no Ti and Nl, ferrite with a crystal grain size of 5 um or less reaches an area ratio of 509' or more in the surface layer (2, steel plate containing T1 and Nb: 3 uA or less). The crystal grain size (ferrite) reaches a surface area ratio of 50% or more, and both Kea (-50°C) force ≧ 400 kg f −
We have found two materials that exhibit stable brittle crack propagation arrest characteristics.

Also, after finish rolling in the reheating process, the temperature at that time is 5~
It was found that when held for 300 seconds, the brittle crack propagation arresting property was further improved by randomizing the L position of the refined crystals [7]. Ta.

Furthermore, the steel plate let after finishing this finish rolling and undergoing reheating.

It has been found that the strength and toughness of the base metal improve when the temperature is then strongly increased to 650°C or less at a cooling rate of 40°C/second or less.

Based on these findings, the above 1. The present invention has been made to achieve the object.

(Example) 1. Test steel shown in Table 1.

2. Rolling conditions are shown in Table 2.

3. Cooling conditions are shown in Table 2.

4. Cooling stop conditions shown in 2.

5. Recuperation conditions: Shown in N2.

6. Controlled cooling conditions after hot rolling: Shown in Table 2.

7. Brittle crack propagation stop L1. Properties - Shown in Table 3.

8. Toughness and other properties: Shown in Table 3.

Furthermore, the brittle crack propagation arresting property is the ESSO test value (Kea)
The toughness is determined by the fracture surface transition temperature (vT) in the Charpy test.
rs) was 1, and the crimp of the center porosity was evaluated by the aperture M (RAz) by a Z direction tensile test.

The brightness number A-1-A-23 of the example of the present invention is Kc, a(-50
℃) is 4(〕〕5-125-12O0-3, which is much superior to the conventional example by 17.
/2 to vTrs of -101,1 to -160℃, 1/
It showed excellent characteristics with RAz of 2t being 72-85%.

On the other hand, the comparative examples B-1 to B-19 are Kea (-50
°C) is 102 to 385 kg f-mm-"', which is the same as before, and 1/2 t vT rs is -50 to -90 °C
, 1/2t RAZ was obtained only 43-53%.

(Effects of the Invention) The present invention provides rolling in parallel with ferrite recrystallization during heating up after one or more reverse transformations, or rolling in parallel with ferrite recrystallization during heating up after one or more reverse transformations. Since rolling is performed during the approximate transformation from ferrite to austenite, the structure becomes finer throughout the steel sheet, improving the shear lip effect on the surface layer of the steel sheet, and improving the brittle crack propagation arresting property, which is one of the issues.
50℃) ≧ 400) cg f-Ryu-3'', and since strong pressure is effectively applied to the center of the steel plate in the thickness direction, the toughness of the center of the plate thickness, which is another issue, is vTrs≦ -1
00 (''C), and the productivity and economic efficiency of this steel plate are high, and the effect it brings to the fields where this type of steel plate is manufactured and used is extremely large.

[Brief explanation of drawings]

FIG. 1(a) shows the position in the thickness direction of the slab and steel plate that are regulated by the present invention, and FIG.
The relationship between the above position and cooling/recuperation temperature in the example of the present invention,
(d) is a diagram of the relationship between the above-mentioned position and cooling/recuperation temperature in the conventional example (example of JP-A No. 81-235534), FIG. 2 is the rolling pattern of the present invention, and FIG. 3 is the rolling pattern of the present invention. Figure 4 (a) shows the relationship between the number of times of reverse transformation and the brittle crack propagation arresting characteristic, and (b) shows the relationship between the number of reverse transformations and the brittle crack propagation arresting property. A chart of the relationship between the number of times and toughness (fracture surface transition temperature vTrs in Charpy test) is shown. agent

Claims (1)

[Claims] 1. In weight%: C: 0.01-0.30% Si: ≦0.5% Mn: ≦2.0% Al: ≦0.1% N: 0.001-0 .01% A structural steel material consisting of other Fe and other unavoidable components is cooled at a cooling rate of 2°C/sec or more from a temperature of Ar_3 points or more in the upper and lower surface areas corresponding to 2 to 33% of the steel material thickness. Start rolling, cool to Ar_3 point or less, stop the cooling, and reheat at least once, and finish rolling is performed until the reheating after the last cooling is completed. The upper and lower surface areas of the steel plate after finish rolling are reduced to less than 3 points or Ac_
A method for producing a steel plate with excellent brittle crack propagation arresting properties and low-temperature toughness, characterized by recuperating heat at 3 points or more or at the Ac_3 point and a temperature range above and below it. 2. In weight%, Cr: ≦0.5% Ti: ≦0.1% Ni: ≦1.0% Nb: ≦0.05% Mo: ≦0.5% B: ≦0.0015% V: The method for producing a steel sheet with excellent brittle crack propagation arresting properties and low-temperature toughness according to claim 1, characterized in that it contains one or more of the following: ≦0.1% Cu:≦0.9%. 3. The method for producing a steel plate with excellent brittle crack propagation arresting properties and low-temperature toughness according to claim 1 or 2, wherein the steel plate temperature is maintained for 5 to 300 seconds after finish rolling. 4. After completion of reheating, the steel plate is cooled at a cooling rate of 40℃/second or less.
Claims 1 and 2 characterized in that cooling is performed to 50°C or less.
Or the method for producing a steel plate with excellent brittle crack propagation arresting properties and low-temperature toughness according to 3.