JPH04308036A - Production of thick steel plate excellent in brittle crack arrest property - Google Patents

Abstract

PURPOSE:To economically produce a steel plate for structural use excellent in crack arrest property with superior productivity. CONSTITUTION:In the rolling where rolling for a cast slab of structural steel heated to 900-<1000 deg.C after solidification is finished at a temp. not lower than the Ar3 point, cooling is done in the course between the starting of rolling of this cast slab and the completion of rolling so that the required cooling rate V according to plate thickness (t) satisfies the relation in V>(18/t)<0.5>. By this method, the production of the steel plate having superior arresting property is made possible without using the means so for inevitably used in the course of rolling, such as temp. waiting, low temp. heating and rolling, and reheating and rolling, and further, the productivity of the steel plates of this kind can be increased and manufacturing costs can be remarkably reduced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for economically producing structural steel sheets with excellent brittle crack propagation arresting properties (hereinafter referred to as arresting properties) with high productivity.

0002

[Prior Art] In recent years, demands on the material properties of large welded structural steel for offshore structures, ships, storage tanks, etc. have become more and more severe, especially for tanks that store LNG, LPG, etc., and lines that transport them. Pipes are required to have improved arresting characteristics due to the magnitude of damage caused by their destruction and the magnitude of social unrest caused by them. Specifically, -46℃
The arrest characteristics of steel materials for tanks that store LPG are required to exhibit a toughness value Kca of 400 kgf/mm1.5 or more measured in a temperature gradient type ESSO test.

On the other hand, there have been proposals focusing on the arresting properties of steel materials, for example, the proposal in Japanese Patent Application Laid-Open No. 19431/1983, and the proposal in Japanese Patent Application Laid-open No. 19431/1983, which uses alloying elements such as Nb and Ni. There is a proposal in Publication No. 60-29452. However, the arrest characteristics guaranteed by these proposals are such that the temperature at which the Kca value is 300 kgf/mm1.5 is -30 DEG C., which does not meet the recent demands described above.

As a proposal to meet the above-mentioned recent demands, Japanese Patent Application Laid-Open No. 77419/1983 discloses a method for producing high tensile strength steel with excellent arrest characteristics. The high tensile strength steel obtained from the manufacturing method disclosed herein is -50°C Kca
Value is 440kgf/mm1.5 ~ 720kgf/mm
1.5, which satisfies the recent demands mentioned above.

This proposal uses Ni and Nb and sets C to 0.
．． A low alloy steel slab containing 05 to 0.20% to ensure strength and non-recrystallization temperature range is heated to 900℃ or higher to uniformly austenite, and then rolled at a finishing temperature of Ar3 point -20℃ or higher. After generating a fine ferrite structure and obtaining a preferable texture, Ar3 point -2 at the temperature
The ferrite structure is transformed into a layered structure by cooling at an air cooling rate from 0°C or above to the Ar3 point -80°C, and then water-cooled from that temperature to a lower temperature, thereby forming a quenched two-phase structure of ferrite and layered bainite or martensite. A high tensile strength steel with excellent strength and toughness is obtained by producing a high tensile strength steel and then subjecting it to a tempering treatment by heating it to a temperature below the Ac1 point of the steel. This proposal uses a hardened two-phase structure of ferrite and layered bainite or martensite generated during air cooling to disperse stress by creating multiple fine separations in the crack propagation area when cracks occur and propagate. Improves brittle properties through a synergistic effect with
This greatly improves arrest characteristics.

[0006] In addition, a method using controlled cooling was published in Japanese Unexamined Patent Publication No. 5
There is a publication No. 9-47323. In this proposal, the arresting properties are improved by limiting the rolling conditions and controlled cooling conditions to obtain a steel sheet whose main structure is ferrite and finely dispersed martensite.
(Drop Weight Tear Test) test, and there is no description of the temperature gradient type ESSO test, making comparison impossible.

[0007]

[Problem to be solved by the invention] The above-mentioned Japanese Patent Application Laid-Open No. 62-77
In achieving its purpose, the proposal in Publication No. 419:
After heating the low alloy slab containing Ni and Nb to 900°C or higher, rolling is finished by the Ar3 point of -20°C, and Ar
It is essential to air cool from point 3 -20°C to -80°C at point Ar3 and then water cool. Ni and Nb are expensive alloy components, and their addition significantly increases the cost of steel materials. In addition, the addition of a large amount of Nb causes the weld heat affected zone (hereinafter referred to as HA) to
(referred to as Z).

[0008] The present invention is free from these problems and has a Kca of at least 400 kgf/mm1.5 at -50°C.
It is an object of the present invention to provide a method for manufacturing steel materials having the above arresting characteristics with good productivity and economically.

[0009]

[Means for Solving the Problems] In order to achieve the above-mentioned problems, the present invention provides the following steps in rolling a slab of structural steel heated to 900°C or more and less than 1000°C after completion of solidification to a temperature of Ar3 point or more. From the start of rolling of the slab to the temperature at which recrystallization ends, the slab thickness t (mm) and cooling rate V (°C/
As a first means, a method for producing a thick steel plate with excellent brittle crack propagation arresting properties is characterized in that cooling is performed such that V>(18/t)0.5 (sec) satisfies the relationship:

[0010] In rolling a structural steel slab heated to 900° C. or more and less than 1000° C. after completion of solidification at a temperature of Ar3 point or higher, from the time after recrystallization of the slab to the end of the rolling, Slab thickness t (mm) and cooling rate V (°C/sec)
As a second means, a method for producing a thick steel plate with excellent brittle crack propagation arresting properties is characterized in that cooling is performed such that V>(18/t)0.5.

[0011] After completion of solidification, during rolling of a structural steel slab heated to 900°C or higher and lower than 1000°C at an Ar3 point temperature or higher, the rolling process is performed from the start of rolling of the slab to the end of rolling. Piece thickness t (mm) and cooling rate V (°C/sec)
As a third means, a method for producing a thick steel plate with excellent brittle crack propagation arresting properties is characterized in that cooling is performed so that V>(18/t)0.5.

[0012] After the completion of rolling, the first means is characterized in that accelerated cooling is performed at a cooling rate of 5° C./sec or more and a cooling stop temperature of 650° C. or less;

[0013] After completion of rolling, the second means is characterized in that accelerated cooling is performed at a cooling rate of 5° C./sec or more and a cooling stop temperature of 650° C. or less;

[0014] After the rolling, the third means is characterized in that accelerated cooling is performed at a cooling rate of 5° C./sec or more and a cooling stop temperature of 650° C. or less;

[0015] A seventh method is to continue quenching and tempering after rolling,

[0016] After the rolling is completed, the eighth method is to continue quenching and tempering,

[0017] The ninth means is to subsequently perform quenching and tempering treatment after the rolling is completed.

Structural steels to which the present invention is directed include, for example,
As described in Japanese Patent Publication No. 58-14849, as described below, in order to obtain the required material quality of ordinary welded structural steel,
Equivalent actions and effects can be obtained by using the types and amounts of additive elements that have been determined based on the action-effect relationship that has been conventionally confirmed in the field of art. Therefore, the present invention targets structural steels containing these materials. The reason for addition and amount of each of these component elements will be shown below.

[0019] C is added as an effective component to improve the strength of steel, but if the content exceeds 0.20%, island martensite will precipitate and the HAZ toughness will deteriorate significantly. .Regulated below 20%.

[0020]Si is added as a deoxidizing element and strength increasing element for molten steel, but if it is less than 0.01%, the deoxidizing effect is insufficient, and if it is added in excess of 1.0%, the workability of the steel will decrease. However, since the toughness of HAZ decreases, the amount added is 0.01~
It will be regulated at 1.0%.

Mn is also necessary as a deoxidizing component element, and 0
．． If it is less than 3%, the cleanliness of the steel decreases and the workability is impaired. Further, it is necessary to add 0.3% or more of C as a component to improve the strength of steel materials. However, excessive addition of Mn significantly deteriorates weldability. Therefore, the upper limit is set at 2.0%.

[0022] Al and N are necessary because the crystal grain size of steel can be made finer by means of Al nitride. However, if the amount of Al added is small, it will not be effective, and if it is excessive, the toughness of the steel will deteriorate. .020% or less.

The basic components of the structural steel to which the present invention is directed are as described above. Based on this, when adding alloying elements according to required properties with the aim of increasing base metal strength or joint toughness, if too much is added, it will be difficult to ensure weldability. Therefore, the amount of alloy added is Ni,
Cr, Mo, Cu, W, P, Co, V, Nb, Ti, Z
r, Ta, Hf, rare earth elements, Y, Ca, Mg, Te,
The total content of one or more of Se and B is regulated within 4.5%.

[0024] In addition, the finishing temperature of rolling is set at the Ar3 point of the rolling finishing temperature in the present invention, because if the surface of the steel plate, which is the lowest temperature in the steel plate, becomes less than the Ar3 point temperature, the austenite grains will become coarse and the surface toughness will deteriorate. The temperature above includes the surface temperature.

[0025]

[Operation] In order to solve the problems of the prior art as well as achieve the objects of the present invention, the present inventors have developed the steel types shown in Table 1 of Examples as test steels representative of general structural steels. We repeated various experimental studies using 2.

It is known that the austenite grain size of this structural steel sheet changes depending on the heating temperature. The present inventors investigated the relationship between the heating temperature and grain size of the above-mentioned structural steel by varying the heating temperature of the above-mentioned structural steel slab, and found that the center temperature of the above-mentioned structural steel It has been found that the heated austenite crystal grains become even finer if the temperature is lower than 1000°C. However, the lower temperature limit is 90% due to the electric power consumption of rolling.
It has been found that a temperature of 0°C is desirable.

[0027] In order to search for a method of improving arrest characteristics by grain refinement with good productivity, the inventors of the present invention set the cooling rate during normal rolling, ie, rolling, to 0.4 to 0.5°C.
An investigation into the relationship between slab thickness and cooling rate in conventional rolling, which is recognized to be around 1/2, revealed that the actual cooling rate corresponds to the thickness of the rolled material, which has not been utilized at all in conventional rolling technology. . The actual situation is shown by curve A in FIG.

[0028] The inventors of the present invention have achieved a method that is equivalent to or similar to that obtained with the prior art, without adding a large amount of alloying elements, holding/waiting for temperature adjustment, and reheating and rolling in a low temperature range. In order to establish a manufacturing method for structural steel sheets having the above-mentioned arrest characteristics, repeated experiments were conducted from the following three points.

[0029] ■ By cooling the slab in which fine austenite grains have been formed by low-temperature heating in a high temperature range from the start of rolling to the end temperature of recrystallization, the residence time of the slab is reduced, the recrystallization temperature is lowered, and the crystal grains of the steel sheet are reduced. Relationship between grain refinement and grain growth suppression effect. ■ Relationship between the accumulation of strain in austenite before transformation by cooling the slab from the end of recrystallization to the end of rolling, and the transformed ferrite grain size. ■ Relationship between the combination of ■ and ■ and arrest characteristics.

The upper part of FIG. 2 shows these three applicable ranges. The recrystallization end temperature, which is the boundary between ■ and ■ in this applicable range, is
As is well known, it is not constant because it is controlled by the relationship between the historical temperature of the steel material to be processed and the historical processing amount. Therefore, the illustrated recrystallization end temperature and the corresponding plate thickness of the rolled material indicate an example position.

[0031] According to this experimental study, the Kca value in the temperature gradient type ESSO test was 545 kgf/m as arrestability.
■ which showed m1.5, ■ which showed 540 kgf/mm1.5, and ■ which showed 608 kgf/mm1.5
The cooling rate for each thickness is shown in FIG. 2 by curve B.

[0032] This curve is expressed as (1
It turns out that it can be approximated by 8/t)0.5. As a result, it was found that even if the thickness of the material to be rolled changes during rolling, the objects of the present invention can be achieved if the cooling rate V (°C/sec) is maintained at (18/t) 0.5 or more. . FIG. 3 shows the relationship between the cooling condition t×V2 [mm·(°C/sec)2] and the arrest characteristics of the steel sheet after rolling.

In order to improve the strength of the structural steel plate obtained as described above by accelerated cooling, after the rolling is completed, a cooling agent such as water, steam, or a mixture of steam and water is used at a cooling rate of 5°C/
It is sufficient to carry out accelerated cooling at a cooling stop temperature of 650°C or less for at least 2 seconds, and if the structural steel plate of the present invention obtained as described above is subjected to quenching and tempering after rolling, the strength and toughness can be improved without impairing the effects of the present invention. It was found that it was possible to improve The present invention has been made based on the above knowledge.

[0035]

[Example] The composition of the test steel of the present invention may be any combination of the elements and addition amounts of the general structural steels mentioned above, but this test was carried out as a typical structural steel with different strength levels. The chemical composition of the steel used in the example is shown in Table 1, and the manufacturing conditions are shown in Table 2.
The rolling pass schedule and cooling conditions during rolling used at that time are shown in Tables 3 and 4, and the obtained materials are shown in Table 5, together with a conventional example.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

[0041] Of the test steels shown in Table 1, steel numbers 1 to 3 are 40 kg class steel, steel numbers 4 to 7 are 50 kg class steel, and steel numbers 8 to 10 are 60 kg class steel.
It is kilo class steel. Alloying elements are added to each as necessary. As shown in Table 2, No. 1 of the present invention example using steel types 1 to 7. A1-1 to A7-3 have arrest characteristics of 5
00kgf/mm1.5 and using steel types 8 to 10. A8-1~A10-3 is 700k
Arrest characteristics exceeding gf/mm1.5 were obtained. We were able to manufacture steel sheets with excellent arrest characteristics in both designated pass cooling and zone cooling with good productivity and economy.

On the other hand, among the conventional examples in which the temperature was waited during rolling and the required cooling was not performed during rolling, steel types 1 to 1
No. 7 was used. B1 to B7 are 500kgf/mm1
．． No. 5 using steel types 8 to 10 has an arrest characteristic of less than 5. The arrest characteristic of B8 to B10 is 700kgf
/mm1.5, which was low, and the strength did not reach the expected level. Also, conventional example N where the heating temperature exceeded 1000℃
o. B1 to B4 have even lower arrest characteristics at 400kgf
/mm1.5.

[0044]

[Effects of the Invention] As is clear from the above description, the present invention has the following features:
By utilizing the above-mentioned action generated by the above-mentioned means, a technology has been established to smoothly, stably, and economically manufacture structural steel plates with excellent arrest characteristics with high productivity, and the present invention The effects brought about in the field of application and the ripple effects on related fields are extremely large.

[Brief explanation of drawings]

FIG. 1 shows the relationship between heating temperature and austenite grain size of a structural steel slab.

FIG. 2 shows the relationship between the slab thickness during rolling and the average cooling rate in the thickness direction of the slab.

FIG. 3 shows the relationship between cooling conditions and arrest characteristics of the present invention.

Claims (9)

[Claims] Claim 1: After the completion of solidification, the temperature is 900°C or higher and 1000°C.
In rolling a structural steel slab heated below ℃ to a temperature above the Ar3 point, the thickness t (mm) of the slab and the cooling rate V from the start of rolling to the temperature at which recrystallization is completed are determined.
A method for producing a thick steel plate with excellent brittle crack propagation arresting properties, characterized in that cooling is performed so that (°C/sec) satisfies the relationship of V>(18/t)0.5. Claim 2: After the completion of solidification, the temperature is 900°C or higher and 1000°C.
When rolling a structural steel slab heated to less than ℃ to a temperature above the Ar3 point, the thickness t (mm) of the slab and the cooling rate between the recrystallization of the slab and the temperature at which the rolling is completed are determined. V
A method for producing a thick steel plate with excellent brittle crack propagation arresting properties, characterized in that cooling is performed so that (°C/sec) satisfies the relationship of V>(18/t)0.5. Claim 3: After the completion of solidification, the temperature is 900°C or higher and 1000°C.
In rolling a structural steel slab heated below ℃ to a temperature above the Ar3 point, the thickness t (mm) of the slab and the cooling rate V between the start of rolling and the temperature at which rolling is completed
A method for producing a thick steel plate with excellent brittle crack propagation arresting properties, characterized in that cooling is performed so that (°C/sec) satisfies the relationship of V>(18/t)0.5. [Claim 4] After finishing rolling, the cooling rate is 5°C/sec or more,
The method according to claim 1, characterized in that accelerated cooling is performed at a cooling stop temperature of 650° C. or lower. [Claim 5] After finishing rolling, the cooling rate is 5°C/sec or more,
The method according to claim 2, characterized in that accelerated cooling is performed at a cooling stop temperature of 650° C. or lower. [Claim 6] After finishing rolling, the cooling rate is 5°C/sec or more,
The method according to claim 3, characterized in that accelerated cooling is performed at a cooling stop temperature of 650° C. or lower. 7. The method according to claim 1, further comprising performing quenching and tempering treatment after the rolling is completed. 8. The method according to claim 2, further comprising performing quenching and tempering treatment after the rolling is completed. 9. The method according to claim 3, further comprising performing quenching and tempering treatment after the rolling is completed.