JPH10121131A - Manufacture of thick high tensile steel plate excellent in brittle fracture propagation stop characteristic and weldability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a thick high tensile steel plate capable of stopping brittle fracture propagation and excellent in weldability even when applied under the condition of >=880 Mpa tensile strength, at <=(vTs-80 deg.C) and at -30 deg.<=. SOLUTION: The steel contains, by weight, 0.07-0.18% C, 0.4-1.5% Mn, <=0.2% Si, 3.5-6.5% Ni, 0.1-1% Cr, 0.1-1% Mo, <=1.5% Cu, 0.01-0.1% V, <=0.03% Nb, <=0.006% N, <=0.05% sol. Al and 0.0003-0.002% B, has an index of C(%)+Si(%)+1.5 sol. Al(%) in a range of 0.1-0.25%. When the steel is heated to a range of 900-1100 deg.C, and hot rolling is carried out, and after finishing rolling is executed at a temperature range of 850-700 deg.C, the steel is quenched as it is, and then tempered at <=700 deg.C and water-cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a material having a thickness of 100 mm or more and a tensile strength of 8 which is excellent in brittle fracture propagation stopping characteristics and weldability.
The present invention relates to a method for manufacturing a high-strength steel sheet having a thickness of 80 MPa or more.

[0002]

2. Description of the Related Art In recent years, the tendency of welded structures to become larger has become remarkable, and accordingly, demands for higher strength and thicker steel plates used for these structures have been increasing. . For example, thick-walled 780 pipes are used for penstocks of pumped-storage power plants and rack materials for jack-up type drilling rigs for offshore structures.
Mpa-class high-strength steel sheets have been used.

[0003] At present, it has been desired to supply high-tensile steel having a tensile strength exceeding 880 MPa. Increasing the strength of thick steel plates is not limited to simply reducing the weight of the structure, but also results in a significant reduction in welding work costs. However, at present, there is no established technology capable of stably supplying a high-strength steel sheet having a thickness of 100 mm or more having such high strength, high toughness, excellent brittle fracture arrestability and weldability.

[0004] Since high-strength high-strength steel sheets are often used for structures that support huge forces, in order to prevent the occurrence of a major accident affecting the life of citizens, it is necessary to reduce potential cracks (for example, welding defects). Even if a brittle fracture occurs, it is required to have a brittle fracture propagation arresting property for stopping the propagated brittle fracture in the thick high-strength steel sheet. In some cases, this brittle fracture propagation arresting property can be considered the same as so-called toughness, but when brittle fracture propagation becomes a problem,
With particular attention to brittle fracture propagation arrest properties, toughness should be discussed separately for brittle fracture initiation properties and brittle fracture arrest properties.

As a method for evaluating toughness, a small-sized Charpy test (mainly for evaluating brittle fracture initiation characteristics) has been frequently and simply used. Therefore, when toughness or low-temperature toughness is generally referred to, evaluation by the Charpy test, that is, Refers to characteristics corresponding to the tendency of brittle fracture to occur. Also in this description, toughness or low-temperature toughness refers to evaluation based on the Charpy test. In most cases, steel with good brittle fracture arrestability also has good evolving properties, and both show the same tendency. However, the transition temperature (v
Although the Ts) is improved by several tens of degrees Celsius, the brittle fracture arrestability may not be improved at all. In the case of the thick high-strength steel sheet used for the above-mentioned important structure, both have different properties. Must be discussed as:

[0006] In order to evaluate the brittle fracture propagation arresting property, it is necessary to simulate a steel plate incorporated in a structure, to once generate and propagate brittle fracture, and to stop the brittle fracture. Larger tests such as the double tensile test or ESSO test used are usually used.

FIG. 1A is a drawing schematically showing a test piece shape and a test state of an ESSO test for evaluating the brittle fracture propagation arresting property of a thick steel plate.

FIG. 1 (b) is a drawing showing a test piece and a test state of a double tensile test for evaluating the brittle fracture arrestability.

In the ESSO test, a wedge is driven to generate a brittle crack. On the other hand, in a double tensile test, there is a difference that a brittle crack is generated by a tensile load dedicated to the generation of a brittle crack. Not a law. In both tests, the brittle crack that has occurred once progresses through the part where the stress and the temperature rise gradient that cause the brittle crack to grow, and stops at a certain temperature position. At this time, the fracture toughness value Kca (MPa · m ^0.5 ) at the stop of brittle fracture propagation, which is an index of the property of stopping brittle fracture propagation, is expressed by the following equation.

Kca = σ (πc) ^1/2 × f (a / W) where σ is a stress (MPa) that causes a brittle crack to propagate,
c: Stopping crack length (m) and f (a / W) are shape factors that change depending on the test piece shape related to the width and thickness of the test piece. This Kca is obtained for each stop temperature, and Kc
When "a" is plotted against the reciprocal of the absolute temperature, a substantially straight line is obtained. From this straight line, Kca at the target temperature can be known.

[0011] Manufacturing methods for improving the toughness and brittle fracture arrestability of thick high strength steel sheets have been proposed in the past. For example, Japanese Patent Publication No.
The Mn-Ni-Cr-Mo steel containing 0.35% C has a thickness of 4 by controlling the Mn / Cr ratio to 1.5 or less.
A method for manufacturing a high-strength steel sheet having a tensile strength of 0 mm exceeding 90 kgf / mm ² (880 MPa) has been proposed.
The toughness and weldability are not sufficient for a steel plate exceeding mm.

Japanese Patent Publication No. 6-70250 discloses a method for increasing the toughness of a 100 kgf / mm ² (980 MPa) grade steel sheet having a thickness of more than 100 mm by adding Ni—Cr—M containing Nb.
A double quenching treatment of oB-Nb steel is disclosed. This is because a very fine martensite structure is obtained from the surface layer to the center of the thick steel plate through the fine austenite grains by the double quenching treatment, so that the high strength and high toughness (Charpy (Transition temperature of −60 ° C. or less in an impact test). However, the refining of the martensitic structure alone does not sufficiently improve the toughness of the thick steel plate, not only the brittle fracture propagation arresting property is also insufficient, but also the toughness of the welded part needs to be improved.

[0013]

SUMMARY OF THE INVENTION The present invention has a tensile strength of 8%.
An object of the present invention is to provide a method for manufacturing a high-strength high-strength steel sheet having a weldability of 80 mm or more (especially 950 MPa or more), excellent in brittle fracture initiation characteristics and excellent brittle fracture propagation stopping performance, and excellent in weldability with a thickness of 100 mm or more. Specifically, an object of the present invention is to provide a method for producing a thick high-tensile steel sheet having the following performance.

(1) Base material performance: (a) Tensile strength of 880 MPa or more, (b) Transition temperature (vTs) of Charpy impact test -80 ° C or less from the surface layer to the center, (c) -30 ° C Prevents the propagation of brittle fracture.

(2) Weldability: (a) vT of HAZ (Heat Affected Zone)
s −80 ° C. or less (b) Weld preheating temperature 100 ° C. or less The transition temperature vTs of the above Charpy impact test of −80 ° C. or less means that brittle fracture is extremely caused by welding defects having a size that cannot be easily detected by ordinary ultrasonic testing. It means that it hardly occurs, and that it hardly occurs at least at -30 ° C. The lower the vTs, the better the brittle fracture initiation characteristics.

Preventing the propagation of brittle fracture at −30 ° C. means that, even if a brittle fracture occurs at −30 ° C., the brittle fracture can be stopped by the base material. means. In order for the brittle fracture to be stopped at -30 ° C, the fracture toughness value Kca is usually 200 at -30 ° C.
It is said that it is sufficient if the ^{pressure is at} least MPa · m ^0.5 . -30 ° C
These temperatures cover most of the world's civilized areas.

In the following description, the welded portion is assumed to be composed of "weld metal", "HAZ in contact with weld metal", and "HAZ". Usually, HAZ includes "HAZ in contact with weld metal", but "HAZ in contact with weld metal" may be distinguished as "bond".

[0018]

Means for Solving the Problems The present inventors have proposed 880M
In order to impart good low-temperature toughness and excellent brittle crack propagation arrestability to a thick steel plate having a tensile strength of Pa or more and a thickness of 100 mm or more without impairing weldability, the following items are required. It was confirmed.

(1) As for the microstructure, the refinement of austenite grains and the optimization of the ratio of martensite to bainite as in the conventional method are not sufficient. In addition to such a microstructure, it is necessary to uniformly disperse fine stable retained austenite. In particular, as a necessary condition for obtaining a predetermined brittle fracture arrest performance in a thick steel plate having a tensile strength of 880 MPa or more, this retained austenite is 10% or less.
Preferably, it should be controlled to 1 to 5%.

Optimum range of retained austenite 1 to 10%
An increase in excess of that causes a reduction in the concentration of austenite stabilizing elements such as Ni in the same portion, and tends to transform into high-hardness martensite by plastic deformation occurring before the occurrence of brittle fracture or by shock waves during propagation, It has an adverse effect on both toughness and brittle fracture arrestability.

(2) Further improve the brittle fracture arresting property of the surface layer (the excellent properties of the surface layer enable the entire steel sheet
(Improving the same characteristics as (original thickness)), the combination of the refining of austenite grains by heating at a low temperature and the processing of the unrecrystallized austenite region are also performed simultaneously. Specifically, Cu-Ni-C containing 3.5 to 6.5% Ni
It is necessary to perform a direct quenching process on the r-Mo-Nb-Ti-B steel after low-temperature heating and low-temperature finish rolling.

(3) In order to stably obtain a good balance between strength and toughness, in addition to the above, C (%) + Si (%)
The index of + 1.5solAl (%) is 0.1 to 0.25
% In order to suppress the formation of a hard phase (martensite) that adversely affects the toughness and brittle fracture propagation arrestability of the base material and HAZ.

(4) By applying a small amount of Ti treatment to a low C-high Ni-extremely small amount B-low N steel, the effect of B can be utilized to impart appropriate hardenability over the entire thickness of a thick steel plate. Can be.

The present invention has been made based on the above findings.
The following chemical composition, rolling and heat treatment methods are summarized.

"C: 0.07 to 0.18 by weight ratio"
%, Mn: 0.4 to 1.5%, Si: 0.2% or less, N
i: 3.5 to 6.5%, Cr: 0.1 to 1%, Mo:
0.1-1%, Cu: 1.5% or less, V: 0.01-
0.1%, Nb: 0.03% or less, N: 0.006% or less, solAl: 0.05% or less, and B: 0.000
3 to 0.002%, C (%) + Si (%) + 1.
For a steel having an index of 5solAl (%) in the range of 0.1 to 0.25%, the heating temperature before hot rolling is 900 to
In the temperature range of 1100 ° C., finish rolling of hot rolling was performed for 8 hours.
Production of thick high-strength steel sheet excellent in brittle fracture propagation arrestability and weldability characterized by being quenched as it is in a temperature range of 50 to 700 ° C., and then tempered and water-cooled at 700 ° C. or less. Method. The composition of the steel targeted by the present invention described above corresponds to all low- to medium-alloy steels satisfying the above composition and index. However, the main elements other than the trace elements are limited to those described above.

In the above description, “finishing rolling is performed at 850-7
"Quenching as it is in a temperature range of 00 ° C." refers to directly quenching after finish rolling. After finishing rolling and before direct quenching, descaling, straightening,
Temperature uniform heating, carry-in to a quenching facility, and other necessary processing may be performed.

"Tempering with water cooling" means that after cooling to a tempering temperature for a predetermined time, a temperature range of 550 ° C. or less is water-cooled for the purpose of preventing temper brittleness and stabilizing retained austenite.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION

 1. Chemical Composition First, the reasons for limiting each alloy component will be described.

C: C is added in order to secure the strength of the steel sheet. However, if the content is less than 0.07%, hardenability becomes insufficient, and it is difficult to secure a tensile strength of 880 MPa.
Also, the toughness is not sufficient. On the other hand, if it exceeds 0.18%, not only the toughness and brittle crack propagation stopping performance of the base material decrease, but also the hardness of the HAZ increases and the susceptibility to welding cold cracking increases, so that 0.07 to 0.18. %.

Mn: Mn is added to improve the hardenability of the steel sheet and increase the strength. However, if it is less than 0.4%, it is difficult to secure the strength, and on the other hand, it exceeds 1.5%. And since the toughness of both the base material and HAZ is reduced,
To 1.5%.

Si: Si is added for the purpose of improving the yield of Al in final deoxidation due to its deoxidizing action. In the present description, the term “Si contained in steel” refers to Si that exceeds the amount that worked for deoxidation remaining in steel, but does not have to be intentionally left. That is, Si does not need to be substantially contained. Si remaining in the steel is effective in increasing the strength, but if it exceeds 0.2%, the base metal and H
Since the toughness of AZ is reduced, the content is set to 0.2% or less even when intentionally remaining.

Ni: Ni is an element indispensable for improving the low-temperature toughness, brittle fracture propagation stopping performance and weldability of a high-strength thick steel plate. In particular, it is effective in improving the brittle fracture propagation stopping performance, and the target brittle fracture propagation stopping performance can be obtained by containing 3.5% or more. When the content is 3.5% or more, the mixed structure of fine martensite and bainite has a structure in which several percent of stable retained austenite is mixed, and the low-temperature toughness and the ability to stop brittle fracture propagation are dramatically improved. On the other hand, when the content exceeds 6.5%, not only is the margin of improvement of the effect small for the cost increase, but also the amount of residual austenite generated by the double quenching-tempering treatment is increased, and the yield strength may be reduced. 3.5-
6.5%.

Cr: Cr improves quenchability and improves strength and toughness by precipitation hardening during tempering. 0.1
If it is less than 1%, the effect is not sufficient, while if it exceeds 1%, the strength is excessively increased and the toughness of the base material and the HAZ is impaired.
0.1 to 1%.

Mo: Mo has a greater hardenability improvement effect and precipitation hardening than Cr when compared in the same amount. In particular, when Mo coexists, the hardenability improvement effect is remarkably exhibited. 0.1
%, It is not sufficient to insert "baking" up to the center of the thick steel plate and obtain a tensile strength of 880 MPa or more,
On the other hand, if it exceeds 1%, "burning" is excessive in the surface layer portion, and the toughness of the surface layer portion is deteriorated.

Cu: Cu need not be added. However, if added, the hardenability is improved, and if it exceeds 0.3%, the base material is strengthened by precipitation hardening during tempering, so that the tensile strength is higher than 880 MPa, for example, 950 MPa.
When it is set to be not less than MPa, it is desirable to add. However, when the content exceeds 1.5%, not only the toughness of the base material and the HAZ is impaired, but also the hot ductility is greatly reduced.
1.5% or less.

V: V precipitates during tempering and increases tempering softening resistance by precipitation hardening, so that tempering at high temperatures is possible and the balance between strength and toughness is improved.
If it is less than 0.01%, the effect is not enough, while
If it exceeds 0.1%, the strength becomes excessively high and the toughness is impaired.

Nb: Nb may not be added. However, when a small amount is added, fine austenitic grains are formed by forming fine Nb carbonitride in a low temperature region of austenite, and a fine martensite structure is formed from the surface layer portion to the center portion of the thick steel plate. It improves the toughness of the high-strength steel sheet, especially the low-temperature toughness and brittle fracture arrestability of the surface layer. Therefore, it is desirable to add it especially when these properties of the surface layer are improved. However, if it exceeds 0.03%, not only will lateral cracks occur in the weld metal at the time of welding, but it also lowers the low-temperature toughness and brittle fracture propagation stopping characteristics of the base metal. I do. In order to prevent the lateral cracking of the weld metal and obtain the above effect stably, 0.0
It is desirably set to be from 0.05 to 0.02%.

N: N is an unavoidable impurity, and the smaller the better, the better. If more than 0.006%, base metal and H
Since the toughness of AZ significantly decreases, the content is made 0.006% or less.

SolAl (acid-soluble Al): solAl
Is a case in which Al in excess of the amount worked for deoxidation remains in the molten steel, and there are both cases where Al is intentionally left and cases where surplus components added only for deoxidation remain. The present invention includes both cases. Therefore, solAl may not be substantially contained. solAl combines with N after solidification to form AlN or becomes solid-solution Al. Al
N refines austenite grains to improve toughness, while solute Al uses a hard phase as a base material and HA as described later.
It forms in Z and degrades toughness. When N is relatively high (when it cannot be lowered), it is desirable to intentionally include solAl. However, even in this case, if the content exceeds 0.05%, not only the properties of the HAZ are deteriorated due to the solid solution Al, but also the structure is refined by performing low-temperature heating, low-temperature finish rolling and direct quenching. In addition, the toughness and brittle fracture propagation stopping characteristics of the base material are deteriorated, so the content is made 0.05% or less.

B: Since B improves the hardenability in a very small amount, it is necessary to obtain martensite or a mixed structure of martensite and bainite at the center of the thickness of a thick steel plate, and particularly coexist with Mo. is required. 0.0
If it is less than 003%, the effect is not sufficient.
If the content exceeds 003%, the toughness of the base material and the welded portion, particularly the toughness of the welded portion, is impaired.
And

C (%) + Si (%) + 1.5solAl
(%): In addition to the above component limitation, an index of C (%) + Si (%) + 1.5solAl (%) is used in order to improve the toughness of the base material and the HAZ and to improve the brittle fracture propagation stopping characteristics of the base material. It needs to be limited. That is, if this index is less than 0.1%, the required tensile strength cannot be obtained, and if it exceeds 0.25%, the base material and HAZ have a fine but hard and brittle structure (hard martensite). ) Is mixed, and the toughness of the base material and the welded portion and the brittle fracture arrestability of the base material are reduced. Therefore, C
(%) + Si (%) + 1.5solAl (%) within the range of 0.1 to 0.25%, preferably 0.13 to 0.25%.
It is necessary to limit the range to 0.20%.

The composition of the steel according to the present invention may be any steel as long as the above composition and index are satisfied. However, they fall into the category of low-alloy steel to medium-alloy steel, and the main elements other than the trace elements fall within the above range.

Other trace elements include, for example, Ti
It is desirable to include Ti suppresses cracking of the surface of the continuously cast slab by stably fixing N as TiN, and improves the toughness of the HAZ. Further, by stabilizing the effect of improving the hardenability of B, the uniformity of the strength of the thick steel plate in the thickness direction, that is, the hardness distribution in the thickness direction can be improved by U.
It has a remarkable effect on making it flat rather than character-shaped. 0.00
Below 5%, such an effect is not apparent, while
If it exceeds 0.02%, on the contrary, the toughness of the base material and the HAZ deteriorates.
% Is desirable.

Inevitably remaining impurities (P, S, Sn)
Etc.) should be as low as possible.

Next, the reasons for limiting the rolling and heat treatment conditions will be described.

2. Rolling and heat treatment conditions Heating temperature: If the slab heating temperature is lower than 900 ° C., austenitization is insufficient, so that even if the rolling and heat treatment conditions are changed later, a sufficient property improvement cannot be obtained. On the other hand, if the heating temperature exceeds 1100 ° C., the austenite grains do not become finer, and not only the base material toughness of the steel sheet significantly decreases, but also the N
When b is contained, the amount of Nb solid solution increases, so that Nb carbonitride precipitates during rolling and hardly recrystallizes, and a strong texture is formed. Will occur. Therefore, the slab heating temperature before rolling is 900 to 1100 ° C.

Rolling finishing temperature: The rolling finishing temperature is limited to improve brittle fracture propagation stopping performance, particularly at the surface layer. By limiting the finishing temperature to a temperature range of 850 to 700 ° C., the structure of the surface layer becomes fine unrecrystallized austenite, and the quenching start temperature of the subsequent direct quenching is appropriately reduced. Prevents excessive hardening of the surface layer and improves brittle fracture propagation stopping performance and toughness. In order to sufficiently obtain such an effect, it is desirable to set the cumulative rolling reduction in the non-recrystallized austenite region to 25 to 75% and to control the quenching start temperature to a range of 650 to 800 ° C. Here, the “cumulative rolling reduction in the unrecrystallized austenite region” refers to the total rolling reduction in the temperature range of 975 ° C. to the rolling finishing temperature when Nb is included, and 900 ° C. to rolling finishing in the case where Nb is not included. Temperature refers to the total reduction in the temperature range.

As described above, descaling, straightening, heating at a uniform temperature, and carrying into a quenching device may be inserted between the finish rolling and the direct quenching.

Tempering conditions: The tempering in the method of the present invention improves the balance between strength and toughness in addition to improving the balance between strength and toughness by removing the strain caused by quenching and precipitating fine carbides in the same manner as general tempering. The purpose is to finely disperse austenite. 3.
A steel containing 5-6.5% Ni is tempered in a temperature range of 700 ° C or less, preferably 550-650 ° C, and 10%
In the following, desirably, when 1 to 5% of stable retained austenite is finely dispersed in martensite, the toughness and brittle fracture propagation stopping performance of the base material are dramatically improved. 70
When heating to a temperature range exceeding 0 ° C., austenite is destabilized, and the above-mentioned effects are reduced.
It should be below ° C. The reason why the low-temperature toughness and the brittle fracture initiation characteristics deteriorate when the amount of austenite exceeds 10% and becomes unstable is as described above.

It is necessary to perform water cooling after tempering. This is because the temperature range in which temper embrittlement proceeds at 475 to 550 ° C. is quenched to suppress temper embrittlement, and at the same time, quenched to a lower temperature range to stabilize retained austenite.

[0051]

EXAMPLES Table 1 is a list showing the chemical compositions of steels used in the practice and comparison of the present invention. Table 2 is a list showing P and S among impurities of these steels. P and S are controlled low. These steels were melted in a 70-ton converter and cast in an Ar atmosphere to form a steel ingot having a thickness of 600 mm × 1000 mm width × 2500 mm height.

[0052]

[Table 1]

[0053]

[Table 2]

The columns such as the rolling heating temperature and the finishing temperature in Tables 3 to 6 show the conditions of the rolling and heat treatment applied to these steel ingots. Thick steel plates having a thickness of 150 to 200 mm were produced by rolling and heat treatment shown in these tables, test pieces were cut out from each steel plate, and mechanical properties and weldability were evaluated.

[0055]

[Table 3]

[0056]

[Table 4]

[0057]

[Table 5]

[0058]

[Table 6]

The toughness was determined by the Charpy impact test (JIS Z 2242:
The test piece was evaluated by the transition temperature (vTs: JIS Z 2242) in the JIS Z 2202 No. 4 test piece), and the brittle fracture arrestability was evaluated by the fracture toughness (Kca) at 0 ° C and -30 ° C in the double tensile test. . The tensile and Charpy impact tests of the base material were evaluated at the surface layer, (1/4) t portion and (1/2) t portion of the plate thickness. In the case of the Charpy test, the surface layer is positioned at a position 6 mm from the surface at the center line of the test piece, and is subjected to a tensile test (JIS Z 2241: test piece JISZ 2201).
In the case of No. 4 test piece), the position 10 mm from the surface was aligned with the center line of the test piece.

In the evaluation of the weldability, a steel plate having a thickness of 40 mm centering on the thickness (1/4) t portion of the thick steel plate was cut out, and six layers were formed on one side with a heat input of 45 kJ / cm at the groove. Was subjected to submerged arc welding (SAW) and subjected to a test. In addition, in order to evaluate weld cracking, 100 kg /
A weld bead was placed at a preheating temperature of 100 ° C. on a y-groove constraint crack test specimen (JIS Z 3158) by manual welding using a mm ² class welding rod, and after a predetermined time had elapsed, the cross section was cut out and examined for cracks. This welding bead holder is used when the welding material
After leaving it to stand for 1 hour in an environment of 0 ° C. and 80% humidity and humidifying, the test was performed in an environment of a temperature of 30 ° C. and 80% humidity.

The toughness of the welded joint was evaluated by the above-mentioned Charpy test. The notch was placed at the interface (bond) between the weld metal and the HAZ (bond). Evaluation was made in both included tests.

In Tables 3 to 6, the columns of the mechanical properties, Kca in the double tensile test, the toughness of the welded joint, and the y-groove constraint cracking test show these results. Table 3
As shown in Table 5 and Table 5, the thick steel sheet produced by the method of the present invention has a desired tensile strength (880 MPa or more) and good toughness (vTs-80 ° C. or less at each position of the sheet thickness) and is excellent. Brittle fracture arrest characteristics (K at -30 ° C)
ca is 200MPa · m ^0.5 or more) and excellent joint toughness (v
Ts-80 ° C or less) and welding crack resistance (preheating temperature 10)
0 ° C. or less). On the other hand, in Test Nos. 6 to 9 (Table 4), the steel composition (Steel A) was within the range of the present invention, but the rolling heat treatment conditions were out of the range. It can be seen that the propagation stop characteristics are significantly inferior. Further, as shown in Table 6, in Test Nos. 14 to 19 in which the composition of the steel was out of the range of the present invention, not only the performance of the base material but also the weldability was remarkably inferior to the performance of the examples of the present invention. You can see that.

[0063]

According to the method of the present invention, brittle crack propagation can be stopped at a plate thickness of 100 mm or more, good weldability, a tensile strength of 880 MPa or more, a Charpy impact transition temperature of -80 ° C or less, and -30 ° C. The stable supply of the thick high-tensile steel plate that can be performed becomes possible. As a result, a large-sized structure such as a penstock and a marine structure with high reliability can be manufactured while reducing the welding work cost, which is very useful in industry.

[Brief description of the drawings]

FIG. 1A is a schematic diagram showing an ESSO test piece and a test situation. (B) is a schematic diagram showing a double tensile test piece and a test situation.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Notch 2 ... Stop crack tip 3 ... Wedge 4 ... Impact load 5 ... Brittle crack generation load σ ... Stress that causes brittle crack to propagate

Claims (1)

[Claims] C: 0.07 to 0.18 by weight
%, Mn: 0.4 to 1.5%, Si: 0.2% or less, N
i: 3.5 to 6.5%, Cr: 0.1 to 1%, Mo:
0.1-1%, Cu: 1.5% or less, V: 0.01-
0.1%, Nb: 0.03% or less, N: 0.006% or less, solAl: 0.05% or less, and B: 0.000
3 to 0.002%, C (%) + Si (%) + 1.
For a steel having an index of 5solAl (%) in the range of 0.1 to 0.25%, the heating temperature before hot rolling is 900 to
In the temperature range of 1100 ° C., finish rolling of hot rolling was performed for 8 hours.
Production of thick high-strength steel sheet excellent in brittle fracture propagation arrestability and weldability characterized by being quenched as it is in a temperature range of 50 to 700 ° C., and then tempered and water-cooled at 700 ° C. or less. Method.