JP3327065B2 - Method for producing tempered high-strength steel sheet excellent in brittle crack propagation arrestability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an extremely excellent tensile strength of 880 MPa or more, which is suitable for use in penstocks of pumped-storage power plants and offshore structures in ice-sea areas where a high degree of safety is required. The present invention relates to a method for producing a thick high-strength steel sheet having improved toughness.

[0002]

2. Description of the Related Art In recent years, the tendency of welded steel structures to become larger has become remarkable, and the thick steel plates used for these structures have also become stronger and thicker. For example, a 780 MPa class high-tensile steel plate having a thickness of more than 150 mm is applied to a penstock of a pumped-storage type hydroelectric power plant, or a 780M plate having a thickness of 100 mm or more is used for a rack material of a jack-up drilling rig in an offshore structure.
Pa-class high-strength steel sheets have been used.

[0003] From such a background, it has been desired to supply an ultra-high-strength thick steel plate having a tensile strength exceeding 900 MPa. However, technology to simultaneously impart high toughness and excellent weldability to such ultra-high strength thick steel plates,
And the technology which can supply this stably in large quantities has not been established yet.

[0004] For example, Japanese Patent Publication No. 6-4889 discloses Mn-Ni-Cr-Mo containing 0.2 to 0.35% of C.
Manufacture of tempered high-strength steel sheet exceeding 90 kgf / mm ² (880 MPa) with a plate thickness of 40 mm or more in which the Mn / Cr ratio of the steel is 1.5 or less in weight ratio to improve low-temperature toughness. A method has been proposed. However, this method is insufficient to improve the toughness and weldability of a thick steel plate having a thickness of more than 100 mm.

The applicant of the present invention is characterized by a double quenching treatment of Nb-added Ni-Cr-Mo-B steel as a method for increasing the toughness of a 100 kgf / mm ² (980 MPa) class high-strength steel sheet having a thickness of more than 100 mm. (See Japanese Patent Publication No. 6-70250). The purpose of this method is to obtain a fine martensitic structure from the surface layer portion to the center portion of a thick steel plate stably through fine-grained austenite by twice quenching. As a result, the plate thickness was 150 mm.
Strength and transition temperature of Charpy impact test-6
High toughness of 0 ° C. or less is obtained. However, there is a limit to the improvement in toughness by miniaturization by simple double quenching alone, and in particular, the brittle fracture crack propagation characteristic of the thick steel plate aimed at by the present invention is insufficient. There is also room for improvement in the toughness of the heat affected zone.

[0006]

SUMMARY OF THE INVENTION The present invention provides a steel sheet having a tensile strength of at least 880 MPa and a Charpy
Transition temperature (vTs) is -80 ° C or less, plate thickness 100mm
Even if the above, the fracture toughness value in the brittle crack propagation arrest test is 200 MPa · m ^0.5 or more at −30 ° C., the welding crack arrest preheating temperature is 75 ° C. or less, and the vT
It is an object of the present invention to provide a method of manufacturing a thick-walled high-tensile steel having all the performances of s of -60 ° C or less.

[0007]

Means for Solving the Problems The inventors of the present invention have a thickness of 100
As a result of examining the improvement of the weldability and toughness of the 880 MPa class steel sheet having a tensile strength of at least 8.8 mm or more, particularly the improvement of brittle crack propagation arresting properties, the following findings were obtained.

Microstructure: Refinement of austenite grains and optimization of the ratio of martensite to bainite, as in the conventional production method, are not sufficient to prevent the propagation of brittle cracks. It is necessary to disperse fine residual austenite which is stable at low temperature. When the above-mentioned brittle crack propagation arresting property is given to a steel plate having a tensile strength of 880 MPa and a plate thickness of 100 mm or more, it is necessary to control this retained austenite to 10% or less, preferably 1 to 5%.

FIG. 1 is a drawing showing conditions of rolling and heat treatment in the manufacturing method according to the present invention. (A) is a method of re-heating and quenching and tempering twice after rolling,
(B) shows a method in which the first quenching of the second quenching is replaced by direct quenching after rolling. The method shown in the figure is applied to a Cu-Ni-Cr- containing 4.5-6.0% Ni.
By using the Mo-Nb-B steel, it is possible to finely disperse the retained austenite that is stable even at a low temperature.

To obtain better performance, C (%)
+ Si (%) + 1.5solAl (%) is set to 0.
It must be limited to 30% or less. As a result, the formation of a hard martensite phase in the base metal and the heat affected zone is suppressed, and the toughness, in particular, the brittle crack propagation stopping property is improved.

The combined use of the above methods and can significantly improve the toughness, particularly the brittle crack propagation arrestability.

Here, the gist of the present invention is a method of rolling and heat-treating steel having a specific composition shown below in order to finely disperse stable retained austenite and at the same time avoid formation of hard martensite. (See FIG. 1) (1) By weight%, C: 0.08 to 0.15%, Mn:
0.4 to 1.2%, Si: 0.20% or less, N: 0.0
05% or less, Cu: 0.15 to 1.5% , Ni: 4.5
６6.0% (excluding 4.5%) , Cr: 0.1 to 1.0
%, Mo: 0.1-0.8%, Nb: 0.005-0.
03%, V: 0.005 to 0.10%, solAl:
0.05% or less and B: 0.0003 to 0.0020
%, The balance consisting of Fe and unavoidable impurities, and the value of C (%) + Si (%) + 1.5solAl (%) being 0.30% or less is heated to a temperature range of 950 to 1250 ° C. 850 to 105 after hot rolling
Reheat to the temperature range of 0 ° C and perform the first quenching b
After that, a second quenching is performed by further heating to a temperature in a temperature range of 750 to 950 ° C. and lower than the first quenching temperature, followed by tempering at a temperature of 700 ° C. or lower and water cooling. A method for producing a tempered high-strength high-strength steel sheet excellent in brittle crack propagation stopping characteristics and weldability characterized by the following.

(2) The steel having the composition described in the above (1) is
After heating to a temperature range of 50 to 1250 ° C. and performing hot rolling, quenching is performed directly from a temperature range of 650 ° C. or higher,
Next, reheating is performed to a temperature range of 750 to 950 ° C. to perform quenching, followed by tempering at a temperature of 700 ° C. or less and cooling with water. Method for manufacturing thick high-strength steel sheet.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The steel composition and the rolling heat treatment method will be described separately below.

1. Chemical composition C: 0.08 to 0.15% C is added for the purpose of securing the strength of the steel sheet. 0.08%
If it is less than 3,000, hardenability will be insufficient, it will be difficult to secure a tensile strength of 880 MPa, and toughness will also be insufficient.

On the other hand, if the content exceeds 0.15%, not only the toughness and brittle crack propagation stopping characteristics of the base material decrease, but also the hardness of the weld heat-affected zone increases to increase the weld cracking susceptibility. 0.15%. Usually, in order to prevent welding cracks during welding work, the steel material and the welding material are preheated to 100 to 200 ° C. When welding penstock at a construction site of a pumped storage power plant, for example, in a narrow tunnel in rock, if the preheating temperature exceeds 150 ° C, the working environment becomes worse, and the frequency of changing welding operators must be increased. Costs increase. In order to keep the welding crack prevention preheating temperature at 75 ° C. or less, it is desirable that the C content be 0.13% or less.

Mn: 0.4 to 1.2% Mn is an element added to improve the hardenability of the steel sheet and increase the strength, but if it is less than 0.4%, it is difficult to secure the strength. On the other hand, if it exceeds 1.2%, the toughness of both the base metal and the welded portion is reduced.

Si: 0.20% or less Si is added during refining for the purpose of deoxidizing steel. As a result, the amount that remains in the steel may be included. However, 0.
If it exceeds 20%, the toughness of the base metal and the heat affected zone of welding deteriorates, so the content is made 0.20% or less.

N: 0.005% or less N is an unavoidable impurity, and the smaller the content, the better.
If the content exceeds 0.005%, the toughness of the base metal and the heat affected zone of the weld is significantly reduced, so the content must be 0.005% or less.

Cu: 0.15 to 1.5% Cu improves hardenability, and when contained in excess of 0.8%, is effective for toughening the base metal by precipitation hardening during tempering. There is. When the content is less than 0.15%, no improvement in hardenability is clearly observed, but when the content exceeds 1.5%, not only the toughness of the base material and the welded portion is impaired, but also the hot ductility is greatly reduced. To 1.5%.

Ni: 4.5-6.0% Ni is an element indispensable for improving the low-temperature toughness of ultra-high-strength ultrahigh-strength steel sheets, in particular, brittle crack arrestability and weldability. When the content is 4.5% or more, a structure in which a mixed structure of fine martensite and bainite is mixed with several percent of stable residual austenite by a double quenching and tempering treatment is obtained. As a result, the low-temperature toughness, especially the brittle crack propagation arresting property, is dramatically improved. On the other hand, if it exceeds 6.0%, not only the improvement is reduced compared to the cost increase, but also the amount of residual austenite generated by the double quenching and tempering treatment increases, and the yield strength decreases. So N
The i amount is 4.5 to 6.0%. Ni lowers the transformation temperature without increasing the hardness, and causes compression stress in the vicinity of the weld due to expansion accompanying the transformation in a low temperature range. Therefore, by setting the Ni content to 4.5% or more, the welding crack prevention preheating temperature can be lowered.

Nb: 0.005 to 0.03% Nb refines austenite grains by forming fine Nb carbonitrides in the low temperature range of austenite.
Since the martensite structure is divided into austenite grain boundaries, the structure is refined accordingly. Since this fine martensitic structure is formed from the surface layer to the center of the thick steel plate, the toughness of the ultra-high strength thick steel plate is greatly improved. To obtain this effect, 0.005% or more is necessary. However, if it exceeds 0.03%, the toughness of the heat affected zone is significantly deteriorated.

Cr: 0.1-1.0% Cr improves the internal strength and toughness of the thick steel plate mainly through the improvement of hardenability. If it is less than 0.1%, a clear effect cannot be obtained, and if it exceeds 1.0%, stable retained austenite cannot be obtained, and the brittle crack propagation arresting property deteriorates. %.

Mo: 0.1 to 0.8% Mo has a great effect of improving hardenability and tempering softening resistance. If it is less than 0.1%, the required strength and toughness cannot be obtained, but if it exceeds 0.8%, the strength becomes too high, and the brittle crack propagation arresting property deteriorates.
And

V: 0.005 to 0.10% V contributes to the improvement of strength and toughness by increasing the tempering softening resistance and allowing the tempering temperature to be sufficiently high. If it is less than 0.005%, the effect is small, and if it exceeds 0.10%, the toughness is deteriorated, so the content is made 0.005 to 0.10%.

B: 0.0003% to 0.0020% B improves hardenability with a very small amount of solid solution, and makes the structure at the center of the sheet thickness martensite or a mixed structure of martensite and bainite. If the content is less than 0.0003%, the effect is small, and if it exceeds 0.0020%, the base material toughness and the toughness of the heat affected zone are significantly deteriorated.
To 0.0020%.

SolAl: 0.05% or less Al is generally added for deoxidation and finer structure. It may be added for the purpose of deoxidizing the steel, so that the amount remaining in the steel may be included. However, 880MPa
If the content exceeds 0.05%, the toughness of the weld heat affected zone deteriorates, and even if the structure is refined by heat treatment, the toughness, particularly brittle crack propagation arresting characteristics, is adversely affected. 0.05% or less. A desirable range is 0.005 to 0.025%. 0.00
If it is less than 5%, the effect on the refining of the structure is not clearly exhibited, and it is desirable that it does not exceed 0.025% in order to secure higher brittle crack propagation characteristics.

C (%) + Si (%) + 1.5solAl
(%): 0.3% or less In addition to the above-mentioned limitation of each element, in order to improve the toughness of the base material and the weld heat affected zone, particularly to improve the arrest of brittle crack propagation, C (%) + Si (%) + 1 .5solAl (%)
It is necessary to limit by some index. The limitation by this index is required when the Ni content is increased to utilize the retained austenite and when the hardenability is increased. In other words, the limitation by this index is the conventional tensile strength of 5
In the case of 80 MPa class or 780 MPa class steel, the purpose is to eliminate a toughness deterioration factor which did not cause a problem. The specific reasons for the limitation are as follows.

In the present invention, the base material structure is made to have a mixed structure of fine martensite and a small amount of fine stable austenite by adjusting the amount of Ni and optimizing the heat treatment conditions, thereby toughness, especially brittle crack propagation arresting characteristics. Has been improved. Fine austenite forms in the tempered martensite during the final heat treatment and is brought to room temperature by cooling. However, at this time, not all austenite remains without being transformed to room temperature, and some austenite is decomposed during cooling. Here, if the above index exceeds 0.3%, austenite is transformed into brittle hard martensite while carbon is dissolved in a supersaturated state, and the toughness of the base material, particularly the brittle crack propagation stopping characteristics, is remarkably reduced. .
By setting the above index to 0.3% or less, it is possible to suppress the formation of hard martensite in austenite, increase the stability of retained austenite, and improve the toughness of the base material.

The thick ultra-high strength steel targeted by the present invention has extremely high hardenability, and most of the weld heat affected zone has a martensitic structure. At this time, the index is 0.
If it exceeds 3%, carbide (mainly cementite) hardly precipitates during cooling, resulting in hard and brittle martensite. On the other hand, when the above index is 0.3% or less, carbide precipitation during cooling becomes easy, and martensite having high toughness is easily formed, so that the toughness of the heat affected zone is greatly improved.

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

2. Rolling and heat treatment conditions (a) When quenching and tempering are performed twice by reheating after rolling (1) Heating temperature: If the heating temperature is lower than 950 ° C, various precipitates generated when the slab is solidified are sufficient. Does not form a solid solution. As a result, the hardenability and the refinement of austenite grains before quenching become insufficient. At a heating temperature exceeding 1250 ° C., austenite grains are coarsened and the base material toughness of the steel sheet is significantly reduced, and at the same time, the cost of fuel for heating increases and the frequency of repair of the furnace increases. So the heating temperature of the slab is 950
１２1250 ° C. In addition, the slab before rolling of the thick steel plate which is the object of the present invention may be manufactured by slab rolling after casting the ingot, or the slab may be directly cast by the continuous casting method. Normally, a slab by a continuous casting method is used up to a plate thickness of about 100 mm, and a slab obtained by slab rolling from an ingot is used for a plate thickness larger than 100 mm.

(2) First quenching conditions: The purpose of the first quenching (b in FIG. 1) is mainly at the following two points. By making the prestructure a quenched structure, the austenite grain size at the time of the second quenching is refined, and the final structure is refined. In the case of a thick steel plate, since the steel plate is rolled into a thick steel plate and then gradually cooled for dehydrogenation, coarse carbonitrides of B, Al or Nb precipitate at the austenite grain boundaries and the like during the slow cooling. Unless these precipitates are dissolved, the effect of improving the hardenability of B and the effect of reducing austenite grains such as Nb and Al cannot be obtained. Therefore, to achieve these two objectives, 85
Heat to 0-1050 ° C. If the temperature is lower than 850 ° C., the above-mentioned carbonitride does not form a solid solution. On the other hand, when the temperature exceeds 1050 ° C., AlN forms a solid solution and solute N increases, and B precipitates as BN on austenite grain boundaries during quenching, so that the effect of improving the hardenability of B cannot be obtained. If the temperature exceeds 1050 ° C, A
Since 1N forms a solid solution, the structure becomes coarse. Quenching is performed using a device such as a roller quench. If the composition has a relatively high hardenability, it may be quenched in a water tank or quenched using a forced cooling device.

(3) Second quenching conditions: The second quenching (C in FIG. 1) is carried out by ensuring the hardenability of B completely and quenching from fine-grained austenite. The purpose is to obtain a martensite structure finely divided by austenite grain boundaries. In order to secure the hardenability and the refining effect, the second hardening temperature is 95%.
The temperature must be 0 ° C. or lower. However, when the temperature is lower than 750 ° C., austenitization is insufficient and sufficient strength cannot be obtained. This quenching is also performed using a roller quench device or the like as in the first quenching.

(4) Tempering conditions: The effect of tempering (d in FIG. 1) in the method of the present invention is not limited to the same effect as general tempering. That is, it is not only an effect of improving the balance between strength and toughness by removing strain caused by quenching and precipitating fine carbides.

A steel containing 4.5 to 6.0% of Ni is heated to 700 ° C.
By maintaining the temperature in a temperature range of preferably 550 to 650 ° C., fine and stable residual austenite of 10% or less, preferably 1 to 5% is dispersed in the martensite structure, and the toughness, particularly the brittle crack Dramatically improve propagation stop characteristics. The soaking time for tempering is desirably 10 to 60 minutes per 25 mm of plate thickness. Soak time 1
If the time is not longer than 0 minute, the entire steel sheet does not have uniform mechanical properties, and the reason why the time is within 60 minutes is that the strength is reduced by tempering. For example, plate thickness 15
In the case of 0 mm, the soaking time is preferably 60 minutes or more and 360 minutes or less. Here, the soaking time is a holding time after the surface temperature of the steel sheet reaches a predetermined tempering temperature. When the temperature exceeds 700 ° C., the amount of austenite increases,
Since the amount of Ni enriched in them relatively decreases, the retained austenite becomes unstable at a low impact test temperature, and the above-described effect of improving toughness cannot be obtained.

Further, the water cooling treatment after the heat retention of the tempering is as follows:
This is performed for the purpose of suppressing temper embrittlement by quenching the temperature range in which temper embrittlement occurs, and stabilizing retained austenite.

(B) Direct quenching after rolling, followed by reheating to perform quenching and tempering The rolling heating temperature, the second quenching condition and the tempering condition are the same as those described above. Direct quenching does not exist in the case of the above-mentioned manufacturing method (a), and its conditions will be described below.

(1) Direct quenching conditions: Direct quenching (E in FIG. 1) is an alternative to the first quenching in the double quenching treatment, in which finish rolling is performed at 650 ° C. or more and water cooling is performed from the temperature range. At this time, if the quenching start temperature is lower than 650 ° C., coarse bainite is mixed in the quenched structure and adversely affects the strength and toughness.
The temperature was set to 0 ° C. or higher.

[0040]

EXAMPLES Table 1 is a list showing the chemical compositions of seven steels having compositions within the scope of the present invention and six steels outside the range used in the present invention. Tables 2 to 4 are lists showing the rolling and heat treatment conditions performed on the steel slabs having these compositions. The tempering time for tempering was 2 per 25 mm plate thickness.
0 minutes. Table 2 shows steel A in the composition range of the present invention,
Both those obtained by subjecting B, C and D to rolling and heat treatment according to the present invention and those subjected to conditions outside the scope of the present invention are shown together. Table 3 shows steels E, F and G having the composition of the present invention subjected to rolling and heat treatment according to the present invention. Table 4 shows the rolling and heat treatment conditions for steels H, I, J, K, L and M whose compositions were outside the composition range of the present invention. These steel sheets were evaluated for strength, toughness and weldability. Toughness is vT in Charpy impact test
s, and the brittle crack arrestability was evaluated by the fracture toughness value in the ESSO test.

FIG. 2 is a drawing showing an outline of a test piece and a crack in a temperature gradient type ESSO test. In this test, a uniform stress was applied to a large test piece with a temperature gradient, a crack that was forcibly generated at the end of the test piece was forced into the test piece, and the temperature and crack length at the stop point were measured. Ask. From these, the fracture toughness value Kca can be determined. In the temperature gradient type ESSO test, Kca in a wide temperature range can be obtained by a small number of tests by utilizing the temperature dependency of Kca.

The welding crack susceptibility was evaluated by the presence or absence of cracks in a y-groove constraint crack test at 75 ° C. The welding rod was allowed to stand for 1 hour in an atmosphere of a temperature of 30 ° C. and a humidity of 80% to absorb moisture, and then welding was performed under an atmosphere of the same conditions.

The toughness of the welded joint was evaluated by subjecting a steel plate of each thickness to an X-shaped groove processing, and multi-layer welding (25 to 35) by sub-mark welding at a heat input of about 30,000 J / cm. Layers, 60-7
0 pass, preheating / interpass temperature of 125 ° C.). The test piece was processed by aligning the notch position of the Charpy test piece with the center of the bond and the heat-affected zone and subjected to the test.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

The test results are shown in Tables 2 to 4. The extra-thick steel plate manufactured by the method of the present invention has a tensile strength of 880 MPa or more in all of the surface layer of the sheet thickness, 1 / 4t and 1 / 2t,
And vTs -80 ° C or less. Further, the fracture toughness value at −30 ° C. in the ESSO test according to the method of the present invention sufficiently satisfies 200 MPa · m ^0.5 .
All of these results show that the method of the present invention is a very effective method for improving the strength and toughness of ultra-high-strength ultra-high-strength steel sheets, in particular, the property of stopping the propagation of brittle cracks.

[0049]

According to the method of the present invention in which rolling and heat treatment under specific conditions are applied to the steel having the specific composition described above, in addition to the refinement of the structure, the stable dispersion of retained austenite and the fine dispersion of the retained austenite can be achieved. Prevention of generation of hard martensite can be realized.
As a result, the tensile strength of 880 MPa or more and vTs-8 at all the thickness positions of the steel sheet having a thickness of 100 mm or more.
A performance with a fracture toughness value of 200 MPa · m ^0.5 or more at 0 ° C. or less and at −30 ° C. in an ESSO test is obtained.
In addition, since the preheating temperature during welding can be reduced to 75 ° C or less, the steel plate does not have to be harsh in a narrow place and the toughness of the welded joint is extremely good at vTs -60 ° C or less. It is. The steel sheet manufactured by the method of the present invention is applied to a penstock for pumped-storage power generation or a marine structure in an ice sea area where a high degree of safety is required, and even if a brittle crack occurs due to an unexpected cause, the brittle crack is generated. By stopping propagation, the range affected by the propagation can be suppressed to a certain range.

The steel sheet to which the method of the present invention is applied greatly contributes to the development of the industry in which these structures are manufactured and used.

[Brief description of the drawings]

FIG. 1 is a drawing showing the steps of rolling and heat treatment of the method of the present invention. (A) shows a method of reheating and then quenching and tempering twice after rolling, and (b) shows a method of replacing the first quenching of the twice quenching by direct quenching after rolling.

FIG. 2 is a drawing showing an outline of a test piece and a crack in a temperature gradient type ESSO test.

[Explanation of symbols]

B: Hot rolling, B: First quenching, C: Second quenching, D: Tempering E: Direct quenching

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-63-105921 (JP, A) JP-A-8-269543 (JP, A) JP-B-6-70250 (JP, B2) JP-B-57- 17925 (JP, B1) JP 5-18888 (JP, B2) (58) Fields surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (2)

(57) [Claims] (1) C: 0.08 to 0.15% by weight, M
n: 0.4 to 1.2%, Si: 0.20% or less, N:
0.005% or less, Cu: 0.15 to 1.5% , Ni:
4.5 to 6.0% (excluding 4.5%) , Cr: 0.1 to
1.0%, Mo: 0.1 to 0.8%, Nb: 0.005
-0.03%, V: 0.005-0.10%, solA
l: 0.05% or less and B: 0.0003 to 0.00
20%, the balance being Fe and unavoidable impurities, and C (%) + Si (%) + 1.5solAl
(%) Is 950-12
After hot-rolling by heating to a temperature range of 50 ° C.,
The first quenching is performed by reheating to a temperature range of ５０1050 ° C. and then a second quenching by heating to a temperature range of 750 to 950 ° C. and lower than the first quenching temperature. , Followed by tempering at a temperature of 700 ° C or less and water cooling to produce a tempered thick high-tensile steel sheet excellent in brittle crack propagation stopping characteristics and weldability. 2. A steel having the composition described in claim 1 having a composition of 950 to 1
After performing hot rolling by heating to a temperature range of 250 ° C.,
Direct quenching from a temperature range of 0 ° C or higher, followed by 750
A tempered die with excellent brittle crack propagation arrestability and weldability characterized by being reheated to a temperature range of up to 950 ° C. for quenching, followed by tempering at a temperature of 700 ° C. or less and water cooling. Manufacturing method of high tension steel sheet.