JP3387371B2 - High tensile steel excellent in arrestability and weldability and manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel having a tensile strength of 900 MPa or more (hereinafter referred to as "TS") and excellent in arrestability and weldability, and is used for transporting natural gas or crude oil. The present invention relates to high-strength steel suitable for use in line pipes, various pressure vessels, etc., and a method for producing the same.

[0002]

2. Description of the Related Art In pipelines for long-distance transportation of natural gas, crude oil, etc., reduction of transportation cost is a universal need, and improvement of transportation efficiency by increasing operating pressure has been pursued. To increase the operating pressure, it is possible to increase the wall thickness of conventional strength grade pipes.
There is a problem that the increase in wall thickness lowers the welding work efficiency at the site and causes a decrease in work efficiency due to an increase in the weight of the structure. To avoid this problem, there has been a consistent aim to increase the strength of pipe materials and limit the increase in wall thickness. As a result, the American Petroleum Institute (API) standardized X80 grade steel. It is put to practical use.

Although a standard has not been established for a line pipe material having a strength higher than that of X80, it has been clarified that it is possible to manufacture up to about X100 grade steel based on the manufacturing technology of X80 grade steel. Further, in recent years, a high-strength steel exceeding X100 grade steel utilizing aging precipitation of Cu and a manufacturing method thereof have been proposed (JP-A-8-104922 and JP-A-8-209287).
JP-A-8-209288). Further, as similar high strength steel exceeding X100 grade steel and its manufacturing method, in addition to the method utilizing the aging precipitation of Cu, Mn
Also disclosed is a steel having a high content of iron and a method for producing the same (Japanese Patent Application Laid-Open Nos. 8-209290 and 8-2092).
91 publication).

However, although the former method of utilizing Cu precipitation strengthening provides both high strength of the base metal and excellent field weldability, it also has excellent arrestability due to the presence of Cu precipitates dispersed in the matrix. Is difficult. Moreover, in the latter high Mn steel, low cost Mn
Although there is a merit that expensive alloy elements such as Ni and Mo can be reduced by using, the required arrestability cannot be secured when Ni is lowered.

The term "arrestability" as used herein means the ability to suppress the development of cracks in the pipe body even if brittle fracture should occur from a defective portion inevitably occurring in a welded portion during construction. Therefore, the arrestability may be referred to as brittle crack propagation stopping property. The characteristic that is paired with this characteristic, that is, the characteristic that brittle fracture is less likely to occur from the defect portion is called brittle crack generation prevention characteristic. These two properties are not completely independent and unrelated properties, and both properties deteriorate together when, for example, the precipitates are co-deposited and hardened. However, another factor, for example, the refinement of the structure, has a large effect of improving the brittle crack initiation inhibiting property, but there is a difference that the brittle crack propagation arresting property has an improving effect, but the degree thereof is not so large. When discussing these two properties,
It should be noted that some impact test methods produce results that include both properties. The Charpy impact test yields results that include both properties, but is said to include more of the properties of brittle crack initiation inhibition properties.
In order to obtain the result of only the arrestability, it is necessary to use a relatively large test piece such as a DWTT or a double tensile test described in Examples described later, in which a crack generation part and a crack stopping part are separated to some extent. There is. Historically, the property obtained by the Charpy test without making such a distinction was called "toughness". Even today, it is usually called toughness including the brittle crack propagation arresting property and the brittle crack initiation arresting property. In the present specification, the term toughness refers to both unless otherwise specified.

[0006]

DISCLOSURE OF THE INVENTION The object of the present invention is 90
It is to provide a high-strength steel excellent in arrestability and weldability that satisfies TS of 0 MPa or more. Specifically, it is to provide a high-strength steel having all of the following properties and a manufacturing method thereof.

Base material strength: TS ≧ 900 MPa Base material toughness: vE-40 ≧ 200J Arrestability: 85% ductile fracture transition temperature at DWTT
(FATT) ≤ -40 ° C Welded joint strength: TS ≥ 900MPa Welded joint toughness: vE-20 ≥ 100J Local weldability: y No cracking in pre-heating condition in groove welding crack test

[0008]

[Means for Solving the Problems]
Of 900 MPa or more, excellent composition for arresting, and various compositions in order to obtain high-strength steel excellent in weld toughness even when welding with relatively large heat input (3 to 10 kJ / mm) is performed,
The following items were able to be confirmed by diligently studying the steel material having a structure.

(A) Even when utilizing the precipitation hardening of Cu, by suppressing the Mn content and setting the Ni content within an appropriate range, the toughness is secured and a desired steel sheet excellent in weldability is obtained. . In particular, it is possible to obtain excellent arrestability in high-strength steel by adjusting the Ni content appropriately.

(B) When the aspect ratio of the prior austenite grains is 3 or more, even in the lower bainite produced at the same transformation temperature, the transformation sites become dense and the structure becomes fine. The refined structure extends not only to the lath length of the lower bainite but also to the lath thickness, cementite, and the like. As a result, the matrix is toughened and the toughness, especially the arrestability is improved.

The present invention has been completed through basic studies and trial production at the manufacturing site based on the above matters, and its gist resides in the following high-strength steel and its manufacturing method.

(1) By weight ratio, C: 0.02 to 0.1
%, Si: 0.6% or less, Mn: 0.2 to 2.5%, N
i: 0.2 to 1.2%, Nb: 0.01 to 0.1%, T
i: 0.005-0.03%, sol Al: 0.1% or less, N: 0.001-0.006%, B: 0.0005
~ 0.0025%, Cu: 0 ~ 0.6%, Cr: 0 ~
0.8%, Mo: 0 to 0.6%, V: 0 to 0.1% and Ca: 0 to 0.006%, P: 0.015% or less of inevitable impurity elements, S: Carbon equivalent (Ceq) and V defined by the following formula at 0.003% or less
s is 0.42-0.58%, 0.28-0.42%
With a chemical composition in the range of, the ratio of the mixed structure of lower bainite and martensite is 9% of the whole structure.
0% or more, the ratio of lower bainite in the mixed structure is 1
A high-strength steel (referred to as [invention 1]) having 0% or less and an aspect ratio of old austenite grains of 3 or more and having excellent arrestability and weldability.

Ceq (%) = C + (Mn / 6) + {(Cu + Ni) / 15} + {(Cr + Mo + V) / 5} Vs (%) = C + (Mn / 5) + 5P- (Ni / 10)-(Mo / 15) + (Cu / 10) where each element symbol is% by weight of the content of that element
Is displayed. (2) By weight ratio, C: 0.02-0.1%, Si:
0.6% or less, Mn: 0.2% or more and less than 1.7%, N
i: 0.2 to 1.2%, Nb: 0.01 to 0.1%, T
i: 0.005-0.03%, sol Al: 0.1% or less, N: 0.001-0.006%, B: 0.0005
~ 0.0025%, Cu: 0 ~ 0.6%, Cr: 0 ~
0.8%, Mo: 0 to 0.6%, V: 0 to 0.1% and Ca: 0 to 0.006%, P: 0.015% or less of inevitable impurity elements, S: Carbon equivalent (Ceq) and V defined by the following formula at 0.003% or less
s is 0.42-0.58%, 0.28-0.42%
With a chemical composition in the range of, the ratio of the mixed structure of lower bainite and martensite is 9% of the whole structure.
0% or more, the ratio of lower bainite in the mixed structure is 1
A high-strength steel (referred to as [invention 2]) having an arrestability and weldability of 0% or less and an aspect ratio of the former austenite grains of 3 or more.

Ceq (%) = C + (Mn / 6) + {(Cu + Ni) / 15} + {(Cr + Mo + V) / 5} Vs (%) = C + (Mn / 5) + 5P- (Ni / 10)-(Mo / 15) + (Cu / 10) where each element symbol is% by weight of the content of that element
Is displayed. (3) A high-strength steel (referred to as [invention 3]) described in [invention 1] or [invention 2] whose metallographic structure satisfies one or both of the followings.

The lath structure of the mixed structure of lower bainite and martensite has an average thickness of 1 μm or less, and the lath structure of martensite and lower bainite has an average length in the longitudinal growth direction of 20 μm or less.

The major diameter of the cementite particles precipitated inside the lath of the lower bainite is 0.5 μm or less.

(4) A slab having the chemical composition described in [Invention 1] or [Invention 2] is heated to 900 to 1200 ° C. and then rolled to obtain a cumulative reduction ratio of austenite in the non-recrystallization temperature range of 50. % Or more, and the rolling ends when Ar is ₃ points or more,
A method for producing a high-strength steel excellent in arrestability and weld characteristics, characterized by cooling from an Ar ₃ point or more to a cooling rate of 10 to 45 ° C / sec, and tempering at an Ac less than ₁ point if necessary ( [Invention 4]).

(5) A steel plate having the chemical composition described in [Invention 1] or [Invention 2] is processed and welded to produce a steel pipe, and the steel pipe is heated to a temperature range of Ac ₃ point to 980 ° C. A method for producing a high-strength steel pipe excellent in arrestability and weld property (which is referred to as [invention 5]) in which quenching is performed and tempering is performed at a temperature of less than Ac ₁ point if necessary.

In each of the above inventions, when austenite remains in the metal structure, its ratio is determined by X-ray diffraction. Other upper bainite, pearlite, etc. can be distinguished from the mixed structure of lower bainite and martensite by observing the metal surface etched by picral with an optical microscope. In addition, since the carbides formed in these tissues also have morphological characteristics in the respective tissues, they can be identified by observing the replica extracted from the carbides with an electron microscope at a magnification of about 2000 times,
Therefore, the ratio of the mixed structure of lower bainite and martensite can be obtained. This ratio refers to a ratio averaged over 10 to 30 fields of view.

The length in the longitudinal direction of the laths of the lower bainite and martensite is an average length, and can be measured in the visual field of an optical microscope, the visual field of an electron microscope of the extracted replica, or the like. It goes without saying that it is possible to take a picture and measure it on the picture. The average thickness of the lath is measured by observing the thin film with a transmission electron microscope. Other methods are not impossible, but errors will increase. The major axis of cementite precipitated in the lower bainite can be measured by observing the thin film with a transmission electron microscope or observing an extraction replica. In each case, 10 to 30 visual fields are measured and averaged.

The "non-recrystallizing temperature range" refers to a temperature range in which high-density dislocations introduced by processing such as rolling rapidly disappear with movement of the interface, and includes Nb targeted by the present invention. In the case of steel, it means a temperature range of 975 ° C or lower and Ar ₃ points or higher. “Cumulative rolling reduction” refers to the cumulative rolling reduction in this non-recrystallization temperature range, and is (thickness at 975 ° C.−thickness at Ar ₃ point) / 9
The wall thickness at 75 ° C. "Temperature" and "Cooling rate"
Is the value at the center of the wall thickness.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The reason for limiting the present invention as described above will be described in detail below. In the following description, “%” of the content rate of the alloying element represents “% by weight”.

1. Chemical composition C: 0.02 to 0.1% C is an element effective for increasing strength, and in the present invention, 0.02% or more is necessary to obtain desired strength. However, if it exceeds 0.1%, not only the arrestability and toughness of the steel are deteriorated, but also the welding workability at the site is significantly deteriorated. Therefore, it is necessary to limit the upper limit to 0.1%.

Si: 0.6% or less Si is an element effective for deoxidation, but if it exceeds 0.6%, not only does the toughness of the weld heat affected zone (hereinafter referred to as "HAZ") deteriorate, , The upper limit of 0.
6%. The lower limit of Si may be 0, but if it is set to 0, the loss of Al becomes large at the time of deoxidation. Therefore, the Si content rate that remains after deoxidation, for example, about 0.01% is desirable as the lower limit.

Mn: 0.2-2.5% Mn is an element effective in increasing strength, and for that purpose,
0.2% or more is required. However, if it exceeds 2.5%, the arrestability of the base material and the toughness of the welded portion deteriorate, so in the case of the present invention in which TS is 900 MPa or more,
It is necessary to limit Mn to 2.5% or less. In addition, since excessive segregation of Mn promotes center segregation during casting, it must be avoided when manufacturing high strength steel having a TS of 900 MPa or more. In order to avoid center segregation and further improve arrestability and toughness, Mn should be 1.
It is desirable to set it to less than 7%. In the steel of [Invention 2], even if a brittle crack is generated from a small accident, the development of the brittle crack is extremely unlikely to occur.
It is a high-strength steel containing Mn of 0.2% or more and less than 1.7% with the upper limit of n reduced.

Ni: 0.2 to 1.2% Ni is effective in increasing strength and also has an effect of improving toughness and arrestability. For this reason, Ni is 0.
2% or more is required. However, even if it exceeds 1.2%, the increase in strength and the improvement in toughness commensurate with the cost increase cannot be obtained, so the upper limit is made 1.2%.

Nb: 0.01 to 0.1% Nb is an element effective for refining austenite crystal grains in controlled rolling, and for this reason, it is set to 0.01% or more. However, if it exceeds 0.1%, not only the toughness deteriorates but also the weldability at the site deteriorates remarkably, so the upper limit is made 0.1%.

Ti: 0.005 to 0.03% Ti is an element effective for refining austenite crystal grains during slab heating, and is 0.005% or more. Especially Nb
In the case of containing steel, a trace amount of Ti is effective for suppressing cracking of the surface of the continuously cast slab promoted by Nb. Such an effect can be obtained with 0.005% or more. However, if it exceeds 0.03%, TiN becomes coarse and the effect of refining the austenite crystal grains disappears, so the upper limit of Ti is made 0.03%.

Sol Al: 0.1% or less Al is usually added to steel as a deoxidizer. Further, it has an effect of refining the structure, and is a useful element from the viewpoint of improving the toughness of the base material. Excessive Al causes coarsening of inclusions and impairs the cleanliness of steel, so the upper limit of sol Al is made 0.1%.
A preferred upper limit value is 0.06%, more preferably 0.05
%.

N: 0.001 to 0.006% N forms TiN together with Ti and has an effect of suppressing coarsening of austenite grains during slab reheating and welding. The lower limit for obtaining such an effect is 0.001
%. On the other hand, excessive N causes deterioration of slab quality and deterioration of HAZ toughness due to an increase in solid solution N, so the upper limit is made 0.006%.

B: 0.0005 to 0.0025% In order to secure the strength, 0.0005% or more is necessary. However, if it exceeds 0.0025%, the toughness deteriorates, so the upper limit is made 0.0025%.

Cu: 0 to 0.6% Cu may be added without addition. However, since Cu is effective in increasing the strength, Cu is added when the strength is to be increased. If it exceeds 0.6%, the toughness deteriorates, so the upper limit is made 0.6% when it is added. 0.2 to ensure sufficient strength
It is desirable to set it to be at least%.

Cr: 0 to 0.8% Cr may be added without addition. When it is added to secure the strength, if it is less than 0.3%, the effect is not clearly shown.
It is desirable to be 3% or more. On the other hand, if it exceeds 0.8%, the toughness deteriorates, so the upper limit is made 0.8%.

Mo: 0 to 0.6% Mo may be added without addition. When added to secure the strength, if less than 0.1%, the effect is small, so 0.1% or more is desirable. On the other hand, if it exceeds 0.6%, the toughness deteriorates, so the upper limit is made 0.6%.

V: 0 to 0.1% V may be added without addition. However, when it is added to secure the strength, if less than 0.01%, the effect does not appear, so 0.0
It is desirable to be 1% or more. On the other hand, if it exceeds 0.1%, the toughness deteriorates, so the upper limit is made 0.1%.

Ca: 0 to 0.006% Ca may be added without addition. Ca controls the morphology of MnS and improves the toughness of the steel in the direction perpendicular to the rolling direction, so is added when the effect is obtained. If the content is less than 0.001%, the effect is small.
It is desirable to set it to be at least%. On the other hand, if it exceeds 0.006%, nonmetallic inclusions in the steel increase and cause internal defects, so the upper limit is made 0.006%.

Inevitable impurity element P: 0.015%,
S: 0.003% or less It is desirable to lower the inevitable impurity element. Among the inevitable impurity elements, the contents of P and S have a significant effect on the toughness of the steel, so it is necessary to reduce them. The reduction of P reduces the center segregation of the slab and reduces brittle fracture at grain boundaries. S becomes MnS and precipitates in the steel, which stretches by rolling and adversely affects toughness. In order to suppress these adverse effects, it is necessary to set P to 0.015% or less and S to 0.003% or less.

Ceq: 0.42-0.58% In addition to the limits of the individual alloying elements, Ceq is further used in the present invention.
Limit (carbon equivalent). It is possible to obtain a steel having a desired structure in a wide production range without deterioration of toughness by forming a mixed structure of lower bainite and martensite not only in the base material but also in the HAZ part due to the limitation of carbon equivalent. . When the carbon equivalent is less than the lower limit, it becomes difficult to maintain the tensile strength of the base material at 900 MPa or more due to insufficient hardenability. On the other hand, if the carbon equivalent exceeds the upper limit, the HAZ toughness and the toughness on the surface of the steel sheet deteriorate due to an excessive increase in hardenability.

Vs: 0.28 to 0.42% Vs is added to the limitation of individual alloying elements in order to reduce center segregation of continuously cast slabs. Vs value is 0.4
If it exceeds 2%, center segregation will occur strongly and TS900MP
In the case of high-strength steel of a or more, the toughness of the central part deteriorates. On the other hand, if it is less than 0.28%, TS900 MPa or more cannot be secured although center segregation does not occur, so the lower limit is made 0.28%.

2. Metal Structure Next, the metal structure will be described. In order to simultaneously satisfy the strength and toughness of the base material, it is necessary to have a mixed structure of lower bainite and martensite, and the ratio of both structures is 90% or more in total. The lower bainite referred to here is a structure in which cementite is precipitated inside lath-like bainitic ferrite. The reason why the toughness of the mixed microstructure of lower bainite and martensite is excellent is that the lower bainite formed prior to martensite forms a “wall” that divides austenite grains, and the growth of martensite and packets (fracture unit of brittle fracture) This is because it suppresses the size expansion of (). Since lower bainite has lower strength than martensite, if the ratio of lower bainite becomes too high, the strength of the entire steel decreases. Therefore,
In order for TS to satisfy 900 MPa or more, the ratio of lower bainite must be 10% or less of the total of martensite and lower bainite.

Further, in order to further improve the toughness in the mixed structure of lower bainite and martensite, it is important to disperse the lower bainite formation positions. For this purpose, it is necessary to process austenite sufficiently and then transform it from the non-recrystallized state. After processing, the unrecrystallized austenite contains a high concentration of lower bainite nucleation sites, and lower bainite can be generated from the unrecrystallized austenite grain boundaries and many nucleation sites in the grains. The flatness of the non-recrystallized austenite grains necessary for exhibiting these effects is required to have an aspect ratio of 3 or more. Here, the aspect ratio of the non-recrystallized austenite grains refers to a value obtained by dividing the diameter (major axis) of the austenite grains stretched in the rolling direction by the diameter (minor axis) in the plate thickness direction.

In order to satisfy high strength and high arrestability, it is necessary for the metallographic structure to satisfy one or both of the following two characteristics.

[1] The average thickness of lath in the mixed structure of lower bainite and martensite, which is composed of lath-like structure, is 1 μm or less. The lath thickness of bainite changes depending on the transformation temperature, and the higher the temperature is, the thicker it is. Bainite having a high transformation temperature cannot obtain high toughness, and if the average thickness of lath exceeds 1 μm, the toughness targeted by the present invention cannot be obtained. The lower limit is not particularly limited, but in the case of the C content of the steel targeted by the present invention, it is desirable to set it to about 0.3 μm, and it is difficult to make it thinner than that.

The aggregate unit (called a packet) of the mixed structure of martensite and bainite having the same orientation must be small. Since the crack length of brittle fracture is considered to correspond to this packet diameter, it is necessary to reduce the packet diameter in order to shorten the crack length. The strongest effect on the packet size is the length of the lath structure of martensite and bainite in the longitudinal growth direction. If this length exceeds 20 μm, excellent toughness cannot be obtained, and the average lath length is 20 μm for improving toughness.
Must be: The smaller the lower limit is, the more preferable. It is not particularly limited, but about 10 μm is usually the limit that can be realized.

[2] Cementite precipitates in the lath in the lower bainite structure. At this time, the major diameter of the cementite particles is set to 0.5 μm or less. Even in the lower bainite where the cementite precipitates in the lath, when the lower bainite is formed when the recrystallized austenite is cooled relatively slowly, the cementite particles are coarse and sufficient arrestability cannot be obtained. In the case of lower bainite formed from unrecrystallized austenite, the dislocation density in austenite is inherited by lower bainite, so the cementite precipitation sites in lower bainite increase and cementite precipitates finely. In order to obtain the target arrestability in the present invention, the major axis of cementite needs to be 0.5 μm or less.

3. Manufacturing Method Next, a manufacturing method such as rolling and heat treatment of [Invention 4] will be described.

The most important point in the production method of the present invention is that not only the austenite grain boundaries but also the austenite in which the integrated dislocations introduced by the hot rolling are preserved, that is, the austenite grains in the unrecrystallized state also form the lower bainite and martensite. Is to nucleate and make this an appropriate volume ratio.

The slab heating temperature is set to 1200 ° C. or lower in order to prevent coarsening of austenite crystal grains during heating, while Nb effective for refining crystal grains during rolling and precipitation strengthening after rolling is used. The temperature is set to 900 ° C. or higher to form a solid solution. In order to nucleate the lower bainite from within the austenite grains and to suppress the growth of the lower bainite, high density dislocations are necessary. For that purpose, the austenite unrecrystallized temperature range (975 ° C or lower and Ar ₃ or higher) is 50 It is essential to carry out rolling of at least%. On the other hand, if the rolling reduction ratio of austenite in the non-recrystallization temperature region exceeds 90%, the anisotropy of mechanical properties becomes remarkable, so the rolling reduction ratio in the non-recrystallization temperature region is preferably 90% or less.

In order to suppress the formation of upper bainite which is generated when the temperature is slowly cooled below Ar ₃ point, it is necessary to cool from the Ar ₃ point or higher at a cooling rate within a certain range. The cooling rate after rolling is limited in order to obtain a mixed microstructure of lower bainite and martensite in good balance and finally obtain a fine microstructure, and is set in the range of 10 to 45 ° C / sec.
If the cooling rate is less than 10 ° C / sec, the upper bainite is 1
It is formed in an amount of more than 0%, or the proportion of the lower bainite in the mixed structure is more than 10%, and the transformation product at 500 ° C. or less is less than 60% by volume. As a result, the strength, toughness, and arrestability that the high-strength steel targeted by the present invention should have are not ensured. On the other hand, if it exceeds 45 ° C / sec, lower bainite is not formed and only martensite is formed, and the toughness and arrestability deteriorate.

300 to 450 without cooling to room temperature
From the viewpoint of improving the toughness due to the tempering effect and preventing hydrogen-like defects, it is desirable to stop at 0 ° C. and then gradually cool.
The midway stop temperature is limited to a temperature range of 300 to 450 ° C., and cooling stop at higher temperatures directly leads to insufficient quenching. Therefore, in order to obtain a further tempering effect, tempering is performed at less than Ac ₁ point.

[Invention 5] is to manufacture a steel sheet having the chemical composition of [Invention 1] or [Invention 2] using the production method of [Invention 4] and the like, and process the steel sheet, for example. This is a method of manufacturing a steel pipe by a method of processing into a U-shape, then into an O-shape, welding and expanding by a usual method, quenching, and tempering if necessary. The above UO pressing, welding and pipe expansion can be carried out by using an apparatus provided in a normal pipe manufacturing factory. The above-mentioned welding can be carried out by a submerged arc welding method or the like using a commercially available welding material.

[0052]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

Tables 1 and 2 are tables showing the chemical compositions of the steels used in the examples.

[0054]

[Table 1]

[0055]

[Table 2]

The test steel sheets are obtained by melting steels having the chemical compositions shown in Table 1 by a conventional method and casting the obtained slabs, which are rolled under various conditions, and have a plate thickness of 12 to 35 mm. .

Table 3 shows the method of manufacturing the test steel sheet.

[0058]

[Table 3]

Table 4 shows the metallographic structure obtained by the above manufacturing method.

[0060]

[Table 4]

Test pieces were taken from the center of the thickness of these steel plates and subjected to a tensile test (JIS Z 2241, test piece JIS Z 2201 No. 4).
And 2mmV notch Charpy impact test (JIS Z 224
2. Test piece JIS Z 2202 No. 4) and DWTT test were conducted. In the DWTT (Drop Weight Tear Test), a press notch is introduced into a steel plate of an original thickness, and impact judgment is applied by a large pendulum type falling weight at each test temperature, and the fracture surface of the fractured test piece is judged. In an effective test, a brittle fracture surface occurs from the bottom of the press notch, but after that, it changes to a ductile fracture surface, and when the ductile fracture surface is 85% or more of the entire fracture surface, the arrestability is at the test temperature. It is judged to be in.
If brittle cracks do not occur from the notch bottom, this is not an effective test. In this case, the notch bottom is carburized to further embrittle it so that brittle cracks develop from the notch bottom. In each of the present examples, a brittle fracture surface was generated from the bottom of the notch that had been press-notched in all the steels.

The weldability test is performed for tensile test and Charpy impact test of welded joint, and y for evaluation of field weldability.
A groove weld cracking test (JIS Z 3158) was performed. The welded joint was made by submerged arc welding (heat input 4 kJ / mm) with the above thick steel plate having a thickness of 25 mm using a V-groove single-sided 4 layer for tensile test and a die groove single-sided 4 layer for Charpy impact test. )
Then, each test piece was collected. As the flux and the wire, commercially available products for 100 kilo high tension steel were used. The tensile test piece was JIS Z 3121 No. 1 test piece. The Charpy impact test was carried out in accordance with JIS Z 3128, and the sample thickness was sampled from the 1/2 position so that the bottom position of the notch coincided with the melting line of the macro etch. The test temperature in the Charpy impact test is -40 ° C in the base metal and -20 ° C in the welded part.
And In the y-groove weld cracking test, a commercially available hand welding rod for 100 kilo high-tensile steel was used, and the welding bead was placed without preheating (25 ° C.). The amount of diffusible hydrogen measured by gas chromatography was 1.2 cc / 100 g.

Table 5 is a list showing the results of these tests.

[0064]

[Table 5]

In Test No. 8, which is a comparative example, the aspect ratio of the prior austenite grains was less than 3 and the toughness of the base material was low because the cumulative rolling reduction in hot rolling was small.
In test number 9, the quenching cooling rate was small, the ratio of the lower bainite was excessive, and the average thickness of the lath was also thick, and the TS of the base material and the welded joint was less than 900 MPa. Test Nos. 10 to 20 had poor performance because the chemical compositions were not within the limits of the present invention. In the test No. 10 which is a comparative example, since the B content was low, the ratio of the lower bainite and the average thickness of the lath were out of the limited range of the present invention without quenching, and the TS did not reach the target value. Test number 11 is Ni
The target value was not reached in both the Charpy test and the DWTT test because of the low value. Test No. 12 did not contain Nb, and therefore the unrecrystallized temperature range was limited to the low temperature side, so that the cumulative reduction ratio in the unrecrystallized range could not be sufficiently secured, and the toughness of the base material deteriorated. Test No. 13 has an excessive Cu content, Test No. 14 has an excessive Mn content, Test No. 15 has an excessive Cr content, Test No. 16 has an excessive Mo content, Test No. 17
Has an excessively large Ceq and test No. 18 has an excessively large Vs, so that the toughness and arrestability of the base material are deteriorated. Further, since the test number 19 was Vs, and the test number 20 was Ceq, both were too low, the TS of the base material decreased.

On the other hand, test number 1 which is an example of the present invention
No. 7 is an operation within the limits of the present invention in terms of chemical composition, metal structure, rolling conditions, etc., and TS is 900 MPa.
As described above, 200 in the Charpy impact test at -40 ° C.
An absorbed energy of J or more was obtained. In the DWTT test for evaluating arrestability, 85% transition temperature (FA
The TT) became -40 ° C or lower. The characteristics of the welded joint are
TS is 900MPa or more, -2 in Charpy impact test
The result was that the absorbed energy at 0 ° C. satisfied 100 J or more. Furthermore, no cracks were generated in the weld even if welding was performed without preheating in the field welding work.

[0067]

EFFECTS OF THE INVENTION According to the present invention, it is possible to inexpensively provide a high-strength steel of TS900 MPa or more having good arrestability, toughness, and weldability, and it is possible to dramatically improve the transportation efficiency of a pipeline. Becomes

Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 38/58 C22C 38/58 (72) Inventor Yuichi Komizo 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries ( 58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (5)

(57) [Claims] 1. A weight ratio of C: 0.02 to 0.1%, S
i: 0.6% or less, Mn: 0.2 to 2.5%, Ni:
0.2-1.2%, Nb: 0.01-0.1%, Ti:
0.005-0.03%, sol Al: 0.1% or less,
N: 0.001-0.006%, B: 0.0005-
0.0025%, Cu: 0 to 0.6%, Cr: 0 to 0.
8%, Mo: 0 to 0.6%, V: 0 to 0.1% and C
a: 0 to 0.006%, P: 0.015% or less and S: 0.003% or less of the unavoidable impurity elements, and the carbon equivalent (Ceq) and Vs defined by the following formulas, respectively. The chemical composition is in the range of 0.42 to 0.58% and 0.28 to 0.42%, and the metal structure has a ratio of a mixed structure of lower bainite and martensite of 90% or more of the entire structure, The ratio of lower bainite in the structure is 10% or less, and the aspect ratio of the former austenite grains is 3
The high-strength steel excellent in arrestability and weldability characterized by the above. Ceq (%) = C + (Mn / 6) + {(Cu + Ni) / 15} + {(Cr + Mo + V) / 5} Vs (%) = C + (Mn / 5) + 5P- (Ni / 10)-(Mo / 15) + (Cu / 10) where all element symbols are% by weight of the content of that element
Is displayed. 2. By weight ratio, C: 0.02-0.1%, S
i: 0.6% or less, Mn: 0.2% or more and less than 1.7%,
Ni: 0.2 to 1.2%, Nb: 0.01 to 0.1%,
Ti: 0.005-0.03%, sol Al: 0.1% or less, N: 0.001-0.006%, B: 0.0005
~ 0.0025%, Cu: 0 ~ 0.6%, Cr: 0 ~
0.8%, Mo: 0 to 0.6%, V: 0 to 0.1% and Ca: 0 to 0.006%, P: 0.015% or less of inevitable impurity elements, S: Carbon equivalent (Ceq) and V defined by the following formula at 0.003% or less
s is 0.42-0.58%, 0.28-0.42%
With a chemical composition in the range of, the ratio of the mixed structure of lower bainite and martensite is 9% of the whole structure.
0% or more, the ratio of lower bainite in the mixed structure is 1
A high-strength steel excellent in arrestability and weldability, characterized in that it is 0% or less and the aspect ratio of the former austenite grains is 3 or more. Ceq (%) = C + (Mn / 6) + {(Cu + Ni) / 15} + {(Cr + Mo + V) / 5} Vs (%) = C + (Mn / 5) + 5P- (Ni / 10)-(Mo / 15) + (Cu / 10) where all element symbols are% by weight of the content of that element
Is displayed. 3. The high-strength steel according to claim 1, wherein the metallographic structure satisfies one or both of the following or: The average thickness of the lath of the mixed structure of lower bainite and martensite is 1 μm or less, and the average length of the lath structures of martensite and lower bainite in the longitudinal growth direction is 2
It is 0 μm or less. The major axis of the cementite particles precipitated inside the lath of the lower bainite is 0.5 μm or less. 4. A slab having the chemical composition according to claim 1 or 2 is heated to 900 to 1200 ° C. and then rolled to obtain a cumulative reduction of austenite in the unrecrystallized temperature range of 50% or more. , Ar ₃ points or more, rolling is completed, cooling is performed at a cooling rate of 10 to 45 ° C./sec from ₃ points or more of Ar, and if necessary, tempering is performed at less than ₁ point of Ac. A method for producing high-strength steel with excellent properties. 5. A steel pipe having the chemical composition according to claim 1 or 2 is processed and welded to produce a steel pipe, and the steel pipe is heated to a temperature range of Ac ₃ point to 980 ° C. for quenching treatment. Then
A method for producing a high-strength steel pipe having excellent arrestability and weld properties, which is characterized by performing a tempering treatment at a temperature of less than Ac _{1 as} required.