JPH11279639A - Production of steel plate for high strength linepipe excellent in hic resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for economically and stably producing a steel excellent in HIP resistance which is a high strength material of X70 and X80 as well as X65 in the API standard. SOLUTION: A steel contg., by weight, 0.03 to 0.08% C, 0.05 to 0.5% Si, 1 to 1.8% Mn, <=0.01% P, <=0.002% S, 0.02 to 0.05% Nb, 0.005 to 0.02% Ti, 0.01 to 0.07% Al and 0.0005 to 0.0025% Ca, in which Ceq>=0.28 is also satisfied, and the balance Fe with inevitable impurities, and in which the total Fe+MnO in the slag at the time of ladle refining furthermore satisfies 0.5 to 3% is heated to 1000 to 1200 deg.C to be subjected to hot rolling and is subjected to accelerated cooling till the steel plate surface temp. reaches <=500 deg.C, thereafter, the cooling is once stopped, and it is recuperated till the steel plate surface temp. reaches >=500 deg.C. Then, it is again subjected to accelerated cooling at the steel plate average cooling rate of 3 to 50 deg.C/sec to the steel plate surface temp. of <=600 deg.C, where Ceq=C%+Mn%/6+(Cu%+Ni%)/15+(Cr%+Mo%+V%)/5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a tube by hot plate mill or hot rolling mill, forming the tube by UOE forming, press bend forming, roll forming, etc., and by submerged arc welding or electric resistance welding. The present invention relates to a method for manufacturing a steel sheet which is welded and used as a line pipe for transporting crude oil and natural gas, and which is excellent in hydrogen-induced cracking resistance and has a strength level of API standard X65 grade or higher. The present invention relates to a method for manufacturing a steel sheet used as a material.

[0002]

2. Description of the Related Art In general, line pipes used for transporting crude oil and natural gas containing hydrogen sulfide have properties required for pipelines such as strength, toughness and weldability, as well as hydrogen-induced cracking resistance (HIC resistance). Resistance) and stress corrosion cracking resistance (SS resistance)
CC performance) is required. Here, the HIC is that hydrogen ions generated by the corrosion reaction are adsorbed on the steel surface, penetrate into the steel as atomic hydrogen, and diffuse around the non-metallic inclusions such as MnS in the steel and the hard second phase structure. It accumulates and it is said that the internal pressure causes cracks. For this reason, the following methods have been devised so far in order to prevent the occurrence of HIC. (1) While reducing the S content in steel, Ca and REM
By adding an appropriate amount of MnS or the like, the formation of long-extended MnS is suppressed, and the generation and propagation of cracks is suppressed by changing the form into finely dispersed spherical inclusions with low stress concentration (see, for example, No. 110119). (2) Regarding cracks at the central segregation part, steel is required to suppress the formation of island-like martensite which can be a starting point and to suppress the formation of hardened structures such as martensite and bainite which are likely to be crack propagation paths. C, Mn,
Reduce the content of elements with high segregation tendency such as P, apply soaking to eliminate segregation of alloying elements in the slab heating stage before rolling, or diffuse C during transformation during cooling after rolling. In order to prevent the formation of a hardened structure due to the above, accelerated cooling is performed (for example, JP-A-61-60866, JP-A-61-165207, etc.).

(3) A microstructure having low crack susceptibility is obtained, for example, by performing heat treatment such as quenching and tempering, or by setting the finishing temperature of rolling to be equal to or higher than the recrystallization temperature of austenite (Japanese Patent Application Laid-Open No. 54-12782, JP-A-62-7819, JP-A-6-73450). (4) By adding Cu to steel, a protective film is formed on the surface to suppress hydrogen intrusion into steel (Japanese Patent Laid-Open No. 52-11 / 1982)
No. 1815).

By adopting these methods, HI resistance can be improved.
The C property has been improved, and line pipes requiring sour resistance have been mass-produced up to API standard X65 grade.

However, in recent years, there has been an increasing demand for higher-strength steel pipes in order to increase transportation efficiency and reduce laying costs, and line pipes used in sour environments have also been required to have higher strength up to X80 grade. The possibility has come out. However, since HIC is likely to occur as the strength increases, the methods (1) to (4) cannot completely suppress the occurrence of HIC. When such a high-strength material is used, cracks also occur from the inclusions subjected to the shape control of (1) above,
Cracks occur in portions other than the central portion where the countermeasures against the central segregation of (2) are taken. In addition, the structure control by the quenching and tempering treatment or the recrystallization temperature range finish in (3) is inappropriate for mass production of line pipes from the viewpoint of cost and efficiency, and it is difficult to obtain sufficient low-temperature toughness. Furthermore, the effect of the Cu coating of (4) cannot be expected in an environment with a low pH, and 5% NaCl + 0.
In a 5% CH ₃ COOH aqueous solution (so-called NACE solution),
The effect of the coating has not been obtained.

In order to cope with such a problem, recently, several methods for producing a sour-resistant X80 grade linepipe steel plate have been disclosed. The essence is low S
While controlling the morphology of inclusions by adding Ca, the center segregation is suppressed as low C and low Mn, and the accompanying decrease in strength is reduced by C.
addition of r (JP-A-5-9575), addition of Cr-Mo (JP-A-5-271766, JP-A-7-1095)
No. 19), Ni-Cr-Mo addition (Japanese Unexamined Patent Publication No.
No. 3536) and accelerated cooling after rolling.

[0007]

However, a manufacturing technique (Japanese Unexamined Patent Publication No. 5-9) related to the above-described method of manufacturing X80.
575, JP-A-5-271766, JP-A-7-109519, and JP-A-7-173536) are methods for preventing the occurrence of HIC in the central segregation part, and the method for preventing the occurrence of HIC in the central segregation part The prevention of HIC generated in the above is not a specific measure against cracking. In other words, when the strength level of the steel pipe used in the sour environment increases, HIC is likely to occur even if the morphology control of the inclusions of the steel sheet as the material and the structure control of the central segregation part are performed. The hardness of the material near the surface increases,
HIC is likely to occur. HI near such a surface
Preventing the generation of C is a major issue.

An object of the present invention is to provide a method for economically and stably producing a steel material having high strength such as X70 and X80 and excellent HIC resistance in addition to API standard X65.

[0009]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) In the production method of the present invention, C: 0.03-
0.08%, Si: 0.05 to 0.5%, Mn: 1
~ 1.8%, P: 0.01% or less, S: 0.002
% Or less, Nb: 0.005 to 0.05%, and Ti:
0.005 to 0.02%, Al: 0.01 to 0.07
% And Ca: 0.0005 to 0.004%,
And a carbon equivalent: Ceq ≧ 0.28%, the balance being Fe and unavoidable impurities, and a method for producing a steel sheet in which the total Fe + MnO of slag at the time of ladle refining satisfies 0.5 to 3%. 1000-1200 ° C steel
Step of heating and hot rolling, and hot-rolled steel sheet, after accelerated cooling until the steel sheet surface temperature is 500 ℃ or less, temporarily suspend cooling, until the steel sheet surface temperature becomes 500 ℃ or more Reheating process and steel plate surface temperature 500 ℃
A step of accelerating and cooling the steel sheet that has been reheated to the above temperature again to a steel sheet surface temperature of 600 ° C. or less at an average cooling rate of the steel sheet of 3 to 50 ° C./sec.
This is a method for producing a high-strength linepipe steel sheet having excellent HIC resistance.

However, carbon equivalent: Ceq = C% + Mn% /
6+ (Cu% + Ni%) / 15+ (Cr% + Mo% + V
%) / 5 (2) In the production method of the present invention, as a steel component, Cu: 0.5% or less, Ni: 0.5% or less, C
r: 0.5% or less, Mo: 0.5% or less, and V: 0.
The method for producing a high-strength linepipe steel sheet having excellent HIC resistance according to the above (1), comprising one or more selected from the group of 1% or less.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In order to solve the above-mentioned problems, the present inventors prepared base materials having various component systems and microstructures by changing additive elements and heat treatment conditions, and developed HIC resistance and strength. , And toughness.

As a result, the amount of the added element, the carbon equivalent represented by the formula (1), the total Fe + MnO of the ladle slag,
It has been found that by performing intermittent cooling type accelerated cooling on the steel defined as above, an increase in the hardness of the surface layer portion is suppressed, and high strength, toughness, and good HIC resistance are obtained.

Carbon equivalent: Ceq = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (Cr% + Mo% + V%) / 5 (1) Based on the above findings, the present inventors have determined the specific amount Excellent in HIC resistance by controlling the hot rolling conditions of steel with the total Fe + MnO of ladle slag and the accelerated cooling conditions including the cooling interruption and recuperation steps within a certain range. The present inventors have found a method for producing a high-strength linepipe steel and completed the present invention. That is, the present invention provides a method for inexpensively and stably producing an API standard X80 grade linepipe steel sheet having excellent HIC resistance by limiting the steel composition and production conditions to the following ranges. it can.

The reasons for adding the components, the reasons for limiting the components, and the reasons for limiting the manufacturing conditions of the present invention are described below. (1) Component composition range C: 0.03 to 0.08% C is necessary as a strengthening element for steel, and in order to secure a predetermined strength of X65 to X80, the content of 0.03% or more is necessary. . On the other hand, excessive C content exceeding 0.08% causes deterioration of the toughness and HIC resistance of the steel sheet.
It is necessary to reduce the amount of C from the viewpoint of weldability and sulfide stress corrosion cracking resistance.
08%. Si: 0.05-0.5% Si is added for deoxidation. If it is less than 0.05%, a sufficient deoxidizing effect cannot be obtained, while if it exceeds 0.5%, toughness and weldability deteriorate. , It is 0.05 to 0.5%.

Mn: 1 to 1.8% Mn is added as a basic element of steel effective for improving the strength and toughness of steel.
On the other hand, if it exceeds 1.8%, the weldability and the HIC resistance are remarkably deteriorated, so that the content is 1 to 1.8%.

P: 0.01% or less In the case of the steel of the present invention, P is an impurity element deteriorating weldability and HIC resistance, and it is desirable to reduce P as much as possible. However, excessive removal of P causes a rise in cost. The upper limit is 0.01%. S: 0.002% or less Even if the form is controlled from MnS to CaS-based inclusions by adding Ca, in the case of a high-strength material of X80 grade, the finely dispersed CaS-based inclusions are also the starting points of cracking. Therefore, it is necessary to reduce the S content to 0.002% or less.

Nb: 0.005 to 0.05% Nb is a component necessary for miniaturizing the microstructure by suppressing grain growth during rolling and quenching and imparting sufficient toughness as a line pipe. . The effect is remarkable at 0.005% or more, and when it exceeds 0.05%, the effect is almost saturated and the toughness of the heat affected zone is deteriorated.

Ti: 0.005 to 0.02% Ti forms TiN and suppresses grain growth during slab heating and quenching, resulting in an effect of improving microstructure and improving toughness. But the effect is 0.005
% Or more, and if it exceeds 0.02%, the toughness deteriorates conversely, so it is 0.005 to 0.02%. Al: 0.01 to 0.07% Al is added as a deoxidizing agent, and its effect is remarkable when it is 0.01% or more, and when it exceeds 0.07%, the cleanliness is reduced and the HIC resistance is deteriorated. Cause 0.01 to 0.
07%.

Ca: 0.0005% to 0.004% Ca is an element indispensable for controlling the form of sulfide inclusions, and its effect appears at 0.0005% or more.
If it exceeds 4%, the effect is saturated, and conversely, the cleanliness is reduced and the HIC resistance is deteriorated.
4%. Carbon equivalent: Ceq ≧ 0.28%, provided that carbon equivalent: Ce
q = C% + Mn% / 6 + (Cu% + Ni%) / 15+
(Cr% + Mo% + V%) / 5 Since the carbon equivalent Ceq needs to be 0.28% or more in order to obtain sufficient strength as X65 to X80, the lower limit is 0.28%. The upper limit is not particularly limited. Ceq is shown by the following equation.

Ceq = C% + Mn% / 6 + (Cu% + N
i%) / 15+ (Cr% + Mo% + V%) / 5 Total Fe + MnO: 0.5-3% If the total Fe + MnO of the slag during ladle refining exceeds 3%, HIC occurs near the steel sheet surface, so the upper limit is set. Is 3
%. In addition, total Fe is set to be less than 0.5%.
Controlling + MnO impairs economic efficiency. In the present invention, one or more selected from the following selected component groups may be contained, if necessary, in addition to the above components.

(Selective component group) Cu: 0.5% or less Cu is one of the elements effective in improving toughness and increasing strength, but containing Cu exceeding 0.5% inhibits weldability. Therefore, when added, it must be limited to 0.5% or less.

Ni: 0.5% or less Ni is one of the elements effective in improving toughness and increasing the strength, but when it exceeds 0.5%, the effect is saturated and stress corrosion cracking is likely to occur. Therefore, when it is added, it is 0.5% or less.

Mo: 0.5% or less Mo is one of the elements effective in improving the toughness and increasing the strength, but when it exceeds 0.5%, the effect is saturated and the weldability and the H resistance are reduced.
In order to inhibit IC properties, the content is 0.5% or less when added. Cr: 0.5% or less Cr is an effective element for obtaining sufficient strength as X80 grade even with low C, together with Mn. However, if added in excess of 0.5%, the weldability is adversely affected. 0.5%. V: 0.1% or less Addition of an appropriate amount of V enhances the strength without deteriorating toughness, weldability and sour resistance, and is an optional addition effective for obtaining sufficient strength as X80 grade even with low C with Cr. Although it is an element, if it exceeds 0.1%, the weldability is significantly impaired, so that the content is 0.1% or less. By adjusting to the above component composition range, it becomes possible to obtain a high-strength steel sheet such as X65, X70, X80 grade having good toughness in addition to good HIC resistance.

The steel sheet having such characteristics can be manufactured by the following manufacturing method. (2) Steel plate manufacturing process (Manufacturing method) A steel slab obtained by melting steel adjusted to the above-described component composition range and continuously casting is used to produce a steel slab of 1000 to 1200.
After being heated to ℃ and hot-rolled, accelerated cooling until the steel sheet surface temperature becomes 500 ° C or lower, cooling is once suspended, and the steel sheet is reheated until the steel sheet surface temperature becomes 500 ° C or higher. Then
The steel sheet is again cooled at an average cooling rate of 3 to 50 ° C./sec to a steel sheet surface temperature of 600 ° C. or less.

A. Slab heating temperature If the slab heating temperature is lower than 1000 ° C, sufficient strength cannot be obtained. If the slab heating temperature exceeds 1200 ° C., good toughness cannot be obtained. Therefore, the slab heating temperature is 1
000-1200 ° C. Further, the hot rolling end temperature is desirably equal to or higher than the Ar ₃ transformation temperature. b. Accelerated cooling start temperature If the accelerated cooling start temperature is low, it takes time to regain heat,
750 ° C. + 300 / t or more is desirable because the HIC resistance also deteriorates. Here, t is the thickness of the steel plate (mm).

C. When the surface temperature at the time of the accelerated cooling interruption is higher than 500 ° C., transformation near the surface is not sufficiently advanced, and the material is transformed into bainite or the like at the time of quenching after recuperation and hardens. Therefore, the surface temperature when the accelerated cooling is interrupted is 500 ° C. or less. d. Surface recuperation temperature When the surface recuperation temperature is lower than 500 ° C, the hardness of the transformed surface layer does not decrease when cooled to 500 ° C or lower, which causes HIC. Therefore, the surface recuperation temperature is 500 ° C or higher.

E. Accelerated cooling stop temperature If the accelerated cooling stop temperature exceeds 600 ° C. at the surface temperature, sufficient strength may not be obtained. Therefore, the accelerated cooling stop temperature is 600 ° C. or less. f. Cooling rate If the average cooling rate of the steel sheet is less than 3 ° C / sec, sufficient strength may not be obtained. On the other hand, if the temperature exceeds 50 ° C./sec, the strength increases and the HIC resistance is deteriorated. Therefore, the cooling rate is 3-50 ° C / sec.

As long as the above conditions are satisfied, other rolling conditions of the steel sheet are not particularly defined. The method of forming the steel pipe is not particularly limited as long as it is cold. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0029]

EXAMPLES Steels having the chemical components shown in Table 1 (A to K: steels of the present invention, LR to comparative steels) were hot-rolled under accelerated rolling conditions shown in Table 2 (A-4, 7, B-1 to K-1: steel sheet of the present invention, A-1 to 3, 5, 6, 8, 9, L-1, 2,
M-1 to R-1: Comparative steel sheet). Table 2 shows the mechanical properties (yield strength, tensile strength, toughness), HIC resistance, and weldability of the steel sheet. The HIC test was performed in a 5% NaCl + 0.5% CH ₃ COOH aqueous solution (commonly called NACE solution) saturated with hydrogen sulfide having a pH of about 3, and the crack length ratio (CLR) was 1
The HIC resistance was judged to be good at 5% or less. The toughness was determined to be good when the fracture surface transition temperature in the Charpy impact test was −60 ° C. or less. The strength was determined to be good when the yield strength was 448 MPa or more. In addition, the weldability was examined by performing a submerged arc welding equivalent to seam welding of an actual steel pipe, and observing the presence or absence of welding hot cracks and cold cracks by observing the cross section of the welded portion. When there was no crack in the weld, the weldability was judged to be good.
Inventive steel sheets A-4, A-7, B-1, C-1, D-1, E-E in which the steel of the present invention is subjected to the accelerated rolling treatment of the present invention.
1, F-1, G-1, H-1, I-1, J-1, K-1
In each case, sufficient strength and good HIC resistance were obtained. On the other hand, comparative steel sheets A-1, A-2, A-3, A-5, which do not perform the accelerated rolling of the present invention even when using the steel of the present invention.
A-6, A-8, and A-9 do not provide sufficient performance. Further, comparative steel sheets L-1, Q-1, R-1 obtained by subjecting a non-invented steel to the accelerated rolling of the present invention, or comparative steel sheet L-2 obtained by subjecting a non-invented steel to the accelerated rolling-in non-inventive cooling , M-1, N-1, O-1, and P-1 did not provide sufficient performance.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

As described above, according to the present invention, by specifying the steel composition and the manufacturing conditions, it is possible to inexpensively and stably manufacture an API standard X80 grade line pipe steel sheet having excellent HIC resistance. It became possible to do.

Claims (2)

[Claims] C: 0.03 to 0.08% by weight
And Si: 0.05 to 0.5% and Mn: 1 to 1.8%
And P: 0.01% or less, S: 0.002% or less,
Nb: 0.005 to 0.05%, Ti: 0.005 to
0.02%, Al: 0.01 to 0.07%, and Ca:
0.0005 to 0.004%, and the carbon equivalent: Ceq ≧ 0.28%, the balance being Fe and unavoidable impurities, and the total Fe + MnO of the slag at the time of ladle refining is 0.5 to 50%. In a method for producing a steel sheet satisfying 3%, a step of heating the steel to 1000 to 1200 ° C. and hot rolling, and accelerating cooling the hot-rolled steel sheet to 500 ° C. or less at a steel sheet surface temperature. After the cooling, the cooling is interrupted, and the steel sheet is reheated until the steel sheet surface temperature becomes 500 ° C. or more. Up to 3-50 ° C
And a step of accelerated cooling at an average cooling rate of the steel sheet per second. 2. A method for manufacturing a high-strength line pipe steel sheet excellent in HIC resistance, comprising: However, carbon equivalent: Ce
q = C% + Mn% / 6 + (Cu% + Ni%) / 15+
(Cr% + Mo% + V%) / 5 2. The steel composition further comprises Cu:
One or more selected from the group of 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, and V: 0.1% or less The method for producing a high-strength steel sheet for line pipes having excellent HIC resistance according to claim 1, characterized by containing: