JP5991175B2 - High-strength steel sheet for line pipes with excellent material uniformity in the steel sheet and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength steel sheet for line pipes, which is excellent in material uniformity and elongation characteristics in steel sheets, used in the fields of architecture, offshore structures, shipbuilding, civil engineering, construction industry machines, line pipes, etc. It is about the method.

  The line pipe is manufactured by forming a steel plate manufactured by a thick plate mill or a hot rolling mill into a steel pipe shape by UOE forming, press bend forming, roll forming or the like.

  In particular, line pipes used for transporting crude oil and natural gas containing hydrogen sulfide are resistant to hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC) in addition to strength, toughness and weldability. It is necessary to provide so-called sour resistance.

  In hydrogen induced cracking (HIC) of steel, hydrogen ions from the corrosion reaction are adsorbed on the surface of the steel, penetrate into the steel as atomic hydrogen, and include non-metallic inclusions such as MnS in the steel and hard second phase structure. It is said that it diffuses and accumulates around, and cracks occur due to its internal pressure.

  Several methods have been proposed to prevent such hydrogen-induced cracking. For example, in Patent Document 1, while lowering the S content in steel and adding an appropriate amount of Ca, REM, or the like, the formation of long extended MnS is suppressed, and a finely dispersed spherical CaS inclusion is formed. Has been proposed to improve the HIC resistance by reducing stress concentration due to sulfide inclusions and suppressing the occurrence and propagation of cracks.

  In Patent Documents 2 and 3, there is a reduction in elements that have a high segregation tendency (C, Mn, P, etc.), a reduction in segregation by soaking in the slab heating stage, and an acceleration during transformation after cooling after rolling. Techniques for cooling have been proposed. This suppresses the generation of island martensite that becomes the starting point of cracks in the center segregation part and the generation of hardened structures such as martensite that becomes the propagation path of cracks.

  In Patent Documents 4 and 5, for high-strength steel sheets, while controlling the form of sulfide inclusions with low S and Ca addition, the center segregation is suppressed by low C-low Mn, There has been proposed a method of compensating for the accompanying strength reduction by adding Cr, Mo, Ni or the like and accelerated cooling.

  On the other hand, from the viewpoint of increasing the size of steel structures and reducing costs, there is an increasing demand for steel sheets having higher strength and higher toughness. For the purpose of improving the properties of steel sheets, reducing alloy elements, and omitting heat treatment, high-strength steel sheets are usually manufactured by applying so-called TMCP technology, which combines controlled rolling and controlled cooling.

  In order to increase the strength of steel using the TMCP technology, it is effective to increase the cooling rate during controlled cooling. However, when controlled cooling is performed at a high cooling rate, the surface layer portion of the steel sheet is rapidly cooled, so that the hardness of the surface layer portion is higher than that inside the steel plate, and the hardness distribution in the thickness direction varies. Therefore, it becomes a problem from the viewpoint of ensuring the material uniformity in the steel plate.

  Patent Document 6 discloses a method of suppressing an increase in surface hardness with respect to the center portion of the plate thickness by controlling the cooling rate to a relatively low cooling rate of 3 to 12 ° C./s during controlled cooling.

  In Patent Document 7, by waiting in a cooling process at a temperature range where ferrite precipitates, the steel sheet has a two-phase structure of ferrite and bainite, and the difference in hardness between the surface layer and the center of the plate thickness is reduced. A method of manufacturing a steel sheet having a small material difference in the thickness direction is disclosed.

  Patent Document 8 and Patent Document 9 include a steel plate having a small material difference in the plate thickness direction, which has been subjected to controlled cooling at a high cooling rate for reheating the surface after the rolling before the surface layer portion completes the bainite transformation. A manufacturing method is disclosed.

  Patent Document 10 and Patent Document 11 disclose a method for manufacturing a steel plate for a line pipe in which the surface of the steel plate after accelerated cooling is heated to a higher temperature from the inside by using a high-frequency induction heating device to reduce the hardness of the surface layer portion. Has been.

  Patent Document 12 discloses that control cooling is performed at an average cooling rate of 5 to 15 ° C./s at the central part of the steel sheet until the steel sheet surface temperature becomes 500 ° C. or lower, and then at the latter stage cooling. A method for suppressing the hardened structure of the surface layer by cooling to a plate thickness direction average temperature of 600 ° C. or lower at 20 to 50 ° C./s is disclosed.

  On the other hand, if there is unevenness in the scale properties of the steel sheet surface, the cooling rate of the lower steel sheet varies depending on the scale thickness during cooling, that is, the cooling stop temperature varies partially within the steel sheet, Corresponding to the scale properties, the steel plate material varies in the plate width direction.

  Patent Document 13 and Patent Document 14 disclose a method of reducing the uneven cooling due to the scale properties and improving the steel plate shape by performing descaling immediately before cooling.

Japanese Patent Laid-Open No. 54-110119 JP 61-60866 A JP-A-61-165207 JP-A-5-271766 JP 7-173536 A Japanese Patent Publication No.7-116504 Japanese Patent No. 3911834 Japanese Patent No. 3951428 Japanese Patent No. 3951429 JP 2002-327212 A Japanese Patent No. 3711896 Japanese Patent No. 3546726 JP-A-9-57327 Patent No. 3796133

  However, all of the techniques described in Patent Documents 1 to 5 are directed to the center segregation part, and no part other than the center segregation part is considered.

  In high-strength steel sheets with API standard X65 grade strength or higher manufactured by controlled cooling or direct quenching, the steel sheet surface portion with a high cooling rate is hardened compared to the inside, so that hydrogen-induced cracking occurs from the vicinity of the surface. There's a problem.

The technology described in Patent Document 6 cannot fully utilize the effect of controlled cooling such as high strength by high cooling rate, reduction of alloy elements, simplification of controlled rolling, etc. due to limitation of cooling rate. In the manufacturing method of Patent Document 7, the ferrite is precipitated in the cooling standby at the Ar ₃ transformation point or lower, so that the strength is lowered and the cooling standby time is required, so that the manufacturing efficiency is deteriorated.

In the production methods described in Patent Document 8 and Patent Document 9, if the transformation behavior differs depending on the components of the steel sheet, a sufficient material homogenization effect due to recuperation may not be obtained. Moreover, since highly accurate cooling control is required, the application range is limited and the manufacturing efficiency is deteriorated. In the manufacturing methods described in Patent Document 10 and Patent Document 11, if the cooling rate of the surface layer portion in accelerated cooling is large, the hardness of the surface layer portion may not be sufficiently reduced only by heating the steel sheet surface.
In the production method described in Patent Document 12, if the transformation behavior varies depending on the components of the steel sheet, the hardened structure of the surface layer may not be suppressed, and a sufficient material homogenizing effect may not be obtained.

  In addition, the methods described in Patent Document 13 and Patent Document 14 improve the steel plate shape by reducing the surface property failure due to the indentation of the scale during hot correction and the variation in the cooling stop temperature of the steel plate by descaling. However, there is no description about cooling conditions for obtaining a uniform material.

  Since the cooling state of the steel sheet is affected not only by the surface properties but also by the strength of the cooling, there is a risk that the hardness may vary if the cooling rate of the steel sheet surface varies. Depending on the cooling rate, in the cooling state of the steel sheet surface, a film of bubbles is generated between the steel sheet surface and the cooling water, and “nucleate boiling” where the bubbles are separated from the surface by the cooling water before forming the film. "May be mixed, and the surface cooling rate may vary. As a result, the hardness of the steel sheet surface varies.

  Therefore, the present invention solves the problem that, conventionally, when a low-cost component and high cooling rate cooling are combined, a high-strength steel sheet with material uniformity and HIC resistance in the steel sheet could not be produced, Uniformity and elongation in steel sheet with excellent HIC resistance against HIC generated from the vicinity of the surface together with HIC in the central segregation part, and reduced variation in hardness in the sheet thickness direction and sheet width direction. It aims at providing the high strength steel plate for line pipes which was excellent in the characteristic, and its manufacturing method.

In order to solve the above-mentioned problems, the present inventors have prevented the occurrence of HIC from the vicinity of the surface together with the central segregation part in the high strength steel plate having the strength of API standard X70 grade, and the hardness in the plate thickness direction and the plate width direction. In order to reduce the variation of the material and improve the material uniformity in the steel sheet, we intensively studied the chemical composition, microstructure and manufacturing method of the steel material, the cooling pattern including the cooling rate of the surface layer and the average in the steel sheet The present invention was completed with the knowledge that it is important to control the cooling rate. The object of the present invention can be achieved by the following means.
(1) By mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 0.5 to 1.8%, P: 0.01% or less, S: 0.001% or less, Al: 0.01 to 0.08%, Ca: 0.0005 to 0.005%, CP value (mass%) represented by the following formula (1) is 1.1 or less The Ceq value (mass%) represented by the following formula (2) is 0.35 or more and 0.45 or less, the balance is made of Fe and unavoidable impurities, and the metal structure is a region within 3 mm below the surface layer, A ferrite structure having an average particle size of 20 μm or less and a volume ratio of 80% or less (including 0%), a bainite structure in other internal regions, and a hardness variation in the thickness direction of ΔH _V10 30 or less, variations in the width direction stiffness is at [Delta] H _{V10 30} below, maximum hardness of the steel sheet surface layer portion H _{V 0,} characterized in that it is 230 or less, high-strength steel sheet for line pipe superior in material homogeneity within the steel sheet.
CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.74Cu (%) + 1.7Ni (%)} / 15+ {1.18Cr (%) + 1.95Mo (%) + 1.74V ( %)} / 5 + 22.36P (%) (1)
Ceq = C (%) + Mn (%) / 6+ (Cu (%) + Ni (%)) / 15+ (Cr (%) + Mo (%) + V (%)) / 5 (2)
However, each element symbol in each formula is the content (% by mass).
2. Furthermore, it is characterized by containing one or more of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less in mass%. The high-strength steel sheet for line pipes having excellent material uniformity in the steel sheet according to 1.
3. Furthermore, it contains one or more of Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% in mass%. A high-strength steel sheet for line pipes, which is characterized by excellent material uniformity in the steel sheet according to 1 or 2.
After the steel having the chemical composition according to any one of 4.1 to 3 is heated to a temperature of 1000 ° C. or higher and 1300 ° C. or lower and the rolling finish temperature is hot rolling at a steel sheet surface temperature of Ar ₃ or higher. , impact pressure of the injection flow on the steel sheet surface just before the controlled cooling is carried out descaling at 1MPa or higher, the steel sheet surface temperature of the cooling start is (Ar 3 _-80) the cooling rate of the steel sheet surface from ° C. over 20 ° C. / s to 100 ° C./s or less, and the steel sheet surface temperature is 300 ° C. to 600 ° C. to satisfy the formula (3), and then the steel sheet is cooled at an average cooling rate of 15 ° C./s or more. A method for producing a high-strength steel sheet for line pipes having excellent material uniformity in the steel sheet, wherein the second stage cooling is performed to an average temperature of 200 ° C. to 600 ° C.
3 ≦ (700−T) / V (3)
However, T: Finishing temperature of steel sheet surface cooling at 1st stage cooling (° C.), V: Steel sheet surface cooling rate at 1st stage cooling (° C./s)
Using the steel plate manufactured by the manufacturing method described in 5.4, the hardness variation in the pipe thickness direction is ΔH _v10 30 or less, and the hardness variation in the pipe circumferential direction is _{ΔH v10} 30 or less, A high-strength steel pipe for line pipes having excellent material uniformity, characterized in that the maximum hardness of the steel pipe surface layer portion is _Hv10 248 or less.

  According to the present invention, it is possible to obtain a steel plate for a line pipe and a method for producing the same, which are excellent in material uniformity in the steel plate even with low-cost chemical components, and excellent in HIC resistance and elongation properties, and are extremely useful industrially.

Schematic which shows an example of the manufacturing line for enforcing the manufacturing method of this invention.

  The chemical components of the high-strength steel sheet according to the present invention will be described. In the following description, all units represented by% are mass%.

C: 0.02 to 0.08%
If C is less than 0.02%, sufficient strength cannot be ensured, and if it exceeds 0.08%, the hardness of the surface layer portion increases during accelerated cooling, and the HIC resistance and toughness are deteriorated. It is specified at 02 to 0.08%.

Si: 0.01 to 0.5%
Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, the toughness and weldability are deteriorated. %.

Mn: 0.5 to 1.8%
Mn is added for strength and toughness, but if it is less than 0.5%, its effect is not sufficient, and if it exceeds 1.8%, the weldability and HIC resistance deteriorate, so the content is 0.5 to 1 .8% is specified.

P: 0.01% or less P is an unavoidable impurity element, which deteriorates weldability and HIC resistance by increasing the hardness of the central segregation part. Since the tendency will become remarkable when it exceeds 0.01%, the upper limit of content is prescribed | regulated to 0.01%. Preferably it is 0.008% or less.

S: 0.001% or less Generally, S is preferably as small as possible because it becomes MnS inclusions in steel and deteriorates the HIC resistance. However, since there is no problem if it is 0.001% or less, the upper limit of the content is specified to 0.001%.

Al: 0.01 to 0.08%
Al is added as a deoxidizer, but if it is less than 0.01%, there is no effect, and if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the content is 0.01-0. .08% is specified.

Ca: 0.0005 to 0.005%
Ca is an element effective for improving the HIC resistance by controlling the form of sulfide inclusions, but the effect is not sufficient if it is less than 0.0005%, and the effect is saturated even if added over 0.005%. However, since the HIC resistance is deteriorated due to a decrease in the cleanliness of the steel, the content is specified to be 0.0005 to 0.005%. The balance other than the above is Fe and inevitable impurities.

CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.74Cu (%) + 1.7Ni (%)} / 15+ {1.18Cr (%) + 1.95Mo (%) + 1.74V ( %)} / 5 + 22.36P (%) (1)
However, C (%), Mn (%), Cu (%), Ni (%), Cr (%), Mo (%), V (%), P (%) are the contents of each element (mass% The element not added is 0.

  The CP value is an expression devised for estimating the material of the central segregation part from the content of each alloy element, and the CP value (mass%) represented by the expression (1) is 1.0 or less. The higher the CP value, the higher the concentration of the center segregation part and the higher the hardness of the center segregation part. By setting the CP value to 1.0 or less, HIC can be suppressed. Further, the lower the CP value, the lower the hardness of the center segregation part. Therefore, when higher HIC resistance is required, the upper limit is desirably set to 0.95.

Ceq = C (%) + Mn (%) / 6+ (Cu (%) + Ni (%)) / 15+ (Cr (%) + Mo (%) + V (%)) / 5 (2)
However, C (%), Mn (%), Cu (%), Ni (%), Cr (%), Mo (%), and V (%) are the contents (mass%) of each element and added The element not to be used is 0.

  The Ceq value correlates with the hardness of the surface layer and the tensile strength characteristics of the entire steel sheet (total thickness). The Ceq value represented by the formula (2) is 0.35 or more and 0.45 or less.

  If the Ceq value is less than 0.35, the tensile strength of 570 MPa or more of the API standard X70 targeted in the present invention cannot be obtained, and if the Ceq value exceeds 0.45, the surface hardness HV ≦ 230 is suppressed. Therefore, the Ceq value is in the range of 0.35 to 0.45.

  The above is the basic chemical component of the present invention, but when further improving the strength toughness of the steel sheet, it may contain one or more of Cu, Ni, Cr, Mo, Nb, V, and Ti.

Cu: 0.50% or less Cu is an element effective for improving toughness and increasing strength, but if added in a large amount, weldability deteriorates. Therefore, when added, the upper limit is 0.50%.

Ni: 0.50% or less Ni is an element effective for improving toughness and increasing strength. However, if added in a large amount, it is disadvantageous in cost, and the weld heat affected zone toughness deteriorates. The upper limit is 0.50%.

Cr: 0.50% or less Cr is an element effective for obtaining sufficient strength even at low C like Mn, but if added in a large amount, the weldability deteriorates, so when added, the upper limit is 0.50% And

Mo: 0.50% or less Mo is an element effective for improving toughness and increasing strength, but if added in a large amount, weldability deteriorates, so when added, the upper limit is 0.50%.

One or more of Nb, V, and Ti Nb, V, and Ti are selective elements that are added to increase the strength and toughness of the steel sheet, and one or more of Nb, V, and Ti are added depending on the required strength. be able to. If each element is less than 0.005%, there is no effect, and if it exceeds 0.1%, the toughness of the welded portion deteriorates. Therefore, when added, the content is preferably in the range of 0.005 to 0.1%.
[Metal structure]
The metal structure is a bainite structure in order to achieve an increase in tensile strength of 570 MPa or more. However, in a region within 3 mm below the surface layer, a ferrite structure having a bainite structure and an average particle size of 20 μm or less and a volume ratio of 80% or less (including 0%) is used.

  When the area within 3 mm below the surface layer is a bainite main structure in which a ferrite structure having an average particle diameter of 20 μm or less and a volume ratio of 80% or less (including 0%) and a bainite structure having a volume ratio of 20% or more are mixed, the surface layer hardness is Low and improved HIC resistance. When a hard phase such as martensite or island martensite (MA) is formed in the surface layer, the surface layer hardness is increased, the HIC resistance is deteriorated, and the hardness variation in the steel sheet is increased, resulting in material uniformity. Decreases. The surface layer portion is defined as 5 mm in the plate thickness direction from the steel plate surface.

  In addition to the bainite structure, when one or more metal structures such as martensite, pearlite, island martensite, and retained austenite are mixed, the toughness deteriorates and the surface hardness increases. Although it is good, when the volume fraction is less than 5%, the influence thereof can be ignored, so it is within the scope of the present invention.

[Hardness variation]
The variation in hardness in the plate thickness direction is ΔH _V10 30 or less in terms of Vickers hardness (hereinafter referred to as H _V10 ) at a load of 10 kg, and the variation in hardness in the plate width direction is set to be ΔH _V10 30 or less. ΔH _V10 is the difference between the highest hardness and the lowest hardness.

  In particular, in the case of a high-strength steel sheet for line pipes, it is required to suppress hardness variation in the steel sheet from the viewpoint of satisfying the strength and elongation of the steel sheet, formability, HIC resistance, SCC resistance, and the like.

When the variation in hardness in the sheet thickness direction exceeds ΔH _V10 30 or when the variation in hardness in the sheet width direction exceeds ΔH _V10 30, the above characteristics are adversely affected. For example, when the hardness of the steel sheet surface layer portion exceeds ΔH _V10 30 as compared with the inside of the steel sheet, springback is likely to occur after forming, and cracking susceptibility to hydrogen sulfide is increased.

Also, if the variation in hardness in the plate width direction exceeds ΔH _V10 30, the difference in deformation amount between the hard part and the soft part during molding is such that the desired shape cannot be obtained and cut into small plates In addition, the strength and elongation of each platelet differ.

From the viewpoint of material uniformity in the steel plate and HIC resistance, it is more preferable that the variation in hardness in the thickness direction is ΔH _V10 25 or less, and the variation in hardness in the plate width direction is ΔH _V10 25 or less.

[Maximum hardness]
In the high-strength steel sheet having the strength of API standard X70 grade, when the hardness of the steel sheet surface layer portion increases, the risk of generating hydrogen-induced cracking (HIC) increases. In order to suppress hydrogen-induced cracking (HIC) from the steel plate surface layer portion, the hardness of the steel plate surface layer portion (hardness at 1 mm below the surface) is set to H _V10 230 or less.

It is desirable hardness of the steel tube surface portion after the pipe by cold forming (subsurface 1mm) is H _{V10 248} or less. This takes into account the work hardening amount during cold forming.

The high-strength steel sheet according to the present invention can be manufactured by performing hot-rolling and then controlled cooling in which two-stage cooling is performed, and the conditions in each step are as follows.
[Slab heating temperature]
Slab heating temperature shall be 1000-1300 degreeC. If the heating temperature is less than 1000 ° C., the solid solution of the carbide is insufficient and the required strength cannot be obtained, and if it exceeds 1300 ° C., the toughness deteriorates, so the temperature is set to 1000 to 1300 ° C. Here, the temperature is the furnace temperature of the heating furnace, and the slab is sufficiently heated to this temperature to the center.

[Hot rolling]
In the hot rolling, from the viewpoint of strength and HIC resistance, the rolling end temperature is set to the Ar ₃ temperature or higher at the steel sheet surface temperature. The Ar ₃ temperature can be obtained by the following equation.

Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo, where each element symbol is the content (% by mass).

  In order to obtain high base metal toughness, it is desirable that the rolling reduction in a temperature range of 950 ° C. or lower corresponding to the austenite non-recrystallization temperature range be 60% or more. In addition, the surface temperature of a steel plate can be measured with a radiation thermometer or the like.

[Descaling]
After hot rolling, just before controlled cooling, high impact pressure descaling is performed so that the impact pressure of the jet flow on the steel sheet surface is 1 MPa or more. If the impinging pressure of the jet flow on the steel sheet surface is less than 1 MPa, descaling may be insufficient and unevenness in scale may occur, resulting in variations in surface hardness.

Immediately before the controlled cooling, a preliminary test was performed in which the collision pressure of the jet flow on the surface of the steel sheet was changed in various ways, and the collision pressure of the jet flow at which the scale thickness of the steel sheet after descaling cooling was 15 μm or less was determined to be 1 MPa or more. When the scale thickness of the steel sheet after controlled cooling is 15 μm or less, it is possible to make the hardness variation in the plate thickness direction ΔH _V10 30 or less and the hardness variation in the plate width direction be ΔH _V10 30 or less. is there.

Descaling is performed using high-pressure water, but other jet streams may be used as long as the collision pressure of the jet stream on the steel sheet surface is 1 MPa or more.
[Controlled cooling]
The present invention performs two-stage cooling, and the first-stage cooling defines the cooling start temperature, cooling stop temperature, and cooling rate at the steel sheet surface temperature, and the second-stage cooling determines the average temperature of the steel sheet (in the thickness direction). It indicates the average temperature).
[First stage cooling]
In the first stage cooling, the steel sheet surface temperature is (Ar ₃ −80 ° C.) or higher to 300 ° C. or higher and 600 ° C. or lower, and the cooling rate of the steel plate surface is 20 ° C./s or higher and 100 ° C./s or lower. To meet.

The steel sheet surface temperature at the start of cooling is set to (Ar ₃ -80 ° C.) or higher. When the steel sheet surface temperature at the start of cooling is less than (Ar ₃ -80 ° C.), the amount of ferrite generated before controlled cooling increases, the strength decreases and the HIC resistance deteriorates (Ar ₃ -80 ° C.). That's it.

  It is desirable to start cooling within 5 seconds after descaling. When controlled cooling is performed for more than 5 seconds after descaling, the scale grows and the cooling rate of the surface layer increases, and the scale thickness of the steel sheet after controlled cooling exceeds 15 μm, increasing the surface hardness. In addition, the hardness variation in the steel sheet increases, and the material uniformity and the deterioration of the HIC resistance become remarkable.

  When the cooling rate exceeds 100 ° C./s, a hard phase such as martensite and island martensite (MA) is generated, and the surface hardness increases and the HIC resistance deteriorates. The following. Preferably, it is 80 degrees C / s or less.

  On the other hand, if it is less than 20 ° C./s, the average cooling rate of the steel sheet decreases and the desired tensile strength characteristics cannot be obtained.

Furthermore, in the first stage cooling, in order to suppress the occurrence of a hardened structure on the steel sheet surface, the steel sheet surface cooling end temperature (° C.) and the steel sheet surface cooling rate are controlled so as to satisfy the expression (3).
3 ≦ (700−T) / V (3)
T: Finishing temperature of steel sheet surface cooling in 1st stage cooling (° C.), V: Cooling speed of steel sheet surface in 1st stage cooling (° C./s)
[Second stage cooling]
After the first stage cooling, the second stage cooling is performed at an average cooling rate of the steel sheet of 15 ° C./s or more until the average temperature of the steel sheet is 200 ° C. or more and 600 ° C. or less. If the average cooling rate of the steel sheet is less than 15 ° C./s, the bainite structure cannot be obtained, the strength is lowered, the HIC resistance is deteriorated, and the variation in hardness is increased. More preferably, it is 20 ° C./s or more.

  When the cooling stop temperature is 200 ° C. or more and 600 ° C. or less as the average temperature of the steel plate, the difference between the steel plate surface temperature and the steel plate center temperature generated after accelerated cooling is eliminated by heat conduction, resulting in a substantially uniform temperature distribution in the steel plate. . The average cooling rate and the average temperature of the steel sheet both indicate the average value in the thickness direction.

The hardened structure is suppressed by the reheating of the surface of the steel sheet, the hardness variation in the steel sheet is suppressed, the maximum value of the hardness is suppressed to _HV10 230 or less, and the deterioration of the HIC resistance can be suppressed.

  When the cooling stop temperature is less than 200 ° C., the reheating effect of the steel sheet surface layer portion is reduced, so that the increase in hardness becomes significant, the variation in hardness increases, and the HIC resistance deteriorates. Further, the steel sheet is easily distorted, and formability deteriorates.

  FIG. 1 shows an example of equipment suitable for manufacturing a steel sheet according to the present invention. In the rolling line, a hot rolling mill 2, a high collision pressure deske league device 3, and a control cooling device 4 are arranged from upstream to downstream. A hot straightening machine can also be installed in front of the descaling device. By improving the shape of the steel sheet 1 with a hot straightening machine, it is possible to increase the collision pressure of the jet flow, and therefore it is possible to perform descaling more efficiently at a low cost.

  The high-strength steel sheet according to the present invention is formed into a tubular shape by press bend forming, roll forming, UOE forming, etc., and then the butt portion is welded to provide excellent material uniformity in the steel sheet, high strength, and HIC resistance. High strength steel pipes (UOE steel pipes, ERW steel pipes, spiral steel pipes, etc.) suitable for transportation of crude oil and natural gas containing excellent hydrogen sulfide can be manufactured.

  For example, in UOE steel pipe, the end of a steel plate is grooved and formed into a steel pipe shape by C press, U press, and O press, and then the butt portion is seam welded by inner surface welding and outer surface welding. Manufactured through a tube expansion process. Any welding method may be used as long as sufficient joint strength and joint toughness can be obtained, but it is preferable to use submerged arc welding from the viewpoint of excellent welding quality and manufacturing efficiency.

  Steel of chemical composition (steel types A to L) shown in Table 1 was made into a slab by a continuous casting method, and a thick steel plate (No. 1 to 25) having a plate thickness of 25 mm to 38 mm was produced using this.

  After the slab was heated, immediately after it was made to have a predetermined thickness by hot rolling, or after descaling the high collision pressure just before the controlled cooling, cooling was performed using a water-cooled control cooling device. Table 2 shows the production conditions of each steel plate (No. 1 to 25).

  The microstructure and scale properties of the obtained steel sheet were observed with an optical microscope and a scanning electron microscope. A cross-sectional structure photograph of 10 fields of view was obtained, the scale thickness was measured, and the average value of 10 fields of view was evaluated.

  In addition, tensile properties, hardness, and HIC resistance of each steel plate were measured. For tensile properties, a tensile test was performed using a full thickness test piece (API standard) in the direction perpendicular to the rolling as a tensile test piece, and yield strength (0.5% yield strength), tensile strength, and total elongation were measured.

  Further, the hardness in the plate thickness direction and the hardness in the plate width direction were measured with a Vickers hardness tester. The thickness in the thickness direction was measured at a pitch of 1 mm, and the total thickness was measured at a pitch of 20 mm. In addition, the hardness in the plate width direction is the surface layer 1 mm position (1 mm from the surface layer to the plate thickness (t) direction), plate thickness (t) / 4 position, plate thickness (t) / 2 position (plate thickness center). Although measured, all the steel plates showed the largest variation in hardness at the surface layer position of 1 mm, so the variation in hardness in the plate width direction was evaluated at the surface layer position of 1 mm. The hardness was measured at a load of 10 kg.

  As for the HIC resistance, an HIC test with an immersion time of 96 hours according to NACE Standard TM-02-84 was conducted, and when no crack was observed, it was judged that the HIC resistance was good.

  As for the base material toughness, three full-size Charpy V-notch test pieces in the vertical direction of rolling were sampled, Charpy test was performed, the absorbed energy at −30 ° C. was measured, and the average value was obtained. The absorption energy at −30 ° C. was determined to be 200 J or more.

  For the weld heat affected zone (HAZ) toughness, three specimens with a heat history corresponding to a heat input of 70 kJ / cm were collected by a reproducible thermal cycle apparatus and subjected to a Charpy test. The absorbed energy at a test temperature of −30 ° C. was measured, and the average value was obtained. Those having Charpy absorbed energy at −30 ° C. of 100 J or more were considered good.

  Further, a DWTT test piece conforming to API-5L is taken from the steel sheet, tested at a test temperature of 0 to -80 ° C., and a transition temperature at which the SA value (Shear Area: ductile fracture surface ratio) is 85% is obtained. It was.

The scope of the present invention is a ferrite structure and a bainite structure having a yield strength of 485 MPa or more, a tensile strength of 570 MPa or more as a high-strength steel sheet, and an average grain size of 20 μm or less and a volume ratio of 80% or less (including 0%). The microstructure at the t / 2 position (t is the plate thickness (mm)) is the bainite structure, the hardness variation between the plate thickness direction and the plate width direction is ΔH _V10 30 or less, the maximum hardness value is H _V10 230 or less, the mother VE-30 in the Charpy test of the material part is 200 J or more, 85% SATT in the DWTT test is −50 ° C. or less, vE-30 in the reproduction HAZ Charpy test is 100 J or more, and no crack is observed in the HIC test. did.

  Table 3 shows the measurement results.

No. 1 to 12 are examples of the present invention in which the chemical components and the production method are within the scope of the present invention. In any case, the yield strength is 485 MPa or more, the tensile strength is 570 MPa or more, the variation in hardness in the sheet thickness direction and the sheet width direction is ΔH _V10 30 or less, the maximum value of the hardness is H _V10 230 or less, and the microstructure of the steel sheet is The surface layer 1 mm position is an average grain size of 20 μm or less and a volume ratio of 80% or less (including 0%) ferrite structure and bainite structure, t / 2 position (t is the plate thickness (mm)) is a bainite structure, and the base material toughness The HAZ toughness and the HIC resistance (when the crack was not observed was judged as good when the crack was generated, and when the crack occurred, the mark was shown as x).

  On the other hand, no. Examples 13 to 20 are examples in which the chemical components are within the scope of the present invention but the production methods are outside the scope of the present invention. No. No. 13 had a low slab heating temperature, insufficient homogenization of the microstructure and solid solution of carbide, and low strength.

  No. No. 14 had a low cooling start temperature and ferrite precipitated too much, so it was low in strength and inferior in HIC resistance. No. No. 15 had a controlled cooling condition outside the range of the present invention, and pearlite was excessively precipitated in the center of the plate thickness as a microstructure. Therefore, the strength was low and the HIC resistance was inferior.

No. No. 16 has a low cooling stop temperature, and a hard phase of martensite or island martensite (MA) was generated. Therefore, the hardness variation exceeded ΔH _V10 30 and the material uniformity in the steel sheet and the HIC resistance were It was inferior. No. No. 17, since the impact pressure of descaling just before the control cooling is low and the cooling control rejection condition is outside the scope of the present invention, the hardness variation in the plate thickness direction and the plate width direction exceeds ΔH _V10 30 The material uniformity in the steel plate was poor.

No. 18-No. No. 20 has not been descaled immediately before control cooling, or because the collision pressure is low, the cooling rate of the surface layer portion is increased and martensite is generated, resulting in variations in hardness in the plate thickness direction and plate width direction. Was over ΔH _V10 30 and the material uniformity in the steel sheet and the HIC resistance were inferior.

No. 21-No. In No. 25, the chemical component was outside the scope of the present invention, and the hardness variation exceeded ΔH _V10 30 or the HIC resistance was inferior.

1 Steel plate 2 Hot rolling mill 3 High collision pressure descaling device 4 Control cooling device

Claims (5)

In mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 0.5 to 1.8%, P: 0.01% or less, S: 0.001 % Or less, Al: 0.01 to 0.08%, Ca: 0.0005 to 0.005%, CP value (mass%) represented by the following formula (1) is 1.1 or less, following (2) Ceq value represented by the formula (wt%) 0.35 0.45, the balance being Fe and unavoidable impurities, metal structure mean particle bainite structure in the region of the front side under 1 m m A ferrite structure having a diameter of 20 μm or less and a volume ratio of 80% or less (including 0%), a bainite structure at a t / 2 position (t is a plate thickness), and a variation in hardness in the plate thickness direction is ΔH _V10 30 or less , and the variation in hardness in the plate width direction is at [Delta] H _{V10 30} below, the steel sheet surface layer portion of the subsurface 1mm Height hardness is at H _{V10 230} or less, the tensile strength is equal to or is more than 570 MPa, high strength steel plate for line pipe superior in material homogeneity within the steel sheet.
CP = 4.46C (%) + 2.37Mn (%) / 6+ {1.74Cu (%) + 1.7Ni (%)} / 15+ {1.18Cr (%) + 1.95Mo (%) + 1.74V (% )} / 5 + 22.36P (%) (1)
Ceq = C (%) + Mn (%) / 6+ (Cu (%) + Ni (%)) / 15+ (Cr (%) + Mo (%) + V (%)) / 5 (2)
However, each element symbol in each formula is the content (% by mass).   Furthermore, it is characterized by containing one or more of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less in mass%. The high-strength steel sheet for line pipes excellent in material uniformity in the steel sheet according to claim 1.   Furthermore, it contains one or more of Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% in mass%. The high-strength steel sheet for line pipes, which is characterized by excellent material uniformity in the steel sheet according to claim 1 or 2. After heating the steel having the chemical component according to any one of claims 1 to 3 to a temperature of 1000 ° C or higher and 1300 ° C or lower and hot rolling at a rolling end temperature of Ar ₃ or higher at a steel sheet surface temperature. , impact pressure of the injection flow on the steel sheet surface just before the controlled cooling is carried out descaling at 1MPa or higher, the steel sheet surface temperature of the cooling start is (Ar 3 _-80) the cooling rate of the steel sheet surface from ° C. over 20 ° C. / s to 100 ° C./s or less, and the steel sheet surface temperature is 300 ° C. to 600 ° C. to satisfy the formula (3), and then the steel sheet is cooled at an average cooling rate of 15 ° C./s or more. wherein the average temperature is carried out in the second stage of cooling to 200 ° C. or higher 600 ° C. or less, the metal structure by volume of 80% or less in average particle size below 20μm and bainite structure in the region of the front side under 1 m m (0 % Ferrite structure) Yes, (the t plate thickness) t / 2 position a bainite structure, the variation in the sheet thickness direction stiffness is at [Delta] H _V10 30 below, the variation of the hardness in the plate width direction is at [Delta] H _V10 30 below, maximum hardness of the steel sheet surface layer portion of the subsurface 1mm is at H _{V10 230} or less, a tensile strength of not less than 570 MPa, method of manufacturing the material homogeneity in excellent line high strength steel sheet for pipes in steel.
3 ≦ (700−T) / V (3)
However, T: Finishing temperature of steel sheet surface cooling at 1st stage cooling (° C.), V: Steel sheet surface cooling rate at 1st stage cooling (° C./s) Using the steel plate produced by the production method according to claim 4, the hardness variation in the tube thickness direction is ΔH _V10 30 or less, and the hardness variation in the tube circumferential direction is ΔH _V10 30 or less, A method for producing a high-strength steel pipe for a line pipe excellent in material uniformity, characterized in that a steel pipe having a maximum hardness of _HV10 248 or less at a surface layer portion of 1 mm below the surface is produced.