JP2013139630A - High strength steel sheet for sour-resistant line pipe excellent in material uniformity in the steel sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet for a sour-resistant line pipe excellent in material uniformity in the steel sheet which is suitable for use in a field, such as a line pipe, and a method for producing the same.SOLUTION: The steel sheet includes, in mass%, 0.02-0.08% of C, 0.01-0.5% of Si, 0.5-1.8% of Mn, 0.01% or less of P, 0.001% or less of S, 0.01-0.08% of Al, 0.0005-0.005% of Ca, and if necessary, one or two or more of Cu, Ni, Cr, Mo, Nb, V, and Ti, with the balance of Fe and unavoidable impurities, wherein metal structure is bainite structure, variation of hardness in the through-thickness direction is ΔH25 or less, the variation of hardness in the sheet width direction is ΔH25 or less in Vickers hardness, and the maximum hardness in the steel sheet surface layer part is ΔH220 or less in Vickers hardness. The steel with the component composition is heated to a specific temperature, and is subjected to hot rolling, then accelerated cooling, and reheat treatment.

Description

  The present invention relates to a high-strength steel plate for sour line pipes having excellent material uniformity in a steel plate used in the fields of architecture, offshore structures, shipbuilding, civil engineering, construction industry, line pipes, and the like, and a method for producing the same. It is about.

  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.

  Line pipes used to transport crude oil and natural gas containing hydrogen sulfide have strength, toughness and weldability, as well as hydrogen-induced crack resistance (HIC resistance) and stress corrosion crack resistance (SCC resistance). 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, It has excellent HIC resistance against HIC generated from the vicinity of the surface together with HIC in the central segregation part, and it has excellent material uniformity in the steel sheet with reduced variation in hardness in the sheet thickness direction and sheet width direction. Another object of the present invention is to provide a high-strength steel sheet for sour-resistant pipes and a method for producing the same.

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 portion in the high strength steel plate having the strength of API standard X65 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: 0.01-0.08%, Ca: 0.0005-0.005%, the balance is made of Fe and inevitable impurities, the metal structure is a bainite structure, The variation in hardness in the thickness direction is ΔH _V10 25 or less, the variation in hardness in the sheet width direction is ΔH _V10 25 or less, and the maximum hardness of the steel sheet surface layer portion is H _V10 220 or less. High strength steel plate for sour line pipes with excellent material uniformity in the steel plate.
(2) Further, by mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, or one or more of them are contained. The high-strength steel plate for sour line pipes having excellent material uniformity in the steel plate according to (1), characterized in that
(3) Further, by mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, or one or more A high-strength steel sheet for a sour line pipe excellent in material uniformity in the steel sheet according to (1) or (2).
(4) The CP value (mass%) represented by the following formula (1) is 1.0 or less, and the Ceq value (mass%) represented by the following formula (2) is 0.30 or more and 0.37 or less. The high-strength steel plate for sour line pipes having excellent material uniformity in the steel plate according to any one of (1) to (3).
CP = 4.46C (%) + 2.37Mn (%) + {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).
(5) The steel having the chemical component according to any one of (1) to (4) is heated to a temperature of 1000 ° C. or higher and 1300 ° C. or lower, and the rolling end temperature is a steel plate surface temperature of Ar ₃ temperature or higher. After hot rolling, just before the controlled cooling, the impact pressure of the jet flow on the steel sheet surface is descaled at 1 MPa or more, and the steel sheet surface temperature is cooled from (Ar ₃ -10) ° C. or more at the start of cooling. The rate is 200 ° C./s or less, the average cooling rate of the steel plate is 15 ° C./s or more, the average temperature of the steel plate is controlled to 250 ° C. to 550 ° C., and then immediately after induction cooling, the cooling stop temperature is over And the manufacturing method of the high-strength steel plate for sour-line pipes excellent in the material uniformity in the steel plate characterized by performing reheating to 500-700 degreeC with a steel plate surface temperature and 350-600 degreeC with a steel plate average temperature.
(6) A high-strength steel pipe for a sour line pipe excellent in material uniformity using a steel plate produced by the production method described in (5).

  According to the present invention, a high-strength sour line pipe steel plate having excellent material uniformity in a steel plate even with low-cost chemical components and excellent in HIC resistance and a method for producing the same can be obtained, which is 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 (%) + {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.

  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]
In order to achieve a high strength with a tensile strength of 520 MPa or more, the metal structure is a bainite main structure that is uniform in the plate thickness direction and mixed with the bainite structure. Here, the bainite structure is a transformation structure generated by accelerated cooling from the austenite temperature range, and includes bainitic ferrite and granular bainite.

  When at least the surface layer portion has a bainite structure, the surface layer hardness is low and the HIC resistance is improved. 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, cementite, and retained austenite are mixed, the toughness deteriorates and the surface hardness increases. The smaller the volume, the better. However, 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 sheet thickness direction is ΔH _V10 25 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 sheet width direction is defined as ΔH _V10 25 or less. ΔH _V10 is the difference between the highest hardness and the lowest hardness.

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

When the variation in hardness in the plate thickness direction exceeds ΔH _V10 25 or the variation in hardness in the plate width direction exceeds ΔH _V10 25, the above characteristics are adversely affected. For example, when the hardness of the steel sheet surface layer part exceeds ΔH _V10 25 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 25, 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 it is cut into small plates In addition, the strength and elongation of each platelet differ.

From the viewpoint of material uniformity in the steel sheet and HIC resistance, it is more preferable that the variation in hardness in the thickness direction is ΔH _V10 20 or less, and the variation in hardness in the plate width direction is ΔH _V10 20 or less.

[Maximum hardness]
In the high-strength steel sheet having the strength of API standard X65 grade, when the hardness of the steel sheet surface layer portion is increased, 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 220 or less. When a steel plate is used in a more severe sour environment, the hardness of the steel plate surface layer portion (hardness at 1 mm below the surface) is preferably H _V10 210 or less from the viewpoint of sour resistance characteristics.

It is desirable hardness of the steel tube surface portion after the pipe by cold forming (subsurface 1mm) is H _{V10 235} or less. When a steel plate is used in a more severe sour environment, it is preferable that the hardness (hardness at 1 mm below the surface) of the steel plate surface layer is _HV10 225 or less from the viewpoint of sour resistance characteristics. 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 hot rolling, control cooling, and then reheating, 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 plate after controlled cooling is 15 μm or less, it is possible to make the hardness variation in the plate thickness direction ΔH _V10 25 or less and the hardness variation in the plate width direction be ΔH _V10 25 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 cooling start temperature of controlled cooling is (Ar ₃ −10 ° C.) or higher in terms of the steel sheet surface temperature. If the steel sheet surface temperature at the start of cooling is low, the amount of ferrite produced before controlled cooling increases, and if the temperature drop from the Ar ₃ transformation point exceeds 10 ° C., ferrite exceeding 5% may be produced, and the strength is high. Since the sour resistance characteristics deteriorate with decreasing, the steel sheet surface temperature at the start of cooling is set to (Ar ₃ -10 ° C) or higher.

[Cooling rate]
The cooling rate of the steel sheet surface is 200 ° C./s or less, and the average cooling rate of the steel plate is 15 ° C./s or more. It is important to control both the cooling rate of the surface layer part and the average cooling rate of the steel sheet in order to reduce the variation in hardness in the steel sheet and improve the material uniformity while increasing the strength. The average cooling rate of a steel sheet refers to the average value of cooling rates in the thickness direction.

  When the cooling rate of the steel sheet surface exceeds 200 ° C / s, a hard phase such as martensite and island martensite (MA) is generated, and the surface layer hardness increases and the sour resistance property deteriorates. The cooling rate is set to 200 ° C./s or less. Preferably, it is 150 ° C./s or less.

  Further, if the average cooling rate of the steel sheet is less than 15 ° C / s, the average cooling rate of the steel sheet is 15 ° C because the bainite structure is not obtained, the strength is lowered, the sour resistance is deteriorated, and the hardness is increased. / s or more. More preferably, it is 20 ° C./s or more.

  Furthermore, when the cooling rate of the steel plate surface is 200 ° C./s or less and the average cooling rate of the steel plate is 15 ° C./s or more, variation in the cooling stop temperature can be suppressed, and the steel plate shape is improved.

[Cooling stop temperature]
Cooling stop temperature shall be 250-550 degreeC by the average temperature of a steel plate. After rolling, the bainite phase is generated by quenching to 250 to 550 ° C., which is the temperature range of bainite transformation, by controlled cooling. In addition, the average temperature of a steel plate is the temperature averaged in the cross section, when there exists temperature distribution in the steel plate thickness direction.

  When the cooling stop temperature exceeds 550 ° C., the bainite transformation is incomplete and sufficient strength cannot be obtained. Further, when the cooling stop temperature is less than 250 ° C., martensite and island martensite (MA) are generated, and particularly the hardness of the surface layer portion of the steel sheet is remarkably increased, so that the variation in hardness is increased and the sour resistance is deteriorated. To do. Further, the steel sheet is easily distorted, and formability deteriorates.

  Therefore, the cooling stop temperature of the controlled cooling is set to 250 to 550 ° C. in order to suppress deterioration of material uniformity and sour resistance in the steel plate. Note that the difference between the steel plate surface temperature and the steel plate center temperature generated after accelerated cooling becomes a substantially uniform temperature distribution within the steel plate due to heat conduction after a while, so the cooling stop temperature is the temperature of the steel plate soaked after cooling. It is good also as steel plate surface temperature.

[Reheating temperature]
Immediately after accelerated cooling, reheating is performed by induction heating to a temperature equal to or higher than the cooling stop temperature, to a steel plate surface temperature of 500 to 700 ° C., and to a steel plate average temperature of 350 to 600 ° C. In order to temper the surface layer portion by reheating, the reheating temperature is set to the cooling stop temperature or higher. Here, the induction heating device is used because rapid heating is possible, and the heating temperature of the steel sheet can be changed between the steel sheet surface layer part and the plate thickness center part.

  Even when a hard phase is generated in a part of the surface layer portion by accelerated cooling at a high cooling rate, the hard phase is decomposed by heating the surface layer portion of the steel sheet, and the hardness of the surface layer portion is reduced. However, if the steel sheet surface temperature is less than 500 ° C., the hard phase is not sufficiently decomposed, so that the hardness is insufficiently reduced. Invite. Therefore, the steel sheet surface temperature in reheating by induction heating is set to 500 to 700 ° C.

  In addition, the surface layer part is heated by induction heating, and the inside of the steel sheet is heated by heat conduction, so the temperature of the surface layer part becomes higher than the inside of the steel sheet, but the increase in the average temperature of the steel sheet is suppressed, and the hardness of the surface layer part is increased. In order to reduce effectively, it is desirable that the surface temperature of the steel plate by induction heating is higher than the average temperature of the steel plate by 100 ° C or more.

  By induction heating after accelerated cooling, not only the surface layer portion of the steel sheet but also the strength variation inside the steel sheet can be reduced. However, when the heating temperature at the central part of the steel sheet is less than 350 ° C., the variation is insufficiently reduced. On the other hand, when it exceeds 600 ° C., not only does the strength decrease due to tempering, but also the DWTT performance deteriorates. Therefore, the average temperature of the steel sheet during reheating by induction heating is set to 350 to 600 ° C.

  Furthermore, in order to suppress the variation in strength and the deterioration of the DWTT performance, it is desirable that the average temperature of the steel sheet in the reheating by induction heating is in the range of 400 to 550 ° C.

  Since the difference between the steel plate surface temperature and the steel plate center temperature generated immediately after induction heating becomes a substantially uniform temperature distribution in the steel plate due to heat conduction after a while, it may be the steel plate surface temperature of the steel plate soaked after induction heating. .

  In the reheating temperature, it is not necessary to set the temperature holding time in particular, and it may be cooled immediately after reaching the reheating temperature. In the case where the temperature is maintained at the reheating temperature, if the temperature is maintained for more than 30 minutes, the strength may be reduced. Moreover, although the cooling rate after reheating may be selected arbitrarily, air cooling is desirable because the steel plate shape may deteriorate when rapidly cooled.

  FIG. 1 shows an example of equipment suitable for manufacturing a steel sheet according to the present invention. In the rolling line 1, a hot rolling mill 2, a high collision pressure descaling device 3, a control cooling device 4, and an in-line induction heating device 5 are arranged from upstream to downstream. A hot straightening machine may be installed in front of the high collision pressure descaling device 3.

  By improving the shape of the steel plate 6 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.

  Since the in-line induction heating device 5 is installed on the rolling line, the steel sheet after the completion of rolling and cooling can be quickly reheated. That is, the steel sheet after rolling and accelerated cooling is immediately over the cooling stop temperature without being excessively cooled from the cooling stop temperature, and the steel sheet surface temperature is 500 to 700 ° C., and the steel sheet average temperature is 350 to 600 ° C. Can be reheated.

  A hot straightening machine may be installed in front of the inline type induction heating device 5. While the shape of the steel sheet is improved by the hot straightening machine, the reheating by the induction heating device can be made more uniform.

According to the present invention, high impact pressure descaling is performed immediately before the control cooling, and reheating treatment is performed by induction heating immediately after the control cooling, whereby the variation in hardness in the plate thickness direction is ΔH _V10 25 or less, the plate The variation in the hardness in the width direction can be ΔH _V10 25 or less.

  Furthermore, since a uniform thin scale with good adhesion is formed, scale peeling that is likely to occur in a reheat-treated steel sheet is suppressed, excellent material uniformity within the steel sheet, and excellent resistance to scale peeling. High strength steel sheets for sour line pipes can be manufactured.

  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 for sour line pipes (UOE steel pipes, ERW steel pipes, spiral steel pipes, etc.) suitable for transporting 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 shown in Table 1 (steel types A to L) was made into a slab by a continuous casting method, and thick steel plates (No. 1 to 27) having a thickness of 25 mm and 34 mm were manufactured 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.

  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.

  Further, the tensile properties, hardness, HIC resistance, and scale peel resistance of each steel plate were measured. Tensile properties were measured by performing a tensile test using a full thickness test piece in the vertical direction of rolling as a tensile test piece, and measuring the tensile strength.

  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.

  The HIC resistance was evaluated by performing an HIC test with an immersion time of 96 hours in accordance with NACE Standard TM-02-84. If no crack was observed, the HIC resistance was judged good. As evaluated.

  The peel resistance of the scale is determined by conducting a cross-cut test in which the tape is peeled after making a grid-like cut on the scale surface of the steel sheet. The peel resistance was evaluated.

  The DWTT performance was evaluated by performing a drop weight test at a test temperature of −37 ° C. using a DWTT test piece whose thickness was reduced to 19 mm from both the front and back surfaces at the center of the plate thickness of the steel sheet, and evaluated by the ductile fracture surface ratio.

The scope of the present invention is that the tensile strength is 520 MPa or more as a high-strength steel plate, the microstructure is a bainite structure at both the 1 mm position and the t / 2 position of the surface layer, the variation in hardness in the plate thickness direction and the plate width direction is ΔH _V10 25 or less, No cracks were observed, the peel resistance of the scale was good, and the DWTT performance was a ductile fracture surface ratio of 85% or more.
The manufacturing conditions and measurement results for each steel plate (No. 1 to 27) are shown in Table 2.

Nos. 1 to 12 are examples of the present invention in which chemical components and production methods are within the scope of the present invention. In any case, the tensile strength is 520 MPa or more, the variation in hardness in the sheet thickness direction and the sheet width direction is ΔH _V10 25 or less, and the microstructure of the steel sheet is a bainite structure in both the 1 mm position of the surface layer and the t / 2 position. Properties, scale peel resistance and DWTT performance were also good.

  On the other hand, Nos. 13 to 22 are examples whose chemical components are within the scope of the present invention, but whose production methods are outside the scope of the present invention. In No. 13, the slab heating temperature was low, the homogenization of the microstructure and the solid solution of carbides were insufficient, and the strength was low.

  No. 14 had a low cooling start temperature and ferrite precipitated, so that it had low strength and inferior HIC resistance. In No. 15, the controlled cooling condition was outside the range of the present invention, and a bainite structure was not obtained as the microstructure at the center of the plate thickness, and pearlite was precipitated, so that the strength was low and the HIC resistance was inferior.

No. 16 has a low cooling stop temperature, and a hard phase of martensite or island-like martensite (MA) was generated. Therefore, the maximum hardness of the steel sheet surface layer part exceeded _HV10 220, and the thickness direction and the sheet width direction The hardness variation of the steel sheet also exceeded ΔH _V10 25, and the material uniformity in the steel sheet and the HIC resistance were inferior.

No. 17 has a low descaling collision pressure just before the control cooling and the cooling control rejection condition is outside the scope of the present invention. Therefore, the maximum hardness of the steel sheet surface layer exceeds _HV10 220 and the hardness varies. Further, ΔH _V10 25 was exceeded, the material uniformity in the steel sheet was inferior, and the scale peel resistance was also inferior.

No. 18 to No. 20 were not subjected to descaling immediately before control cooling, and because the collision pressure was low, the cooling rate of the surface layer portion increased, martensite was generated, and the maximum hardness of the steel plate surface layer portion Exceeds _HV10 220, and the variation in hardness in the plate thickness direction and the plate width direction also exceeds ΔH _V10 25, and the material uniformity in the steel sheet, the HIC resistance, and the scale peel resistance were inferior.

No. 21 has a low reheating temperature and is outside the scope of the present invention, the maximum hardness of the steel sheet surface layer portion exceeds H _V10 220, and the hardness variation also exceeds ΔH _V10 25. It was inferior. No. 22 had a high reheating temperature, and cementite precipitated coarsely at the center of the plate thickness, so the DWTT performance was inferior. No.23~No.27, the chemical component is outside the scope of the present invention, or maximum hardness of the steel sheet surface layer portion exceeds the _H V10 220, variation in hardness is greater than [Delta] H _V10 25, HIC resistant The characteristics were inferior.

DESCRIPTION OF SYMBOLS 1 Rolling line 2 Hot rolling mill 3 High collision pressure descaling device 4 Control cooling device 5 Inline type induction heating device 6 Steel plate

Claims (6)

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%, the balance is composed of Fe and inevitable impurities, the metal structure is a bainite structure, The variation in hardness is ΔH _V10 25 or less, the variation in hardness in the sheet width direction is ΔH _V10 25 or less, and the maximum hardness of the steel sheet surface layer portion is H _V10 220 or less. High-strength steel plate for sour line pipes with excellent material uniformity.   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 sour-resistant pipes with excellent 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 plate for a sour-resistant pipe excellent in material uniformity in the steel plate according to claim 1 or 2, characterized by being characterized by the following. The CP value (mass%) represented by the following formula (1) is 1.0 or less, and is excellent in material uniformity in the steel sheet according to any one of claims 1 to 3. High strength steel plate for sour line pipes.
CP = 4.46C (%) + 2.37Mn (%) + {1.74Cu (%) + 1.7Ni (%)} / 15+ {1.18Cr (%) + 1.95Mo (%) + 1.74V (%) } /5+22.36P (%) (1)
However, each element symbol in the formula is the content (% by mass). After heating the steel having the chemical component according to any one of claims 1 to 4 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. Immediately before the control cooling, descaling is performed when the impinging pressure of the jet flow on the steel sheet surface is 1 MPa or more, and the steel sheet surface temperature at the start of cooling is (Ar ₃ −10) ° C. or more, and the steel sheet surface cooling rate is 200 ° C. / s or less, and the average cooling rate of the steel sheet is 15 ° C./s or more, and the steel sheet is controlled to be cooled at an average temperature of 250 ° C. or more to 550 ° C., and then immediately after induction heating, the cooling stop temperature or more and the steel plate surface temperature. A method for producing a high-strength steel sheet for a sour line pipe excellent in material uniformity in a steel sheet, wherein reheating is performed to a temperature of 500 to 700 ° C. and an average steel sheet temperature of 350 to 600 ° C.   A high-strength steel pipe for a sour line pipe excellent in material uniformity using a steel plate produced by the production method according to claim 5.