JP5098256B2 - Steel sheet for high-strength line pipe with low yield stress reduction due to the Bauschinger effect with excellent hydrogen-induced cracking resistance and method for producing the same - Google Patents

Description

  The present invention is suitable as a steel plate for high-strength line pipes used for oil and natural gas transportation and a method for producing the same, and is excellent in hydrogen-induced cracking resistance, and yields of steel plates before being formed into steel pipes by the Bauschinger effect. The present invention relates to a steel plate for high-strength line pipes and a method for producing the same, in which a decrease in yield stress in the circumferential direction of the steel pipe after being formed into a steel pipe from stress is small.

  In general, when cold working is applied to a steel sheet to impart tensile or compressive strain, and then strain is applied in the opposite direction, the yield stress is lowered by the Bauschinger effect compared to that of the steel sheet before cold working.

  The Bauschinger effect is caused by the occurrence of reverse stress due to local strain gradients that occur in hard second phases such as cementite, pearlite, and island martensite (hereinafter referred to as MA), inclusions, and grain boundaries in the initial deformation stage. It is the cause.

  In order to obtain a bainite structure excellent in high strength and high toughness, the steel sheet for line pipe is generally manufactured by a process of controlled rolling and accelerated cooling in many cases. In the accelerated coolant, C is concentrated between the lath of bainite and untransformed austenite after accelerated cooling, and the C enriched portion is transformed into cementite and MA in the air cooling stage after accelerated cooling. The structure has two phases.

  In general, an accelerated coolant has a surface cooling rate that is higher than that of the central portion of the plate thickness, so that the difference between the surface hardness and the central portion of the plate thickness is increased. The presence of such a hard second phase and uneven strength in the plate thickness direction cause local strain gradients during UOE steel pipe molding and correction of tensile specimens taken in the pipe circumferential direction, and in the steel pipe circumferential direction. The yield strength is reduced by the Bauschinger effect compared to the yield strength of the steel plate.

  In line pipes used for transporting crude oil and natural gas containing hydrogen sulfide, in addition to strength, toughness, and weldability, hydrogen-induced crack resistance (HIC resistance), stress corrosion crack resistance (SCC resistance), etc. So-called sour resistance is required.

  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, around non-metallic inclusions such as MnS in the steel and hard second phase structure. It diffuses and accumulates on the surface, and is cracked by its internal pressure. The higher the strength of the steel, the higher the sensitivity.

  The strength of the pipe blank is designed to be high in anticipation of the yield strength reduction due to the Bauschinger effect, so reducing the decrease in yield strength due to the Bauschinger effect will lead to the relaxation of the strength design of the steel sheet, and cost reduction by reducing alloy elements Improvement of weld heat affected zone toughness and further improvement of HIC performance are expected.

  As a technique for suppressing a decrease in yield strength due to the Bauschinger effect, a method using steel having a low C-high Cr composition is known (see, for example, Patent Document 1). However, this method causes a decrease in weldability and a cost increase due to the addition of a large amount of Cr.

  As a method that does not depend on the addition of a large amount of Cr, a method is known in which the controlled rolling end temperature and the accelerated cooling stop temperature are defined, and the yield ratio and yield elongation of the steel sheet are optimized (see, for example, Patent Document 2).

  However, in this method, it is necessary to increase the yield ratio of the steel sheet to 90% or more, which deteriorates the formability of the steel pipe and causes a decrease in productivity. In addition, in order to keep the surface hardness low, the target microstructure is a soft ferrite structure, so it is necessary to add alloying elements to obtain high strength, and deterioration of weld heat affected zone toughness and weldability Is concerned.

  In addition, in order to prevent HIC, the addition of Ca and Ce to increase the HIC, suppression of the formation of acicular MnS, suppression of the hardened structure of the central segregation part, which is the HIC propagation path by optimizing slab soaking and cooling methods Techniques are disclosed (see, for example, Patent Documents 3 and 4).

  However, all the methods for improving the above-mentioned HIC resistance are for the center segregation part. For high-strength steel sheets exceeding X65 grade such as API X80 grade, even if measures against HIC in the central segregation part are taken, HIC other than the central segregation part starting from sulfide-based or oxide-based inclusions should be eliminated. It is difficult.

As a technique for suppressing the HIC other than the central segregation part, a technique is disclosed in which the microstructure is made into a ferrite single-phase structure with low HIC sensitivity. (For example, Patent Documents 5 and 6)
However, in the above technology, the manufacturing process is complicated with quenching and tempering, cold working and re-tempering in order to obtain a ferrite structure, and it is necessary to add Mo to secure the strength. To do.
Japanese Patent Publication No.53-25801 JP 2000-212680 A Japanese Patent Laid-Open No. 54-110119 JP-A-61-165207 Japanese Patent Laid-Open No. 61-227129 JP-A-7-70697

  As described above, the conventional technology produces a steel sheet with a small decrease in yield strength due to the Bausinger effect, which has excellent resistance to hydrogen-induced cracking, without incurring toughness deterioration, productivity reduction, and cost increase in the weld heat affected zone. It was difficult.

  Therefore, the present invention is a steel plate that can be manufactured at high productivity and low cost without deteriorating the toughness of the weld heat affected zone, has excellent resistance to hydrogen-induced cracking, and has a small decrease in yield strength due to the Bauschinger effect, and its An object is to provide a manufacturing method.

  In order to solve the above-mentioned problems, the present inventors performed a microstructure of the steel sheet and a manufacturing method for achieving the microstructure, particularly accelerated cooling after controlled rolling, and a cooling rate of 5 ° C./s or more. The inventors studied diligently about the manufacturing process to be heated and obtained the following knowledge.

  1. Steel pipe molding by reducing cementite, pearlite, and MA, which are hard second phases in the microstructure of the steel sheet, and further reducing the difference in hardness between the surface layer and the center of the plate thickness to provide a uniform strength distribution in the plate thickness direction. It is possible to relieve the local strain gradient that occurs around the hard phase during stage or tensile specimen correction and to suppress the yield stress drop due to the Bauschinger effect.

  2. Furthermore, steel sheet strength by properly controlling the amount of Ca, S, P, and O in steel and reducing the hard second phase that becomes the HIC propagation path, lowering the surface hardness, and suppressing the yield stress drop due to the Bausinger effect The relaxation of the design can improve the HIC performance.

  3. In addition, it is important to reheat so that the surface layer part becomes higher than the center part of the plate thickness immediately after accelerated cooling, and an induction heating apparatus is preferable as an apparatus for carrying out such heating, without reducing productivity, It has also been found that the steel sheet can be manufactured.

In addition, the yield stress reduction by the Bausinger effect of the present invention is small, a 10φ round bar test piece is taken from the 1/4 thickness position, and after introducing a compression pre-strain of 1 to 3%, a tensile test is performed, A value obtained by dividing the 0.5% yield strength obtained by the tensile test by the 0.5% yield strength at the time of compression was evaluated as a yield strength ratio, and a yield strength ratio of 0.8 or more was considered to be small in yield stress reduction. Here, 0.5% proof stress during tension and compression means equivalent stress at equivalent strain of 0.5% during tensile test and compression test, and the value was derived from the stress-strain curve. .

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. In mass%, C: 0.03-0.06%, Si: 0.01-0.5%, Mn: 0.8-1.5%, P: 0.01% or less, S: 0.0015 %: Ti: 0.005 to 0.04%, Nb: 0.005 to 0.06%, Al: 0.08% or less, Ca: 0.001 to 0.005%, O: 0.0030% Containing the remainder, Fe and inevitable impurities, the content of Ca, O, S satisfies the following formula (1), the volume fraction of the hard second phase structure in the metal structure is 0.8 % or less A steel sheet for a high-strength line pipe with a low yield stress reduction due to the Bauschinger effect excellent in hydrogen-induced cracking resistance, characterized in that the Vickers hardness difference between the surface layer and the center of the plate thickness is within 40.
1.0 ≦ {[Ca] − (0.18 + 130 × [Ca]) × [O]} / (1.25 × [S]) ≦ 4.5 (1)
However, [Ca], [O], and [S] are the contents (% by mass).
2. Furthermore, it contains 1 type or 2 types chosen from Mo: 0.05-0.4% or less and V: 0.005-0.07% by mass%. Steel plate for high-strength line pipes with low yield stress reduction due to the Bauschinger effect, which has excellent resistance to hydrogen-induced cracking.
3. Furthermore, by mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and B: 0.005% or less, containing one or more kinds The steel sheet for high-strength line pipes having a low yield stress reduction due to the Bauschinger effect, which is excellent in hydrogen-resistant cracking performance according to 1 or 2, characterized in that:
4). Furthermore, by mass%, it contains 1 type or 2 types chosen from Mg: 0.005% or less, REM: 0.02% or less, As described in any one of 1 thru | or 3 characterized by the above-mentioned. Steel plate for high-strength line pipes with low yield stress reduction due to the Bauschinger effect, which has excellent resistance to hydrogen-induced cracking.
5.1 to steel having a component composition according to any one of 4, was heated to a temperature of 1000 to 1300 ° C., after the steel sheet was hot rolled at Ar ₃ transformation point temperature or more rolling end temperature , Accelerated cooling from 400 ° C. to 600 ° C. at a cooling rate of 5 ° C./s or higher from a temperature not lower than the Ar ₃ transformation point, and immediately followed by a temperature rise rate of 0.5 ° C./s or higher at a steel sheet surface temperature of 600 ° C. or higher. A bow with excellent resistance to hydrogen-induced cracking, characterized in that it is reheated to a thickness center temperature of 550 to 700 ° C., and the temperature difference between the steel plate surface and the thickness center at the end of reheating is 20 ° C. or more. A method for producing a steel sheet for high-strength line pipes, in which the yield stress drop due to the singer effect is small.

  According to the present invention, the steel pipe circumferential direction is excellent in hydrogen-induced cracking resistance and has a small decrease in yield stress due to the Bauschinger effect, i.e., the yield stress of the steel sheet before being formed into a steel pipe decreases after being formed into a steel pipe. It is possible to manufacture a steel sheet with a small decrease in yield stress at a low cost without deteriorating the toughness of the weld heat affected zone or reducing the productivity, which is extremely useful industrially.

  The steel sheet for high-strength line pipes according to the present invention, which has a low yield stress reduction due to the Bauschinger effect, which has excellent resistance to hydrogen-induced cracking, is as follows. Component composition, 2. 2. Microstructure and Specifies the hardness characteristics in the thickness direction.

[Micro structure]
In the present invention, the volume fraction of the second phase structure in the metal structure is set to 3% or less. In the present invention, the second phase structure is a hard phase such as cementite or MA, which prevents the propagation of HIC and the occurrence of reverse stress due to the local strain gradient generated around it, suppresses the HIC, and yields due to the Bausinger effect. In order to suppress the stress drop, the volume fraction is set to 3% or less in the metal structure.

  If the volume fraction exceeds 3%, not only the HIC performance deteriorates, but also the amount of decrease in yield stress due to the Bausinger effect increases, and it is necessary to increase the strength design of the steel sheet. Invite rise. From the viewpoint of reducing the Bausinger effect, it is more preferably 1% or less.

[Hardness characteristics in thickness direction]
The difference in Vickers hardness between the steel plate surface and the center of the plate thickness is 40 or less. By reducing the difference in hardness between the steel plate surface and the center of the plate thickness to within 40, the strain distribution during steel pipe molding and sample correction becomes uniform, the local strain gradient is reduced, and the Bausinger effect can be suppressed. . The test load for the Vickers hardness test is 98N.

  In addition, since the surface-cured portion has high HIC sensitivity, generation of HIC in the surface layer portion can be suppressed by uniforming the hardness. From the viewpoint of obtaining a more uniform strain distribution and HIC resistance, it is more preferably within 30.

[Ingredient composition]
In the following description, all units represented by% are mass%.
C
The C content is 0.03 to 0.06%. C is an element that enhances the hardenability and is important for securing the strength, but if it is less than 0.03%, sufficient strength cannot be secured. Moreover, addition over 0.06% increases the volume fraction of MA and cementite in the structure, increases the resistance to HIC, and increases the Bauschinger effect, so the C content is 0.03 to 0.06%. Stipulate.

Si
The Si content is set to 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 MA volume fraction increases and weldability deteriorates. Set to .5%. More preferably, it is 0.01 to 0.3%.

Mn
The Mn content is 0.8 to 1.5%. Mn is an element effective for improving strength and toughness. However, if it is less than 0.8%, the effect is not sufficient, and if it exceeds 1.5%, the hardenability increases and the MA volume fraction increases and the surface hardness increases. In order to cause weldability deterioration, it is defined as 0.8 to 1.5%. From a viewpoint of MA production | generation suppression, it is 0.8 to 1.3% more suitably.

P
The P content is 0.01% or less. Since P is an inevitable impurity element that deteriorates weldability and HIC resistance, the upper limit is defined as 0.01%.

S
S content shall be 0.0015% or less. In general, 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 0.0015% or less, the upper limit of the S content is specified to be 0.0015%.

Al
Al content shall be 0.08% or less. Al is added as a deoxidizer, but if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so it is specified to be 0.08% or less. Preferably, the content is 0.01 to 0.08%.

Ca
The Ca content is 0.001 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 0.001%, 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 0.001 to 0.0050%.

O
The O content is 0.003% or less. If it exceeds 0.003%, clusters of Ca and Al-based oxides are generated and the HIC resistance is deteriorated, so the content is made 0.003% or less.

1.0 ≦ (1-130 × [O]) × [Ca] / (1.25 × [S]) ≦ 4.5 (1)
This parameter formula defines the mutual relationship between the contents of Ca, O, and S in steel in order to improve the HIC resistance. Usually, Ca suppresses the generation of MnS that causes HIC and lamination, In order to form harmless CaS, it is added so as to be stoichiometrically surplus with respect to the amount of S in the steel.

  However, the effective Ca amount excluding CaO generation is important for securing the HIC performance, and it is necessary to optimize the O, Ca, and S amounts as shown in the equation (1). When the value of (1-130 × [O]) × [Ca] / (1.25 × [S]) is less than 1.0, S is excessive, MnS is generated, and the HIC performance is deteriorated.

  On the other hand, when it exceeds 4.5, Ca is excessive, and HIC deterioration occurs due to coarse clusters of CaO, so (1-130 × [O]) × [Ca] / (1.25 × [S]) The value of 1.0 to 4.5. In the formula (1), each element has a content (mass%).

  The above is the basic component composition, but it is possible to contain one or more of Mo, Ti, Nb, and V shown below for the purpose of further improving the strength toughness of the steel sheet.

Mo
Mo is an element that improves hardenability and greatly contributes to an increase in strength. However, if less than 0.05%, the effect cannot be obtained, and addition exceeding 0.4% leads to an increase in the MA volume fraction and deterioration of the weld heat affected zone toughness. Is specified to be 0.05 to 0.4%. More preferably, it is 0.3% or less.

Ti
Ti is an effective element for suppressing the austenite coarsening during heating due to the pinning effect of TiN and improving the toughness of the base metal and the weld heat affected zone. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.04%, TiN becomes coarse and conversely deteriorates the weld heat affected zone toughness. Therefore, when Ti is added, the content is 0.0. It is specified to be 005 to 0.04%. Furthermore, when the Ti content is less than 0.02%, more excellent toughness is exhibited.

Nb
Nb is an element that enhances the effect of controlled rolling and improves strength and toughness by refining the structure. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.06%, the toughness of the weld heat affected zone deteriorates. Therefore, when Nb is added, the content is specified to be 0.005 to 0.06%. To do.

V
V is an element contributing to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.07%, the toughness of the weld heat-affected zone deteriorates. Therefore, when V is added, the content is specified to be 0.005 to 0.07%. To do.

  Furthermore, when improving the strength toughness of a steel plate, you may contain 1 type (s) or 2 or more types of Cu, Ni, Cr, B, Mg, and REM.

Cu
Cu is an element effective for improving toughness and increasing strength. In order to obtain the effect, it is preferable to add 0.1% or more, but adding a large amount causes deterioration of weldability and an increase in the MA volume fraction. .

Ni
Ni is an element effective for improving toughness and increasing strength. In order to obtain the effect, it is preferable to add 0.1% or more, but adding a large amount is disadvantageous in terms of cost, and the weld heat affected zone toughness deteriorates. Is the upper limit.

Cr
Cr, like Mn, is an element effective for obtaining sufficient strength even at low C. In order to obtain the effect, it is preferable to add 0.1% or more, but adding a large amount causes deterioration of weldability and an increase in the MA volume fraction. .

B
B is an element contributing to strength increase and HAZ toughness improvement. In order to obtain the effect, it is preferable to add 0.0005% or more, but if added over 0.005%, the weldability is deteriorated, so when added, the content is made 0.005% or less.

Mg
Mg is an element that contributes to improving the toughness of the base material by finely dispersing alumina clusters (Al ₂ O ₃ ) as Al and Mg-based oxides. If the amount exceeds 0.005%, the toughness of the base metal decreases due to an increase in oxide, so when added, the amount is made 0.005% or less.

REM
REM, like Ca, is an element that is effective in controlling the morphology of MnS and contributes to the improvement of the base material toughness. Addition exceeding 0.02% causes excessive generation of REM oxysulfide and deteriorates base metal toughness.

  The balance other than the above is Fe and unavoidable impurities, and N is treated as an unavoidable impurity in the present invention. However, if it exceeds 0.007%, the weld heat affected zone toughness deteriorates, so preferably 0.007% or less. Restrict.

  Furthermore, by optimizing Ti / N which is the ratio of Ti amount and N amount, it is possible to suppress austenite coarsening of the weld heat affected zone by TiN particles, and to obtain good weld heat affected zone toughness, Preferably Ti / N is set to 2-8, more preferably 2-5.

  Next, the suitable manufacturing method of the high strength steel plate which concerns on this invention is demonstrated. In the manufacturing method, slab heating temperature, hot rolling, accelerated cooling, and reheating conditions after accelerated cooling are defined.

  The heating temperature, rolling end temperature, and cooling stop temperature are the average temperature of the steel sheet. The average temperature is obtained by calculation from the surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity.

  The cooling rate is an average cooling rate obtained by dividing the temperature difference from the start of cooling to the cooling stop temperature by the time required to perform the cooling.

[Slab heating temperature]
Slab heating temperature shall be 1000-1300 degreeC. If the heating temperature is less than 1000 ° C., sufficient strength cannot be obtained, and if it exceeds 1300 ° C., the base material toughness deteriorates, so the temperature is set to 1000 to 1300 ° C.

[Hot rolling conditions]
The hot rolling is performed at a rolling end temperature: Ar ₃ transformation point temperature or higher. In the present invention, it is important to have a uniform structure with few hard phases. However, if the rolling end temperature is lower than the Ar ₃ transformation point temperature, proeutectoid ferrite is formed, and the metal structure after cooling is a mixture of ferrite and bainite. Since the Bausinger effect increases and the HIC deteriorates, the rolling end temperature is set to the Ar ₃ transformation temperature or higher.

[Accelerated cooling conditions]
Immediately after the end of rolling, accelerated cooling is performed at a cooling rate of 5 ° C./s or higher immediately above the Ar ₃ transformation point temperature. When the cooling start temperature is lower than the Ar ₃ transformation point temperature, pro-eutectoid ferrite is generated and becomes a mixed structure, so that the Bauschinger effect is increased and the strength is further insufficient.

Further, when the cooling rate is less than 5 ° C./s, pearlite which is a hard phase is generated at the time of cooling. Therefore, the cooling start is defined as the Ar ₃ transformation point temperature or more, and the cooling rate after the rolling is finished is 5 ° C./s or more.

The accelerated cooling stop temperature is 400 to 600 ° C. If the accelerated cooling stop temperature is less than 400 ° C., island martensite is generated during cooling, and aggregated cementite is generated even if decomposed by subsequent reheating. Furthermore, when it becomes less than 400 degreeC, surface hardness will rise.
On the other hand, when it exceeds 600 ° C., the untransformed austenite fraction at the time of accelerating cooling stop increases, and MA and pearlite are generated during air cooling after reheating. Such agglomerated cementite and pearlite cause the HIC propagation path and local strain gradient, and the HIC performance deteriorates and the yield stress decreases due to the Bausinger effect during steel pipe molding. Specified at ~ 600 ° C. More preferably, it is 400-530 degreeC.

  As the cooling equipment, any cooling equipment can be used depending on the manufacturing process. For example, a water cooling type accelerated cooling equipment can be used.

[Reheating conditions after accelerated cooling]
As described above, the cementite and MA hard phases in the accelerated coolant are generated between untransformed austenite and bainite lath where C is concentrated during air cooling after accelerated cooling.

In the present invention, fine carbonitride is precipitated during reheating immediately after accelerated cooling and C is consumed, thereby suppressing C concentration to untransformed austenite and suppressing formation of MA and cementite.
Furthermore, by making the steel plate surface temperature higher than the plate thickness center temperature during reheating, the surface can be softened and a uniform hardness distribution in the plate thickness direction can be obtained.

  Therefore, immediately after accelerated cooling, reheating is performed to a steel plate surface temperature of 600 ° C. or higher and a plate thickness center temperature of 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more. The temperature difference at the center is set to 20 ° C. or more. The rate of temperature rise is 0.5 ° C./s or more at the plate surface and the plate center.

  When the rate of temperature rise is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency is deteriorated and pearlite transformation occurs, so that the Bausinger effect is increased.

  If the reheating temperature at the center of the plate thickness is less than 550 ° C., sufficient precipitation of cementite and carbonitride cannot be obtained and MA is generated. When the temperature exceeds 700 ° C., cementite aggregation and coarsening occur, so the reheating temperature at the center of the plate thickness is regulated to 550 to 700 ° C.

  Furthermore, when the steel sheet surface temperature is less than 600 ° C. and the temperature difference between the steel sheet surface and the center of the plate thickness is less than 20 ° C., the surface hardness cannot be reduced, and the uneven thickness distribution in the thickness direction in which the surface is cured. Thus, not only the HIC performance is deteriorated but also the Baudinger effect is increased, so that the steel plate surface temperature is set to 600 ° C. or higher and the temperature difference between the steel plate surface and the plate thickness center portion is set to 20 ° C. or higher. The cooling process after reheating is not particularly defined, but it is desirable to use air cooling.

  In addition, after accelerated cooling, the lath of bainite enriched with C and the untransformed austenite part are transformed into cementite and MA by air cooling, and thus it is necessary to immediately start heating for reheating. In the present invention, “immediately” means within 180 seconds, preferably within 120 seconds.

  As equipment for performing reheating after accelerated cooling, a heating device is installed on the downstream side of the cooling equipment. As the heating device, it is preferable to use an induction heating device that can easily heat the surface of the steel plate and easily eliminate the temperature difference after the accelerated cooling as compared with the heating at the center of the plate thickness.

  As equipment for carrying out the manufacturing method described above, it is preferable to sequentially arrange a hot rolling mill, a cooling device, an induction heating device, and a hot leveler from the upstream side to the downstream side of the rolling line.

  By installing an induction heating device or other heat treatment device on the same line as the hot rolling mill that is the rolling equipment and the cooling device arranged on the outlet side, the reheating treatment can be performed quickly after the completion of rolling and accelerated cooling. Since it can be performed, it is possible to heat the steel sheet after accelerated cooling without excessively reducing the steel sheet temperature.

  In the steel sheet of the present invention manufactured by the combination of the manufacturing method and the component composition described above, the island-like martensite fraction in the metal structure is 3% or less, and further, a hardness difference of 40 or less is obtained as the hardness difference between the surface and the center of the plate thickness.

  Steels (steel types A to L) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and steel plates (Nos. 1 to 18) having thicknesses of 18 and 26 mm were manufactured using the manufacturing conditions shown in Table 2.

  After the heated slab was rolled by hot rolling, it was immediately subjected to accelerated cooling using a water-cooling type cooling facility, and then reheated using an induction heating furnace. The induction heating furnace was installed on the same line as the cooling equipment.

  The heating temperature, rolling end temperature, cooling start and stop temperature were the average temperature of the steel sheet. The average temperature was determined from the surface temperature of the slab or steel plate by parameters and calculations such as plate thickness and thermal conductivity.

  The accelerated cooling rate was an average cooling rate obtained by dividing the temperature difference required for cooling to the accelerated cooling stop temperature by the time required for the cooling after the accelerated cooling was started.

  The reheating temperature increase rate was the average temperature increase rate divided by the time required for reheating the temperature difference required for reheating up to the reheating temperature at the center of the plate thickness after accelerated cooling.

  The steel plate surface temperature at the end of reheating was measured with a radiation thermometer, and the steel plate thickness center temperature was determined from the surface temperature of the steel plate by parameters such as plate thickness and thermal conductivity, and calculation.

  Using the steel plate manufactured under the above conditions, a hardness difference measurement, a tensile property measurement, and a Bausinger test were conducted between the surface and the center of the plate thickness. The measurement results are also shown in Table 2.

  The hardness difference is a value of a Vickers hardness with a load of 10 kgf, and indicates a difference between the hardness of the surface layer (the hardness at a position 1 mm from the surface of the cross section in the width direction of the steel sheet) and the hardness at the center of the thickness.

  Tensile properties were obtained by collecting two full thickness tensile test specimens in the vertical direction of rolling, performing a tensile test, measuring the tensile properties, and evaluating the average value. The tensile strength of 540 MPa or more was determined as the strength required for the present invention.

  In the Bausinger test, a 10φ round bar test piece was sampled from a 1/4 thickness position, and after introducing a compression pre-strain of 1 to 3%, a tensile test was performed, and the 0.5% yield strength obtained in the tensile test was measured. The value divided by 0.5% proof stress during compression was evaluated as the proof stress ratio. It can be evaluated that the yield stress reduction due to the Bauschinger effect is smaller as the yield strength ratio is higher, and the yield strength ratio is set to a value necessary for the present invention of 0.8 or more.

  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 cracks were observed, the HIC resistance was judged good. Indicated.

  For the weld heat affected zone (HAZ) toughness, a Charpy test was performed using a test piece to which a heat history corresponding to a heat input of 40 kJ / cm was added by a reproducible heat cycle apparatus. Charpy absorption energy at a test temperature of −10 ° C. was 100 J or more.

  The second phase volume fraction is obtained by averaging the area fraction from image analysis of five SEM photographs observed at a magnification of 1000 times, and it is assumed that the second phase is uniformly dispersed in the steel sheet. And volume fraction.

Table 3 shows the results obtained. Comparative Example No. Nos. 8 to 18 are Examples Nos. Any characteristic is inferior compared with 1-7.

Claims (5)

In mass%, C: 0.03-0.06%, Si: 0.01-0.5%, Mn: 0.8-1.5%, P: 0.01% or less, S: 0.0015 %: Ti: 0.005 to 0.04%, Nb: 0.005 to 0.06%, Al: 0.08% or less, Ca: 0.001 to 0.005%, O: 0.0030% Containing the remainder, Fe and inevitable impurities, the content of Ca, O, S satisfies the following formula (1), the volume fraction of the hard second phase structure in the metal structure is 0.8 % or less A steel sheet for a high-strength line pipe with a low yield stress reduction due to the Bauschinger effect excellent in hydrogen-induced cracking resistance, characterized in that the Vickers hardness difference between the surface layer and the center of the plate thickness is within 40.
1.0 ≦ {[Ca] − (0.18 + 130 × [Ca]) × [O]} / (1.25 × [S]) ≦ 4.5 (1)
However, [Ca], [O], and [S] are the contents (% by mass).   Furthermore, it contains 1 type or 2 types chosen from Mo: 0.05-0.4% or less and V: 0.005-0.07% by the mass%. Steel sheet for high-strength line pipes with low yield stress reduction due to the Bauschinger effect, which has excellent hydrogen-induced cracking resistance.   Furthermore, by mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and B: 0.005% or less, containing one or more kinds The steel sheet for high-strength line pipes according to claim 1 or 2, wherein the decrease in yield stress due to the Bauschinger effect is excellent in hydrogen-resistant cracking resistance.   Furthermore, by mass%, it contains 1 type or 2 types chosen from Mg: 0.005% or less and REM: 0.02% or less. Steel sheet for high-strength line pipes with low yield stress reduction due to the Bauschinger effect, which has excellent hydrogen-induced cracking resistance. After the steel having the component composition according to any one of claims 1 to 4, and heated to a temperature of 1000 to 1300 ° C., was steel and hot rolled at Ar ₃ transformation point temperature or more rolling end temperature , Accelerated cooling from 400 ° C. to 600 ° C. at a cooling rate of 5 ° C./s or higher from a temperature not lower than the Ar ₃ transformation point, and immediately followed by a temperature rise rate of 0.5 ° C./s or higher at a steel sheet surface temperature of 600 ° C. or higher. A bow with excellent resistance to hydrogen-induced cracking, characterized in that it is reheated to a thickness center temperature of 550 to 700 ° C., and the temperature difference between the steel plate surface and the thickness center at the end of reheating is 20 ° C. or more. A method for producing a steel sheet for high-strength line pipes, in which the yield stress drop due to the singer effect is small.