JP2007138210A - Steel sheet for high strength line pipe in with reduced lowering of yield stress caused by bauschinger effect and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for a high strength line pipe in which the lowering of the yield stress in the steel sheet before being formed into a steel pipe to the yield stress in the circumferential direction of the steel pipe after being formed into the steel pipe caused by Bauschinger effect is reduced; and to provide its production method. <P>SOLUTION: A steel having a componential composition comprising, by mass, 0.03 to 0.06% C, 0.01 to 0.5% Si, 0.5 to 2.0% Mn and ≤0.08% Al, and, if required, comprising one or more kinds selected from Mo, Ti, Nb, V, Cu, Ni, Cr and B, and the balance Fe with inevitable impurities, and in which the volume fraction of the second phase structure in its metallic structure is ≤3%, and the difference in Vickers hardness between the surface layer and the central part of the sheet thickness is ≤40 is hot-rolled at a rolling finishing temperature of an Ar<SB>3</SB>transformation point or above in order to form a steel sheet. The steel sheet is thereafter subjected to accelerated cooling from an Ar<SB>3</SB>transformation point or above to 300 to 600°C at a cooling rate of ≥5°C/s, and is immediately reheated, and also, the difference in temperature between the surface of the steel sheet and the central part of the sheet thickness upon the end of the heating is controlled to ≥20°C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a steel plate for a high-strength line pipe used for oil and natural gas and a steel pipe circumference after being formed into a steel pipe from the yield stress of the steel plate before being formed into a steel pipe due to the Bauschinger effect, which is suitable as a manufacturing method thereof. The present invention relates to a steel plate for a high-strength line pipe with a small decrease in yield stress in the direction and a method for producing the same.

  In general, when a tensile or compressive strain is applied to a steel sheet in a cold state, and then a strain is applied in the opposite direction, the yield stress is reduced compared to that of the steel sheet due to the Bauschinger effect. The Bausinger 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, current steel plates for line pipes are generally manufactured by controlled rolling and accelerated cooling processes. In the accelerated cooling steel sheet, C is concentrated between the lath of bainite and in the untransformed austenite part after accelerated cooling, and the C concentrated part is transformed into cementite and MA in the air cooling stage after accelerated cooling. The structure has two phases.

  In general, the accelerated cooling steel sheet has a surface cooling rate that is higher than that of the central part of the plate thickness, so that the difference between the surface hardness and the central part hardness of the plate thickness increases. The existence of such a hard second phase and uneven strength in the plate thickness direction cause local strain gradients during UOE steel pipe molding and pipe circumferential direction tensile specimen correction, and the yield strength in the steel pipe circumferential direction is The singer effect lowers the yield strength of the steel sheet.

  In consideration of this yield strength reduction allowance, it is necessary to design the strength of the original pipe plate to be high, and reducing the yield strength reduction due to the Bauschinger effect leads to the relaxation of the steel strength design, reducing the cost by reducing alloy elements, welding heat Improvement of toughness of affected area is 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, the target structure is a ferrite structure in terms of surface hardness and the like, and it is necessary to increase the amount of alloy components to be added in order to obtain strength, and there is concern about deterioration in weldability and HAZ toughness.
Japanese Patent Publication No.53-25801 JP 2000-212680 A

  As described above, with the conventional technology, it has been difficult to produce a steel sheet with a small decrease in yield strength due to the Bauschinger effect without incurring toughness deterioration, productivity reduction, and cost increase in the weld heat affected zone.

  Therefore, an object of the present invention is to provide a steel plate that can be produced at high productivity and low cost without deteriorating the toughness of the heat affected zone, and a method for producing the same, with a small decrease in yield strength due to the Bauschinger effect. .

In order to solve the above-mentioned problems, the inventors of the present invention have a microstructure of a steel sheet and a manufacturing method for achieving the microstructure, particularly accelerated cooling after controlled rolling, a cooling rate of 5 ° C./s or more, and subsequent reheating. We have earnestly examined the manufacturing process and obtained the following knowledge.
That is, by reducing cementite, pearlite, MA, which are the hard second phase in the microstructure of the steel sheet, and further reducing the hardness difference between the surface layer part and the sheet thickness center part to obtain a uniform strength distribution in the sheet thickness direction, It is possible to relieve the local strain gradient generated around the hard phase at the time of steel pipe molding and tensile specimen correction, and to suppress the yield stress drop due to the Bauschinger effect.

  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 Bauschinger effect of the present invention is small. The 10% round bar test piece was sampled from the 1/4 thickness position and the compression pre-strain of 1 to 3% was introduced. The value obtained by dividing the 0.5% proof stress during tension by the 0.5% proof stress during compression was evaluated as the proof stress ratio.

The present invention has been made based on further studies based on the obtained knowledge, that is, the present invention,
1% by mass, C: 0.03 to 0.06%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.0%, Al: 0.08% or less, the balance A Bausinger effect characterized by comprising Fe and inevitable impurities, having a volume fraction of the second phase structure in the metal structure of 3% or less, and having a Vickers hardness difference of 40 or less between the surface layer and the center of the plate thickness. Steel sheet for high-strength line pipes with low yield stress reduction due to the

  2 Further, by mass, Mo: 0.05 to 0.4%, Ti: 0.005 to 0.04%, Nb: 0.005 to 0.06%, V: 0.005 to 0.07% The steel plate for high-strength line pipes having a small yield stress drop due to the Bauschinger effect according to 1, which contains one or more selected from among the above.

  3 Further, by mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, B: 0.005% or less A steel sheet for high-strength line pipes, which contains a small drop in yield stress due to the Bauschinger effect according to 1 or 2.

  4 Further, it is characterized by containing one or more selected from Ca: 0.001 to 0.005%, Mg: 0.005% or less, REM: 0.02% or less in mass%. A steel plate for a high-strength line pipe with a low yield stress reduction due to the Bauschinger effect according to any one of 1 to 3.

5 1 to a steel having the component composition according to any one of 4, was heated to a temperature of 1000 to 1300 ° C., after the hot rolled steel sheet at Ar ₃ transformation point temperature or more rolling end temperature, Ar Accelerated cooling is performed from a temperature of ₃ transformation points or higher to 300 to 600 ° C. at a cooling rate of 5 ° C./s or more, and immediately after that, a steel sheet surface temperature of 600 ° C. or more at a temperature rising rate of 0.5 ° C./s or more. A high-strength line with low yield stress reduction due to the Bausinger effect, characterized in that the temperature difference between the steel plate surface and the thickness center at the end of heating is 20 ° C. or more. Manufacturing method of steel plate for pipes.

  According to the present invention, the decrease in yield stress due to the Bauschinger effect is small, that is, the steel sheet has a small decrease in yield stress in the circumferential direction of the steel pipe that decreases after forming into the steel pipe from the yield stress of the steel sheet before forming into the steel pipe. Can be manufactured at a low cost without degrading the toughness of the weld heat affected zone or reducing the productivity, which is extremely useful industrially.

  The component composition, microstructure, and hardness characteristic in the plate thickness direction of the steel plate for high-strength line pipes with low yield stress reduction due to the Bauschinger effect according to the present invention are defined.

[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, and the metal structure prevents the occurrence of reverse stress due to the local strain gradient generated around the second phase structure and suppresses the yield stress reduction due to the Bauschinger effect. The volume fraction is 3% or less.

  If it exceeds 3%, the yield stress drop due to the Bauschinger effect will increase, and it will be necessary to increase the strength design of the steel sheet, leading to an increase in manufacturing costs such as alloy costs. 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, 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. From the viewpoint of obtaining a more uniform strain distribution, it is more preferably within 30.

[Ingredient composition]
In the following description, all units represented by% are mass%.

C
C: Set to 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. Addition exceeding 0.06% increases the volume fraction of MA and cementite in the structure and increases the Bauschinger effect, so the C content is regulated to 0.03 to 0.06%.

Si
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 MA volume fraction increases and weldability deteriorates. It is specified to be 0.01 to 0.5%. More preferably, it is 0.01 to 0.3%.

Mn
Mn: 0.5 to 2.0%. Mn is an element effective for improving strength and toughness. However, if it is less than 0.5%, the effect is not sufficient, and if it exceeds 2.0%, the hardenability increases and the MA volume fraction increases and the surface hardness increases. In order to cause weldability deterioration, the Mn content is specified to be 0.5 to 2.0%. From a viewpoint of MA production | generation suppression, it is 0.5 to 1.5% more suitably.

Al
Al: 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 the Al content is specified to be 0.08% or less. Preferably, the content is 0.01 to 0.08%.

  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 defined as 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, Ca, Mg, REM, and N shown below.

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.

N
In the present invention, N is treated as an inevitable impurity. However, if it exceeds 0.007%, the weld heat affected zone toughness deteriorates, so the content is preferably limited to 0.007% or less.

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.
Ca
In the steelmaking process, it is preferable to add 0.001% or more of Ca in order to secure CaS by controlling the deoxidation reaction and obtain a toughness improving effect. However, if it exceeds 0.005%, coarse CaO is likely to be generated. In addition to a decrease in toughness, this causes clogging of the nozzle of the ladle and impedes productivity, 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. When the amount exceeds 0.005%, the base material toughness is lowered due to an increase in oxides. When added, the amount is made 0.005% or less.

REM
REM is an effective element for controlling the morphology of MnS and contributes to the improvement of the base material toughness. Addition of 0.02% or more causes excessive generation of REM oxysulfide and deteriorates the toughness of the base metal.

  The balance other than the above is Fe and inevitable impurities.

  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 based on 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 required for cooling to the cooling stop temperature (300 to 600 ° C.) by the time required for the cooling after the start of cooling.
[Slab heating temperature]
Slab heating temperature: 1000-1300 ° C. 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 it becomes a structure, the rolling end temperature is set to the Ar ₃ transformation point 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 temperature. When the cooling start temperature is lower than the Ar ₃ 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.

  Accelerated cooling stop temperature: 300 to 600 ° C. If the accelerated cooling stop temperature is less than 300 ° C., island martensite is generated during cooling, and aggregated cementite is generated even if decomposed by subsequent reheating. Further, when the temperature is less than 300 ° C., the surface hardness increases.

  On the other hand, if it exceeds 600 ° C., the untransformed austenite fraction at the time of accelerated cooling stop increases, and MA and pearlite are generated during air cooling after reheating. Such agglomerated cementite and pearlite cause local strain gradients, and a decrease in yield stress due to the Bauschinger effect at the time of steel pipe molding becomes large, so the accelerated cooling stop temperature is regulated to 300 to 600 ° C. Preferably it is 350-550 degreeC, More preferably, it is 400-530 degreeC.

As a cooling method, any cooling equipment can be used depending on the manufacturing process, and for example, a water-cooled accelerated cooling equipment can be used.
[Reheating conditions after accelerated cooling]
As described above, hard phases such as cementite and MA in the accelerated coolant are generated between untransformed austenite and bainite lath in which 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.
After cooling, between the lath of bainite enriched with C and the untransformed austenite part are transformed into cementite and MA by air cooling, so heating needs to be started immediately within 180 seconds. Preferably, it is within 120 seconds.

  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., agglomeration and coarsening of cementite occur. Therefore, the reheating temperature range is defined as 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 surface is hardened in an uneven thickness direction. Since the hardness distribution is increased and the Baudinger effect is increased, 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.

  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 generate a temperature difference between the surface of the steel plate and the central portion 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 a combination of the above-described manufacturing method and component composition, 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.

  FIG. 1 shows a microstructure of a steel sheet of the present invention (0.05 mass% C-1.4 mass% Mn-0.01 mass% Ti-0.04 mass% Nb) observed with a scanning electron microscope (SEM).

  Fig. 1 shows the results of observation of a sample that had been subjected to electrolytic etching after nital etching. Cementite (indicated by an arrow) was observed as a black hole because it was dissolved by electrolytic etching, and MA was present on the needle between the bainite laths. It is observed as a lump in the vicinity of the grain boundary. As is apparent from FIG. 1, the cementite and MA hard phases in the bainite structure are hardly observed.

  Steels (steel types A to L) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and thick steel plates (Nos. 1 to 21) having a thickness of 18 and 26 mm were manufactured.

  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.

  Table 2 shows the production conditions of each steel plate (No. 1 to 21). 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 to reheat 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 Bau Singer 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%, tension was performed, and 0.5% proof stress during tension was reduced to 0. The value divided by the 5% yield strength was evaluated as the yield 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. 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 thermal 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.

  In Table 2, all of Nos. 1 to 9, which are examples of the present invention, have chemical components and production methods within the scope of the present invention, have a high tensile strength of 540 MPa or more, and a volume fraction of the second phase is 1 %, And the proof stress ratio was 0.8 or more.

  Nos. 10 to 16 have chemical components within the scope of the present invention, but because the production method is outside the scope of the present invention, either the second phase volume fraction or the hardness difference cannot be satisfied, and the yield strength ratio Is less than 0.8.

  In Nos. 17 to 21, since the chemical component is outside the scope of the present invention, the second phase volume fraction exceeds 3%, and the proof stress ratio is also less than 0.8.

The photograph which observed the steel plate of the present invention with the scanning electron microscope (SEM).

Claims (5)

  In mass%, C: 0.03 to 0.06%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.0%, Al: 0.08% or less, and the balance Fe And the inevitable impurities, the volume fraction of the second phase structure in the metal structure is 3% or less, and the difference in Vickers hardness between the surface layer and the thickness center is within 40. Steel plate for high-strength line pipe with low yield stress reduction.   Further, in terms of mass%, Mo: 0.05 to 0.4%, Ti: 0.005 to 0.04%, Nb: 0.005 to 0.06%, V: 0.005 to 0.07% The steel sheet for high-strength line pipes having a low yield stress reduction due to the Bauschinger effect according to claim 1, comprising one or more selected from the inside.   Further, by mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, B: 0.005% or less The steel sheet for high-strength line pipes containing a small yield stress drop by the Bauschinger effect according to claim 1 or 2, wherein the steel sheet is contained.   Furthermore, it is characterized by containing one or more selected from Ca: 0.001 to 0.005%, Mg: 0.005% or less, and REM: 0.02% or less in mass%. A steel plate for a high-strength line pipe, wherein the yield stress drop due to the Bauschinger effect according to any one of claims 1 to 3 is small. After heating the steel having the component composition according to any one of claims 1 to 4 to a temperature of 1000 to 1300 ° C and hot rolling at a rolling end temperature equal to or higher than the Ar ₃ transformation point temperature, Accelerated cooling is performed from a temperature not lower than the Ar ₃ transformation point to 300 to 600 ° C. at a cooling rate of 5 ° C./s or higher, and immediately after that, the steel sheet surface temperature is 600 ° C. or higher at a heating rate of 0.5 ° C./s or higher. Reheating to a center temperature of 550 to 700 ° C., and the difference in temperature between the steel sheet surface and the thickness center at the end of heating is 20 ° C. or more, and the strength is small in yield stress reduction due to the Bauschinger effect Manufacturing method of steel plate for line pipe.