JPWO2016051727A1 - Welded steel pipe, thick steel plate and method for producing them - Google Patents

Abstract

An object of the present invention is to provide a high toughness welded steel pipe and high tough steel plate having excellent base metal toughness while ensuring excellent HIC resistance according to the required HIC environment, and a method for producing them. It has a specific component composition, Ca / O is 2.5 or less, ACRM represented by formula (1) is 0 or more, PHIC represented by formula (2) satisfies formula (3), and the balance Is composed of Fe and inevitable impurities, and the maximum value HV of the micro Vickers hardness of the hard second phase contained in the central segregation part and the microstructure of the surface layer and the back layer satisfies the formula (4), and the surface layer and the back layer In a low hydrogen sulfide concentration environment characterized in that the Vickers hardness is 248 or less and the degree of integration of the (211) plane of the rolled surface at the tube thickness center position obtained by X-ray diffraction is 1.6 or more High toughness welded steel pipe with excellent sour resistance.

Description

  TECHNICAL FIELD The present invention relates to a welded steel pipe, a thick steel plate, and a manufacturing method thereof for process piping and line pipe, and particularly to a high toughness welded steel pipe excellent in sour resistance performance in a low hydrogen sulfide concentration environment (0.5 to 50 Vol%) and a high The present invention relates to tough steel plates and methods for producing them.

  Oil and natural gas transported by pipelines may contain hydrogen sulfide depending on their properties, and may be laid in an environment where hydrogen sulfide is contained in the seabed or soil.

A line pipe exposed to such an environment requires HIC (hydrogen induced cracking) resistance and SSC (hydrogen sulfide stress corrosion cracking) resistance. Line pipes that excel in their performance are called sour-resistant line pipes. Conventionally, the HIC resistance and the SSC resistance were evaluated under the condition that 100% H ₂ S (hydrogen sulfide) was blown into the solution A defined by NACE-TM0284 and NACE-TM0177.

  However, this condition is a severe condition in which HIC and SSC are very likely to occur in an actual environment, and is an excessively safe evaluation in evaluating the HIC resistance and SSC resistance. Therefore, when producing a sour-resistant pipe, various restrictions for ensuring the HIC-resistant performance (for example, addition of elements concentrated in segregation parts such as Mn, suppression of multiphase formation of the microstructure, etc.). ) As a result of this restriction, it is difficult to make other performance such as strength and toughness compatible with HIC resistance and SSC resistance. As for SSC resistance, it is widely known that the occurrence of SSC is suppressed when the Vickers hardness is suppressed to 248 or less. In particular, in order to improve the DWTT performance often required for line pipes, it is necessary to obtain a fine microstructure or texture by controlled rolling, which requires rolling at a low temperature. This easily deteriorates the HIC resistance in order to facilitate the formation of a multi-phase microstructure, and in order to suppress this, a large amount of Ni, which has a large effect of lowering the transformation point, is added, leading to an increase in cost.

On the other hand, recently, a concept called Fit for Purpose has been proposed. This is to perform the HIC test and the SSC test under conditions that consider the severity of the real environment in order to obtain the requirements of the HIC resistance and SSC resistance for the line pipe. Regarding SSC resistance, ISO 15156 classifies environmental severity by pH and H ₂ S fraction. Therefore, compared with the condition where 100% H ₂ S is blown into the solution A defined by the conventional NACE-TM0288 and NACE-TM0177, it is easy to satisfy the target of HIC resistance and SSC resistance, and other performance Can be set to manufacturing conditions compatible with the above.

With regard to such a background, Non-Patent Document 1 evaluates the severity of the HIC test environment by pH and H ₂ S fraction, and derives the limit length of MnS generated in the central segregation part. In Non-Patent Document 2, the severity of the HIC test environment is evaluated by the pH and H ₂ S fraction, and the limit hardness at which HIC occurs is derived.

Hara, Asahi, Terada, Shigenobu, Ogawa HIC generation conditions of X65 line pipe under humid H2S environment, 45th Materials and Environmental Discussion, 341-344 (1998) Y. Inohara, N .; Ishikawa, S .; Endo Development Development in High Strength Linepipe for Source Environment, Proceedings of The Thirteenth International Offensive and Polar Eng. 60-66 (2003)

  However, Non-Patent Document 1 is based on the premise that MnS exists in the center segregation part, and cannot satisfy the strict HIC resistance performance required for an actual line pipe. Further, Non-Patent Document 1 does not fully disclose the limit of the HIC resistance performance when MnS is not present. Further, Non-Patent Document 2 does not quantitatively disclose the chemical composition or manufacturing method of steel for satisfying the target hardness upper limit. Further, Non-Patent Document 2 does not disclose a method for achieving both strength and toughness required for use as a line pipe and HIC resistance.

  The present invention has been made in view of the above circumstances, and ensures a high toughness welded steel pipe and a high toughness thick steel plate having excellent base metal toughness while ensuring excellent HIC resistance according to the required HIC environment. An object of the present invention is to provide a production method thereof.

In the present invention, in order to obtain a method of achieving both excellent HIC resistance performance and excellent base material toughness according to the required HIC environment, the steel material according to the pH and H ₂ S fraction of the HIC test environment is used. The following findings were obtained through intensive studies on the crack generation limit and rolling conditions and microstructure for obtaining excellent DWTT performance.

As is clear from conventional knowledge, steel materials are more susceptible to HIC as the pH is lower and the H ₂ S fraction is higher. As a result of investigating the crack, it was found that when the microstructure is uneven or the center segregation is large, the hard second phase cracks and propagates. And in the state where generation of MnS in the steel material segregation part and coarse CaO clusters on the front and back surfaces is suppressed, the maximum value HV of the micro-Vickers hardness of the center segregation part, and the microstructure of the surface layer and the back layer It was found that cracking occurs when the maximum value HV of the micro Vickers hardness of the hard second phase contained in the above does not satisfy formula (4) described later. Further, it was found that the _{PHIC calculated} by the expression (2) described later needs to satisfy the expression (3) described later. Note that the P _HIC is a parameter obtained by multiplying the thickening degree and carbon equivalent of the alloy elements in the center segregation area, it is possible to quantify the hardness of the center segregation area by P _HIC.

Next, after securing the HIC resistance, a method for securing the DWTT performance was examined. In order to ensure the HIC resistance, it is essential that the vicinity of the center segregation portion has a uniform microstructure, and the hot rolling is terminated so that the hot rolling temperature _TF satisfies the formula (6) described later. In addition, it is necessary to start the accelerated cooling so that the accelerated cooling start temperature _TF satisfies the formula (6) described later. In addition, in order to ensure excellent DWTT performance within the constraints, it is necessary to perform intensive rolling on the low temperature side of the non-recrystallized region, the cumulative rolling reduction is 50% or more, and a formula described later It performs rolling at a rolling start temperature consistent with T _s as indicated by (5), it was found that it is necessary to integrate the (211) plane.

The present invention has been made by further studying the above findings and is as follows.
[1] By mass%, C: 0.02 to 0.10%, Si: 0.40% or less, Mn: 1.00 to 2.00%, Nb: 0.005 to 0.060%, Ti: 0.005 to 0.025%, Ca: 0.0010 to 0.0040%, N: 0.0010 to 0.0100%, Ca / O is 2.5 or less, and the following formula (1) in ACRM shown is not less than 0, _{P HIC} satisfies the following formula (3) represented by the following formula (2), the balance being Fe and unavoidable impurities,
The maximum value HV of the micro Vickers hardness of the hard second phase contained in the microstructure of the center segregation part and the surface layer and the back layer satisfies the following formula (4),
The Vickers hardness of the surface layer and the back layer is 248 or less,
High toughness welding with excellent sour resistance in a low hydrogen sulfide concentration environment characterized in that the degree of integration of the (211) surface of the rolled surface at the tube thickness center position obtained by X-ray diffraction is 1.6 or more Steel pipe.
ACRM = ([Ca] − (1.23 [O] −0.000365)) / (1.25 [S]) (1)
P _HIC = 4.46 [C] +2.37 [Mn] / 6 + (1.74 [Cu] +1.7 [Ni]) / 15+ (1.18 [Cr] +1.95 [Mo] +1.74 [ V]) / 5 + 22.36 [P] (2)
P _HIC ≦ 1.35 + (pH−12) (1 + log (P _H2S )) / 60 (3)
HV ≦ 400 + 50 (pH−12) (1 + log (P _H2S )) / 9 (4)
However, in the formulas (1) to (4),
[Ca], [O], [S], [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [P]: Content of each element ( Mass%), and when not included, pH is 0: pH of the HIC test environment
P _{H2S: H 2} S fraction of HIC test environment (Vol%)
And
[2] Further, by mass%, Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.060% or less, B : A high toughness welded steel pipe excellent in sour resistance performance in a low hydrogen sulfide concentration environment according to [1], comprising at least one selected from 0.0030% or less.
[3] By mass%, C: 0.02 to 0.10%, Si: 0.40% or less, Mn: 1.00 to 2.00%, Nb: 0.005 to 0.060%, Ti: 0.005 to 0.025%, Ca: 0.0010 to 0.0040%, N: 0.0010 to 0.0100%, Ca / O is 2.5 or less, and the following formula (1) in ACRM shown is not less than 0, _{P HIC} satisfies the following formula (3) represented by the following formula (2), the balance being Fe and unavoidable impurities,
The maximum value HV of the micro Vickers hardness of the hard second phase contained in the microstructure of the center segregation part and the surface layer and the back layer satisfies the following formula (4),
The Vickers hardness of the surface layer and the back layer is 248 or less,
High toughness thickness excellent in sour resistance in low hydrogen sulfide concentration environment, characterized in that the degree of integration of the (211) plane of the rolled surface at the thickness center position obtained by X-ray diffraction is 1.6 or more steel sheet.
ACRM = ([Ca] − (1.23 [O] −0.000365)) / (1.25 [S]) (1)
P _HIC = 4.46 [C] +2.37 [Mn] / 6 + (1.74 [Cu] +1.7 [Ni]) / 15+ (1.18 [Cr] +1.95 [Mo] +1.74 [ V]) / 5 + 22.36 [P] (2)
P _HIC ≦ 1.35 + (pH−12) (1 + log (P _H2S )) / 60 (3)
HV ≦ 400 + 50 (pH−12) (1 + log (P _H2S )) / 9 (4)
However, in the formulas (1) to (4),
[Ca], [O], [S], [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [P]: Content of each element ( Mass%), and when not included, pH is 0: pH of the HIC test environment
P _{H2S: H 2} S fraction of HIC test environment (Vol%)
And
[4] Further, by mass%, Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.060% or less, B : A high tough steel plate excellent in sour resistance in a low hydrogen sulfide concentration environment according to [3], containing one or more selected from 0.0030% or less.
[5] [3] or a continuously cast slab steel material having a composition as set forth in [4], was heated to 1000 to 1200 ° C., the cumulative rolling reduction rolling start temperature _{T S} is to satisfy the following formula (5) Hot rolling is performed at 50% or more, then the hot rolling is finished so that the rolling end temperature _TF satisfies the following formula (6), and then the accelerated cooling start temperature T _ACS satisfies the following formula (7). The method for producing a high toughness thick steel plate excellent in sour resistance in a low hydrogen sulfide concentration environment is characterized in that accelerated cooling is started, the accelerated cooling is stopped at a cooling stop temperature of 600 ° C. or less, and then air cooling is performed. .
T _S ≦ 174 log ([Nb] ([C] +12 [N] / 14)) + 1444-1.2t (5)
T _F ≧ 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6t (6)
T _ACS ≧ 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6 t (7)
However, in the above formulas (5), (6), (7),
[Nb], [C], [N], [Mn], [Cu], [Ni], [Cr], [Mo]: content (% by mass) of each element, 0 if not included T: Thickness at the end of rolling (mm)
And
[6] The method for producing a high toughness thick steel plate excellent in sour resistance performance in a low hydrogen sulfide concentration environment according to [5], wherein after the air cooling, tempering to 480 to 720 ° C.
[7] A thick steel plate produced by the method of [5] or [6] is cold worked into a cylindrical shape, and the butt portion is welded to form a welded steel pipe. A method of manufacturing high toughness welded steel pipes with excellent sour performance.

  According to the present invention, rational component design is possible according to the required test environment of the HIC test. Further, it is possible to ensure high toughness, which is extremely effective in the industry.

In addition, the test environment of the HIC test in the present invention is 0.5 to 50% in terms of H ₂ S fraction.

  Hereinafter, the present invention will be specifically described.

<Ingredient composition>
Below, the reason for limitation of the component composition of the welded steel pipe or thick steel plate which concerns on this invention is demonstrated. In addition,% of the unit which shows a component composition means the mass% altogether.

C: 0.02-0.10%
C is the most effective element for increasing the strength of the steel sheet produced by accelerated cooling. However, if the C content (content) is less than 0.02%, sufficient strength cannot be secured, and if it exceeds 0.10%, the toughness and HIC resistance deteriorate. Therefore, the C content is 0.02% or more, preferably 0.03% or more, 0.10% or less, preferably 0.08% or less.

Si: 0.40% or less Si is added for deoxidation. For deoxidation, the Si content is preferably 0.01% or more. However, when the Si content exceeds 0.40%, toughness and weldability deteriorate. Therefore, the Si content is within the range of 0.40% or less, preferably 0.35% or less.

Mn: 1.00 to 2.00%
Mn is added to improve the strength and toughness of the steel. However, if the amount of Mn is less than 1.00%, the effect is not sufficient, and if it exceeds 2.00%, weldability and HIC resistance deteriorate. Therefore, the amount of Mn is 1.00% or more, preferably 1.10% or more, 2.00% or less, preferably 1.90% or less.

Nb: 0.005 to 0.060%
When Nb is present in the steel as solute Nb, it suppresses grain growth during rolling and improves toughness by making the particles finer. However, when the Nb content is less than 0.005%, the effect is not obtained. When the Nb content exceeds 0.060%, not only the toughness of the HAZ is deteriorated, but also the formation of coarse Nb carbonitrides is caused and the HIC resistance is deteriorated. Therefore, the Nb content is 0.005% or more, preferably 0.010% or more, 0.060% or less, preferably 0.040% or less.

Ti: 0.005-0.025%
Ti not only suppresses grain growth during slab heating by forming TiN, but also suppresses HAZ grain growth and improves toughness by making the base material and HAZ finer. However, if the amount of Ti is less than 0.005%, there is no effect, and if it exceeds 0.025%, the toughness deteriorates. Therefore, the Ti content is 0.005% or more and 0.025% or less, preferably 0.020% or less.

Ca: 0.0010 to 0.0040%
Ca is an effective element for controlling the form of oxysulfide inclusions and improving ductility. However, if the Ca content is less than 0.0010%, the effect is not obtained, and if the Ca content exceeds 0.0040%, the effect is saturated, but rather the toughness deteriorates due to a decrease in cleanliness. Therefore, the Ca content is 0.0010% or more, preferably 0.0015% or more, 0.0040% or less, preferably 0.0035% or less.

N: 0.0010 to 0.0100%
N is an element effective for suppressing the austenite coarsening during heating by the pinning effect of TiN and improving the toughness of the base metal and the weld heat affected zone. However, if the amount of N is less than 0.0010%, there is no effect, and the addition exceeding 0.0100% conversely causes the deterioration of the toughness of the weld heat affected zone due to the coarsening of TiN and the increase of solute N. Therefore, the N content is 0.0010% or more, preferably 0.0020% or more, and is specified to be 0.0100% or less, preferably 0.0055% or less. Further, N is preferably 0.0010 to 0.0060% and Ti / N (Ti content (mass%) / N content (mass%)) is preferably 1 to 5 from the viewpoint of improving toughness. More preferably, when it is 2 to 4, more excellent toughness is exhibited.

O: 0.0030% or less O is an element inevitably contained in the steel, and is usually present as an oxide combined with Al or Ca. When O is excessively contained, the content of these Al and Ca-based oxides in the steel becomes too large to form clusters, and the HIC resistance is deteriorated. For this reason, it is preferable to make content of O into 0.0030% or less, and it is more preferable to set it as 0.0025% or less.

Ca / O: 2.5 or less Ca / O is an index for quantifying the CaO cluster generation limit. When Ca / O exceeds 2.5, Ca clusters are likely to be generated, and the HIC resistance in the vicinity of the surface layer or inclusion inclusion zone deteriorates. For this reason, it is preferable that the upper limit of Ca / O is 2.5 and the upper limit is 2.3.

ACRM: 0 or more ACRM is an index for quantifying the morphology control of MnS by Ca. When ACRM is 0 or more, the generation of MnS due to center segregation is suppressed, and the HIC resistance performance at the center of the plate thickness (tube thickness) is improved. Therefore, it is preferable to set the lower limit of ACRM to 0 and the lower limit to 0.2.
The ACRM is defined by the following formula (1).
ACRM = ([Ca] − (1.23 [O] −0.000365)) / (1.25 [S]) (1)
However, in said formula (1), [Ca], [O], [S] represents content (mass%) of each element.

_PHIC is 1.35+ (pH-12) (1 + log ( _PH2S )) / 60 or less _PHIC is a parameter for quantifying the hardness of center segregation, and is defined by the following formula (2).
P _HIC = 4.46 [C] +2.37 [Mn] / 6 + (1.74 [Cu] +1.7 [Ni]) / 15+ (1.18 [Cr] +1.95 [Mo] +1.74 [ V]) / 5 + 22.36 [P] (2)
However, in the above formula (2), [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [P] are the contents (% by mass) of each element. ) And 0 if not included.
The value of P _HIC higher the higher the hardness of the center segregation large, it promotes HIC generation of sheet thickness (wall thickness) center. The limit P _{HIC at which} HIC is generated increases as the H ₂ S fraction of the HIC test environment is lower and the pH is higher. The present inventors have studied, P _HIC is as long as it satisfies the following expression (3), HIC resistance performance can be ensured. For this reason, the upper limit of _PHIC is set to 1.35+ (pH-12) (1 + log ( _PH2S )) / 60.
P _HIC ≦ 1.35 + (pH−12) (1 + log (P _H2S )) / 60 (3)
However, in the above formula (3),
pH: pH of the HIC test environment
P _{H2S: H 2} S fraction of HIC test environment (Vol%)
It is.

  In the present invention, P, S and O are inevitably contained impurity elements, and the smaller the number, the better. However, the following ranges are acceptable. O is as described above.

P: 0.015% or less P is an element that easily segregates and concentrates in the central portion, and even if contained in a small amount, the hardness of the central segregation is remarkably increased and sour resistance is deteriorated. For this reason, the smaller the amount of P, the better. However, the amount of P can be allowed up to 0.015%. More preferably, it is 0.010% or less.

S: 0.0015% or less S combines with Mn to generate MnS. Further, since S is an element that is also likely to concentrate Mn into central segregation, if the amount of S is large, a large number of central segregation of MnS is generated, and sour resistance is significantly deteriorated. Therefore, it is desirable to reduce the amount of S as much as possible, but it is acceptable up to 0.0015%. More preferably, it is 0.0010% or less.

  In order to ensure strength and toughness, one or more elements selected from the following can be added.

Cu: 0.50% or less Cu is an element effective for improving the toughness of the base metal and increasing the strength, and the Cu content is preferably 0.10% or more for exhibiting this effect. However, when the amount of Cu exceeds 0.50%, weldability deteriorates. Therefore, when adding Cu, it is preferable to make Cu amount into 0.50% or less, and it is more preferable that it is 0.30% or less.

Ni: 1.00% or less Ni is an element effective for improving the toughness of the base metal and increasing the strength. In order to exhibit this effect, the Ni content is preferably 0.10% or more. However, if the Ni content exceeds 1.00%, the weldability deteriorates. Therefore, when adding Ni, it is preferable to make Ni amount into 1.00% or less, and it is more preferable that it is 0.50% or less.

Cr: 0.50% or less Cr is an element effective for increasing the strength by enhancing the hardenability, and the Cr content is preferably 0.10% or more in order to exhibit this effect. However, if the Cr content exceeds 0.50%, the weldability deteriorates. Therefore, when adding Cr, the Cr content is preferably 0.50% or less, and more preferably 0.30% or less.

Mo: 0.50% or less Mo is an element effective for improving the toughness of the base metal and increasing the strength, and the Mo amount is preferably 0.10% or more in order to exert this effect. However, if the Mo content exceeds 0.50%, the HAZ toughness and weldability deteriorate. Therefore, when adding Mo, it is preferable to make Mo amount into 0.50% or less, and it is more preferable that it is 0.30% or less.

V: 0.060% or less V is an element that increases the strength, and in order to exhibit this effect, the V content is preferably 0.010% or more. However, if the V content exceeds 0.060%, the HAZ toughness and weldability are significantly impaired. Therefore, when adding V, it is preferable to make V amount into 0.060% or less, and it is more preferable that it is 0.050% or less.

B: 0.0030% or less B is an element effective for increasing the strength, and the B content is preferably 0.0005% or more in order to exhibit this effect. However, if the B content exceeds 0.0030%, the HAZ toughness and weldability deteriorate. Therefore, when adding B, the amount of B is preferably 0.0030% or less, and more preferably 0.0025% or less.

  The remainder other than the said component in the welded steel pipe or thick steel plate of this invention is Fe and an unavoidable impurity. However, the content of elements other than the above is not a problem as long as the effects of the present invention are not impaired.

<Hardness>
Central segregation part and maximum value HV of micro Vickers hardness of hard second phase contained in microstructure of surface layer and back layer are 400 + 50 (pH-12) (1 + log (P _H2S )) / 9 or less, respectively. HIC is more likely to occur as the hardness of the second layer and the hardness of the hard second phase contained in the microstructures of the surface layer and the back layer increase. As a result of investigations by the present inventors, it has been found that HIC occurs when the maximum value of each micro Vickers hardness does not satisfy the following formula (4). In addition, a surface layer and a back layer refer to the area | region to 5 mm from the surface to a plate | board thickness direction.
HV ≦ 400 + 50 (pH−12) (1 + log (P _H2S )) / 9 (4)
However, in Formula (4),
pH: pH of the HIC test environment
P _{H2S: H 2} S fraction of HIC test environment (Vol%)
And
Therefore, in the present invention, the upper limit of the maximum value HV of the micro-Vickers hardness of the hard second phase contained in the center segregation portion and the microstructures of the surface layer and the back layer is set to 400 + 50 (pH-12) (1 + log (P _H2S )). / 9. The micro Vickers load is set in the range of 5 to 50 g in accordance with the cross-sectional dimension of the hard second phase, and is measured so that no indentation protrudes from the hard second phase to be measured. If the microstructure is almost uniform (for example, a bainite single phase structure) and there is no hard second phase that can be measured even with 5 g, an arbitrary portion is measured with 50 g. In addition, since the micro Vickers test is a test with a large measurement variation, the maximum value of the micro Vickers hardness of the hard second phase contained in the microstructure of the central segregation part, the surface layer and the back layer is a result of measurement of 5 points or more. Use the maximum value.

It is known that the Vickers hardness of the surface layer and the back layer is generally 248 or less. In order to ensure the SSC resistance, it is generally necessary to suppress the Vickers hardness to 248 or less. In the welded steel pipe and the thick steel plate manufactured by TMCP (Thermo-Mechanical Control Process) handled in the present invention, the surface layer and the back layer are the hardest, so the hardness of the surface layer and the back layer needs to be suppressed to 248 or less. Note that the load of Vickers hardness is 10 kg, and the measurement position is preferably measured at a depth of 1.5 mm in the tube thickness direction from the surface layer and the back layer in the cross section in the tube thickness direction.

<Group organization>
The accumulation degree of the (211) plane of the rolled surface at the tube thickness center position or the sheet thickness center position obtained by X-ray diffraction is 1.6 or more. Base material toughness such as DWTT performance required for a line pipe is Influenced by microstructure and texture. The present inventors have a good correlation between the degree of integration of the (211) plane of the rolled surface at the tube thickness center position or the sheet thickness center position and the base material toughness that develop when transforming from austenite to bainite. I found out. Since the base material toughness becomes good when the integration degree is 1.6 or more, the lower limit of the integration degree is set to 1.6. More preferably, it is 1.8 or more. Here, the degree of integration of the (211) plane is a numerical value representing the degree of integration of the (211) crystal plane of the target material, and is the value of the plate surface taken in parallel to the steel sheet rolling surface from the tube thickness center position of the target material ( 211) the ratio of the X-ray diffraction intensity of the reflection _{(I (211))} and, (211) reflection of X-ray diffraction intensity _{(I 0 (211)} without random standard sample of texture) _{(I (211)} / I _{0 (211)} ).

<Production conditions>
Production method of the present invention, a continuous cast slab steel material having the above-mentioned composition of ingredients, and heated to 1000 to 1200 ° C., rolling start temperature perform hot rolling at a cumulative rolling reduction of 50% or more below T _S, then Hot rolling is terminated so that the rolling end temperature satisfies _TF or higher, and then accelerated cooling is performed so that the cooling stop temperature is 600 ° C. or lower, followed by air cooling.

  Next, the reasons for limiting each manufacturing condition will be described. In the present invention, the temperature under the production conditions is the surface temperature of the steel material or steel plate. The surface temperature of a steel material or a steel plate can be measured by, for example, a radiation thermometer.

  As the steel material, a continuously cast slab can be used.

Heating temperature: 1000-1200 ° C
In order to obtain a minimum amount of Nb solid solution while austenizing the slab, the lower limit temperature of the slab heating temperature is set to 1000 ° C. On the other hand, when the slab is heated to a temperature exceeding 1200 ° C., the pinning effect by NbC and TiN is weakened, austenite grains grow significantly, and the base material toughness deteriorates. For this reason, slab heating temperature shall be 1000-1200 degreeC.

Rolling start temperature T _S : 174 log ([Nb] ([C] +12 [N] / 14)) + 1444-1.2t or less when applying rolling reduction of 50% or more to reduce the base metal toughness It is desirable to perform rolling to increase the cumulative rolling reduction at the low temperature side of the austenite non-recrystallization temperature range. T _S represented by the following formula (5) is determined in accordance with the composition of steel plate thickness at the completion of rolling (mm), it shows a rolling start temperature during rolling necessary for the base material toughness ensure is there.
T _S ≦ 174 log ([Nb] ([C] +12 [N] / 14)) + 1444-1.2t (5)
However, in the above formula (5),
[Nb], [C], [N]: content (% by mass) of each element, 0 if not included t: thickness at the end of rolling (mm)
And
In the present invention, when the rolling start temperature when applying a reduction of 50% or more of the cumulative reduction ratio exceeds 174 log ([Nb] ([C] +12 [N] / 14)) + 1444-1.2t, the base material toughness Deteriorates. For this reason, the upper limit of the rolling start temperature is set to 174 log ([Nb] ([C] +12 [N] / 14)) + 1444-1.2 t.

Rolling end temperature _TF : 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6t or more The lower the rolling end temperature, the base material Toughness is improved. On the other hand, when the rolling end temperature is low, the HIC resistance is deteriorated. When the rolling end temperature does not satisfy the following formula (6), processed ferrite is generated near the surface, the hardness of the hard second phase is increased, and the HIC resistance near the surface layer is deteriorated.
T _F ≧ 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6t (6)
However, in the above formula (6),
[C], [Mn], [Cu], [Ni], [Cr], [Mo]: content (mass%) of each element and 0 when not included t: plate at the end of rolling Thickness (mm)
And
Therefore, the lower limit of the rolling end temperature _{T F} and 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6t. Further, the rolling end temperature is preferably 950 ° C. or lower for the purpose of ensuring the minimum base metal toughness required for line pipe applications. If the lower T _F also accelerated cooling start temperature, ferrite is generated in the vicinity of the center segregation area, and hard cured second phase significantly central polarized析近near, since the HIC resistance may deteriorate, the lower limit T _F is preferable.

Accelerated cooling start temperature _{T ACS: 910-310 [C] -80} [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] accelerated cooling start temperature or higher -0.6t is as low Base material toughness is improved. On the other hand, when the accelerated cooling start temperature is low, the HIC resistance is deteriorated. If the accelerated cooling start temperature is less than _TACS represented by the following formula (7), processed ferrite is generated near the surface, the hardness of the hard second phase is increased, and the HIC resistance near the surface layer is deteriorated.
T _ACS ≧ 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6 t (7)
However, in the above formula (7),
[C], [Mn], [Cu], [Ni], [Cr], [Mo]: content (mass%) of each element and 0 when not included t: plate at the end of rolling Thickness (mm)
And
Therefore, the lower limit of the accelerated cooling start temperature _{T ACS} and 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6t.

Note that T _S , T _F, and T _ACS are steel plate surface temperatures. The steel sheet surface temperature can be measured by, for example, a radiation thermometer.

Cooling stop temperature of accelerated cooling is 600 ° C. or less After the rolling, accelerated cooling is performed. The lower the cooling stop temperature, the greater the strength, and the higher the temperature, the better the flatness of the steel sheet. On the other hand, from the viewpoint of securing HIC resistance, if the temperature exceeds 600 ° C., ferrite and pearlite transformation occurs after cooling is stopped, the microstructure becomes non-uniform, and the HIC performance deteriorates. For this reason, an upper limit shall be 600 degreeC. Further, the cooling stop temperature is preferably 300 ° C. or higher for the purpose of suppressing the deterioration of the HIC performance caused by increasing the strength more than necessary. The cooling rate during accelerated cooling is preferably 10 ° C./s or higher.

Tempering temperature: 480-720 ° C
In the present invention, in order to adjust the strength and toughness after accelerated cooling, or to reduce the change in characteristics at the time of stress-relieving annealing, after air cooling after accelerated cooling, tempering heat treatment can be performed as necessary. The effect cannot be obtained when the tempering temperature is less than 480 ° C., and when it exceeds 720 ° C., a part of the microstructure is reversely transformed, and the HIC performance is deteriorated. For this reason, when performing tempering heat processing, it is preferable to perform tempering temperature in the range of 480-720 degreeC.

  According to the above production conditions, the high toughness thick steel plate of the present invention can be obtained. Furthermore, the manufacturing method of the high toughness welded steel pipe of this invention is demonstrated.

  The present invention forms a steel pipe using the above-described thick steel plate. Examples of the method for forming a steel pipe include a method for forming a steel pipe into a shape by cold forming such as a UOE process or a press bend (also called a bending press).

  In the UOE process, after performing groove processing on the width direction end of the thick steel plate used as a raw material, the end bending of the width direction end of the steel plate is performed using a press machine, and then the steel plate is processed using a press machine. By forming it into a U shape and an O shape, the steel plate is formed into a cylindrical shape so that the widthwise ends of the steel plate face each other. Next, the opposing widthwise ends of the steel plates are brought together and welded. This welding is called seam welding. In this seam welding, a cylindrical steel plate is constrained, the widthwise ends of opposing steel plates are butted against each other in a tack welding process, and welding is performed on the inner and outer surfaces of the butt portion of the steel plate by the submerged arc welding method. A method having a two-stage process, that is, a main welding process for performing the above-described process is preferable. After seam welding, pipe expansion is performed to remove residual welding stress and improve roundness of the steel pipe. In the pipe expansion process, the pipe expansion ratio (ratio of the outer diameter change amount before and after the pipe expansion to the outer diameter of the pipe before the pipe expansion) is usually performed in the range of 0.3% to 1.5%. From the viewpoint of the balance between the roundness improvement effect and the capacity required for the tube expansion device, the tube expansion rate is preferably in the range of 0.5% to 1.2%. Thereafter, a coating treatment can be carried out for the purpose of preventing corrosion. As the coating treatment, for example, after heating to a temperature range of 200 to 300 ° C., a known resin may be applied to the outer surface.

  In the case of a press bend, a steel pipe having a substantially circular cross-sectional shape is manufactured by sequentially forming a steel plate by repeating three-point bending. Thereafter, seam welding is performed in the same manner as the above-described UOE process. Also in the case of press bend, tube expansion may be performed after seam welding, and coating may also be performed.

  Steel of chemical composition shown in Table 1 was made into a slab by continuous casting. Thereafter, the slab was reheated under the conditions shown in Tables 2 and 3, then hot-rolled, accelerated cooled, and then air-cooled to room temperature to obtain a thick steel plate. Some thick steel plates were tempered after air cooling to room temperature. Furthermore, except for some of the thick steel plates, the steel plate was made into a tubular shape by cold working so that the width end of the thick steel plate became the butt portion, and submerged arc welding was performed at the butt portion to obtain a welded steel pipe.

  These thick steel plates and welded steel pipes were evaluated for performance by the following methods.

  The micro Vickers hardness of the hard second phase contained in the microstructure of the central segregation part, surface layer and back layer is set in the range of 5 to 50 g according to the cross-sectional dimension of the hard second phase, and the hard second phase measured by the indentation is measured. It measured so that it might not protrude from two phases. In addition, when the microstructure was almost uniform (for example, a bainite single phase structure) and there was no hard second phase that could be measured using 5 g, an arbitrary portion was measured at 50 g. As the value of the micro Vickers hardness of the hard second phase contained in the microstructure of the center segregation part, the surface layer and the back layer, the maximum value obtained by measuring 10 points was used.

The degree of integration of the (211) plane of the rolled surface at the center position of the tube thickness and the plate thickness was measured by an inverse method using an X-ray diffractometer by collecting a 5 mm thick thin film so that the rolled surface becomes the measurement surface. The values obtained were used. Here, the degree of integration of the (211) plane is a numerical value representing the degree of integration of the (211) crystal plane of the target material, and is the value of the plate surface taken in parallel to the steel sheet rolling surface from the tube thickness center position of the target material ( 211) the ratio of the X-ray diffraction intensity of the reflection _{(I (211))} and, (211) reflection of X-ray diffraction intensity _{(I 0 (211)} without random standard sample of texture) _{(I (211)} / I _{0 (211)} ).

  The strength was evaluated using a tensile test piece having a width of 38.1 mm defined by ASTM A516, and a minimum tensile strength of 535 MPa, which is the minimum strength of API 5L X65M, was accepted. The base material toughness is evaluated by DWTT specified by API RP 5L3. When the plate thickness and the tube thickness are 30 mm or less, the full thickness test piece is used at -10 ° C., and when it exceeds 30 mm, the 19 mm thickness test piece is used. Each test was conducted at −27 ° C., and the average value of the ductile fracture surface ratio of the two tested was 85% or more.

In the HIC test, the pH at the start of the test was adjusted by appropriately changing the ratio of NaCl, CH ₃ COOH, and CH ₃ COONa in the buffer solution in an atmosphere of 1 to 100% hydrogen sulfide gas (balance gas: N ₂ ). Adjustment was made between 2.7 and 5.8. About other conditions, it carried out by the method according to NACE TM0284. The evaluation is performed by measuring the cracked area ratio by ultrasonic flaw detection performed from the surface of the test piece (when the plate thickness and the tube thickness exceed 30 mm, the average value of the test pieces taken by dividing in the plate thickness direction is measured), and 5% The following was accepted.

  Table 3 shows the test results obtained.

  All of the examples of the present invention satisfy the target performance, whereas in the comparative example, either the DWTT performance or the HIC performance does not meet the target.

Claims (7)

In mass%, C: 0.02-0.10%, Si: 0.40% or less, Mn: 1.00-2.00%, Nb: 0.005-0.060%, Ti: 0.005 -0.025%, Ca: 0.0010-0.0040%, N: 0.0010-0.0100% is contained, Ca / O is 2.5 or less, and is shown by following formula (1) ACRM is not less than 0, satisfying _{P HIC} has the following formula (3) represented by the following formula (2), the balance being Fe and unavoidable impurities,
The maximum value HV of the micro Vickers hardness of the hard second phase contained in the microstructure of the center segregation part and the surface layer and the back layer satisfies the following formula (4),
The Vickers hardness of the surface layer and the back layer is 248 or less,
A welded steel pipe having an accumulation degree of the (211) plane of the rolled surface at the tube thickness center position obtained by X-ray diffraction of 1.6 or more.
ACRM = ([Ca] − (1.23 [O] −0.000365)) / (1.25 [S]) (1)
P _HIC = 4.46 [C] +2.37 [Mn] / 6 + (1.74 [Cu] +1.7 [Ni]) / 15+ (1.18 [Cr] +1.95 [Mo] +1.74 [ V]) / 5 + 22.36 [P] (2)
P _HIC ≦ 1.35 + (pH−12) (1 + log (P _H2S )) / 60 (3)
HV ≦ 400 + 50 (pH−12) (1 + log (P _H2S )) / 9 (4)
However, in the formulas (1) to (4),
[Ca], [O], [S], [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [P]: Content of each element ( Mass%), and when not included, pH is 0: pH of the HIC test environment
P _{H2S: H 2} S fraction of HIC test environment (Vol%)
And   Further, in terms of mass%, Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.060% or less, B: 0.0. The welded steel pipe according to claim 1, comprising one or more selected from 0030% or less. In mass%, C: 0.02-0.10%, Si: 0.40% or less, Mn: 1.00-2.00%, Nb: 0.005-0.060%, Ti: 0.005 -0.025%, Ca: 0.0010-0.0040%, N: 0.0010-0.0100% is contained, Ca / O is 2.5 or less, and is shown by following formula (1) ACRM is not less than 0, satisfying _{P HIC} has the following formula (3) represented by the following formula (2), the balance being Fe and unavoidable impurities,
The maximum value HV of the micro Vickers hardness of the hard second phase contained in the microstructure of the center segregation part and the surface layer and the back layer satisfies the following formula (4),
The Vickers hardness of the surface layer and the back layer is 248 or less,
A thick steel plate having a degree of integration of the (211) plane of the rolled surface at the plate thickness center position obtained by X-ray diffraction of 1.6 or more.
ACRM = ([Ca] − (1.23 [O] −0.000365)) / (1.25 [S]) (1)
P _HIC = 4.46 [C] +2.37 [Mn] / 6 + (1.74 [Cu] +1.7 [Ni]) / 15+ (1.18 [Cr] +1.95 [Mo] +1.74 [ V]) / 5 + 22.36 [P] (2)
P _HIC ≦ 1.35 + (pH−12) (1 + log (P _H2S )) / 60 (3)
HV ≦ 400 + 50 (pH−12) (1 + log (P _H2S )) / 9 (4)
However, in the formulas (1) to (4),
[Ca], [O], [S], [C], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [P]: Content of each element ( Mass%), and when not included, pH is 0: pH of the HIC test environment
P _{H2S: H 2} S fraction of HIC test environment (Vol%)
And   Further, in terms of mass%, Cu: 0.50% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.060% or less, B: 0.0. The thick steel plate according to claim 3, containing one or more selected from 0030% or less. Heat continuously cast slab steel material, with heating to 1000 to 1200 ° C., initial rolling temperature T _S the following formula (5) to meet as cumulative rolling reduction of 50% or more having a composition as set forth in claim 3 or 4 Then, the hot rolling is finished so that the rolling end temperature _TF satisfies the following formula (6), and then the accelerated cooling is performed so that the accelerated cooling start temperature T _ACS satisfies the following formula (7). A method for producing a thick steel plate, which is started and air-cooled after stopping cooling at a cooling stop temperature of 600 ° C. or less.
T _S ≦ 174 log ([Nb] ([C] +12 [N] / 14)) + 1444-1.2t (5)
T _F ≧ 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6t (6)
T _ACS ≧ 910-310 [C] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo] -0.6 t (7)
However, in the above formulas (5), (6), (7),
[Nb], [C], [N], [Mn], [Cu], [Ni], [Cr], [Mo]: content (% by mass) of each element, 0 if not included T: Thickness at the end of rolling (mm)
And   The manufacturing method of the thick steel plate of Claim 5 which tempers to 480-720 degreeC after the said air cooling. A method for producing a welded steel pipe, wherein the thick steel plate produced by the method according to claim 5 or 6 is cold-worked into a cylindrical shape and the butt portion is welded to form a welded steel pipe.