JP2010209461A - High-strength steel sheet and high-strength steel pipe having excellent hydrogen-induced cracking resistance for use in line pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet and a steel pipe having excellent HIC resistance that are ideal for, for instance, a line pipe used to convey petroleum or natural gas. <P>SOLUTION: The steel sheet and the steel pipe are formed from steel having a steel composition that comprises, by mass, C: 0.02 to 0.08%, Si: 0.01 to 0.5%, Mn: 1.2 to 1.6%, Nb: 0.001 to 0.10%, N: 0.0010 to 0.0050%, Ca: 0.0001 to 0.0050%, P: 0.01% or less, S: 0.0020 or less, Ti: 0.030% or less, Al: 0.030% or less, and O: 0.0035% or less, and the balance Fe with inevitable impurities and that satisfies S/Ca&lt;0.5, and having a maximum Mn degree of segregation of 2.0% or less, a maximum Nb degree of segregation of 4.0 or less, and a maximum Ti degree of segregation of 4.0 or less. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a steel plate for a line pipe and a steel pipe for a line pipe, which are excellent in hydrogen-induced cracking resistance (referred to as HIC resistance), which is optimal for applications such as oil and natural gas transportation line pipes.

There is a concern that hydrogen-induced cracking (referred to as HIC) may occur in transportation pipes for petroleum, natural gas, and the like that contain a large amount of water-containing hydrogen sulfide (H ₂ S). This is because in an H ₂ S environment containing moisture (referred to as a sour environment), hydrogen easily enters the steel from the surface. HIC accumulates around defects in steel, especially stretched MnS, accumulated Ti and Nb carbonitrides, or oxide inclusions in oxide accumulation zones, present in the central segregation of steel. Caused by hydrogen.

That is, in the sour environment, hydrogen that has entered the steel accumulates around the defect to become a gas, and cracking occurs when the pressure exceeds the fracture toughness value (K _IC ) of the steel. Furthermore, if the center segregation part of steel, the periphery of inclusions, and the like are hardened, cracks are likely to propagate. Therefore, conventionally, in line pipes used in sour environments, the production of stretched MnS, the accumulation of carbonitrides of Ti and Nb, the accumulation of oxides, or the formation of hardened phases due to center segregation are suppressed. Measures are taken such as.

  For example, Mn is an element that easily segregates at the center of a steel sheet, and methods for suppressing the segregation of Mn have been proposed (for example, Patent Documents 1 to 3). Patent Document 1 proposes a steel sheet in which the ratio of the Mn content of the segregation part to the average Mn content in the steel is suppressed. Patent Documents 2 and 3 propose a high-strength line pipe that limits the P concentration of the segregation part in addition to the size of the Mn segregation spot and further utilizes Ca.

  In addition to Mn segregation, hot rolled steel sheets with excellent HIC resistance have been proposed that focus on Nb segregation (for example, Patent Document 4). Furthermore, methods for suppressing inclusions such as carbides and nitrides of Ti and Nb have been proposed (for example, Patent Documents 5 and 6).

Japanese Patent Laid-Open No. 6-220577 JP-A-6-256894 JP-A-6-271974 JP 2002-36389 A JP 2006-63351 A JP 2008-7841 A

  Conventionally, development related to suppression of segregation of Mn and morphology control of MnS using Ca has been actively carried out, but (maximum Mn content of segregation part) / (average Mn content in steel), Mn It has been found that HIC cannot be completely prevented only by controlling the size of the segregation spot, and it is necessary to control it more strictly.

  Further, when the segregation of Mn was eliminated, the segregation of Nb became a problem. As for the segregation of Nb, it was found that control of (maximum Nb content of segregation part) / (average Nb content in steel) is insufficient, and it is necessary to control it more strictly. Moreover, even if the length of Nb—Ti—C—N inclusions and the surface density and length of (Ti, Nb) (C, N) inclusions are controlled, generation of HIC can be prevented. could not.

  The present invention has been made in view of such circumstances, and is suitable for steel pipes used for transportation line pipes of petroleum, natural gas, etc., and has excellent HIC resistance and steel pipe for line pipes and line pipes The issue is to provide steel pipes.

  The present inventors conducted intensive research on the conditions that should be satisfied by a steel material for obtaining a steel sheet for a high-strength line pipe and a steel pipe for a high-strength line pipe excellent in hydrogen-induced crack resistance with a tensile strength of 500 MPa or more. It came to invent the steel plate for ultra high strength line pipes, and the steel pipe for high strength line pipes. The gist of the present invention is as follows.

(1) In mass%,
C: 0.02 to 0.08%,
Si: 0.01 to 0.5%,
Mn: 1.2-1.6%
Nb: 0.001 to 0.10%,
N: 0.0010 to 0.0050%,
Ca: 0.0001 to 0.0050%
Including
P: 0.01% or less,
S: 0.0020% or less,
Ti: 0.030% or less,
Al: 0.030% or less,
O: Limited to 0.0035% or less, S, Ca content,
S / Ca <0.5
And the balance consists of Fe and inevitable impurity elements,
Furthermore,
Maximum Mn segregation degree: 2.0 or less,
Nb segregation degree: 4.0 or less,
Ti segregation degree: Steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance, characterized by being limited to 4.0 or less.
(2) In mass%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01-0.60%,
W: 0.01-1.0%
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.050%,
B: 0.0001 to 0.0020%
The steel sheet for high-strength line pipes having excellent resistance to hydrogen-induced cracking as described in (1) above, further comprising one or more of the above.
(3) By mass% REM: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Y: 0.0001 to 0.005%,
Hf: 0.0001 to 0.005%,
Re: 0.0001 to 0.005%
The steel sheet for high-strength line pipes having excellent resistance to hydrogen-induced cracking as described in (1) or (2) above, further comprising one or more of them.
(4) The steel sheet for a high-strength line pipe excellent in hydrogen-induced crack resistance according to any one of the above (1) to (3), wherein the center segregation portion has a maximum hardness of 300 Hv or less.

(5) The base material is mass%,
C: 0.02 to 0.08%,
Si: 0.01 to 0.5%,
Mn: 1.2-1.6%
Nb: 0.001 to 0.10%,
N: 0.0010 to 0.0050%,
Ca: 0.0001 to 0.0050%
Including
P: 0.010% or less,
S: 0.0020% or less,
Ti: 0.030% or less,
Al: 0.030% or less,
O: Limited to 0.0035% or less, S, Ca content,
S / Ca <0.5
And the balance consists of Fe and inevitable impurity elements,
Furthermore, the maximum Mn segregation degree of the base material: 2.0 or less,
Nb segregation degree: 4.0 or less,
Ti segregation degree: Steel pipe for high-strength line pipe excellent in resistance to hydrogen-induced cracking, characterized by being limited to 4.0 or less.
(6) The base material is mass%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01-0.60%,
W: 0.01-1.0%
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.050%,
B: 0.0001 to 0.0020%
The steel pipe for high-strength linepipe excellent in hydrogen-induced crack resistance according to (5) above, further comprising one or more of the above.
(7) The base material is mass%,
REM: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Y: 0.0001 to 0.005%,
Hf: 0.0001 to 0.005%,
Re: 0.0001 to 0.005%
The steel pipe for high-strength line pipes having excellent resistance to hydrogen-induced cracking as described in (5) or (6) above, further comprising one or more of them.
(8) The high-strength line pipe excellent in hydrogen-induced crack resistance according to any one of the above (5) to (7), wherein the maximum hardness of the center segregation part of the base material is 300 Hv or less Steel pipe.

  According to the present invention, the segregation degree of Mn, Nb, and Ti is reduced, the increase in the maximum hardness of the center segregation part is suppressed, and the production of a steel plate for line pipe and a steel pipe for line pipe excellent in hydrogen-induced crack resistance. The industrial contribution is extremely remarkable.

It is a figure which shows the relationship between ratio S / Ca of S and Ca content, and CAR in a HIC test.

  The present inventors performed a NACE (National Association of Corrosion and Engineer) test using various steel plates for line pipes, and evaluated the presence or absence of HIC. The NACE test is a test method in which hydrogen sulfide gas is saturated in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7 to investigate whether cracks are generated after 96 hours.

  After the test, a test piece was collected from the cracked steel plate, and the location where HIC was generated was observed in detail. As a result, the following three HIC occurrence locations were observed. That is, 1) stretched MnS, 2) accumulated Ti, Nb carbonitride, and 3) accumulated oxide. Furthermore, as a result of repeated studies, it has been found that if all three of these are suppressed, the occurrence of HIC in the steel pipe for line pipe and the steel pipe for line pipe can be remarkably prevented.

  First, in order to suppress stretched coarse MnS, it is necessary to satisfy the following conditions. By making the S amount less than 0.002%, making the ratio S / Ca of S and Ca less than 0.5, and further making the maximum Mn segregation degree of the steel plate and the steel pipe less than 2.0. is there. FIG. 1 shows the relationship between CAR (crack area ratio) and S / Ca in the HIC test of 0.04% C-1.25% Mn steel. As shown in FIG. 1, when the S / Ca ratio is 0.5 or more, HIC starts to be generated, so S / Ca needs to be less than 0.5.

  Next, to suppress the accumulation of Ti and Nb carbonitrides, particularly Nb (C, N) and TiC, the following conditions must be satisfied. The N content is 0.0050% or less, the C content is 0.06% or less, and the segregation degrees of Nb and Ti are 4.0 or less, respectively.

Here, the maximum Mn segregation degree is a ratio of the maximum Mn amount of the central segregation portion to the average Mn amount excluding the central segregation portion in the steel plate and the steel pipe.
Similarly, the Nb segregation degree and the Ti segregation degree are ratios of the average Nb amount (Ti amount) of the central segregation portion to the average Nb amount (Ti amount) excluding the central segregation portion in the steel plate and the steel pipe.

  The degree of Mn segregation can be determined by measuring the Mn concentration distribution of steel plates and steel pipes using EPMA (Electron Probe Micro Analyzer) or CMA (Computer Aided Micro Analyzer) capable of image processing the measurement results by EPMA. it can.

  At that time, the numerical value of the maximum Mn segregation degree varies depending on the probe diameter of EPMA (or CMA). The present inventors have found that the segregation of Mn can be properly evaluated by setting the probe diameter (beam diameter) to 2 μm. Specifically, the measurement can be performed as follows.

  The concentration distribution of Mn in a measurement region of 20 mm width (HIC specimen width) × 20 mm thickness (HIC specimen thickness) is measured with an EPMA at a beam diameter of 50 μm. Next, the Mn concentration in a region of 1 mm (width) × 1 mm (thickness) is further measured with a beam diameter of 2 μm at the place where the Mn amount is most concentrated (center segregation portion). Then, the maximum Mn segregation degree is obtained from this Mn concentration distribution. At that time, data of 500 points × 500 points are accumulated. The ratio of the maximum Mn concentration in the 250,000 points to the average Mn concentration excluding the central segregation portion was defined as the maximum Mn segregation degree, and the value was obtained.

  Similarly, the Nb segregation degree and the Ti segregation degree can be obtained by measuring the Nb concentration distribution and the Ti concentration distribution by EPMA or CMA. At that time, it was also found that the segregation degree can be properly evaluated by setting the beam diameter to 2 μm in the same manner for the Nb segregation degree and the Ti segregation degree.

  Actually, regarding the segregation degree of Nb and Ti, the concentration distribution of Nb and Ti in a measurement area of 20 mm width (HIC test piece width) × 20 mm thickness (HIC test piece thickness) is measured by EPMA with a beam diameter of 50 μm. Then, after obtaining the average Nb (Ti) concentration, a region of 1 mm (width) × 1 mm (thickness) at a beam diameter of 2 μm at the place where the Nb and Ti amounts were most concentrated (central segregation portion). The Nb and Ti concentrations of are measured. At that time, the average of 500 points measured in the plate width direction is taken, and the average Nb and Ti concentrations of the center segregation part are derived. Then, the ratio of the average Nb and Ti concentration to the average Nb and Ti concentration in the central segregation part is defined as Nb and Ti segregation degree, and the value is obtained.

  If inclusions such as MnS, TiN, and Nb (C, N) are present, the Mn segregation degree, Ti segregation degree, and Nb segregation degree increase apparently. It shall be.

  Finally, in order to suppress the accumulation of oxides, it is necessary to make the O content 0.0035% or less and the Al content 0.030% or less. It has been clarified that when the amount of O is large, coarse oxides are easily accumulated, and when Al is added in an amount of more than 0.030%, Al oxide clusters are easily accumulated.

  Furthermore, it is preferable that the maximum hardness of the central segregation part of the steel plate and the steel pipe in which segregation of Mn, Nb, and Ti is suppressed is 300 Hv or less. By setting the upper limit of the center segregation portion maximum hardness to 300 Hv or less, generation of HIC can be reliably prevented. Mn and Nb are elements that enhance the hardenability, and Ti contributes to precipitation strengthening. Therefore, by suppressing the segregation of these elements, the hardening of the central segregation part can be suppressed.

  The central segregation part is a part where the concentration of Mn measured by EPMA or CMA becomes maximum, and the maximum hardness of the central segregation part conforms to JIS Z 2244 after corroding with 3% nitric acid + 97% nital solution. Then, a Vickers hardness test may be performed with a load of 25 g.

The present invention made based on the above examination results will be described in detail below.
First, the reasons for limiting the base material components in the steel plate and steel pipe of the present invention will be described.

  C: C is an element that improves the strength of steel, and as its effective lower limit, addition of 0.02% or more is necessary. On the other hand, if the amount of C exceeds 0.08%, the formation of carbides is promoted and the HIC resistance is impaired, so the upper limit is made 0.08% or less. Moreover, in order to suppress deterioration of HIC property, weldability, and toughness, the upper limit of the C content is preferably set to 0.06% or less.

  Si: Si is a deoxidizing element and needs to be added in an amount of 0.01% or more. On the other hand, if the Si content exceeds 0.5%, the toughness of the weld heat affected zone (HAZ) is lowered, so the upper limit is made 0.5% or less.

  Mn: Mn is an element that improves strength and toughness, and needs to be added in an amount of 1.2% or more. On the other hand, if the amount of Mn exceeds 1.6%, the HAZ toughness is lowered, so the upper limit is made 1.8% or less. Moreover, in order to suppress HIC, it is preferable that the upper limit of the amount of Mn be less than 1.5%.

  Nb: Nb is an element that forms carbides and nitrides and contributes to improvement in strength. In order to obtain the effect, it is necessary to add 0.0001% or more of Nb. However, if Nb is added excessively, the degree of segregation of Nb increases, and the accumulation of Nb carbonitrides is invited, resulting in a decrease in HIC resistance. Therefore, in the present invention, the upper limit of the Nb amount is 0.10% or less. In consideration of the HIC property, the upper limit of the Nb amount is preferably 0.05% or less.

  N: N is an element that forms nitrides such as TiN and NbN, and in order to make the austenite grain size at the time of heating using nitrides, the lower limit value of N amount is 0.0010% or more. Is necessary. However, if the N content exceeds 0.0050%, Ti and Nb carbonitrides are likely to accumulate, and the HIC resistance is impaired. Therefore, the upper limit of the N amount is set to 0.0050% or less. In addition, when toughness etc. are requested | required, in order to suppress the coarsening of TiN, it is preferable to make the upper limit of N amount 0.0035% or less.

  P: P is an impurity. If the content exceeds 0.01%, the HIC resistance is impaired, and the toughness of the HAZ is lowered. Therefore, the upper limit of the P content is limited to 0.01% or less.

  S: S is an element that reduces the HIC resistance by generating MnS that extends in the rolling direction during hot rolling. Therefore, in the present invention, it is necessary to reduce the amount of S, and the upper limit is limited to 0.0020% or less. In order to improve toughness, the S content is preferably 0.0010% or less. The smaller the amount of S, the better. However, it is difficult to make it less than 0.0001%, and the lower limit is preferably made 0.0001% or more from the viewpoint of manufacturing cost.

  Ti: Ti is an element that is usually used for grain refinement as a deoxidizer or nitride-forming element. In the present invention, an element that lowers HIC resistance and toughness by forming carbonitrides. It is. Therefore, the upper limit of the Ti content is limited to 0.030% or less.

  Al: Al is a deoxidizing element. However, in the present invention, when the addition amount exceeds 0.030%, an accumulation cluster of Al oxide is confirmed, so it is limited to 0.030% or less. When toughness is required, the upper limit of Al content is preferably set to 0.017% or less. Although the lower limit of the amount of Al is not particularly limited, it is preferable to add Al in an amount of 0.0005% or more in order to reduce the amount of oxygen in the molten steel.

  O: O is an impurity, and the upper limit is limited to 0.0035% or less in order to suppress oxide accumulation and improve HIC resistance. In order to suppress the formation of oxides and improve the base material and the HAZ toughness, the upper limit value of the O content is preferably set to 0.0030% or less. The optimum upper limit of the amount of O is 0.0020% or less.

  Ca: Ca is an element that generates sulfide CaS, suppresses the generation of MnS extending in the rolling direction, and contributes significantly to the improvement of HIC resistance. If the addition amount of Ca is less than 0.0001%, the effect cannot be obtained, so the lower limit is made 0.0001% or more. 0.0005% or more is preferable. On the other hand, if the amount of Ca exceeds 0.0050%, oxides accumulate and the HIC resistance is impaired, so the upper limit is made 0.0050% or less.

  In the present invention, since S is fixed by adding Ca to form CaS, the ratio of S / Ca in the content of S and Ca is an important index. MnS produces | generates that ratio of S / Ca is 0.5 or more, and MnS extended at the time of rolling is formed. As a result, the HIC resistance deteriorates. Accordingly, the S / Ca ratio is set to less than 0.5.

  In the present invention, one or more elements among Ni, Cu, Cr, Mo, W, V, Zr, Ta, and B may be added as elements for improving strength and toughness. it can.

  Ni: Ni is an element effective for improving toughness and strength, and in order to obtain the effect, addition of 0.01% or more is necessary. However, addition of 2.0% or more requires HIC and weldability. Therefore, the upper limit is preferably made 2.0%.

  Cu: Cu is an element effective for increasing the strength without reducing toughness, but if it is less than 0.01%, there is no effect, and if it exceeds 1.0%, cracking is likely to occur during heating of the steel slab or during welding. To do. Therefore, the content is preferably 0.01 to 1.0% or less.

  Cr: Cr is effective to improve the strength of steel by precipitation strengthening, but addition of 0.01% or more is effective, but if added in a large amount, the hardenability is increased, the bainite structure is generated, and the toughness is decreased. . Therefore, the upper limit is preferably 1.0%.

  Mo: Mo is an element that improves hardenability and at the same time forms carbonitrides and improves strength. To obtain the effect, addition of 0.01% or more is preferable. On the other hand, when Mo is added in a large amount exceeding 0.60%, the cost increases, so the upper limit is preferably made 0.60% or less. Moreover, since the HIC property and toughness may decrease when the strength of steel increases, the more preferable upper limit is made 0.40% or less.

  W: W is an element effective for improving the strength, and is preferably added in an amount of 0.01% or more. On the other hand, if W exceeding 1.0% is added, the toughness may be lowered, so the upper limit is preferably made 1.0% or less.

  V: V is an element that forms carbides and nitrides and contributes to improvement in strength. In order to obtain the effect, addition of 0.01% or more is preferable. On the other hand, if V exceeding 0.10% is added, the toughness may be lowered, so the upper limit is preferably made 0.10% or less.

  Zr, Ta: Zr and Ta are elements that form carbides and nitrides as well as V and contribute to the improvement of strength, and in order to obtain the effect, 0.0001% or more is preferably added. On the other hand, if Zr and Ta are added excessively in excess of 0.050%, the toughness may be lowered, so the upper limit is preferably made 0.050% or less.

  B: B is an element that segregates at the grain boundaries of steel and contributes significantly to improving the hardenability. In order to obtain this effect, 0.0001% or more of B is preferably added. Further, B is an element that generates BN, lowers the solid solution N, and contributes to the improvement of the toughness of the weld heat affected zone. Therefore, addition of 0.0005% or more is more preferable. on the other hand. When B is added excessively, segregation to the grain boundary becomes excessive and the toughness may be lowered, so the upper limit is preferably made 0.0020%.

  Furthermore, in order to control inclusions such as oxides and sulfides, one or more of REM, Mg, Zr, Ta, Y, Hf, and Re may be included.

  REM: REM is an element added as a deoxidizer and a desulfurizer, and 0.0001% or more is preferably added. On the other hand, if added over 0.010%, a coarse oxide is formed, which may reduce the HIC property and the toughness of the base material and HAZ. The preferable upper limit is 0.010% or less.

  Mg: Mg is an element added as a deoxidizing agent and a desulfurizing agent. In particular, a fine oxide is generated and contributes to improvement of HAZ toughness. In order to obtain this effect, 0.0001% or more of Mg is preferably added. On the other hand, if Mg is added in an amount exceeding 0.010%, the oxide tends to aggregate and coarsen, which may lead to deterioration in HIC properties and toughness of the base material and HAZ. Therefore, it is preferable that the upper limit of the amount of Mg is 0.010% or less.

  Y, Hf, Re: Y, Hf, and Re are elements that, like Ca, generate sulfides, suppress the generation of MnS elongated in the rolling direction, and contribute to improvement in HIC resistance. In order to obtain such an effect, it is preferable to add 0.0001% or more of Y, Hf, and Re. On the other hand, if the amount of Y, Hf, Re exceeds 0.0050%, the oxides increase, and if aggregation or coarsening impairs the HIC resistance, the upper limit is preferably made 0.0050% or less.

  Furthermore, in the present invention, the maximum Mn segregation degree, the Nb segregation degree, and the Ti segregation degree in the base material of the steel plate and the steel pipe are 2.0 or less, 4.0 or less, and 4.0 or less, respectively.

  By setting the maximum Mn segregation degree to 2.0 or less, generation of coarse MnS is suppressed, and generation of HIC starting from MnS stretched in the rolling direction can be prevented. Further, when the Nb segregation degree is 4.0 or less, the formation of accumulated Nb (C, N) is suppressed, and when the Ti segregation degree is 4.0 or less, the formation of accumulated TiN is suppressed, and the HIC property is deteriorated. Can be prevented.

  The maximum Mn segregation degree is the ratio of the maximum Mn amount of the central segregation portion to the average Mn amount excluding the central segregation portion of the steel plate and steel pipe, and the Mn concentration of the steel plate and steel pipe by EPMA or CMA with a beam diameter of 2 μm. Distribution can be measured and determined. The same applies to the degree of Nb segregation and the degree of Ti segregation. The Nb concentration distribution and the Ti concentration distribution were measured by EPMA or CMA with a beam diameter of 2 μm, respectively, and the average Nb excluding the central segregation portion of the steel plate and the steel pipe was measured. The ratio of the average amount of Nb of the center segregation part to the amount (Nb segregation degree), the ratio of the average Ti amount of the center segregation part to the average Ti amount excluding the center segregation part of the steel plate and steel pipe (Ti segregation degree) Is to be sought.

  A method for suppressing the maximum Mn segregation degree, Nb segregation degree, and Ti segregation degree will be described below.

  In order to suppress segregation of Mn, Nb and Ti, light reduction at the time of final solidification in continuous casting is optimal. The light reduction at the time of final solidification is applied to eliminate the mixing of solidified and unsolidified parts due to non-uniform cooling of the casting. it can.

  In continuous casting, the steel slab is usually water-cooled, but the end in the width direction is cooled quickly, and the cooling in the center in the width direction is strengthened. Therefore, even if the steel piece is solidified at the end portion and the center portion in the width direction, solidification is delayed at a quarter portion in the width direction, and an unsolidified portion remains inside the steel piece. Therefore, in the width direction of the steel slab, the solidified part and the unsolidified part are not uniform, and for example, the shape of the interface between the solidified part and the unsolidified part may be W-shaped in the width direction. If such non-uniform solidification occurs in the width direction, segregation is promoted and the HIC resistance is deteriorated.

  On the other hand, in continuous casting, when light reduction is performed at the time of final solidification, an unsolidified portion is pushed out and can be solidified uniformly in the width direction. Further, if light pressure is applied after uneven solidification occurs in the width direction, the unsolidified portion cannot be effectively pushed out due to the large deformation resistance of the solidified portion.

  Therefore, in order to prevent such W-type solidification from occurring, it is preferable to lightly reduce the amount of reduction while controlling the amount of reduction according to the distribution in the width direction of the central solid fraction at the final solidification position of the slab. . Thereby, center segregation is suppressed also in the width direction, and the maximum Mn segregation degree, Nb segregation degree, and Ti segregation degree can be further reduced.

  Steel containing the above components is made into a steel slab by continuous casting after melting in the steel making process, and the steel slab is reheated and subjected to thick plate rolling to obtain a steel plate. In this case, if the steel plate is rolled at a reheating temperature of 1000 ° C. or higher, a reduction ratio in the recrystallization temperature range of 2 or higher, and a reduction ratio in the non-recrystallization range of 3 or higher, The austenite particle size can be 20 μm or less. Furthermore, although water cooling is performed after completion | finish of rolling, it is preferable to perform water cooling start temperature from the temperature below 750 degreeC, and to make water cooling stop temperature into 400-500 degreeC.

  The recrystallization temperature range is a temperature range where recrystallization occurs after rolling, and is generally over 900 ° C. for the steel components of the present invention. On the other hand, the non-recrystallization temperature range is a temperature range in which recrystallization and ferrite transformation do not occur after rolling, and is generally 750 to 900 ° C. for the steel components of the present invention. Rolling in the recrystallization temperature range is called recrystallization rolling or rough rolling, and rolling in the non-recrystallization temperature range is called non-recrystallization rolling or finish rolling.

  After non-recrystallization rolling, water cooling is started from a temperature of 750 ° C. or higher, and the water cooling stop temperature is set to 400 ° C. or higher, so that the maximum hardness of center segregation can be 300 Hv or less as described below. First, when the water cooling start temperature is less than 750 ° C., a large amount of ferrite is generated before the start of cooling, and C (carbon) is discharged from the ferrite to austenite. Thereafter, when cooled, the austenite phase enriched with C is transformed into hard martensite containing a large amount of C.

  Therefore, if the water cooling start temperature is set to 750 ° C. or higher to suppress the formation of hard martensite, the hardness can be suppressed to 300 Hv or less. Further, when the water cooling stop temperature is set to 400 ° C. or higher, similarly, the hard martensite after transformation is partially decomposed, and the hardness can be suppressed to 300 Hv or lower. Moreover, since intensity | strength will fall when water-cooling stop temperature is too high, 500 degrees C or less is preferable.

  Next, the present invention will be described in further detail with reference to examples.

  Steel having chemical components shown in Table 1 was melted, and a steel piece having a thickness of 240 mm was obtained by continuous casting. In continuous casting, light reduction during final solidification was performed. The obtained steel slab was heated to 1100 to 1250 ° C., hot-rolled in a recrystallization temperature range exceeding 900 ° C., and subsequently hot-rolled in a non-recrystallization temperature range of 750 to 900 ° C. After hot rolling, water cooling was started at 750 ° C. or higher, and water cooling was stopped at a temperature of 400 to 500 ° C., and steel plates having various plate thicknesses shown in Table 2 were produced.

  Further, the steel plate was formed into a tubular shape by a C press, U press, and O press, end surfaces were tack welded, main welding was performed from the inner and outer surfaces, and then expanded to obtain a steel pipe. In addition, this welding employ | adopted submerged arc welding and performed it with the heat input shown in Table 3.

Tensile test pieces, HIC test pieces, and macro test pieces were collected from the obtained steel plates and steel pipes and used for each test.
The HIC test was performed according to NACETM0284. Moreover, the segregation degree of Mn, Nb, and Ti was measured by EPMA using a macro test piece. The segregation degree by EPMA is measured with a 50 μm beam system in a total thickness × 20 mm width measurement area to measure the concentration distribution of Mn, Nb and Ti, and then each element in the specimen thickness direction is concentrated. The concentration of each element was measured in a 1 mm × 1 mm region with a 2 μm beam system at the location (center segregation part).
Furthermore, the Vickers hardness of center segregation was measured according to JIS Z 2244. Vickers hardness was measured at a site where the load was 25 g and the Mn concentration was highest in the distribution of Mn concentration in the thickness direction measured by EPMA.

  Table 2 shows the thicknesses of steel sheets obtained from the steels 1 to 33 shown in Table 1, the maximum Mn segregation degree, the Nb segregation degree, the Ti segregation degree, the maximum hardness of the central segregation part, the tensile strength, and the HIC test. The area ratio (CAR) of the cracks produced is shown. Table 3 shows the thicknesses of the steel pipes obtained from the steels 1 to 33 shown in Table 1, the heat input amount of main welding, and the crack area ratio determined by the HIC test. The maximum Mn segregation degree, the Nb segregation degree, the Ti segregation degree, and the maximum hardness of the central segregation portion of the steel pipe are the same as those of the steel sheet, and the tensile strength of the steel pipe is about several percent greater than that of the steel sheet.

  Steels 1 to 23 are examples of the present invention, and the steel sheets obtained from these steels have a maximum Mn segregation degree of 1.6 or less, an Nb segregation degree of 4.0 or less, and a Ti segregation degree of 4.0 or less. The maximum hardness of the center segregation part is 300 Hv or less, and no cracks are generated by the HIC test. The same applies to steel pipes made of these steel plates.

  On the other hand, steel 24-33 shows the comparative example which is outside the scope of the present invention. That is, since any element of the basic components is outside the scope of the present invention, the CAR is over 3% in the HIC test.

Claims (8)

% By mass
C: 0.02 to 0.08%,
Si: 0.01 to 0.5%,
Mn: 1.2-1.6%
Nb: 0.001 to 0.10%,
N: 0.0010 to 0.0050%,
Ca: 0.0001 to 0.0050%
Including
P: 0.01% or less,
S: 0.0020% or less,
Ti: 0.030% or less,
Al: 0.030% or less,
O: Limited to 0.0035% or less, S, Ca content,
S / Ca <0.5
And the balance consists of Fe and inevitable impurity elements,
Furthermore,
Maximum Mn segregation degree: 2.0 or less,
Nb segregation degree: 4.0 or less,
Ti segregation degree: Steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance, characterized by being limited to 4.0 or less. % By mass
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01-0.60%,
W: 0.01-1.0%
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.050%,
B: 0.0001 to 0.0020%
The steel sheet for high-strength line pipe excellent in resistance to hydrogen-induced cracking according to claim 1, further comprising one or more of the following. REM: 0.0001 to 0.01% by mass%,
Mg: 0.0001 to 0.01%
Y: 0.0001 to 0.005%,
Hf: 0.0001 to 0.005%,
Re: 0.0001 to 0.005%
The steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance according to claim 1 or 2, further comprising one or more of them.   The steel sheet for a high-strength line pipe excellent in resistance to hydrogen-induced cracking according to any one of claims 1 to 3, wherein the central segregation part has a maximum hardness of 300 Hv or less. The base material is mass%.
C: 0.02 to 0.08%,
Si: 0.01 to 0.5%,
Mn: 1.2-1.6%
Nb: 0.001 to 0.10%,
N: 0.0010 to 0.0050%,
Ca: 0.0001 to 0.0050%
Including
P: 0.010% or less,
S: 0.002% or less,
Ti: 0.030% or less,
Al: 0.030% or less,
O: Limited to 0.0035% or less, S, Ca content,
S / Ca <0.5
And the balance consists of Fe and inevitable impurity elements,
Furthermore, the maximum Mn segregation degree of the base material: 2.0 or less,
Nb segregation degree: 4.0 or less,
Ti segregation degree: Steel pipe for high-strength line pipe excellent in resistance to hydrogen-induced cracking, characterized by being limited to 4.0 or less. The base material is mass%.
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01-0.60%,
W: 0.01-1.0%
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.050%,
B: 0.0001 to 0.0020%
The steel pipe for high-strength line pipe excellent in resistance to hydrogen-induced cracking according to claim 5, further comprising one or more of the following. The base material is mass%.
REM: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Y: 0.0001 to 0.005%,
Hf: 0.0001 to 0.005%,
Re: 0.0001 to 0.005%
The steel pipe for high-strength line pipe excellent in hydrogen-induced crack resistance according to claim 5 or 6, further comprising one or more of them.   The steel pipe for a high-strength line pipe excellent in resistance to hydrogen-induced cracking according to any one of claims 5 to 7, wherein the maximum hardness of the central segregation part of the base material is 300 Hv or less.

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