KR20220098786A - steel plate and steel pipe - Google Patents

Abstract

This steel sheet has a predetermined chemical composition, and the metal structure in the center of the sheet thickness is 0 to 80% polygonal ferrite, and one or two types selected from acicular ferrite and bainite, in area%, , the balance is M-A phase, the effective crystal grain size is 15.0 µm or less, and the metal structure in the surface layer in the range from the surface to 1.0 mm in the thickness direction is 95% or more in area% in total from needle-shaped ferrite and bainite 1 type or 2 types selected are included, remainder is M-A phase, and the highest hardness in the said surface layer is 250HV0.1 or less.

Description

steel plate and steel pipe

The present invention relates to a steel plate and a steel pipe.

In recent years, mining conditions for oil wells and gas wells such as crude oil and natural gas (hereinafter, the oil wells and gas wells are collectively referred to as "oil wells") have become harsh. The mining environment of an oil well is accompanied by an increase in the depth of mining, and the atmosphere contains CO ₂ , H ₂ S, Cl ^- , etc., and crude oil and natural gas mined also contain a lot of H ₂ S.

Therefore, the requirements for the performance of the line pipe transporting them are also becoming stricter, and high sulfide stress cracking resistance (hereinafter also referred to as "SSC resistance") and hydrogen induced cracking resistance (hereinafter also referred to as "HIC resistance") ), and the demand for a steel plate for a line pipe used as a material of the steel pipe is increasing.

The steel used in the environment containing _H2S needs to suppress the highest hardness of steel low from a viewpoint of SSC-resistance improvement. Therefore, in steels that require sulfide resistance (such as SSC resistance), improvement of a technique for suppressing hardness is an important subject.

For example, Patent Document 1 discloses a method for producing high tensile strength steel having a tensile strength of 60 kgf/mm 2 class excellent in SSC resistance. In addition, Patent Document 2 discloses a thick steel sheet having a tensile strength of 570 to 720 N/mm 2 and a small difference in hardness between a welded heat affected zone and a base material, and a method for manufacturing the same. In addition, Patent Document 3 discloses a method for manufacturing a high strength steel sheet for a sour line pipe having an X60 class or higher strength capable of reducing surface hardness while preventing a decrease in strength and deterioration of DWTT characteristics.

According to patent documents 1 - 3, it becomes possible to reduce the hardness of the steel plate surface by tempering after quenching. However, in these documents, evaluation of hardness WHEREIN: Vickers hardness test which set the test force to 98 N (10 kgf) is performed. The higher the test force, the larger the measuring area. That is, the average hardness of the metal structure included in a wide area is measured. Moreover, when the test force is high, the size of the indentation itself will also be several 100 micrometers. Therefore, the hardness in the range of several 100 micrometers from the outermost layer of a steel plate, for example, a surface layer cannot be measured.

However, as a result of the investigations by the present inventors, it was found that even if the average hardness of the surface layer was suppressed to some extent, when a tissue with high hardness locally existed, there was a possibility that SSC could occur from there. That is, since SSC is a crack generated from the surface layer, it was found that if a structure having a high hardness was present in the outermost layer, there was a possibility that SSC was generated from there.

Therefore, in order to further improve SSC resistance, it is necessary to also control low the local maximum hardness obtained by performing a Vickers hardness test with a lower test force. However, as mentioned above, although the Vickers hardness test which set the test force to 98 N (10 kgf) in patent documents 1 - 3 was performed, control of local hardness was not performed.

In addition, not only SSC resistance and HIC resistance but also low-temperature toughness is required for steel sheets and steel pipes for line pipes used in cold regions.

Patent Document 4 discloses a steel sheet suitable for line pipes in which the maximum hardness in the surface layer portion is set to 270 Hv or less to improve SSC resistance, and a steel pipe using the steel sheet as a base material. Further, Patent Document 5 discloses a steel sheet suitable for line pipes and a steel pipe using the steel sheet as a base material, in which the SSC resistance is improved by setting the maximum hardness in the surface layer portion to 250 Hv or less.

However, in the technique described in these documents, in cooling the steel sheet, the surface layer hardness is reduced by using cooling including recuperative heat to slow down the cooling rate of the surface layer on average. Therefore, with these techniques, it is not possible to sufficiently control the structure of the center portion, and there are cases where it is not possible to respond to a higher demand for low-temperature toughness (DWTT).

Therefore, a steel plate and a steel pipe having a low surface layer hardness and excellent low-temperature toughness (DWTT) have been desired.

Japanese Patent Laid-Open No. 2-8322 Japanese Patent Laid-Open No. 2001-73071 Japanese Patent Laid-Open No. 2002-327212 International Publication No. 2019/058420 International Publication No. 2019/058422

The Japan Iron and Steel Association Basic Research Group, Bainite Research and Research Section, 「Legendary Bainite Photo Collection 1」, published by the Japan Steel Association, June 1992

An object of the present invention is to solve the above problems and provide a steel sheet and a steel pipe having excellent SSC resistance and HIC resistance, and excellent low-temperature toughness.

The present invention has been made in order to solve the above problems, and has the following steel plate and steel pipe as its gist.

(1) The steel sheet according to an aspect of the present invention has a chemical composition, in mass%, C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.60%, Nb: 0.001 to 0.100%, N : 0.0010 to 0.0100%, Ca: 0.0001 to 0.0050%, P: 0.030% or less, S: 0.0025% or less, Ti: 0.005 to 0.030%, Al: 0.010 to 0.040%, O: 0.0040% or less, Mo: 0 to 2.00 %, Cr: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, W: 0 to 1.00%, V: 0 to 0.200%, Zr: 0 to 0.0500%, Ta: 0 to 0.0500% , B: 0 to 0.0020%, REM: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0050%, Re: 0 to 0.0050%, balance: Fe and impurities, satisfying the following formula (i) Ceq represented by the following formula (ii) is 0.30 to 0.50, and the metal structure in the center of the plate thickness is 0 to 80% of polygonal ferrite, 1 selected from acicular ferrite and bainite in area% The metal structure in the surface layer including the species or two types, the balance being M-A phase, the effective crystal grain size of 15.0 µm or less, and the range from the surface to 1.0 mm in the thickness direction is 95% in area% in total 1 type or 2 types selected from the above needle-shaped ferrite and bainite are included, remainder is M-A phase, and the highest hardness in the said surface layer is 250HV0.1 or less.

0.05≤Mo+Cr+Cu+Ni≤2.00 … (i)

Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 … (ii)

However, each element symbol in a formula shows content (mass %) of each element contained in steel, and when not contained, it is set as zero.

(2) In the steel sheet according to (1) above, the area % of polygonal ferrite in the metal structure of the central portion of the sheet thickness may be 0 to less than 20%.

(3) In the steel sheet according to (1) above, the area % of polygonal ferrite in the metal structure in the central portion of the sheet thickness is 20 to 80%, and the effective crystal grain size may be 10.0 µm or less.

(4) The steel sheet according to any one of (1) to (3), wherein the chemical composition is, in mass%, W: 0.01 to 1.00%, V: 0.010 to 0.200%, Zr: 0.0001 to 0.050%, Ta: 0.0001 to 0.0500%, and B: You may contain 1 or more types chosen from 0.0001 to 0.0020%.

(5) The steel sheet according to any one of (1) to (4), wherein the chemical composition is, in mass%, REM: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0050%, Re: You may contain 1 or more types chosen from 0.0001 to 0.0050%.

(6) A steel pipe according to another aspect of the present invention has a base material part made of a cylindrical steel plate, and a welding part provided in a butt part of the steel plate and extending in the longitudinal direction of the steel plate, wherein the steel plate has a chemical composition , in mass%, C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.60%, Nb: 0.001 to 0.100%, N: 0.0010 to 0.0100%, Ca: 0.0001 to 0.0050%, P: 0.030 % or less, S: 0.0025% or less, Ti: 0.005 to 0.030%, Al: 0.010 to 0.040%, O: 0.0040% or less, Mo: 0 to 2.00%, Cr: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, W: 0 to 1.00%, V: 0 to 0.200%, Zr: 0 to 0.0500%, Ta: 0 to 0.0500%, B: 0 to 0.0020%, REM: 0 to 0.0100%, Mg : 0 to 0.0100%, Hf: 0 to 0.0050%, Re: 0 to 0.0050%, balance: Fe and impurities, satisfy the following formula (i), and Ceq represented by the following formula (ii) is 0.30 to 0.50 , the metal structure in the thickness center contains 0 to 80% polygonal ferrite in area %, and one or two types selected from needle-shaped ferrite and bainite, the balance being M-A phase, effective crystal grain size 15.0 µm or less, and the metal structure in the surface layer that is in the range from the surface to 1.0 mm in the thickness direction contains one or two types selected from acicular ferrite and bainite in a total of 95% or more in area%, , remainder is M-A phase, and the highest hardness in the said surface layer is 250HV0.1 or less.

0.05≤Mo+Cr+Cu+Ni≤2.00 … (i)

Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 … (ii)

However, each element symbol in a formula shows content (mass %) of each element contained in steel, and when not contained, it is set as zero.

(7) In the steel pipe according to (6), the area % of polygonal ferrite in the metal structure in the central portion of the thickness may be 0 to less than 20%.

(8) In the steel pipe described in (6) above, the area % of polygonal ferrite in the metal structure in the central portion of the thickness is 20 to 80%, and the effective crystal grain size may be 10.0 µm or less.

In this invention, "HV0.1" means the "hardness symbol" at the time of implementing a Vickers hardness test by making a test force 0.98N (0.1kgf) (refer JIS Z 2244:2009).

According to the above aspect of the present invention, it becomes possible to obtain a steel sheet and a steel pipe having excellent SSC resistance and HIC resistance, and excellent low-temperature toughness. Such a steel pipe is suitable for a line pipe use, and a steel plate is suitable as a raw material of the steel pipe for the line pipe.

Hereinafter, each requirement of the steel plate (steel plate according to this embodiment) according to one embodiment of the present invention and the steel pipe (steel pipe according to the present embodiment) according to one embodiment of the present invention will be described in detail.

<Steel plate>

First, a steel sheet according to the present embodiment will be described.

1. Chemical composition

The reasons for limiting each element are as follows. In the following description, "%" regarding content means "mass %." In addition, in the numerical limitation range sandwiching "to", the value is included in the range as a lower limit and an upper limit. On the other hand, the numerical value indicated by "exceeding" or "less than" is not included in the numerical range.

C: 0.020 to 0.080%

C is an element which improves the strength of steel. When the C content is less than 0.020%, the effect of improving the strength cannot be sufficiently obtained. Therefore, the C content is made 0.020% or more. It is preferable that C content is 0.030 % or more.

On the other hand, when C content exceeds 0.080 %, the hardness of surface layer will rise and it will become easy to generate|occur|produce SSC. Therefore, the C content is made 0.080% or less. In order to ensure SSC resistance and suppress the fall of weldability and toughness, it is preferable that it is 0.060 % or less, and, as for C content, it is more preferable that it is 0.055 % or less.

Si: 0.01 to 0.50%

Si is an element added for deoxidation. When the Si content is less than 0.01%, the deoxidation effect is not sufficiently obtained, and the manufacturing cost greatly increases. Therefore, the Si content is made 0.01% or more. It is preferable that it is 0.05 % or more, and, as for Si content, it is more preferable that it is 0.10 % or more.

On the other hand, when Si content exceeds 0.50 %, the toughness of a weld part will fall. Therefore, the Si content is made 0.50% or less. It is preferable that it is 0.40 % or less, and, as for Si content, it is more preferable that it is 0.30 % or less.

Mn: 0.50 to 1.60%

Mn is an element that improves strength and toughness. When Mn content is less than 0.50 %, the effect by containing is not fully acquired. Therefore, the Mn content is made 0.50% or more. It is preferable that it is 1.00 % or more, and, as for Mn content, it is more preferable that it is 1.20 % or more.

On the other hand, when the Mn content exceeds 1.60%, the hydrogen-induced cracking resistance (HIC resistance) decreases. Therefore, the Mn content is set to 1.60% or less. The Mn content is preferably 1.50% or less.

Nb: 0.001 to 0.100%

Nb is an element that forms carbides and nitrides and contributes to the improvement of the strength of steel. Further, since Nb has an action of expanding the non-recrystallization temperature range to the high temperature range, it is an element contributing to the improvement of toughness due to grain refinement. When Nb content is less than 0.001 %, the said effect is not fully acquired. Therefore, the Nb content is made 0.001% or more. It is preferable that it is 0.005 % or more, and, as for Nb content, it is more preferable that it is 0.010 % or more.

On the other hand, when the Nb content exceeds 0.100%, coarse carbides and nitrides are formed, and the HIC resistance and toughness are lowered. Therefore, the Nb content is made 0.100% or less. It is preferable that it is 0.080 % or less, and, as for Nb content, it is more preferable that it is 0.060 % or less.

N: 0.0010 to 0.0100%

N is an element which forms a nitride with Ti or Nb, and contributes to refinement|miniaturization of the austenite grain size at the time of heating. When the N content is less than 0.0010%, the above effects are not sufficiently obtained, and it requires a great manufacturing cost to make the N content less than 0.0010% in a commercial manufacturing process. Therefore, the N content is made 0.0010% or more. The N content is preferably 0.0020% or more.

On the other hand, when the N content exceeds 0.0100%, coarse carbonitrides are formed and the HIC resistance and toughness are deteriorated. Therefore, the N content is made 0.0100% or less. The N content is preferably 0.0060% or less.

Ca: 0.0001 to 0.0050%

Ca is an element contributing to the improvement of HIC resistance by forming CaS and suppressing the formation of MnS extending in the rolling direction. When Ca content is less than 0.0001 %, the said effect is not fully acquired. Therefore, the Ca content is made 0.0001% or more. It is preferable that it is 0.0005 % or more, and, as for Ca content, it is more preferable that it is 0.0010 % or more.

On the other hand, when the Ca content exceeds 0.0050%, oxides accumulate and the HIC resistance decreases. Therefore, the Ca content is made 0.0050% or less. It is preferable that it is 0.0045 % or less, and, as for Ca content, it is more preferable that it is 0.0040 % or less.

P: 0.030% or less

P is an element contained as an impurity. When P content exceeds 0.030 %, SSC resistance and HIC resistance will fall. Moreover, when welding is performed, the toughness of a weld part falls. Therefore, the P content is made 0.030% or less. It is preferable that it is 0.015 % or less, and, as for P content, it is more preferable that it is 0.010 % or less. Since excessive reduction of P content causes a significant increase in manufacturing cost, 0.001% is a practical lower limit.

S: 0.0025% or less

S is contained as an impurity and is an element that forms MnS that is elongated in the rolling direction at the time of hot rolling and inhibits HIC resistance. When the S content exceeds 0.0025%, the HIC resistance remarkably decreases. Therefore, the S content is made 0.0025% or less. It is preferable that it is 0.0015 % or less, and, as for S content, it is more preferable that it is 0.0010 % or less. Since excessive reduction of the S content causes a significant increase in manufacturing cost, 0.0001% is a practical lower limit.

Ti: 0.005 to 0.030%

Ti is an element which forms a nitride and contributes to refinement|miniaturization of a crystal grain. When the Ti content is less than 0.005%, the effect cannot be sufficiently obtained. Therefore, the Ti content is made 0.005% or more. The Ti content is preferably 0.008% or more.

On the other hand, when the Ti content exceeds 0.030%, not only the toughness decreases, but also coarse nitrides are formed and the HIC resistance decreases. Therefore, the Ti content is made 0.030% or less. The Ti content is preferably 0.020% or less.

Al: 0.010 to 0.040%

Al is an element added for deoxidation. When Al content is less than 0.010 %, the said effect cannot fully be acquired. Therefore, the Al content is made 0.010% or more. It is preferable that Al content is 0.015 % or more.

On the other hand, when Al content exceeds 0.040 %, Al oxide accumulates and HIC resistance falls. Therefore, the Al content is made 0.040% or less. It is preferable that Al content is 0.035 % or less.

O: 0.0040% or less

O is an impurity element which unavoidably remains after deoxidation. When the O content exceeds 0.0040%, oxides are formed, and toughness and HIC resistance are lowered. Therefore, the O content is made 0.0040% or less. The O content is preferably 0.0030% or less. The smaller the O content, the more preferable. However, since excessive reduction of the O content causes a significant increase in manufacturing cost, 0.0010% is a practical lower limit.

Mo: 0 to 2.00%

Cr: 0 to 2.00%

Cu: 0 to 2.00%

Ni: 0 to 2.00%

0.05≤Mo+Cr+Cu+Ni≤2.00 … (i)

However, each element symbol in a formula shows content (mass %) of each element contained in steel, and sets it as 0 (zero) when not contained.

Mo, Cr, Cu and Ni are elements contributing to the improvement of hardenability. In order to adjust Ceq which is the parameter|index of quenching property mentioned later, the total content of these elements is made into 0.05 % or more. It is preferable that it is 0.07 % or more, and, as for the total content of these elements, it is more preferable that it is 0.10 % or more.

On the other hand, when the total content of Mo, Cr, Cu, and Ni exceeds 2.00%, the hardness of steel increases and the SSC resistance decreases. Therefore, the total content of Mo, Cr, Cu and Ni is set to 2.00% or less. It is preferable that it is 1.00 % or less, and, as for total content, it is more preferable that it is 0.90 % or less. Moreover, 1.00 % or less is preferable and, as for each content of Mo, Cr, Cu, and Ni, 0.50 % or less is more preferable.

W: 0 to 1.00%

W is an element effective for the improvement of the intensity|strength of steel. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.01 % or more, and, as for W content, it is more preferable that it is 0.05 % or more.

However, when W content exceeds 1.00 %, hardness may rise, SSC resistance may fall, or toughness may fall. Therefore, even when it is made to contain, W content shall be 1.00 % or less. It is preferable that it is 0.50 % or less, and, as for W content, it is more preferable that it is 0.30 % or less.

V: 0 to 0.200%

V is an element that forms carbides and nitrides and contributes to the improvement of the strength of steel. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.010 % or more, and, as for V content, it is more preferable that it is 0.030 % or more.

However, when V content exceeds 0.200%, the toughness of steel will fall. Therefore, even when it is made to contain, V content shall be 0.200 % or less. It is preferable that it is 0.100 % or less, and, as for V content, it is more preferable that it is 0.080 % or less.

Zr: 0 to 0.0500%

Zr is an element that, like V, forms carbides and nitrides and contributes to the improvement of the strength of steel. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.0001 % or more, and, as for Zr content, it is more preferable that it is 0.0005 % or more.

However, when Zr content exceeds 0.0500 %, the toughness of steel may fall. Therefore, even when it is made to contain, Zr content shall be 0.0500 % or less. It is preferable that it is 0.0200 % or less, and, as for Zr content, it is more preferable that it is 0.0100 % or less.

Ta: 0 to 0.0500%

Ta is an element that, like V, forms carbides and nitrides and contributes to the improvement of strength. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.0001 % or more, and, as for Ta content, it is more preferable that it is 0.0005 % or more.

However, when Ta content exceeds 0.0500%, the toughness of steel may fall. Therefore, even when it is made to contain, Ta content is made into 0.0500 % or less. It is preferable that it is 0.0200 % or less, and, as for Ta content, it is more preferable that it is 0.0100 % or less.

B: 0 to 0.0020%

B is an element which segregates at the grain boundary of steel and contributes remarkably to the improvement of hardenability. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.0001 % or more, and, as for B content, it is more preferable that it is 0.0005 % or more.

However, when B content exceeds 0.0020 %, the toughness of steel may fall. Therefore, even when it is made to contain, B content shall be 0.0020 % or less. It is preferable that it is 0.0015 % or less, and, as for B content, it is more preferable that it is 0.0012 % or less.

REM: 0 to 0.0100%

REM is an element that controls the shape of sulfide inclusions and contributes to improvement of SSC resistance, HIC resistance, and toughness. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.0001 % or more, and, as for REM content, it is more preferable that it is 0.0010 % or more.

However, when the REM content exceeds 0.0100%, coarse oxides are generated, and not only the cleanliness of the steel is lowered, but also the HIC resistance and toughness are lowered. Therefore, even in the case of containing, the REM content is made 0.0100% or less. The REM content is preferably 0.0060% or less.

Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoids, and the content of REM means the total content of these elements.

Mg: 0 to 0.0100%

Mg is an element which produces|generates a fine oxide, suppresses coarsening of a crystal grain, and contributes to the improvement of toughness. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.0001 % or more, and, as for Mg content, it is more preferable that it is 0.0010 % or more.

However, when the Mg content exceeds 0.0100%, the oxide aggregates and coarsens, the HIC resistance decreases, and the toughness decreases. Therefore, even when it is made to contain, Mg content shall be 0.0100 % or less. It is preferable that Mg content is 0.0050 % or less.

Hf: 0 to 0.0050%

Hf, like Ca, is an element contributing to the improvement of HIC resistance by generating sulfides and suppressing the generation of MnS elongated in the rolling direction. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.0001 % or more, and, as for Hf content, it is more preferable that it is 0.0005 % or more.

However, when the Hf content exceeds 0.0050%, oxides increase, agglomerate and coarsen, and HIC resistance deteriorates. Therefore, even in the case of containing, the Hf content is made 0.0050% or less. It is preferable that it is 0.0040 % or less, and, as for Hf content, it is more preferable that it is 0.0030 % or less.

Re: 0 to 0.0050%

Re is, like Ca, an element contributing to the improvement of HIC resistance by generating sulfides and suppressing the generation of MnS elongated in the rolling direction. Therefore, you may make it contain as needed. In order to acquire the said effect, it is preferable that it is 0.0001 % or more, and, as for Re content, it is more preferable that it is 0.0005 % or more.

However, when the Re content exceeds 0.0050%, oxides increase, agglomerate and coarsen, and HIC resistance deteriorates. Therefore, even when it is made to contain, Re content shall be 0.0050 % or less. It is preferable that it is 0.0040 % or less, and, as for Re content, it is more preferable that it is 0.0030 % or less.

In the chemical composition of the steel sheet according to the present embodiment, the balance is Fe and impurities. Here, "impurity" is a component that is mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when steel is industrially manufactured, and is permitted in a range that does not adversely affect the steel sheet according to the present embodiment. it means.

Ceq: 0.30 to 0.50

In the steel sheet according to the present embodiment, after controlling the content of each element as described above, it is necessary to make Ceq calculated by the content of each element into a predetermined range. Ceq is a value used as an index of quenching property, and is represented by the following formula (ii).

If Ceq is less than 0.30, the required strength is not obtained. On the other hand, when Ceq exceeds 0.50, surface layer hardness will become high and SSC resistance will fall. Therefore, Ceq is set to 0.30 to 0.50. It is preferable that Ceq is 0.33 or more, and it is preferable that it is 0.45 or less.

Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 … (ii)

However, each element symbol in a formula shows content (mass %) of each element contained in steel, and when not contained, it is set as zero.

2. Metallic organization

<The metal structure in the center of the sheet thickness contains 0 to 80% polygonal ferrite, one or two types selected from acicular ferrite and bainite, in area %, and the balance is M-A phase>

In the steel sheet according to the present embodiment, the metal structure in the center of the sheet thickness contains 0 to 80% polygonal ferrite in area%, and one or two selected from needle-shaped ferrite and bainite. It is included and the balance is M-A phase.

When martensite is contained in the metal structure inside steel, the intensity|strength of steel will rise too much, and it will become difficult to suppress surface layer hardness low. Therefore, while adjusting the chemical composition of steel and making the value of Ceq into an appropriate range especially, it suppresses the production|generation of martensite by performing controlled cooling after hot rolling so that it may mention later.

Therefore, in consideration of the balance between strength and surface hardness, the metal structure at the center of the plate thickness is a structure containing polygonal ferrite, acicular ferrite and/or bainite.

When the area ratio of polygonal ferrite exceeds 80%, not only does it become difficult to obtain the required strength, but also the HIC resistance deteriorates. Therefore, the area ratio of polygonal ferrite is set to 80% or less. The area ratio of polygonal ferrite is preferably 60% or less.

By incorporating polygonal ferrite into the steel, it becomes possible to improve toughness. Therefore, when more excellent low-temperature toughness is required, it is preferable that the area ratio of polygonal ferrite be 20% or more.

On the other hand, when more excellent HIC resistance is requested|required, it is preferable to make the metal structure in the center part of plate|board thickness into the structure|tissue mainly of needle-shaped ferrite and bainite. In this case, it is preferable that the area ratio of polygonal ferrite be less than 20% and the total area ratio of acicular ferrite and bainite be 80% or more. More preferably, the total area ratio of acicular ferrite and bainite is 90% or more.

In the metal structure of the central plate thickness, the remainder other than polygonal ferrite, needle-shaped ferrite, and bainite is M-A phase. It is preferable that M-A phase is 5.0 % or less. The M-A award does not have to be included.

<Effective grain size at the center of plate thickness: 15.0 µm or less>

In addition, the effective crystal grain diameter in the center part of plate|board thickness is 15.0 micrometers or less. It becomes possible to ensure favorable low-temperature toughness by refining the crystal|crystallization in the center part of plate|board thickness. When ensuring better low-temperature toughness, it is preferable that the effective grain size is 10.0 micrometers or less.

<The metal structure in the surface layer contains, in area%, one or two types selected from acicular ferrite and bainite of 95% or more in total, and the balance is M-A phase>

In the steel sheet according to the present embodiment, the metal structure in the surface layer that is in the range from the surface to 1.0 mm in the thickness direction contains one or two types selected from needle-shaped ferrite and bainite, and the balance is M-A phase. do it with

The cooling rate of the surface layer is relatively high compared to the inside of the steel, and martensite is easily generated in the cooling process after hot rolling. When this martensite remains in the final tissue without receiving sufficient tempering effect, the SSC resistance is lowered. Therefore, the metal structure in the surface layer is a structure mainly composed of acicular ferrite and/or bainite. In addition, in order to make the highest hardness of a surface layer into the range mentioned later, it is preferable to make the hardness of a surface layer as uniform as possible. When acicular ferrite or bainite is contained in the surface layer, since the effect of making hardness uniform is acquired, it is preferable.

The total area ratio of acicular ferrite and bainite is preferably 97% or more, more preferably 98% or more, still more preferably 99% or more, and may be 100%.

The metal structure in the surface layer WHEREIN: The remainder is an M-A phase. However, the M-A phase does not need to be included.

Here, in the steel sheet according to the present embodiment, "acicular ferrite" is defined in Non-Patent Document 1, pseudo polygonal ferrite (αq), Widmanstein ferrite ( _αw ), granular bainite (αB) It shall refer to an organization consisting of one or more types selected from Bainite means a structure containing bainitic ferrite (α° _B ) having an underlying structure in the grain. In addition, the MA phase (Martensite-Austenite constituent) means a complex of martensite (α'm) and austenite (γ).

The area ratio of each phase of a metal structure and the effective crystal grain size of the plate|board thickness center are calculated|required as follows.

First, two specimens of full thickness from the position of 1/4 of the plate width (1/4 width) from the end of the steel plate in the width direction so that the cross section in the L (length) direction becomes the observation surface from the steel sample. They were cut out, and each was provided for tissue observation and particle size measurement.

About the test piece for structure observation, after wet-polishing and finishing to a mirror surface, the metal structure is made to stand out using an etching solution. As the etchant, nital is used. Then, with respect to the L-direction cross section, using an optical microscope or SEM, observe the tissue of the surface layer and the center of the plate thickness at a magnification of 100 times to 1000 times, and after confirming each tissue, each tissue at a magnification of 200 times or 500 times Check the type.

As described in Non-Patent Document 1, polygonal ferrite αp has a round, polygonal shape, and there is no underlying structure such as lath or block seen in cementite, retained austenite, M-A phase, bainite or martensite in the grain, recovered, is an organization Pseudo-polygonal ferrite exhibits a complex shape, and is particularly similar to granular bainite, but like polygonal ferrite, it does not include an underlying structure due to diffusion transformation and is a structure that spans the former austenite grain boundary. Widmansteten ferrite is ferrite in the shape of a needle. Granular bainite is similar to pseudo polygonal ferrite because it exhibits a complex shape and does not show a clear underlying structure compared to bainite, but contains cementite, retained austenite, M-A phase in the grains, or has old austenite grain boundaries. It is different from pseudo polygonal ferrite in that it is a structure that does not span the However, in this embodiment, pseudo polygonal ferrite, Widmannsteten ferrite, and granular bainite are defined as needle-like ferrite, so that pseudo polygonal ferrite and granular bainite can be distinguished. No need.

Bainite is a structure containing bainitic ferrite having an underlying structure in the grain. Bainite consists of upper bainite (BI type) containing retained austenite or M-A phase between bainitic ferrites of the lath phase, upper bainite containing cementite between bainitic ferrites of the lath phase (BII type), bainitic of lath phase Although it can be divided into lath-like lower bainite containing cementite in ferrite (BIII type), and lower bainite containing cementite in plate-shaped bainitic ferrite, in this embodiment, both are contained in bainite.

Therefore, when judging each structure, it is judged based on the said characteristic.

In the M-A phase, after wet-polishing the test piece for structure observation and finishing it to a mirror surface, the metal structure is exposed using an etching solution. The etching solution is Repera. Then, with respect to the L-direction cross section, the tissue is observed at a magnification of 500 times using an optical microscope, and the area ratio is measured.

For the test piece for particle size measurement, the central plate thickness is observed using the SEM-EBSD device, the area surrounded by the diagonal grain boundaries with an inclination angle of 15° or more is defined as a crystal grain, and the effective crystal grain size is obtained by determining the grain size of the crystal grains. Specifically, a region surrounded by grain boundaries with an angular difference of 15° or more measured by OIM Analysis of TSL Solutions, an EBSD analysis software, is called a crystal grain, and the average diameter of a circle (equivalent circle diameter) of the same area as the crystal grain is called a crystal grain size. . However, the area|region with an equivalent circle diameter of 0.5 micrometer or less is disregarded. In this embodiment, among the average particle diameters calculated by OIM Analysis, the average value by the area fraction method is called effective crystal grain diameter. As for the fraction of polygonal ferrite, as described above, the area ratio of polygonal ferrite may be measured by the difference in shape in observation using an optical microscope or SEM. Since there is no substructure such as visible laths or blocks, an equivalent polygonal ferrite fraction is obtained even if the area ratio of the structure without angular differences in particles due to laths or blocks is measured. In the case of measuring the area ratio of the tissue without the angular difference within the particle, the area where the angular difference to the second proximity by KAM (Karnel Average Misorientation) method using OIM Analysis of TSL Solutions is 1° or less is defined as polygonal ferrite, Obtain the polygonal ferrite fraction.

The step interval during EBSD measurement is 0.5 μm so that the angle difference between substructures such as laths and blocks of bainite structure is measured.

3. Mechanical properties

The highest hardness of the surface layer: 250HV0.1 or less

As mentioned above, in order to improve SSC resistance, it is necessary to suppress the highest hardness of the steel of a surface layer low. Moreover, even if the average hardness in a comparatively wide range of the surface layer is suppressed, if a structure|tissue with high hardness exists locally, there exists a possibility that SSC may generate|occur|produce as a starting point. Therefore, in this embodiment, the hardness in a surface layer is evaluated by the Vickers hardness test which set the test force to 0.98 N (0.1 kgf). When the highest hardness of the surface layer is 250HV0.1 or less, SSC resistance improves. Therefore, the highest hardness of the surface layer shall be 250HV0.1 or less.

In this embodiment, the measurement of the highest hardness of the surface layer which is the range from the surface to a depth of 1.0 mm is performed as follows.

First, from the position of 1/4, 1/2 and 3/4 of the plate width in the width direction of the steel plate from the end of the steel plate in the width direction, a steel plate of a square (300 mm × 300 mm) with a side of 300 mm is gas cut , and a block test piece having a length of 20 mm and a width of 20 mm is taken from the center of the cut steel sheet by machine cutting, and polished by mechanical polishing. For one block test piece, with a Vickers hardness tester (load: 0.1 kgf), starting at a position of 0.1 mm from the surface in the plate thickness direction, 10 points at 0.1 mm intervals in the plate thickness direction, in the width direction for the same depth 10 points, a total of 100 points, are measured at intervals of 1.0 mm. That is, a total of 300 points|pieces are measured by three block test pieces.

As a result of the above measurement, even if there is one measurement point exceeding 250 HV, if two or more points do not appear continuously in the sheet thickness direction, the point is not adopted as an abnormal point, and the next highest value is set as the highest hardness. On the other hand, when two or more measurement points exceeding 250 HV exist continuously in the plate thickness direction, the highest value among them is employ|adopted as the highest hardness.

Tensile strength: 480 MPa or more

In the steel sheet according to the present embodiment, although there is no particular limitation on the tensile strength, the line pipe used in the H ₂ S environment in which the steel sheet according to the present embodiment is assumed to be used is generally X52, X60, or X65 grade. materials are often used. In order to satisfy the request|requirement, it is preferable that it is 480 MPa or more, and, as for tensile strength, it is more preferable that it is 500 MPa or more.

On the other hand, when tensile strength exceeds 700 MPa, SSC resistance and HIC resistance may deteriorate. Therefore, it is preferable that tensile strength is 700 MPa or less.

The tensile strength is obtained by processing a round bar tensile test piece according to API5L and performing a tensile test so that the longitudinal direction of the test piece is parallel to the width direction of the steel sheet.

4. Plate thickness

There is no restriction|limiting in particular about the plate|board thickness of the steel plate which concerns on this embodiment. However, from the viewpoint of improving the transport efficiency of the fluid passing through the line pipe when the steel sheet according to the present embodiment is used as a line pipe, the sheet thickness is preferably 16.0 mm or more, and more preferably 19.0 mm or more.

On the other hand, the hardness of the surface layer is increased by work hardening at the time of forming a steel pipe, and the hardness of the surface layer increases as the thickness increases. Moreover, when it thickens, refinement|miniaturization of the crystal|crystallization in the center part of plate|board thickness becomes difficult. Therefore, it is preferable that plate|board thickness is 35.0 mm or less, It is more preferable that it is 30.0 mm or less, It is still more preferable in it being 25.0 mm or less.

<steel pipe>

Next, the steel pipe according to the present embodiment will be described.

1. base material

<Chemical composition, metal structure and mechanical properties>

The steel pipe according to the present embodiment has a base material portion made of a cylindrical steel sheet, and a weld portion provided in a butt portion of the steel sheet and extending in the longitudinal direction of the steel sheet. Such a steel pipe can be obtained by processing the steel plate which concerns on this embodiment into a cylinder shape, and welding a butt part.

Therefore, the reasons for limiting the chemical composition of the base material portion (steel sheet) of the steel pipe according to the present embodiment, the metal structure, and the maximum hardness of the surface layer are the same as those of the steel sheet according to the present embodiment.

However, as for the observation surface of the metal structure in the steel pipe, two full-thickness test pieces are cut from the position of 90° from the seam weld in the steel pipe so that the L (length) direction cross-section becomes the observation surface, and each It is provided for observation and particle size measurement. The 90 degree position corresponds to a 1/4 or 3/4 position of the plate width of the steel sheet.

In addition, the measurement of the highest hardness of a surface layer is performed by the following method.

First, a square (300 mm × 300 mm) with one side of 300 mm from the positions of 3 o'clock, 6 o'clock, and 9 o'clock respectively (positions of 90°, 180° and 270° from the seam weld) when the welded part of the steel pipe is set to 0 o'clock. mm) is cut by gas cutting, and a block test piece having a length of 20 mm and a width of 20 mm is taken from the center of the cut steel sheet by machine cutting and polished by mechanical polishing. For one block test piece, using a Vickers hardness tester (load: 0.1 kgf), starting at 0.1 mm from the surface, 10 points at intervals of 0.1 mm in the plate thickness direction, 10 points at intervals of 1.0 mm in the width direction with respect to the same depth, A total of 100 points are measured. That is, a total of 300 points|pieces are measured by three block test pieces.

As a result of the measurement, if two or more measurement points exceeding 250HV do not appear continuously in the thickness direction, it is determined that the highest hardness of the surface layer is 250HV0.1 or less.

Tensile strength: 480 MPa or more

In the steel pipe according to the present embodiment, there is no particular limitation on the tensile strength, but generally a material of grade X52, X60, or X65 is used as a line pipe used in an H ₂ S environment in many cases. In order to satisfy the request|requirement, it is preferable that it is 480 MPa or more, and, as for tensile strength, it is more preferable that it is 500 MPa or more.

On the other hand, when tensile strength exceeds 700 MPa, SSC resistance and HIC resistance may deteriorate. Therefore, it is preferable that tensile strength is 700 MPa or less.

The tensile strength is obtained by taking a test piece of a round bar from a position 180° from the core of the steel pipe so that the longitudinal direction is parallel to the width direction of the steel sheet, and performing a tensile test in accordance with API5L.

thickness

There is no restriction|limiting in particular with respect to the thickness of the steel pipe which concerns on this embodiment. However, from the viewpoint of improving the transport efficiency of the fluid passing through the line pipe, the thickness is preferably 16.0 mm or more, and more preferably 19.0 mm or more.

On the other hand, the hardness of the surface layer is increased by work hardening at the time of forming a steel pipe, and the hardness of the surface layer increases as the thickness increases. Moreover, when it thickens, refinement|miniaturization of the crystal|crystallization in the center part of plate|board thickness becomes difficult. Therefore, it is preferable that thickness is 35.0 mm or less, It is more preferable that it is 30.0 mm or less, It is still more preferable in it being 25.0 mm or less.

2. Weld

In general, in steel pipe welding, the welded portion is constructed to have a greater thickness than the base material portion. The welded part is constructed so that the strength is higher than that of the base metal part, but in order to suppress the occurrence of SSC, as long as the hardness of the welded part is 250Hv or less as described in NACE MR0175/ISO15156-2, the welded part of the steel pipe according to the present embodiment is SAW welding It will not specifically limit, as long as it is obtained under normal conditions. For example, when the steel sheet according to the present embodiment is used as a raw material, by SAW welding or the like, by welding at 3 or 4 electrodes, depending on the thickness of the sheet, the heat input is 2.0 kJ/mm to 10 kJ/mm in the condition range, Since the highest hardness will be 250 Hv or less, it is preferable. Moreover, you may perform the tempering process (deep heat treatment) which heats a welding part after welding.

Since the welding of the steel pipe is performed after controlled cooling, the surface layer of the welded portion is not hardened by controlled cooling. Therefore, the hardness of the weld may be measured with a load of 0.1 kgf as in the base metal, but may be measured with a load of 10 kgf or a load of 5 kgf as described in NACE MR0175/ISO15156-2.

<Production method>

If the steel plate according to the present embodiment and the steel pipe according to the present embodiment have the above-described configuration, the effect can be obtained. That is, although it can manufacture by the following method, it is not limited to this method.

That is, the steel plate according to the present embodiment can be obtained by a manufacturing method including the following steps.

(I) hot rolling process

(II) first cooling process

(III) maintenance process

(IV) the second cooling step (performed as needed)

(V) third cooling process

(VI) fourth cooling process

Moreover, the steel pipe which concerns on this embodiment can be obtained by the manufacturing method including the following processes in addition to the above.

(VII) forming process

(VIII) Welding process

Preferred conditions for each step will be described.

[Hot rolling process]

After melting the steel having the above-described chemical composition in a furnace, the slab produced by casting is heated to perform hot rolling.

In the hot rolling step, it is preferable that the heating temperature before hot rolling be 1000 to 1300°C, the finish rolling start temperature of the hot rolling is Ar3 to 900°C, and the finish rolling end temperature is Ar3°C or higher.

When heating temperature exceeds 1300 degreeC, there is a concern that crystal grains become coarse and a predetermined effective crystal grain size cannot be obtained. On the other hand, if the heating temperature is less than 1000°C, there is a possibility that the predetermined finish rolling temperature cannot be ensured.

Moreover, when a rolling start temperature is more than 900 degreeC, we are anxious that a crystal grain will coarsen and a predetermined|prescribed effective crystal grain size will not be obtained. On the other hand, when the rolling start temperature is less than Ar3°C, there is a possibility that the predetermined finish rolling temperature cannot be ensured.

When the finish rolling end temperature is less than Ar3°C, deformed ferrite is produced. If deformed ferrite has defects in steelmaking, it causes cracks during use. Therefore, when deformed ferrite is generated, it is necessary to perform strict control in the steelmaking step. Therefore, the finish rolling end temperature is set to Ar3°C or higher. Ar3 changes depending on the chemical composition, heating temperature, hot rolling conditions, and plate thickness (cooling rate during air cooling), but within the range of the chemical composition, plate thickness, and strength of the steel plate according to the present embodiment, it is approximately 760 to 790 ° C. .

As described above, in the steel sheet and the steel pipe according to the present embodiment, it is necessary to make both the miniaturization of the grain size in the center of the sheet thickness (thickness in the steel pipe) and the reduction of the highest hardness in the surface layer. In order to make the center part of plate|board thickness (thickness) into a fine structure|tissue, it is desired to make high the cooling rate after completion|finish of hot rolling. However, when a cooling rate is high, there arises a possibility that the hardness of a surface layer rises. Therefore, in order to satisfy both, controlled cooling after completion of hot rolling becomes important.

Specifically, the metal structure described below by sequentially performing the first cooling step, the holding step, the second cooling step, the third cooling step, and the fourth cooling step shown below on the steel sheet (hot rolled steel sheet) after the hot rolling step. It becomes possible to manufacture a steel plate having However, about a 2nd cooling process, it is arbitrary and does not need to perform it.

[First cooling process]

After the completion of hot rolling, in the surface temperature of the steel sheet, from a temperature of Ar3 °C or higher, for example, 790 to 830 °C, to the bainite transformation zone of the Bs point to the Ms point, accelerated cooling at an average cooling rate of 30 °C/s or more . By performing accelerated cooling to the said bainite transformation zone, it can suppress that polygonal ferrite and martensite are produced|generated in the metal structure in the steel plate surface layer.

When the cooling stop temperature in the first cooling step becomes higher than the Bs point, there is a possibility that polygonal ferrite is generated in the metal structure of the surface layer in the next holding step. On the other hand, there exists a possibility that martensite may produce|generate in the metal structure of a surface layer that the cooling stop temperature in a 1st cooling process is less than the Ms point. Moreover, even if the average cooling rate is less than 30°C/s, there is a possibility that polygonal ferrite is generated during cooling. There is no restriction|limiting in particular about the upper limit of an average cooling rate.

The average cooling rate in the first cooling step is a cooling rate calculated by dividing the change in surface temperature by the difference between the cooling start time and the cooling end time.

Here, the Bs point (°C) is expressed by the following formula (iii), and means the production start temperature of acicular ferrite and bainite.

Bs=830-270×C-90×Mn-37×Ni-70×Cr-83×Mo… (iii)

However, each element symbol in a formula shows content (mass %) of each element contained in steel, and when not contained, it is set as zero.

In addition, the Ms point can be calculated by the following formula (iv).

Ms=545-330×C+2×Al-14×Cr-13×Cu-23×Mn-5×Mo-4×Nb-13×Ni-7×Si+3×Ti+4×V… (iv)

However, each element symbol in a formula shows content (mass %) of each element contained in steel, and when not contained, it is set as zero.

[Maintenance process]

After the first cooling step, the temperature of the surface layer is maintained within the temperature range of the Ms point to the Bs point (bainite transformation region) by performing gentle cooling. By holding the bainite transformation zone for 3.0 seconds or more, the metal structure in the surface layer is controlled to be a metal structure mainly composed of needle-shaped ferrite and bainite. When the holding temperature is lower than the Ms point, martensite is formed, and the maximum hardness of the surface layer cannot be 250HV0.1 or less. On the other hand, when the holding temperature range exceeds the Bs point, polygonal ferrite is generated, and carbon is concentrated in untransformed austenite from polygonal ferrite having a low carbon solid solution limit, so the subsequent steps (second cooling step and/or In the third cooling step), martensite is generated, and the maximum hardness of the surface layer cannot be less than 250HV0.1.

In addition, if the holding time is not sufficient, untransformed austenite is transformed into martensite in a subsequent step, and the maximum hardness of the surface layer cannot be set to 250 HV0.1 or less. Therefore, in the holding step, the temperature of the surface layer is maintained in the bainite transformation zone for 3.0 seconds or more in order to control the metal structure mainly of acicular ferrite and bainite.

In this step, in order to maintain the surface layer temperature for 3.0 seconds or more in the bainite transformation region while performing slow cooling, from the first cooling step to the bainite transformation region of the Bs point to the Ms point, average cooling of 30° C./s or more Accelerated cooling at a speed is very important. When cooling at a fast average cooling rate of 30°C/s or more in the first cooling step, the plate thickness center temperature at the start of the holding step is maintained at a temperature higher than the surface layer temperature for a certain period of time. Therefore, the surface layer after the 1st cooling process is going to recuperate (heat up) by heat conduction with the plate|board thickness center. Here, by performing gentle cooling at a low water quantity capable of suppressing recuperation by heat conduction, the temperature of the surface layer can be maintained in the bainite transformation region for 3.0 seconds or more.

As described above, during the first cooling process and the holding process, the surface layer is cooled and maintained as described above, but with respect to the central part of the plate thickness, it is slowly cooled. It is preferable that the temperature of the center part of the plate thickness at the completion of the holding process is 700°C or more, and the average cooling rate of the plate thickness central part during the first cooling process and the holding process is preferably 15°C/s or less.

[Second cooling process]

After the control of the metal structure in the surface layer is completed by the holding step, the cooling rate of the center is controlled by repeating the cooling of the surface and the reheating in which the surface temperature after reheating becomes 550 ° C. or higher in the second cooling step twice or more, , it is possible to increase the polygonal ferrite fraction. If the fraction of the fine polygonal ferrite particles generated in the second cooling step is increased, the average particle diameter of the overall metal structure of the finally obtained composite structure can be reduced. In addition, the surface layer of the steel sheet can be self-tempered by reheating, and as a result, there is also an effect of reducing the highest hardness of the surface layer.

Therefore, when it is desired to obtain more excellent low-temperature toughness, it is preferable to perform the second cooling step.

When the steel sheet is cooled by accelerated cooling, the surface temperature is cooled to a lower temperature compared to the internal temperature. The surface temperature recovers so that the difference between the internal temperature and the surface temperature becomes small due to heat conduction from the inside when the accelerated cooling is temporarily stopped. Since the core temperature is cooled by heat conduction due to a temperature difference with the surface layer, the cooling rate of the core decreases when the surface layer temperature recovers. Therefore, by repeating the recuperation and cooling of the surface layer, the cooling rate of the center can be controlled, and the polygonal ferrite fraction can be increased. For example, after the holding step, the central temperature is maintained in the ferrite transformation zone by reducing the surface temperature to 500° C. or less by accelerated cooling, and repeating the cooling and recuperating of the surface layer to recover to 550° C. or more twice or more, effectively can increase the polygonal ferrite fraction.

In the second cooling step, when cooling and recuperating are less than two times, there is a fear that a sufficient transformation time to increase the polygonal ferrite fraction in the central portion of the plate thickness cannot be secured. In addition, since recuperation on the surface of the steel sheet is generated by heat conduction with the internal temperature, when the recuperation temperature is only less than 550°C, the central plate thickness is also maintained in the bainite transformation region, and the polygonal ferrite fraction is high. There is a fear that it will not be supported.

[Third cooling process]

After the control of the metal structure in the surface layer is completed by the holding step, or after increasing the polygonal ferrite fraction in the central plate thickness in the second cooling step, accelerated cooling is performed at an average cooling rate of 10° C./s or more. At this time, the surface temperature is accelerated to below the Ms point, and the final recuperation temperature after cooling is stopped is below the Bs point. By performing accelerated cooling immediately after the control of the structure of the surface layer is completed, internal cooling is promoted, and it becomes possible to make the central plate thickness into a structure containing fine acicular ferrite and/or bainite.

When the average cooling rate is less than 10°C/s, there is a possibility that the crystal grains in the central portion of the plate thickness may be granulated. Therefore, in a 3rd cooling process, for the purpose of raising an average cooling rate, the surface temperature of a steel plate is acceleratedly cooled to below Ms point. When the surface temperature of the steel sheet is cooled to below the Ms point, the cooling rate of the central portion of the sheet thickness can be increased by heat conduction. In the general accelerated cooling method, when quenching to the Ms point or lower, the hardness of the surface of the steel sheet increases. However, in the manufacturing method of the present embodiment, since the metal structure on the surface of the steel sheet is controlled, the surface of the steel sheet is rapidly cooled to the Ms point or lower. The hardness of the sea-island steel sheet surface layer does not rise. Therefore, an average cooling rate can be raised, without providing an upper limit to the cooling rate in the steel plate surface.

The said average cooling rate is the average cooling rate of the thickness center part obtained by dividing the temperature change of a plate|board thickness center part by cooling time (difference between cooling start time and cooling end time). The temperature change at the center of the plate thickness can be obtained from the surface temperature by heat conduction calculation.

The plate thickness central portion is cooled by accelerated heat conduction with the surface, but when cooling is stopped, the surface recovers heat by thermal conduction with the plate thickness central portion. Since recuperation proceeds until the temperature of the surface coincides with the central part of the plate thickness, the final recuperation temperature after cooling corresponds to the cooling stop temperature in the central part of the plate thickness. By setting the final recuperation temperature to the Bs point or lower, the central plate thickness can be made into a structure containing fine acicular ferrite and/or bainite. When the final recuperation temperature exceeds the Bs point, the generated polygonal ferrite grows and the structure becomes coarse.

[Etc]

During the third cooling step, if necessary, cooling may be temporarily stopped and the surface temperature of the steel sheet may be at least one Ms point or higher by reheating. When the steel sheet is cooled by accelerated cooling, the surface temperature is cooled to a lower temperature compared to the internal temperature. The surface temperature can be recovered by thermal conduction with the internal temperature when accelerated cooling is temporarily stopped. For example, even if the surface temperature is lowered to 400°C or lower by accelerated cooling, if the internal temperature at the time of cooling is stopped is 700°C or higher, it can be reheated to a temperature of 550°C or higher by giving an appropriate recuperation time.

Since the self-tempering effect is high compared with the case where normal accelerated cooling is performed by recuperating, surface layer hardness can be reduced. Even after that, accelerated cooling can be performed intermittently, and cooling and recuperating can be repeated. It is more preferable that the recombination is performed, for example, twice or more.

(4th cooling process)

After the 3rd cooling process, it cools to 300 degrees C or less so that the average cooling rate to 300 degreeC may become 200 degrees C/hr or more. When the average cooling rate to 300°C is less than 200°C/hr, the desired strength cannot be obtained.

(Forming process)

The steel pipe according to the present embodiment is formed by forming the steel sheet according to the present embodiment into a cylindrical shape, and by butt welding (seam welding) both ends of the steel sheet formed into the cylindrical shape.

The forming of the steel sheet into a steel pipe according to the present embodiment is not limited to a specific forming method. For example, it can manufacture by performing UO canning. In the UO manufacturing method, for example, with respect to a rolled steel sheet (material) that has been processed by cutting the edge portion, a C press is performed to form a C shape, then a U press is performed to form a U shape, and an O press is performed to It is molded into an O-shape and molded into a cylindrical shape.

(Welding process)

After forming the steel sheet, the joint (the core) as the end portion is put together, and temporary welding, inner surface welding, and outer surface welding are performed, and further, if necessary, the pipe is expanded. Although welding is also not limited to a specific welding, Submerged arc welding (SAW) is preferable. As long as the maximum hardness of the welded portion of the steel pipe according to the present embodiment is in the range described above, welding conditions and the like are not limited. However, when the steel sheet according to the present embodiment is used as a raw material, by SAW welding or the like, by welding at three or four electrodes, depending on the thickness, the heat input is in the range of 2.0 kJ/mm to 10 kJ/mm, so that the surface layer Since the highest hardness will be 250HV0.1 or less, it is preferable.

In the manufacturing method of the steel pipe which concerns on this embodiment, you may perform deep heat processing which heats a weld part to Ac1 point (degreeC) or less and tempers it.

Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

Example

(Example 1)

Steels having the chemical compositions shown in Tables 1-1 and 1-2 were melted, and steel pieces were obtained by continuous casting. The thickness at this time was 300 mm in steel types J-N, and was 240 mm in steel types A-I and O-S other than that. As shown in Tables 2-1 and 2-3, the obtained steel piece is heated to a temperature range of 1100 to 1250 ° C., hot-rolled in a recrystallization temperature range exceeding 900 ° C., and then Ar3 to 900 ° C. Hot rolling (finish rolling) was performed in the non-recrystallization temperature range, and the hot rolling was completed at the temperature shown in Tables 2-1 and 2-3 that is equal to or higher than the temperature of Ar3 (°C).

After that, for some examples, the first cooling step, the holding step, the third cooling step, and the fourth cooling step are sequentially performed under the conditions shown in Tables 2-1 to 2-4, and then cooling and recuperation are performed. It cooled to room temperature while repeating, and the steel plate was manufactured (2nd cooling process was not performed).

In addition, about the example other than that, a 1st cooling process, a holding process, a 2nd cooling process, a 3rd cooling process, and a 4th cooling process are performed in order under the conditions shown in Table 2-1 - Table 2-4, and it is carried out to room temperature. After cooling, a steel plate was prepared. In the 2nd cooling process, in the cooling before recuperation, the surface temperature was once lowered to 500 degrees C or less in all.

[Table 1-1]

[Table 1-2]

[Table 2-1]

[Table 2-2]

[Table 2-3]

[Table 2-4]

Tissue observation specimens, particle size measurement specimens, tensile specimens, hardness measurement specimens, DWTT specimens, impact specimens, SSC specimens, and HIC specimens were collected from the steel sheet and used for each test.

<Organization observation>

For the test piece for tissue observation, the test piece is collected from the position of W/4 in the plate width direction so that the L-direction cross section becomes the observation surface, wet-polished and mirror-finished, followed by nital corrosion to reveal the metal structure made it Then, the L-direction cross section was observed using an optical microscope at a magnification of 500 times in 4 fields of view, and the area ratio of each tissue in the surface layer (position 0.1 mm from the surface) and the plate thickness center was measured.

<Measurement of effective crystal grain size>

In addition, with respect to the test piece for particle size measurement, the test piece is taken from the same position as the test piece for tissue observation so that the L-direction cross section becomes the observation surface, and the central plate thickness is observed using the SEM-EBSD apparatus, and the inclination angle is 15° or more. The average effective grain size was obtained by determining the grain size of grains surrounded by diagonal grain boundaries.

<Tensile Test>

In the tensile test, according to API5L, a round bar tensile test piece was processed so that the longitudinal direction of the test piece was parallel to the width direction of the steel sheet, and a tensile test was performed. From the result, tensile strength (MPa) was calculated|required.

When the tensile strength was 480 MPa or more, it was judged that the steel sheet for a line pipe had a suitable strength.

<Hardness test>

Next, using the test piece for hardness measurement, the highest hardness of the surface layer was measured. Specifically, from the edge of the steel sheet in the width direction, from the positions of 1/4, 1/2 and 3/4 of the steel sheet in the width direction, a square (300 mm × 300 mm) steel sheet with a side of 300 mm is cut by gas cutting. From the center of the steel sheet cut out and cut out, a block test piece having a length of 20 mm and a width of 20 mm was taken by machine cutting and polished by machine polishing. For one block test piece, using a Vickers hardness tester (load: 0.1 kgf), starting at 0.1 mm from the surface, 10 points at intervals of 0.1 mm in the plate thickness direction, 10 points at intervals of 1.0 mm in the width direction with respect to the same depth, A total of 100 points were measured. That is, a total of 300 points|pieces were measured by three block test pieces. As a result of the above measurement, even if there was one measurement point exceeding 250 HV, if two or more points did not appear continuously in the plate thickness direction, the point was not adopted as an abnormal point, and the next highest value was set as the highest hardness. On the other hand, when two or more measurement points exceeding 250 HV exist continuously in the plate thickness direction, the highest value among them was employ|adopted as the highest hardness.

<DWTT Test>

The DWTT test piece was taken so that the longitudinal direction of the test piece was parallel to the width direction of the steel sheet from the 1/4 position in the width direction of the steel sheet. Using this DWTT test piece, the DWTT test at test temperatures of -20 degreeC and -30 degreeC was done, and the DWTT ductile fracture factor was measured. The DWTT test was performed based on API standard 5L3.

When the DWTT ductile fracture ratio after the DWTT test was 85% or more, it was judged that the toughness at the test temperature was excellent.

<Charpy Impact Test>

The impact test piece was a 2 mm V-notch test piece with a width of 10 mm. Each of the three test pieces was cut from the 1/4 position in the width direction of the steel sheet so that the longitudinal direction of the test piece was parallel to the width direction of the steel sheet, and the Charpy impact test was performed at -100 ° C. did.

When the average absorbed energy after the Charpy impact test was 150 J or more, it was judged that the toughness at -100°C or more was excellent.

<SSC exam>

In the SSC test, a four-point bending test using the inner surface of a steel pipe as a test surface was performed in accordance with NACE TM 0316 in order to evaluate the SSC sensitivity of the outermost layer. The test piece was taken so that the longitudinal direction of the test piece was parallel to the width direction of the steel sheet from the center of the width direction and the 1/4 position in the width direction of the steel sheet. In that case, load stress was made into 90% of the actual YS (Yield Strength: yield stress) of a test piece, and NACE Solution A prescribed|regulated to NACE TM 0177 was used for a test solution. Specifically, the presence or absence of SSC occurrence was observed after immersion for 720 hours under conditions in which 0.1 MPa of hydrogen sulfide was saturated in a solution containing 5% salt and 0.5% acetic acid. Other conditions were performed in accordance with NACE TM 0177. And the thing which SSC did not generate|occur|produce was judged as pass (OK), and the thing which SSC generate|occur|produced was judged as fail (NG).

<HIC test>

The HIC test piece was made into a 100 mm length and 20 mm width full-thickness test piece. And the HIC test was performed based on NACETM0284. Specifically, the crack area ratio after 96 hours of immersion was calculated|required on the conditions which made 0.1 MPa of hydrogen sulfide into the solution containing 5% salt and 0.5% acetic acid saturated. A crack area ratio of 6% or less was judged as pass (OK), and a thing exceeding 6% was judged as failing (NG). Moreover, it was judged that it was especially excellent (Ex) that the crack area ratio was 3 % or less.

Those results are shown collectively in Tables 3-1 to 3-4.

[Table 3-1]

[Table 3-2]

[Table 3-3]

[Table 3-4]

As can be seen from Tables 3-1 to 3-4, Test Nos. 1, 2, 6 to 9, 13 to 26, 101, 102, 108, 109, 111 to 123 satisfying all of the provisions of the present invention While the highest hardness of silver and the surface layer was 250HV0.1 or less, the crack by the SSC test was not seen. In addition, 85% or more of DWTT ductile fracture factor after DWTT test at -20°C is obtained, absorbed energy of Charpy impact test at -100°C is 150J or more, tensile strength is also 480MPa or more, and crack area ratio after HIC test 6% or less.

In Test Nos. 1, 2, 6 to 9, and 13 to 26, the polygonal ferrite area ratio was less than 20%, and since the structure was mainly composed of needle-shaped ferrite and bainite, the crack area ratio after the HIC test was also 3% or less. In particular, the HIC resistance was excellent. Further, in Test Nos. 101, 102, 108, 109, 111 to 123, the polygonal ferrite area ratio was 20% or more and the effective crystal grain size was 10.0 µm or less, so the DWTT ductile fracture surface after the DWTT test at -30°C A ratio of 85% or more was obtained, and the absorbed energy in the Charpy impact test at -100°C was 150J or more, and particularly, the low-temperature toughness was excellent.

In contrast, Test Nos. 3 to 5, 10 to 12, 27 to 29, 103-107, 110, 124, and 125 did not satisfy any of the provisions of the present invention.

In Test Nos. 3, 10, 103, and 104, in the 1st cooling process, since the surface layer average cooling rate was less than 30 degreeC/s, or the holding temperature exceeded the Bs point, martensite was produced|generated in the surface layer. As a result, the highest hardness of the surface layer could not be reduced to 250HV0.1 or less.

In Test Nos. 4, 11, 105, and 110, since the surface layer cooling stop temperature was lower than the Ms point in the first cooling step, martensite was generated, and the maximum hardness of the surface layer could not be reduced to 250 HV0.1 or less.

In Test Nos. 5, 12, and 106, since the holding time in the holding step was shorter than the effective time during which the needle-shaped ferrite/bainite transformation proceeded, the austenite remaining untransformed in the holding step was converted into martens in the subsequent cooling. It became a site, and the highest hardness of the surface layer could not be reduced to 250HV0.1 or less.

In test No. 107, since the average cooling rate of the 3rd cooling process was less than 10 degreeC/s, the effective grain size of the center part of plate|board thickness became coarse, and the DWTT ductile fracture factor fell.

In Test Nos. 27 and 124, since the C content was higher than the specified range, the maximum hardness of the surface layer exceeded 250 HV0.1. Further, in Test No. 125, since the value of Ceq was lower than the specified range, the polygonal ferrite fraction exceeded 20% and sufficient strength was not obtained.

In Test No. 28, the value of Ceq was higher than the specified range, and in Test No. 29, the total content of Mo, Cr, Cu and Ni was higher than the specified range, so the cooling process similar to the present invention was performed. Even if given, martensite was generated and the maximum hardness of the surface layer could not be reduced to 250HV0.1 or less.

(Example 2)

Among the steel sheets obtained in Example 1, the steel sheet obtained with desirable properties was formed into a cylindrical shape by the UO manufacturing method, welded from the inner and outer surfaces of the steel pipe by submerged arc welding, and expanded to obtain a UOE steel pipe. As for welding conditions, three electrodes were used for the inner surface side, and four electrodes were used for the outer surface side, and the heat input was set in the range of 2.0 kJ/mm - 10 kJ/mm according to the plate|board thickness.

The obtained steel pipe was subjected to metal structure observation, effective crystal grain size measurement, tensile test, surface hardness measurement, DWTT test, Charpy impact test, SSC test, and HIC test, similarly to the steel plate.

However, as for the observation surface of the metal structure in the steel pipe, two full-thickness test pieces are cut from the position of 90° from the seam weld in the steel pipe so that the L (length) direction cross-section becomes the observation surface, and each It was provided for observation and particle size measurement. Using this test piece, observation of the structure and measurement of the effective crystal grain size were performed in the same manner as in Example 1.

In addition, the measurement of the highest hardness of the surface layer is 300 on one side from the positions of 3 o'clock, 6 o'clock, and 9 o'clock, respectively (positions of 90°, 180°, and 270° from the seam welded part) when the welding part of the steel pipe is set to 0 o'clock. A square (300 mm x 300 mm) steel sheet of mm was cut by gas cutting, and a block test piece having a length of 20 mm and a width of 20 mm was taken from the center of the cut steel sheet by machine cutting, and polished by mechanical polishing. For one block test piece, with a Vickers hardness tester (load: 0.1 kgf), starting at 0.1 mm from the surface, 10 points at intervals of 0.1 mm in the plate thickness direction, 10 points at intervals of 1.0 mm in the width direction with respect to the same depth, A total of 100 points are measured. That is, a total of 300 points|pieces were measured by three block test pieces.

In addition, the DWTT test piece was extract|collected from the position of 90 degrees from the seam welding part of a steel pipe.

In addition, the Charpy impact test piece was extract|collected from the position 90 degree|times from the seam welding part of a steel pipe.

For the tensile test piece, a round bar test piece was taken from a position 180° from the core of the steel pipe so that the longitudinal direction was parallel to the width direction of the steel sheet, and a tensile test was performed in accordance with API5L.

In addition, similarly to Example 1, the SSC test and the HIC test were implemented.

The results are shown in Table 4-1 and Table 4-2.

[Table 4-1]

[Table 4-2]

As can be seen from Tables 4-1 and 4-2, a steel pipe manufactured using a steel sheet having excellent SSC resistance and HIC resistance, and high low-temperature toughness has excellent SSC resistance and HIC resistance, and It had high low-temperature toughness.

According to the present invention, it becomes possible to obtain a steel sheet and a steel pipe having excellent SSC resistance and HIC resistance, and high low-temperature toughness. Therefore, the steel plate and the steel pipe which concern on this invention can be used suitably as a line pipe for transporting the crude oil and natural gas containing a lot of H ₂ S.

Claims (8)

The chemical composition, in mass %,
C: 0.020 to 0.080%;
Si: 0.01 to 0.50%,
Mn: 0.50 to 1.60%;
Nb: 0.001 to 0.100%;
N: 0.0010 to 0.0100%,
Ca: 0.0001 to 0.0050%,
P: 0.030% or less;
S: 0.0025% or less;
Ti: 0.005 to 0.030%;
Al: 0.010 to 0.040%,
O: 0.0040% or less;
Mo: 0 to 2.00%,
Cr: 0 to 2.00%,
Cu: 0 to 2.00%,
Ni: 0 to 2.00%,
W: 0 to 1.00%,
V: 0 to 0.200%,
Zr: 0 to 0.0500%,
Ta: 0 to 0.0500%,
B: 0 to 0.0020%;
REM: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Hf: 0 to 0.0050%,
Re: 0 to 0.0050%,
balance: Fe and impurities,
The following formula (i) is satisfied,
Ceq represented by the following formula (ii) is 0.30 to 0.50,
The metal structure in the central plate thickness contains 0 to 80% polygonal ferrite in area%, and one or two selected from needle-shaped ferrite and bainite, the balance being MA phase, and effective crystal grain size is 15.0 μm or less,
The metal structure in the surface layer, which is in the range from the surface to 1.0 mm in the thickness direction, contains one or two types selected from needle-shaped ferrite and bainite in a total of 95% or more in area%, the remainder being MA phase. ,
The highest hardness in the said surface layer is 250HV0.1 or less, The steel plate.
0.05≤Mo+Cr+Cu+Ni≤2.00 … (i)
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 … (ii)
However, each element symbol in a formula shows content (mass %) of each element contained in steel, and when not contained, it is set as zero. According to claim 1,
The steel sheet, wherein the area % of polygonal ferrite in the metal structure of the central portion of the sheet thickness is 0 to less than 20%. According to claim 1,
The steel sheet, wherein the area % of polygonal ferrite in the metal structure of the central portion of the plate thickness is 20 to 80%, and the effective crystal grain size is 10.0 µm or less. 4. The method according to any one of claims 1 to 3,
The chemical composition is, in mass%,
W: 0.01 to 1.00%,
V: 0.010 to 0.200%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.0500%, and
B: 0.0001 to 0.0020%
A steel sheet containing at least one selected from 5. The method according to any one of claims 1 to 4,
The chemical composition is, in mass%,
REM: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0100%,
Hf: 0.0001 to 0.0050%,
Re: 0.0001 to 0.0050%
A steel sheet containing at least one selected from A base material part made of a cylindrical steel plate,
A welding part provided in the butt portion of the steel plate and extending in the longitudinal direction of the steel plate
have,
The steel plate is
The chemical composition, in mass %,
C: 0.020 to 0.080%;
Si: 0.01 to 0.50%,
Mn: 0.50 to 1.60%;
Nb: 0.001 to 0.100%;
N: 0.0010 to 0.0100%,
Ca: 0.0001 to 0.0050%,
P: 0.030% or less;
S: 0.0025% or less;
Ti: 0.005 to 0.030%;
Al: 0.010 to 0.040%,
O: 0.0040% or less;
Mo: 0 to 2.00%,
Cr: 0 to 2.00%,
Cu: 0 to 2.00%,
Ni: 0 to 2.00%,
W: 0 to 1.00%,
V: 0 to 0.200%,
Zr: 0 to 0.0500%,
Ta: 0 to 0.0500%,
B: 0 to 0.0020%;
REM: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Hf: 0 to 0.0050%,
Re: 0 to 0.0050%,
balance: Fe and impurities,
The following formula (i) is satisfied,
Ceq represented by the following formula (ii) is 0.30 to 0.50,
The metal structure in the thickness center contains 0 to 80% polygonal ferrite in area%, one or two types selected from needle-shaped ferrite and bainite, the balance being MA phase, and the effective crystal grain size is 15.0 μm or less,
The metal structure in the surface layer, which is in the range from the surface to 1.0 mm in the thickness direction, contains one or two types selected from needle-shaped ferrite and bainite in a total of 95% or more in area%, the remainder being MA phase. ,
The highest hardness in the surface layer is 250HV0.1 or less, the steel pipe.
0.05≤Mo+Cr+Cu+Ni≤2.00 … (i)
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 … (ii)
However, each element symbol in a formula shows content (mass %) of each element contained in steel, and sets it as zero when not contained. 7. The method of claim 6,
The steel pipe, wherein the area % of polygonal ferrite in the metal structure of the central portion of the thickness is 0 to less than 20%. 7. The method of claim 6,
An area % of polygonal ferrite in the metal structure in the central portion of the thickness is 20 to 80%, and the effective crystal grain size is 10.0 µm or less.