JP2008101242A - High-strength steel plate with excellent hic resistance for line pipe, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel plate having a strength of API X70 grade or higher and particularly having excellent hydrogen-induced cracking resistance (HIC resistance) and also to provide its manufacturing method. <P>SOLUTION: The steel plate has a steel composition consisting of, by mass, 0.02 to 0.08% C, 0.01 to 0.5% Si, 0.5 to 1.8% Mn, ≤0.01% P, ≤0.002% S, 0.05 to 0.5% Mo, 0.05 to 1% Cr, ≤0.07% Al, 0.01 to 0.05% Ti, ≤0.007% N and the balance Fe with inevitable impurities, satisfying limiting inequalities specified by Ceq and further containing, if necessary, either or both of Nb and V and one or more elements selected from Cu, Ni and Ca and also has a metallic structure which is composed of a two-phase structure of ferrite and bainite and in which fraction of island martensite (MA) is ≤5% by volume fraction and fine complex precipitates containing Ti, Mo and either or both of Nb and V and having <10 nm diameter are dispersedly precipitated. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-strength steel plate having a strength of API standard X70 grade or higher and a method for producing the same, and particularly to a material excellent in hydrogen-induced crack resistance (HIC resistance).

  In addition to strength, toughness and weldability, line pipes used for transporting crude oil and natural gas containing hydrogen sulfide are resistant to hydrogen-induced cracking resistance (hereinafter referred to as HIC resistance) and stress corrosion cracking resistance (hereinafter referred to as SCC resistance). So-called sour resistance is required.

  Hydrogen-induced cracking (hereinafter referred to as HIC) in steel materials is that hydrogen ions from the corrosion reaction are adsorbed on the steel material surface and penetrate into the steel as atomic hydrogen, and non-metallic inclusions such as MnS in the steel and a hard second phase structure. It is said that it diffuses and accumulates around the surface and cracks occur due to its internal pressure.

  In Patent Document 1, by adding an appropriate amount of Ca or Ce to the amount of S, the formation of acicular MnS is suppressed, and the shape is changed to a finely dispersed spherical inclusion with a small stress concentration, and cracks are not generated. A method for producing steel for line pipes that suppresses generation and propagation and has excellent HIC resistance is disclosed.

  In Patent Documents 2 and 3, cracks in the central segregation part are caused by reduction of elements having a high segregation tendency (C, Mn, P, etc.), soaking in the slab heating stage, and accelerated cooling during transformation during cooling. Steel having excellent HIC resistance that suppresses the formation of hardened structures such as island-like martensite that is the starting point of martensite, martensite that is a propagation path of cracks, and bainite is disclosed.

  The methods for improving the HIC resistance described in Patent Documents 1 to 3 are all about the center segregation part, manufactured by accelerated cooling or direct quenching, and the steel plate surface part having a high cooling rate is hardened compared to the inside, and the surface It is not intended for high-strength steel sheets exceeding the API X70 grade that generate hydrogen-induced cracks from the vicinity.

  Since the microstructure of these high-strength steel sheets obtained by accelerated cooling is a relatively high cracking susceptibility of bainite or acicular ferrite not only to the surface but also to the inside, it is even more stringent for HIC resistance. Not only segregation suppression is required, but also countermeasures against HIC starting from sulfide-based or oxide-based inclusions are necessary.

  That is, in the case of high-strength steel sheets exceeding API X70 grade, in addition to the component design and microstructure control that enables further strict segregation control while ensuring the strength, the HIC or sulfide system on the surface of the steel sheet And measures against HIC starting from oxide inclusions are necessary.

  Patent Document 4 discloses that API X80 grade HIC resistance, which is a ferrite-bainite two-phase structure, as a high-strength steel excellent in HIC resistance that does not contain block bainite or martensite with high cracking sensitivity. A high-strength steel material excellent in the above is disclosed.

In Patent Documents 5 and 6, SCC (SSCC) resistance and HIC resistance are improved by making the microstructure a ferrite single phase structure, and the strength is precipitation of carbide obtained by adding a large amount of Mo or Ti. A high strength steel secured by strengthening is disclosed.
Japanese Patent Laid-Open No. 54-110119 JP 61-60866 A JP-A-61-165207 JP 7-216500 A Japanese Patent Laid-Open No. 61-227129 JP-A-7-70697

  However, the bainite structure of the high-strength steel described in Patent Document 4 is a structure that is not as high as block bainite and martensite but is relatively high in cracking susceptibility. The amount of S and Mn is strictly limited, and Ca treatment is essential. As a result, it is necessary to improve the HIC resistance. Moreover, it is difficult to stably obtain a ferrite-bainite two-phase structure using the rolling / cooling method described in Patent Document 4.

  On the other hand, the ferrite phases described in Patent Documents 5 and 6 have a highly ductile structure and extremely low cracking susceptibility, so that the HIC resistance is greatly improved as compared with a steel having a bainite structure or an acicular ferrite structure.

  However, since the strength of the ferrite single phase is low, the steel described in Patent Document 5 is made stronger by precipitating a large amount of carbide using steel added with a large amount of C and Mo. In the band, Ti-added steel is wound around the steel band at a specific temperature, and the strength is increased by utilizing precipitation strengthening of TiC.

  However, in order to obtain a ferrite structure in which Mo carbide is dispersed as described in Patent Document 5, it is necessary to perform cold working after quenching and tempering, and further tempering again, resulting in an increase in manufacturing cost.

  In addition, since the particle size of Mo carbide is as large as about 0.1 microns and the effect of increasing the strength is low, it is necessary to increase the content of C and Mo and obtain a predetermined strength by increasing the amount of carbide, which is expensive. It becomes an ingredient composition.

  TiC used for securing strength in the high-strength steel described in Patent Document 6 is finer than Mo carbide and effective for precipitation strengthening, but is easily coarsened under the influence of temperature during precipitation. In patent document 6, since the countermeasure for coarsening is not sufficient, precipitation strengthening is small, and a large amount of Ti is required.

  Therefore, the present invention solves the above-described problems, and is a high-strength steel sheet for API X70 grade linepipe, which has excellent HIC resistance near the central segregation part and the surface and inclusions. An object is to provide a high-strength steel sheet and a method for producing the same.

The present inventors diligently studied the components and microstructure of the steel material and the manufacturing method of the steel plate in order to achieve both high HIC resistance and high strength, and obtained the following knowledge.
1． In order to achieve both high strength and HIC resistance, it is necessary to have a component composition that can suppress segregation and ensure high strength, and contains Cr, Cu, and Ni that have relatively small segregation. , Ceq1) and Ceq in consideration of segregation (hereinafter, Ceq2) are set within a certain range, and a system in which precipitation strengthening is obtained is obtained.
2. It is most effective to make the microstructure a ferrite + bainite two-phase structure with a small strength difference between the ferrite structure and the bainite structure.

That is, the two-phase structure of ferrite and bainite is generally a mixed structure of a soft phase (ferrite phase) and a hard phase (bainite phase), and a steel material having such a structure is a soft phase (ferrite phase). ) And a hard phase (bainite phase), hydrogen tends to accumulate, and the interface serves as a crack propagation path, resulting in poor HIC resistance. However, by reducing the difference in strength between the two, it is possible to achieve both HIC resistance and high strength.
3. As a component contributing to precipitation strengthening in the above component composition, Mo or Ti, and further, one or two of (Nb, V) are added in combination, the amount of Ti added to N is optimized, and Mo and Ti to C are added. If the addition amount is optimized, precipitation strengthening due to carbide can be utilized to the maximum.
4). With accelerated cooling after hot rolling and subsequent reheating, precipitates containing one or two of Ti, Mo, (Nb, V) are dispersed and precipitated, and the strength of the ferrite phase can be increased, and the hard phase can be achieved. Softening of a certain bainite phase occurs, and a ferrite + bainite two-phase structure with a small strength difference can be obtained, and the formation of MA can be reduced.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. Steel composition is mass%, C: 0.02-0.08%, Si: 0.01-0.5%, Mn: 0.5-1.8%, P: 0.01% or less, S : 0.002% or less, Mo: 0.05 to 0.5%, Cr: 0.05 to 1%, Al: 0.07% or less, Ti: 0.01 to 0.05%, N: 0.00. 007% or less, the balance Fe and inevitable impurities, satisfy the following formulas (1) to (4),
The metal structure is a two-phase structure of ferrite and bainite, the fraction of island martensite (MA) is 5% or less in volume fraction, and the composite precipitate containing Ti and Mo is dispersed and precipitated. A high-strength steel sheet for line pipes with excellent HIC resistance.
0.35 ≦ Ceq1 ≦ 0.45 (1)
However, Ceq1 = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5
Ceq2 ≦ 0.6 (2)
However, Ceq2 = 1.5C + 1.6Mn / 6 + (1.3Cu + 1.3Ni) / 15 + (1.1Cr + 1.2Mo + V) / 5
However, each element is contained (mass%), and elements not contained are set to 0.
Ti / N> 4 (3)
0.5 ≦ C / (Mo + Ti + Nb + V) ≦ 3 (4)
However, the element which does not contain each element in atomic% is set to 0.
2. The steel composition further contains one or two of Nb: 0.005 to 0.07% and V: 0.005 to 0.1% by mass%, and the composite precipitate is Ti, Mo, and Nb. 1. A high-strength steel sheet for line pipes having excellent HIC resistance according to 1, which comprises one or two of V.
3. The steel composition further contains one or two or more kinds selected from Cu: 0.5% or less, Ni: 0.5% or less, and Ca: 0.0005 to 0.005% by mass%. 3. A high-strength steel sheet for line pipes having excellent HIC resistance according to 1 or 2.
4). The high strength steel sheet for a line pipe excellent in HIC resistance according to any one of 1 to 3, wherein the composite precipitate is a fine precipitate having a diameter of less than 10 nm.
The steel containing the chemical component according to any one of 5.1 to ₃ is hot-rolled at a heating temperature of 1000 to 1300 ° C. and a rolling end temperature of Ar ₃ or higher, and then a cooling rate of 5 HIC resistance, characterized in that accelerated cooling is performed at 400 ° C./s or higher to 400 to 600 ° C., and immediately after cooling, the heating rate is reheated to a temperature of 600 to 700 ° C. at 0.5 ° C./s or higher. Manufacturing method for high-strength steel sheets for line pipes.
A line pipe excellent in HIC resistance, characterized by being manufactured using the steel sheet according to any one of 6.1 to 4.

  According to the present invention, steel pipes such as ERW steel pipes, spiral steel pipes and UOE steel pipes having high strength of API X70 grade or higher and excellent in HIC resistance can be manufactured without adding a large amount of alloy elements. It is extremely useful in industry.

In the present invention, the microstructure and component composition are defined. Hereinafter, the reason for limitation will be described in detail.
[Microstructure]
In the present invention, the microstructure is substantially a ferrite + bainite two-phase structure.
Since the ferrite phase is rich in ductility and has low cracking sensitivity, high HIC resistance can be obtained, and the bainite phase has excellent strength toughness.

  Since MA is a very hard hard phase, it is easy for hydrogen to accumulate at the interface between the parent phase and MA, and the interface is likely to become a crack propagation path. HIC characteristics deteriorate rapidly.

  In the present invention, the production of MA is suppressed and the HIC resistance can be improved by a manufacturing process of accelerated cooling after hot rolling and subsequent reheating.

  If different metal structures such as martensite and pearlite are mixed in the two-phase structure with ferrite + bainite, HIC is likely to occur due to hydrogen accumulation and stress concentration at the interface between different phases. The lower the rate, the better.

  However, since the influence is negligible when the volume fraction of the structure other than the ferrite phase and the bainite phase is low, one or more kinds of other metal structures with a volume fraction of 5% or less, that is, martensite, pearlite, etc. are used. You may contain.

  The bainite fraction is not particularly defined, but is preferably 10% or more from the viewpoint of securing the toughness of the base material and 80% or less from the viewpoint of HIC resistance. More preferably, it is 20 to 60%.

  Next, the precipitate that is dispersed and precipitated in the ferrite phase will be described. In the steel sheet according to the present invention, a precipitate containing Mo and Ti is dispersed and precipitated in the ferrite phase, whereby the ferrite phase is strengthened, and the strength difference between ferrite and bainite is reduced, so that excellent HIC resistance is obtained. be able to.

  Since the precipitate is extremely fine, the HIC resistance is not deteriorated. In the present invention, Mo and Ti are added in combination, and a composite carbide containing both Mo and Ti is finely precipitated in the steel. Compared to the precipitation strengthening of MoC and / or TiC, it is characterized in that a greater strength improvement effect can be obtained.

  This unprecedented strength improvement effect is presumed to be due to the fact that the composite carbide containing both Mo and Ti is thermally stable and has a slow growth rate, so that the particle size is extremely fine with a particle size of less than 10 nm. Is done.

  In addition, when the composite carbide containing Mo and Ti is composed only of Mo, Ti, and C, the total amount of Mo and Ti and the amount of C are combined in the vicinity of 1: 1 by atomic ratio.

  In the present invention, in order to further improve the toughness of the heat affected zone, one or two of Nb and V are added in combination, but the precipitate is one or two of Mo, Ti, and (Nb, V). Thus, precipitation strengthening similar to that of composite carbide containing Mo and Ti is obtained.

  Composite carbides containing Mo and Ti and further composite carbides containing one or two of (Nb, V) are fine carbides, and mainly precipitate in the ferrite phase, but depending on the chemical composition and production conditions, the bainite phase It may also be deposited from

  In this case, further strengthening is possible, but even if the bainite phase is strengthened by precipitation, if the hardness difference between the ferrite phase and the bainite phase is HV70 or less, the HIC resistance is not impaired.

In order to obtain a high-strength steel sheet having a yield strength of 500 MPa or more (API × 70 grade or more), it is preferable to deposit 2 × 10 ³ pieces / μm ³ or more of these precipitates of less than 10 nm. The form of precipitation may be random or in line, and is not particularly defined.

  In the present invention, even if precipitates other than the composite carbide mainly composed of Mo and Ti are generated as long as the effect of increasing the strength by the composite carbide of Mo and Ti is not impaired and the HIC resistance is not deteriorated. good. However, the number of precipitates of less than 10 nm is preferably 95% or more of the total number of precipitates excluding TiN.

[Chemical composition]
Next, chemical components will be described. Unless otherwise specified in the following description, all units shown in% are% by mass.

C
C is 0.02 to 0.08%. C is an element that contributes to precipitation strengthening as a carbide. However, if it is less than 0.02%, sufficient strength cannot be secured, and if it exceeds 0.08%, toughness and HIC resistance are deteriorated. It is specified at 02 to 0.08%.

Si
Si is set to 0.01 to 0.5%. Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, the toughness and weldability are deteriorated. %.

Mn
Mn is 0.5 to 1.8%. Mn is added for strength and toughness, but if less than 0.5%, the effect is not sufficient, and if it exceeds 1.8%, the weldability and HIC resistance deteriorate, so the Mn content is 0.5 to It is specified to 1.8%. Preferably, it is 0.5 to 1.5%.

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

S
S is made 0.002% or less. In general, S is preferably as small as possible because it becomes MnS inclusions in steel and deteriorates the HIC resistance. However, since there is no problem if it is 0.002% or less, the upper limit of the S content is defined as 0.002%.

Mo
Mo is set to 0.05 to 0.5%. Mo is an important element in the present invention, and by containing 0.05% or more, fine composite precipitates with Ti are formed while suppressing pearlite transformation during cooling after hot rolling, thereby increasing strength. A big contribution.

  However, if added over 0.5%, a hardened phase such as martensite is formed and the HIC resistance is deteriorated, so the Mo content is specified to be 0.05 to 0.5%. Preferably, it is 0.05 to less than 0.3%.

Cr
Cr is 0.05 to 1%. Similar to Mn, Cr is an element effective for obtaining sufficient strength even at low C, and segregation is relatively small. Therefore, Cr is an element effective for ensuring strength and improving HIC resistance. If it is less than 0.05%, the effect is not sufficient, and if it exceeds 1%, weldability deteriorates, so the Cr content is specified to be 0.05 to 1%.

Al
Al is made 0.07% or less. Al is added as a deoxidizer, but if it exceeds 0.07%, the cleanliness of the steel is lowered and the HIC resistance is deteriorated, so the Al content is specified to be 0.07% or less. Preferably, it is 0.01 to 0.07%.

Ti
Ti is set to 0.01 to 0.05%. Ti is an important element in the present invention, and is added in combination with Mo. By adding 0.01% or more, a composite precipitate is formed with Mo, which greatly contributes to an increase in strength.

  However, if added over 0.05%, the weld heat affected zone toughness is deteriorated, so the Ti content is specified to be 0.01 to 0.05%. Furthermore, if it is less than 0.025%, more excellent toughness is exhibited. For this reason, when adding 1 type or 2 types of Nb and V, it is preferable to make Ti content into 0.01 to less than 0.025%.

N
N is set to 0.007% or less. If added over 0.007%, the toughness of the welded part is deteriorated and slab cracking in the steel making stage is also caused, so the content is made 0.007% or less.

  N forms precipitates with Ti, but the TiN precipitates are finely dispersed in the high temperature region of the weld heat affected zone reaching 1350 ° C. or more, and the old austenite grains in the weld heat affected zone are refined to produce welding heat. Significantly contributes to improved toughness of the affected area It is preferable to set it as 0.004 to 0.006% as content with which the effect becomes sufficient.

0.35 ≦ Ceq1 ≦ 0.45
However, Ceq1 = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5, and each element is content (mass%), and elements not contained are 0.

  In order to ensure strength exceeding X70 grade such as API X80 grade, Ceq1 is set to 0.35 or more. Further, from the viewpoint of weldability, Ceq1 is set to 0.45 or less in order to ensure good weldability. The steel material according to the present invention is not dependent on the plate thickness of Ceq1 in the range of plate thickness from 10 mm to 30 mm, and can be designed with the same Ceq1 up to about 30 mm.

Ceq2 ≦ 0.6
However, Ceq2 = 1.5C + 1.6Mn / 6 + (1.3Cu + 1.3Ni) / 15 + (1.1Cr + 1.2Mo + V) / 5, and each element is content (mass%) and elements not contained are 0.

Ceq2 is an index indicating the degree of segregation of the alloy elements, and is expressed by the following formula using the segregation coefficient of each alloy element. In the formula, * indicates multiplication.
_{Ceq2 = f C * C + f} Mn * Mn / 6 + (f Cu * Cu + f Ni * Ni) / 15 + (f Cr * Cr + f Mo * Mo + f V * V) / 5
The segregation coefficient of each alloy element is f _C = 1.5, f _Mn = 1.6, f _Cu = 1.3, f _Ni = 1.3, f _Cr = 1.1, f _Mo = 1.2, f _V = 1.

  If Ceq2 exceeds 0.6, a hardened structure is likely to be formed at the segregated portion, the cracking sensitivity is increased, and the HIC resistance is deteriorated, so Ceq2 ≦ 0.6.

Ti / N> 4
Ratio of Ti amount and N amount in mass%: Ti / N> 4. Strengthening according to the present invention is due to fine precipitation of composite precipitates (mainly carbides) containing Ti and Mo. However, when Ti / N ≦ 4, Ti is consumed for precipitation of TiN, which is effective for precipitation strengthening. Since a composite precipitate containing Ti and Mo cannot be obtained sufficiently, the ratio of Ti amount to N amount by mass%: Ti / N> 4.

C / (Mo + Ti + Nb + V)
However, the element which does not contain each element in atomic% is set to 0.

  C / (Mo + Ti + Nb + V), which is the ratio of the amount of C and the total amount of Mo, Ti, Nb, and V in atomic%, is set to 0.5-3. The increase in strength according to the present invention is due to composite precipitates (mainly carbides) containing Ti, Mo, Nb, and V. The case of including one or two of Nb and V will be described later.

  In order to make effective use of precipitation strengthening by this composite precipitate, the relationship between the amount of C and the amounts of Mo, Ti, Nb, and V that are carbide forming elements is important. In this case, a thermally stable and very fine composite precipitate can be obtained.

  When the value of C / (Mo + Ti + Nb + V) represented by the atomic% content of each element is less than 0.5 or more than 3.0, the amount of any element is excessive, and the HIC resistance due to the formation of a hardened structure In order to cause deterioration of characteristics and toughness, the value of C / (Mo + Ti + Nb + V) is regulated to 0.5-3.

  In addition, when using content of the mass%, the value of (C / 12.01) / (Mo / 95.9 + Ti / 47.9) is prescribed | regulated to 0.5-3. When the value of C / (Mo + Ti) is 0.7-2, a finer precipitate having a particle size of 5 nm or less is obtained, which is more preferable.

  The above is the basic component composition of the present invention. In the present invention, Nb and V may be contained in one or two kinds for the purpose of further improving the strength and weld zone toughness of the steel sheet.

When 1 type or 2 types of Nb and V contain 1 type or 2 types of Nb and V, Nb shall be 0.005-0.07%. Nb improves toughness by refining the structure, but forms a composite precipitate with Ti and Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.07%, the toughness of the weld heat affected zone deteriorates, so the Nb content is specified to be 0.005 to 0.07%.

  V is 0.005 to 0.1%. V similarly forms a composite precipitate with Ti and Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.1%, the toughness of the weld heat affected zone deteriorates, so the V content is specified to be 0.005 to 0.1%.

  When one or two of Nb and V are contained, C / (Mo + Ti + Nb + V), which is a ratio of the amount of C and the total amount of Mo, Ti, Nb, and V, is set to 0.5 to 3.

  Strengthening according to the present invention depends on precipitates containing Ti and Mo, but when one or two of Nb and V are contained, they become composite precipitates (mainly carbides) containing them. At this time, when the value of C / (Mo + Ti + Nb + V) represented by the content of atomic% of each element is less than 0.5 or more than 3, the amount of any element is excessive, and HIC resistance due to formation of a hardened structure In order to cause deterioration of characteristics and toughness, the value of C / (Mo + Ti + Nb + V) is regulated to 0.5-3.

  However, each element symbol is a content in atomic%. In addition, when content of mass% is used, the value of (C / 12.01) / (Mo / 95.9 + Ti / 47.9 + Nb / 92.91 + V / 50.94) is specified to 0.5-3. . More preferably, it is 0.7-2, and a finer precipitate having a particle size of 5 nm or less is obtained.

  In the present invention, for the purpose of further improving the strength and HIC resistance of the steel sheet, one or more of Cu, Ni and Ca shown below may be contained.

Cu
Cu is an element effective for improving toughness and increasing strength, but if added in a large amount, weldability deteriorates, so when added, the upper limit is 0.5%.

Ni
Ni is an element effective for improving toughness and increasing strength, but if added in a large amount, the HIC resistance decreases, so when added, the upper limit is 0.5%.

Ca
Ca is an element effective for improving the HIC resistance by controlling the form of sulfide inclusions. However, if it is less than 0.0005%, its effect is not sufficient, and even if added over 0.005%, it is effective. Saturated, but rather deteriorates the HIC resistance due to a decrease in the cleanliness of the steel, so when added, the Ca content is specified to be 0.0005 to 0.005%.
[Production conditions]
FIG. 1 schematically shows the tissue control method of the present invention. The steel having the above-described component composition is accelerated and cooled from the austenite region of Ar ₃ or higher to the bainite region to obtain a mixed structure of untransformed austenite and bainite.

  Reheat immediately after cooling. Untransformed austenite is transformed into ferrite, and fine precipitates are dispersed and precipitated in the ferrite phase. On the other hand, the bainite phase is tempered to become tempered bainite.

  By adopting a two-phase structure of a ferrite phase that has been precipitation strengthened by fine precipitates and a bainite phase that has been tempered and softened, both high strength and HIC resistance can be achieved.

The high-strength steel sheet for line pipes according to the present invention uses steel having the above composition, and is hot-rolled at a heating temperature of 1000 to 1300 ° C. and a rolling end temperature of Ar ₃ temperature or higher, and then 5 ° C./s or higher. By cooling to 400 to 600 ° C. at a cooling rate of 5 ° C., and immediately after cooling, it is reheated to a temperature of 600 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more. A composite structure in which composite carbide is dispersed and precipitated and the bainite phase is softened is provided. The temperature is the average temperature of the steel sheet.

Heating temperature If the heating temperature is less than 1000 ° C, the required strength cannot be obtained because the solid solution of the carbide is insufficient, and if it exceeds 1300 ° C, the toughness deteriorates. Preferably, it is 1050-1250 degreeC.

Rolling end temperature When the rolling end temperature becomes Ar ₃ temperature or less, the subsequent ferrite transformation rate decreases, so that sufficient precipitation of fine precipitates cannot be obtained during ferrite transformation by reheating, and the strength decreases. The end temperature is set to Ar ₃ temperature or higher.

The Ar ₃ temperature can be obtained by the following equation.
Ar ₃ = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
Cooling condition Immediately after the rolling, cooling is performed at a cooling rate of 5 ° C./s or more, and cooling is stopped at 400 to 600 ° C. When the product is allowed to cool or gradually cool after the completion of rolling, precipitates are precipitated from a high temperature range, and the precipitates are easily coarsened and sufficient strengthening cannot be obtained. Therefore, rapid cooling (accelerated cooling) is performed to a temperature optimal for precipitation strengthening, and precipitation from a high temperature region is prevented.

  If the cooling rate is less than 5 ° C./s, the effect of preventing precipitation in a high temperature region is not sufficient and the strength is lowered. Any cooling equipment can be used as the cooling method.

  The cooling stop temperature is 400 to 600 ° C. Accelerated cooling after the end of rolling and rapid cooling to 400 to 600 ° C. in the bainite transformation region generate a bainite phase and increase the driving force for ferrite transformation during reheating.

  By increasing the driving force, the ferrite transformation in the reheating process is promoted, and the ferrite transformation can be completed by reheating for a short time. When the cooling stop temperature is less than 400 ° C., even if a bainite, martensite single-phase structure, or a ferrite + bainite two-phase structure is obtained, island-like martensite (MA) is generated, so that the HIC resistance is deteriorated.

  On the other hand, if the temperature exceeds 600 ° C., ferrite transformation at the time of reheating is not completed, and pearlite is precipitated to deteriorate the HIC resistance. Therefore, the accelerated cooling stop temperature is defined as 400 to 600 ° C.

Reheating Immediately after accelerated cooling, reheating is performed to a temperature of 600 to 700 ° C. at a heating rate of 0.5 ° C./s or more. This process is an important production condition in the present invention, and fine precipitates that contribute to strengthening of the ferrite phase are precipitated simultaneously with the ferrite transformation during reheating.

  In order to simultaneously strengthen the ferrite phase with fine precipitates and soften the bainite phase and obtain a structure with a small difference in strength between the ferrite phase and the bainite phase, reheat to 600 to 700 ° C immediately after accelerated cooling. is required.

  Further, it is desirable that the reheating is performed at least 50 ° C. higher than the temperature after cooling. When the heating rate during reheating is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that production efficiency deteriorates and pearlite transformation occurs, so that fine precipitates are dispersed and precipitated. Cannot be obtained and sufficient strength cannot be obtained.

  If the reheating temperature is less than 600 ° C., the ferrite transformation is not completed, and untransformed austenite transforms to pearlite during subsequent cooling, so that the HIC resistance is deteriorated.

  On the other hand, if the temperature exceeds 700 ° C., the precipitates become coarse and sufficient strength cannot be obtained. Therefore, the reheating temperature range is regulated to 600 to 700 ° C.

  Even if it cools immediately after reheating if the manufacturing method of this invention is used, since ferrite transformation will fully advance, the holding time in reheating temperature is not prescribed | regulated.

  However, if the temperature is maintained for more than 30 minutes, precipitates may be coarsened and the strength may be reduced.

  The cooling rate after reheating is not specified as long as it is set appropriately. However, air cooling is preferable because the ferrite transformation proceeds in the cooling process after reheating. As long as the ferrite transformation is not hindered, it is possible to perform cooling at a cooling rate faster than air cooling.

  As equipment for performing reheating to a temperature of 600 to 700 ° C., a heating device can be installed on the downstream side of the accelerated cooling equipment. As a heating device, it is preferable to use an induction heating device or a gas combustion furnace capable of rapid heating of a steel plate.

  The induction heating device is particularly preferable because temperature control is easier than in a soaking furnace, the cost is relatively low, and the cooled steel sheet can be heated quickly. In addition, by arranging a plurality of induction heating devices in series, even if the line speed and the type and size of the steel sheet are different, the number of induction heating devices to be energized and the supply power can be set by arbitrarily setting them. It is possible to freely control the temperature rate and the reheating temperature. In addition, since the cooling rate after reheating may be arbitrary, it is not necessary to install special equipment in the downstream of a heating apparatus.

  In FIG. 2, the schematic of an example of the manufacturing line for enforcing the manufacturing method of this invention is shown. In the rolling line 1, a hot rolling mill 3, an accelerated cooling device 4, an in-line induction heating device 5, and a hot leveler 6 are arranged from upstream to downstream.

  By installing the in-line type induction heating device 5 or other heat treatment device on the same line as the hot rolling mill 3 as a rolling facility and the accelerated cooling device 4 as a subsequent cooling facility, the rolling and cooling can be quickly performed. Since the reheating treatment can be performed, the steel sheet after being rolled and accelerated and cooled can be immediately heated to 600 ° C. or higher.

  Since the steel sheet according to the present invention does not increase in hardness at the surface layer portion unlike a conventional bainite or acicular ferrite structure steel plate obtained by accelerated cooling or the like, HIC from the surface layer portion does not occur.

  Furthermore, the two-phase structure of the ferrite phase and bainite phase, which have a small strength difference, has extremely high resistance to cracking and can reduce the generation of hard phase MA that can serve as the origin and propagation path of HIC generation. HIC can also be suppressed.

  Therefore, it is suitable as a steel pipe (an electric resistance steel pipe, a spiral steel pipe, a UOE steel pipe) that is formed into a steel pipe by press bend molding, roll molding, UOE molding, or the like and transports crude oil or natural gas.

  Steels (steel types A to N) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and thick steel plates (Nos. 1 to 24) having thicknesses of 22 and 28 mm were produced.

  After the heated slab was rolled by hot rolling, it was immediately cooled using a water-cooled accelerated cooling facility and reheated using an induction heating furnace or a gas combustion furnace. The cooling equipment and induction heating furnace were in-line type.

The microstructure of the steel sheet produced as described above was observed with an optical microscope (magnification × 400) and a transmission electron microscope (TEM). Observation was carried out for a cross section perpendicular to the rolling direction and 10 views for an area of 12 mm ² with a plate thickness of ½, and the average value for each view.

  The components of the precipitate were analyzed by energy dispersive X-ray spectroscopy (EDX). The tensile properties and HIC resistance of each steel plate were measured.

  Tensile properties were measured by performing a tensile test using a full thickness test piece in the rolling vertical direction as a tensile test piece, and measuring yield strength and tensile strength.

  In consideration of manufacturing variations, a steel having a yield strength of 500 MPa or more and a tensile strength of 600 MPa or more was evaluated as a high strength steel plate of API X70 grade or more.

  The HIC resistance was evaluated by performing an HIC test with an immersion time of 96 hours in accordance with NACE Standard TM-02-84. If no cracks were observed, the HIC resistance was judged good. Indicated.

  In Table 2, the manufacturing conditions and measurement result of each steel plate (No. 1-24) are shown collectively. No. which is an example of the present invention. All of Nos. 1 to 13 were within the scope of the present invention in terms of chemical composition and production method, had high yield strength of 500 MPa or higher, tensile strength of 600 MPa or higher, and excellent HIC resistance.

  As for Ti, Mo, and some of the steel plates, fine carbide precipitates containing Nb and / or V and having a particle size of less than 10 nm were dispersed and precipitated. Moreover, the structure of the steel sheet was substantially a ferrite + bainite two-phase structure, the fraction of the bainite phase was in the range of 10 to 80%, and the fraction of MA was 5% or less.

  No. In Nos. 14 to 18, the chemical components are within the scope of the present invention, but the manufacturing method is outside the scope of the present invention, so that the structure is not a ferrite + bainite two-phase structure, and fine carbides are dispersed and precipitated. Therefore, cracks occurred in the strength and HIC tests.

  No. Since the chemical components of Nos. 19 to 24 are outside the scope of the present invention, coarse precipitates are not formed, and precipitates containing Ti and Mo are not dispersed and precipitated. Cracking occurred in the HIC test due to excessive formation of.

  In addition, when the reheating was performed in the induction heating furnace or in the gas combustion furnace, there was no particular difference in the results.

The figure explaining the process heat history and structure | tissue change of the structure | tissue control method based on this invention. Schematic which shows an example of the manufacturing line for enforcing the manufacturing method of this invention.

Explanation of symbols

1: Rolling line 2: Steel plate 3: Hot rolling mill 4: Accelerated cooling device 5: In-line induction heating device 6: Hot leveler

Claims (6)

Steel composition is mass%, C: 0.02-0.08%, Si: 0.01-0.5%, Mn: 0.5-1.8%, P: 0.01% or less, S : 0.002% or less, Mo: 0.05 to 0.5%, Cr: 0.05 to 1%, Al: 0.07% or less, Ti: 0.01 to 0.05%, N: 0.00. Less than 007%, satisfying the following formulas (1) to (4), the balance Fe and inevitable impurities, the metal structure is a two-phase structure of ferrite and bainite, and the fraction of island martensite (MA) is volume fraction A high-strength steel sheet for line pipes excellent in HIC resistance, characterized in that a composite precipitate containing Ti and Mo is dispersed and precipitated at a rate of 5% or less.
0.35 ≦ Ceq1 ≦ 0.45 (1)
However, Ceq1 = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5
Ceq2 ≦ 0.6 (2)
However, Ceq2 = 1.5C + 1.6Mn / 6 + (1.3Cu + 1.3Ni) / 15 + (1.1Cr + 1.2Mo + V) / 5
However, each element is contained (mass%), and elements not contained are set to 0.
Ti / N> 4 (3)
0.5 ≦ C / (Mo + Ti + Nb + V) ≦ 3 (4)
However, the element which does not contain each element in atomic% is set to 0.   The steel composition further contains one or two of Nb: 0.005 to 0.07% and V: 0.005 to 0.1% by mass%, and the composite precipitate is Ti, Mo, and The high-strength steel sheet for line pipes having excellent HIC resistance according to claim 1, comprising one or two of (Nb, V).   The steel composition further contains one or two or more kinds selected from Cu: 0.5% or less, Ni: 0.5% or less, and Ca: 0.0005 to 0.005% by mass%. The high-strength steel sheet for line pipes having excellent HIC resistance according to claim 1 or 2.   The high strength steel sheet for line pipes having excellent HIC resistance according to any one of claims 1 to 3, wherein the composite precipitate is a fine precipitate having a diameter of less than 10 nm. The steel containing the chemical component according to any one of claims 1 to 3 is hot-rolled under conditions of a heating temperature of 1000 to 1300 ° C and a rolling end temperature of Ar ₃ or more, and then a cooling rate. : Accelerated cooling to 400 to 600 ° C. at 5 ° C./s or more, and immediately after cooling, temperature rising rate: reheating to 600 to 700 ° C. at 0.5 ° C./s or more, A manufacturing method for high strength steel sheets for line pipes with excellent HIC characteristics.   A line pipe excellent in HIC resistance, characterized by being manufactured using the steel plate according to any one of claims 1 to 4.

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