JP2017002349A - High tensile steel plate and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile steel plate having a plate thickness of 35 to 100 mm, a yield stress of 460 MPa or more and excellent in CTOD (Crack Tip Opening Displacement) property of a heat affected zone which is multi-pass welded, and a production method therefor.SOLUTION: There is provided the high tensile steel plate which has a component composition containing, by mass%, C:0.010 to 0.080%, Si:0.01 to 0.50%, Mn:0.20 to 1.80%, P:0.012% or less, S:0.0035% or less, sol.Al:0.010 to 0.060%, Ni:0.1 to 2.0%, Cr:1.0 to 4.0%, Nb:0.005 to 0.040%, Ti:0.005 to 0.025%, N:0.0020 to 0.0050%, satisfying 35Cr+8Mn≥63, and the balance Fe with inevitable impurities, and which has a yield stress YS of 460 MPa or more and absorption energy vE-80°C of 200 J or more at a test temperature of -80°C in a Charpy impact test.SELECTED DRAWING: None

Description

  The present invention relates to a high-strength steel plate used for steel structures such as ships, offshore structures, pressure vessels, and penstocks, and a method for producing the same, and in particular, a plate thickness of 35 to 100 mm, a yield stress YS of 460 MPa or more, The present invention relates to a thick high-strength steel sheet that not only excels in strength and toughness of the material, but also excels in low-temperature toughness in the weld heat-affected zone subjected to multi-layer welding.

  Steel plates (thick steel plates) used for steel structures such as ships, marine structures, pressure vessels, and penstock are welded and finished as structures having desired shapes and dimensions. Therefore, in these steel plates, from the viewpoint of ensuring safety as a steel structure, the strength of the base material is high and the toughness is excellent, as well as the welded portion (welded metal or weld heat affected zone). The toughness is also required to be excellent.

  As a standard for evaluating the toughness of a steel sheet, absorption energy by a Charpy impact test has been mainly used, but in recent years, a more reliable crack opening displacement test (CTOD test; Crack Tip Opening Displacement Test) is used. It is increasingly used. This test is intended to evaluate the resistance to brittle fracture by bending a specimen with fatigue precracking in the toughness evaluation section at three points and measuring the amount of crack opening (plastic deformation) just before fracture. Is. Hereinafter, the evaluation result obtained in the “CTOD test” is also referred to as “CTOD value” or “CTOD characteristic”.

  By the way, since the fatigue precrack is used in the CTOD test, a very small region serves as a toughness evaluation part. Therefore, when a local embrittlement region (hereinafter also referred to as a local embrittlement region) exists, a Charpy impact test is performed. Even when good toughness is obtained, a low CTOD value may be exhibited. The local embrittlement region is likely to occur in a weld heat affected zone (HAZ) that receives a complex thermal history when multi-layer welding is performed on a thick steel plate or the like, and in particular, a bond portion (welding) The boundary between the metal and the base material) and the weld heat affected zone, which is close to the bond part, and is reheated to the two-phase region during multi-layer welding (coarse grains are formed in the first cycle welding, resulting from subsequent welding) A region that is reheated to a two-phase region of ferrite and austenite by heat input, hereinafter also referred to as a “two-phase region reheating part”) tends to be a local embrittlement region. The toughness of the weld heat affected zone in the vicinity of the bond portion is reduced because the weld heat affected zone in the vicinity of the bond portion is exposed to a high temperature just below the melting point, the austenite grains become coarse, and the lower toughness is lower during subsequent cooling. This is because the tissue is easy to generate. Moreover, in the two-phase region reheated part near the bond part of the welding heat affected zone, an embrittled structure such as Woodman Stetten structure and island martensite MA is likely to be generated. That is why the toughness is reduced.

  Therefore, in order to suppress the toughness degradation of the above-mentioned weld heat affected zone, conventionally, a technology to finely disperse TiN in steel to suppress coarsening of austenite grains or to use as a ferrite transformation nucleus has been put into practical use. Has been. However, in the vicinity of the bond portion of the welding heat affected zone, there is a problem that the TiN precipitation effect disappears because it may be heated to a temperature range where TiN dissolves.

As other technologies, for example, in Patent Document 1 and Patent Document 2, rare earth elements (REM: Rare Earth Metal) are added together with Ti, and fine particles are dispersed in steel, thereby austenite grain growth. Techniques for suppressing and improving the toughness of the weld are disclosed.
In addition, the technology of finely dispersing Ti oxide in steel, the technology of combining BN ferrite nucleation ability and oxide dispersion, and further adding Ca and REM to control the form of sulfide, Techniques for increasing toughness have also been proposed. However, these techniques are intended for a steel sheet having a relatively low strength, a small amount of alloy elements, and a weld heat-affected zone having ferrite.

  Thus, as a technique for promoting the generation of ferrite in the weld heat affected zone, for example, Patent Document 3 discloses a technique for increasing the amount of Mn added to 2.0 mass% or more. However, when Mn is a steel material (slab) produced by continuous casting, it is an element that is easily segregated at the center. Easy to start. Therefore, excessive addition of Mn causes not only the weld heat affected zone but also the toughness of the base material.

  On the other hand, the two-phase region reheat part is a fragile bainite structure containing carbon in the martensite MA during cooling due to carbon re-concentration in the region transformed back to austenite by reheating to the two-phase region during multi-layer welding. As a result, the toughness is reduced. Therefore, a technique has been disclosed in which the steel component is made low C and low Si to suppress the formation of island martensite MA to improve toughness, and further, Cu is added to increase the base material strength (for example, Patent Documents 4 and 5). These techniques are intended to increase the strength by precipitating Cu by aging treatment. However, since a large amount of Cu is added, there is a problem that hot workability is lowered and productivity is hindered.

  By the way, in recent years, steel structures such as ships, offshore structures, pressure vessels, and penstocks have tended to increase in size, and accordingly, steel sheets used for steel structures have been made thicker and stronger. It has been. For steel plates used in steel structures, thick steel plates with a plate thickness of about 35 to 100 mm are mainly used. In order to obtain a yield stress YS of 420 MPa or higher, addition of alloying elements is required. Larger amounts are advantageous. However, as described above, the addition of a large amount of the alloy element makes it difficult to ensure the toughness of the bond portion and the two-phase region reheated portion.

  With respect to this problem, Patent Document 6 discloses a technique for optimizing the addition amounts of Mn and Cr in order to reduce the island-like martensite MA in the two-phase region reheating part of the welding heat affected zone. Yes. However, this technology is a technology related to a steel material for large heat input welding with a heat input amount exceeding 500 kJ / cm, and is not intended for a steel plate that is multilayer-welded with small to medium heat inputs with a heat input amount of 100 kJ / cm or less. Absent. Moreover, since the technique of patent document 6 does not add Nb, there is a problem that the control rolling technique cannot be applied and the toughness of the base material is not sufficient.

  Further, Patent Document 7 describes that by optimizing the carbon equivalent Ceq under a predetermined component composition, a yield stress of 420 MPa or more and good low-temperature toughness, particularly CTOD characteristics, even for a steel component with a large amount of alloy elements. A technique for achieving both has been proposed. This technology can be applied to steel structures, and it is possible to produce a high-tensile steel material having a yield stress of 420 MPa or more and excellent CTOD characteristics of a weld heat-affected zone obtained by multi-layer welding by small to medium heat input. became.

Japanese Patent Publication No. 03-053367 JP 60-184663 A JP 2003-147484 A JP 05-186823 A Japanese Patent Laid-Open No. 2001-335484 Japanese Patent No. 5365146 JP 2012-184500 A

  As described above, in recent years, steel structures tend to be heavy and long, and accordingly, in the field of ships and marine structures, high strength (high yield stress), thick plate thickness, and welding heat There is a growing demand for steel plates having excellent low temperature toughness of the affected zone, particularly steel plates having a plate thickness of 35 to 100 mm, a yield stress of 460 to 620 MPa, and a multilayer heat-welded weld heat affected zone having excellent CTOD characteristics. .

  In response to the above-described demand, the technique disclosed in Patent Document 7 described above realizes a yield stress of 420 MPa or more and good low-temperature toughness (hereinafter also referred to as CTOD characteristics) even in a steel component system with many alloying elements. The way to do it was pioneered. However, steel sheets having a thickness of more than 50 mm have not yet achieved the same strength characteristics as those of steel sheets having a thickness of 50 mm or less. That is, in the technique described in Patent Document 7, a steel sheet having a thickness of 50 mm or less can provide a strength with a yield stress of 500 MPa or more. For example, a steel sheet with a thickness of 70 mm can have a yield stress of at most 462 MPa. It has been. However, simply adding a large amount of an alloy element with the aim of increasing the strength will degrade the CTOD characteristics. Thus, depending on the conventional technique including the technique disclosed in Patent Document 7, the plate thickness is 35 to 100 mm, the yield stress is 460 to 620 MPa, and the CTOD characteristics in which the weld heat-affected zone is excellent in multi-layer welding. A steel plate having no has not yet been obtained.

  The present invention has been made in view of the above-described problems of the prior art, and the purpose thereof is multilayer overlay welding with a plate thickness of 35 to 100 mm, a yield stress of 460 MPa or more, and a heat input of 100 kJ / cm or less. An object of the present invention is to provide a high-tensile steel sheet having excellent CTOD characteristics in the heat affected zone and to propose an advantageous manufacturing method thereof.

In order to solve the above-mentioned problems, the inventors have conducted intensive studies focusing on the influence of the composition of steel on the weld. As a result, the following was found.
i) Since the CTOD characteristic is evaluated with a test piece having a full thickness of the steel sheet, the central segregation portion is the starting point of the fracture. Therefore, in order to improve the CTOD characteristics of the weld heat affected zone, the content of elements that are easily segregated at the center, specifically, the contents of C, Mn, P, Ni, and Nb are controlled within an appropriate range, and the hardness of the center segregated zone is increased. It is important to suppress the increase in height.
ii) In order to improve the toughness of the weld heat affected zone, TiN dispersed uniformly and finely in steel is precipitated to suppress the austenite grain coarsening in the vicinity of the bond portion of the weld heat affected zone. It is valid.
iii) Since the CTOD value and the strength are in a trade-off relationship, simply increasing Ceq to increase the strength decreases the CTOD characteristics, but the low C-low P-high Ni component system By doing so, the balance of strength-CTOD characteristics can be improved.
iv) Further, in order to further improve the toughness of the weld heat affected zone, the Ca compound (CaS) added for the purpose of controlling the form of sulfide is crystallized to improve the toughness of the weld heat affected zone. It is effective to use.
v) Moreover, the steel plate excellent in intensity | strength and toughness can be manufactured by controlling a slab heating temperature and rolling conditions in the manufacturing method of a steel plate, setting a steel raw material as a specific component composition.

The gist of the present invention developed based on the above findings is as follows.
[1] By mass%, C: 0.010 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.80%, P: 0.012% or less, S: 0.0035% or less, sol. Al: 0.010-0.060%, Ni: 0.1-2.0%, Cr: 1.0-4.0%, Nb: 0.005-0.040%, Ti: 0.005- 0.025%, N: 0.0020 to 0.0050%, further containing Cr satisfying 35Cr + 8Mn ≧ 63, with the balance being composed of Fe and inevitable impurities, yield stress YS Is a high-tensile steel plate having an absorption energy vE-80 ° C. of 200 J or more at a test temperature of −80 ° C. in a Charpy impact test.

  [2] Further, in mass%, Cu: less than 1.0%, Mo: 0.05 to 0.50%, V: 0.005 to 0.05%, B: 0.0005 to 0.0030%, The high-tensile steel plate according to the above [1], containing one or more selected from Ca: 0.0005 to 0.0050% and Mg: 0.0002 to 0.0030% .

[3] The high-tensile steel plate according to [2], further containing, by mass%, O: 0.0030% or less, and the Ca, S, and O satisfying the following formula (1):
0 <{Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) <1 (1)
However, each element symbol in the above formula is the content (% by mass) of each element.

  [4] After heating the steel material having the component composition according to any one of [1] to [3] to a heating temperature of 1030 to 1200 ° C., the cumulative rolling reduction at a temperature of 950 ° C. or more is 30% or more, Manufacture of high-strength steel sheets that are hot-rolled at a cumulative reduction rate of 30 to 70% at a temperature of less than 950 ° C and accelerated to a cooling stop temperature of 600 ° C or less at a cooling rate of 1.0 ° C / s or more. Method.

  [5] The method for producing a high-strength steel sheet according to [4], further including performing tempering at a temperature of 450 to 650 ° C. after the accelerated cooling.

  In the present invention, the high-tensile steel plate refers to a steel plate having a yield stress of 460 MPa or more.

  According to the present invention, welding heat that is suitable for large steel structures such as ships and marine structures, and is multi-layer welded with a plate thickness of 35 to 100 mm, a yield stress of 460 MPa or more, and a heat input of 100 kJ / cm or less. It is possible to stably produce and provide a high-tensile steel plate having excellent CTOD characteristics in the affected area.

  The high-tensile steel plate according to the present invention is mass%, C: 0.010 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.80%, P: 0.00. 012% or less, S: 0.0035% or less, sol. Al: 0.010-0.060%, Ni: 0.1-2.0%, Cr: 1.0-4.0%, Nb: 0.005-0.040%, Ti: 0.005- 0.025%, N: 0.0020 to 0.0050%, further containing Cr satisfying 35Cr + 8Mn ≧ 63, with the balance being composed of Fe and inevitable impurities, yield stress YS Is 460 MPa or more, and the absorbed energy vE-80 ° C. at a test temperature of −80 ° C. in a Charpy impact test is 200 J or more. The high-tensile steel plate of the present invention is used with a plate thickness of 35 to 100 mm.

  Hereinafter, the high-tensile steel plate of the present invention will be described. First, the component composition that the high-tensile steel sheet of the present invention should have will be described. In the following, “%” notation of the component composition means mass%.

C: 0.010-0.080%
C is an element necessary for ensuring the strength of the base material as a high-tensile steel plate. When the C content is less than 0.010%, the hardenability is lowered. Therefore, in order to ensure the target strength (yield stress YS ≧ 460 MPa), a hardenability improving element such as Cu, Ni, Cr, or Mo is used. It is necessary to contain a large amount, leading to an increase in raw material costs and a decrease in weldability. On the other hand, if the C content exceeds 0.080%, the toughness of the welded portion decreases. Therefore, the C content is in the range of 0.010 to 0.080%. Preferably, the C content is in the range of 0.015 to 0.075%. More preferably, the C content is in the range of 0.020 to 0.065%.

Si: 0.01 to 0.50%
Si is an element to be contained as a deoxidizer and to increase the strength of the base material, and it is necessary to contain 0.01% or more. However, if Si is contained in a large amount exceeding 0.50%, the weldability and the toughness of the weld are lowered. Therefore, the Si content is in the range of 0.01 to 0.50%. Preferably, the Si content is in the range of 0.01 to 0.35%.

Mn: 0.20 to 1.80%
Mn needs to be contained in an amount of 0.20% or more in order to ensure the strength of the base material and the weld. However, when Mn is contained exceeding 1.80%, not only the weldability is lowered, but also the hardenability becomes excessive and the toughness of the base material and the welded portion is lowered. Therefore, the Mn content is in the range of 0.20 to 1.80%. Preferably, the Mn content is in the range of 0.30 to 1.80%.

P: 0.012% or less P is an impurity element inevitably mixed in the steel sheet, and is also an element that lowers the toughness of the base material and the weld. In particular, when the P content exceeds 0.012%, the CTOD characteristics are remarkably lowered. Therefore, in the present invention, the upper limit of the P content is limited to 0.012%. Preferably, the P content is 0.008% or less.

S: 0.0035% or less S is an impurity element inevitably mixed in the steel sheet, and if contained in excess of 0.0035%, the toughness of the base metal and the welded portion is lowered. Therefore, the upper limit of the S content is 0.0035%. Preferably, the S content is 0.0030% or less.

sol. Al: 0.010 to 0.060%
Al is an element contained for deoxidizing molten steel. It is necessary to contain 0.010% or more of Al. On the other hand, sol. When the content of Al exceeds 0.060%, the toughness of the base metal and the welded portion is lowered, and it is mixed into the weld metal by dilution by welding, so that the toughness is lowered. The upper limit of Al is 0.060%. Preferably, sol. The range of Al is 0.017 to 0.055%. More preferably, sol. It is 0.020 to 0.055% of Al.

Ni: 0.1 to 2.0%
Ni is an element effective not only for improving the strength and toughness of the steel sheet but also for improving the CTOD characteristics of the weld. In order to obtain these effects, it is necessary to contain 0.1% or more of Ni. On the other hand, Ni is an expensive element, and excessively containing Ni causes slab surface defects during casting, so the upper limit of Ni content is 2.0%. Preferably, the Ni content is in the range of 0.1-1.8%.

Cr: 1.0-4.0%
Cr is an element useful for improving the hardenability of the steel sheet and ensuring the strength and toughness of the base material. Moreover, it is a ferrite stabilizing element and has the effect of preventing excessive austenite stabilization by Mn and suppressing the formation of island martensite MA. That is, the inclusion of Mn alone only enhances hardenability, but by containing Cr as a ferrite stabilizing element, excessive austenite stabilization by Mn is mitigated, and the formation of island martensite MA is suppressed. The In order to acquire such an effect, it is necessary to contain 1.0% or more of Cr. However, if Cr is excessively contained, the hardness of the weld heat affected zone increases and the toughness decreases, so the upper limit of the Cr content is 4.0%. Preferably, the Cr content is in the range of 1.0 to 3.8%.

  In the present invention, in order to obtain the effect of suppressing the formation of island martensite MA in the weld heat affected zone, it is necessary to satisfy 7Cr + 18Mn ≦ 63, but as described above, the Mn content is 0.00. Since it is 20 to 1.80% and the Cr content is 1.0 to 4.0%, the above 7Cr + 18Mn ≦ 63 is necessarily satisfied.

Nb: 0.005 to 0.040%
Nb is an element that contributes to the formation of a non-recrystallized region in the low temperature region of austenite. By hot rolling in this temperature region, the microstructure of the base material can be refined and the toughness can be increased. It is also effective in improving hardenability and resistance to temper softening and is an element effective in improving the strength of the base material. In order to acquire said effect, it is necessary to contain Nb 0.005% or more. On the other hand, if the Nb content exceeds 0.040%, the toughness of the weld heat affected zone decreases, so the upper limit of the Nb content is 0.040%. Preferably, the Nb content is in the range of 0.007 to 0.035%.

Ti: 0.005-0.025%
Ti precipitates as TiN when the molten steel is solidified, and suppresses austenite coarsening in the welded portion, thereby contributing to improvement in the toughness of the welded portion. However, when Ti is contained at less than 0.005%, the effect of improving the toughness of the welded portion is small. On the other hand, when Ti is contained exceeding 0.025%, TiN becomes coarse and the toughness improving effect of the base material and the welded portion cannot be obtained. Therefore, Ti content is taken as 0.005 to 0.025% of range. Preferably, the Ti content is in the range of 0.006 to 0.023%.

N: 0.0020 to 0.0050%
N has an effect of refining crystal grains by combining with Ti or Al to form precipitates and improving the toughness of the base material. It is also an element necessary for forming TiN that suppresses the coarsening of the structure of the heat affected zone. In order to express these effects, N is contained 0.0020% or more. However, if N is contained in excess of 0.0050%, the toughness of the base metal and the welded portion is remarkably reduced due to an increase in the solid solution N, and the strength is lowered due to the decrease in the solid solution Nb accompanying the formation of TiNb composite precipitates. Therefore, the upper limit of the N content is set to 0.0050%. Preferably, the N content is in the range of 0.0020 to 0.0047%.

35Cr + 8Mn ≧ 63
Cr is a ferrite stabilizing element, and has an effect of relaxing excessive austenite stabilization by Mn and suppressing generation of island martensite MA in the weld heat affected zone. That is, the addition of Mn alone only enhances the hardenability, but by adding Cr, which is a ferrite stabilizing element, excessive austenite stabilization by Mn is mitigated and the formation of island martensite MA is suppressed. . In order to further enhance the above effect, it is necessary to contain Cr so as to satisfy 35Cr + 8Mn ≧ 63 in relation to Mn.

  In the high-tensile steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities. However, even if it is other than the said component, if it is in the range which does not injure the effect of this invention, it does not refuse containing. In addition, as an inevitable impurity, O can be contained, for example in 0.01% or less of range. Preferably, O is contained in the range of 0.003% or less.

  The high-tensile steel sheet of the present invention can further contain one or more selected from Cu, Mo, V, B, Ca, and Mg in the following range in addition to the above component composition.

Cu: less than 1.0% Cu is an element effective for increasing the strength of the base material, and in order to obtain the above effect, it is preferable to contain 0.1% or more of Cu. However, when Cu is contained in an amount of 1.0% or more, the hot workability is lowered, so the Cu content is preferably less than 1.0%. More preferably, the Cu content is 0.7% or less.

Mo: 0.05 to 0.50%
Mo is an effective element for increasing the strength of the base material, and has a particularly large strength improvement effect in a high-tensile steel plate. In order to acquire the said effect, it is preferable to contain 0.05% or more of Mo. However, since excessive inclusion of Mo adversely affects toughness, the upper limit of the Mo content is preferably 0.50%. More preferably, the Mo content is in the range of 0.10 to 0.45%.

V: 0.005-0.05%
V is an element effective for improving the strength and toughness of the base material, and therefore can be contained in an amount of 0.005% or more. However, if the V content exceeds 0.05%, the toughness is reduced, so when V is contained, the content is preferably in the range of 0.005 to 0.05%. More preferably, the V content is in the range of 0.005 to 0.045%.

B: 0.0005 to 0.0030%
B is segregated at the austenite grain boundary when cooled from the austenite region, and has the effect of increasing the strength by improving the hardenability. In order to acquire this effect, it is preferable to contain B 0.0005% or more. This effect is achieved when the B content is saturated at 0.0030%, and when it exceeds 0.0030%, the formation of island martensite MA is promoted in the weld heat affected zone, and the toughness is reduced. There is. Therefore, it is preferable to contain B in 0.0005 to 0.030% of range.

Ca: 0.0005 to 0.0050%
Ca combines with S to form CaS, and by fixing S, the form of sulfide is controlled and the toughness is improved. In order to acquire this effect, it is preferable to contain 0.0005% or more of Ca. However, even if Ca is contained exceeding 0.0050%, the effect is saturated. Therefore, Ca is preferably contained in the range of 0.0005 to 0.0050%.

Mg: 0.0002 to 0.0030%
Mg is an element that improves toughness by fixing S in the same manner as Ca. In order to acquire this effect, it is preferable to contain 0.0002% or more of Mg. However, even if Mg exceeds 0.0030%, the effect is saturated. Therefore, it is preferable to contain Mg in the range of 0.0002 to 0.0030%.

O: 0.0030% or less, and {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) is greater than 0 and less than 1, As described above, the high-tensile steel sheet of the present invention is unavoidable. O may be contained as an impurity, but if O is contained in an amount exceeding 0.0030%, the toughness of the base material may be lowered. Therefore, the upper limit of the O content is preferably 0.0030%. More preferably, the O content is 0.0025% or less.

Moreover, it is preferable that Ca, S, and O demonstrated above satisfy | fill the following (1) Formula, and the high-tensile steel plate of this invention contains.
0 <{Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) <1 (1)
However, each element symbol in the above formula (1) is the content (% by mass) of each element.
{Ca− (0.18 + 130 × Ca) × O} / (1.25 × S), which is the middle side of the above formula (1), indicates the ratio of the atomic concentrations of Ca and S that are effective for sulfide morphology control. It is an index value (also called ACR (Atomic Concentration Ratio)), and the form of sulfide can be estimated.

  Since CaS, which is a sulfide of Ca, crystallizes at a lower temperature than oxides, it is advantageous for uniform fine dispersion. Therefore, if CaS is crystallized and solid solution S is ensured even after CaS crystallization, MnS precipitates on the surface of the crystallized CaS to form a composite sulfide that is difficult to dissolve even at high temperatures. . Further, since a Mn dilute band is formed around the MnS, the ferrite transformation is further promoted.

In order to finely disperse and crystallize CaS as described above, it is necessary to control the Ca content and the S and O contents in the molten steel at the time of Ca addition to an appropriate range. By controlling in the range of (1) above, CaS serving as ferrite transformation nuclei can be finely dispersed.
When the ACR is 0 or less, CaS does not crystallize, and S precipitates in the form of MnS alone, so that the ferrite formation nuclei in the weld heat affected zone may not be obtained. Further, MnS precipitated alone can be elongated at the time of rolling to cause a decrease in the toughness of the base material. On the other hand, when the ACR is 1 or more, S is completely fixed by Ca, and MnS acting as a ferrite nuclei does not precipitate on CaS, and fine dispersion of the composite sulfide that becomes a ferrite nuclei cannot be realized. The toughness improving effect of the weld heat affected zone may not be obtained. Therefore, when the ACR value is greater than 0 and less than 1, MnS is deposited on CaS to form a composite sulfide, which effectively functions as a ferrite nuclei. A more preferable ACR value is in the range of 0.2 to 0.8.

Next, the manufacturing method of the high-tensile steel plate of this invention is demonstrated.
The high-strength steel sheet of the present invention described above is prepared by melting steel in a converter, electric furnace, vacuum melting furnace, etc., as in the case of conventional high-strength steel sheet manufacturing methods. After continuously casting a molten steel adjusted to a composition suitable for the present invention in a conventional refining process for performing next refining to obtain a steel material (slab), the slab is reheated and hot-rolled to obtain a desired plate. It can be manufactured through a thickness and accelerated cooling process. Moreover, it can also manufacture through the process of further tempering the steel plate after the said accelerated cooling.

However, in the above process, it is important that the slab reheating temperature and the rolling reduction in hot rolling, the cooling rate of accelerated cooling, and the conditions of the tempering treatment to be performed as necessary thereafter satisfy the following requirements. .
In the present invention, unless otherwise specified, the steel plate temperature is the temperature at the center of the plate thickness (1/2 t portion of the plate thickness t). The temperature at the central portion of the plate thickness can be obtained by heat transfer calculation using a difference method or the like from the plate thickness, surface temperature, cooling conditions, and the like.

Heating temperature (slab reheating temperature): 1030 to 1200 ° C
The slab reheating temperature before the hot rolling needs to be 1030 ° C. or higher in order to surely press-bond the casting defects existing inside the slab by hot rolling. However, when heated to a temperature exceeding 1200 ° C., TiN precipitated during solidification becomes coarse and the toughness of the base material and the welded portion decreases, so the upper limit of the heating temperature is 1200 ° C. Preferably it is the range of 1030-1170 degreeC.

Cumulative rolling reduction of hot rolling in a temperature range of 950 ° C. or higher: 30% or higher Cumulative rolling in a recrystallization temperature range of 950 ° C. or higher in order to refine austenite grains using recrystallization during hot rolling The rate needs to be 30% or more. If the cumulative rolling reduction in the temperature range of 950 ° C. or higher is less than 30%, abnormal coarse particles generated during reheating of the slab may remain, which may adversely affect the toughness of the base material. Preferably, the cumulative rolling reduction in the temperature range of 950 ° C. or higher is 35% or higher. The upper limit of the cumulative rolling reduction at this stage is not particularly limited as long as the rolling reduction in a temperature range of less than 950 ° C. can be secured.

Cumulative rolling reduction of hot rolling in a temperature range below 950 ° C .: 30 to 70%
Since austenite grains rolled in a temperature range below 950 ° C. do not recrystallize sufficiently, the austenite grains after rolling remain flatly deformed and have a high internal strain containing a large amount of defects such as deformation bands inside. Become an organization. The internal strain acts as a driving force for ferrite transformation and promotes ferrite transformation. However, if the cumulative rolling reduction in the temperature range of less than 950 ° C. is less than 30%, the strain energy is not sufficiently accumulated and ferrite transformation is difficult to occur, so that the toughness of the base material is lowered. On the other hand, if the cumulative rolling reduction in the temperature range of less than 950 ° C. exceeds 70%, the formation of polygonal ferrite is promoted and high strength and high toughness cannot be achieved at the same time. Preferably, the cumulative rolling reduction in the temperature range below 950 ° C. is in the range of 40 to 65%.

  In addition, it is preferable to make the completion | finish temperature of hot rolling into the range of 650-850 degreeC. If the end temperature of hot rolling is less than 650 ° C., the processed ferrite may remain and the toughness may decrease. On the other hand, when the end temperature of hot rolling exceeds 850 ° C., the structure becomes coarse and the toughness may be lowered.

Cooling stop temperature: 600 ° C. or lower, and cooling rate to the cooling stop temperature: 1.0 ° C./s or higher After the hot rolling is completed, the cooling rate is set to 1.0 ° C./s or higher and cooling is stopped at 600 ° C. It is important to accelerate cooling to temperature. When the cooling rate is less than 1.0 ° C./s, the formation of ferrite with low strength cannot be suppressed, and thus sufficient strength of the base material cannot be obtained. Therefore, the cooling rate is set to 1.0 ° C./s or more. Preferably, the cooling rate is 1.2 ° C./s or higher. In addition, although there is no restriction | limiting in particular about the upper limit of a cooling rate, From a viewpoint of ensuring the toughness of a base material, said cooling rate has preferable 30 degrees C / s or less.
The reason why the cooling stop temperature is 600 ° C. or lower is that when the cooling stop temperature is higher than 600 ° C., the fraction of the structure such as ferrite + pearlite and upper bainite becomes high, and both high strength and high toughness are achieved. Because it will not. However, the cooling stop temperature for accelerated cooling is preferably 350 ° C. or higher from the viewpoint of suppressing the generation of a hard phase inferior toughness such as island martensite MA. In addition, when performing a tempering process after the said accelerated cooling, there is no restriction | limiting in particular in the minimum of the cooling stop temperature of the said accelerated cooling.

Tempering temperature: 450-650 ° C
When the tempering treatment is performed after the accelerated cooling, the tempering temperature is preferably in the range of 450 to 650 ° C. at the plate thickness center temperature of the steel plate. If the tempering temperature is less than 450 ° C., sufficient tempering effect may not be obtained. On the other hand, when tempering is performed at a temperature exceeding 650 ° C., carbonitrides are coarsely precipitated and the toughness may be reduced or the strength may be reduced. Therefore, the tempering temperature is preferably in the range of 450 to 650 ° C. Moreover, it is preferable to perform the heating method of a tempering process by induction heating from a viewpoint of suppressing the coarsening of the carbide | carbonized_material at the time of tempering.
The tempering time is preferably in the range of 10 to 300 min.

  As described above, the high-tensile steel plate of the present invention suppresses the austenite grain coarsening in the weld heat affected zone, and further finely disperses ferrite transformation nuclei that do not melt even at high temperatures, thereby providing a weld heat affected zone. Refine the structure. Therefore, high toughness can be obtained even in the weld heat affected zone. In particular, in the region reheated to the two-phase region by the thermal cycle during multi-layer welding, the structure of the weld heat-affected zone is refined by the first welding, so only the toughness of the untransformed region is improved. In addition, since the austenite grains that are retransformed can also be made finer, a reduction in toughness due to heat input during welding can be greatly reduced.

After smelting steels of symbols A to X having various component compositions shown in Table 1 and making them steel materials (slabs) by a continuous casting method, they are hot-rolled under various conditions shown in Table 2 and accelerated cooling. Or a further tempering treatment to obtain a thick steel plate No. 50 having a thickness of 50 to 100 mm. 1-30 were manufactured.
Subsequently, the said thick steel plate was used for the following evaluation tests.

<Evaluation of base material>
(Strength characteristics)
A JIS No. 4 test piece having a length direction perpendicular to the rolling direction of the steel plate was taken from the position of the steel plate thickness 1/2, and yield stress YS, tensile strength TS, and yield ratio YR (= ( YS × 100) / TS (%)) was measured, and those satisfying YS ≧ 460 MPa and TS ≧ 570 MPa were evaluated as having good strength properties of the base material.
(Toughness characteristics)
A V-notch test piece defined in JIS Z2202 whose length direction is perpendicular to the rolling direction of the steel plate is taken from the position of the steel plate thickness 1/2, and the absorbed energy at −80 ° C. in the Charpy impact test. When vE- _{80 ° C} was measured, a material satisfying vE- _{80 ° C} ≥ 200 J was evaluated as having good toughness characteristics of the base material.

<Evaluation of weld zone>
(Toughness characteristics)
After collecting a test piece from a thick steel plate and giving a K-shaped groove, multi-layer welding is performed by submerged arc welding with a welding heat input of 35 kJ / cm to produce a welded joint. A V-notch test piece having the straight bond portion as a notch position was collected, and the absorbed energy vE- _{80 ° C} at _{-80 ° C} was measured by a Charpy impact test. The above test was performed three times under each condition, and a welded joint having an average vE- _{80 ° C} of 150 J or more was evaluated as having good toughness.
(CTOD characteristics)
A welded joint was prepared in the same manner as described above, and a CTOD test piece having a straight bond portion as a notch position of a three-point bending CTOD test piece was collected, and a CTOD value (−60 ° C.) at −60 ° C. was measured. . Three tests were performed under each condition, and those having a CTOD value (δ-60 ° C.) of 0.50 mm or more were evaluated as having good CTOD characteristics of the welded joint.

The results of the above measurements are also shown in Table 2. From Table 2, steels of reference signs A to E shown in Table 1 all satisfy the composition of the present invention, and steel plates of inventive examples manufactured under conditions suitable for the present invention using the steel slabs. Are excellent in the strength characteristics and toughness of the base material and the weld.
On the other hand, the steel plate of the comparative example manufactured on the conditions which deviate from this invention using the steel slab which satisfy | fills the component composition of this invention, or the present invention using the slab of the steel which does not satisfy | fill the component composition of this invention It can be seen that the steel plate of the comparative example manufactured under the conditions to be used is inferior in strength characteristics and toughness of the base material and the welded portion to the steel plate of the above-described invention example.

  The high-tensile steel plate of the present invention can be applied to civil engineering / architectural fields such as buildings and bridges, as well as steel structures such as ships, offshore structures, pressure vessels, and penstocks.

Claims (5)

% By mass
C: 0.010 to 0.080%,
Si: 0.01 to 0.50%,
Mn: 0.20 to 1.80%
P: 0.012% or less,
S: 0.0035% or less,
sol. Al: 0.010 to 0.060%,
Ni: 0.1 to 2.0%,
Cr: 1.0-4.0%,
Nb: 0.005 to 0.040%,
Ti: 0.005 to 0.025%,
N: 0.0020 to 0.0050% is contained, and
Cr is contained satisfying 35Cr + 8Mn ≧ 63, the remainder has a component composition consisting of Fe and inevitable impurities, the yield stress YS is 460 MPa or more, and the absorbed energy vE-80 at a test temperature of −80 ° C. in the Charpy impact test A high-tensile steel plate having a temperature of 200J or higher.   Furthermore, by mass%, Cu: less than 1.0%, Mo: 0.05 to 0.50%, V: 0.005 to 0.05%, B: 0.0005 to 0.0030%, Ca: 0 The high-tensile steel sheet according to claim 1, containing one or more selected from .0005 to 0.0050% and Mg: 0.0002 to 0.0030%. Furthermore, the high-tensile steel plate according to claim 2, which is contained by mass%, O: 0.0030% or less, and wherein the Ca, S and O satisfy the following formula (1).
0 <{Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) <1 (1)
However, each element symbol in the above formula is the content (% by mass) of each element.   After heating the steel raw material which has a component composition of any one of Claims 1-3 to the heating temperature of 1030-1200 degreeC, the cumulative reduction rate in the temperature of 950 degreeC or more is 30% or more and less than 950 degreeC. A method for producing a high-strength steel sheet, which is hot-rolled at a cumulative rolling reduction of 30 to 70% and accelerated cooling to a cooling stop temperature of 600 ° C. or lower at a cooling rate of 1.0 ° C./s or higher.   The method for producing a high-tensile steel sheet according to claim 4, further comprising a tempering treatment at a temperature of 450 to 650 ° C after the accelerated cooling.