KR20210053218A - High-tension steel plate having excellent low-temperature toughness of base metal and joint, and process for producing same - Google Patents

Abstract

The present invention provides a high-tension steel plate suppressing an NI amount to 1.1 mass% or lower and having excellent low-temperature toughness, particularly, low-temperature toughness of a joint and base metal while having high strength and a manufacturing method thereof. The high-tension steel plate has a prescribed component composition. BI defined by the following formula (1) is higher than or equal to 5.3 and lower than or equal to 6.2. The fraction of ferrite occupying in the entire structure is 85 area% or higher, and the fraction of perlite is lower than 10 area%. The average circle-equivalent crystal grain size of the ferrite is 7μm or lower, and the standard deviation of the average circle-equivalent crystal grain size is 3.7μm or lower. BI=12×(C+5Nb)+2Mn+Cu+Ni+300B · · · (1) In formula (1), C, Nb, Mn, Cu, Ni, and B represent the content of C, Nb, Mn, Cu, Ni, and B in steel represented in mass%. Elements which are not included are calculated with 0 mass%.

Description

High-tension steel plate having excellent low-temperature toughness of base material and joint, and its manufacturing method {HIGH-TENSION STEEL PLATE HAVING EXCELLENT LOW-TEMPERATURE TOUGHNESS OF BASE METAL AND JOINT, AND PROCESS FOR PRODUCING SAME}

The present invention relates to a high-tensile steel sheet having excellent low-temperature toughness between a base material and a joint, and a method for producing the same.

Steel plates applied to pressure vessels, ships, offshore structures, etc. are often used in a low-temperature environment, and are required to have high strength and excellent low-temperature toughness (hereinafter sometimes referred to as "low temperature toughness"). In particular, in recent years, from the viewpoint of safety, it is required to exhibit high toughness at a lower temperature. In addition, in particular, with the increase in size of structures such as LPG tanks, there is an increasing demand for steel sheets having excellent low-temperature toughness of joints formed by welding together with the high strength and excellent low-temperature toughness of the base material. Moreover, excellent weldability is also required for the steel sheet.

Although the addition of an alloy is effective for improving the strength, since the addition of the alloy causes a decrease in the low-temperature toughness of the base material and the joint, it is extremely difficult to achieve both high strength and low-temperature toughness.

As one of the effective methods for improving both the strength and toughness properties of the steel sheet, it is possible to include Ni, which is an alloying element. Until now, many steel sheets containing Ni have been proposed, but as represented by 3.5% Ni steel or 9% Ni steel, it is a reality that the effect cannot be exhibited to the maximum unless a large amount of Ni is contained. On the other hand, as a steel sheet containing about 0.5 to 2% of Ni, for example, the technique of Patent Document 1 has been proposed. In Patent Document 1, when the structure is mainly composed of bainite and martensite, or bainite or martensite, and the minimum short side length of the lath-like structure is 1.3 μm or less, and further includes a bainite structure, the bay By making the ratio of the MA transformation product containing retained austenite as a type of island martensite contained in the knight structure and having an aspect ratio of 5 or more and an area ratio of less than 5%, the resistance to fatigue crack propagation is increased. It is obtaining excellent high-strength steel.

Japanese Patent No. 3741078

Although the steel sheet of Patent Document 1 has high strength, the base metal toughness at a lower temperature is not realized, and it is difficult to satisfy both the high strength and the excellent low-temperature toughness of the base material. Further, it is also required to have excellent joint toughness at low temperatures, but in Patent Document 1, the improvement of the low-temperature toughness of the joint has not been studied. In addition, it is required to further reduce the amount of Ni from the viewpoint of cost and to satisfy both characteristics of the high strength and low temperature toughness.

The present invention has been made in view of the above circumstances, and its object is to provide a high-strength steel sheet having high strength and excellent low-temperature toughness, in particular, low-temperature toughness of a base material and a joint, and a method for producing the same, in addition to suppressing the amount of Ni to 1.1% by mass or less. It's in what it offers.

Aspect 1 of the present invention, the component composition,

C: 0.03% by mass to 0.10% by mass,

Si: 0.05% by mass to 0.40% by mass,

Mn: 0.90 mass% to 1.60 mass%,

P: more than 0 mass%, 0.010 mass% or less,

S: more than 0 mass%, 0.010 mass% or less,

Al: 0.010% by mass to 0.060% by mass,

Ni: 0.50 mass% to 1.1 mass%,

Nb: 0.007% by mass to 0.022% by mass,

Ti: 0.007% by mass to 0.017% by mass,

N: 0.0025 mass% to 0.0060 mass%, and

The balance consists of iron and unavoidable impurities,

BI defined by the following formula (1) is 5.30 or more and 6.2 or less,

The low-temperature toughness of the base material and the joint, wherein the proportion of ferrite occupied in the entire structure is 85 area% or more, and the proportion of pearlite is less than 10 area%, and the average crystal grain size per source of ferrite is 7 μm or less, and its standard deviation is 3.7 μm or less. This is an excellent high-tensile steel plate.

BI=12×(C+5Nb)+2Mn+Cu+Ni+300B...(1)

In formula (1), C, Nb, Mn, Cu, Ni, and B represent the content in the steel of C, Nb, Mn, Cu, Ni, and B represented by mass%, respectively, and elements not included are 0% by mass. It is calculated as.

Aspect 2 of the present invention, the component composition, further,

B: more than 0% by mass, 0.002% by mass or less,

Ca: more than 0 mass%, 0.003 mass% or less, and

Cu: A high-tensile strength steel sheet having excellent low-temperature toughness between the base metal and the joint according to the first aspect, containing at least one element selected from the group consisting of more than 0% by mass and 0.35% by mass or less.

Aspect 3 of the present invention is a method of manufacturing a high-tensile steel sheet according to aspect 1 or 2,

After heating the steel sheet having the component composition described in the aspect 1 or 2, hot rolling is performed so as to satisfy the conditions of the following (a) to (c), and after hot rolling, the rolling end temperature-(Ar ₃ transformation point -30° C.) The high-tensile strength steel sheet with excellent low-temperature toughness of the base material and the joint is cooled at an average cooling rate of 0.6°C/s or more and 10°C/s or less from the control cooling start temperature to _{the control cooling end temperature of the Ar 3 transformation point to 500°C.} It is a manufacturing method.

(a) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is 950 to 875°C, it is reduced by a cumulative reduction ratio of 35% or more.

(b) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is 820°C or less and the Ar ₃ transformation point or more, it is reduced by a cumulative reduction ratio of 30% or more.

(c) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is in a temperature range of less than 875°C and more than 820°C, and in a two-phase temperature range, no reduction is performed.

According to the present invention, it is possible to provide a high-tensile steel sheet having high strength and excellent low-temperature toughness, particularly low-temperature toughness of a base material and a joint, and a method for producing the same, in addition to suppressing the amount of Ni to 1.1% by mass or less.

1 is a graph showing the relationship between the MA (island martensite) fraction of the joint and the joint toughness vE.
2 is a graph showing the relationship between the BI and the MA fraction of the joint.
3 is a graph showing the relationship between BI and the product of the tensile strength and the low-temperature toughness of the base material (TS×vTrs).

In order to provide a high-strength steel sheet having high strength and low temperature toughness, in particular, excellent in base metal toughness at low temperature and joint toughness at low temperature, and a method for producing the same, the present inventors have suppressed the amount of Ni to 1.1% by mass or less. The structure, component composition, and manufacturing method of the steel sheet, which can maximize the effect of improving the strength-base metal low-temperature toughness balance due to Ni, were studied. As a result, when the component composition is within the range of the present invention, the parameter BI is within a predetermined range, and manufactured by the method of the present invention, and the structure is controlled, it is possible to achieve both high strength and low temperature toughness of the base material. In addition, it was found that a steel sheet having excellent joint toughness was obtained.

In the present invention, in the component composition of the steel sheet, the BI specified by the following formula (1) satisfies 5.30 or more and 6.2 or less. Hereinafter, first, this parameter BI will be described.

BI=12×(C+5Nb)+2Mn+Cu+Ni+300B...(1)

In formula (1), C, Nb, Mn, Cu, Ni, and B represent the content in the steel of C, Nb, Mn, Cu, Ni, and B represented by mass%, respectively, and elements not included are 0% by mass. It is calculated as.

The present inventor investigated the relationship between the low-temperature toughness of the joint and the structure of the joint in order to ensure the joint toughness at a low temperature. As shown in Examples to be described later, in order to evaluate the toughness of the joint obtained by welding, the Charpy absorbed energy VE in a temperature range of -65°C or lower and -70°C or higher was measured. 1 is a graph showing the relationship between this VE and the MA (island martensite) fraction of the joint. It was found that in order to achieve the excellent low-temperature toughness of 27J or more, which is the target of the present invention, it is necessary to suppress the fraction of MA occupied in the structure of the joint to 8 area% or less, as shown in Fig. 1. On the other hand, in Fig. 1, the portion enclosed by the broken line is an example in which the amount of Ni was lower than the range specified in the present invention, and therefore the value of VE became low.

In order to suppress the MA fraction in the structure of the joint, the present inventor studied the means. Fig. 2 is a graph showing the relationship between the MA fraction of the joint and the BI represented by Equation (1). The MA fraction in Figs. 1 and 2 is obtained by observing the structure of a joint in a welded product after welding in Examples to be described later.

Nb constituting the formula of BI is an element that suppresses recrystallization of austenite grains, enlarges the non-recrystallized area, and contributes to the acceleration of refinement of ferrite grains by rolling. Further, Mn, Cu, Ni, and B constituting the formula of BI are elements contributing to the microstructure by rolling by _{stabilizing austenite and lowering the transformation temperature, i.e., lowering the Ar 3 transformation point.} The expression of BI in the present invention contains these elements that contribute to the refinement of ferrite grains, and is obtained by obtaining the coefficient of the element from the experimental data.

As shown in Fig. 2, when the BI represented by the formula (1) is suppressed to 6.2 or less, the MA fraction in the structure of the joint is suppressed to 8 area% or less. From the viewpoint of further suppressing the MA fraction in the structure of the joint and further increasing the VE, BI is preferably 6.1 or less, and more preferably 6.0 or less.

On the other hand, the present invention also aims to achieve both high strength and excellent low-temperature toughness (vTrs) of the base material. In particular, the tensile strength is 490 MPa or more, and vTrs is -80°C or less, and the product (TS×vTrs) thereof is -41000 (MPa·°C) or less. When the present inventor studied the relationship between these characteristics and BI, it was found that mixing of coarse ferrite grains and fine ferrite grains of the base material can be suppressed by increasing the BI. By suppressing the mixing, the standard deviation of the crystal grain size per circle of ferrite can be reduced, and as a result, a desired TS x VTrs can be obtained. The increase in BI also contributes to the microstructure of the joint. Moreover, by increasing BI, when heat input is applied during welding, coarse bainite formation from the pearlite portion of the base material can be suppressed, and the low-temperature toughness of the joint can be improved. From the above, in the present invention, it is necessary to control the BI within a predetermined range.

As shown in Fig. 3, the present inventors have found that in order to achieve the above-described level of TS x vTrs (hereinafter, referred to as "excellent strength-base metal low-temperature toughness balance" in some cases), BI should be 5.30 or higher. In order to achieve a better strength-base metal low-temperature toughness balance, the BI is preferably 5.40 or more, more preferably 5.45 or more, still more preferably 5.50 or more, and even more preferably 5.60 or more.

(Strong organization)

In the steel sheet of the present invention, the fraction of ferrite occupied in the entire structure is 85 area% or more, the average crystal grain size per element of the ferrite is 7 μm or less, and its standard deviation is 3.7 μm or less. In the present invention, as described above, a steel sheet having both high strength and excellent low-temperature toughness can be realized by optimizing the ferrite fraction and further miniaturizing and uniformizing the ferrite grains. The ferrite has an average crystal grain size per circle, preferably 6.9 μm or less. Considering the manufacturing conditions of the steel sheet of the present invention, etc., the lower limit of the average crystal grain size per element of the ferrite is about 4.5 μm. Further, by setting the standard deviation of the crystal grain size per circle to 3.7 μm or less, it is possible to realize a steel sheet having more reliably high strength and excellent low-temperature toughness. The standard deviation is preferably 3.6 μm or less, more preferably 3.5 μm or less. From the viewpoint of further uniforming the ferrite grains, the smaller the standard deviation is, the more preferable it is, and there is no particular lower limit, but the lower limit is, for example, about 2.0 μm.

In order to improve the characteristics of the crystal grains by fine and uniform ferrite, the fraction of ferrite occupied in the entire structure is set to 85 area% or more as described above. The ferrite fraction is preferably 88 area% or more, more preferably 90 area% or more. The upper limit of the ferrite fraction is approximately 95 area% in consideration of the component composition and manufacturing method of the steel sheet of the present invention. The structure of the remainder other than ferrite is at least one of pearlite, bainite, martensite, and MA. Among these, the fraction of pearlite is less than 10 area%, preferably 9.0 area% or less, and more preferably 8.5 area% or less. The lower the fraction of pearlite is, the more preferable, and the lower limit is not particularly provided, but considering the ferrite fraction, the lower limit of the fraction of pearlite may be 5 area%. Bainite, martensite, and MA are preferably 3 area% or less in total, and more preferably 0 area%. The fraction of the structure is obtained at a position of 6 to 7 mm from the surface in the thickness direction of the steel sheet.

(Ingredient composition)

Next, the component composition of the steel sheet of the present invention will be described.

[C: 0.03% by mass to 0.10% by mass]

Since C is an element contributing to high strength, it is contained in an amount of 0.03 mass% or more. The amount of C is preferably 0.04% by mass or more, and more preferably 0.050% by mass or more. On the other hand, when the amount of C is excessive, the pearlite fraction increases, resulting in a decrease in the toughness of the base metal, a decrease in the joint toughness, and further deterioration of the weldability, so the amount of C is made 0.10 mass% or less. The amount of C is preferably 0.090 mass% or less, and further, 0.080 mass% or less.

[Si: 0.05% by mass to 0.40% by mass]

Si acts as a deoxidizing agent when melting steel, and also exhibits an effect of increasing the strength of the steel. In order to exhibit such an effect, it contains 0.05 mass% or more. The amount of Si is preferably 0.07% by mass or more, and more preferably 0.10% by mass or more. On the other hand, when the amount of Si becomes excessive, the toughness of the base metal and the toughness of the joint portion decrease, so that the amount of Si is set to 0.40% by mass or less. Si amount is preferably 0.35 mass% or less, more preferably 0.30 mass% or less.

[Mn: 0.90 mass%-1.60 mass%]

Mn is an element effective for refining the structure by rolling by stabilizing austenite and lowering the transformation temperature. It is also an element effective in increasing the strength. Therefore, 0.90 mass% or more of Mn is contained. The amount of Mn is preferably 1.00 mass% or more, more preferably 1.10 mass% or more. On the other hand, if Mn is contained excessively, the coarsening of MnS and the increase in the pearlite fraction are caused, resulting in deterioration of the toughness of the base material and the joint, and MA is formed in the joint, resulting in a further decrease in the toughness of the joint. The upper limit of is set to 1.60% by mass. The amount of Mn is preferably 1.55% by mass or less.

[P: more than 0 mass%, 0.010 mass% or less]

P, which is an unavoidable impurity, adversely affects the toughness of the base metal and the welded portion, and therefore is suppressed to 0.010% by mass or less. Industrially, it is difficult to set the amount of P to 0% by mass, and the lower limit of the amount of P is about 0.002% by mass.

[S: more than 0 mass%, 0.010 mass% or less]

Since S is an element that forms MnS and deteriorates toughness, it needs to be suppressed to 0.010% by mass or less. S amount is preferably 0.005 mass% or less. Industrially, it is difficult to set the amount of S to 0% by mass, and the lower limit of the amount of S is about 0.001% by mass.

[Al: 0.010% by mass to 0.060% by mass]

Al is an element necessary for deoxidation, and in order to exhibit the effect, it is contained in an amount of 0.010% by mass or more. Al amount is preferably 0.015 mass% or more. On the other hand, when Al is contained excessively, alumina-based coarse inclusions are formed and the toughness decreases, so the upper limit of the amount of Al is set to 0.060% by mass. The amount of Al is preferably 0.050% by mass or less.

[Ni: 0.50% by mass to 1.1% by mass]

Ni is an element useful for ensuring good low-temperature toughness in a steel sheet and improving both the strength and low-temperature toughness of the steel sheet. In the present invention, Ni is an element useful _{for stabilizing austenite, lowering the transformation temperature, that is, lowering the Ar 3 transformation point, as described above.} By lowering the Ar ₃ transformation point, the microstructure by rolling can be achieved, and the above properties can be improved. In order to exhibit this effect, the amount of Ni is made 0.50 mass% or more. The amount of Ni is preferably 0.60 mass% or more, more preferably 0.65 mass% or more, and still more preferably 0.70 mass% or more. On the other hand, when the amount of Ni becomes excessive, the balance of the effect on strength and toughness due to Ni is broken, the effect of increasing the strength is superior to the effect of suppressing ductile fracture at low temperature, and the low-temperature toughness deteriorates. In the present invention, as described above, in order to improve the strength and improve the toughness of the base metal at a low temperature, the amount of Ni is set to 1.1% by mass or less. The amount of Ni is preferably 1.0% by mass or less, and more preferably 0.80% by mass or less.

[Nb: 0.007 mass%-0.022 mass%]

Nb is an element having an effect of refining ferrite grains through an effect of suppressing recrystallization of austenite grains. In order to obtain the effect, Nb is contained in an amount of 0.007% by mass or more. The amount of Nb is preferably 0.010% by mass or more. On the other hand, when the amount of Nb becomes excessive, the toughness decreases, so the upper limit thereof is set to 0.022% by mass. The amount of Nb is preferably 0.020% by mass or less.

[Ti: 0.007 mass%-0.017 mass%]

Ti is a strong nitride-forming element, and exhibits an effect of miniaturizing crystal grains by fine precipitation of TiN in a trace amount. In order to exhibit the effect, the amount of Ti is made 0.007% by mass or more. Ti amount is preferably 0.010 mass% or more. On the other hand, if the amount of Ti is excessive, the toughness of the joint is rather lowered. Therefore, the amount of Ti is set at 0.017% by mass or less, preferably 0.015% by mass or less.

[N: 0.0025 mass%-0.0060 mass%]

N is an element effective in generating AlN, preventing coarsening of γ grains during heating before hot rolling and during welding, and improving the toughness of the base material or joint. In order to exhibit the effect, N is contained in an amount of 0.0025% by mass or more. The amount of N is preferably 0.0030 mass% or more. On the other hand, when N is contained excessively, the toughness of the base metal deteriorates due to an increase in the solid solution N. Therefore, the amount of N is made 0.0060 mass% or less, preferably 0.0050 mass% or less.

It contains the above elements, and the balance consists of iron and unavoidable impurities. As unavoidable impurities, the incorporation of trace elements brought in depending on circumstances such as raw materials, materials, and manufacturing facilities is allowed. As the inevitable impurity, there is a case where any one or more of 0.05% by mass or less of Cr, 0.05% by mass or less of Mo, and 0.005% by mass or less of V may be included. In addition, as the unavoidable impurities, at least one of Mg, REM, and Zr, which are oxide-forming elements, may be contained within a total of 0.0010% by mass or less. However, if the oxide-forming element is about the level of the inevitable impurity, the effect on the properties is small. On the other hand, for example, like P and S, usually, the smaller the content is, the more preferable, and therefore, is an unavoidable impurity, but there is an element that is separately prescribed as described above with respect to the composition range. For this reason, in the present specification, the case of "inevitable impurities" constituting the remainder is a concept excluding elements whose composition range is separately defined.

The steel sheet of the present invention may be composed of the above elements and the remainder of iron and unavoidable impurities, and the optional elements described below do not need to be included. It contributes to further improvement of the back.

[B: more than 0% by mass, 0.002% by mass or less, Ca: more than 0% by mass, 0.003% by mass or less, and Cu: more than 0% by mass, at least one element selected from the group consisting of 0.35% by mass or less]

These elements contribute to the improvement of strength or toughness, and contribute to further raising the balance of high strength and low temperature toughness. About each element, it shows below.

[B: more than 0 mass%, 0.002 mass% or less]

B has an effect of lowering the solid solution N which adversely affects toughness by generating BN. It is also an element contributing to the microstructure by rolling by stabilizing austenite _{and lowering the Ar 3 transformation point.} In the case where the effect is exhibited as necessary, the amount of B is preferably more than 0% by mass, more preferably 0.0003% by mass or more. On the other hand, when the B content is too large, the precipitate of B is increased and the toughness is rather deteriorated, so it is preferable to suppress the content to 0.002% by mass or less.

[Ca: more than 0% by mass, 0.003% by mass or less]

Ca is an element effective in improving the toughness of a steel sheet by controlling inclusions. In the case where the effect is exhibited as necessary, the amount of Ca is preferably more than 0% by mass, more preferably 0.0005% by mass or more. On the other hand, when Ca is contained excessively, the toughness decreases, and therefore, the amount of Ca is preferably 0.003% by mass or less.

[Cu: more than 0 mass%, 0.35 mass% or less]

Cu is an element effective in improving strength. When exerting the effect as necessary, it is preferable to make the amount of Cu more than 0 mass %, More preferably, it is 0.05 mass% or more. When the Cu content is too large, cracks tend to occur during hot working, so the amount of Cu is preferably 0.35% by mass or less, and more preferably 0.30% by mass or less.

(characteristic)

The high-tensile steel sheet of the present invention has tensile strength, low-temperature toughness of the base material (vTrs), the product of tensile strength and the low-temperature toughness of the base material (TS×vTrs), and the joint toughness in a temperature range of -65°C or less and -70°C or more, They are all on a high level. These characteristics of the high-tensile steel sheet of the present invention will be described in detail below.

(1) Tensile strength (TS)

It has a TS of 490 MPa or more. Thereby, sufficient strength can be ensured. TS is preferably 500 MPa or more, more preferably 510 MPa or more, and still more preferably 520 MPa or more. The higher the tensile strength is, the more preferable, and the upper limit of the tensile strength is not particularly limited, but may be, for example, about 700 MPa.

(2) Low temperature toughness of the base material

ｖTrs is below -80℃. The said vTrs becomes like this. Preferably it is -90 degreeC or less, More preferably, it is -100 degreeC or less. The lower vTrs is, the more preferable, and the lower limit of vTrs is not particularly limited, but may be, for example, about -160°C.

(3) Product of tensile strength and low-temperature toughness of base metal (ｖTrs) (TS×ｖTrs)

TS×vTrs is -41000 (MPa·°C) or less. TS×vTrs is preferably -42000 (MPa·°C) or less, more preferably -43000 (MPa·°C) or less, and still more preferably -46000 (MPa·°C) or less. The smaller TS×vTrs, the more preferable, and the lower limit of TS×vTrs is not particularly limited, but may be, for example, about -70000 (MPa·°C).

(4) Toughness of the joint in the temperature range below -65℃ and above -70℃ ｖE

The steel sheet of the present invention has excellent low-temperature toughness when welding is performed with a heat input of 4 to 5 kJ/mm, as shown in the later examples. Specifically, the Charpy absorbed energy VE in the temperature range of -65°C or lower and -70°C or higher of the joint is 27J or higher. Said VE is preferably 40J or more, more preferably 50J or more, and still more preferably 80J or more. The higher VE is, the more preferable, and the upper limit of VE is not particularly limited, but may be, for example, about 250J.

The high-tensile steel sheet of the present invention can be advantageously applied as a so-called thick steel sheet, and in this case, the sheet thickness is about 6 mm or more, preferably 10 mm or more, and more preferably 15 mm or more. The upper limit of the plate thickness is not particularly limited, but, for example, when used in the above-described structure, it is about 50 mm or less, preferably 45 mm or less, and more preferably 40 mm or less.

(Manufacturing method)

In order to manufacture the high-tensile steel sheet of the present invention having the above structure, the manufacturing conditions thereof are controlled as follows. That is, after heating the steel sheet satisfying the above-described component composition, hot rolling is performed under the following conditions. In the heating process before rolling, heating a steel piece, such as a slab, at 1000-1250 degreeC is mentioned, for example.

Hot rolling is performed so as to satisfy the following conditions (a) to (c). Hereinafter, each condition is demonstrated.

(a) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is 950 to 875°C, it is reduced by a cumulative reduction ratio of 35% or more.

(b) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is 820°C or less and the Ar ₃ transformation point or more, it is reduced by a cumulative reduction ratio of 30% or more.

(c) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is less than 875°C and more than 820°C, and in the two-phase temperature range, no reduction is performed.

The two-phase temperature region refers to a temperature region in which austenite and ferrite two phase regions are at or below the _{Ar 3 transformation point.}

[(A) The cumulative reduction rate is 35% or more when the temperature at the position of 1/4 of the sheet thickness of the steel sheet is 950 to 875°C]

In order to refine the austenite grain, it is necessary to sufficiently reduce it in the recrystallization temperature range after heating. By applying a reduction of 35% or more of the cumulative reduction rate in the recrystallization temperature range, dislocations are accumulated in the austenite grains, and new grains can be generated using this dislocation as a driving force, which contributes to the refinement of the grains. In the component composition of the steel sheet of the present invention, recrystallization occurs when a reduction is applied at 875°C or higher. On the other hand, when the temperature at which the reduction is applied is too high, the effect of contributing to miniaturization is small. Therefore, the temperature at which the reduction is applied was set to 950°C or less. That is, in the present invention, the rolling reduction temperature range (effective recrystallization temperature range) effective for miniaturization of austenite grains was set to 950 to 875°C. In addition, in the present invention, rolling may be mentioned as the rolling-down method, and in addition, forging and the like may be mentioned.

In the present invention, the reduction in the effective recrystallization temperature range is performed at a cumulative reduction ratio of 35% or more, thereby generating new crystal grains useful for the formation of the structure of the present invention. The cumulative reduction ratio is preferably 40% or more. On the other hand, the upper limit of the cumulative reduction ratio is approximately 80%.

[(B) The cumulative reduction rate when the temperature at the position 1/4 of the sheet thickness of the steel sheet is 820°C or less and the Ar ₃ transformation point or more is 30% or more]

[(C) When the temperature at the position of 1/4 of the thickness of the steel plate is less than 875℃, exceeds 820℃, and is in the two-phase temperature range, there is no reduction]

In order to increase the strain zone that can become the generation nuclei of ferrite grains, a sufficient reduction is required even in the non-recrystallized temperature range. By applying a reduction in the lower temperature region than the recrystallization temperature region, the austenite grains do not generate new crystal grains and become a flat structure, so that a strain zone can be introduced into the grains. However, even if it is lower than the recrystallization temperature range, when the reduction is performed on the high temperature side of the non-recrystallization temperature range, a mixed structure is likely to be formed, and coarse ferrite grains are likely to be generated. Therefore, in the present invention, the temperature range in which the reduction is applied on the low temperature side of the non-recrystallization temperature range is set to 820°C or less and the Ar ₃ transformation point or more. In addition, in a temperature range of less than 875°C and more than 820°C, which is the high temperature side of the non-recrystallization temperature range, it is assumed that no reduction is performed.

In order to sufficiently obtain the effect of introducing the strain zone, the pressure reduction in the temperature range of 820°C or lower and Ar _{3 transformation point or higher is set to 30% or more.} The cumulative reduction ratio is preferably 35% or more. On the other hand, the upper limit of the cumulative reduction ratio is approximately 80%.

Further, when the reduction is performed in a two-phase temperature range that is lower than the non-recrystallization temperature range, the strength of the steel sheet is improved, but the stress concentration accompanying the work strengthening becomes remarkable, and the toughness of the steel sheet is deteriorated. Therefore, the reduction is not performed even in the two-phase temperature range.

The Ar ₃ transformation point is obtained based on the following equation (2).

Ar ₃ transformation point=868-369×[C]+24.6×[Si]-68.1×[Mn]-36.1×[Ni]-20.7×[Cu]-24.8×[Cr]+29.6×[Mo]... (2)

In formula (2), [C], [Si], [Mn], [Ni], [Cu], [Cr] and [Mo] are each of C, Si, Mn, Ni, Cu, Cr, and Mo. It represents the content (mass%) in the steel, and the element not contained is calculated as 0% by mass.

The cumulative reduction ratio was calculated by the following equation.

Cumulative reduction rate in the temperature range of 950 to 875℃ (%) = (H1-H2)/H1×100

Cumulative reduction rate (%) = (H2-t)/H2×100 at 820°C or lower and above the Ar _{3 transformation point}

In the above,

H1 is the plate thickness at the start of rolling in the temperature range of 950 to 875°C (for example, the thickness of the slab),

H2 is the sheet thickness at the end of rolling in a temperature range of 950 to 875°C = 820°C or less, and the sheet thickness at the start of rolling in a temperature range above the _{Ar 3 transformation point,}

t is the finish thickness, all in mm.

After the hot rolling, from the controlled cooling start temperature of the rolling end temperature to (Ar _{3 transformation point -30° C.) to the controlled cooling end temperature of the Ar 3} transformation point to 500° C., an average of 0.6° C./s or more and 10° C./s or less. Cool at a cooling rate. Cooling at the above average cooling rate from a temperature below (Ar ₃ transformation point -30°C) results in cooling from the two-phase region of ferrite and austenite, concentrating elements in austenite to form bainite or MA. Because it is not desirable.

In the above temperature range, by performing accelerated cooling with an average cooling rate of 0.6°C/s or more, generation of the second phase other than ferrite can be suppressed, growth of ferrite can be suppressed, and fine ferrite grains can be secured. The average cooling rate is preferably 0.7°C/s or more, more preferably 0.8°C/s or more, and still more preferably 2.0°C/s or more. On the other hand, when the average cooling rate exceeds 10° C./s and is too fast, the desired ferrite fraction cannot be secured and the toughness decreases. Therefore, the average cooling rate is 10°C/s or less, preferably 9.5°C/s or less, more preferably 9.0°C/s or less, and even more preferably 8.5°C/s or less.

If the cooling at the average cooling rate is _{terminated at a temperature higher than the Ar 3} transformation point, since the ferrite coarsens and the pearlite fraction increases, desired properties cannot be obtained. On the other hand, when it is performed to a temperature lower than 500°C, the MA fraction increases, and there arises a problem that the base metal toughness decreases. Therefore, the temperature at which the cooling ends at the average cooling rate is from the Ar ₃ transformation point to 500°C. On the other hand, from the viewpoint of sufficiently securing the structure defined in the present invention by the controlled cooling, the temperature difference between the controlled cooling start temperature and the controlled cooling end temperature (the controlled cooling start temperature-the controlled cooling end temperature) is preferably Is 40°C or higher, more preferably 60°C or higher, and still more preferably 80°C or higher.

After the control cooling, for example, it can be cooled to room temperature.

Example

Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited by the following embodiments, and can be carried out with appropriate modifications within a range that can be consistent with the above and the purpose described later, and all of them are included in the technical scope of the present invention.

A steel piece (slab) satisfying the component composition shown in Table 1 was obtained by a conventional method. After heating the said steel piece to the heating temperature shown in Table 2, hot rolling and cooling after hot rolling were performed under the conditions shown in Table 2. From the control cooling end temperature shown in Table 2 to room temperature, it air-cooled. By these manufacturing methods, a steel sheet having a sheet thickness shown in Table 2 as a finish thickness was obtained.

Each temperature in heating and hot rolling in the manufacturing process is a temperature at a position of 1/4 of the sheet thickness of the steel sheet, calculated from the surface temperature of the steel sheet using parameters such as sheet thickness and thermal conductivity. In addition, the controlled cooling start temperature and the controlled cooling end temperature are surface temperatures. On the other hand, during heating, the crack was sufficiently maintained so that the temperature difference between the surface and the center of the sheet thickness became sufficiently small.

With respect to the obtained steel sheet, the steel structure, tensile strength, low-temperature toughness of the base material, and low-temperature toughness of the joint were evaluated in the following manner.

[Observation of strong structure]

The impact test piece described later, that is, at a position of 6 to 7 mm in the plate thickness direction from the surface of each steel plate, which is the same position as the collecting position of the Charpy test piece, using an optical microscope at a magnification of 100 times and a field of view of 600 μm×800 μm Was observed, and the fractions of ferrite and pearlite were measured using image analysis software. In addition, the ferrite grain size was observed at a magnification of 100 times using an optical microscope at a position of 6 to 7 mm from the surface of each steel plate in the plate thickness direction, and the diameter of the ferrite grain assuming a circle was determined per circle. It was calculated|required as a particle diameter, and its average value (crystal grain size per average circle) and standard deviation were obtained.

[Evaluation of tensile strength]

From the total thickness of each steel sheet, a JIS Z 2201 No. 1B test piece was taken in a direction perpendicular to the rolling direction, and a tensile test was conducted in accordance with JIS Z 2241 to measure the tensile strength (TS). And what had a tensile strength of 490 MPa or more was evaluated as high strength.

[Evaluation of the low-temperature toughness of the base material (impact test using the base material)]

From the surface of each steel plate, the test piece was taken so that the position of 6 to 7 mm in the plate thickness direction became the same as the center of the Charpy test piece, and the long direction of the test piece was perpendicular to the rolling direction. Then, the Charpy impact test was performed in accordance with the method of JIS Z 2242, and the wavefront transition temperature vTrs was measured. And it evaluated that the fracture surface transition temperature vTrs was -80°C or less as excellent in low-temperature toughness.

[Evaluation of low-temperature toughness of joints (impact test using joints)]

A test piece was taken from the weldment obtained by welding at 4 to 5 kJ/mm of heat input. In the joint of the weldment, as in the evaluation of the low-temperature toughness of the base metal, the position of 6 to 7 mm from the surface to the plate thickness direction is the same as the center of the Charpy test piece, and the long direction of the test piece is rolled at right angles to the welding line direction. The test piece was taken so as to be perpendicular to the direction. Then, a Charpy impact test was performed in accordance with JIS Z 2242, and the Charpy absorbed energy at -65°C or -70°C was calculated to evaluate the toughness of the joint.

In addition, the structure of the joint was also observed. Specifically, the sample of the joint part was corroded using a 3% nital solution or a Lepera solution depending on the object to be observed, and the grain boundaries and MA were exposed. Then, at a position of 6 to 7 mm in the plate thickness direction from the surface, the exposed structure was observed with an optical microscope, and the fractions of ferrite, bainite and martensite, and MA were calculated. Among them, the fraction of MA is also shown in Table 3.

Table 3 shows the results of these evaluations.

The following can be seen from the results of Tables 1 to 3 above. No. In 3 to 14, the component composition of the present invention was satisfied, and the production conditions were also satisfied, and the obtained steel sheet had a desired structure, high strength, and excellent strength-base metal low-temperature toughness balance, and low-temperature toughness of the joint. On the contrary, No. In 1 and 2, since the amount of Ni was insufficient, and the BI was less than the scope of the present invention, a certain amount or more of ferrite having a uniform particle diameter could not be secured, and as a result, high strength and low-temperature toughness of the base metal and the joint could not be obtained. . Also No. In 15, since the amount of Nb was excessive and BI exceeded the range of the present invention, the MA fraction in the structure of the joint became excessive, and the low-temperature toughness was inferior.

Claims (3)

Ingredient composition,
C: 0.03% by mass to 0.10% by mass,
Si: 0.05% by mass to 0.40% by mass,
Mn: 0.90 mass% to 1.60 mass%,
P: more than 0 mass%, 0.010 mass% or less,
S: more than 0 mass%, 0.010 mass% or less,
Al: 0.010% by mass to 0.060% by mass,
Ni: 0.50 mass% to 1.1 mass%,
Nb: 0.007% by mass to 0.022% by mass,
Ti: 0.007% by mass to 0.017% by mass,
N: 0.0025 mass% to 0.0060 mass%, and
The balance consists of iron and unavoidable impurities,
BI defined by the following formula (1) is 5.30 or more and 6.2 or less,
Low temperature between the base material and the joint, wherein the proportion of ferrite occupied in the entire structure is 85 area% or more, and the proportion of pearlite is less than 10 area%, the average crystal grain size per source of ferrite is 7 μm or less, and its standard deviation is 3.7 μm or less. High-tensile steel sheet with excellent toughness.
BI=12×(C+5Nb)+2Mn+Cu+Ni+300B...(1)
In formula (1), C, Nb, Mn, Cu, Ni, and B represent the content in the steel of C, Nb, Mn, Cu, Ni, and B represented by mass%, respectively, and elements not included are 0% by mass. It is calculated as. The method of claim 1,
The component composition, further,
B: more than 0% by mass, 0.002% by mass or less,
Ca: more than 0 mass%, 0.003 mass% or less, and
Cu: A high-tensile steel sheet having excellent low-temperature toughness of a base material and a joint containing at least one element selected from the group consisting of more than 0% by mass and not more than 0.35% by mass. As a method of manufacturing the high-tensile steel sheet according to claim 1 or 2,
After heating the steel sheet having the component composition according to claim 1 or 2, hot rolling is performed so as to satisfy the following conditions (a) to (c), and after hot rolling, rolling end temperature to (Ar ₃ transformation point -30 ℃) from the control cooling start temperature to _{the control cooling end temperature of Ar 3} transformation point to 500°C, cooling at an average cooling rate of 0.6°C/s or more and 10°C/s or less, excellent low-temperature toughness of the base metal and the joint Manufacturing method of high-tensile steel sheet.
(a) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is 950 to 875°C, it is reduced by a cumulative reduction ratio of 35% or more.
(b) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is 820°C or less and the Ar ₃ transformation point or more, it is reduced by a cumulative reduction ratio of 30% or more.
(c) When the temperature at the position of 1/4 of the sheet thickness of the steel sheet is in a temperature range of less than 875°C and more than 820°C, and in a two-phase temperature range, no reduction is performed.

Images