JP6897450B2 - High-strength thick steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a high-strength thick steel sheet and a method for producing the same.

With the increase in size of penstocks and pressure vessels, high-strength thick steel plates with a tensile strength of 780 MPa or more are used for structures from the viewpoint of reducing procurement, transportation, and construction costs. On the other hand, steel sheets used for penstocks and pressure vessels are required to have excellent low-temperature toughness and arrestability, and in recent years, large heat input welding has been applied for the purpose of reducing the welding cost of high-strength thick steel sheets. The demand for is also increasing.

The arrest property is a brittle crack propagation stopping property, and even if a crack occurs in the steel sheet, the propagation of the crack can be stopped before the plate thickness penetrates. Therefore, it is possible to inspect and repair the structure before it is destroyed, and it is possible to minimize the damage. So far, the development of high-strength thick steel sheets with excellent low-temperature toughness and arrestability has been studied.

For example, in Patent Document 1, hardenability is ensured by adding an alloying element, and hardenability in the vicinity of the surface layer portion is reduced by rolling under light reduction in a low temperature region after rolling in a high temperature region. A method for ensuring homogeneity in the thickness direction is disclosed.

Patent Document 2 discloses an invention in which the toughness of the surface layer portion is improved by reducing the pressure in a low temperature range by utilizing the function of adding Ti to contribute to the improvement of the hardenability of B.

Further, in Patent Document 3, N is surely fixed by adding Ti to secure the effect of B, and in addition, Nb is added to recover dislocations and delay recrystallization, resulting in reduction in a low temperature range. An invention is disclosed that obtains a structure miniaturization effect and improves low temperature toughness and arrestability.

Japanese Unexamined Patent Publication No. 2-77521 Japanese Unexamined Patent Publication No. 62-196326 Japanese Unexamined Patent Publication No. 9-263828

In Patent Document 1, nothing has been examined for Ti-based composite inclusions that are optimal for suppressing coarsening of austenite grains by welding and for microstructure miniaturization, HAZ toughness is not sufficient, and even for large heat input welding. I can not cope.

In Patent Document 2, nothing is examined regarding HAZ toughness.
Furthermore, the properties disclosed in Patent Document 3 are only for steel sheets, and no study has been made on HAZ toughness, which is a problem when applying large heat input welding.

An object of the present invention is that it is used for large structures such as penstocks and pressure vessels, has excellent low temperature toughness and arrestability at a tensile strength of 780 MPa or more, and has excellent HAZ toughness in large heat input welding. Is applicable, that is, a high-strength thick steel plate having excellent welding efficiency, that is, a high-strength thick steel plate having a plate thickness of 25 mm or more excellent in low-temperature toughness, arrestability and HAZ toughness, and a method for producing the same.

In order to solve the above problems, the present inventors have investigated a chemical composition and a manufacturing method having a tensile strength of 780 MPa or more, excellent low temperature toughness and arrestability, and excellent HAZ toughness even in high heat input welding.

First, in the production of slabs, slabs with controlled inclusions in steel are produced, and the slabs are hot-rolled and hardened directly from that temperature after hot rolling, and then tempered to achieve a high tensile thickness. A steel sheet was manufactured and this high-strength thick steel sheet was investigated.

First, as a result of examining the carbon equivalent Ceq, which is an index of hardenability defined by the Japan Welding Engineering Society standard (WES) represented by the following formula (1), in order to secure the desired tensile strength, the carbon equivalent Ceq is set to 0. It turned out that it is necessary to secure .45 or more.
Ceq = [C%] + [Si%] / 24+ [Mn%] / 6+ [Ni%] / 40+ [Cr%] / 5+ [Mo%] / 4+ [V%] / 14 ・ ・・ Equation (1)
In the formula (1), the elements with [] represent the content (mass%) of each element.

Therefore, as a result of examining the chemical composition and metallographic structure for obtaining strength, low temperature toughness and arrest property in this range, the findings A to D listed below were obtained.
(A) The most effective element for improving the low temperature toughness and arrest property of thick steel sheets is Ni, and a required amount is added.

(B) Nb can delay recrystallization and introduce high-density dislocations by rolling at a low temperature to increase the number of metamorphic nucleation sites, thereby micronizing the structure and improving toughness and arrestability. be able to.

(C) Mo is necessary because it can suppress the softening of HAZ due to the influence of welding heat by precipitation strengthening due to the formation of carbides, and therefore it can suppress the local strain concentration on the softened zone when it receives an impact. Add in quantity.

By forming a nitride with N in (D) Ti, coarsening of austenite grains can be suppressed during heating and welding before rolling, so that low temperature toughness and HAZ toughness are improved. Further, under the condition that the Al content is limited, Ti preferentially forms an oxide. As a result of examining the composition of suitable inclusions that contribute to microstructure miniaturization as intragranular transformation nuclei, Ti-based inclusions in which Ti is bound to oxygen are effective.

That is, in order to effectively grow the intragranular ferrite in the old austenite grains during welding, it is essential to control the inclusions that are the nucleating nuclei of the intragranular ferrite. In particular, in a thick steel plate with a thick plate thickness, it is difficult to control the chemical composition and the number of inclusions in the plate thickness direction due to the difference in the cooling rate between the surface and the inside. Therefore, it is necessary to control inclusions that are nucleation nucleation of intragranular ferrite. Therefore, as a result of investigating the growth mechanism of intragranular ferrite, the following findings EG were obtained.

(E) If MnS is precipitated around the Ti-based oxide in the steelmaking stage to generate a composite inclusion containing the Ti-based oxide and MnS, Mn is deficient at the matrix interface between MnS and the base metal. A region is formed. In this Mn-deficient region (hereinafter, referred to as “initial Mn-deficient region”), the ferrite growth start temperature rises significantly. Therefore, when the base metal is welded, intragranular ferrite grows preferentially from this Mn-deficient region in the cooling process.

(F) When the base metal is welded, Mn in the matrix of the base metal existing in the vicinity of the Ti-based oxide is diffused and absorbed by the atomic vacancies existing inside the Ti-based oxide. As a result, a Mn-deficient region is formed at the interface between the HAZ of the base metal and the Ti-based oxide that has undergone thermal history by welding. This Mn-deficient region (hereinafter, referred to as “welded Mn-deficient region”) also serves as a starting point for preferential growth of intragranular ferrite.

(G) Since the structure of HAZ can be miniaturized by both actions (E) and (F), the required low temperature toughness of HAZ can be obtained.

(H) Based on the above mechanism, the present inventors have found that intragranular ferrite can be effectively nucleated by controlling the amount of MnS compounded with inclusions and the number density. Furthermore, the present inventors have found that the inclusions in the steel must satisfy the requirements [1] to [3] listed below in order to obtain the above-mentioned grain refinement effect.

[1] A composite inclusion having MnS around a Ti oxide in steel, and the area ratio of MnS in the cross section of the composite inclusion is 10 to 50%.
[2] The ratio of MnS to the circumference of the composite inclusion is 10% or more.
[3] The number density of the composite inclusions having a particle size of 0.5 to 5.0 μm is 10 to 40 pieces / mm ² .

(I) In order to obtain an appropriate amount of such Ti-based inclusions, the present inventors need to control at the steelmaking stage, preferentially suppress the amount of Al added which tends to form oxides, and Ti. It was found that it is important to adjust the oxygen potential after adding a certain amount or more of. This makes it possible to improve the HAZ toughness even in large heat input welding.

(J) Microstructure miniaturization is effective for ensuring arrestability, but since the austenite grain size varies in the plate thickness direction due to heating before hot rolling, a fine structure is ensured for the entire thickness of the steel sheet. It is difficult. However, it was found that when Ti-based inclusions, which are the nuclei of intragranular transformation, are included, the austenite grains are transformed inside the coarse austenite grains, so that a fine structure can be obtained with the full thickness of the steel sheet and the arrest property is also improved.

The present invention is based on these findings A to J and is as listed below.
(1) The chemical composition is C: 0.05 to 0.13%, Si: 0.10 to 0.50%, Mn: 1.0 to 1.6%, P: 0.015% in mass%. Hereinafter, S: 0.001 to 0.005%, Al: 0.0028% or less, Cu: 0.25 to 0.50%, Ni: 0.6 to 2.0%, Cr: 0.3 to 1 0.0%, Mo: 0.25 to 0.8%, Nb: 0.010 to 0.030%, Ti: 0.010 to 0.030%, N: 0.0020 to 0.0040%, O: 0.0015 to 0.0035%, B: 0.0005 to 0.0020%, V: 0 to 0.05%, the balance is Fe and impurities.
The carbon equivalent Ceq defined by the following formula (1) is 0.45 to 0.60.
In addition, the steel contains composite inclusions in which MnS is present around the Ti oxide.
The area ratio of the MnS in the cross section of the composite inclusion is 10 to 50%.
The proportion of MnS at the interface is 10% or more,
The number density of the composite inclusions having a particle size of 0.5 to 5.0 μm ^{is 10 to 40 pieces / mm 2} .
A high-strength thick steel sheet having an average effective crystal grain size of 10 μm or less, a tensile strength of 780 MPa or more, and a plate thickness of 25 mm or more, excellent in low-temperature toughness, arrestability, and HAZ toughness.
Ceq = [C%] + [Si%] / 24+ [Mn%] / 6+ [Ni%] / 40+ [Cr%] / 5+ [Mo%] / 4+ [V%] / 14 ・ ・・ (1)
In the formula (1), the elements with [] represent the content (mass%) of each element.

(2) The high-strength thick steel sheet according to Item 1, which contains V: 0.01 to 0.05% by mass.

(3) Before the RH vacuum degassing treatment, the oxygen potential in the molten steel is set to 10 to 30 ppm, and the chemical composition is adjusted in the RH vacuum degassing treatment to produce the molten steel.
A slab is produced by a continuous casting method using the molten steel.
The slab is heated and leveled to a temperature of 950 to 1100 ° C.
Hot rolling is performed in a temperature range of 900 ° C. or lower so as to finish to a predetermined plate thickness with a cumulative reduction rate of 50% or more.
The method for producing a high-strength thick steel sheet according to item 1 or 2, wherein the hot rolling is directly performed by quenching directly from a temperature of 700 ° C. or higher.

(4) The method for producing a high-strength thick steel sheet according to Item 3, wherein the direct quenching is further performed and then tempering is performed at a temperature of 600 to 650 ° C.

In the present invention, the thickness of the thick steel sheet is specified to be 25 mm or more, but the upper limit is not specified because the characteristics of the present invention can be obtained if the requirements of the chemical composition, inclusions, and metal structure specified in the present invention are satisfied. .. However, considering the production of the high-strength thick steel sheet of the present invention, the sheet thickness is 80 mm or less.

To provide a high-strength thick steel sheet having a tensile strength of 780 MPa or more, excellent low-temperature toughness and arrestability, and excellent HAZ toughness in large heat-affected welding by the high-strength thick steel sheet having a plate thickness of 25 mm or more and the manufacturing method according to the present invention. Is possible. As a result, welding work efficiency can be improved in the manufacture of large structures such as penstocks and pressure vessels, and welding work costs can be significantly reduced, resulting in an extremely large industrial effect.

FIG. 1 is a diagram showing a test piece collection procedure for evaluating HAZ toughness in Examples.

Hereinafter, the present invention will be described in detail.
1. 1. Chemical Composition The reason for limiting the chemical composition of the high-strength thick steel sheet according to the present invention as described above will be described. In the following description, "%" with respect to chemical composition or concentration means "% by mass" unless otherwise specified.

First, the essential elements will be explained.
(1-1) C: 0.05 to 0.13%
C is the most important element that determines the strength of the high-strength thick steel sheet according to the present invention. If the C content is less than 0.05%, the required strength of 780 MPa or more cannot be obtained. Therefore, the C content is 0.05% or more, preferably 0.06% or more, and more preferably 0.07% or more.

On the other hand, when the C content exceeds 0.13%, the toughness, arrest property and HAZ toughness of the high-strength thick steel sheet deteriorate. Therefore, the C content is 0.13% or less, preferably 0.12% or less, and more preferably 0.10% or less.

(1-2) Si: 0.10 to 0.50%
Si is an element effective for pre-deoxidation of molten steel, and has an effect of improving the strength of a high-strength thick steel sheet without deteriorating the toughness. If the Si content is less than 0.10%, these effects cannot be sufficiently obtained. Therefore, the Si content is 0.10% or more, preferably 0.12% or more, and more preferably 0.14% or more.

On the other hand, when the Si content exceeds 0.50%, the surface texture and HAZ toughness of the high-strength thick steel sheet are deteriorated. Therefore, the Si content is 0.50% or less, preferably 0.44% or less, and more preferably 0.39% or less.

(1-3) Mn: 1.0 to 1.6%
Mn is important for improving the strength of a high-strength thick steel sheet through improving hardenability, and is necessary for obtaining an inclusion form suitable for improving HAZ toughness. Therefore, the Mn content is 1.0% or more, preferably 1.1% or more, and more preferably 1.2% or more.

On the other hand, if the Mn content exceeds 1.6%, the toughness of the high-strength thick steel sheet deteriorates. Therefore, the Mn content is 1.6% or less, preferably 1.5% or less, and more preferably 1.4% or less.

In order to obtain a good balance of strength, low temperature toughness, arrest property and HAZ toughness, the Mn content is preferably 1.2 to 1.6%.

(1-4) P: 0.015% or less Since P segregates at the grain boundaries and deteriorates toughness, the P content is preferably as low as possible. When the P content exceeds 0.015%, the toughness is significantly deteriorated. Therefore, the P content is 0.015% or less, preferably 0.007% or less, and more preferably 0.006% or less.

(1-5) S: 0.001 to 0.005%
S precipitates MnS on the surface of the oxide and forms a Mn-deficient region at the matrix interface between MnS and the base material. Therefore, when the base metal is welded, intragranular ferrite is preferentially nucleated from this Mn-deficient region, so that the structure becomes finer and the low-temperature toughness of the HAZ portion can be ensured. Therefore, the S content is 0.001% or more.

However, if the S content exceeds 0.005%, it causes deterioration of ductility and base material toughness of the high-strength thick steel sheet. Therefore, the S content is 0.005% or less, preferably 0.003% or less, and more preferably 0.002% or less.

(1-6) Al: 0.0028% or less Al is an element added to obtain the cleanliness of molten steel. However, since Al forms an oxide preferentially over other elements, it becomes impossible to obtain a Ti-based oxide that contributes to the improvement of low temperature toughness and HAZ toughness of the present invention. Therefore, the Al content is 0.0028% or less, preferably 0.0022% or less, and more preferably 0.0021% or less.

(1-7) Cu: 0.20 to 0.50%
Cu can improve hardenability and improve the strength of high-strength thick steel sheets without significantly impairing weldability and toughness. Therefore, the Cu content is 0.20% or more, preferably 0.22% or more, and more preferably 0.23% or more.

On the other hand, if the Cu content exceeds 0.50%, the toughness of the high-strength thick steel sheet deteriorates. Therefore, the Cu content is 0.50% or less, preferably 0.45% or less, and more preferably 0.39% or less.

(1-8) Ni: 0.6 to 2.0%
Ni is a useful element that not only improves hardenability to obtain strength, but also improves low temperature toughness and arrestability. Therefore, the Ni content is 0.6% or more, preferably 0.7% or more, and more preferably 0.8% or more.

On the other hand, Ni is an expensive alloying element, and if it is contained in excess of 2.0%, it does not meet economic rationality. Therefore, the Ni content is 2.0% or less, preferably 1.9% or less, and more preferably 1.8% or less.

(1-9) Cr: 0.3 to 1.0%
Cr can improve the strength without impairing low temperature toughness and arrest property. Therefore, the Cr content is 0.3% or more, preferably 0.4% or more, and more preferably 0.5% or more.

On the other hand, when the Cr content exceeds 1.0%, the toughness of the high-strength thick steel sheet is deteriorated. Therefore, the Cr content is 1.0% or less, preferably 0.9% or less, and more preferably 0.8% or less.

(1-10) Mo: 0.25 to 0.8%
Mo has the effect of improving the strength of the high-strength thick steel sheet and alleviating the deterioration of toughness due to the addition of B. Further, in welding of a high-strength thick steel sheet having a tensile strength of more than 780 N / mm ² , softening occurs due to the heat-affected zone of welding, but Mo has the effect of suppressing softening by forming precipitates due to the heat of welding. Have. As a result, local stress concentration on the softened portion is alleviated when an impact is applied to the high-strength thick steel sheet, and deterioration of toughness is suppressed. Therefore, the Mo content is 0.20% or more, preferably 0.3% or more, and more preferably 0.4% or more.

On the other hand, if the Mo content exceeds 0.8%, the toughness of the high-strength thick steel sheet deteriorates. Therefore, the Mo content is 0.8% or less, preferably 0.7% or less, and more preferably 0.6% or less.

(1-11) Nb: 0.010 to 0.030%
Nb can delay recrystallization and introduce high-density dislocations by hot rolling at low temperature to increase metamorphic nucleation sites. Therefore, the structure can be refined and the toughness and arrestability of the high-strength thick steel sheet can be improved. Therefore, the Nb content is 0.010% or more, preferably 0.012% or more, and more preferably 0.017% or more.

On the other hand, when the Nb content exceeds 0.030%, the toughness of the high-strength thick steel sheet deteriorates. Therefore, the Nb content is 0.030% or less, preferably 0.029% or less, and more preferably 0.028% or less.

(1-12) Ti: 0.010 to 0.030%
Ti is necessary for forming nitrides to suppress coarsening of crystal grains during heating and for forming complex inclusions that become intragranular transformation nuclei. However, if the Ti content is less than 0.010%, this effect is not exhibited. Therefore, the Ti content is 0.010% or more, preferably 0.011% or more, and more preferably 0.015% or more.

On the other hand, if the Ti content exceeds 0.030%, Ti carbides are excessively precipitated, which adversely affects the toughness of the base metal and the toughness of the weld. Therefore, the Ti content is 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less.

(1-13) N: 0.0020 to 0.0040%
N contributes to the improvement of the toughness of the high-strength thick steel sheet because it suppresses the texture coarsening during heating by forming a nitride with Ti. Therefore, the N content is 0.0020% or more, preferably 0.0024% or more, and more preferably 0.0025% or more.

On the other hand, when the N content exceeds 0.0040%, the toughness of the high-strength thick steel sheet deteriorates due to the coarsening of the nitride. Therefore, the N content is 0.0040% or less, preferably 0.0038% or less, and more preferably 0.0037% or less.

(1-14) O: 0.0015 to 0.0035%
O (oxygen) is essential for the formation of complex oxides that serve as intragranular transformation nuclei. Therefore, the O content is 0.0015% or more, preferably 0.0017% or more, and more preferably 0.0020% or more.

On the other hand, if O is contained in a large amount, the cleanliness is significantly deteriorated, so that it becomes difficult to secure practical toughness for the base metal, the weld metal portion, and the HAZ. Therefore, the O content is 0.0035% or less, preferably 0.0033% or less, and more preferably 0.0030% or less.

(1-15) B: 0.0005 to 0.0020%
Since B can improve hardenability with a small amount, it is extremely effective in improving the strength of a high-strength thick steel sheet. Further, during welding, segregation occurs at the austenite grain boundaries, and by lowering the grain boundary energy, the grains are transformed from the inside of the grains and the structure is made finer, which is also effective in improving HAZ toughness. Therefore, the B content is 0.0005% or more, preferably 0.0006% or more, and preferably 0.0007% or more.

On the other hand, if the B content exceeds 0.0020%, the hardenability becomes excessive and the toughness deteriorates. Therefore, the B content is 0.0020% or less, preferably 0.0017% or less, and more preferably 0.0014% or less.

Next, arbitrary elements will be described.
(1-16) V: 0 to 0.05%
V is an optional element contained as necessary in the present invention, and is generally effective in improving hardenability and suppressing softening by precipitating during welding. However, when the V content exceeds 0.05%, the precipitate becomes coarse and the toughness deteriorates. Therefore, the V content is 0.05% or less, preferably 0.04% or less, and more preferably 0.03% or less.

In order to surely obtain the above effect, the V content is preferably 0.01% or more, and more preferably 0.02% or more.

(1-17) Carbon equivalent Ceq: 0.45 to 0.60
Further, in order to secure the required tensile strength in the present invention, the carbon equivalent Ceq, which is an index of quenching hardness defined by the Japan Welding Association Standard (WES) as shown in the following formula (1), is set to 0.45. It is set to ~ 0.60.
Ceq = [C%] + [Si%] / 24+ [Mn%] / 6+ [Ni%] / 40+ [Cr%] / 5+ [Mo%] / 4+ [V%] / 14 ・ ・・ (1)
In the formula (1), the elements with [] represent the content (mass%) of each element.

The carbon equivalent Ceq defined by the above formula (1) is an index showing the hardenability of the steel sheet. In order to secure the tensile strength, the content of C, Si, Mn, Ni, Cr, Mo, and V contained in the steel sheet should be set to 0.45 or more with the carbon equivalent Ceq defined by the above formula (1). Adjust to do so. If the carbon equivalent Ceq is less than 0.45, sufficient tensile strength cannot be obtained due to insufficient hardenability.

The larger the carbon equivalent Ceq, the higher the tensile strength. However, when the carbon equivalent Ceq exceeds 0.60, the tensile strength becomes excessive, and the absorbed energy of Charpy is remarkably reduced accordingly. Therefore, the carbon equivalent Ceq defined by the above formula (1) is 0.45 to 0.60.

(1-18) Remaining Remaining other than the above is Fe and impurities. Examples of impurities include those contained in raw materials such as ore and scrap, and those contained in the manufacturing process.

2. Composite inclusions Further, the present invention includes composite inclusions in which MnS is present around the Ti oxide in the steel as the composite inclusions that contribute to the microstructure miniaturization of the steel sheet and HAZ. The area ratio of MnS in the cross section of this composite inclusion, the ratio of MnS at the interface, the particle size and the number density of the inclusion are in the following ranges.

(2-1) Area ratio of MnS in the cross section of the composite inclusions: 10% to 50%
In the present invention, the amount of MnS in the composite inclusions is defined by analyzing the composite inclusions appearing on an arbitrary cut surface and measuring the area ratio of MnS in the cross-sectional area of the composite inclusions.

When the area ratio of MnS in the cross section of the composite inclusion is less than 10%, the amount of MnS in the composite inclusion is small, and the initial Mn-deficient layer is not sufficiently formed at the interface between MnS and the matrix. As a result, it becomes difficult to generate intragranular ferrite when welding. Therefore, the area ratio of MnS in the cross section of the composite inclusion is 10% or more, preferably 12% or more, and more preferably 14% or more.

On the other hand, when the proportion of MnS in the cross section of the composite inclusions exceeds 50%, the composite inclusions are mainly MnS. In this case, the amount of Mn absorbed by the atomic vacancies in the Ti-based oxide is small, the welded Mn-deficient layer is not formed, and it becomes difficult to generate intragranular ferrite. Therefore, the area ratio of MnS in the cross section of the composite inclusion is 50% or less, preferably 47% or less, and more preferably 46% or less.

(2-2) Percentage of MnS at the interface of Ti-based composite inclusions: 10% or more MnS in the composite inclusions is formed around the Ti-based oxide. When the ratio of MnS to the peripheral length of the composite inclusions is less than 10%, the initial Mn-deficient region formed at the interface between MnS and the matrix is small, and intragranular ferrite nucleation is not sufficient even after welding. Therefore, good low temperature HAZ toughness cannot be obtained. Therefore, the ratio of MnS to the circumference of the composite inclusions with the matrix is 10% or more, preferably 14% or more, and more preferably 17% or more.

On the other hand, the larger the proportion of MnS, the larger the initial Mn-deficient layer and the easier it is to form intragranular ferrite. Therefore, the upper limit of the ratio of MnS is not set, but it is usually 80% or less.

(2-3) Number density of composite inclusions (number density of composite inclusions having a particle size of 0.5 to 5.0 μm): 10 to 40 pieces / mm ²

The number density of composite inclusions refers to the number of composite inclusions having a specified particle size per unit area. When the particle size of the composite inclusions is less than 0.5 μm, the amount of Mn that can be absorbed from the periphery of the composite inclusions is small, and as a result, the amount of intragranular ferrite produced decreases. On the other hand, when the particle size of the composite inclusions is larger than 5.0 μm, the composite inclusions become the starting point of fracture. Therefore, in the present invention, the particle size of the target composite inclusion is set to 0.5 to 5.0 μm.

The number density of the composite inclusions having such a particle size is related to the amount of Mn absorbed. In order to nucleate stable intragranular ferrite, it is necessary that at least one of each composite inclusion is contained in the old austenite. Therefore, the number density of the composite inclusions is 10 pieces / mm ² or more, preferably 12 pieces / mm ² or more, and more preferably 13 pieces / mm ² or more.

On the other hand, if the amount of composite inclusions is excessive, the absorbed energy of ductile fracture decreases. Therefore, the number density of the composite inclusions is 40 pieces / mm ² or less, preferably 39 pieces / mm ² or less, and more preferably 34 pieces / mm ² or less.

3. 3. Average effective crystal grain size: 10 μm or less Further, in the present invention, the size of the effective crystal grain size is in the following range in order to ensure arrestability.

In order to ensure low temperature toughness and arrestability, it is effective to reduce the effective crystal grain size, which is the fracture unit of the crystal. When the average effective crystal grain size is 10 μm or less, the propagation energy of brittle cracks can be absorbed and the arrest property required for the present invention can be secured. If the average effective crystal grain size exceeds 10 μm, the arrest property cannot be ensured.

Therefore, the average effective crystal grain size is 10 μm or less, preferably 9.9 μm or less, and more preferably 9.5 μm or less. The smaller the average effective crystal grain size, the better the low temperature toughness and arrest property. Therefore, the lower limit of the average effective crystal grain size is not specified, but is usually 5 μm.

4. Manufacturing Method Next, even if the high-strength thick steel sheet according to the present invention has the above-mentioned chemical composition, if the manufacturing method is not appropriate in order to secure the required arrestability and HAZ toughness, The above composite inclusions cannot be obtained.

(4-1) Step I
In the production of slabs of high-strength pressure steel sheets according to the present invention, Ar gas is blown into the molten steel from above before the RH (Ruhrstahl-Hausen) vacuum degassing treatment in order to control inclusions in the steel. The surface slag reacts with the molten steel. Thereby, the total amount of Fe in the slag is adjusted, and the oxygen potential in the molten steel is controlled to 10 to 30 ppm.

Here, the flow rate of Ar gas is adjusted between 100 and 200 L / min, and the blowing time is adjusted between 5 and 15 min.
After that, each element is added by RH vacuum degassing treatment to adjust the composition, and a slab having a thickness of 300 mm is cast by continuous casting.

(4-2) Steps II to IV
Next, the following steps II to IV are sequentially performed on the slab obtained in step I.
Step II: Heating to a temperature range of 950 to 1100 ° C. and soaking. Step III: Hot rolling to a desired plate thickness so that the cumulative reduction rate in the temperature range of 900 ° C. or lower is 50% or more Step IV: Heat Forced cooling (direct quenching) from a temperature of 700 ° C or higher after the end of inter-rolling
Further, if necessary, step V may be performed after step IV.
Step V: Tempering at a temperature of 600-650 ° C

In step II, if the heating temperature is less than 950 ° C., the solid solution Nb is insufficient and the effect of widening the unrecrystallized region is insufficient, so that high-density dislocations cannot be introduced. For this reason, the structure cannot be miniaturized due to the lack of metamorphic nucleation sites, and low temperature toughness and arrestability cannot be ensured. On the other hand, when the heating temperature exceeds 1100 ° C., the coarsening of the austenite grains becomes remarkable, and the microstructure cannot be sufficiently refined. Therefore, low temperature toughness and arrestability cannot be ensured. Therefore, the heating temperature is 950 to 1100 ° C.

In step III, even in a state where the solid solution Nb is secured and the unrecrystallized region is widened, the dislocations introduced by rolling in the temperature range exceeding 900 ° C. gradually recover during rolling. , Dislocations cannot be accumulated enough to make the structure sufficiently fine. Rolling at 900 ° C or lower is preferable for the accumulation of dislocations, but on the other hand, rolling in the low temperature region has a large deformation resistance of the steel sheet and a large load on the rolling mill, so from the equipment side, this temperature range It is preferable that the rolling pressure is small.

If hot rolling is performed in a temperature range of 900 ° C. or lower with a cumulative reduction rate of 50% or more, preferably 50 to 60%, the required arrestability is ensured and the load on the equipment is allowed. However, if there is no problem with the equipment, there is no problem with the arrest property even if the rolling is performed at a cumulative rolling reduction rate of more than 60%. Here, the 900 ° C. cumulative reduction rate is defined as the following (Equation 2).

(Cumulative reduction rate [%] below 900 ° C) = {(Steel plate thickness [mm] at 900 ° C rolled)-(Steel plate thickness after rolling [mm])} / {(Cast thickness [mm])-(Rolling Rear steel plate thickness [mm])} × 100 ・ ・ ・ (2)

In step IV, forced cooling is performed shortly after the completion of hot rolling. It is preferable to forcibly cool the product as soon as possible after the hot rolling is completed, but it may not be possible to cool the product immediately due to the positional relationship between the rolling mill and the cooling device. Even in such a case, a sufficient quenching effect can be obtained by forcibly cooling from at least 700 ° C. or higher. In order to obtain sufficient quenching strength, it is preferable that forced cooling is performed at a cooling rate such that a cooling medium such as water uniformly hits the entire surface of the steel sheet and the central portion of the plate thickness is 1 ° C./sec or more.

Further, step V can be performed if necessary, and the toughness can be improved by tempering at 600 to 650 ° C.
In this way, the high-strength thick steel sheet according to the present invention can be manufactured. Further, the high-strength thick steel sheet according to the present invention and the method for producing the same will be specifically described, but this is an example of the present invention, and the present invention is not limited thereto.

In the present invention, a 300 mm thick slab having the chemical composition shown in Tables 1 and 2 was prepared by a continuous casting method by melting in a converter. Here, from the viewpoint of controlling the composite inclusions, the oxygen potential in the molten steel before the RH vacuum degassing treatment in the converter was adjusted to the amounts shown in Tables 3 and 4, and then Ti and the like were added to adjust the components.

After that, the temperature of the molten steel is not excessively raised in the continuous casting process, and the difference between the solidification temperature determined by the chemical composition of the molten steel is controlled to be within 50 ° C. It was reduced to a 300 mm thick slab.

The slabs having the chemical compositions shown in Tables 1 and 2 are heated, soaked, hot-rolled, and hardened according to the processing conditions shown in Tables 3 and 4, and some samples are further tempered. , A high-strength thick steel plate having a plate thickness of 25 to 80 mm was obtained.

For the calculation of the MnS area ratio and the MnS peripheral length ratio in the cross section of the Ti-based composite inclusions, a test piece for analyzing the composite inclusions collected from a 1/4 t portion of the high-strength thick steel plate was used. For the composite inclusions, the MnS area ratio and the MnS circumference ratio at the interface of the composite inclusions were measured from the mapping image obtained by surface-analyzing the composite inclusions using an electron probe microanalyzer (EPMA). The MnS area ratio and the MnS peripheral length ratio at the interface of the composite inclusions were determined by performing an EPMA analysis of 20 samples for each sample and calculating an average value.

Further, the number density of Ti-based composite inclusions is a composite having a particle size in the range of 0.5 to 5.0 μm from the shape measurement data of the composite inclusions obtained from the automatic inclusion analyzer combined with SEM-EDX. The number density was calculated by calculating the number of inclusions. The calculated results are shown in Tables 3 and 4.

The average effective crystal grain size was identified using the Orientation Imaging Microscopy method (OIM method) using the electron backscatter diffraction method (EBSD) of a field emission scanning electron microscope (FE-SEM). In the EBSP-OIM method, a highly inclined sample is irradiated with an electron beam in a scanning electron microscope (SEM), and the Kikuchi pattern formed by backscattering is photographed with a high-sensitivity camera and irradiated by computer image processing. It consists of a device and software for measuring the crystal orientation of a point in a short waiting time.

This makes it possible to quantitatively analyze the microstructure and crystal orientation of the surface of the bulk sample. The grain is visualized from the mapped image by defining 15 °, which is generally recognized as the threshold that contributes to the brittle crack propagation stop characteristic, as the grain boundary, and the average effective crystal grain size is obtained. It was.

Specifically, a test piece was collected from a 1 / 4t portion of a high-strength thick steel plate, the cross section of the plate thickness was mirror-polished, and then an electric field polishing was performed to prepare a sample whose surface was adjusted. Then, the crystal grain size was measured using FE-SEM manufactured by JEOL Ltd. (for example, JEOL JSM 7800F). The measurement area was 400 μm × 400 μm, and the measurement step interval was 0.4 μm. The OIM software manufactured by TSL was used for EBSD measurement and analysis. The average effective crystal grain size obtained is shown in Tables 3 and 4.

No. 4 tensile test piece (round bar) (diameter = 14 mm) conforming to JIS Z2241-2016 in the direction perpendicular to rolling from the center of the thickness of the high-strength thick steel sheet obtained in Tables 3 and 4, and JIS Z2242- A Charpy test piece (2 mm V notch test piece) conforming to the 2016 Charpy test piece was collected. The notch position was in the plate thickness direction.

Each mechanical property test was carried out, and the tensile strength, yield strength, and Charpy absorption energy at −60 ° C. were measured. For the brittle crack propagation stop characteristic, a temperature gradient type ESSO test was performed, and the temperature at which ^{Kca was 200 MPa · m 1/2 was evaluated.}

FIG. 1 is a diagram showing a test piece collection procedure for evaluating HAZ toughness in Examples. In FIG. 1, reference numeral 1 indicates a 2 mm V notch Charpy test piece, reference numeral 2 indicates a back plate, reference numeral 3 indicates a weld bead, and reference numeral 4 indicates a steel plate.

For HAZ toughness, after abutting the steel plate 4 with a re-shaped groove, the solid wire for submerged arc welding with a diameter of 4.8 mm for 80 k class steel Y-80 and the welding flux brand name NB-80 (both Nippon Steel & Sumitomo Metal) Welding industry Co., Ltd.) was used to perform multi-layered welding as shown in FIG. 1 at a welding heat input of 60 to 80 kJ / cm.

After that, it is cut perpendicular to the longitudinal direction of welding, and as shown in FIG. 1, a 2 mm V notch Charpy test piece is taken from the surface layer with a 1/4 t melted portion as a notch position, and a Charpy impact test is performed at −40 ° C. to absorb the test piece. Evaluated the energy.

The evaluation criteria for the characteristics are as follows.
-Yield strength: No acceptance criteria.
-Tensile strength: 780 MPa or more was regarded as acceptable.
-Steel toughness: -60 ° C Charpy absorption energy of 100 J or more was accepted.
-Arestability: A temperature at which Kca was 200 MPa · m ^1/2 was -10 ° C or less was accepted.
-HAZ part toughness: -40 ° C Charpy absorption energy of 47J or more was accepted.
The obtained test results are shown in Tables 3 and 4.

Symbols A01 to A30 in Table 3 are examples of the present invention that satisfy all the provisions of the present invention, and symbols B01 to B31 in Table 4 are comparative examples that do not satisfy the provisions of the present invention.

Symbols A01 to A30 have a tensile strength of 780 MPa or more, a -60 ° C Charpy absorption energy of 100 J or more, a temperature at which Kca is 200 MPa · m ^1/2 , a temperature of -10 ° C or less, and a -40 ° C Charpy absorption energy of 47 J or more. It has mechanical properties, is excellent in low temperature toughness and arrest property, and is excellent in HAZ toughness in high heat input welding. Therefore, it is a high-strength thick steel sheet to which high heat input welding can be applied, which is excellent in welding work efficiency, and which is excellent in low temperature toughness, arrest property and HAZ toughness, and has a plate thickness of 25 mm or more. Therefore, the symbols A01 to A30 are preferably used for large structures such as penstocks and pressure vessels.

On the other hand, the symbol B01 had insufficient tensile strength because the C content was below the lower limit of the range of the present invention.
In symbol B02, the toughness of the steel sheet was insufficient because the C content exceeded the upper limit of the range of the present invention.
In symbol B03, the toughness of the steel sheet was insufficient because the Si content was below the lower limit of the range of the present invention.

In symbol B04, the toughness of the steel sheet was insufficient because the Si content exceeded the upper limit of the range of the present invention.
The symbol B05 lacked HAZ toughness because the Mn content was below the lower limit of the range of the present invention and the MnS area ratio and the MnS peripheral length ratio were insufficient.
In symbol B06, the toughness of the steel sheet was insufficient because the Mn content exceeded the upper limit of the range of the present invention.

The symbol B07 lacked HAZ toughness because the Al content exceeded the upper limit of the range of the present invention and the number density was insufficient.
Symbol B08 has insufficient tensile strength because the Cu content is below the lower limit of the range of the present invention.
The symbol B09 lacks the toughness of the steel sheet because the Cu content exceeds the upper limit of the range of the present invention.

Symbol B10 lacked arrestability because the Ni content exceeded the lower limit of the range of the present invention.
Symbol B11 has insufficient tensile strength because the Cr content is below the lower limit of the range of the present invention.
The symbol B12 lacks the toughness of the steel sheet because the Cr content exceeds the upper limit of the range of the present invention.
Symbol B13 has insufficient tensile strength because the Mo content is below the lower limit of the range of the present invention.

The symbol B14 has insufficient toughness of the steel sheet because the Mo content exceeds the upper limit of the range of the present invention.
The symbol B15 has insufficient toughness of the steel sheet because the Nb content is below the lower limit of the range of the present invention.
The symbol B16 has insufficient toughness of the steel sheet because the Nb content exceeds the upper limit of the range of the present invention.
In symbol B17, the toughness of the steel sheet was insufficient because the V content exceeded the upper limit of the range of the present invention.

Symbol B18 lacked arrestability because the Ti content was below the lower limit of the range of the present invention.
The symbol B19 has insufficient toughness of the steel sheet because the Ti content exceeds the upper limit of the range of the present invention.
The symbol B20 has insufficient toughness of the steel sheet because the N content is below the lower limit of the range of the present invention.
The symbol B21 has insufficient toughness of the steel sheet because the N content exceeds the upper limit of the range of the present invention.

The symbol B22 indicates that the oxygen potential in the molten steel before the RH vacuum degassing treatment is below the lower limit of the range of the present invention and the O content is below the lower limit of the range of the present invention, so that the number density is insufficient and the HAZ toughness is reduced. There was a shortage.

In symbol B23, the toughness of the steel sheet was insufficient because the oxygen potential in the molten steel before the RH vacuum degassing treatment exceeded the upper limit of the range of the present invention and the O content exceeded the upper limit of the range of the present invention.

The symbol B24 has insufficient toughness of the steel sheet because the B content is below the lower limit of the range of the present invention.
The symbol B25 lacks the toughness of the steel sheet because the B content exceeds the upper limit of the range of the present invention.
The symbol B26 has insufficient tensile strength because the carbon equivalent Ceq is below the lower limit of the range of the present invention.

The symbol B27 lacked the toughness of the steel sheet because the carbon equivalent Ceq exceeded the upper limit of the range of the present invention.
Symbol B28 lacked arrestability because the cumulative reduction rate of 900 ° C. or lower was below the lower limit of the range of the present invention.

In symbol B29, the oxygen potential in the molten steel before the RH vacuum degassing treatment was below the lower limit of the range of the present invention, and the number density was insufficient, so that the HAZ toughness was insufficient.
In symbol B30, the oxygen potential in the molten steel before the RH vacuum degassing treatment exceeded the upper limit of the range of the present invention, and the MnS area ratio and the MnS peripheral length ratio became insufficient, so that the HAZ toughness was insufficient.

Further, the symbol B31 lacked HAZ toughness because the S content was below the lower limit of the range of the present invention and the MnS area ratio and the MnS peripheral length ratio were insufficient.

1 2mm V notch Charpy test piece 2 Back plate 3 Welded bead 4 Steel plate

Claims (4)

The chemical composition is mass%,
C: 0.05 to 0.13%,
Si: 0.10 to 0.50%,
Mn: 1.0 to 1.6%,
P: 0.015% or less,
S: 0.001 to 0.005%,
Al: 0.0028% or less,
Cu: 0.25 to 0.50%,
Ni: 0.6-2.0%,
Cr: 0.3-1.0%,
Mo: 0.25 to 0.8%,
Nb: 0.010 to 0.030%,
Ti: 0.010 to 0.030%,
N: 0.0020 to 0.0040%,
O: 0.0015 to 0.0035%,
B: 0.0005 to 0.0020%,
V: 0-0.05%,
The rest is Fe and impurities,
The carbon equivalent Ceq defined by the following formula (1) is 0.45 to 0.60.
In addition, the steel contains composite inclusions in which MnS is present around the Ti oxide.
The area ratio of the MnS in the cross section of the composite inclusion is 10 to 50%.
The proportion of MnS at the interface is 10% or more,
The number density of the composite inclusions having a particle size of 0.5 to 5.0 μm ^{is 10 to 40 pieces / mm 2} .
A high-strength thick steel sheet having an average effective crystal grain size of 10 μm or less, a tensile strength of 780 MPa or more, and a plate thickness of 25 mm or more, excellent in low-temperature toughness, arrestability, and HAZ toughness.
Ceq = [C%] + [Si%] / 24+ [Mn%] / 6+ [Ni%] / 40+ [Cr%] / 5+ [Mo%] / 4+ [V%] / 14 ・ ・・ (1)
In the formula (1), the elements with [] represent the content (mass%) of each element. The high-strength thick steel sheet according to claim 1, which contains V: 0.01 to 0.05%. Before the RH vacuum degassing treatment, the oxygen potential in the molten steel was set to 10 to 30 ppm, and the chemical composition was adjusted in the RH vacuum degassing treatment to produce the molten steel.
A slab is produced by a continuous casting method using the molten steel.
The slab is heated and leveled to a temperature of 950 to 1100 ° C.
Hot rolling is performed in a temperature range of 900 ° C. or lower so as to finish to a predetermined plate thickness with a cumulative reduction rate of 50% or more.
The method for producing a high-strength thick steel sheet according to claim 1 or 2, wherein immediately after the hot rolling, quenching is performed directly from a temperature of 700 ° C. or higher. The method for producing a high-strength thick steel sheet according to claim 3, wherein after the direct quenching, a tempering treatment is further performed at a temperature of 600 to 650 ° C.

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