JP2019056145A - High tensile strength thick steel plate and method of producing the same - Google Patents

Abstract

To provide a high tensile strength thick steel plate which has high strength with a tensile strength at a plate thickness central part of 570 MPa or more and is excellent in low temperature toughness, is prevented from occurring low temperature cracking even when the steel plate is welded at a steel plate temperature of 0°C, is excellent in HAZ toughness even when large heat input welding is applied to the steel plate and has plate thickness of 60 mm or more.SOLUTION: The high tensile strength thick steel plate is provided which contains C:0.05-0.14%, Mn:0.8-1.8%, Cu:0.30-0.50%, Cr:0.30-0.60%, Nb:0.015-0.045%, Ti:0.005-0.030%, 0.0003≤B%≤0.0005+[Cu%]/1000+[Ni%]/500+[Mo%]/500, O:0.0005-0.0050%, and Mo:0-0.40%, which has a value of weld cracking parameter Pcm of 0.17-0.24, includes a composite inclusion where Mns is present in the periphery of a Ti oxide in the steel, can be welded without being preheated, has a plate thickness of 60 mm or more and has a tensile strength of 570 MPa or more. An area ratio of Mns in a cross section of the composite inclusion is 10-50%, a ratio of Mns in a peripheral length of a Ti-based composite inclusion is 10% or more, and a number density of the composite inclusion having a particle diameter of 0.5-5.0 μm is 10-40 pieces/mm.SELECTED DRAWING: None

Description

  The present invention relates to a high-strength thick steel plate and a method for manufacturing the same, and more specifically, is used for large structures such as bridges and high-rise buildings, can be applied with high heat input welding, and does not require preheating before welding. The present invention relates to a high-tensile thick steel plate having excellent properties and a method for producing the same.

  High-tensile thick steel plates having a tensile strength of 570 MPa or more are used for structures such as bridges and high-rise buildings. In addition, steel sheets are becoming thicker because of the desire to reduce the main girder of bridges and the design of wide building spaces. However, since thickening the steel plate increases the welding construction cost, examples of applying high heat input welding are increasing for the purpose of reducing the welding construction cost.

  However, if the thickness of the steel sheet is increased, it becomes difficult to ensure the toughness of the steel sheet and the HAZ toughness by high heat input welding. The reason for this is that if the plate thickness of the steel plate is increased, the cooling rate becomes slower during forced cooling, so the transformation starts at a high temperature, thereby forming a soft and coarse structure, making it difficult to ensure strength and toughness. Because it becomes.

  As a method of ensuring strength and toughness by transforming at a low temperature even when the cooling rate is slow, there is a method of adding an alloy element that improves hardenability. However, the addition of the alloy element hardens the HAZ at the time of welding, so that the HAZ toughness is deteriorated and the low temperature cracking sensitivity is also increased.

  Generally, high-strength steel has a high sensitivity to cold cracking, so when welding high-strength steel, the steel sheet is preheated to prevent cold cracking. However, the load of preheating work for the thick steel plate at the welding construction site is large, leading to an increase in welding construction cost. For this reason, the steel plate which does not require preheating is also calculated | required. In winter construction, welding may be performed in an environment where the temperature is 0 ° C., and there is also a demand that preheating is unnecessary even in an environment at 0 ° C.

  Conventionally, many high strength steel plates excellent in toughness and weldability and techniques for their production have been disclosed. However, thick steel plates with a strength exceeding 570 MPa are mainly intended for plate thicknesses of about 50 mm or less, and most steel plates with a plate thickness of 50 mm or more with high welding efficiency and low crack prevention preheating temperature are disclosed. It has not been.

For example, Patent Document 1 discloses a low-yield-ratio high-tensile steel sheet having a plate thickness of 100 mm, a yield ratio of 80% or less, and a tensile strength of 590 MPa or more, and a method for manufacturing the same.
Patent Document 2 discloses an invention of a 600 MPa class steel plate excellent in weldability and a manufacturing method thereof for a plate thickness of 40 to 100 mm.

  Further, Patent Document 3 discloses an invention of a high-tensile thick steel plate excellent in weldability and toughness with good toughness and a manufacturing method thereof. In the invention disclosed in Patent Document 3, the weld crack-preventing steel plate temperature is 0 ° C., indicating excellent low-temperature crack susceptibility.

Japanese Patent Laid-Open No. 9-3596 Japanese Patent Laid-Open No. 9-13143 JP 2000-54064 A

In the invention disclosed in Patent Document 1, the preheating temperature for preventing cracking of welding is set to 50 ° C. or less.
As long as judged from the examples of Patent Document 2, the preheating temperature for preventing welding cracks of the invention disclosed by Patent Document 2 is 25 ° C.

  Furthermore, the toughness in the invention disclosed by Patent Document 3 is only about the steel plate characteristics, and no study has been made on the HAZ toughness which becomes a problem when applying high heat input welding.

  The object of the present invention is to have a tensile strength of 570 MPa or more, excellent toughness, a steel plate temperature for preventing weld cracking to 0 ° C., and a plate thickness of 60 mm or more excellent in HAZ toughness in high heat input welding. It is providing the high-tensile thick steel plate and its manufacturing method.

  In order to solve the above-mentioned problems, the inventors have a tensile strength of 570 MPa or more at the central portion of the plate thickness and excellent toughness, can be welded without preheating even when the steel plate temperature during welding is 0 ° C., and further to HAZ toughness. The chemical composition and production method of excellent high-tensile thick steel plate were studied.

  First, in the production of the slab, a slab with controlled inclusions in the steel is produced, and the slab is subjected to hot rolling and quenching directly from the temperature after completion of the hot rolling to obtain a high-tensile thick steel plate. Manufactured and investigated it.

In order to reduce the low-temperature cracking susceptibility of the high-tensile thick steel plate, it is necessary to lower the value of the weld cracking sensitivity index Pcm defined by the following formula (2).
Pcm = [C%] + [Si%] / 30+ [Mn%] / 20+ [Cu%] / 20+ [Ni%] / 60+ [Cr%] / 20+ [Mo%] / 15+ [ V%] / 10 + 5 × [B%] (2)
In formula (2), the elements with [] represent the content (% by mass) of each element.

  As a result of investigating the relationship between the value of the weld crack susceptibility index Pcm according to this steel sheet composition and the crack stop temperature, in order to set the temperature of the high-tensile thick steel sheet to 0 ° C., the weld crack susceptibility index Pcm is set to 0.24 or less. I understood that I should do. Then, as a result of examining the chemical composition for obtaining desired strength and toughness in the range where the weld crack sensitivity index Pcm is 0.24 or less, the knowledge listed below was obtained.

(A) In order to improve the hardenability without impairing the toughness of the high-tensile thick steel plate, Cr and Cu are effective. Therefore, Cr and Cu are positively used.
(B) Since Mo tends to deteriorate toughness when the content increases, it does not contain much.

  (C) B can significantly improve the hardenability by containing a small amount, and is extremely effective in securing the strength even with a thick steel plate. However, if B is contained in a certain amount or more, the toughness is deteriorated. The presence of Cu, Ni, and Mo is effective for suppressing the deterioration of toughness due to the inclusion of B and increasing the B content. When Cu, Ni, and Mo are contained, even if B is contained in an amount more than the content that deteriorates toughness, deterioration of toughness can be suppressed, and B can be effectively used. This range can be defined by a mathematical formula.

  (D) V was thought to be effective in improving hardenability and suppressing softening due to welding by precipitation hardening, but in a thick steel plate, the cooling rate at the center of the plate thickness is slow, and the cooling process In V, since the V precipitates are coarsened to deteriorate toughness, V is not contained as much as possible.

  (E) Nb is effective as an element replacing V. Nb expands the non-recrystallized region, and when hot rolling is performed in that region, Nb can refine the structure by increasing the number of transformation nucleation sites by introducing high-density dislocations and improve strength and toughness. it can.

  (F) As a result of examining the composition of suitable inclusions that contribute to the refinement of the structure as intragranular transformation nuclei, Ti-based inclusions having MnS around are effective. That is, in order to effectively grow intragranular ferrite within the prior austenite grains during welding, it is essential to control the inclusions that form the intragranular ferrite formation nuclei.

  In particular, with thick steel plates, it is difficult to control the chemical composition and number of inclusions in the thickness direction due to the difference in cooling rate between the surface and inside, so that the inclusions that form the intranuclear ferrite nuclei You need to control things. Then, as a result of examining the growth mechanism of intragranular ferrite, the following (i) to (iii) were found.

    (I) If a composite inclusion containing Ti-based oxide and MnS is generated by precipitation of MnS around the Ti-based oxide in the steelmaking stage, Mn is deficient at the matrix interface of MnS and the base material. A region is formed. In this Mn-deficient region (hereinafter referred to as “initial Mn-deficient region”), the ferrite growth start temperature is greatly increased. Therefore, when the base material is welded, intragranular ferrite grows preferentially from this Mn-deficient region in the cooling process.

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

    (Iii) Since the ferrite amount of the HAZ can be secured by both actions (i) and (ii), the required low temperature toughness of the HAZ can be obtained.

  (G) Intragranular ferrite can be effectively precipitated by controlling the amount of MnS compounded in the inclusion and the number density based on the mechanism of the item (F). Furthermore, in order to obtain the above-described grain refinement effect, inclusions in the steel must satisfy the requirements [1] to [3] listed below.

[1] A composite inclusion having MnS around 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 composite inclusions having a particle size of 0.5 to 5.0 μm is 10 to 40 / mm ² .

In order to obtain an appropriate amount of composite inclusions, it is necessary to control at the steelmaking stage, and the Al content, which tends to form oxides preferentially, is suppressed, and after containing a certain amount of Ti, the oxygen potential is increased. It is important to adjust.
The present invention is based on these findings A to G and is listed below.

(1) Chemical composition is mass%, C: C: 0.05-0.14%, Si: 0.10-0.5%, Mn: 0.8-1.8%, P: 0.00. 015% or less, S: 0.001 to 0.005%, Al: 0.003% or less, Cu: 0.30 to 0.50%, Ni: 0.15 to 0.50%, Cr: 0.30 ˜0.60%, Nb: 0.015 to 0.045%, Ti: 0.005 to 0.030%, B: a range represented by the following formula (1), N: 0.0020 to 0.0050% , O: 0.0005 to 0.0050%, Mo: 0 to 0.40%, V: 0 to 0.008%,
The value of the weld crack sensitivity index Pcm defined by the following formula (2) is 0.17 to 0.24,
The balance is Fe and impurities,
The steel contains a composite inclusion in which MnS is present around Ti oxide in the steel, the area ratio of the MnS in the cross section of the composite inclusion is 10 to 50%, and MnS occupies the circumference of the Ti-based composite inclusion 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 / mm ² , the tensile strength is 570 MPa or more, and welding is possible without preheating. A high-tensile steel plate with a thickness of 60 mm or more.
0.0003 ≦ [B%] ≦ 0.0005 + [Cu%] / 1000+ [Ni%] / 500+ [Mo%] / 500 (1)
Pcm = [C%] + [Si%] / 30+ [Mn%] / 20+ [Cu%] / 20+ [Ni%] / 60+ [Cr%] / 20+ [Mo%] / 15+ [ V%] / 10 + 5 × [B%] (2)
In formulas (1) and (2), the elements with [] represent the content (% by mass) of each element.

  (2) The high-tensile thick steel plate according to item 1, further containing Mo: 0.03 to 0.4% by mass.

(3) Before the RH vacuum degassing treatment, the oxygen potential in the molten steel is 10 to 30 ppm,
In the RH vacuum degassing process, the chemical composition is adjusted to produce molten steel,
A slab is produced by continuous casting using the molten steel,
The slab is heated and soaked to a temperature of 1050 to 1250 ° C., then hot-rolled to a predetermined thickness at 800 to 900 ° C., and quenched immediately after the hot rolling. 3. A method for producing a high-tensile thick steel plate according to item 2.

  (4) The method for producing a high-tensile thick steel plate according to item 3, wherein tempering is performed at a temperature of 350 to 550 ° C. after the quenching.

  In the present invention, the thickness of the high-tensile thick steel plate is defined as 60 mm or more, but the characteristics of the present invention can be obtained if the chemical composition and inclusion requirements defined in the present invention are satisfied. Is not specified. However, considering the production of the high-tensile steel plate according to the present invention, the plate thickness is 100 mm or less.

  With the high-strength thick steel plate having a thickness of 60 mm or more and the manufacturing method according to the present invention, the tensile strength at the central portion of the plate thickness has a high strength of 570 MPa or more and excellent low-temperature toughness. In addition, it is possible to provide a high-strength thick steel plate that has no cold cracking and that has excellent HAZ toughness even when high heat input welding is applied. Thereby, welding construction efficiency can be raised in manufacture of large structures, such as a bridge, and welding construction cost can be reduced significantly.

FIG. 1 is an explanatory view showing a groove shape used for evaluating the HAZ toughness in Examples. FIG. 2 is an explanatory diagram showing a specimen collection procedure for evaluating the HAZ toughness in Examples.

Hereinafter, the present invention will be described in detail. In the following description, “%” regarding chemical composition means “% by mass” unless otherwise specified.
1. Chemical composition The reason for limiting the chemical composition of the high-tensile thick steel plate according to the present invention as described above will be described. First, the essential elements are explained.

(1-1) C: 0.05 to 0.14%
C is the most important element that determines the strength of the high-tensile thick steel plate. If the C content is less than 0.05%, the required strength of 570 MPa or more cannot be obtained. Therefore, the C content is 0.05% or more, preferably 0.07% or more, and preferably 0.08% or more.

On the other hand, if the C content exceeds 0.14%, the steel plate toughness and the HAZ toughness deteriorate and the low-temperature cracking sensitivity increases. Therefore, the C content is 0.14% or less, preferably 0.13% or less, and more preferably 0.12% or less.
In order to obtain characteristics in which the strength, low-temperature toughness and HAZ toughness of the high-tensile thick steel plate are balanced, the C content is preferably 0.07 to 0.11%.

(1-2) Si: 0.10 to 0.5%
Si is an element effective for preliminary deoxidation of molten steel, and has an effect of improving strength without deteriorating toughness. If the Si content is less than 0.10%, this effect cannot be sufficiently obtained. Therefore, the Si content is 0.10% or more, preferably 0.11% or more, and more preferably 0.12% or more.

  On the other hand, if the Si content exceeds 0.5%, the surface properties and HAZ toughness of the steel sheet deteriorate. Therefore, the Si content is 0.5% or less, preferably 0.49% or less, and more preferably 0.42% or less.

(1-3) Mn: 0.8 to 1.8%
Mn is important for improving the strength through improvement of hardenability and is necessary for obtaining a form of inclusion suitable for improvement of HAZ toughness. Therefore, the Mn content is 0.8% or more, preferably 0.83% or more, and more preferably 0.85% or more.

On the other hand, if the Mn content exceeds 1.8%, the toughness of the steel sheet deteriorates. Therefore, the Mn content is 1.8% or less, preferably 1.79% or less, and more preferably 1.60% or less.
In order to obtain characteristics in which the strength, toughness and HAZ toughness of the high-tensile thick steel plate are balanced, the Mn content is preferably 1.0 to 1.4%.

(1-4) P: 0.015% or less P is segregated at the grain boundaries and deteriorates the toughness of the steel sheet. Therefore, the P content is desirably 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 0.011% or less, and more preferably 0.010% or less.

(1-5) S: 0.001 to 0.005%
S is effective to precipitate MnS and form a Mn-deficient region at the matrix interface between MnS and the base material. When the base material is welded, intragranular ferrite grows preferentially from this Mn-deficient region. Therefore, the low temperature toughness of the HAZ can be ensured. Therefore, the S content is 0.001% or more. However, if the S content exceeds 0.005%, the ductility and base metal toughness of the steel sheet may be deteriorated. Therefore, the S content is 0.005% or less, preferably 0.004% or less. More preferably, it is 0.003% or less.

(1-6) Al: 0.003% or less Al is an element contained in order to obtain the cleanliness of molten steel. Since Al forms oxides preferentially over other elements, Ti-based oxides that contribute to the improvement of low temperature toughness and HAZ toughness cannot be obtained. Therefore, the Al content is 0.003% or less, preferably 0.002% or less, and more preferably 0.001% or less.

(1-7) Cu: 0.30 to 0.50%
Cu can improve strength by improving hardenability without significantly impairing weldability and toughness. Therefore, the Cu content is 0.30% or more, preferably 0.31% or more, and more preferably 0.32% or more.

  On the other hand, if the Cu content exceeds 0.50%, the toughness deteriorates. Therefore, the Cu content is 0.50% or less, preferably 0.47% or less, and more preferably 0.45% or less.

(1-8) Ni: 0.15 to 0.50%
Ni improves both hardenability and toughness, and further prevents hot brittleness of a steel sheet containing a large amount of Cu. Therefore, the Ni content is 0.15% or more, preferably 0.17% or more, and more preferably 0.18% or more.

  On the other hand, Ni is an expensive alloy element, and the cost increases when the Ni content exceeds 0.5%. Therefore, the Ni content is 0.50% or less, preferably 0.49% or less, and more preferably 0.47% or less.

(1-9) Cr: 0.30 to 0.60%
Cr can improve the strength without impairing weldability and toughness like Cu. Therefore, the Cr content is 0.30% or more, preferably 0.33% or more, and more preferably 0.40% or more.

On the other hand, if the Cr content exceeds 0.60%, the toughness deteriorates. Therefore, the Cr content is 0.60% or less, preferably 0.59% or less, and more preferably 0.56% or less.
In order to stably secure the strength, the Cr content is preferably 0.40 to 0.60%.

(1-10) Nb: 0.015 to 0.045%
Nb expands the non-recrystallized region, introduces high-density dislocations by rolling in that region, and increases the number of transformation nucleation sites, thereby making it possible to refine the structure and improve toughness. be able to. Accordingly, the Nb content is 0.015% or more, preferably 0.016% or more, and more preferably 0.025% or more.

  On the other hand, if the Nb content exceeds 0.045%, the above effects are saturated and the cost is increased, and the toughness and weldability are deteriorated. Therefore, the Nb content is 0.045% or less, preferably 0.042% or less, and more preferably 0.035% or less.

  The Nb content is preferably 0.025 to 0.035% as an optimum range for obtaining an effect of improving toughness.

(1-11) Ti: 0.005 to 0.030%
Ti is necessary to generate nitrides and suppress the coarsening of crystal grains, and to generate composite inclusions that become intragranular transformation nuclei. However, when the Ti content is less than 0.005%, this effect is not achieved. Therefore, the Ti content is 0.005% or more, preferably 0.010% or more.

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

(1-12) B: Range indicated by the above formula (1) B is extremely effective in improving strength because it can improve the hardenability with a small amount. Furthermore, B segregates at the austenite grain boundaries during welding, transforms from within the grains by lowering the grain boundary energy, and the structure is refined. For this reason, it is effective in improving the HAZ toughness. Therefore, the B content is 0.0003% or more, preferably 0.0005% or more, and more preferably 0.0006% or more.

  On the other hand, if more B is contained, the toughness tends to deteriorate. This tendency of toughness degradation is mitigated by the presence of Cu, Ni and Mo. Therefore, the B content is preferably [B%] ≦ 0.0005 + [Cu%] / 1000+ [Ni%] / 500+ [Mo%] / 500. In this formula, the elements with [] represent the content (% by mass) of each element.

(1-13) N: 0.0020 to 0.0050%
N contributes to toughness improvement by suppressing the coarsening of the structure during heating by forming a nitride. Therefore, the N content is 0.0020 or more, preferably 0.0024%, and more preferably 0.0031%.

  On the other hand, if the N content exceeds 0.0050%, the nitride becomes coarse and the toughness deteriorates. Therefore, the N content is 0.0050% or less, preferably 0.0043% or less, and more preferably 0.0038% or less.

(1-14) O: 0.0005 to 0.0050%
O (oxygen) is effective in producing a composite oxide that becomes an intragranular transformation nucleus. Therefore, the O content is 0.0005% or more, preferably 0.0008% or more, and more preferably 0.0011% or more.

  On the other hand, when O is contained in a large amount, the cleanliness deteriorates remarkably, making it difficult to ensure practical toughness for both the base metal, the weld metal part and the HAZ. Therefore, the O content is 0.0050% or less, preferably 0.0038% or less, and more preferably 0.0035% or less.

Next, arbitrary elements will be described.
(1-15) Mo: 0 to 0.40%
Mo is preferably contained because it has the effect of improving strength and mitigating toughness deterioration due to the addition of B. However, when the Mo content exceeds 0.40%, the toughness is significantly deteriorated. For this reason, in order not to deteriorate toughness even if Mo is contained, the Mo content is 0.40% or less, preferably 0.15%.

  In order to reliably obtain the above effects, the Mo content is preferably 0.03% or more, and more preferably 0.04% or more.

(1-16) V: 0.008% or less V is generally effective in improving hardenability and suppressing softening due to welding by precipitation hardening. However, as described above, in a thick steel plate having a thickness of 60 mm or more, the cooling rate at the central portion of the plate thickness at the time of cooling is slow, and V precipitates are coarsened and the toughness deteriorates during the cooling process. Therefore, even if V is present as an impurity, the V content is 0.008% or less, preferably 0.005% or less, and more preferably 0.004% or less. Most preferably, V is not contained.

(1-17) Value of weld cracking sensitivity index Pcm defined by the above formula (2): 0.17 to 0.24
The higher the value of the weld crack sensitivity index Pcm, the higher the preheating temperature for preventing cold cracking in welding. If the weld cracking sensitivity index Pcm exceeds 0.24%, cold cracking occurs at a steel plate temperature of 0 ° C. Therefore, the weld crack sensitivity index Pcm is 0.24 or less, preferably 0.23 or less, and more preferably 0.22 or less.

  On the other hand, if the value of the weld crack sensitivity index Pcm is less than 0.17%, the required tensile strength cannot be ensured. Therefore, the weld crack sensitivity index Pcm is 0.17 or more, preferably 0.18 or more, and more preferably 0.20 or more.

(1-18) Balance The balance 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 Inclusion The high-tension thick steel plate according to the present invention includes a composite inclusion in which MnS is present around Ti oxide in steel as a composite inclusion contributing to the refinement of the HAZ structure. And the area ratio of MnS in the cross section of a composite inclusion, the ratio of MnS in an interface, the particle size and number density of the inclusion take the following ranges.

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

  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 produce intragranular ferrite when welding. Therefore, the area ratio of MnS in the cross section of the composite inclusion is 10% or more, preferably 11% or more, and more preferably 16% or more.

  On the other hand, when the ratio of MnS in the cross section of the composite inclusion is more than 50%, the composite inclusion is mainly MnS. In this case, the amount of Mn absorbed in the atomic vacancies in the Ti-based oxide is small, the welded Mn-deficient layer is not formed, and the formation of intragranular ferrite becomes difficult. For this reason, the area ratio of MnS in the cross section of the composite inclusion is 50% or less, preferably 49% or less, and more preferably 42% or less.

(2-2) Ratio of MnS in circumference of composite inclusion: 10% or more MnS in the composite inclusion is formed around the Ti-based oxide. If the ratio of MnS in the circumference of the composite inclusion is less than 10%, the initial Mn-deficient region formed at the interface between MnS and the matrix is small, and the amount of intragranular ferrite formed is not sufficient even after welding. Good low-temperature HAZ toughness cannot be obtained. Therefore, the ratio of MnS in the circumference with the matrix of composite inclusions is 10% or more.

  The larger the MnS ratio, the larger the initial Mn-deficient layer and the easier formation of intragranular ferrite. Therefore, the upper limit of the MnS ratio is not defined, but is usually 80% or less.

(2-3) Number density of composite inclusions (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 per unit area of composite inclusions having a prescribed particle size. When the particle size of the composite inclusion is less than 0.5 μm, the amount of Mn that can be absorbed from the periphery of the composite inclusion is small, and as a result, the amount of intragranular ferrite produced decreases. On the other hand, when the particle size of the composite inclusion is larger than 5.0 μm, the composite inclusion becomes a starting point of destruction. For this reason, in this invention, the particle size of the composite inclusion made into object shall be 0.5-5.0 micrometers.

The number density of the composite inclusion having this particle size is related to the Mn absorption amount. In order to generate stable intragranular ferrite, at least one of each composite inclusion needs to be included in the prior austenite. Therefore, the number density of the composite inclusions is set to 10 pieces / mm ² or more. On the other hand, when the composite inclusion is excessively large, the absorbed energy of ductile fracture is reduced. For this reason, the number density of the composite inclusion is set to 40 pieces / mm ² or less.

3. Manufacturing Method Next, even if the high-tensile thick steel plate according to the present invention has the above-described chemical composition, in order to ensure the required HAZ toughness, if the manufacturing method is not appropriate, the above composite Inclusions cannot be obtained.

  In the production of slabs of this steel material, Ar gas is blown into the molten steel from the top before the RH (Ruhrstahl-Hausen) vacuum degassing treatment in order to control the inclusions in the steel, and the slag on the molten steel surface reacts with the molten steel. Thereby, the total Fe amount in the slag is adjusted, and the oxygen potential in the molten steel is controlled to 10 to 30 ppm. The flow rate of Ar gas is exemplified as 100 to 200 L / min, and the blowing time is exemplified as 5 to 15 min.

  Thereafter, each element is added by RH vacuum degassing to adjust the chemical composition to produce molten steel, and a slab of, for example, 300 mm thickness is produced by continuous casting using this molten steel.

Next, this slab is sequentially subjected to the following steps I to III.
Step I: Heat and soak the slab to a temperature range of 1050 to 1250 ° C.
Step II: Hot rolling is performed to a desired plate thickness so that the finishing temperature of hot rolling is 800 to 900 ° C.
Step III: After the hot rolling is completed, forced cooling (direct quenching) is performed immediately.
Furthermore, step IV may be performed after step III as necessary.
Process IV: Tempering is performed at a temperature of 350 to 550 ° C.

  In Step I, if the heating temperature is less than 1050 ° C., the solid solution Nb is insufficient and the effect of expanding the non-recrystallized region is insufficient, so that high-density dislocations cannot be introduced and the transformation nucleation sites are insufficient. For this reason, the structure cannot be refined, and the tensile strength and the low temperature toughness cannot be ensured. On the other hand, when the heating temperature exceeds 1250 ° C., the amount of scale adhesion increases, and soot may be generated during hot rolling. For this reason, the heating temperature of a slab shall be 1050-1250 degreeC.

  In step II, if the finishing temperature of hot rolling is less than 800 ° C., the transformation temperature will be much lower, and sufficient quenching strength will not be obtained. On the other hand, when the finishing temperature exceeds 900 ° C., the toughness of the high-tensile steel plate may be reduced. For this reason, the finishing temperature of hot rolling shall be 800-900 degreeC.

  In Step III, forced cooling is performed without taking time after the end of hot rolling. It is preferable to perform forced cooling as soon as possible after the hot rolling is completed, but in reality, there is a distance from the rolling mill to the cooling device, and there is a limit to the sheet feeding speed, so time is required for cooling. Even if it is not possible to immediately cool in this way, a sufficient quenching effect can be obtained if forced cooling is performed by 200 seconds after the end of hot rolling. Further, the forced cooling is preferably performed at a cooling rate such that a cooling medium such as water is uniformly applied to the entire surface of the steel plate, and the central portion of the plate thickness is 1 ° C./sec or more.

  Furthermore, process IV can be performed as needed and toughness can be improved by tempering at 350-550 degreeC.

  In this way, the high-tensile thick steel plate according to the present invention can be manufactured. The high-tensile thick steel plate according to the present invention has a plate thickness of 60 mm or more, a tensile strength of 570 MPa or more, and can be welded without preheating.

Further, the high-tensile thick steel plate and the manufacturing method thereof according to the present invention will be specifically described with reference to examples.
In the present invention, a 300 mm-thick slab having a chemical composition shown in Tables 1 and 2 (the balance being Fe and impurities) was produced by a continuous casting method. Here, from the viewpoint of controlling composite inclusions, the oxygen potential Oxp in the molten steel before the RH vacuum degassing treatment was adjusted to the amounts shown in Tables 3 and 4 in the converter, and then Ti and the like were added to adjust the components. Then, in the continuous casting process, the temperature of the molten steel is not excessively raised, and the difference is controlled within 50 ° C with respect to the solidification temperature determined from the molten steel composition. Further, electromagnetic stirring immediately before solidification and reduction during solidification To make a slab of 300 mm thickness.

  This slab is heated and soaked under the conditions shown in Tables 3 and 4 and hot-rolled to finish the sheet thickness of 60 to 90 mm, quenched immediately after the hot-rolling, and partly further tempered. By carrying out, a high-tensile thick steel plate was obtained.

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

  Furthermore, the number density of Ti-based composite inclusions is a composite whose particle size is in the range of 0.5 to 5.0 μm from the shape measurement data of composite inclusions obtained from an 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.

  Further, No. 4 tensile test piece (round bar) (diameter = 14 mm) in accordance with JIS Z2241-2016 in the direction perpendicular to the rolling from the center of the thickness of the high-tensile steel plate obtained under the processing conditions shown in Tables 3 and 4. ) And a Charpy test piece (2 mmV notch test piece) based on the Charpy test piece based on JIS Z2242-2016. The notch position was in the thickness direction. Each was subjected to a mechanical property test to measure tensile strength, yield strength, and Charpy absorbed energy at -10 ° C.

  FIG. 1 is an explanatory view showing a groove shape used for evaluating the HAZ toughness, and FIG. 2 is an explanatory view showing a specimen collecting procedure for evaluating the HAZ toughness. 2, reference numeral 1 indicates a steel plate, reference numeral 2 indicates a weld bead, and reference numeral 3 indicates a 2 mmV notch Charpy test piece.

  As shown in Fig. 1, HAZ toughness is obtained by matching high-tensile thick steel plates with X-shaped grooves, then submerged arc welding solid wire product name YD for 60k grade steel and welding flux product. Using the name NF-320M (both manufactured by Nippon Steel & Sumikin Welding Co., Ltd.), multi-layer welding shown in FIG. 2 was performed at a welding heat input of 100 to 120 kJ / cm.

  Then, it cut | disconnected perpendicularly | vertically with the welding longitudinal direction, and as shown in FIG. 2, the Charpy test piece was extract | collected by making the 1 / 2t fusion | melting part into a notch position, the Charpy impact test was performed at -10 degreeC, and the absorbed energy was evaluated. .

Evaluation of cold cracking resistance was performed by a method based on JIS Z3158-1993 (y-type weld cracking test method). A high-tensile thick steel plate is grooved into a y-type with a gap of 1 mm to produce a y-shaped weld crack specimen, and the diameter for 60k class steel at a welding location under controlled atmosphere at a temperature of 0 ° C and a humidity of 60%. Using 1.2 mm gas shielded arc welding wire trade name YM-60C (manufactured by Nippon Steel & Sumikin Welding Co., Ltd.), using CO ₂ gas as the shielding gas, welding heat input 16.0 to 17.5 kJ / The test was conducted under the condition of cm, and the crack ratio was measured by observing the cross section of the welded specimen.

The evaluation criteria for the characteristics were determined to be acceptable when the criteria described below were satisfied.
-Tensile strength: 570 MPa or more--10 ° C absorbed energy: 100 J or more.
-HAZ part-10 degreeC absorbed energy: It must be 47J or more.
-Cold cracking resistance: The cross-sectional cracking rate is 0 at 0 ° C.
The obtained test results are summarized in Tables 3 and 4.

Symbols A01 to A35 in Tables 1 and 3 are examples of the present invention that satisfy all the conditions defined in the present invention, and symbols B01 to B30 in Tables 2 and 4 are comparative examples that do not satisfy the conditions defined in the present invention. is there.
As shown in Tables 1 and 3, the symbols A01 to A35 are tensile strength: 570 MPa or more, −10 ° C. absorbed energy: 100 J or more, and HAZ part −10 ° C. absorbed energy: 47 J or more, and a cross section at 0 ° C. Cracking ratio: 0 weldability.

  For this reason, the high-strength thick steel plates of symbols A01 to A35 have a high strength of 570 MPa or more at the center of the plate thickness and are excellent in low-temperature toughness. Even when welding at a steel plate temperature of 0 ° C., low-temperature cracking occurs. There is no occurrence, and even if high heat input welding is applied, the HAZ toughness is excellent, so that, for example, welding construction efficiency can be increased in the production of large structures such as bridges, and the welding construction cost can be greatly reduced.

On the other hand, the symbol B01 in Tables 2 and 4 was insufficient in tensile strength because the C content was below the lower limit of the range of the present invention.
The symbol B02 has insufficient weldability because the C content exceeds the upper limit of the range of the present invention.
The symbol B03 was insufficient in toughness of the steel sheet because the Si content was below the lower limit of the range of the present invention.

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

The symbol B07 has insufficient HAZ toughness because the Al content exceeds the upper limit of the range of the present invention.
The symbol B08 was insufficient in tensile strength because the Cu content was below the lower limit of the range of the present invention.
The symbol B09 has insufficient toughness of the steel sheet because the Cu content exceeds the upper limit of the range of the present invention.
The symbol B10 was insufficient in toughness of the steel sheet because the Ni content was below the lower limit of the range of the present invention.

The 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 had insufficient Cr toughness because the Cr content exceeded the upper limit of the range of the present invention.
In symbol B13, the Mo content exceeded the upper limit of the range of the present invention, so that the toughness of the steel sheet was insufficient.
In the symbol B14, since the Nb content is below the lower limit of the range of the present invention, the toughness of the steel sheet was insufficient.

In the symbol B15, the Nb content exceeded the upper limit of the range of the present invention, so that the toughness of the steel plate was insufficient.
The symbol B16 has insufficient toughness of the steel sheet because the V content exceeds the upper limit of the range of the present invention.
For the symbol B17, the Ti content was below the lower limit of the range of the present invention, and the number density became insufficient, so that the HAZ toughness was insufficient.
The symbol B18 has insufficient HAZ toughness because the Ti content exceeds the upper limit of the range of the present invention.

The symbol B19 had insufficient HAZ toughness because the B content was below the lower limit of the range of the present invention.
In the symbol B20, since the B content exceeds the upper limit of the range of the present invention, the toughness of the steel sheet was insufficient.
The symbol B21 had insufficient HAZ toughness because the N content was below the lower limit of the range of the present invention.
In symbol B22, since the N content exceeds the upper limit of the range of the present invention, the toughness of the steel sheet was insufficient.

  The symbol B23 indicates that the oxygen potential Oxp in the molten steel before the RH vacuum degassing treatment is lower than the lower limit of the range of the present invention, the O content is lower than the lower limit of the range of the present invention, and the number density becomes insufficient. The toughness was insufficient.

  Symbol B24 indicates that the oxygen potential Oxp in the molten steel before the RH vacuum degassing treatment exceeds the upper limit of the range of the present invention, the O content exceeds the upper limit of the range of the present invention, and the number density becomes insufficient. The toughness was insufficient.

The symbol B25 has insufficient weldability because the weld crack sensitivity index Pcm exceeds the upper limit of the range of the present invention.
The symbol B26 has a lack of tensile strength because the weld crack sensitivity index Pcm is below the lower limit of the range of the present invention.

  For symbol B27, the oxygen potential Oxp in the molten steel before the RH vacuum degassing treatment was below the lower limit of the range of the present invention, and the MnS area ratio and number density were insufficient, so the HAZ toughness was insufficient.

  For symbol B28, the oxygen potential Oxp 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 number density were insufficient, so that the HAZ toughness was insufficient.

In symbol B29, the oxygen potential Oxp in the molten steel before the RH vacuum degassing treatment was below the lower limit of the range of the present invention, the MnS circumferential length ratio was insufficient, and the HAZ toughness was insufficient.
Further, in the symbol B30, the S content was below the lower limit of the range of the present invention, and the MnS area ratio and the MnS circumference ratio became insufficient, so that the HAZ toughness was insufficient.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Weld bead 3 2mmV notch Charpy test piece

Claims (4)

Chemical composition is mass%,
C: 0.05 to 0.14%,
Si: 0.10 to 0.5%,
Mn: 0.8-1.8%
P: 0.015% or less,
S: 0.001 to 0.005%,
Al: 0.003% or less,
Cu: 0.30 to 0.50%,
Ni: 0.15-0.50%,
Cr: 0.30 to 0.60%,
Nb: 0.015 to 0.045%,
Ti: 0.005 to 0.030%,
B: a range represented by the following formula (1),
N: 0.0020 to 0.0050%,
O: 0.0005 to 0.0050%,
Mo: 0 to 0.40%,
V: 0 to 0.008%,
The value of the weld crack sensitivity index Pcm defined by the following formula (2) is 0.17 to 0.24,
The balance is Fe and impurities,
Including a composite inclusion in which MnS is present around Ti oxide in the steel;
The area ratio of MnS in the cross section of the composite inclusion is 10 to 50%,
The proportion of MnS in the circumference of the Ti-based composite inclusion 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-tensile steel plate with a tensile strength of 570 MPa or more and a thickness of 60 mm or more that can be welded without preheating.
0.0003 ≦ [B%] ≦ 0.0005 + [Cu%] / 1000+ [Ni%] / 500+ [Mo%] / 500 (1)
Pcm = [C%] + [Si%] / 30+ [Mn%] / 20+ [Cu%] / 20+ [Ni%] / 60+ [Cr%] / 20+ [Mo%] / 15+ [ V%] / 10 + 5 × [B%] (2)
In formulas (1) and (2), the elements with [] represent the content (% by mass) of each element.   Furthermore, the high-tensile steel plate of Claim 1 containing Mo: 0.03-0.4 mass%. Before the RH vacuum degassing treatment, the oxygen potential in the molten steel is 10 to 30 ppm,
In the RH vacuum degassing process, the chemical composition is adjusted to produce molten steel,
A slab is produced by continuous casting using the molten steel,
The slab is heated and soaked to a temperature of 1050 to 1250 ° C, and then hot-rolled to a predetermined thickness at 800 to 900 ° C, and quenched immediately after hot rolling. A method for producing a high-tensile thick steel plate according to 1 or 2.   The method for producing a high-tensile thick steel plate according to claim 3, wherein tempering is performed at a temperature of 350 to 550 ° C after the quenching.

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