WO2023149157A1

WO2023149157A1 - Steel sheet and method for manufacturing same

Info

Publication number: WO2023149157A1
Application number: PCT/JP2023/000227
Authority: WO
Inventors: 将臣奥谷; 祐介寺澤; 浩文大坪
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-02-03
Filing date: 2023-01-06
Publication date: 2023-08-10
Also published as: JP7493140B2; JPWO2023149157A1

Abstract

Provided is a thick steel sheet of high strength and excellent multi-pass welded joint CTOD characteristics, even having a thickness exceeding100 mm. A steel sheet with a prescribed component composition and Ti/N, Ceq, and Pcm values in specified ranges, wherein the average effective crystal grain size in the center of the sheet thickness is 20 μm or less, and the number of pores in the steel sheet with an equivalent circle diameter of at least 180 μm per 1 mm2 is 0.10 or less.

Description

Steel plate and its manufacturing method

The present invention relates to steel materials suitably used for steel structures such as ships, offshore structures, pressure vessels, line pipes, and offshore wind power generators. In particular, the present invention relates to a thick high-strength steel plate that not only has excellent base material strength and toughness, but also excellent joint CTOD characteristics in multi-layer welds, and a method for producing the same, with respect to steel plates having a thickness of more than 100 mm.

Conventionally, the Charpy test has been mainly used to evaluate the toughness of steel. In recent years, as a method for evaluating fracture resistance with higher accuracy, a crack opening displacement test (hereinafter referred to as "CTOD test") has been applied to steel plates used in steel structures. It is being applied more and more.
In this test, a test piece in which a fatigue pre-crack is introduced in the toughness evaluation part is bent at three points at a low temperature, and the crack opening amount (plastic deformation amount) immediately before fracture is measured to evaluate the brittle fracture generation resistance. It is.

Multi-layer welding is used when thick steel plates are applied to steel structures such as ships, offshore structures, pressure vessels, line pipes, and wind power generators as described above. In the weld heat affected zone (Heat Affected Zone: hereinafter also referred to as "multilayer welding HAZ") of this multi-layer welding, there is a region near the weld line (Coarse Grain Heat Affected) that has become a coarse structure due to the previous welding pass. Zone: hereinafter also referred to as "CGHAZ") is reheated to a two-phase region of ferrite + austenite by a subsequent welding pass, and island-like martensite (Martensite-Austenite Constituent: hereinafter, (Also referred to as "MA".) It is known that a region (Inter-Critically Reheated Coarse Grain Heat Affected Zone: hereinafter also referred to as "ICCG HAZ") is included in which the structure is mixed and the toughness is remarkably low.

Here, since the CTOD test of the welded joint is basically performed on the full plate thickness, when the multi-layer welded HAZ is to be evaluated, the ICCGHAZ structure is included in the region where the fatigue pre-crack is introduced. In addition, since the joint CTOD characteristics obtained by the joint CTOD test are governed by the toughness of the most embrittled structure in the evaluation region, the joint CTOD characteristics of the multi-layer welded HAZ reflect not only the CGHAZ structure but also the ICCGHAZ structure toughness. be done.
Therefore, in order to improve the joint CTOD characteristics of the multi-layer welded HAZ, it is necessary to improve not only the toughness of the CGHAZ structure but also the toughness of the ICCGHAZ structure.

Conventionally, as a technique for improving the toughness of the heat affected zone (HAZ) of a weld, the use of TiN as ferrite transformation nuclei and the suppression of austenite grain coarsening in the CGHAZ by finely dispersing TiN have been performed. Here, since the bond part may be heated to a temperature range in which TiN melts, when the low-temperature toughness requirement of the weld part is severe, it is difficult to satisfy the requirement only with the effect of using such TiN. is coming.

In addition, grain growth suppression of austenite grains by dispersing REM-based oxysulfides generated by adding REM (rare earth metal), and grain growth of austenite grains by dispersing Ca-based oxysulfides generated by adding Ca. Techniques have been used that combine the growth inhibition, ferrite nucleation ability of BN, and oxide dispersion.

For example, Patent Literature 1 and Patent Literature 2 disclose techniques for suppressing austenite grain growth and improving the toughness of weld zones by adding REM together with Ti to disperse fine particles in the steel. ing.
Further, Patent Document 3 proposes a technology for improving HAZ toughness using CaS and a technology for improving base material toughness by hot rolling.

Furthermore, as a countermeasure against the deterioration of the toughness of ICCGHAZ, Patent Document 4 proposes a technique of increasing the base material strength by adding Cu after suppressing the formation of MA by reducing C and Si.
In addition, Patent Document 5 proposes a technique of using BN as ferrite transformation nuclei in the heat affected zone of high heat input welding to refine the HAZ structure and improve the HAZ toughness.

By the way, in recent years, steel structures such as ships, offshore structures, pressure vessels, line pipes, and offshore wind power generators have tended to grow in size. Strengthening is underway. Although it is necessary to increase the amount of alloying elements added in order to achieve both thickening and high strength of the steel sheet, the addition of a large amount of alloying elements makes it difficult to ensure the toughness of the multi-layer welded HAZ. To address this problem, Patent Document 6 discloses a technique for improving low-temperature toughness by controlling the hardness of the central segregation portion.

JP-A-60-152626 JP-A-60-184663 Japanese Unexamined Patent Application Publication No. 2012-184500 JP-A-05-186823 JP-A-61-253344 WO2014/038200

Here, the CTOD specification temperature in the standard (for example, API (American Petroleum Institute) standard RP (Recommended Practice)-2Z) that defines joint CTOD characteristics is usually -10°C.
However, in order to secure new resources in response to the recent increase in energy demand, construction areas for offshore structures, etc. are shifting to cold regions and deep sea regions where resource mining has not been possible so far. . For this reason, there is an increasing demand for steel sheets that are high in strength and thick in wall thickness and that can withstand a lower CTOD specification temperature (for example, about -40°C) than the CTOD specification temperature defined by API standards.

According to the studies of the inventors, the conventional techniques described in Patent Documents 1 to 6 are high-strength and thick-walled steel plates with a thickness of more than 100 mm, which are required in recent years. It was not possible to sufficiently satisfy the joint CTOD characteristics required for welded joints.

For example, Patent Documents 1 and 2 propose a technology for suppressing coarsening of the austenitic structure of the HAZ by adding REM together with Ti to disperse fine particles in the steel. Since this technique is intended for steel materials with relatively low strength and a small amount of alloying elements, it cannot be applied to steel materials with a high strength and a large amount of alloying elements because the HAZ structure does not contain ferrite.

It should be noted that REM-based oxysulfides and Ca-based oxysulfides in Patent Documents 1 and 2 are effective in suppressing austenite grain growth. However, the effect of improving toughness by suppressing coarsening of austenite grains in the HAZ alone cannot satisfy the joint CTOD characteristics at the low temperature specified above.

Also, according to the technology proposed in Patent Document 3, it is possible to satisfy the joint CTOD characteristics at the normal operating temperature (-10°C). However, the joint CTOD characteristics at the above low temperature specification temperature have not been studied.

Similarly, in Patent Document 4, the joint CTOD characteristics at the above low temperature specification temperature are not studied, and it is considered that the low temperature CTOD specification cannot be satisfied only by improving the ICCGHAZ toughness by reducing the base material composition. be done. In addition, reducing the content of alloying elements in the base metal in order to improve the toughness of ICCGHAZ is a technical idea that conflicts with securing strength for thickening. difficult to apply to

The technology proposed in Patent Document 5 is effective when the cooling rate in the weld heat affected zone is slow and the HAZ has a ferrite-based structure, as in the case of high heat input welding. However, in the case of a thick steel plate having a thickness of more than 100 mm, the amount of alloy components contained in the base metal is relatively large, while the amount of heat input is relatively small in multi-layer welding. Therefore, in multi-layer welding of thick steel plates, the HAZ structure is mainly composed of bainite, and the effect of improving the joint CTOD characteristics cannot be obtained.

The technique described in Patent Document 6 proposes a technique for satisfying the joint CTOD characteristics in a low temperature range in a thick steel plate with a thickness of 100 mm or less. Mechanical properties equivalent to those of thick steel plates having a thickness of 100 mm or less have not yet been obtained.

In this way, it is difficult to say that conventionally, the technology for improving the toughness of CGHAZ and ICCGHAZ in the heat-affected zone of multi-layer welding of high-strength steel plates with a thickness of over 100 mm has been established. That is, there is a problem to be solved in order to improve the joint CTOD characteristics in which the notch position is the bond portion where CGHAZ and ICCGHAZ coexist.

The present invention has been made in view of the above problems of the prior art, and its object is to provide excellent CTOD characteristics of joints subjected to multi-layer welding (hereinafter referred to as multi-layer welding joint CTOD characteristics). is more than 100 mm and has high strength, and to provide a method for producing the same.

In addition, high strength in the present invention means that the yield strength at the plate thickness center position in a tensile test is 320 MPa or more, and excellent multi-layer welded joint CTOD characteristics in the present invention means notch position CGHAZ and SC/ It means that the crack opening displacement amount is 0.30 mm or more at the test temperature of −40° C. at each ICHAZ boundary.

In order to solve this problem, the inventors have earnestly studied a technique for improving the joint CTOD characteristics. As a result, the following findings were obtained.
(1) If porosity generated in the slab manufacturing process remains without pressure bonding during rolling, it may become a defect in the steel sheet and become a starting point of fracture. In particular, in order to crimp the porosity at the center of the plate thickness, it is necessary to appropriately introduce strain in the center of the plate thickness during rolling, but it is difficult to do so with thick steel plates with a thickness of over 100 mm. Therefore, the residual porosity of uncrimped parts becomes a problem. However, as a result of examination by the inventors, it was found that at a high temperature of 950 ° C. or higher at the center of thickness of the steel sheet, the average value of the deformation resistance ratio between the center of thickness and the surface of the steel sheet is 0.70 or less, and the reduction ratio / pass It has been found that sufficient strain can be introduced at the center of the plate thickness and sufficient porosity can be crimped by rolling at a cumulative rolling reduction of 30% or more when the rolling is 3% or more.
In addition, in the present invention, the plate thickness center portion is a region having a thickness of 10% of the plate thickness in both surface directions of the steel plate from the plate thickness center.

(2) In addition, there is a problem that an element segregation region exists in the thickness central portion of the slab, and coarse inclusions are dispersed at a low density due to the concentration of the alloying elements in this region. However, as described above, the temperature at the center of the thickness of the steel sheet is at a high temperature of 950 ° C. or higher, and the average value of the deformation resistance ratio between the center of the thickness of the steel sheet and the surface of the steel sheet is 0.70 or less, and the rolling reduction / pass is 3. % or more with a cumulative rolling reduction of 30% or more, the strain applied to the thickness center can be increased. As a result, it was found that coarse inclusions can be elongated and split, and fine inclusions can be dispersed at high density. In addition, the inventors have found that as a result of such dispersion, the HAZ toughness improvement effect of the inclusions can be ensured.

(3) Furthermore, in order to finely disperse and precipitate TiN in the steel, which is effective for suppressing the growth of austenite grains, Ti and N have a relationship of 1.50 ≤ Ti/N ≤ 5.00 with respect to the components of the steel sheet. In addition to containing satisfactorily, carbon equivalent Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) ≤ 0.540%, weld crack susceptibility index Pcm = [C] + [Si] / 30 + ([Mn] + [Cu] + [Cr]) / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5 [B]) By controlling each in the range of ≤ 0.250%, it is possible to improve the toughness of the base structure of the HAZ subjected to multi-layer welding (hereinafter referred to as multi-layer welding joint HAZ), and the low temperature CTOD specification It was found that good joint CTOD characteristics compatible with

In addition, the inventors have found that the base material transformation region / The joint CTOD characteristics at the SC/ICHAZ (Sub-Critically reheated HAZ/Inter-Critically reheated HAZ) boundary, which is the boundary of the untransformed region, were also studied. As a result, the following findings were obtained.
(4) In order to satisfy the joint CTOD characteristics at the test temperature of -40°C at the SC/ICHAZ boundary, the base material toughness is dominant over the joint CTOD characteristics at the SC/ICHAZ boundary. It was found that it is necessary to improve the toughness of the base material by making the crystal grain size 20 μm or less and refining the crystal grains.

(5) In a thick steel plate having a thickness of more than 100 mm, the cooling rate at the central portion of the plate thickness is low, so the crystal grains at this portion become coarse. However, at a plate thickness center temperature of less than 950 ° C., the average value of the deformation resistance ratio between the plate thickness center and the surface of the steel plate is 0.70 or less. It has been found that it is possible to introduce sufficient strain to the portion, and that the crystal grain refinement can be achieved to the above crystal grain size.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
1. % by mass, C: 0.02 to 0.12%, Si: 0.70% or less, Mn: 0.3 to 3.0%, P: 0.050% or less, S: 0.0050% or less, Al: 0.002 to 0.100%, Ti: 0.002 to 0.060%, N: 0.0130% or less, and O: 0.0100% or less, the balance being Fe and inevitable impurities has a component composition that satisfies the following formulas (1) to (3),
A steel sheet having an average effective crystal grain size of 20 μm or less at the center of the sheet thickness, and having 0.10 or less porosities per 1 mm ² having an equivalent circle diameter of 180 μm or more in the steel sheet.
1.50≤Ti/N≤5.00 (1)
0.280% ≤ Ceq (= [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) ≤ 0.540% ... (2)
P cm (= [C] + [Si] / 30 + ([Mn] + [Cu] + [Cr]) / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5 [B]) ≤ 0.250 % ... (3)
(However, the parentheses in the formulas (1) to (3) represent the content (% by mass) of the elements in the parentheses, and are zero when the elements are not contained.)

2. The component composition is further, in mass%, Ni: 2.0% or less, Ca: 0.0180% or less, Cu: 2.00% or less, Cr: 2.00% or less, Mo: 2.00% or less. , Nb: 0.070% or less, V: 0.20% or less, W: 0.50% or less, B: 0.0050% or less, REM: 0.030% or less, and Mg: 0.0150% or less 2. The steel sheet according to 1 above, including one or more selected from the group consisting of:

3. 3. A method for manufacturing the steel sheet according to 1 or 2 above,
A steel slab having the chemical composition described in 1 or 2 above is heated to a temperature in the range of 990° C. or higher and 1200° C. or lower, and the condition of the following formula (4) is satisfied, and the temperature at the thickness center is 950° C. or higher. In rolling, hot rolling is performed with a cumulative reduction ratio of 30% or more when the reduction ratio/pass is 3% or more, and a cumulative reduction ratio of 40% or more in rolling at a temperature at the center of the thickness of less than 950 ° C. Then, when cooling to a cooling stop temperature of 600 ° C. or less at an average cooling rate of 1.0 ° C./s or more at the center of the plate thickness, when the cooling stop temperature is 500 ° C. or less, from 700 ° C. to 500 ° C. The average value of the cooling rate is the average cooling rate, and when the cooling stop temperature is higher than 500 ° C., the average value of the cooling rate from 700 ° C. to the cooling stop temperature higher than 500 ° C. is the average cooling rate. A method for manufacturing a steel plate.
k _fm (plate thickness center)/k _fm (surface) ≤ 0.70 (4)
(Here, k _fm is according to formula (5))

(However, [C] in formulas (5) to (7) is the mass % of C, _Tk is the absolute temperature (K) at the center of the plate thickness or the surface of the steel plate, _h0 is the plate thickness at the entry side of rolling, and _h1 is Plate thickness on the rolling exit side, n is the roll rotation speed (rpm), r is the rolling reduction, and R is the roll radius (mm))

4. 3. The method for producing a steel sheet according to 3 above, wherein after cooling to the cooling stop temperature, tempering is performed at a temperature of 700° C. or less.

According to the present invention, it is possible to provide a thick steel plate that has high strength even if it has a thickness of more than 100 mm and that is excellent in multi-layer welded joint CTOD characteristics.

Hereinafter, reasons for limiting each constituent element of the present invention will be described.
[Component composition]
First, the reasons for limiting the chemical composition of the steel plate and the steel billet in the present invention to the above range will be explained. In addition, "%" regarding a component composition means "mass %" unless otherwise indicated.
C: 0.02-0.12%
C is an element that enhances hardenability and strength of steel, and should be contained in an amount of 0.02% or more. However, if the C content exceeds 0.12%, the hardness of the C-enriched portion increases and the joint CTOD characteristics deteriorate. Therefore, the C content should be in the range of 0.02 to 0.12%. Preferably, the lower limit is 0.04% and the upper limit is 0.09%.

Si: 0.70% or less Si is an element that is inevitably contained as an impurity, and has the effect of improving the strength. However, if the Si content exceeds 0.70%, the joint CTOD characteristics are degraded. Therefore, the upper limit of the Si content is limited to 0.70%. Preferably, it is 0.50% or less. On the other hand, the lower limit is not particularly limited, but about 0.04% is preferable.

Mn: 0.3-3.0%
Mn is an element that has the effect of improving the strength of the base metal and the weld zone through the improvement of the hardenability of steel. In order to obtain such effects, addition of 0.3% or more is necessary. Preferably, it is 0.5% or more. On the other hand, adding more than 3.0% not only lowers weldability, but also causes excessive hardenability and lowers the toughness of the base metal and the weld zone, thereby deteriorating the joint CTOD characteristics. Therefore, the Mn content should be in the range of 0.3 to 3.0%. Preferably, it is 2.8% or less.

P: 0.050% or less P is an element that has a large effect of embrittlement of grain boundaries, and if added in a large amount, reduces HAZ toughness and joint CTOD characteristics. Therefore, the P content is limited to 0.050% or less. Preferably, it is 0.030% or less. On the other hand, since it is desirable to reduce the P content as much as possible, the lower limit of the P content is not particularly limited. Therefore, the P content is preferably 0.001% or more.

S: 0.0050% or less S is an element that degrades joint CTOD characteristics, so the upper limit of the S content is limited to 0.0050%. Preferably, it is 0.0030% or less. On the other hand, since it is desirable to reduce the S content as much as possible, the lower limit of the S content is not limited. Therefore, the S content is preferably 0.0001% or more.

Al: 0.002-0.100%
Al is an element necessary for forming inclusions for improving the toughness of the multi-layer welded HAZ and improving the joint CTOD characteristics, and must be added in an amount of 0.002% or more. Preferably, it is 0.005% or more. On the other hand, if it is added in excess of 0.100%, the joint CTOD characteristics in the low temperature range will deteriorate. Therefore, the Al content should be in the range of 0.002 to 0.100%. Preferably, it is 0.075% or less.

Ti: 0.002-0.060%
Ti precipitates in steel as TiN. The precipitated TiN has the effect of suppressing coarsening of austenite grains in the base metal and HAZ, refines the HAZ structure, and improves joint CTOD characteristics. In order to obtain such effects, addition of 0.002% or more is necessary. Preferably, it is 0.005% or more. On the other hand, when the Ti content exceeds 0.060%, solid solution Ti and coarse TiC are precipitated, which rather decreases the weld heat affected zone toughness and deteriorates the joint CTOD characteristics. Therefore, the Ti content should be in the range of 0.002 to 0.060%. Preferably, it is 0.050% or less.

N: 0.0130% or less N is an element that lowers the HAZ toughness and deteriorates the joint CTOD characteristics, so the upper limit of the N content is limited to 0.0130%. On the other hand, since it is desirable to reduce the N content as much as possible, the lower limit of the N content is not limited. Therefore, the N content is preferably 0.0005% or more.

O: 0.0100% or less O is an element that lowers the HAZ toughness and deteriorates the joint CTOD characteristics, so the upper limit of the O content is limited to 0.0100%. On the other hand, since it is desirable to reduce the O content as much as possible, the lower limit of the O content is not limited. Therefore, the O content is preferably 0.0005% or more.

The chemical composition of the steel plate in one embodiment of the present invention shall consist of the above elements and the balance of Fe and unavoidable impurities.
In another embodiment of the present invention, Ni, Ca, Cu, Cr, Mo, Nb, V, W, One or more selected from the group consisting of B, REM, and Mg can be further arbitrarily contained in the content shown below.

Ni: 2.0% or less Ni is an element that can increase the strength of a steel plate without significantly deteriorating the toughness of both the base metal and the joint, but the addition of Ni increases the manufacturing cost and environmental load. . Conventionally, inclusion of Ni was essential in order to secure base material toughness and joint toughness. However, in the present invention, by performing rolling with a controlled deformation resistance ratio, it is possible to manufacture a high-strength steel plate having a thickness of more than 100 mm and excellent CTOD characteristics of multi-layer welded joints without containing Ni. On the other hand, Ni may be contained in order to further improve toughness. In that case, the Ni content of 2.0% or more poses problems of an excessive increase in manufacturing cost and an increase in environmental load. Therefore, the Ni content is limited to 2.0% or less. More preferably, it is 1.8% or less. On the other hand, when Ni is added, 0.1% or more is desirable.

Ca: 0.0180% or less Ca is an element that improves the toughness of the multi-layer welding HAZ by forming an oxysulfide that is highly stable at high temperatures. Decrease CTOD characteristics. Therefore, the upper limit of Ca content is limited to 0.0180%. More preferably, it is 0.0160% or less. On the other hand, when adding Ca, 0.0002% or more is desirable.

Cu: 2.00% or less Cu is an element that can increase the strength of steel plates without greatly deteriorating the base metal and joint toughness. Surface cracks due to the Cu-enriched layer formed directly under the tile become a problem. Therefore, the Cu content is limited to 2.00% or less. More preferably, it is 1.50% or less. On the other hand, when adding Cu, 0.05% or more is desirable.

Cr: 2.00% or less Cr is an element that has the effect of improving the strength of steel by improving the hardenability of steel. Limit the Cr content to 2.00% or less. More preferably, it is 1.50% or less. On the other hand, when Cr is added, 0.05% or more is desirable.

Mo: 2.00% or less Mo is an element that has the effect of improving strength by improving the hardenability of steel. Limit the Mo content to 2.00% or less. More preferably, it is 1.50% or less. On the other hand, when adding Mo, 0.05% or more is desirable.

Nb: 0.070% or less Nb is an element that widens the non-recrystallization temperature range of the austenite phase. Therefore, the addition of Nb is effective for efficiently rolling in the non-recrystallized region and obtaining a fine structure. When Nb is added, it is preferably 0.005% or more. On the other hand, if the Nb addition amount exceeds 0.070%, the joint CTOD characteristics deteriorate, so the Nb content is limited to 0.070% or less. More preferably, it is 0.050% or less.

V: 0.20% or less V is an element that improves the strength of the base metal, and when V is added, it is preferably 0.01% or more. On the other hand, if the V content exceeds 0.20%, the HAZ toughness deteriorates and the joint CTOD characteristics deteriorate, so the V content is limited to 0.20% or less. More preferably, it is 0.15% or less.

W: 0.50% or less W is an element that improves the strength of the base metal, and when W is added, it is preferably 0.05% or more. On the other hand, if the W content exceeds 0.50%, the HAZ toughness deteriorates and the joint CTOD characteristics deteriorate, so the W content is limited to 0.50% or less. More preferably, it is 0.40% or less.

B: 0.0050% or less B is an element that can improve the hardenability and thereby improve the strength of the steel sheet when contained in a very small amount. On the other hand, if the B content exceeds 0.0050%, the HAZ toughness decreases and the joint CTOD characteristics deteriorate, so the B content is limited to 0.0050% or less. More preferably, it is 0.0040% or less.

REM: 0.030% or less REM (rare earth metal) is an element that suppresses the growth of austenite grains in the HAZ by forming oxysulfide inclusions and improves the toughness of the HAZ. is preferably 0.001% or more. On the other hand, when the REM content exceeds 0.030%, the base material toughness and HAZ toughness are rather lowered, and joint CTOD characteristics are deteriorated. Therefore, the REM content is limited to 0.030% or less. More preferably, it is 0.025% or less.

Mg: 0.0150% or less Mg is an element that suppresses the growth of austenite grains in the weld heat affected zone by forming oxide inclusions and improves the toughness of the weld heat affected zone. If so, 0.0002% or more is desirable. However, if the Mg content exceeds 0.0150%, the effect of addition is saturated, and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, the Mg content is limited to 0.0150% or less. More preferably, it is 0.0100% or less.

In the present invention, the chemical composition of the steel plate and the steel billet must satisfy the following conditions of Ti/N, Ceq and Pcm, respectively.
1.50≤Ti/N≤5.00 (1)
Ti/N controls the amount of dissolved N in the HAZ and the precipitation state of TiN. When Ti/N is less than 1.50, HAZ toughness deteriorates due to the presence of solid solution N which is not fixed as TiN, and joint CTOD characteristics deteriorate. On the other hand, if Ti/N is greater than 5.00, the HAZ toughness deteriorates due to the precipitation of coarse TiN, and the joint CTOD characteristics deteriorate. Therefore, the range of Ti/N is set to 1.50 to 5.00. Preferably, the lower limit is 1.80 and the upper limit is 4.50.

Ceq: 0.280% or more and 0.540% or less When the carbon equivalent Ceq defined by the following formula (2) increases, the amount of structures with poor toughness such as island martensite and bainite in the HAZ structure increases. Toughness deteriorates. That is, if the Ceq is greater than 0.540%, the required joint CTOD characteristics cannot be satisfied even if the HAZ toughness improvement technique using inclusions is used due to the deterioration of the toughness of the HAZ base structure itself. On the other hand, if the Ceq is less than 0.280%, the target strength cannot be secured. Therefore, the range of Ceq is set to 0.280 to 0.540%. The lower limit is preferably 0.300% and the upper limit is 0.500%.
Ceq(%)=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5 (2)

Pcm: 0.250% or less As the weld crack susceptibility index Pcm defined by the following formula (3) increases, structures with poor toughness such as island martensite and bainite in the HAZ structure increase, and as a result, the HAZ toughness deteriorates. do. If Pcm exceeds 0.250%, the necessary joint CTOD characteristics cannot be obtained due to deterioration in the toughness of the HAZ base structure itself. Therefore, Pcm is made 0.250% or less. Preferably, it is 0.240% or less. On the other hand, the lower limit is not particularly limited, but if Pcm is excessively reduced, the value of Ceq becomes too low, so about 0.140% is preferable.
Pcm (%) = [C] + [Si] / 30 + ([Mn] + [Cu] + [Cr]) / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5 [B] (3 )

The parentheses in the above formulas (1) to (3) all represent the content (% by mass) of the element shown in the parentheses, and are zero when the element is not contained.

[Average effective grain size]
Average effective grain size at the center of plate thickness: 20 µm or less In the present invention, the average effective grain size of the microstructure at the plate thickness center of a steel plate having a thickness of more than 100 mm is set to 20 µm or less. By improving the toughness of the base material by refining the crystal grains at the thickness center where segregation tends to occur as described above, the joint CTOD characteristics at the SC/ICHAZ boundary can be improved. On the other hand, the smaller the average effective crystal grain size, the more advantageous it is, so the lower limit is not particularly limited, but it is usually about 1 μm.

Here, the "effective crystal grain size" in the present invention is defined as the circle-equivalent diameter of a crystal grain surrounded by grain boundaries with an orientation difference of 15° or more from adjacent crystal grains, that is, large-angle grain boundaries. Also, the average effective crystal grain size at the central portion of the plate thickness can be measured by the method described in the examples below.

[Porosity number density]
Porosity number density: 0.10/mm ² or less As described above, the porosity remaining in the steel sheet serves as a starting point for fracture, and deteriorates the joint CTOD characteristics. In particular, if the number of porosities with an equivalent circle diameter of 180 μm or more in the steel sheet per 1 mm ² (also simply referred to as “the number density of porosities” in the present invention) exceeds 0.10, the cracks in the joint CTOD test It is extremely likely that the cleft displacement (δ) will be an insufficient value. In addition, the higher the number density of porosities, the lower the yield strength at the plate thickness center position of the base material. Therefore, it is important to set the number density of the porosities to 0.10/mm ² or less.

The porosity number density in the present invention refers to the average number density of the total thickness x the total width in a thickness direction cross section (a cross section perpendicular to the rolling direction) parallel to the plate width direction of a thick steel plate. The number density of the porosity can be measured by the method described in the examples below, but the measurement method is not limited to the method described in the examples, and can be measured using a known measurement method. can.

In addition, the measurement frequency of such porosity number density may be determined by measuring 1 to 2 cross sections of any one steel sheet among steel sheets having the same smelting conditions and the same rolling conditions. As long as the smelting method and rolling conditions of the steel slab are not changed, the porosity number density can be produced with good reproducibility, so it can be said that the measurement results at the above measurement frequency represent the whole.

[Production method]
Next, the reasons for limiting each condition in the method for manufacturing a thick steel plate in the present invention will be explained below. Note that the temperature in the following description is the temperature at the center of the sheet thickness unless otherwise specified. The plate thickness center temperature can be actually measured as in the examples described later, but in an actual production line or the like, it may be obtained by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer.

Heating conditions for steel slab In the present invention, the method for melting the steel slab is not particularly limited, and any known melting method such as a converter, an electric furnace, or a vacuum melting furnace is suitable. Such billets are produced, for example, by continuous casting. Further, the molten steel obtained by melting such steel slabs may be further subjected to secondary refining such as ladle refining.
The steel slab manufactured as described above is heated to 990° C. or higher and 1200° C. or lower. If the heating temperature is lower than 990° C., the conditions for hot rolling described below cannot be satisfied, and sufficient effects cannot be obtained. On the other hand, if the heating temperature is higher than 1200° C., the austenite grains become coarse and the desired fine grain structure cannot be obtained after controlled rolling. Therefore, the heating temperature range is set to 990° C. or higher and 1200° C. or lower. Preferably, the lower limit temperature is 990°C and the upper limit temperature is 1180°C.

- Hot rolling conditions In hot rolling, it is important to control the rolling conditions in both the recrystallization temperature range and the non-recrystallization temperature range.
In the recrystallization temperature range, in the rolling at 950 ° C. or higher, the rolling reduction / pass is 3% or more. is 30% or more.

[Deformation resistance ratio between the thickness center of the steel plate and the surface: 0.70 or less]
In the present invention, the average value of the ratio of the deformation resistance k _fm (plate thickness center) at the center of the thickness of the steel plate and the surface deformation resistance k _fm (surface) defined by the following equations (5) to (7) is 0.70 or less (equation (4)). Specifically, while adjusting the roll rotation speed, roll radius, and roll gap to appropriate values, rolling is performed at the timing when the temperature difference between the thickness center and the surface becomes an appropriate value according to the mass% of C. By doing so, the average value of the deformation resistance ratio between the plate thickness central portion and the surface is set to 0.70 or less.
k _fm (plate thickness center)/k _fm (surface) ≤ 0.70 (4)
(Here, k _fm is according to formula (5))

However, [C] in formulas (5) to (7) is the mass% of C, T _k is the location where k _fm is obtained, that is, the absolute temperature (K) at the center of the thickness or the surface of the steel plate, h ₀ is the entry side of the rolling _h1 is the plate thickness on the delivery side, n is the roll rotation speed (rpm), r is the rolling reduction, and R is the roll radius (mm).
The surface temperature of the steel sheet can be measured with a radiation thermometer, and the temperature at the center of the thickness of the steel sheet can be measured as in the examples described later. It may be obtained by heat transfer calculation from the steel plate surface temperature.

Under the condition that the average value of the deformation resistance ratio between the thickness center and the surface of the steel plate according to the above formula (4) exceeds 0.70, for a thick steel plate with a thickness of more than 100 mm, sufficient strain at the thickness center cannot be introduced, and porosity remains. As a result, the porosity number density cannot be reduced to 0.10/mm ² or less. For this reason, the ratio of deformation resistance between the central part of the plate thickness and the surface of the steel plate is set to 0.70 or less, and the cumulative reduction ratio of the reduction when the reduction ratio/pass is 3% or more is set to 30% or more.

[950°C or higher rolling]
Here, the purpose of rolling performed at 950° C. or higher is to refine the structure by recrystallization, to refine and disperse coarse inclusions, and to compress porosity. That is, rolling at a temperature of less than 950° C. makes it difficult for recrystallization to occur, resulting in insufficient refining of austenite grains.

[Reduction ratio/pass is 3% or more]
In rolling with a reduction ratio/pass of less than 3%, sufficient strain cannot be introduced at the center of the plate thickness, and even if the reduction ratio/pass is 3% or more, if the cumulative reduction ratio of the reduction is less than 30% Porosity cannot be sufficiently crimped.

[Rolling below 950°C]
In the non-recrystallization temperature range, that is, rolling at less than 950 ° C., the average value of the deformation resistance ratio between the thickness center and the surface of the steel sheet is 0.70 or less as in the recrystallization temperature range, and the cumulative rolling reduction is 40. % or more.

In the steel of the present invention, recrystallization is unlikely to occur when rolled at a temperature of less than 950°C, so the strain introduced by rolling is accumulated without being consumed by recrystallization, and functions as transformation nuclei in the subsequent cooling step. As a result, the structure of the finally obtained thick steel plate can be refined. However, under the condition that the cumulative rolling reduction in this temperature range is less than 40%, the crystal grain refining effect becomes insufficient. In addition, in a thick steel plate with a thickness of more than 100 mm, under the condition that the average value of the deformation resistance ratio between the center of the thickness and the surface of the steel plate exceeds 0.70, it is possible to introduce sufficient strain at the center of the thickness. As a result, the refinement of the final structure at the central portion of the plate thickness becomes insufficient, and the average effective grain size at the central portion of the plate thickness cannot be reduced to 20 μm or less.
For this reason, the rolling in the non-recrystallization temperature range should be such that the cumulative rolling reduction is 40% or more, and the average value of the deformation resistance ratio between the thickness center and the surface of the steel sheet is 0.70 or less.

[cooling]
After completion of the hot rolling, the obtained hot-rolled steel sheet is cooled. Such cooling can be performed by any method, for example, water cooling, as long as the conditions described below are satisfied.

Average cooling rate: 1.0°C/s or more If the average cooling rate at the thickness center temperature is less than 1.0°C/s, a coarse ferrite phase occurs in the base metal structure, resulting in deterioration of SC/ICHAZ joint CTOD characteristics. do. Therefore, the average cooling rate at the plate thickness center position is set to 1.0° C./s or more. On the other hand, if the average cooling rate is higher than 50.0° C./s, the hard bainite phase increases to increase the strength of the base material and deteriorate the SC/ICHAZ joint CTOD characteristics. It is preferable to set it to 0° C./s or less.

In the present invention, when the cooling stop temperature shown in the next paragraph is 500 ° C. or less, the average cooling rate from 700 ° C. to 500 ° C. is the average cooling rate, and this cooling stop temperature is higher than 500 ° C. In this case, the average cooling rate from 700° C. to the cooling stop temperature higher than 500° C. is taken as the average cooling rate.

Cooling stop temperature: 600°C or less In the cooling, the hot-rolled steel sheet is cooled to a cooling stop temperature of 600°C or less at the thickness center temperature. If the cooling stop temperature is higher than 600° C., the structure after transformation becomes coarse, the strength of the base material becomes insufficient, and the SC/ICHAZ joint CTOD characteristics deteriorate. Therefore, the cooling stop temperature is set to 600° C. or lower.

[Tempering treatment]
Tempering temperature: 700° C. or less After stopping the cooling, further tempering treatment can be optionally performed. The tempering treatment can further improve the toughness of the base material. At that time, if the tempering temperature is higher than 700° C., a coarse ferrite phase is generated and the toughness of SCHAZ is deteriorated. Therefore, the tempering temperature is preferably 700° C. or lower. More preferably, it is 650° C. or less. Although the lower limit of the tempering temperature is not particularly limited, it can be about 300°C.

In addition, in the manufacturing method according to the present invention, any item not described in this specification can be used by a conventional method.

Next, the present invention will be described more specifically based on examples. The following examples show preferred examples of the present invention, and the present invention is not limited by the examples.

Using steel slabs with chemical compositions shown in Table 1, thick steel plates were manufactured under the manufacturing conditions shown in Table 2. During hot rolling, thermocouples were attached to the center positions in the longitudinal direction, the width direction, and the plate thickness direction of the steel material to be rolled, and the temperature at the plate thickness center was actually measured. In addition, the surface temperature of the steel material was measured with a radiation thermometer.

For each of the obtained thick steel plates, the average effective grain size, porosity number density, and yield strength were measured by the following methods.

[Average effective grain size]
A sample was taken from the obtained steel sheet so that the center in the longitudinal direction, width direction, and thickness direction of the steel sheet was the measurement position. Then, after the surface of the sample was mirror-polished, EBSP analysis was performed under the following conditions. From the obtained crystal orientation map, the equivalent circle diameter of the structure surrounded by large-angle grain boundaries with an orientation difference of 15° or more from the adjacent crystal grains is obtained. grain size.
EBSP conditions/analysis area: 1 mm x 1 mm area at the center of plate thickness/step size: 0.4 μm

[Porosity number density]
Ultrasonic flaw detection is often used to detect defects inside steel sheets because it can be inspected non-destructively, but in order to accurately confirm the state of defects, direct observation was performed and the porosity number density was measured. First, in the thickness direction cross section parallel to the strip width direction (cross section perpendicular to the rolling direction) of the rolled material, a sample for observation whose observation surface is full thickness x full width size is taken from the center position of the strip length and mirror polished. Finished. The mirror-polished sample was observed with an optical microscope and photographed, and the obtained photograph was image-analyzed to determine the equivalent circle diameter of each existing porosity. The number of porosities with an equivalent circle diameter of 180 μm or more per 1 mm ² was obtained by dividing the number of porosities with a diameter of 180 μm or more by the measurement area (plate thickness×plate width).

[Yield strength]
A tensile test was performed according to EN10002-1 to determine the yield strength (YS) at 1/4 and 1/2 positions of the plate thickness (t) of the steel plate. For the tensile test, round bar tensile test specimens with a parallel portion diameter of 14 mm and a parallel portion length of 70 mm, which were taken parallel to the plate width direction from 1/4 and 1/2 positions of the plate thickness, were used. When the upper yield point appeared in the tensile test, the upper yield stress was defined as the yield strength. Moreover, when the upper yield point did not appear, 0.2% yield strength was taken as the yield strength.

Next, multi-layer welded joints were produced using each of the thick steel plates. A joint CTOD test was performed on each of the multi-layer welded joints obtained, and the crack opening displacement amount in CGHAZ and the crack opening displacement amount in SC/ICHAZ were measured. The conditions for producing a multi-layer welded joint and the conditions for the joint CTOD test will be described below.

[Joint CTOD test]
Welded joints used in the joint CTOD test were produced by submerged arc welding (multilayer welding) with a K groove shape and a heat input of 5.0 kJ/mm. The test method conforms to BS standard EN10225 (2019), using a test piece with a square cross section of t × t (t is the plate thickness), test temperature: -40 ° C crack opening displacement amount [CTOD value (δ)] was evaluated.

In the above-mentioned joint CTOD test, a test where the notch position was the CGHAZ on the linear shape side of the K groove and a test where the SC/ICHAZ boundary was used were performed, and δ of the CGHAZ and δ of the SC/ICHAZ boundary were measured respectively. For each thick steel plate, the test was performed using three test pieces for each notch position, and the lowest value of the measured values was defined as δ.

After the test, it was confirmed on the fracture surface of the test piece that the tip of the fatigue pre-crack was on each of the CGHAZ and SC/ICHAZ boundaries defined by EN10225 (2019). In the case of the multi-layer welding joint CTOD test, even if the notch position is CGHAZ, a certain amount of ICCGHAZ is included, so the test results reflect the toughness of both CGHAZ and ICCGHAZ.
The above measurement results are also shown in Table 2.

As shown in Table 2, the steel plate satisfying the conditions of the present invention (Invention Example) satisfies the range of the invention in all of the manufacturing conditions, the effective crystal grain size of the base material, and the number density of porosity, and is 1/thickness. The yield strength at the 4th position and the plate thickness center position is 320 MPa or more, and both the CGHAZ CTOD value and the SC/ICHAZ boundary CTOD value are 0.30 mm or more at -40°C, indicating high strength and excellent joint CTOD characteristics. I had it.

On the other hand, among the thick steel plates (comparative examples) that do not satisfy the conditions of the present invention, No. 42, 43 and 47 have a yield strength of less than 320 MPa at the 1/4 plate thickness position and the plate thickness center position. No. No. 20 has a yield strength of less than 320 MPa at the quarter thickness position and the thickness center position and a CTOD value of less than 0.30 mm at the SC/ICHAZ boundary. No. 52 has a yield strength of 320 MPa or more at the position of 1/4 of the plate thickness, but less than 320 MPa at the position of the center of the plate thickness. Other comparative examples have one or both of the CGHAZ CTOD value and the SC/ICHAZ boundary CTOD value of less than 0.30 mm. All comparative examples were inferior to the invention examples in base material strength and joint CTOD characteristics.

Claims

in % by mass,
C: 0.02 to 0.12%,
Si: 0.70% or less,
Mn: 0.3 to 3.0%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.002 to 0.100%,
Ti: 0.002 to 0.060%,
N: 0.0130% or less, and O: 0.0100% or less,
The balance is Fe and unavoidable impurities, and has a component composition that satisfies the following formulas (1) to (3),
A steel sheet having an average effective crystal grain size of 20 μm or less at the center of the sheet thickness, and having 0.10 or less porosities per 1 mm 2 having an equivalent circle diameter of 180 μm or more in the steel sheet.
1.50≤Ti/N≤5.00 (1)
0.280% ≤ Ceq (= [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) ≤ 0.540% ... (2)
P cm (= [C] + [Si] / 30 + ([Mn] + [Cu] + [Cr]) / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5 [B]) ≤ 0.250 % ... (3)
(However, the parentheses in the formulas (1) to (3) represent the content (% by mass) of the elements in the parentheses, and are zero when the elements are not contained.)
The component composition further, in mass %,
Ni: 2.0% or less,
Ca: 0.0180% or less,
Cu: 2.00% or less,
Cr: 2.00% or less,
Mo: 2.00% or less,
Nb: 0.070% or less,
V: 0.20% or less,
W: 0.50% or less,
B: 0.0050% or less,
REM: 0.030% or less and
The steel sheet according to claim 1, containing one or more selected from the group consisting of Mg: 0.0150% or less.
A method for manufacturing the steel sheet according to claim 1 or 2,
A steel billet having the chemical composition according to claim 1 or 2 is heated to a temperature in the range of 990° C. or higher and 1200° C. or lower, the condition of the following formula (4) is satisfied, and the temperature at the center of the plate thickness is 950° C. or higher. In the rolling, the cumulative reduction ratio is 30% or more when the reduction ratio/pass is 3% or more, and in the rolling where the temperature at the thickness center is less than 950 ° C, the cumulative reduction ratio is 40% or more. Then, when cooling to a cooling stop temperature of 600 ° C. or less at an average cooling rate of 1.0 ° C./s or more at the center of the plate thickness, when the cooling stop temperature is 500 ° C. or less, from 700 ° C. to 500 ° C. The average value of the cooling rate is the average cooling rate, and when the cooling stop temperature is higher than 500 ° C., the average value of the cooling rate from 700 ° C. to the cooling stop temperature higher than 500 ° C. is the average cooling rate. A steel plate manufacturing method.
k fm (plate thickness center)/k fm (surface) ≤ 0.70 (4)
(Here, k fm is according to formula (5))

(However, [C] in formulas (5) to (7) is the mass % of C, Tk is the absolute temperature (K) at the center of the plate thickness or the surface of the steel plate, h0 is the plate thickness at the entry side of rolling, and h1 is Plate thickness on the rolling exit side, n is the roll rotation speed (rpm), r is the rolling reduction, and R is the roll radius (mm))
The steel sheet manufacturing method according to claim 3, wherein after cooling to the cooling stop temperature, tempering is performed at a temperature of 700°C or less.