JP7493140B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel material suitable for use in steel structures such as ships, marine structures, pressure vessels, line pipes, and offshore wind power generators. In particular, the present invention relates to a thick, high-tensile steel plate that is excellent not only in the strength and toughness of the base material but also in the CTOD characteristics of joints in multi-layer welds, in comparison with steel plates with a thickness of more than 100 mm, and a method for manufacturing the same.

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 Tip Opening Displacement Test (hereinafter, referred to as "CTOD test") has been increasingly applied to thick steel plates used in steel structures.
In this test, a test piece in which a fatigue pre-crack has been introduced into the toughness evaluation portion is bent at three points at low temperature, and the opening of the crack (amount of plastic deformation) just before fracture is measured to evaluate the resistance to brittle fracture.

When thick steel plates are applied to steel structures such as ships, marine structures, pressure vessels, line pipes, and wind turbines as mentioned above, multi-layer welding is used. It is known that the heat affected zone (hereinafter, also referred to as "multi-layer welding HAZ") of this multi-layer welding includes a region (Coarse Grain Heat Affected Zone: hereinafter, also referred to as "CGHAZ") near the weld line, which has a coarse structure due to a previous welding pass, and a region (Inter-Critically Reheated Coarse Grain Heat Affected Zone: hereinafter, also referred to as "ICCGHAZ") in which an island martensite (Martensite-Austenite Constituent: hereinafter, also referred to as "MA") structure is mixed in the coarse base structure and the toughness is significantly reduced.

Here, since the CTOD test of the welded joint is basically performed over the entire plate thickness, when the multi-pass weld HAZ is evaluated, the region where the fatigue pre-crack is introduced includes the ICCGHAZ structure. Also, since the joint CTOD properties obtained by the joint CTOD test are governed by the toughness of the most embrittled structure in the evaluation region, the joint CTOD properties of the multi-pass weld HAZ reflect not only the CGHAZ structure but also the toughness of the ICCGHAZ structure.
For this reason, in order to improve the joint CTOD characteristics of a multi-pass weld HAZ, it is necessary to improve not only the toughness of the CGHAZ structure but also the toughness of the ICCGHAZ structure.

Conventionally, techniques for improving the toughness of weld heat-affected zones (HAZ) have involved the suppression of coarsening of austenite grains in CGHAZ by finely dispersing TiN, and the use of TiN as a ferrite transformation nucleus. However, since the bond zone may be heated to a temperature range where TiN melts, when the low-temperature toughness requirements of the weld zone are strict, it is becoming difficult to meet the requirements with only the effect of using such TiN.

In addition, technologies have been used that inhibit the grain growth of austenite grains by dispersing REM-based oxysulfides formed by adding REM (rare earth metals), inhibit the grain growth of austenite grains by dispersing Ca-based oxysulfides formed by adding Ca, and combine the ferrite nucleation ability of BN with oxide dispersion.

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

Furthermore, as a measure against the decrease in toughness of ICCGHAZ, Patent Document 4 proposes a technology of increasing the strength of the base material by reducing the C and Si content to suppress the generation of MA and then adding Cu.
In addition, Patent Document 5 proposes a technique of utilizing BN as ferrite transformation nuclei in a high heat input weld heat affected zone to refine the HAZ structure and improve the HAZ toughness.

In recent years, steel structures such as ships, marine structures, pressure vessels, line pipes, and offshore wind turbines have tended to become larger, and as a result, the steel plates used in these structures have become thicker and stronger. In order to achieve both thicker and stronger steel plates, it is necessary to increase the amount of alloying elements added, but adding large amounts of alloying elements makes it difficult to ensure the toughness of the multi-layer weld HAZ. To address this issue, Patent Document 6 discloses a technology that improves low-temperature toughness by controlling the hardness of the central segregation area.

Japanese Patent Application Laid-Open No. 60-152626 Japanese Patent Application Laid-Open No. 60-184663 JP 2012-184500 A Japanese Patent Application Laid-Open No. 05-186823 Japanese Patent Application Laid-Open No. 61-253344 International Publication No. 2014/038200

Here, the CTOD specification temperature in standards that define the CTOD characteristics of joints (for example, API (American Petroleum Institute) standard RP (Recommended Practice)-2Z) is usually -10°C.
However, in order to secure new resources in response to the increase in energy demand in recent years, the construction regions of marine structures, etc. are shifting to cold regions and deep sea areas where resource mining has not been possible until now. As a result, there is an increasing demand for steel plates that are high-strength and thick and can withstand CTOD specification temperatures (e.g., about -40°C) that are lower than the CTOD specification temperature defined by the API standard.

According to the inventors' investigations, the conventional technologies described in Patent Documents 1 to 6 were unable to fully satisfy the joint CTOD characteristics required for multi-pass welded joints for low-temperature specifications in the high-strength, thick steel plates (thickness: over 100 mm) that are in demand in recent years.

For example, Patent Documents 1 and 2 propose a technique for suppressing coarsening of the austenite structure in the HAZ by adding REM together with Ti to disperse fine particles in the steel. This technique is intended for steel materials with relatively low strength and low amounts of alloying elements, and cannot be applied to steel materials with higher strength and higher amounts of alloying elements, because the HAZ structure does not contain ferrite.

In addition, the REM-based oxysulfides and Ca-based oxysulfides described in Patent Documents 1 and 2 are effective in suppressing austenite grain growth. However, the effect of improving toughness by suppressing the coarsening of austenite grains in the HAZ alone is not enough to satisfy the joint CTOD characteristics at the above-mentioned low-temperature specification temperatures.

In addition, the technology proposed in Patent Document 3 can satisfy the joint CTOD characteristics at normal use temperatures (-10°C). However, the joint CTOD characteristics at the above-mentioned low-temperature specification temperatures have not been considered.

Similarly, in Patent Document 4, the joint CTOD characteristics at the above-mentioned low-temperature specification temperatures are not considered, and it is considered that the low-temperature CTOD specifications cannot be satisfied only by improving the ICCGHAZ toughness by reducing the base metal composition. In addition, reducing the alloy element content of the base metal in order to improve the toughness of the ICCGHAZ is a technical idea that contradicts the securing of strength for thickening, and is difficult to apply to thick steel plates used in marine structures, etc.

The technology proposed in Patent Document 5 is effective when the cooling rate in the heat-affected zone is slow, as in the case of high heat input welding, and the HAZ has a structure mainly composed of ferrite. However, in the case of thick steel plates with a plate thickness of more than 100 mm, the amount of alloy components contained in the base material is relatively large, while the 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 technology described in Patent Document 6 proposes a technology for satisfying joint CTOD characteristics at low temperatures for thick steel plates with a plate thickness of 100 mm or less, but it does not yet achieve mechanical properties for extra-thick steel plates with a plate thickness of more than 100 mm that are equivalent to those of thick steel plates with a plate thickness of 100 mm or less.

Thus, it is difficult to say that a technology has been established to improve the toughness of the CGHAZ and ICCGHAZ in the heat-affected zone of multi-layer welds in high-strength thick steel plates with a plate thickness of over 100 mm. In other words, there was a problem to be solved in improving the CTOD characteristics of a joint in which the bond part where the CGHAZ and ICCGHAZ are mixed is the notch position.

The present invention has been made in consideration of the above-mentioned problems associated with the conventional technology, and its purpose is to provide a high-strength thick steel plate having a thickness of over 100 mm and excellent CTOD characteristics of joints subjected to multi-layer welding (hereinafter referred to as multi-layer welded joint CTOD characteristics), and a manufacturing method thereof.

In this invention, high strength refers to a yield strength of 320 MPa or more at the center of plate thickness in a tensile test, and excellent CTOD characteristics of a multi-pass welded joint in this invention refers to a crack opening displacement of 0.30 mm or more at a test temperature of -40°C at both the notch position CGHAZ and the SC/ICHAZ boundary.

In order to solve the above problems, the inventors have conducted extensive research into methods for improving the CTOD characteristics of joints, and have obtained the following findings.
(1) If the porosity generated during the slab manufacturing process is not pressed during rolling and remains, it may become a defect in the steel plate and become a fracture origin. In particular, in order to press the porosity in the plate thickness center, it is necessary to appropriately introduce strain into the plate thickness center during rolling, but this is difficult for thick steel plates with a plate thickness of more than 100 mm, so the remaining porosity that is not pressed becomes a problem. However, as a result of the inventors' studies, it was found that if rolling is performed at a high plate thickness center temperature of 950°C or more, while 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, and the rolling reduction rate/pass is 3% or more, and the cumulative rolling reduction rate is 30% or more, sufficient strain can be introduced into the plate thickness center, and the porosity can be sufficiently pressed.
In the present invention, the plate thickness central portion refers to a region having a thickness of 10% of the plate thickness from the center of the plate thickness toward both surface directions of the steel plate.

(2) In addition, there is a problem that the central part of the thickness of the slab has an element segregation region, and the alloy elements are concentrated in this region, which causes coarse inclusions to be dispersed at a low density. However, by performing rolling with a reduction rate/pass of 3% or more and a cumulative reduction rate of 30% or more while setting the average value of the deformation resistance ratio between the central part of the thickness of the steel plate and the surface of the steel plate to 0.70 or less at a high central part temperature of 950°C or more, as described above, the strain applied to the central part of the thickness can be increased. As a result, it was found that the coarse inclusions are elongated and broken, and fine inclusions can be dispersed at a high density. In addition, it was found that the effect of improving the HAZ toughness by the inclusions can be secured as a result of such dispersion.

(3) Furthermore, in order to finely disperse and precipitate TiN in the steel, which is effective in suppressing austenite grain growth, it has been found that by controlling the contents of Ti and N in the steel plate so as to satisfy the relationship 1.50≦Ti/N≦5.00, and the carbon equivalent Ceq = [C] + [Mn]/6 + ([Cu] + [Ni])/15 + ([Cr] + [Mo] + [V])/5) ≦0.540%, and the weld crack susceptibility index Pcm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B]) ≦0.250%, respectively, it is possible to improve the toughness of the base structure of the HAZ where multi-layer welding has been performed (hereinafter referred to as multi-layer weld joint HAZ), and to obtain good joint CTOD characteristics that can meet low-temperature CTOD specifications.

In addition, the inventors also investigated the joint CTOD characteristics of the SC/ICHAZ (Sub-Critically Reheated HAZ/Inter-Critically Reheated HAZ) boundary, which is the boundary between the transformed and untransformed regions of the base metal during welding, as required by the BS standard (British Standards) EN10225 (2019) and the API standard RP-2Z (2005), which specify the joint CTOD test method. As a result, the following findings were obtained.
(4) It was found that in order to satisfy the joint CTOD characteristics at the SC/ICHAZ boundary at a test temperature of -40°C, the base material toughness is dominant over the joint CTOD characteristics at the SC/ICHAZ boundary, and therefore it is necessary to set the effective crystal grain size of the base material microstructure to 20 μm or less and improve the base material toughness by refining the crystal grains.

(5) In thick steel plates with a thickness of over 100 mm, the cooling rate at the center of the plate thickness is slow, causing the crystal grains at that location to become coarse. However, it has been discovered that by rolling at a cumulative reduction rate of 40% or more under conditions where the average deformation resistance ratio between the center of the plate thickness and the surface of the steel plate is 0.70 or less at a plate thickness center temperature of less than 950°C, it is possible to introduce sufficient strain into the plate thickness center, and to achieve grain refinement to the above-mentioned grain size.

The present invention has been completed based on the above findings and further investigations. That is, the gist of the present invention is as follows.
1. Having a component composition, in mass%, containing 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, with the balance being Fe and unavoidable impurities, which satisfies the following formulas (1) to (3);
A steel plate having an average effective crystal grain size of 20 μm or less at the center of the plate thickness, and having porosity with a circular equivalent diameter of 180 μm or more in the steel plate, with the number of porosity pieces per ¹ mm2 being 0.10 or less.
1.50≦Ti/N≦5.00 ... (1)
0.280%≦Ceq(=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)≦0.540% ... (2)
Pcm (= [C] + [Si] / 30 + ( [Mn] + [Cu] + [Cr]) / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5 [B]) ≦ 0.250% ... (3)
(Note that the parentheses in formulas (1) to (3) represent the content (mass%) of the element in the parentheses, and if the element is not contained, the value is set to zero.)

2. The steel plate according to claim 1, wherein the composition further includes, by mass%, one or more selected from the group consisting of 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.

3. A method for producing the steel plate according to 1 or 2,
3. A method for producing a steel plate, comprising: heating a steel slab having the component composition according to 1 or 2 above to a range of 990°C or higher and 1200°C or lower; hot rolling is performed so as to satisfy the condition of the following formula (4), and in rolling where the temperature at the plate thickness center is 950°C or higher, the reduction/pass is 3% or higher, with a cumulative reduction of 30% or higher, and in rolling where the temperature at the plate thickness center is less than 950°C, the cumulative reduction is 40% or higher; and cooling is then performed at an average cooling rate at the plate thickness center of 1.0°C/s or higher to a cooling stop temperature of 600°C or lower, wherein when the cooling stop temperature is 500°C or lower, the average cooling rate is the average value of the cooling rates from 700°C to 500°C, and when the cooling stop temperature is higher than 500°C, the average cooling rate is the average value of the cooling rates from 700°C to a cooling stop temperature higher than 500°C.
k _fm (center of thickness) / k _fm (surface) ≦ 0.70 ... (4)
(where k _fm is based on equation (5))

（ただし、式（５）～（７）における［Ｃ］はＣの質量％、Ｔ_ｋは板厚中心または鋼板表面の絶対温度（Ｋ）、ｈ_０は圧延入側の板厚、ｈ_１は圧延出側の板厚、ｎはロール回転速度（ｒｐｍ）、ｒは圧下率、Ｒはロール半径（ｍｍ）を表す）

(In the formulas (5) to (7), [C] represents the mass% of C, _Tk represents the absolute temperature (K) at the center of the thickness or at the surface of the steel sheet, _h0 represents the thickness at the entry side of rolling, _h1 represents the thickness at the exit side of rolling, n represents the roll rotation speed (rpm), r represents the reduction ratio, and R represents the roll radius (mm).

4. A method for manufacturing a steel sheet as described in 3 above, in which 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 having high strength even when the plate thickness is more than 100 mm, and having excellent multi-pass welded joint CTOD characteristics.

The reasons for limiting each of the constituent elements of the present invention will be explained below.
[Component composition]
First, the reason for limiting the composition of the steel plate and steel billet to the above range in the present invention will be explained. Note that "%" regarding the composition means "mass %" unless otherwise specified.
C: 0.02 to 0.12%
C is an element that improves hardenability and improves the strength of steel, and must be contained at 0.02% or more. However, if C is contained in excess of 0.12%, the hardness of the C-concentrated portion increases, and the CTOD characteristics of the joint deteriorate. Therefore, the C content is set to a 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 strength. However, if the Si content exceeds 0.70%, the joint CTOD characteristics deteriorate. Therefore, the upper limit of the Si content is limited to 0.70%, preferably 0.50% or less. On the other hand, the lower limit is not particularly limited, but is preferably about 0.04%.

Mn: 0.3 to 3.0%
Mn is an element that has the effect of improving the strength of the base material and welded parts by improving the hardenability of steel. To obtain this effect, the addition of 0.3% or more is necessary. Preferably, it is 0.5% or more. On the other hand, the addition of more than 3.0% not only reduces the weldability, but also causes excessive hardenability, which reduces the toughness of the base material and the welded parts, thereby deteriorating the joint CTOD characteristics. For this reason, the Mn content is set to the range of 0.3 to 3.0%, and preferably 2.8% or less.

P: 0.050% or less P is an element that has a large effect of embrittling grain boundaries, and adding a large amount of it reduces HAZ toughness and joint CTOD characteristics. Therefore, the P content is limited to 0.050% or less, and preferably 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, but excessively reducing the P content leads to an increase in refining time and an increase in costs. Therefore, the P content is preferably 0.001% or more.

S: 0.0050% or less S is an element that reduces the CTOD characteristics of joints, so the upper limit of the S content is limited to 0.0050%. It is preferably 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, but excessive reduction in S content leads to an increase in refining time and an increase in costs. Therefore, the S content is preferably 0.0001% or more.

Al: 0.002 to 0.100%
Al is an element necessary for forming inclusions to improve the toughness of multi-pass weld HAZ and improve joint CTOD properties, and must be added in an amount of 0.002% or more. Preferably, it is added in an amount of 0.005% or more. On the other hand, if it is added in an amount exceeding 0.100%, the joint CTOD properties in the low temperature range are reduced. Therefore, the Al content is set to the range of 0.002 to 0.100%, and preferably, it is set to 0.075% or less.

Ti: 0.002 to 0.060%
Ti precipitates in steel as TiN. The precipitated TiN has the effect of suppressing the coarsening of austenite grains in the base material and HAZ, which refines the HAZ structure and improves the joint CTOD characteristics. In order to obtain this effect, it is necessary to add 0.002% or more. Preferably, it is 0.005% or more. On the other hand, if the Ti content exceeds 0.060%, the precipitation of solid solution Ti and coarse TiC will rather reduce the toughness of the welded heat affected zone and deteriorate the joint CTOD characteristics. Therefore, the Ti content is set to 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 reduces HAZ toughness and deteriorates 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, but excessive reduction in N content leads to an increase in refining time and an increase in costs. Therefore, the N content is preferably 0.0005% or more.

O: 0.0100% or less O is an element that reduces HAZ toughness and deteriorates 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, but excessive reduction of O content leads to an increase in refining time and an increase in costs. Therefore, the O content is preferably 0.0005% or more.

The composition of the steel plate in one embodiment of the present invention is made up of the above elements with the balance being Fe and unavoidable impurities.
In another embodiment of the present invention, in order to further improve the strength, base material toughness, joint toughness, and the like, in addition to the above-mentioned component composition, one or more elements selected from the group consisting of Ni, Ca, Cu, Cr, Mo, Nb, V, W, B, REM, and Mg may be optionally further contained in the contents shown below.

Ni: 2.0% or less Ni is an element that can increase the strength of thick steel plates without significantly deteriorating the toughness of both the base material and the joint, but the addition of Ni increases manufacturing costs and environmental load. Conventionally, the inclusion of Ni was essential to ensure the toughness of the base material and the joint. However, in the present invention, by performing rolling with a controlled deformation resistance ratio, it is possible to manufacture high-strength thick steel plates with a plate thickness of over 100 mm that have excellent multi-layer welded joint CTOD characteristics without the inclusion of Ni. On the other hand, Ni may be contained to further improve toughness. In that case, the inclusion of Ni of 2.0% or more causes problems of excessive increase in manufacturing costs and 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, it is preferable that it is 0.1% or more.

Ca: 0.0180% or less Ca is an element that improves the toughness of multi-pass weld HAZ by forming oxysulfides that are highly stable at high temperatures, but a content exceeding 0.0180% actually deteriorates the joint CTOD characteristics. Therefore, the upper limit of the Ca content is limited to 0.0180%. More preferably, it is 0.0160% or less. On the other hand, when Ca is added, it is preferable that the content be 0.0002% or more.

Cu: 2.00% or less Cu is an element that can increase the strength of thick steel plates without significantly deteriorating the base material and joint toughness, but if the Cu content exceeds 2.00%, surface cracking due to a Cu-enriched layer formed just below the scale becomes 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 Cu is added, it is preferable that the content be 0.05% or more.

Cr: 2.00% or less Cr is an element that has the effect of improving the strength by improving the hardenability of steel, but if the Cr content exceeds 2.00%, the joint CTOD properties deteriorate, so the Cr content is limited to 2.00% or less. More preferably, it is 1.50% or less. On the other hand, when Cr is added, it is preferable that the content be 0.05% or more.

Mo: 2.00% or less Mo is an element that has the effect of improving the strength by improving the hardenability of steel, but if the Mo content exceeds 2.00%, the joint CTOD characteristics deteriorate, so the Mo content is limited to 2.00% or less. More preferably, it is 1.50% or less. On the other hand, when Mo is added, it is preferable that the content be 0.05% or more.

Nb: 0.070% or less Nb is an element that expands the non-recrystallization temperature range of the austenite phase. Therefore, the addition of Nb is effective in efficiently performing non-recrystallization region rolling and obtaining a fine structure. When Nb is added, 0.005% or more is preferable. On the other hand, if the amount of Nb added 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 preferable that the content be 0.01% or more. On the other hand, if the V content exceeds 0.20%, the HAZ toughness decreases 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 preferable that the content be 0.05% or more. On the other hand, if the W content exceeds 0.50%, the HAZ toughness decreases 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 hardenability even with a very small amount of content, thereby improving the strength of the steel plate, and when B is added, it is preferable that the content be 0.0005% or more. 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 austenite grain growth in the HAZ by forming oxysulfide inclusions and improves HAZ toughness, and when REM is added, it is preferable to add 0.001% or more. On the other hand, if the REM content exceeds 0.030%, the base material toughness and HAZ toughness decrease, and the joint CTOD characteristics deteriorate. 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 forms oxide-based inclusions to suppress the growth of austenite grains in the weld heat affected zone and improve the toughness of the weld heat affected zone, and when Mg is added, 0.0002% or more is preferable. However, if the Mg content exceeds 0.0150%, the effect of addition becomes saturated, and an 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 compositions of the above-mentioned steel plate and steel slab must further satisfy the conditions of Ti/N, Ceq and Pcm, which will be described below.
1.50≦Ti/N≦5.00 ... (1)
Ti/N controls the amount of solute N in the HAZ and the precipitation state of TiN. If Ti/N is less than 1.50, the presence of solute N that is not fixed as TiN deteriorates the HAZ toughness and deteriorates the joint CTOD characteristics. On the other hand, if Ti/N is more than 5.00, the precipitation of coarse TiN deteriorates the HAZ toughness and deteriorates the joint CTOD characteristics. 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, 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, resulting in deterioration of HAZ toughness. That is, when Ceq is greater than 0.540%, the necessary joint CTOD characteristics cannot be satisfied even if a 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, when Ceq is less than 0.280%, the target strength cannot be secured. Therefore, the range of Ceq is 0.280 to 0.540%. Preferably, the lower limit is 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 When the weld crack susceptibility index Pcm defined by the following formula (3) increases, the amount of structures with poor toughness, such as island martensite and bainite, in the HAZ structure increases, and as a result, the HAZ toughness deteriorates. When Pcm exceeds 0.250%, the necessary joint CTOD characteristics cannot be obtained due to the deterioration of the toughness of the HAZ base structure itself. Therefore, Pcm is set to 0.250% or less. It is preferably 0.240% or less. On the other hand, the lower limit is not particularly limited, but if Pcm is reduced excessively, the value of Ceq becomes too low, so that about 0.140% is preferable.
Pcm(%)=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]...(3)

Note that the parentheses in the above formulas (1) to (3) represent the content (mass%) of the element shown within the parentheses, and if the element is not contained, it is set to zero.

[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 center of plate thickness of a thick steel plate with a plate thickness of more than 100 mm is set to 20 μm or less. By refining the grains at the center of plate thickness, where segregation is likely to exist, as described above to improve the toughness of the base material, it is possible to improve the joint CTOD characteristics at the SC/ICHAZ boundary. On the other hand, since the smaller the average effective grain size, the more advantageous it is, the lower limit is not particularly limited, but is usually about 1 μm.

Here, the "effective grain size" in the present invention is defined as the circle-equivalent diameter of a grain surrounded by a grain boundary having an orientation difference of 15° or more with adjacent grains, i.e., a high-angle grain boundary. The average effective grain size in the center of the sheet thickness can be measured by the method described in the examples below.

[Porosity density]
Number density of porosity: 0.10 pieces/ ^mm2 or less As mentioned above, the porosity remaining in the steel plate becomes the starting point of fracture, and therefore deteriorates the joint CTOD characteristics. In particular, when the number of porosity having a circle equivalent diameter of 180 μm or more in the steel plate per ^mm2 (also simply referred to as "number density of porosity" in the present invention) exceeds 0.10 pieces, the possibility that the crack opening displacement (δ) in the joint CTOD test will be an insufficient value becomes extremely high. In addition, the higher the number density of porosity, the lower the yield strength at the center position of the plate thickness of the base material. Therefore, it is important to set the number density of porosity to 0.10 pieces/ ^mm2 or less.

The number density of porosity in the present invention refers to the average number density in the thickness direction cross section (cross section perpendicular to the rolling direction) parallel to the plate width direction of the thick steel plate at the total thickness x total width. The number density of 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 any known measurement method.

In addition, the frequency of measurement of the porosity number density is sufficient to measure one or two cross sections of any one steel plate among steel plates that have the same slab melting conditions and rolling conditions. As long as the slab melting method and rolling conditions are not changed, the porosity number density can be produced with good reproducibility, so the measurement results at the above measurement frequency can be said to represent the whole.

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

Heating Conditions of Steel Slabs In the present invention, the method of smelting steel slabs is not particularly limited, and any of the known smelting methods such as converter, electric furnace, and vacuum melting furnace is suitable. Such steel slabs are produced, for example, by a continuous casting method. Furthermore, the molten steel produced from such steel slabs may be further subjected to secondary refining such as ladle refining.
The steel slab produced as described above is heated to 990°C or more and 1200°C or less. 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. For this reason, the heating temperature is set to a range of 990°C or more and 1200°C or less. Preferably, the lower limit temperature is 990°C and the upper limit temperature is 1180°C.

- Hot rolling conditions It is important to control the rolling conditions in both the recrystallization temperature region and the non-recrystallization temperature region in hot rolling.
In the recrystallization temperature range, rolling at 950°C or higher is performed with a reduction ratio/pass of 3% or more, so that the cumulative reduction ratio is 30% or more under the condition that the average deformation resistance ratio between the center part of the steel plate and the surface is 0.70 or less.

[Deformation resistance ratio between the center of the steel plate and the surface: 0.70 or less]
In the present invention, the average ratio of deformation resistance k _fm (thickness center) of the steel plate to deformation resistance k _fm (surface) defined by the following formulas (5) to (7) is set to 0.70 or less (formula (4)). Specifically, the roll rotation speed, roll radius, and roll gap are adjusted to appropriate values, and rolling is performed at a timing that results in an appropriate temperature difference between the thickness center and the surface according to the mass % of C, thereby making the average deformation resistance ratio between the thickness center and the surface 0.70 or less.
k _fm (center of thickness) / k _fm (surface) ≦ 0.70 ... (4)
(where k _fm is based on equation (5))

In the formulas (5) to (7), [C] represents mass % of C, _Tk represents the location for which _kfm is obtained, i.e., the absolute temperature (K) of the center of sheet thickness or the surface of the steel sheet, _h0 represents the sheet thickness on the rolling inlet side, _h1 represents the sheet thickness on the rolling outlet side, n represents the roll rotation speed (rpm), r represents the reduction rate, and R represents the roll radius (mm).
The temperature on the surface of the steel plate can be measured by a radiation thermometer, and the temperature at the center of the plate thickness can also be actually measured as in the examples described later. However, in an actual production line, the temperature may also be determined by heat transfer calculation from the steel plate surface temperature measured by the radiation thermometer.

Under conditions where the average value of the deformation resistance ratio between the center and surface of the steel plate according to the above formula (4) exceeds 0.70, sufficient strain cannot be introduced into the center of the thickness of a thick steel plate having a thickness of more than 100 mm, and porosity remains. As a result, the porosity number density cannot be reduced to 0.10 pieces/ ^mm2 or less. For this reason, the deformation resistance ratio between the center and surface of the steel plate is set to 0.70 or less, and the cumulative reduction ratio for reduction with a reduction ratio/pass of 3% or more is set to 30% or more.

[Rolling at 950 ° C or higher]
Here, the purpose of rolling at 950°C or higher is to refine the structure by recrystallization, refine and disperse coarse inclusions, and compress porosity. In other words, with rolling at less than 950°C, recrystallization is difficult to occur, and austenite grains are not sufficiently refined.

[Rolling reduction/pass is 3% or more]
When the reduction rate/pass is less than 3%, sufficient strain cannot be introduced into the center of the plate thickness, and even if the reduction rate/pass is 3% or more, if the cumulative reduction rate of the reduction is less than 30%, the porosity cannot be sufficiently sealed.

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

In the steel of the present invention, recrystallization is difficult to occur when rolling at a temperature below 950°C, so the strain introduced by rolling is accumulated without being consumed in recrystallization and functions as a transformation nucleus in the subsequent cooling process. As a result, the structure of the thick steel plate finally obtained can be refined. However, under conditions where the cumulative rolling reduction in this temperature range is less than 40%, the effect of refining the crystal grains is insufficient. In addition, in thick steel plates with a thickness of more than 100 mm, under conditions where the average value of the deformation resistance ratio between the thickness center and the surface of the steel plate exceeds 0.70, sufficient strain cannot be introduced into the thickness center, the final structure of the thickness center is insufficient, and the average effective crystal grain size of the thickness center cannot be reduced to 20 μm or less.
For this reason, the rolling in the non-recrystallization temperature region is set to a cumulative reduction rate of 40% or more, and the average deformation resistance ratio between the center of the thickness of the steel plate and the surface is set to 0.70 or less.

[cooling]
After the hot rolling is completed, the hot-rolled steel sheet is cooled. This cooling can be performed by any method as long as the following conditions are satisfied, for example, by water cooling.

Average cooling rate: 1.0°C/s or more If the average cooling rate at the plate thickness center temperature is less than 1.0°C/s, a coarse ferrite phase will be generated in the base material structure, deteriorating the CTOD characteristics of the SC/ICHAZ joint. For this reason, the average cooling rate at the plate thickness center is set to 1.0°C/s or more. On the other hand, if the average cooling rate is greater than 50.0°C/s, the hard bainite phase will increase, increasing the base material strength and deteriorating the CTOD characteristics of the SC/ICHAZ joint, so the cooling rate is preferably 50.0°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 is the average value of the cooling rate from 700°C to 500°C, and when this cooling stop temperature is higher than 500°C, the average cooling rate is the average value of the cooling rate from 700°C to a cooling stop temperature higher than 500°C.

Cooling stop temperature: 600°C or less In the cooling, the hot-rolled steel sheet is cooled until the temperature at the center of the sheet thickness reaches a cooling stop temperature of 600°C or less. 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 CTOD characteristics of the SC/ICHAZ joint deteriorate. For this reason, the cooling stop temperature is set to 600°C or less.

[Tempering treatment]
Tempering temperature: 700°C or less After the cooling is stopped, a further tempering treatment can be performed as desired. 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 the SCHAZ deteriorates. Therefore, the tempering temperature is preferably 700°C or less. More preferably, it is 650°C or less. The lower limit of the tempering temperature is not particularly limited, but can be about 300°C.

In the manufacturing method according to the present invention, any items not described in this specification can be performed using conventional methods.

Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred embodiments of the present invention, and the present invention is not limited to these examples.

Steel billets with the chemical composition shown in Table 1 were used to manufacture thick steel plates under the manufacturing conditions shown in Table 2. During hot rolling, thermocouples were attached to the center positions of the steel material being rolled in the longitudinal, transverse and thickness directions, and the temperature at the center of the thickness direction was measured. Additionally, the surface temperature of the steel material was measured with a radiation thermometer.

The average effective grain size, porosity number density, and yield strength of each of the resulting steel plates were measured using the following methods.

[Average effective grain size]
Samples were taken from the obtained steel sheet so that the measurement positions were the centers in the longitudinal direction, width direction, and thickness direction of the steel sheet. Next, the surfaces of the samples were mirror-polished, and then EBSP analysis was performed under the following conditions. From the obtained crystal orientation map, the circle equivalent diameter of the structure surrounded by high-angle grain boundaries with an orientation difference of 15° or more with adjacent crystal grains was obtained, and the average value of the circle equivalent diameter in the following analysis region was taken as the average effective crystal grain size.
EBSP conditions: Analysis area: 1 mm x 1 mm area at the center of plate thickness; Step size: 0.4 μm

[Porosity density]
Ultrasonic testing 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 the defective parts, direct observation was performed to measure the number density of porosity. First, a sample for observation was taken from the center position of the plate length in the thickness direction cross section (cross section perpendicular to the rolling direction) parallel to the plate width direction of the rolled material, with the observation surface being the full thickness x full width size, and was mirror-polished. The mirror-polished sample was observed with an optical microscope and photographed, and the obtained photograph was subjected to image analysis to determine the circle equivalent diameter of each of the existing porosities. The number of porosity with a particle size of 180 μm or more was divided by the measurement area (plate thickness x plate width) to determine the number of porosity with a circle equivalent diameter of 180 μm or more per ¹ mm2.

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

Next, multi-layer welded joints were fabricated using each of the above thick steel plates. A joint CTOD test was performed on each of the resulting multi-layer welded joints to measure the crack opening displacement in the CGHAZ and the crack opening displacement in the SC/ICHAZ. The fabrication conditions for the multi-layer welded joints and the joint CTOD test conditions are described below.

[Joint CTOD test]
The welded joints used in the joint CTOD test were prepared by submerged arc welding (multi-layer welding) with a K groove shape and a heat input of 5.0 kJ/mm. The test method conformed to BS standard EN10225 (2019), and a test specimen with a square cross section of t x t (t is the plate thickness) was used to evaluate the crack opening displacement [CTOD value (δ)] at a test temperature of -40 ° C.

In the joint CTOD test, tests were conducted in which the notch position was the CGHAZ on the straight side of the K groove, and in which it was the SC/ICHAZ boundary, and δ of the CGHAZ and δ of the SC/ICHAZ boundary were measured. For each thick steel plate, tests were conducted using three test pieces for each notch position, and the minimum measured value was recorded as δ.

After the test, it was confirmed that the tip of the fatigue pre-crack was located on the fracture surface of the test specimen in the CGHAZ and the SC/ICHAZ boundary as specified in EN10225 (2019). In the case of a CTOD test of a multi-pass welded joint, even if the notch position is in the CGHAZ, a certain amount of ICCGHAZ is included, so the test results reflect the toughness of both the CGHAZ and the ICCGHAZ.
The above measurement results are shown in Table 2.

As shown in Table 2, the thick steel plate (invention example) satisfying the conditions of the present invention had manufacturing conditions, effective crystal grain size of the base material, and porosity number density all within the range of the invention, had a yield strength of 320 MPa or more at the 1/4 thickness position and the center thickness position, and both the CTOD value of the CGHAZ and the CTOD value of the SC/ICHAZ boundary were 0.30 mm or more at -40°C, combining high strength with excellent joint CTOD characteristics.

In contrast, among the thick steel plates (comparative examples) that do not satisfy the conditions of the present invention, Nos. 42, 43, and 47 have yield strengths of less than 320 MPa at the 1/4 thickness position and at the center of the thickness. No. 20 has a yield strength of less than 320 MPa at the 1/4 thickness position and at the center of the thickness, and a CTOD value of the SC/ICHAZ boundary of less than 0.30 mm. No. 52 has a yield strength of 320 MPa or more at the 1/4 thickness position, but a yield strength of less than 320 MPa at the center of the thickness. In the other comparative examples, one or both of the CTOD values of the CGHAZ and the SC/ICHAZ boundary are less than 0.30 mm. All comparative examples had inferior base material strength and joint CTOD characteristics compared to the inventive examples.

Claims (4)

In mass percent,
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 composition that satisfies the following formulas (1) to (3):
A steel plate having an average effective crystal grain size of 20 μm or less at a center portion of the plate thickness, a porosity having a circle equivalent diameter of 180 μm or more in the steel plate of 0.10 or less per ^mm2 , a yield strength of 545 MPa or less at a position of 1/2 of the plate thickness, and a plate thickness of more than 100 mm.
1.50 ≦Ti/N≦5.00 ... (1)
0.280%≦Ceq(=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)≦0.540% ... (2)
Pcm (= [C] + [Si] / 30 + ( [Mn] + [Cu] + [Cr]) / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5 [B]) ≦ 0.250% ... (3)
(Note that the parentheses in formulas (1) to (3) represent the content (mass%) of the element in the parentheses, and if the element is not contained, the value is set to zero.) The composition further comprises, 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;
The steel plate according to claim 1, further comprising one or more selected from the group consisting of Mg: 0.0150% or less. A method for producing the steel sheet according to claim 1 or 2,
3. A method for producing a steel plate comprising: heating a steel slab having the chemical composition according to claim 1 to a range of 990°C or higher and 1200°C or lower; hot rolling the steel slab, which satisfies the condition of the following formula (4) and in which the rolling reduction/pass is 3% or higher and the cumulative rolling reduction is 30% or higher in rolling where the temperature at the plate thickness center is 950°C or higher, and in which the cumulative rolling reduction is 40% or higher in rolling where the temperature at the plate thickness center is less than 950°C; and then cooling the steel slab at an average cooling rate at the plate thickness center of 1.0°C/s or higher and 50.0°C/s or lower to a cooling stop temperature of 600°C or lower, wherein when the cooling stop temperature is 500°C or lower, the average cooling rate is the average value of the cooling rates from 700°C to 500°C, and when the cooling stop temperature is higher than 500°C, the average cooling rate is the average value of the cooling rates from 700°C to a cooling stop temperature higher than 500°C.
k _fm (center of thickness) / k _fm (surface) ≦ 0.70 ... (4)
(where k _fm is based on equation (5))

(In the formulas (5) to (7), [C] represents the mass% of C, _Tk represents the absolute temperature (K) at the center of the thickness or at the surface of the steel sheet, _h0 represents the thickness at the entry side of rolling, _h1 represents the thickness at the exit side of rolling, n represents the roll rotation speed (rpm), r represents the reduction ratio, and R represents the roll radius (mm). The method for manufacturing steel sheet according to claim 3, in which, after cooling to the cooling stop temperature, tempering is performed at a temperature of 700°C or less.