KR20240134178A - Steel sheet and its manufacturing method - Google Patents

Abstract

A steel plate having a thickness of more than 100 mm and having excellent high strength and multi-welded joint CTOD characteristics is provided. The steel plate has a predetermined component composition and values of Ti/N, Ceq, and Pcm within specific ranges, wherein the average effective crystal grain size at the center of the plate thickness is 20 ㎛ or less, and the number of porosities per 1㎟ having an equivalent circle diameter of 180 ㎛ or more in the steel plate is 0.10 or less.

Description

Steel sheet and its manufacturing method

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-walled, high-strength steel plate having an excellent strength and toughness of a parent material as well as excellent joint CTOD characteristics in multilayer fill welded portions, and a method for manufacturing the same, for a steel plate having a thickness exceeding 100 mm.

In the past, Charpy tests were mainly used to evaluate the toughness of steel. Recently, as a method to evaluate fracture resistance with higher precision, the Crack Tip Opening Displacement Test (hereinafter referred to as the “CTOD Test”) is increasingly being applied to steel plates used in steel structures.

This test evaluates the resistance to brittle fracture by introducing a fatigue pre-crack into a test piece in the toughness evaluation section, performing three-point bending at low temperature, and measuring the amount of crack opening (plastic strain) just before fracture.

When applying the thick steel plate to steel structures such as ships, marine structures, pressure vessels, line pipes, and wind power generators as mentioned above, multi-welding is used. It is known that the weld heat affected zone (Heat Affected Zone: also referred to as “multi-weld HAZ” hereinafter) of this multi-weld includes a region near the weld line that has become a coarse structure by a preceding welding pass (Coarse Grain Heat Affected Zone: also referred to as “CGHAZ” hereinafter), which is reheated into a two-phase region of ferrite and austenite by a subsequent welding pass, and a region in which island-shaped martensite (Martensite-Austenite Constituent: also referred to as “MA” hereinafter) is mixed in the coarse matrix structure and the toughness is significantly reduced (Inter-Critically Reheated Coarse Grain Heat Affected Zone: also referred to as “ICCGHAZ” hereinafter).

Here, since the CTOD test of the welded joint is basically performed over the entire thickness of the plate, when the multi-weld HAZ is evaluated, the area introducing the fatigue pre-crack includes the ICCGHAZ structure. In addition, since the joint CTOD characteristics obtained by the joint CTOD test are governed by the toughness of the most embrittled structure within the evaluated area, the joint CTOD characteristics of the multi-weld HAZ reflect the toughness of not only the CGHAZ structure but also the ICCGHAZ structure.

For this reason, in order to improve the joint CTOD characteristics of a multi-welded HAZ, it is necessary to improve the toughness of the ICCGHAZ organization as well as the CGHAZ organization.

Conventionally, as a technology for improving the toughness of the heat-affected zone (HAZ) of a weld, suppression of austenite grain coarsening in the CGHAZ by fine dispersion of TiN or utilization of TiN as a ferrite transformation nucleus have been performed. Here, since the bonded portion is sometimes heated to a temperature range where TiN melts, when the low-temperature toughness requirement of the weld is strict, it is difficult to satisfy the requirement with only the effect of utilizing TiN.

In addition, techniques have been used to suppress grain growth of austenite grains by dispersion of REM-based acid sulfides produced by adding REM (rare earth metals), to suppress grain growth of austenite grains by dispersion of Ca-based acid sulfides produced by adding Ca, and to combine the ferrite nucleus-forming ability of BN with oxide dispersion.

For example, patent documents 1 and 2 disclose a technology for suppressing grain growth of austenite and improving the toughness of a weld by adding REM together with Ti to disperse fine particles in the steel.

In addition, patent document 3 proposes a technology for improving HAZ toughness by using CaS and a technology for improving base material toughness by hot rolling.

In addition, as a countermeasure for the deterioration of ICCGHAZ's toughness, a technology for increasing the strength of the base material by adding Cu after suppressing the formation of MA by making it low-C and low-Si is proposed in Patent Document 4.

In addition, patent document 5 proposes a technology for refining the HAZ structure and improving the HAZ toughness by using BN as a ferrite transformation nucleus in the heat affected zone of a large-heat input weld.

However, recently, steel structures such as ships, marine structures, pressure vessels, line pipes, and offshore wind power generators have tended to become larger, and accordingly, the steel plates used in the steel structures have been thickened and strengthened. In order to achieve both thickening and high strength of the steel plates, an increase in the amount of alloy elements added is necessary, but the addition of a large amount of alloy elements makes it difficult to secure the toughness of the multi-weld HAZ. Regarding this problem, Patent Document 6 discloses a technology for improving low-temperature toughness by controlling the hardness of the central segregation zone.

Japanese Patent Publication No. 60-152626 Japanese Patent Publication No. 60-184663 Japanese Patent Publication No. 2012-184500 Japanese Patent Publication No. 05-186823 Japanese Patent Publication No. 61-253344 International Publication No. 2014/038200

Here, the CTOD specification temperature in the standard that specifies the joint CTOD characteristics (e.g., API (American Petroleum Institute) standard RP (Recommended Practice)-2Z) is typically -10°C.

However, in order to secure new resources in response to the recent increase in energy demand, construction areas for marine structures, etc. are shifting to cold and deep-sea areas where resource mining was not possible until now. For this reason, the demand for steel plates that are high-strength and thick-walled and can support lower CTOD specification temperatures (e.g., about -40℃) than the CTOD specification temperature specified by API standards is increasing.

According to the inventors' review, the conventional techniques described in Patent Documents 1 to 6 could not sufficiently satisfy the joint CTOD characteristics required for a multi-welded joint for low-temperature specifications in a steel plate having high strength and a thickness exceeding 100 mm, which has been recently demanded.

For example, patent documents 1 and 2 propose a technique for suppressing coarsening of the austenite structure of the HAZ by dispersing fine particles in the steel by compositely adding REM together with Ti. Since this technique targets steels with relatively low strength and a small amount of alloy elements, it cannot be applied to steels with higher strength and a large amount of alloy elements, because the HAZ structure becomes a structure that does not include ferrite.

In addition, in patent documents 1 and 2, REM-based acid sulfides and Ca-based acid sulfides are effective for suppressing austenite grain growth. However, the effect of improving toughness by suppressing austenite grain coarsening in the HAZ alone cannot satisfy the joint CTOD characteristics at the above-mentioned low-temperature specification temperature.

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

Likewise, in Patent Document 4, the joint CTOD characteristics at the above-mentioned low-temperature specification temperature are not examined, and it is thought that the low-temperature CTOD specification cannot be satisfied only by improving the ICCGHAZ toughness by reducing the composition of the parent material components. In addition, reducing the alloy element content of the parent material to improve the toughness of the ICCGHAZ is a technical idea that runs counter to securing strength for thickening, and is difficult to apply to thick-walled steel plates used in marine structures, etc.

The technology proposed in Patent Document 5 is effective in cases where the cooling rate in the heat affected zone of the weld is slow, such as in the case of high heat input welding, and the HAZ is a ferrite-based structure. However, in the case of a thick steel plate having a plate thickness exceeding 100 mm, the amount of alloy components contained in the base material is relatively large, while the heat input is relatively small in multi-welding. Therefore, in multi-welding of a thick steel plate, since the HAZ structure is a bainite-based structure, the effect of improving the joint CTOD characteristics is not obtained.

In the technology described in Patent Document 6, a technology is proposed for satisfying joint CTOD characteristics in a low-temperature range in a thick steel plate having a plate thickness of 100 mm or less, but it does not reach the point of obtaining mechanical characteristics equivalent to those of a thick steel plate having a plate thickness of 100 mm or less for an ultra-thick steel plate having a plate thickness exceeding 100 mm.

In this way, it is difficult to say that a technology has been established for improving the toughness of CGHAZ and ICCGHAZ in the multi-weld heat-affected zone of a high-strength steel plate exceeding 100 mm in thickness. In other words, there was a problem to be solved in improving the joint CTOD characteristics with the bond zone where CGHAZ and ICCGHAZ are mixed as the cutting position.

The present invention has been made in consideration of the above-described problems of the prior art, and its purpose is to provide a high-strength steel plate having a thickness of more than 100 mm and having excellent CTOD characteristics of a joint in which multiple welding is performed (hereinafter referred to as “multiple weld joint CTOD characteristics”), and a method for manufacturing the same.

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

To solve these problems, the inventors conducted a thorough study on a method for improving joint CTOD characteristics. As a result, they obtained the following findings.

(1) If the porosity generated during the slab manufacturing process remains without being compressed during rolling, it may become a defect in the steel plate and a starting point of destruction. In particular, in order to pressurize the porosity in the center of the plate thickness, it is necessary to introduce appropriate strain into the center of the plate thickness during rolling, but this becomes difficult in the case of a thick steel plate having a plate thickness of more than 100 mm, so the remaining porosity that is not compressed becomes a problem. However, as a result of the inventors' examination, it was found that when rolling is performed at a high temperature of 950°C or higher at a center temperature of the plate thickness and a cumulative reduction ratio of a reduction in which the reduction/pass is 3% or more and the average value of the deformation resistance ratio between the center of the plate thickness and the surface of the steel plate is 0.70 or lower, sufficient strain can be introduced into the center of the plate thickness, so that the porosity can be sufficiently compressed.

In addition, in the present invention, the center of the plate thickness is a region having a thickness of 10% of the plate thickness in each direction from the center of the plate thickness toward both surfaces of the steel plate.

(2) In addition, there is a problem that the center of the plate thickness of the slab has an element segregation region, and since alloy elements are concentrated in this region, coarse inclusions are dispersed at a low density. However, by performing rolling at a high temperature of 950°C or higher at the center of the plate thickness as described above, and at an average value of the deformation resistance ratio between the center of the plate thickness and the surface of the steel plate being 0.70 or lower, and at a reduction rate/pass of 3% or higher and a cumulative reduction rate of 30% or higher, it is possible to increase the strain applied to the center of the plate thickness. As a result, it was found that coarse inclusions are elongated and divided, and fine inclusions can be dispersed at a high density. In addition, it was found that as a result of this dispersion, an effect of improving HAZ toughness due to the inclusions can be secured.

(3) In addition, in order to finely disperse and precipitate TiN, which is effective in suppressing austenite grain growth, in the steel, in addition to containing Ti and N satisfying the relationship of 1.50≤Ti/N≤5.00 for the components of the steel plate, by controlling 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%, it is possible to improve the toughness of the base structure of the HAZ in which multiple welding is performed (hereinafter referred to as a multi-welded joint HAZ), and obtain good joint CTOD characteristics that can support the low-temperature CTOD specifications. Found it.

In addition, the inventors also examined the joint CTOD characteristics of the SC/ICHAZ (Sub-Critically reheated HAZ/Inter-Critically reheated HAZ) boundary, which is the boundary between the transformed zone/non-transformed zone of the parent material during welding, as required by BS (British Standards) EN10225 (2019) or API standard RP-2Z (2005), which specify the joint CTOD test method. As a result, the following findings were obtained.

(4) In order to satisfy the joint CTOD characteristics at a test temperature of -40℃ at the SC/ICHAZ boundary, it was found that it was necessary to improve the toughness of the base material by making the effective crystal grain size of the base material microstructure 20 ㎛ or less and refining the crystal grains, since the toughness of the base material becomes dominant for the joint CTOD characteristics at the SC/ICHAZ boundary.

(5) In steel plates with a thickness of more than 100 mm, since the cooling rate in the center of the plate thickness is slow, the crystal grains in these locations become coarsened. However, by performing rolling at a cumulative reduction ratio of 40% or more under the condition that the average value of the deformation resistance ratio between the center of the plate thickness and the surface of the steel plate is 0.70 or less at a center temperature of less than 950°C, it was found that sufficient deformation can be introduced into the center of the plate thickness, and grain refinement can be achieved to the above-mentioned grain size.

The present invention has been completed by further examination in consideration of the above recognition. That is, the gist of the present invention is as follows.

1. A composition comprising, in 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, with the remainder being Fe and unavoidable impurities, and having a component composition satisfying the following formulas (1) to (3):

A steel plate having an average effective crystal grain size of 20 ㎛ or less in the center of the plate thickness and a porosity of 180 ㎛ or more in the equivalent circle diameter in the steel plate of 0.10 or less per ㎟.

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)

(However, the parentheses in formulas (1) to (3) indicate the content (mass%) of the element within the parentheses, and if the element is not contained, it is set to zero.)

2. The steel sheet described in 1 above, wherein the above component composition further includes at least one selected from the group consisting of, 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.

3. A method for manufacturing a steel plate as described in 1 or 2 above,

A method for manufacturing a steel plate, wherein a steel sheet having the component composition described in 1 or 2 above is heated to a temperature in the range of 990°C or more and 1200°C or less, and hot rolling is performed such that, when the temperature at the center of the plate thickness is 950°C or more, a reduction/pass is 3% or more and a cumulative reduction ratio is 30% or more, and when the temperature at the center of the plate thickness is less than 950°C, a cumulative reduction ratio is 40% or more, and then, when cooling to a cooling stop temperature of 600°C or less at an average cooling rate at the center of the plate thickness of 1.0°C/s or more, when the cooling stop temperature is 500°C or less, an average value of the cooling rates from 700°C to 500°C is used as the average cooling rate, and when the cooling stop temperature is higher than 500°C, an average value of the cooling rates from 700°C to a cooling stop temperature higher than 500°C is used as the average cooling rate.

k _fm (center of plate thickness)/k _fm (surface) ≤ 0.70 … (4)

(Here, k _fm is according to equation (5))

(However, in equations (5) to (7), [C] represents the mass% of C, T _k represents the absolute temperature (K) of the center of plate thickness or the surface of the steel plate, h ₀ represents the plate thickness at the rolling entry side, h ₁ represents the plate thickness at the rolling exit side, 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 plate as described in 3 above, wherein after cooling to the above cooling stop temperature, tempering treatment is performed at a temperature of 700°C or lower.

According to the present invention, a steel plate having high strength and excellent multi-welded joint CTOD characteristics can be provided even if the steel plate has a thickness exceeding 100 mm.

(Form for carrying out the invention)

Below, the reasons for limitation of each component requirement of the present invention are explained.

[Ingredients]

First, the reason why the composition of the steel plate and the steel slab in the present invention is limited to the above range is explained. In addition, "%" regarding the composition of the steel plate and the steel slab means "mass%" unless specifically stated otherwise.

C: 0.02∼0.12%

C is an element that improves the strength of steel by increasing hardenability, and requires a content of 0.02% or more. However, if C is excessively contained exceeding 0.12%, the hardness of the part where C is concentrated increases, and the joint CTOD characteristics 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 included as an impurity, and also has the function of improving strength. However, if Si is excessively contained exceeding 0.70%, the joint CTOD characteristics deteriorate. Therefore, the upper limit of the Si content is limited to 0.70%. It is 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∼3.0%

Mn is an element that has the effect of improving the strength of the base material and the weld by improving the quenchability of the steel. In order to obtain this effect, an addition of 0.3% or more is necessary. Preferably, it is 0.5% or more. On the other hand, an addition exceeding 3.0% not only reduces the weldability, but also causes the quenchability to become excessive, reducing the toughness of the base material and the weld, thereby deteriorating the joint CTOD characteristics. Therefore, the Mn content is set to a 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 when added in large amounts, it lowers 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, but excessive reduction in P results in an increase in refining time and an increase in cost. Therefore, the P content is preferably 0.001% or more.

S: 0.0050% or less

Since S is an element that lowers the joint CTOD characteristics, 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, but excessive reduction in S leads to an increase in refining time and an increase in cost. Therefore, it is desirable that the S content be 0.0001% or more.

Al: 0.002∼0.100%

Al is an element necessary for improving the toughness of multi-weld HAZ and forming inclusions to enhance joint CTOD characteristics, and an addition of 0.002% or more is required. Preferably, it is 0.005% or more. On the other hand, if it is excessively added exceeding 0.100%, joint CTOD characteristics in the low-temperature range deteriorate. Therefore, the Al content is set to a range of 0.002 to 0.100%. Preferably, it is 0.075% or less.

Ti: 0.002∼0.060%

Ti precipitates in the steel as TiN. The precipitated TiN has the effect of suppressing the coarsening of austenite grains in the base material and HAZ, thereby refining the HAZ structure and improving the joint CTOD characteristics. In order to obtain this effect, addition of 0.002% or more is necessary. Preferably, it is 0.005% or more. On the other hand, if the Ti content exceeds 0.060%, the weld heat-affected zone toughness rather decreases and the joint CTOD characteristics deteriorate due to the precipitation of solid solution Ti or coarse TiC. Therefore, the Ti content is set to a range of 0.002 to 0.060%. Preferably, it is 0.050% or less.

N: 0.0130% or less

Since N is an element that lowers HAZ toughness and deteriorates joint CTOD characteristics, 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 leads to an increase in refining time and an increase in cost. Therefore, it is desirable to set the N content to 0.0005% or more.

O: 0.0100% or less

Since O is an element that lowers HAZ toughness and deteriorates joint CTOD characteristics, 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 in O results in an increase in refining time and an increase in cost. Therefore, it is desirable to set the O content to 0.0005% or more.

In one embodiment of the present invention, the composition of the steel plate is composed of the above elements, the remainder being Fe and unavoidable impurities.

In addition, in another embodiment of the present invention, in order to further improve strength, toughness of the base material, joint toughness, etc., in addition to the above component composition, one or more selected from the group consisting of Ni, Ca, Cu, Cr, Mo, Nb, V, W, B, REM, and Mg may be additionally optionally contained in the content shown below.

Ni: 2.0% or less

Ni is an element that can increase the strength of a thick-steel plate without significantly deteriorating the toughness of both the base material and the joint, but the manufacturing cost and environmental load increase due to the addition of Ni. Conventionally, Ni content was essential to secure the toughness of the base material and the toughness of the joint. However, in the present invention, by performing rolling while controlling the deformation resistance ratio, it is possible to manufacture a high-strength thick-steel plate having an excellent multi-welded joint CTOD characteristic and a plate thickness of more than 100 mm without containing Ni. On the other hand, Ni may be contained in order to further improve the toughness. In that case, a Ni content of 2.0% or more causes problems of excessive increase in manufacturing cost and increased 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 preferably 0.1% or more.

Ca: 0.0180% or less

Ca is an element that improves the toughness of multi-weld HAZ by forming highly stable acid sulfides at high temperatures, but a content exceeding 0.0180% rather 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 preferably 0.0002% or more.

Cu: 2.00% or less

Cu is an element that can increase the strength of a thick steel plate without significantly deteriorating the toughness of the base material or joints. However, if the Cu content exceeds 2.00%, surface cracks caused by a Cu-concentrated layer formed just below the scale 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, it is preferable that it is 0.05% or more.

Cr: 2.00% or less

Cr is an element that has the effect of improving strength by improving the hardenability of steel, but if the Cr content exceeds 2.00%, the joint CTOD characteristics 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 adding Cr, it is preferable that it is 0.05% or more.

Mo: 2.00% or less

Mo is an element that has the effect of improving strength by improving the quenching property 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 adding Mo, it is preferable that it is 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 for efficiently performing non-recrystallization rolling and obtaining a microstructure. When adding Nb, it is preferable that it be 0.005% or more. 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 material, and when adding V, it is preferable that it be 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 material, and when adding W, it is preferable that it be 0.05% or more. On the other hand, when 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 quenching properties and thus improve the strength of steel plates with trace amounts thereof. When adding B, it is preferable that it be 0.0005% or more. On the other hand, when the B content exceeds 0.0050%, the HAZ toughness deteriorates 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 austenite grain growth in HAZ by forming acid sulfide inclusions and improves HAZ toughness. When adding REM, it is preferable that it be 0.001% or more. On the other hand, if the REM content exceeds 0.030%, the base material toughness and HAZ toughness rather 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 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. When adding Mg, it is preferable that it be 0.0002% or more. However, if the Mg content exceeds 0.0150%, the effect of addition becomes saturated, and it is not possible to expect an effect corresponding to the content, which is economically disadvantageous. Therefore, the Mg content is limited to 0.0150% or less. More preferably, it is 0.0100% or less.

In addition, in the present invention, the component composition of the thick steel plate and the steel piece must further satisfy the conditions of Ti/N, Ceq and Pcm, respectively, described below.

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, the HAZ toughness deteriorates due to the presence of dissolved N that is not fixed as TiN, and the joint CTOD characteristics deteriorate. On the other hand, when 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. Also, preferably, the lower limit is 1.80, and the upper limit is 4.50.

Ceq: 0.280% or more, 0.540% or less

As the carbon equivalent Ceq defined by the following equation (2) increases, the amount of tissues with poor toughness, such as island-shaped martensite or bainite, in the HAZ structure increases, resulting in deterioration of the HAZ toughness. That is, if Ceq is greater than 0.540%, the required joint CTOD characteristics cannot be satisfied even if the HAZ toughness improvement technology by inclusions is used due to deterioration in the toughness of the HAZ matrix structure itself. On the other hand, if Ceq is less than 0.280%, the target strength cannot be secured. Therefore, the range of Ceq is 0.280 to 0.540%. In addition, the lower limit is preferably 0.300%, and the upper limit is preferably 0.500%.

Ceq(%)=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5... (2)

Pcm: 0.250% or less

When the welding crack susceptibility index Pcm defined by the following equation (3) increases, the amount of structures with poor toughness, such as island-shaped martensite or bainite, in the HAZ structure increases, and as a result, the HAZ toughness deteriorates. When Pcm exceeds 0.250%, the required joint CTOD characteristics cannot be obtained due to the deterioration in the toughness of the base structure of the HAZ itself. Therefore, Pcm is set to 0.250% or less. Preferably, it is 0.240% or less. On the other hand, although the lower limit is not particularly limited, if an attempt is made to reduce Pcm excessively, the value of Ceq becomes excessively low, so it is preferably about 0.140%.

Pcm(%)=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]… (3)

In addition, the parentheses in the above formulas (1) to (3) all indicate the content (mass%) of the element shown in the parentheses, and if the element is not contained, it is set to zero.

[Average effective decision particle size]

Average effective crystal grain size at the center of the plate thickness: 20 ㎛ or less

In the present invention, the average effective crystal grain size of the microstructure in the center of the plate thickness of a steel plate exceeding 100 mm in thickness is set to 20 ㎛ or less. By refining the crystal grains in the center of the plate thickness where segregation is likely to exist as described above to improve the toughness of the parent material, the joint CTOD characteristics of the SC/ICHAZ boundary can be improved. On the other hand, since the average effective crystal grain size is more advantageous the smaller it is, the lower limit thereof is not particularly limited, but is usually about 1 ㎛.

Here, the "effective crystal grain size" in the present invention is defined as the circle-equivalent diameter of a crystal grain surrounded by a grain boundary whose orientation difference from that of an adjacent crystal grain is 15° or more, that is, a large-angle grain boundary. In addition, the average effective crystal grain size at the center of the plate thickness can be measured by the method described in the examples described later.

[Porocity number density]

Porocity number density: 0.10/㎟ or less

As described above, the porosity remaining in the steel plate becomes a fracture starting point, thereby worsening the joint CTOD characteristics. In particular, if the number of porosities having an equivalent circle diameter of 180 ㎛ or more per 1㎟ in the steel plate (also simply referred to as the "number density of porosities" in the present invention) exceeds 0.10, the possibility that the crack opening displacement (δ) in the joint CTOD test will be an insufficient value becomes extremely high. In addition, as the number density of porosities increases, the yield strength at the center position of the plate thickness of the base material decreases. Therefore, it is important to make the number density of the porosities 0.10/㎟ or less.

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

In addition, the measurement frequency of the porosity number density can be measured by measuring 1 to 2 cross sections of any one steel sheet among steel sheets having the same flux conditions and rolling conditions. Since the porosity number density can be manufactured with good reproducibility as long as the flux method or rolling conditions of the steel sheet are not changed, it can be said that the measurement results at the above measurement frequency represent the whole.

[Manufacturing method]

Next, the reasons for limitation of each condition for the method for manufacturing a thick steel plate in the present invention are explained below. In addition, the temperature in the following explanation is the temperature at the center of the plate thickness unless otherwise specified. In addition, the temperature at the center of the plate thickness can be measured as in the example described below, but in an actual manufacturing line, etc., it can be obtained by heat transfer calculation from the steel plate surface temperature measured by a radiation thermometer.

·Heating conditions of the steel sheet

In the present invention, the method for melting the steel billet is not particularly limited, and all known melting methods such as a converter, an electric furnace, and a vacuum melting furnace are suitable. Such a steel billet is manufactured by, for example, a continuous casting method. In addition, the molten steel obtained by melting such a steel billet may be subjected to secondary refining such as ladle refining.

The steel sheet 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 later 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 coarsen, and the desired fine grain structure cannot be obtained after controlled rolling. Therefore, the heating temperature range is 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, for rolling at 950℃ or higher, a rolling reduction with a reduction ratio/pass of 3% or higher is performed so that the cumulative rolling reduction is 30% or higher under the condition that the average value of the deformation resistance ratio of the center and surface of the steel plate thickness is 0.70 or lower.

[Deformation resistance ratio of the center and surface of the steel plate: 0.70 or less]

In the present invention, the average value of the ratio of the deformation resistance k _fm (center of plate thickness) at the center of the plate thickness and the deformation resistance k _fm (surface) at the surface, as defined by the following equations (5) to (7), is set to 0.70 or less (equation (4)). Specifically, by adjusting the roll rotation speed, the roll radius, and the roll gap to appropriate values, and performing rolling at a timing at which the temperature difference between the center of plate thickness and the surface becomes an appropriate value according to the mass% of C, the average value of the deformation resistance ratio between the center of plate thickness and the surface is set to 0.70 or less.

k _fm (center of plate thickness)/k _fm (surface) ≤ 0.70 … (4)

(Here, k _fm is according to equation (5))

However, in equations (5) to (7), [C] represents the mass% of C, T _k represents the absolute temperature (K) of the location where k _fm is obtained, i.e., the center of the plate thickness or the surface of the steel plate, h ₀ represents the plate thickness at the rolling entry side, h ₁ represents the plate thickness at the rolling exit side, n represents the roll rotation speed (rpm), r represents the reduction ratio, and R represents the roll radius (mm).

In addition, the temperature of the surface of the steel plate can be measured by a radiation thermometer, and the temperature at the center of the plate thickness can be measured as in the example described below, but in an actual manufacturing line, etc., it can be obtained by heat transfer calculation from the steel plate surface temperature measured by the radiation thermometer.

Under the condition that the average value of the deformation resistance ratio of the center of the plate thickness and the surface of the steel plate according to the above formula (4) exceeds 0.70, sufficient deformation cannot be introduced to the center of the plate thickness for a thick steel plate exceeding 100 mm, and porosity remains. As a result, the porosity number density cannot be 0.10 porosity/㎟ or less. For this reason, the deformation resistance ratio of the center of the plate thickness and the surface of the steel plate: 0.70 or less, and the cumulative reduction ratio of the reduction in which the reduction ratio/pass is 3% or more: 30% or more.

[Rolling above 950℃]

Here, the purpose of rolling at 950℃ or higher is to refine the structure by recrystallization, to refine and disperse coarse inclusions, and to press the porosity. That is, in rolling at less than 950℃, recrystallization is difficult to occur, and the austenite grain refinement becomes insufficient.

[Pressure/pass rate 3% or more]

In rolling with a reduction ratio/pass of less than 3%, sufficient deformation cannot be introduced to the center of the plate thickness, and even if the reduction ratio/pass is 3% or more, the porosity cannot be sufficiently compressed if the cumulative reduction ratio of the reduction is less than 30%.

[Rolling below 950℃]

In the non-recrystallization temperature range, i.e., in rolling below 950℃, the average value of the deformation resistance ratio of the center of the steel plate thickness and the surface is set to 0.70 or less, the same as in the recrystallization temperature range, and rolling is performed so that the cumulative reduction ratio is 40% or more.

In the steel of the present invention, since recrystallization is difficult to occur in rolling at less than 950°C, the strain introduced by rolling is not consumed in recrystallization but is accumulated and functions as a transformation nucleus in the subsequent cooling process. As a result, the structure of the finally obtained thick steel plate can be refined. However, under the condition that the cumulative reduction ratio in this temperature range is less than 40%, the grain refinement effect becomes insufficient. Furthermore, in a thick steel plate having a thickness of more than 100 mm, under the condition that the average value of the deformation resistance ratio between the plate thickness center and the surface of the steel plate exceeds 0.70, sufficient strain cannot be introduced into the plate thickness center, the final structure refinement in the plate thickness center becomes insufficient, and the average effective crystal grain size in the plate thickness center cannot be made 20 μm or less.

For this reason, rolling in the non-recrystallized temperature range is performed with a cumulative reduction ratio of 40% or more and an average value of the deformation resistance ratio of the center and surface of the steel plate thickness of 0.70 or less.

[cooling]

After the above hot rolling is completed, the obtained hot rolled steel sheet is cooled. This cooling can be performed by any method as long as the conditions described below are satisfied, and for example, it can be performed by water cooling.

Average cooling rate: 1.0℃/s or more

If the average cooling rate at the center temperature of the plate thickness is less than 1.0°C/s, a coarse ferrite phase is formed in the base material structure, which deteriorates the joint CTOD characteristics of SC/ICHAZ. For this reason, the average cooling rate at the center temperature of the plate thickness 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 increases, which increases the strength of the base material, thereby deteriorating the joint CTOD characteristics of SC/ICHAZ. Therefore, the cooling rate is preferably set to 50.0°C/s or less.

In addition, in the present invention, when the cooling stop temperature shown in the following paragraph is 500°C or lower, the average value of the cooling rates from 700°C to 500°C is taken as the average cooling rate, and when the cooling stop temperature is higher than 500°C, the average value of the cooling rates from 700°C to the cooling stop temperature higher than 500°C is taken as the average cooling rate.

Cooling stop temperature: 600℃ or less

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

[Tempering treatment]

Tempering temperature: 700℃ or less

After the above cooling is stopped, tempering treatment can be optionally performed additionally. By tempering treatment, the toughness of the base material can be further improved. At that time, if the tempering temperature is higher than 700°C, a coarse ferrite phase is generated, and the toughness of SCHAZ deteriorates. Therefore, the tempering temperature is preferably 700°C or lower. More preferably, it is 650°C or lower. In addition, the lower limit of the tempering temperature is not particularly limited, but can be about 300°C.

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

Example

Next, the present invention will be described more specifically based on examples. The following examples are intended to illustrate suitable examples of the present invention, and the present invention is not limited in any way by the examples.

Using the steel sheet having the composition shown in Table 1, a thick steel plate was manufactured under the manufacturing conditions shown in Table 2. In addition, during hot rolling, thermocouples were attached to the center positions of the longitudinal direction, width direction, and plate thickness direction of the rolled steel, and the temperature at the center of the plate thickness was measured. In addition, the surface temperature of the steel was measured with a radiation thermometer.

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

[Average effective decision particle size]

From the obtained steel plate, a sample was collected so that the center in the longitudinal direction, the width direction, and the thickness direction of the steel plate was the measurement position. Then, the surface of the sample was mirror-polished, and EBSP analysis was performed under the following conditions. From the obtained crystal orientation map, the circle-equivalent diameter of the organization surrounded by angled grain boundaries whose orientation difference from adjacent crystal grains was 15° or more was obtained, and the average value of the circle-equivalent diameters in the following analysis area was taken as the average effective crystal grain size.

EBSP Conditions

·Interpretation area: 1mm x 1mm area centered on the plate thickness

·Step size: 0.4㎛

[Porocity number density]

In order to detect defects inside a steel plate, ultrasonic testing is often used because it can be tested non-destructively. However, in order to accurately confirm the state of the defective part, direct observation was performed and the number density of porosities was measured. First, an observation sample whose observation surface is the total thickness × total width size in the thickness direction cross-section parallel to the plate width direction of the rolled material (the cross-section perpendicular to the rolling direction) was collected from the center position of the plate length, and mirror-polished. This mirror-polished sample was observed with an optical microscope, photographs were taken, and the obtained photographs were subjected to image analysis to obtain the equivalent circle diameter of each existing porosity. By dividing the number of porosities having a grain size of 180 ㎛ or more by the measurement area (plate thickness × plate width), the number per 1㎟ of porosities having an equivalent circle diameter of 180 ㎛ or more was obtained.

[Surrender strength]

A tensile test was performed according to EN10002-1 to obtain the yield strength (YS) at positions of 1/4 and 1/2 the plate thickness (t) of the steel plate. For the above tensile test, a round bar tensile test specimen with a parallel section diameter of 14 mm and a parallel section length of 70 mm, which was taken parallel to the plate width direction from positions of 1/4 and 1/2 the plate thickness, was used. In the above tensile test, when the upper yield point appeared, the upper yield stress was taken as the yield strength. In addition, when the upper yield point did not appear, the 0.2% proof stress was taken as the yield strength.

Next, multi-welded joints were fabricated using each of the above-described thick plates. Joint CTOD tests were performed on each of the obtained multi-welded joints to measure the crack opening displacement in the CGHAZ and the crack opening displacement in the SC/ICHAZ. The fabrication conditions of the multi-welded joints and the conditions of the joint CTOD tests are described below.

[Joint CTOD Test]

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

In the above joint CTOD test, a test was performed with the cut position as the CGHAZ on the straight-line shape side of K improvement and a test with the cut position as the SC/ICHAZ boundary, and δ of the CGHAZ and δ of the SC/ICHAZ boundary were measured, respectively. In addition, for each of the thick plates, a test was performed using three test pieces for each cut position, and the lowest measured value was taken as δ.

After the above test, it was confirmed that the tips of the fatigue precracks were at the CGHAZ and SC/ICHAZ boundaries specified in EN10225 (2019) in the fracture surface of the specimen. In addition, in the case of the CTOD test of a multi-welded joint, even if the cut position is CGHAZ, since a certain amount of ICCGHAZ is included, the test results reflect the toughness of both CGHAZ and ICCGHAZ.

The above measurement results are shown in Table 2.

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

In this regard, 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 position of the plate thickness and the center position of the plate thickness. No. 20 has yield strengths of less than 320 MPa at the 1/4 position of the plate thickness and the center position of the plate thickness, and a CTOD value at 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 position of the plate thickness, but a yield strength at the center position of the plate thickness of less than 320 MPa. In the other comparative examples, one or both of the CTOD value of the CGHAZ and the CTOD value of the SC/ICHAZ boundary are less than 0.30 mm. In all of the comparative examples, the base material strength and the joint CTOD characteristics were inferior to those of the inventive examples.

Claims (4)

In mass %,
C: 0.02∼0.12%,
Si: 0.70% or less,
Mn: 0.3∼3.0%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.002∼0.100%,
Ti: 0.002∼0.060%,
N: 0.0130% or less and,
O: Including 0.0100% or less,
The remainder is Fe and unavoidable impurities, and has a composition satisfying the following equations (1) to (3).
A steel plate having an average effective crystal grain size of 20 ㎛ or less in the center of the plate thickness and a porosity of 180 ㎛ or more in the equivalent circle diameter in the steel plate of 0.10 or less per ㎟.
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)
(However, the parentheses in formulas (1) to (3) indicate the content (mass%) of the element within the parentheses, and if the element is not contained, it is set to zero.) In the first paragraph,
The above composition of ingredients, additionally, 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,
A steel plate comprising one or more types selected from the group consisting of Mg: 0.0150% or less. A method for manufacturing a steel plate as described in claim 1 or claim 2,
A method for manufacturing a steel plate, comprising: heating a steel sheet having the component composition described in claim 1 or 2 to a temperature in the range of 990°C or more and 1200°C or less, and satisfying the condition of the following equation (4), and further performing hot rolling in which, when the temperature at the center of the plate thickness is 950°C or more, a reduction in which a reduction/pass is 3% or more and a cumulative reduction ratio is 30% or more, and when the temperature at the center of the plate thickness is less than 950°C, a cumulative reduction ratio is 40% or more, and then cooling the steel sheet to a cooling-stop temperature of 600°C or less at an average cooling rate at the center of the plate thickness of 1.0°C/s or more, wherein, when the cooling-stop temperature is 500°C or less, an average value of the cooling rates from 700°C to 500°C is used as the average cooling rate, and when the cooling-stop temperature is higher than 500°C, an average value of the cooling rates from 700°C to a cooling-stop temperature higher than 500°C is used as the average cooling rate.
k _fm (center of plate thickness)/k _fm (surface) ≤ 0.70 … (4)
(Here, k _fm is according to equation (5))



(However, in equations (5) to (7), [C] represents the mass% of C, T _k represents the absolute temperature (K) of the center of plate thickness or the surface of the steel plate, h ₀ represents the plate thickness at the rolling entry side, h ₁ represents the plate thickness at the rolling exit side, n represents the roll rotation speed (rpm), r represents the reduction ratio, and R represents the roll radius (mm).) In the third paragraph,
A method for manufacturing a steel plate, comprising cooling to the above cooling stop temperature and then performing tempering treatment at a temperature of 700°C or lower.