KR20230041045A - Steel plate and its manufacturing method - Google Patents

Abstract

A steel sheet capable of exhibiting excellent toughness in a joint after high heat input welding while exhibiting excellent brittle crack propagation stopping characteristics is provided. The steel sheet of the present invention contains Ti and N in a mass % ratio of Ti to N (Ti/N) of 2.00 to 4.00 and satisfies the relationship of 169 ≤ 5158 × Ti + 25563 × N ≤ 360, Ceq TiN particles having a predetermined component composition of 0.400 to 0.500, a structure with a bainite volume fraction of 80% or more, and an average particle diameter of 20 to 50 nm at a depth of 1 mm from the steel sheet surface are 5.0 × 10 ⁸ It is contained at a number density of 1/cm 2 or more, and the (211) plane X-ray intensity ratio at a depth of 1/2 of the plate thickness is 1.60 or more.

Description

Steel plate and its manufacturing method

The present invention relates to a steel plate, particularly a steel plate applicable to high heat input welding, and a method for manufacturing the steel plate. More specifically, the present invention relates to a steel sheet capable of achieving both excellent brittle crack propagation stopping characteristics and excellent toughness at a joint after high heat input welding, and a manufacturing method thereof. In addition, the steel sheet of the present invention can be suitably used for large-scale structures such as ships, offshore structures, low-temperature storage tanks, and architectural and civil engineering structures.

BACKGROUND OF THE INVENTION Large structures such as ships, offshore structures, low-temperature storage tanks, and architectural and civil engineering structures have a great impact on the society, economy and environment when an accident accompanying brittle fracture occurs. For this reason, improvement in safety is always required for large structures, and steel materials used for large structures have, in particular, toughness and strength at operating temperature, and brittle crack propagation stopping properties that prevent brittle crack propagation ( arrest performance) is required at a high level.

Ships such as container ships and bulk carriers do not form a general deck, load cargo even on hatch covers, and travel in the ocean while receiving waves to which a large load is applied. Due to structural and operational reasons, they are subjected to large repeated bending stress. . Therefore, it is common to use a high-strength and thick steel plate base material that can withstand this bending stress for the hull shell plate, and in recent years, with the increase in the size of the hull, high-strength thickening of the steel plate is further progressing.

However, in general, since brittle crack propagation stopping properties of steel sheets tend to decrease as the strength or thickness of the steel sheet increases, the demand for brittle crack propagation stopping properties of steel sheets used for container ships and the like has recently increased.

For example, in Patent Literature 1, in order to improve brittle crack propagation stopping properties in steel materials having a microstructure mainly composed of ferrite-pearlite, a technique for improving brittle crack propagation stopping properties by controlling the shape of ferrite particles is proposed. has been

Further, in Patent Document 2, the {311}<011> plane X-ray intensity ratio at the position of 1/2t of the plate thickness is 2.5 or more, and the X-ray intensity ratio of the {110}<001> plane at the position of 1/4t of the plate thickness is 0.7 or more. In addition, a technique for improving brittle crack propagation stopping properties by setting the Charpy fracture front transition temperature to -40°C or lower has been proposed.

On the other hand, when manufacturing such a container ship, it is usually necessary to continuously join thick steel plates in the longitudinal direction of the hull, so from the viewpoint of work efficiency, high heat input welding such as submerged arc welding and electrogas arc welding is used. are often employed However, when this large heat input welding is performed, the characteristics of the HAZ may be damaged through large heat transmitted to the heat affected zone (hereinafter abbreviated as HAZ) of the steel plate. As an example, in HAZ exposed to a high temperature just below the melting point during high heat input welding, austenite grains tend to coarsen, and these coarsened austenite grains form island martensite with poor toughness by subsequent cooling. Since it is transformed into an upper bainite structure containing , the toughness of HAZ may decrease as a result.

Patent Literature 3 discloses a technique for suppressing the coarsening of the austenite grain size of HAZ by dispersing a large amount of TiN.

Patent Literature 4 discloses a technique for reducing island martensite (MA) in HAZ by reducing the content of C and Si, as well as reducing the content of P.

In the invention described in Patent Literature 5, brittle crack propagation stopping performance and HAZ toughness during large heat input welding are made compatible by dispersing particles such as TiN in a ferrite-based microstructure with a refined crystal grain size.

Japanese Unexamined Patent Publication No. 2002-256375 International Publication No. 2013/099177 International Publication No. 2011/148754 Japanese Unexamined Patent Publication No. 2008-163446 Japanese Unexamined Patent Publication No. 2015-098642

However, in the technology using the above microstructure and X-ray intensity ratio, there was a problem that the high brittle crack propagation stopping characteristics required for recent large container ships could not be stably achieved.

In addition, in the above technology for improving the HAZ toughness during high heat input welding by utilizing TiN, when the high heat input welding is performed, the weld heat affected zone is heated to the melting temperature range of TiN, so TiN is decomposed and the dispersing effect is lost. or the solid-solution Ti and solid-solution N generated by the decomposition of TiN embrittle the ground structure of the steel in the HAZ section, and the toughness of the heat-affected zone is remarkably reduced.

In addition to this, in the above technique of reducing the P content for the purpose of reducing the MA amount, variation occurs in the suppression of the amount of MA due to the distribution of P that tends to segregate at grain boundaries, etc., thereby uniformly reducing the amount of MA in the HAZ structure. From this point of view, it was insufficient.

Then, in view of the above situation, an object of the present invention is to provide a steel sheet capable of exhibiting excellent toughness in a joint after high heat input welding while exhibiting excellent brittle crack propagation stopping characteristics. Moreover, an object of this invention is to provide the method of manufacturing such a steel plate suitably.

In addition, in this specification, “large heat input welding” refers to welding with a heat input of about 150 kJ/cm, and specifically, submerged arc welding is exemplified.

Regarding the high-strength steel used for the hatch side coaming part of the container ship, the inventors have found that the brittle crack propagation stopping characteristic of the base material and the high heat input welding under the low temperature environment that is generally used for the container ship In order to be able to improve the toughness in HAZ (hereinafter also referred to as HAZ toughness by high heat input welding), intensive studies were conducted and new knowledge of the following (1) to (4) was obtained.

As an example, the yield strength of the high-strength steel used in the above study was 390 N/mm or more, the heat input for high heat input welding was 150 kJ/cm, and the use environment was assumed to be a low-temperature environment of about -10°C.

(1) In order to improve HAZ toughness by high heat input welding, it is important to suppress the coarsening of the austenite grain size of HAZ. In order to suppress the coarsening of the austenite grain size in HAZ, it is important to disperse a large amount of TiN, but when subjected to high heat input welding, TiN is decomposed by being heated to the melting temperature range, and the dispersion effect may be lost. In order to suppress this decomposition of TiN, the amount of addition of Ti and N is set within a range where the mass % ratio (Ti/N) of Ti and N is 2.00 or more and 4.00 or less, and the condition of formula (1) described later is satisfied. By design, it was found that it is effective in suppressing the decomposition of TiN during high heat input welding.

(2) Also, when dispersing a large amount of TiN, TiN particles having an average particle diameter within a predetermined range are controlled to precipitate at a number density of 5.0 × 10 ⁸ particles/cm 2 or more, thereby ensuring a good TiN dispersion effect, It was also found that HAZ toughness can be improved by high heat input welding.

(3) In order to improve the HAZ toughness by high heat input welding, it is also important to reduce island martensite in HAZ. In addition, in order to hardly generate MA in HAZ, the carbon equivalent (Ceq) represented by formula (2) described later is controlled to 0.500 or less, the C content contained in the steel sheet is 0.090% or less, and the Si content is 0.10% or less. It is important to do In addition, it is also necessary to set the Mn content to 2.00% or less, the Al content to 0.100% or less, and the Nb content to 0.100% or less in order not to reduce the HAZ toughness.

Here, "MA is hardly produced" in the present invention means that the volume fraction of MA in the microstructure of HAZ is 10% or less.

(4) In order to further achieve both the brittle crack propagation stopping properties of the steel sheet while exhibiting the excellent characteristics of the HAZ described above, adding C, Si, Mn, Al, and Nb in a predetermined amount or more, and setting the Ceq to 0.400 or more In addition to controlling by , it is effective to set the texture of the steel sheet so that the (211) plane X-ray intensity at a depth of 1/2 of the sheet thickness is 1.60 or more. Further, in order to increase the (211) plane X-ray intensity as described above, it is effective to set the volume fraction of bainite in the steel sheet to 80% or more.

The present invention was completed by further examination based on these findings. That is, the gist of the present invention is as follows.

1. In mass %,

C: 0.040% or more and 0.090% or less;

Si: 0.02% or more and 0.10% or less;

Mn: 1.60% or more and 2.00% or less;

P: 0.010% or less;

S: 0.010% or less;

Al: 0.010% or more and 0.100% or less;

Nb: 0.005% or more and 0.100% or less;

O: 0.0100% or less;

Cu: 1.00% or less;

Ni: 1.00% or less;

Cr: 1.00% or less;

Mo: 0.50% or less;

V: 0.50% or less

, and further contains Ti and N within a range where the mass% ratio of Ti and N (Ti/N) is 2.00 or more and 4.00 or less, and further satisfies the following formula (1), and the following (2 ) a component composition in which the carbon equivalent (Ceq) represented by the formula is 0.400 or more and 0.500 or less, and the balance is Fe and unavoidable impurities;

It has a structure in which the volume ratio of bainite is 80% or more,

At a position at a depth of 1 mm from the surface of the steel sheet, TiN particles having an average particle diameter of 20 nm or more and 50 nm or less are contained at a number density of 5.0 × 10 ⁸ particles/cm 2 or more,

A steel sheet having a (211) plane X-ray intensity ratio at a depth of 1/2 of the sheet thickness of 1.60 or more.

169 ≤ 5158 × Ti + 25563 × N ≤ 360 . (One)

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

However, the symbol of each element in formula (1) and formula (2) indicates the content (% by mass) of each element, and when not contained, it is set as 0.

Here, the volume fraction of bainite, the average particle diameter of TiN particles, the number density of TiN particles, and the (211) plane X-ray intensity ratio can each be measured according to the method described in Examples below.

2. Further, in mass %,

W: 0.50% or less;

Co: 0.50% or less;

B: 0.0100% or less;

Ca: 0.0100% or less;

Mg: 0.0100% or less and

REM: 0.0200% or less

The steel sheet according to 1 above, containing one or two or more selected from among.

3. The steel sheet according to the above 1 or 2, wherein the content of Ti and N is Ti: 0.010% or more and 0.031% or less and N: 0.0038% or more and 0.0100% or less.

4. In mass %,

C: 0.040% or more and 0.090% or less;

Si: 0.02% or more and 0.10% or less;

Mn: 1.60% or more and 2.00% or less;

P: 0.010% or less;

S: 0.010% or less;

Al: 0.010% or more and 0.100% or less;

Nb: 0.005% or more and 0.100% or less;

O: 0.0100% or less;

Cu: 1.00% or less;

Ni: 1.00% or less;

Cr: 1.00% or less;

Mo: 0.50% or less;

V: 0.50% or less

, and further contains Ti and N within a range satisfying the following formula (1), in which (Ti/N), the mass percent ratio of Ti and N, is set to 2.00 or more and 4.00 or less, and (2) Melting molten steel having a component composition in which the carbon equivalent (Ceq) represented by the formula is 0.400 or more and 0.500 or less, and the balance is Fe and unavoidable impurities;

When casting the molten steel to obtain a slab-shaped steel material, the average cooling rate at a position at a depth of 1 mm from the slab surface at the time of casting is 100 ° C / min or more and 500 ° C / min or less. So,

The steel material is heated to a temperature of 950 ° C. or more and 1250 ° C. or less, and the rolling start temperature is set to Ar ₃ point + 100 ° C. or more, and the reduction rate per 1 pass in the non-recrystallization region is 5.0% or more and the cumulative reduction After making a hot-rolled sheet by performing hot rolling with a ratio of 50% or more and a rolling end temperature of Ar ₃ or more,

For the hot-rolled sheet, the cooling start temperature is Ar ₃ point (° C.) or more, and the average cooling rate between 600 and 500° C. until the temperature at a depth of 1/2 the sheet thickness is 500° C. or less A method for manufacturing a steel sheet comprising cooling at 2.0 ° C./s or higher.

169 ≤ 5158 × Ti + 25563 × N ≤ 360 . (One)

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

However, the symbol of each element in formula (1) and formula (2) indicates the content (% by mass) of each element, and when not contained, it is set as 0.

Here, the temperature in each process can be measured and calculated using a radiation thermometer. In addition, the "average cooling rate at a position at a depth of 1 mm from the slab surface" is obtained by calculating the temperature at a position at a depth of 1 mm from the surface temperature of the slab measured using a radiation thermometer, and the temperature can be calculated as an average of cooling rates from 1400°C to 1250°C.

5. Further, in mass %,

W: 0.50% or less;

Co: 0.50% or less;

B: 0.0100% or less;

Ca: 0.0100% or less;

Mg: 0.0100% or less and

REM: 0.0200% or less

The manufacturing method of the steel plate as described in said 4 containing 1 type(s) or 2 or more types selected from these.

6. The manufacturing method of the steel plate as described in said 4 or 5 which makes content of said Ti and N into Ti: 0.010 % or more and 0.031 % or less, and N: 0.0038 % or more and 0.0100 % or less.

ADVANTAGE OF THE INVENTION According to this invention, the steel plate which achieved both the excellent brittle crack propagation stopping characteristic and the outstanding toughness in HAZ of the joint after high heat input welding, and its manufacturing method can be provided.

In addition, since the steel sheet obtained in the present invention is suitable for high heat input welding with excellent workability at the time of container ship construction, for example, it exhibits a special industrial effect.

The steel sheet obtained by the manufacturing method of the present invention is excellent in brittle crack propagation stopping properties and can exert excellent toughness in HAZ after high heat input welding. Therefore, the manufacturing method of this invention is suitable for manufacture of large structures, such as a container ship.

An embodiment of the present invention will be specifically described. In addition, the following embodiment shows a preferable example of this invention, It is not limited only to these examples.

(steel)

The steel sheet of the present invention has a predetermined component composition. In the component composition of the steel sheet of the present invention, the content of each element of C, Si, Mn, P, S, Al, Nb, O, Cu, Ni, Cr, Mo, and V is specified, and Ti and N are , the mass % ratio of Ti to N (Ti/N), and the predetermined formula (1) are satisfied. In addition, Ceq according to the predetermined formula (2) is defined. In addition, the volume fraction of bainite is specified in the structure of the steel sheet of the present invention. In addition, in the steel sheet of the present invention, the number density of TiN particles having a predetermined average grain size and the (211) plane X-ray intensity ratio are specified.

Since the steel sheet of the present invention is excellent in brittle crack propagation stopping properties and can exhibit excellent toughness at joints after welding with high heat input, it can be preferably used for large structures such as container ships, particularly for hatch side coaming of container ships. It can be preferably used in the The steel sheet of the present invention is preferably a steel sheet for high heat input welding.

And the steel plate of this invention can be obtained by the manufacturing method of this invention mentioned later, for example.

[Ingredient Composition]

First, the reason for limiting the component composition of the steel sheet in the present invention will be explained.

In addition, hereinafter, "%" regarding the component composition of a steel sheet shall mean "mass %" unless otherwise specified.

C: 0.040% or more and 0.090% or less

C is an element that has an effect of increasing the hardenability of the steel, and is necessary to improve the bainite structure fraction, achieve desired strength in the base material, and exhibit excellent arrest characteristics. In addition, C also affects the development of the texture of steel and is one of the important elements for achieving desired brittle crack propagation stopping characteristics by increasing the (211) plane X-ray intensity. In order to obtain the above effect, the C content is made 0.040% or more. Further, from the viewpoint of reducing the content of other alloying elements and manufacturing a steel sheet at a lower cost, the C content is preferably 0.045% or more, and more preferably 0.050% or more. On the other hand, when the C content is large, the HAZ toughness significantly decreases due to austenite being coarsened and transformed due to high heat input welding or MA being generated. From these viewpoints, the C content is made 0.090% or less. Moreover, from the viewpoint of further suppressing the decrease in HAZ toughness and the viewpoint of suppressing the decrease in weldability, the C content is preferably 0.085% or less, and more preferably 0.080% or less.

Si: 0.02% or more and 0.10% or less;

Si is a component necessary for deoxidation and the like, and is an element that has an effect of increasing the quenchability of steel by suppressing the formation of coarse carbides. Si is required to increase the bainite structure fraction to achieve desired strength in the base material and develop the texture to exhibit excellent arrest characteristics, and is added in an amount of 0.02% or more. From the viewpoint of reducing the content of other alloying elements and manufacturing a steel sheet at a lower cost, the Si content is preferably 0.03% or more, and more preferably 0.04% or more. On the other hand, when the Si content is high, HAZ toughness significantly decreases due to generation of MA due to high heat input welding. Therefore, in order to ensure high HAZ weldability, Si content is made into 0.10 % or less. From the standpoint of improving the HAZ toughness, the Si content is preferably 0.09% or less, and more preferably 0.08% or less.

Mn: 1.60% or more and 2.00% or less

Mn is an element that has an effect of increasing the hardenability of the steel, and is required to increase the bainite structure fraction to achieve desired strength in the base material and to develop the texture to exhibit excellent arrest characteristics. In addition, Mn affects the development of the texture of steel and is one of the important elements for achieving desired brittle crack propagation stopping characteristics by increasing the (211) plane X-ray intensity. In order to obtain the above effects, the Mn content is set to 1.60% or more. Further, from the viewpoint of reducing the content of other alloying elements and manufacturing a steel sheet at a lower cost, the Mn content is preferably 1.65% or more, and more preferably 1.70% or more. On the other hand, when the Mn content is high, in addition to lowering the HAZ toughness and weldability, the alloy cost becomes excessively high. From these viewpoints, the Mn content is made 2.00% or less. Further, from the viewpoint of further suppressing deterioration in toughness and weldability and further suppressing cost, the Mn content is preferably 1.95% or less, and more preferably 1.90% or less.

P: 0.010% or less

P has an adverse effect of lowering HAZ toughness by being segregated at grain boundaries. Therefore, it is preferable to make the P content as low as possible, but 0.010% or less is acceptable. On the other hand, the lower limit of the P content is not particularly limited, and may be 0%. Usually, since P is an element that is unavoidably contained in steel as an impurity, it may be more than 0% industrially. In addition, excessive reduction of P causes an increase in refining cost, so from the viewpoint of cost, the P content is preferably 0.005% or more.

S: 0.010% or less

S exists in steel as sulfide-based inclusions such as MnS, lowers HAZ toughness, and exerts an adverse effect of becoming the starting point of brittle fracture. Therefore, it is desirable to make the S content as low as possible, but 0.010% or less is acceptable. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. Usually, since S is an element that is unavoidably contained in steel as an impurity, it may be more than 0% industrially. In addition, since reducing S excessively causes an increase in refining cost, from the viewpoint of cost, it is preferable to set the S content to 0.005% or more.

Al: 0.010% or more and 0.100% or less

Al is an element that has an effect of increasing the hardenability of the steel, and is required to increase the bainite structure fraction to achieve desired strength in the base material, develop the texture, and exhibit excellent arrest characteristics. In order to obtain these effects, the Al content is set to 0.010% or more. On the other hand, when the Al content exceeds 0.100%, oxide-based inclusions increase, cleanliness decreases, and HAZ toughness decreases. Therefore, the Al content is made 0.100% or less. Further, the Al content is preferably 0.050% or less, and more preferably 0.040% or less.

Nb: 0.005% or more and 0.100% or less

Nb is an element that has an effect of increasing the hardenability of the steel, and is required to increase the bainite structure fraction to achieve desired strength in the base material and to exhibit excellent arrest characteristics. In order to obtain the said effect, Nb content is made into 0.005 % or more. Moreover, it is preferable to make Nb content into 0.007 % or more, and it is more preferable to set it as 0.009 % or more. On the other hand, when the Nb content exceeds 0.100%, MA is generated in HAZ to reduce toughness. Therefore, the upper limit of Nb content is made into 0.100 % or less. From the viewpoint of improving HAZ toughness, it is preferably 0.050% or less, more preferably 0.045% or less, and even more preferably 0.040% or less.

O: 0.0100% or less

O is an element contained as an unavoidable impurity, but since it is an element to be particularly reduced, its content is specified. O forms an oxide, becomes a starting point of occurrence of brittle fracture, and has an adverse effect of lowering HAZ toughness. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited, and may be 0%. Since O is usually an element that is unavoidably contained in steel as an impurity, it may be more than 0% industrially. In addition, since excessive reduction of O causes an increase in refining cost, from the viewpoint of cost, the O content is preferably 0.0005% or more, and more preferably 0.0020% or more.

Cu: 1.00% or less

Cu is an element having an effect of increasing the hardenability of the steel to improve the strength of the steel sheet (base material), and can be added arbitrarily. When adding Cu, it is preferable to make Cu content into 0.01 % or more in order to acquire the said effect. On the other hand, when the Cu content exceeds 1.00%, deterioration of toughness and increase in alloy cost are caused. Therefore, when adding Cu, Cu content is made into 1.00 % or less. Moreover, while 0.075 % or more is more preferable, as for Cu content, 0.50 % or less is more preferable.

Ni: 1.00% or less

Ni, like Cu, is an element that has an effect of improving the strength of the steel sheet (base material), and can be added arbitrarily. When adding Ni, in order to obtain the said effect, it is preferable to make Ni content into 0.01 % or more. On the other hand, when the Ni content exceeds 1.00%, deterioration of weldability and increase in alloy cost are caused. Therefore, when adding Ni, Ni content is made into 1.00 % or less. Moreover, while 0.075 % or more is more preferable, as for Ni content, 0.50 % or less is more preferable.

Cr: 1.00% or less

Cr, like Cu, is an element that has an effect of improving the strength of the steel sheet (base material), and can be added arbitrarily. In order to obtain the said effect, it is preferable to make Cr content into 0.01 % or more. On the other hand, when the Cr content exceeds 1.00%, deterioration of weldability and increase in alloy cost are caused. Therefore, when adding Cr, Cr content is made into 1.00 % or less. Moreover, while 0.05 % or more is more preferable, as for Cr content, 0.75 % or less is more preferable, and 0.50 % or less is still more preferable.

Mo: 0.50% or less

Mo, like Cu, is an element that has an effect of improving the strength of the steel sheet (base material), and can be added arbitrarily. In order to obtain the said effect, it is preferable to make Mo content into 0.01 % or more. On the other hand, when Mo content exceeds 0.50 %, deterioration of weldability and an increase in alloy cost are caused. Therefore, when adding Mo, Mo content is made into 0.50 % or less. Moreover, while 0.05 % or more is more preferable, as for Mo content, 0.25 % or less is more preferable.

V: 0.50% or less

V, like Cu, is an element that has an effect of improving the strength of the steel sheet (base material), and can be added arbitrarily. In order to obtain the above effect, it is preferable to make the V content 0.01% or more. On the other hand, when the V content exceeds 0.50%, deterioration in weldability and an increase in alloy cost are caused. Therefore, when adding V, the V content is made 0.50% or less. Moreover, while 0.05 % or more is more preferable, as for V content, 0.25 % or less is more preferable.

Ti and N are elements that play an important role in the present invention by precipitating as TiN during steel solidification, suppressing granulation of austenite in the heat-affected zone of welding, and contributing to high toughness by becoming ferrite transformation nuclei. contained within the range of

Ti/N: 2.00 or more and 4.00 or less

When Ti/N is less than 2.00, solid solution N that does not become TiN increases, and HAZ toughness decreases. Therefore, Ti/N is made into 2.00 or more. Ti/N is preferably 2.10 or more, more preferably 2.20 or more. Moreover, when Ti/N exceeds 4.00, TiN coarsens and reduces HAZ toughness. Therefore, the upper limit of Ti/N is 4.00. Moreover, from a viewpoint of HAZ toughness improvement, Ti/N is preferably 3.90 or less, and more preferably 3.80 or less. In addition, in Ti/N, each element is taken as content (mass %) in steel.

169 ≤ 5158 × Ti + 25563 × N ≤ 360 . (One)

In the conventional technology for improving toughness at high heat input welding using TiN, when the heat-affected zone is exposed to high heat input welding, TiN is decomposed and the dispersing effect of TiN is lost, or TiN generated by decomposition of TiN And, the ground structure of the steel in the HAZ portion is embrittled by solute N, and the HAZ toughness is remarkably reduced. Therefore, in order to suppress this TiN decomposition, it is important to set the value of (5158 × Ti) + (25563 × N), that is, the value of formula (1) to 169 or more. From the viewpoint of further improving HAZ toughness, it is preferably more than 169, more preferably 175 or more, and even more preferably 180 or more.

On the other hand, when the value of the above formula (1) exceeds 360, a large amount of TiN is generated, rather reducing the HAZ toughness. Therefore, the value of the above formula (1) is 360 or less. From the viewpoint of further improving toughness, it is preferably less than 360, more preferably 330 or less, and even more preferably 300 or less.

The condition of the above formula (1) is based on a regression value found as a result of intensive studies by the present inventors regarding the content ratio of Ti and N for controlling the distribution of TiN particles for the purpose of securing good HAZ toughness. .

In the present invention, the content ranges of Ti and N are the same as those specified above. However, following the above rules, the preferred ranges for the content of Ti and N are specifically as follows.

Ti: 0.010% or more and 0.031% or less

Ti is one of the important elements in the present invention, which precipitates as TiN during steel solidification, suppresses granulation of austenite in the weld heat affected zone, and contributes to high toughness as a ferrite transformation nucleus. In order to secure the required amount of TiN, it is preferable to contain 0.010% of Ti. The Ti content is more preferably 0.012% or more, and still more preferably 0.014% or more. On the other hand, when Ti is added in an amount exceeding 0.031%, a large amount of TiN is easily generated or the TiN particles are coarsened, making it difficult to obtain the expected effect, rather easily reducing the toughness of the welded part. Therefore, it is preferable to make the upper limit of Ti content into 0.031 %. Further, from the viewpoint of improving toughness, the Ti content is more preferably 0.028% or less, still more preferably 0.025% or less, and still more preferably 0.022% or less.

N: 0.0038% or more and 0.0100% or less

N is an element necessary for the production of TiN described above, and in order to secure the required amount of TiN, it is preferable to contain N at 0.0038% or more. Further, the N content is more preferably 0.0040% or more, and even more preferably 0.0042% or more. On the other hand, when N is added in an amount exceeding 0.0100%, a large amount of TiN is easily generated, and the toughness of the welded part is rather easily reduced. Therefore, the upper limit of the N content is preferably 0.0100%. From the viewpoint of improving toughness, the N content is more preferably 0.0090% or less, still more preferably 0.0080% or less, and still more preferably 0.0070% or less.

0.400 ≤ Ceq ≤ 0.500

Ceq = C + Mn/6 + (Cu + Ni)/ It is important that the carbon equivalent defined by the formula (2) of 15 + (V + Mo + Cr)/5 be 0.400 or more. In order to obtain the above effect, Ceq is preferably 0.410 or more, more preferably 0.420 or more, and still more preferably 0.430 or more.

On the other hand, when Ceq exceeds 0.500, MA generation occurs in HAZ generated during high heat input welding, and HAZ toughness decreases. Therefore, Ceq is made into 0.500 or less. Moreover, from a viewpoint of cost, it is preferable to make Ceq 0.490 or less, and it is more preferable to make it 0.480 or less.

The basic component composition of the steel sheet of the present invention includes each element in the content described above, and the balance is Fe and other unavoidable impurities. This basic component composition optionally contains W: 0.50% or less, Co: 0.50% or less, B: 0.0100% or less, and Ca: 0.0100% for the purpose of improving further characteristics, particularly strength or base metal toughness and HAZ toughness. Hereinafter, one or two or more selected from Mg: 0.0100% or less and REM: 0.0200% or less may be further contained.

W: 0.50% or less

W, like Cu, is an element that has an effect of improving the strength of the steel sheet (base material), and can be added arbitrarily. In order to obtain the above effect, it is preferable to make the W content 0.01% or more. On the other hand, when the W content exceeds 0.50%, deterioration of weldability and increase in alloy cost are caused. Therefore, when adding W, W content is made into 0.50 % or less. Moreover, while 0.05 % or more is more preferable, as for W content, 0.25 % or less is more preferable.

Co: 0.50% or less

Co, like Cu, is an element that has an effect of improving the strength of the steel sheet and can be added arbitrarily. In order to obtain the above effect, it is preferable to make the Co content 0.01% or more. On the other hand, when the Co content exceeds 0.50%, deterioration of weldability and increase in alloy cost are caused. Therefore, when adding Co, Co content is made into 0.50 % or less. Moreover, while 0.05 % or more is more preferable, as for Co content, 0.25 % or less is more preferable.

B: 0.0100% or less

B is an element having an effect of remarkably improving quenching properties even when added in a small amount. Therefore, the strength of the steel plate (base material) can be improved. In addition, while suppressing the generation and growth of coarse ferrite structures by contributing to the improvement of quenching properties in HAZ, by forming N and precipitates, they act as transformation nuclei and contribute to refinement of the structure, thereby improving HAZ toughness. can make it In order to obtain the said effect, when adding B, it is preferable to make B content into 0.0001% or more. On the other hand, when the B content exceeds 0.0100%, coarse Fe-B-based carbides are likely to be formed. Such coarse Fe-B-based carbides serve as a starting point for fracture, and the toughness of the base material and HAZ is remarkably reduced.

Therefore, when adding B, B content is made into 0.0100% or less. The B content is more preferably 0.0050% or less, more preferably 0.0030% or less, more preferably 0.0012% or less, and even more preferably 0.0010% or less. Also, from the viewpoint of avoiding high alloying and reducing cost, when B is added, it is preferable to set the upper limit of the B content as described above.

Ca: 0.0100% or less

Ca is an element that binds to S and has an action of suppressing formation of MnS or the like elongated in the rolling direction. Therefore, by adding Ca, the sulfide-based inclusions are morphologically controlled to exhibit a spherical shape, and the toughness of weld joints and the like can be improved. In order to obtain the said effect, when adding Ca, it is preferable to make Ca content into 0.0005 % or more, and it is more preferable to set it as 0.0020 % or more. On the other hand, when Ca content exceeds 0.0100%, the cleanliness of steel will fall and HAZ toughness will fall. Therefore, when adding Ca, Ca content is made into 0.0100% or less. Further, the Ca content is more preferably 0.0075% or less, and still more preferably 0.0050% or less.

Mg: 0.0100% or less

Mg, like Ca, is an element that binds to S and has an action of suppressing formation of MnS or the like elongated in the rolling direction. Therefore, by adding Mg, the shape of the sulfide-based inclusions can be controlled to exhibit a spherical shape, and the toughness of weld joints and the like can be improved. In order to obtain the said effect, when adding Mg, it is preferable to make Mg content into 0.0005 % or more, and it is more preferable to set it as 0.0020 % or more. On the other hand, when Mg content exceeds 0.0100%, the cleanliness of steel will fall and HAZ toughness will fall. Therefore, when adding Mg, Mg content is made into 0.0100% or less. Further, the Mg content is more preferably 0.0075% or less, and still more preferably 0.0050% or less.

REM: 0.0200% or less

REM (rare earth metal), like Ca and Mg, is an element that binds to S and has an action of suppressing formation of MnS or the like elongated in the rolling direction. Therefore, by adding REM, the sulfide-based inclusions can be morphologically controlled to exhibit a spherical shape, and the toughness of weld joints and the like can be improved. In order to obtain the said effect, when adding REM, it is preferable to make REM content into 0.0005 % or more, and it is more preferable to set it as 0.0015 % or more. On the other hand, when REM content exceeds 0.0200%, the cleanliness of steel will fall and HAZ toughness will fall. Therefore, when adding REM, REM content is made into 0.0200 % or less. Further, the REM content is more preferably 0.0100% or less, still more preferably 0.0080% or less, and still more preferably 0.0050% or less.

REM is a general term for 17 elements obtained by adding Y and Sc to the 15 elements of lanthanoid, and one or two or more of these elements may be contained. Moreover, content of REM means the total content of these elements.

[group]

Next, the structure of the steel sheet of the present invention will be described.

At a position at a depth of 1 mm from the steel sheet surface, average particle size: 20 nm or more and 50 nm or less TiN particle number density: 5.0 × 10 ⁸ particles/cm 2 or more

TiN is a precipitate that is precipitated during steel solidification and plays an important role in the present invention by suppressing granulation of austenite in a weld heat affected zone and contributing to high toughness by becoming a ferrite transformation nucleus. If the average particle diameter of TiN particles precipitated at a depth of 1 mm from the steel sheet surface at a predetermined density or more is less than 20 nm, TiN is decomposed during welding and its dispersing effect is lost, or TiN produced by such decomposition Due to solute N, the ground structure of the steel in the HAZ portion is embrittled, and the toughness of the HAZ is remarkably reduced. Therefore, the average particle diameter of TiN particles is 20 nm or more. The average particle diameter of the TiN particles is preferably 25 nm or more, more preferably 30 nm or more, from the viewpoint of the effect of improving HAZ toughness. Moreover, when the average particle diameter of TiN particle|grains exceeds 50 nm, the effect of suppressing granulation of austenite will fall and the toughness of HAZ will fall. Therefore, the average particle diameter of TiN particles is 50 nm or less. The average particle diameter of the TiN particles is preferably 45 nm or less, more preferably 40 nm or less, from the viewpoint of improving HAZ toughness.

In addition, it is important to disperse a large amount of TiN having the above-mentioned average particle diameter in order to obtain the effect of suppressing granulation of austenite in the heat-affected zone of welding and increasing toughness by becoming ferrite transformation nuclei. Therefore, the number density of TiN particles precipitated at a position at a depth of 1 mm from the surface of the steel sheet is set to 5.0 × 10 ⁸ particles/cm 2 or more. The number density of TiN particles is preferably 8.0 × 10 ⁸ particles/cm 2 or more, more preferably 1.0 × 10 ⁹ particles/cm 2 or more, from the viewpoint of the effect of improving HAZ toughness. On the other hand, the upper limit of the number density of TiN particles is not particularly limited, but from the viewpoint of satisfying the relationship between the contents of Ti and N specified in the present invention, and if the number density of TiN particles is too high, the average particle diameter of TiN particles is too fine. From the viewpoint of the tendency to fade, it is substantially 1.0 × 10 ¹⁰ pieces/cm 2 or less.

Here, brittle crack propagation stopping characteristics in the steel plate base material and toughness in the HAZ section tend to be problems especially in the front and back surfaces of the steel plate, and therefore it is desirable to improve these characteristics in the front and back surfaces of the steel plate. do. From this point of view, in the present invention, the average particle diameter and number density of TiN particles precipitated at a position close to the steel sheet surface and at a depth of 1 mm are specified.

In the present invention, TiN particles refer to precipitates each containing 10% or more of Ti and N. In addition, the average particle diameter and number density of the above-mentioned TiN particles are in the range of 10 μm × 10 μm selected arbitrarily by taking a sample so that a position at a depth of 1 mm from the surface of the steel sheet becomes the observation surface and observing with a microscope. , the area equivalent circle diameter and number of TiN particles can be specified, and can be calculated from these.

(211) plane X-ray intensity ratio at a depth of 1/2 of the plate thickness: 1.60 or more

In the steel sheet of the present invention, in order to improve brittle crack propagation stopping properties for cracks propagating in a direction perpendicular to the rolling direction (sheet width direction) while penetrating in the sheet thickness direction, the depth of 1/2 of the sheet thickness, that is, the sheet thickness The (211) plane X-ray intensity ratio in a plane parallel to the steel plate surface (plate surface) located at 1/2t, which is the center of t, is limited to 1.60 or more. In the above 1/2t, assuming a texture in which a (211) plane is developed on a rolling surface parallel to the plate surface, the (001) plane, which is the direction of preferential propagation of cracks, has an angle with respect to the plate width direction, and brittle cracks propagate. Since propagation energy can be absorbed by making the path zigzag, it is effective in improving brittle crack propagation stopping characteristics. From the above viewpoint, the (211) plane X-ray intensity ratio in 1/2t is preferably 1.80 or more, and more preferably 2.00 or more.

On the other hand, there is no upper limit on the (211) plane X-ray intensity ratio in 1/2t, but in order to increase the (211) plane X-ray intensity ratio, the reduction ratio / pass in the non-recrystallization region in controlled rolling must be increased. Since it is effective, if the reduction ratio/pass is excessively increased to increase the (211) plane X-ray intensity ratio excessively, the load on the rolling mill becomes excessively high, so it is preferably set to 3.00 or less.

Here, the (211) plane X-ray intensity ratio is a numerical value representing the degree of integration of (211) crystal planes of the target material (steel sheet), and the X-ray diffraction intensity of (211) reflection of the target material (I ₍₂₁₁₎ ) and the texture It is calculated as the ratio (I ₍₂₁₁₎ /I _{0 (211)} ) of the X-ray diffraction intensity (I _{0 (211) ) of the (211) reflection} of a random standard sample without

Volume ratio of bainite: 80% or more

The (211) plane on the rolling surface parallel to the steel sheet surface develops when the austenite structure processed during rolling transforms into a ferrite and/or bainite structure. However, when transformed into a ferrite-cementite structure, it is difficult for the (211) plane to develop into a uniform aggregate structure due to differences in the formation mechanism of the microstructure. In addition, when it is mainly transformed into a ferrite structure, it is difficult to secure sufficient strength to withstand large structures such as container ships. Therefore, it is effective to increase the (211) plane X-ray intensity ratio to make the microstructure after transformation mainly of bainite structure. From this point of view, in the steel sheet of the present invention, the volume fraction of bainite is specified to be 80% or more, and the volume fraction of bainite in the center 1/2t of the plate thickness, which is 1/2 the depth of the plate thickness, is 80% or more. It is preferable to The volume ratio of such bainite is preferably 90% or more, and of course may be 100%. By setting the volume fraction of bainite within the above range, the (211) plane X-ray intensity ratio can be increased and the brittle crack propagation stopping characteristic of the steel sheet base material can be improved.

In addition, as long as the volume ratio of bainite satisfies the above range, structures other than bainite normally present in steel sheets such as ferrite and pearlite may be present in the remaining microstructure.

The characteristics of the joint produced by high heat input welding are mainly achieved by complying with the above-mentioned requirements, but the preferable plate thickness and strength of the steel plate in the case of use as a hatch side coaming part of a container ship, for example, are as follows.

[plate thickness]

The sheet thickness of the steel sheet of the present invention is not particularly limited, but is preferably 50 mm or more, and more preferably 65 mm or more, when applied, for example, to a hatch side coaming portion in a container ship. On the other hand, the plate thickness is preferably 100 mm or less, and more preferably 80 mm or less. When the plate|board thickness of a steel plate is less than the said lower limit, it will become difficult to load cargo to the hatch cover upper part in a container ship, for example. On the other hand, when the plate thickness of a steel plate exceeds the said upper limit, it becomes difficult to give desired strength.

[robbery]

Although the strength of the steel plate (base material) of the present invention is not particularly limited, for example, when applied to the hatch side coaming part in a container ship, the depth of 1/2 of the plate thickness (at the position of 1/2t of the plate thickness) It is preferable to apply a steel sheet with a yield strength of 390 MPa or more, more preferably 430 MPa or more, and still more preferably 460 MPa or more.

(Method of manufacturing steel sheet)

In the manufacturing method of the present invention, a steel sheet is obtained by performing casting, heating, hot rolling, and cooling under predetermined conditions using molten steel having a predetermined component composition. Here, the manufacturing method of this invention is especially component composition of the molten steel in a melting process; average cooling rate of the steel material in the casting process; Heating temperature of the steel material in a heating process; Rolling start temperature in a hot rolling process, the rolling reduction rate and total rolling reduction rate per 1 pass in a non-recrystallization area|region, and rolling completion|finish temperature; Cooling start temperature and average cooling rate in a cooling process; It is important to define According to the manufacturing method of the present invention, the steel sheet of the present invention described above can be obtained satisfactorily.

[Solvent Process]

In the melting step, the component composition of the molten steel to be obtained is controlled. The component composition of the molten steel is in the same range as the amount of each element, Ti/N, formula (1) and Ceq described above for the steel sheet. Normally, O is an element that is unavoidably contained in steel as an impurity, and since refining costs increase when the content is excessively lowered, it is preferable to set the lower limit of the amount of O in steel and steel materials to 0.0005%.

[Casting process]

Regarding casting conditions for obtaining a slab-shaped steel material from molten steel, the average cooling rate at a predetermined position of the slab is defined as the following conditions.

That is, it is important to set the average cooling rate at a depth of 1 mm from the slab surface at the time of casting to 100 ° C./min or more and 500 ° C./min or less, and in the temperature range of 1400 to 1250 ° C. where TiN precipitates It is preferable to set the average cooling rate to 100 °C/min or more and 500 °C/min or less. When the average cooling rate is less than 100°C/min, the size of TiN in the base material (steel sheet) in the product steel sheet is coarsened, and the average particle size of TiN particles cannot be controlled to a predetermined value or less. In addition, when the size of the TiN particles becomes coarse, the precipitation density of TiN in the base material (steel sheet) decreases, and the number density of the TiN particles cannot be controlled beyond a predetermined level. As a result, the austenite structure in HAZ at the time of large heat input coarsens, and the toughness of HAZ falls. Therefore, the average cooling rate during casting is 100 °C/min or more, preferably 150 °C/min or more, and more preferably 200 °C/min or more.

On the other hand, when the average cooling rate exceeds 500 ° C./min, the TiN precipitation density increases, but the size of the TiN particles becomes excessively refined, and the average particle size of the TiN particles cannot be controlled beyond a predetermined level. As a result, since TiN melts and austenite grains coarsen at the time of high heat input welding, the toughness of HAZ deteriorates. In addition, since cracks occur on the surface of the steel material (slab), there is a possibility that the cost for removing the cracks and the yield of the steel material may decrease. Therefore, the average cooling rate during casting is 500 °C/min or less, preferably 400 °C/min or less, and more preferably 300 °C/min or less.

In addition, when the casting speed is less than 0.3 m/min, the size of TiN particles in the base material (steel sheet) in the product steel sheet may be coarsened. When the size of the TiN particles is coarsened, the TiN precipitation density in the base material (steel sheet) is reduced, the austenite structure in the high heat input HAZ is coarsened, and there is a possibility that the HAZ toughness is reduced. Therefore, it is preferable to set the casting speed at the time of slab casting to 0.3 m/min or more. On the other hand, the upper limit of the casting speed is not particularly limited, but when the casting speed exceeds 2.5 m/min, the TiN precipitation density increases, but the size of the TiN particles becomes fine, and TiN dissolves during high heat input welding to form austenite. Night grains may become coarse, and HAZ toughness may deteriorate. Therefore, it is preferable to set the casting speed at the time of slab casting to 2.5 m/min or less.

The temperature in the manufacturing process described below is the temperature in the plate thickness central part (1/2t) of each steel material unless otherwise specified.

[Heating process]

Heating temperature of steel material: 950 ℃ or more and 1250 ℃ or less

The heating temperature of the steel material needs to be 950°C or more and 1250°C or less. If the heating temperature is less than 950°C, the heating temperature is too low and the reverse transformation to austenite is not sufficiently completed. Even if a steel sheet is manufactured using a hot-rolled sheet in which reverse transformation to austenite has not been completed, desired brittle crack propagation stopping characteristics cannot be obtained because the texture does not develop sufficiently and the bainite structure cannot be sufficiently obtained. In addition, while the strength of the base material decreases, the deformation resistance increases, and the load on the hot rolling mill increases, so subsequent hot rolling becomes difficult. On the other hand, at a high temperature in which the heating temperature exceeds 1250°C, the austenite grains coarsen, the texture does not develop sufficiently, and the bainite structure is not sufficiently obtained, so desired brittle crack propagation stopping characteristics are not obtained. In addition, since there is a possibility that the base metal strength decreases and the oxidation loss increases due to significant oxidation and the yield decreases, the heating temperature is set to 1250°C or lower. The heating temperature is preferably 1000°C or higher, and preferably 1150°C or lower.

[Hot rolling process]

Rolling start temperature: Ar ₃ points + 100 ℃ or more

As described above, when a heated steel material is hot-rolled to obtain a hot-rolled sheet, if the temperature at which rolling is started is less than Ar ₃ point + 100 ° C., recrystallization does not sufficiently occur in the hot-rolled hot-rolled sheet, so austen The particle size of the knight does not become fine. Even if a steel sheet is manufactured using a hot-rolled sheet in which the austenite grain size is not sufficiently refined, the desired brittle crack propagation stopping characteristics cannot be obtained because the texture does not develop sufficiently and the bainite structure cannot be sufficiently obtained. Also, the strength of the base material is lowered. Therefore, the rolling start temperature is set to Ar ₃ points + 100°C or higher. From the viewpoint of securing the rolling time in the non-recrystallization region described later, the rolling start temperature is preferably Ar ₃ point + 150 ° C. or higher, and more preferably Ar ₃ point + 200 ° C. or higher. In addition, the upper limit of the rolling start temperature should normally follow the heating temperature of the raw material steel mentioned above.

In addition, the Ar ₃ point (°C) can be obtained according to the following formula (3).

Ar ₃ points (℃)

= 910 - 273C - 74Mn - 57Ni - 16Cr - 9Mo - 5Cu … (3)

Here, in formula (3), each element symbol represents the strong content (mass %) of the element, and it is set as 0 for an element not contained.

Reduction rate per pass in the non-recrystallized region: 5.0% or more and cumulative reduction rate: 50% or more

In the non-recrystallization region (in the present invention, it means the temperature region below Ar ₃ points + 100 ° C. in the steel material to be hot rolled), the reduction rate per pass (hereinafter also referred to as the reduction rate / pass) If this is less than 5.0% or if the cumulative reduction is less than 50%, sufficient working effect on austenite cannot be obtained. If austenite is not sufficiently worked, the (211) plane X-ray intensity ratio after the cooling step described later decreases, and desired brittle crack propagation stopping characteristics cannot be obtained. Therefore, in the non-recrystallized region, the reduction ratio per pass is specified to be 5.0% or more, and the cumulative reduction ratio is specified to be 50% or more.

(211) From the viewpoint of further increasing the plane X-ray intensity ratio and further improving the brittle crack propagation stopping characteristic, the reduction ratio / pass in the non-recrystallized region is preferably 8.0% or more, and more preferably 10.0% or more It is preferable, and it is more preferable to set it as 12.0 % or more. On the other hand, from the viewpoint of preventing the load on the rolling mill from becoming excessively large, it is preferable to set the reduction ratio/pass in the non-recrystallization region to 20.0% or less.

In addition, the cumulative reduction ratio in the non-recrystallized region is preferably 55% or more, and more preferably 60% or more, from the viewpoint of further improving the brittle crack propagation stopping characteristic. On the other hand, if the cumulative reduction ratio in the non-recrystallized region exceeds 70%, the cumulative reduction ratio in the recrystallized region cannot be sufficiently secured, so that the austenite grain size is not sufficiently refined. If the austenite grain size is not sufficiently refined, the toughness of the steel sheet decreases and the brittle crack propagation stopping property deteriorates.

End of rolling temperature: Ar ₃ points or more

The hot rolling process needs to be completed at a temperature equal to or higher than the Ar ₃ transformation point (°C). When the temperature during hot rolling is lower than the Ar ₃ transformation point (°C), a large amount of ferrite is formed in the steel, so that the volume fraction of bainite cannot be increased. As a result, the (211) plane X-ray intensity ratio is lowered, and excellent brittle crack propagation stopping characteristics cannot be obtained. Also, the strength of the base material is lowered. In addition, since the deformation resistance increases at a lower temperature, a problem arises in that the load on the hot rolling mill increases. In addition, it is preferable that this end temperature is Ar ₃ point + 10 degreeC or more from a viewpoint of setting the cooling start temperature of a post process to Ar ₃ point (°C) or more.

[Cooling process]

Cooling start temperature: Ar ₃ points or more

As described above, it is necessary to start cooling at a temperature equal to or higher than the Ar ₃ transformation point (°C) for the hot-rolled sheet obtained through hot rolling. If the cooling start temperature is below the Ar ₃ transformation point (°C), a large amount of ferrite is formed in the steel, so that the volume fraction of bainite cannot be increased. As a result, the (211) plane X-ray intensity ratio is lowered, and excellent brittle crack propagation stopping characteristics cannot be obtained. Also, the strength of the base material is lowered. Therefore, the cooling start temperature is set to Ar ₃ point (°C) or higher.

Average cooling rate between 600 and 500 °C until 500 °C or less: 2.0 °C/s or more

If the average cooling rate between 600 and 500°C is less than 2.0°C/s until the temperature at a depth of 1/2 of the plate thickness after starting cooling is 500°C or less, a large amount in steel due to slow cooling Since ferrite of is generated, the volume fraction of bainite cannot be increased. As a result, the (211) plane X-ray intensity ratio is lowered, and excellent brittle crack propagation stopping characteristics cannot be obtained. Also, the strength of the base material is lowered. Therefore, the average cooling rate at a depth (1/2t) of 1/2 of the sheet thickness is 2.0°C/s or more, preferably 3.0°C/s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably 20°C/s or less in order to avoid an increase in cooling cost due to excessive rapid cooling.

In addition, the temperature range for measuring this average cooling rate is 600 ° C. or less and 500 ° C. or more, where most of the austenite transformation occurs and greatly contributes to the properties.

Cooling stop temperature: 500 ℃ or less

The above cooling step needs to be carried out until the temperature at 1/2t, which is a depth of 1/2 of the sheet thickness, becomes 500°C or less, in other words, the cooling stop temperature: 500°C or less. there is When the cooling stop temperature exceeds 500°C, a large amount of ferrite is generated in the steel, so that the volume fraction of bainite cannot be increased. As a result, the (211) plane X-ray intensity ratio is lowered, and excellent brittle crack propagation stopping characteristics cannot be obtained. Also, the strength of the base material is lowered. On the other hand, although the lower limit of the cooling stop temperature is not limited, it is preferably about 200°C, more preferably about 300°C, since the shape of the steel sheet deteriorates if the cooling stop temperature is too low.

A steel sheet satisfying the above-described characteristics can be obtained by performing the above-described manufacturing process with respect to a steel material having the above-described component composition. The steel sheet obtained in this way has excellent brittle crack propagation stopping properties. Moreover, the steel plate obtained in this way has high strength. In addition, since the HAZ generated as a result of welding such a steel plate with high heat input has excellent toughness, the weld joint containing the HAZ also has excellent toughness.

Here, in the present invention, regarding the base material properties described in detail in the examples, the brittle crack propagation excellent in the case of Kca value (Kca (-10 ° C)) at -10 ° C.: 6000 N / mm ^3/2 or more It is set as a static characteristic, and yield strength (YS): The case of 390 MPa or more is regarded as an excellent strength characteristic. In addition, absorbed energy at -20 ° C. (vE-20 ° C.): The case of 46 J or more is considered excellent HAZ toughness, the case of 53 J or more is considered more excellent toughness, and the case of 64 J or more is considered particularly excellent toughness.

In addition, the steel sheet of the present invention is manufactured with a component composition in which the amount of carbon and Si is suppressed lower than that of ordinary carbon steel, and TiN is difficult to dissolve during high heat input welding. Therefore, in the HAZ generated when high heat input welding is performed, coarsening of austenite and formation of ferrite and island martensite can be avoided compared to the case of normal carbon steel, and high YS and It is compatible with high vE-20 degreeC. And high YS and high vE-20 degreeC are compatible also in the weld joint containing this HAZ. In this way, the steel sheet and its manufacturing method of the present invention solve the conventional problem of securing the toughness of the joint by high heat input welding, in addition to high brittle crack propagation stopping characteristics, and can be suitably used for high heat input welding.

Example

Hereinafter, the present invention will be specifically described based on examples. In addition, the following Example shows a preferable example of this invention, and does not limit this invention at all. In addition, the following examples can also be implemented with changes within the range that can be suitable for the purpose of the present invention, and such embodiments are also included in the technical scope of the present invention.

Molten steel having the component composition shown in Table 1 was smelted and made into a steel material (slab) under the conditions shown in Table 2, and the obtained steel material was heated, hot rolled, and cooled to obtain a steel sheet.

For each steel sheet obtained, the average particle diameter and number density of TiN particles at a depth of 1 mm from the surface, (211) plane X at 1/2 depth of the sheet thickness (center of the sheet thickness, also described as 1/2t) The line intensity ratio and the volume fraction of bainite were measured. Further, the steel sheet was evaluated for yield strength YS at 1/2t and brittle crack propagation stopping property Kca (-10°C) as base metal properties.

In addition, V improvement processing was performed on the test plate for a joint obtained from each of the steel sheets concerned, and submerged arc welding with a welding heat input of 150 kJ/cm was performed using a commercially available welding wire for low temperature service steel, to achieve high heat input welding. A seam was made by And, toughness was evaluated using the obtained joint. Each test method is as follows. In addition, the characteristic evaluated using the joint obtained in this way was made into the HAZ characteristic.

[Average particle diameter and number density of TiN particles]

Samples were taken from the steel sheet so that a position at a depth of 1 mm from the surface of the steel sheet became the observation surface. A thin film sample was prepared from the collected sample by the extraction replica method, and an area of 10 μm × 10 μm was photographed using a transmission electron microscope (TEM). In addition, for precipitates containing 10% or more of Ti and N, respectively, which are confirmed by EDX analysis, the number of precipitates and the equivalent circle diameter calculated from the area of the precipitate are analyzed from the captured image using an image analysis device Thus, the average particle diameter and number density were calculated.

[(211) plane X-ray intensity ratio]

A test piece for X-ray diffraction was prepared by taking a sample having a sheet thickness of 1 mm with the central portion of the steel sheet as the center of the sheet thickness and mechanically polishing and electrolytically polishing a surface parallel to the surface of the sample. Using this test piece, X-ray diffraction measurement was performed using a Mo radiation source and an X-ray diffractometer, and the (211) plane X-ray intensity ratio was determined.

[Volume ratio of bainite]

Samples were taken from the steel sheet so that the center of the sheet thickness served as the observation surface. The surface of the collected sample was mirror-polished and further subjected to nital corrosion to reveal a structure, and then, using a scanning electron microscope (SEM), it was magnified 500 to 3000 times, and a 10 mm × 10 mm area was photographed. In the SEM image, a structure having a lath-like ferrite structure that grew thin and long and containing carbide with an equivalent circle diameter of 0.05 μm or more was identified as a bainite structure. And, the fraction of the bainite structure was calculated|required by analyzing the photographed image using an image analyzer, and the value was made into the volume fraction.

[Yield strength of parent material]

A No. 14A test piece of JIS Z 2201 was taken from the center of the thickness of the steel sheet in a direction perpendicular to the rolling direction so that the center of the thickness was the center of the test piece. With respect to the sampled test pieces, a tensile test was conducted in accordance with JIS Z 2241, and yield strength YS (unit: MPa) was measured.

[Brittle Crack Propagation Stopping Characteristics of Base Material]

The steel sheet was subjected to a temperature gradient standard ESSO test, and the Kca value at -10°C (Kca (-10°C)) (unit: N/mm ^3/2 ) was measured as a brittle crack propagation stopping characteristic. .

[HAZ Toughness]

An NK U4 impact test piece with a depth of 1 mm as the surface layer of the test piece and HAZ as the notch position was taken from the surface of the joint obtained by high heat input welding. The sampled specimen was subjected to a Charpy impact test at a test temperature of -20°C, and the average value vE-20°C (unit: J) of the absorbed energy of three test pieces performed under the same conditions was determined as HAZ toughness.

The evaluation result obtained in this way is written together in Table 2.

As can be seen from Tables 1 and 2, all of the invention examples exhibit brittle crack propagation stopping characteristics with a Kca value of 6000 N/mm ^3/2 or more of the base material, and a yield strength (YS) of 390 MPa or more As a result, the vE-20°C of HAZ by high heat input welding was 46 J or more, and high strength and toughness were compatible. In this way, it can be seen that the steel sheet of the example of the invention is suitable for high heat input welding.

On the other hand, in steel sheet Nos. 5 to 6 corresponding to the comparative examples, at least either of the TiN average particle diameter and number density did not satisfy the conditions of the present invention, and as a result, the HAZ toughness was poor. In the steel sheet Nos. 7 to 15 corresponding to the comparative examples, at least either of the (211) plane X-ray intensity ratio at the center of the sheet thickness and the volume fraction of bainite did not satisfy the conditions of the present invention, and as a result, the base material brittle crack propagation stopping properties are declining.

In addition, the steel sheet Nos. 27 to 49 corresponding to the comparative examples do not satisfy the conditions of the present invention in the component composition of the raw material steel. Specifically, since the carbon content of steel sheet No. 27 was too low, the degree of development to a desired texture was low, and the brittle crack propagation stopping characteristic of the base material was poor. Since steel sheet No. 28 has too much carbon content, the toughness of HAZ is inferior. For steel sheet Nos. 29 to 49, any of the additive amounts of various elements, the provisions for Ti and N, and Ceq were outside the upper or lower limits specified in the present invention, and any of the brittle crack propagation stopping characteristics of the base material and the toughness of HAZ things are falling

From the above results, it can be seen that according to the present invention, it is possible to provide a steel sheet having both excellent brittle crack propagation stopping properties in a base material and excellent toughness in a joint after high heat input welding, and a method for manufacturing the same.

Claims (6)

in mass %,
C: 0.040% or more and 0.090% or less;
Si: 0.02% or more and 0.10% or less;
Mn: 1.60% or more and 2.00% or less;
P: 0.010% or less;
S: 0.010% or less;
Al: 0.010% or more and 0.100% or less;
Nb: 0.005% or more and 0.100% or less;
O: 0.0100% or less;
Cu: 1.00% or less;
Ni: 1.00% or less;
Cr: 1.00% or less;
Mo: 0.50% or less;
V: 0.50% or less
, and further contains Ti and N within a range where Ti/N, which is the mass% ratio of Ti and N, is 2.00 or more and 4.00 or less, and further satisfies the following formula (1), and the following formula (2) A component composition in which the carbon equivalent Ceq represented by is 0.400 or more and 0.500 or less, and the balance is Fe and unavoidable impurities;
It has a structure in which the volume ratio of bainite is 80% or more,
At a position at a depth of 1 mm from the surface of the steel sheet, TiN particles having an average particle diameter of 20 nm or more and 50 nm or less are contained at a number density of 5.0 × 10 ⁸ particles/cm 2 or more,
A steel sheet having a (211) plane X-ray intensity ratio at a depth of 1/2 of the sheet thickness of 1.60 or more.
169 ≤ 5158 × Ti + 25563 × N ≤ 360 . (One)
Ceq = C + Mn/6 + (Cu + Ni)/15 + (V + Mo + Cr)/5 . (2)
However, the symbol of each element in formula (1) and formula (2) indicates the content (% by mass) of each element, and when not contained, it is set as 0. According to claim 1,
Additionally, in mass %,
W: 0.50% or less;
Co: 0.50% or less;
B: 0.0100% or less;
Ca: 0.0100% or less;
Mg: 0.0100% or less and
REM: 0.0200% or less
A steel sheet containing one or two or more selected from among. According to claim 1 or 2,
A steel sheet wherein the content of Ti and N is Ti: 0.010% or more and 0.031% or less and N: 0.0038% or more and 0.0100% or less. in mass %,
C: 0.040% or more and 0.090% or less;
Si: 0.02% or more and 0.10% or less;
Mn: 1.60% or more and 2.00% or less;
P: 0.010% or less;
S: 0.010% or less;
Al: 0.010% or more and 0.100% or less;
Nb: 0.005% or more and 0.100% or less;
O: 0.0100% or less;
Cu: 1.00% or less;
Ni: 1.00% or less;
Cr: 1.00% or less;
Mo: 0.50% or less;
V: 0.50% or less
, and further contains Ti and N within a range satisfying Ti/N, which is the mass% ratio of Ti and N, of 2.00 or more and 4.00 or less, and satisfying the following formula (1), and the following (2 ) Melting molten steel having a component composition in which the carbon equivalent Ceq represented by the formula is 0.400 or more and 0.500 or less, and the balance is Fe and unavoidable impurities;
When casting the molten steel to obtain a slab-shaped steel material, the average cooling rate at a position at a depth of 1 mm from the slab surface at the time of casting is 100 ° C / min or more and 500 ° C / min or less. So,
The steel material is heated to a temperature of 950 ° C. or more and 1250 ° C. or less, and the rolling start temperature is set to Ar ₃ point + 100 ° C. or more, and the reduction rate per 1 pass in the non-recrystallization region is 5.0% or more and the cumulative reduction After performing hot rolling with a ratio of 50% or more and a rolling end temperature of Ar ₃ or more to obtain a hot-rolled sheet,
For the hot-rolled sheet, the cooling start temperature is Ar ₃ point (° C.) or more, and the average cooling rate between 600 and 500° C. until the temperature at a depth of 1/2 the sheet thickness is 500° C. or less A method for manufacturing a steel sheet comprising cooling at 2.0 ° C./s or higher.
169 ≤ 5158 × Ti + 25563 × N ≤ 360 . (One)
Ceq = C + Mn/6 + (Cu + Ni)/15 + (V + Mo + Cr)/5 . (2)
However, the symbol of each element in formula (1) and formula (2) indicates the content (% by mass) of each element, and when not contained, it is set as 0. According to claim 4,
Additionally, in mass %,
W: 0.50% or less;
Co: 0.50% or less;
B: 0.0100% or less;
Ca: 0.0100% or less;
Mg: 0.0100% or less and
REM: 0.0200% or less
A method for producing a steel sheet containing one or two or more selected from among. According to claim 4 or 5,
The manufacturing method of the steel plate which makes content of said Ti and N into Ti: 0.010% or more and 0.031% or less, and N: 0.0038% or more and 0.0100% or less.