KR102691589B1 - Steel plate and method for producing same - Google Patents

Abstract

For example, a steel sheet excellent in ammonia stress corrosion cracking resistance and low-temperature toughness for use in storage tanks used to contain liquefied gas in energy transport ships is provided. C: 0.05% or more and 0.15% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, Al: 0.060% or less, N: 0.0010% or more and 0.0100% or less, Ti: 0.005% or more and 0.100% or less % or less, P: 0.020% or less, S: 0.010% or less, and O: 0.0100% or less, the component composition of the balance Fe and inevitable impurities, and tempered martensite and tempered bay at a depth of 1 mm from the surface of the steel sheet. The total volume fraction of nite is 90% or more, the total volume fraction of ferrite and bainite at 1/2 of the plate thickness of the steel sheet is 60% or more and 90% or less, and the volume fraction of island martensite is 10%. It is assumed to have the following microstructure.

Description

Steel plate and its manufacturing method {STEEL PLATE AND METHOD FOR PRODUCING SAME}

The present invention relates to a steel sheet excellent in toughness and corrosion resistance, particularly a steel sheet excellent in low-temperature toughness and ammonia stress corrosion cracking resistance for use in a multi-purpose tank mixing liquefied petroleum gas (hereinafter referred to as LPG) and liquid ammonia, and a method for manufacturing the same. It's about.

With the recent increase in energy demand, the transportation of liquefied gas by energy transport ships is being actively carried out. For efficient operation of energy transport ships, the tank sometimes carries not only LPG but also liquid ammonia.

Since these liquefied gases are transported at low temperatures, steel plates used in tanks for storing these liquefied gases are required to have excellent, high low-temperature toughness.

In addition, as tanks have recently become larger in size, steel plates are required to have a high tensile strength (TS) of 490 MPa or more. Additionally, liquefied ammonia is known to cause stress corrosion cracking, and in order to avoid stress corrosion cracking caused by ammonia, a yield strength (YS) of 440 MPa or less is simultaneously required.

As described above, a technique for having the low-temperature toughness required for a liquefied gas storage tank and satisfying the strength range is described in Patent Documents 1 and 2, in which a thick steel plate cooled after hot rolling is heat treated several times, or High low-temperature toughness and specified strength characteristics are achieved by heat-treating thick steel sheets that are hot-rolled and then water-cooled several times.

Japanese Patent No. 3802626 Japanese Patent No. 3848415

In the method described in Patent Documents 1 and 2 above, there was an economic problem as it was necessary to perform heat treatment multiple times and the cost of equipment and energy for the heat treatment was large. In addition, when the first quenching temperature was high, the area for improvement in toughness at 1/4t was small, so there was a concern that the toughness in the surface layer of the steel sheet exposed to high temperatures for a long time may become unstable. .

The present invention solves the above problems and provides, for example, a steel sheet excellent in ammonia stress corrosion cracking resistance and low-temperature toughness, which is provided for storage tanks used to accommodate liquefied gas in energy transport ships, and a method for manufacturing the same. The purpose is to

In order to achieve the above object, the present inventors conducted intensive studies on various factors related to the low-temperature toughness and strength characteristics of steel sheets using an online heating and cooling device. As a result, elements such as C, Si, Mn, and Ti are added in a predetermined amount or more, and the total volume ratio of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel sheet is 90% or more. If the microstructure is controlled such that the total volume ratio of ferrite and bainite at a depth of 1/2 of the plate thickness from the surface of the steel sheet is 60 to 90%, and the volume ratio of island martensite is 10% or less. , it was found that the desired low-temperature toughness and strength characteristics were exhibited, and costly heat treatment could be omitted.

The present invention was completed through further investigation based on this knowledge. That is, the gist of the present invention is as follows.

1. In mass%,

C: 0.05% or more and 0.15% or less,

Si: 0.50% or less,

Mn: 0.50% or more and 2.00% or less,

Al: 0.060% or less,

N: 0.0010% or more and 0.0100% or less,

Ti: 0.005% or more and 0.100% or less,

P: 0.020% or less,

S: 0.010% or less and

O: 0.0100% or less

Contains and has a composition of the balance Fe and inevitable impurities,

The total volume ratio of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel sheet is 90% or more, and the total volume of ferrite and bainite at 1/2 of the sheet thickness of the steel sheet. A steel sheet with a microstructure in which the percentage is 60% or more and 90% or less, and the volume fraction of fine martensite is 10% or less.

2. The above component composition is further expressed in mass%,

Cu: 2.00% or less,

Ni: 2.00% or less

Cr: 1.00% or less,

Mo: 1.00% or less

V: 1.00% or less,

W: 1.00% or less,

Co: 1.00% or less,

Nb: 0.100% or less,

B: 0.0100% or less,

Ca: 0.0200% or less,

Mg: 0.0200% or less and

REM: 0.0200% or less

The steel plate according to 1 above, containing one or more types selected from among.

3. In mass%,

C: 0.05% or more and 0.15% or less,

Si: 0.50% or less,

Mn: 0.50% or more and 2.00% or less,

Al: 0.060% or less,

N: 0.0010% or more and 0.0100% or less,

Ti: 0.005% or more and 0.100% or less,

P: 0.020% or less,

S: 0.010% or less and

O: 0.0100% or less

A steel material containing and having a component composition of the balance Fe and inevitable impurities is subjected to hot rolling at an ending temperature of Ar ₃ or higher, and then cooling is started from a temperature of Ar ₃ or higher, and the steel material is 1 mm from the surface of the steel sheet. Cool at an average cooling rate of 10°C/s or more until the temperature at the depth becomes 600°C or lower, stop cooling once and stop the cooling between 10 and 600 seconds, and then cool the steel sheet to a thickness of 600°C or lower. A method of manufacturing a steel sheet, wherein cooling is performed at an average cooling rate of 5 to 50°C/s per half, and the cooling is terminated in a temperature range where the temperature at the center of the sheet thickness is 200°C or more and 450°C or less.

4. The above component composition is further expressed in mass%,

Cu: 2.00% or less,

Ni: 2.00% or less,

Cr: 1.00% or less,

Mo: 1.00% or less,

V: 1.00% or less,

W: 1.00% or less,

Co: 1.00% or less,

Nb: 0.100% or less,

B: 0.0100% or less,

Ca: 0.0200% or less,

Mg: 0.0200% or less and

REM: 0.0200% or less

The method for producing a steel sheet according to item 3 above, comprising at least one selected from the group consisting of:

According to the present invention, a steel plate suitable for tanks used in environments with low temperatures and corrosive atmospheres, which has excellent impact resistance at low temperatures and ammonia stress corrosion cracking resistance, can be provided at low cost, thereby exhibiting a special industrial effect.

Next, the steel plate of the present invention will be described in detail. In the present invention, it is important that the steel sheet and the steel material used for its production have the above chemical composition. So, first, the reason for limiting the composition of steel in the present invention as described above will be explained. In addition, “%” regarding component composition shall mean “mass%” unless otherwise specified.

[Ingredients Composition]

C: 0.05% or more and 0.15% or less

C is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that needs to be added to achieve high strength. In order to obtain the above effect, the C content is set to 0.05% or more. Additionally, from the viewpoint of reducing the content of other alloy elements and manufacturing at lower cost, it is preferable that the C content is 0.07% or more. On the other hand, when the C content exceeds 0.15%, not only does the strength increase excessively, but toughness and weldability decrease. Therefore, the C content is set to 0.15% or less. Additionally, from the viewpoint of suppressing a decrease in toughness and weldability, it is preferable to set the C content to 0.13% or less.

Si: 0.50% or less

Si is an element that acts as a deoxidizing agent, but on the other hand, it is an element that causes a decrease in toughness and weldability. Therefore, it is desirable to keep the content as low as possible, but it is acceptable as long as it is 0.50% or less. In addition, since deoxidation of steel is sufficiently possible with Al, Ti, etc., the lower limit of the Si content is not particularly limited and may be 0%. From the viewpoint of toughness and weldability, it is preferable to set it to 0.40% or less, and it is more preferable to set it to 0.30% or less.

Mn: 0.50% or more and 2.00% or less

Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that needs to be added to satisfy high strength. In order to obtain the above effect, the Mn content is set to 0.50% or more. Additionally, from the viewpoint of reducing the content of other alloy elements and manufacturing at a lower cost, the Mn content is preferably 0.70% or more, and more preferably 0.90% or more. On the other hand, when the Mn content exceeds 2.00%, the strength becomes excessively high and the toughness and weldability decrease, and the alloy cost becomes excessively high. Therefore, the Mn content is set to 2.00% or less. Additionally, from the viewpoint of suppressing a decrease in toughness and weldability, the Mn content is preferably 1.80% or less, and more preferably 1.60% or less.

Al: 0.060% or less

Al is an element that acts as a deoxidizer and has the effect of refining crystal grains. In order to obtain these effects, it is preferable that the Al content is 0.010% or more. On the other hand, when the Al content exceeds 0.060%, oxide inclusions increase, cleanliness decreases, and toughness decreases. Therefore, the Al content is set to 0.060% or less. Additionally, the Al content is preferably 0.050% or less, and more preferably 0.040% or less.

N: 0.0010% or more and 0.0100% or less

N combines with Ti and precipitates as TiN, contributing to the refinement of the structure and improving toughness. To obtain this effect, the N content is set to 0.0010% or more. Preferably, it is 0.0020% or more. On the other hand, when the N content exceeds 0.0100%, toughness actually decreases. Therefore, from the viewpoint of suppressing a decrease in toughness and weldability, the amount is set to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less.

Ti: 0.005% or more and 0.100% or less

Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing dissolved N. Therefore, the toughness of the base material and weld zone can be improved by adding Ti. To obtain this effect, the Ti content is set to 0.005% or more. The Ti content is preferably 0.012% or more. On the other hand, when the Ti content exceeds 0.100%, the toughness actually decreases. Therefore, the Ti content is set to 0.100%. The Ti content is preferably 0.090% or less, and more preferably 0.080% or less.

P: 0.020% or less

P is an element contained as an inevitable impurity, and has adverse effects, such as lowering toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but it is acceptable as long as it is 0.020% or less. In addition, the lower limit of the P content is not particularly limited and may be 0%, but since P is usually an element inevitably contained in steel as an impurity, it may be industrially more than 0%. Additionally, since excessive reduction results in an increase in refining costs, the P content is preferably set to 0.0005% or more.

S: 0.010% or less

S is an element contained as an inevitable impurity, which exists in steel as a sulfide-based inclusion such as MnS, and is an element that exerts adverse effects, such as becoming a starting point of destruction. Therefore, it is desirable to keep the S content as low as possible, but it is acceptable as long as it is 0.010% or less. In addition, the lower limit of the S content is not particularly limited and may be 0%. Normally, S is an element that is inevitably contained in steel as an impurity, and therefore, industrially, it may be more than 0%. That is, since excessive reduction results in an increase in refining costs, it is desirable to set the S content to 0.0005% or more from the viewpoint of cost.

O: 0.0100% or less

O is an element contained as an inevitable impurity and is limited to 0.0100% or less because it is an element that has adverse effects, such as forming oxides and becoming the origin of destruction. 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%, but since O is usually an element inevitably contained in steel as an impurity, it may be industrially more than 0%. That is, since excessive reduction results in an increase in refining costs, it is desirable to set the O content to 0.0020% or more from the viewpoint of cost.

The component composition including the above components and the balance Fe and inevitable impurities is the basic component composition in the present invention. For the purpose of further improving strength characteristics or toughness, this basic component composition is optionally Cu: 2.00% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 1.00% or less, W: 1.00% or less, Co: 1.00% or less, Nb: 0.100% or less, B: 0.0100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0200% or less. It may contain additional ingredients.

Cu: 2.00% or less

Cu is an element that has the effect of increasing the hardenability of steel and improving the strength of the steel sheet, and can be added arbitrarily. When adding Cu, it is preferable that the Cu content is 0.01% or more in order to obtain the above effect. More preferably, it is 0.20% or more. On the other hand, if the Cu content exceeds 2.00%, it causes deterioration in toughness and an increase in alloy cost. Therefore, when adding Cu, the Cu content is set to 2.00% or less. More preferably, it is 1.00% or less.

Ni: 2.00% or less

Ni, like Cu, is an element that has the effect of improving the strength of the steel sheet, and can be added arbitrarily. When adding Ni, it is preferable to set the Ni content to 0.01% or more in order to obtain the above effect. More preferably, it is 0.20% or more. On the other hand, if the Ni content exceeds 2.00%, it causes deterioration of weldability and an increase in alloy cost. Therefore, when adding Ni, the Ni content is set to 2.00% or less. More preferably, it is 1.00% or less.

Cr: 1.00% or less

Cr, like Cu, is an element that has the effect of improving the strength of the steel sheet, and can be added arbitrarily. In order to obtain the above effect, it is preferable that the Cr content is 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the Cr content exceeds 1.00%, it causes deterioration of weldability and an increase in alloy cost. Therefore, when adding Cr, the Cr content is set to 1.00% or less. More preferably, it is 0.50% or less.

Mo: 1.00% or less

Mo, like Cu, is an element that has the effect of improving the strength of the steel sheet, and can be added arbitrarily. In order to obtain the above effect, it is preferable that the Mo content is 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the Mo content exceeds 1.00%, it causes deterioration of weldability and an increase in alloy cost. Therefore, when adding Mo, the Mo content is set to 1.00% or less. More preferably, it is 0.50% or less.

V: 1.00% or less

V, like Cu, is an element that has the effect of improving the strength of the steel sheet, and can be added arbitrarily. In order to obtain the above effect, it is preferable that the V content is 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the V content exceeds 1.00%, it causes deterioration of weldability and an increase in alloy cost. Therefore, when adding V, the V content is set to 1.00% or less. More preferably, it is 0.50% or less.

W: 1.00% or less

W, like Cu, is an element that has the effect of improving the strength of the steel sheet, and can be added arbitrarily. In order to obtain the above effect, it is preferable that the W content is 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the W content exceeds 1.00%, it causes deterioration of weldability and an increase in alloy cost. Therefore, when adding W, the Mo content is set to 1.00% or less. More preferably, it is 0.50% or less.

Co: 1.00% or less

Co is an element that, like Cu, has the effect of improving the strength of the steel sheet, and can be added arbitrarily. In order to obtain the above effect, it is preferable that the Co content is 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the Co content exceeds 1.00%, it causes deterioration of weldability and an increase in alloy cost. Therefore, when adding Co, the Co content is set to 1.00% or less. More preferably, it is 0.50% or less.

Nb: 0.100% or less

Nb is an element that has the effect of reducing the prior austenite grain size and improving toughness by precipitating as carbonitride. When adding Nb, it is preferable that the Nb content is 0.005% or more to obtain the above effect. Moreover, it is more preferable that the Nb content is 0.007% or more. On the other hand, when the Nb content exceeds 0.100%, a large amount of NbC precipitates and the toughness decreases. Therefore, when adding Nb, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, more preferably 0.060% or less, and even more preferably 0.045% or less.

B: 0.0100% or less

B is an element that has the effect of significantly improving quenching properties even when added in a trace amount. Therefore, the strength of the steel sheet can be improved. In order to obtain the above effect, when adding B, it is preferable that the B content is 0.0001% or more. The B content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, when the B content exceeds 0.0100%, weldability deteriorates. Therefore, when adding B, the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less, and more preferably 0.0030% or less.

Ca: 0.0200% or less

Ca is an element that combines with S and has the effect of suppressing the formation of MnS, etc., which extends long in the rolling direction. Therefore, by adding Ca, the shape of the sulfide-based inclusions can be controlled so that they appear spherical, and the toughness of the weld zone and the like can be improved. In order to obtain the above effect, when adding Ca, it is preferable that the Ca content is 0.0005% or more. More preferably, it is 0.0020% or more. On the other hand, when the Ca content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness causes deterioration of surface properties due to an increase in surface scratches and a decrease in bending workability. Therefore, when adding Ca, the Ca content is set to 0.0050% or less. More preferably, it is 0.0100% or less.

Mg: 0.0200% or less

Mg, like Ca, is an element that combines with S and has the effect of suppressing the formation of MnS, etc., which extends long in the rolling direction. Therefore, by adding Mg, the shape of the sulfide-based inclusions can be controlled to appear spherical, thereby improving the toughness of the weld zone and the like. In order to obtain the above effect, when adding Mg, it is preferable that the Mg content is 0.0005% or more. More preferably, it is 0.0020% or more. On the other hand, when the Mg content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness causes deterioration of surface properties due to an increase in surface scratches and a decrease in bending workability. Therefore, when adding Mg, the Mg content is set to 0.0050% or less. More preferably, it is 0.0100% or less.

REM: 0.0200% or less

REM (rare earth metal), like Ca and Mg, is an element that combines with S and has the effect of suppressing the formation of MnS, etc., which extends long in the rolling direction. Therefore, by adding REM, the shape of the sulfide-based inclusions can be controlled to appear spherical, thereby improving the toughness of the weld zone and the like. In order to obtain the above effect, when adding REM, it is preferable that the REM content is 0.0005% or more. More preferably, it is 0.0020% or more. On the other hand, when the REM content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness causes deterioration of surface properties due to an increase in surface scratches and a decrease in bending workability. Therefore, when adding REM, the REM content is set to 0.0080% or less. More preferably, it is 0.0100% or less.

In addition to having the above chemical composition, the steel sheet of the present invention has a total volume ratio of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel sheet of 90% or more, and It has a microstructure in which the total volume ratio of ferrite and bainite at a depth of half the thickness is 60 to 90%, and the volume ratio of island martensite is 10% or less. The reason for limiting the microstructure of steel as described above will be explained below.

[micro organization]

The microstructure of the steel sheet of the present invention will be described.

[The total volume ratio of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel sheet is 90% or more]

Typically, in steel sheets that have been continuously cooled after hot rolling, the surface structure with the fastest cooling rate is martensite or bainite. In the present invention, as the manufacturing conditions of the steel sheet will be described later, cooling after hot rolling is temporarily stopped and only the surface layer portion of the steel sheet is intentionally tempered, thereby preventing excessive hardening of the surface of the steel sheet, satisfying predetermined strength characteristics, and further improving durability at low temperatures. Improving personality. Therefore, the structure at a depth of 1 mm from the surface of the steel sheet (hereinafter also referred to as the surface layer portion) has a total volume ratio of tempered martensite and tempered bainite of 90% or more. If the remaining structure other than tempered martensite or tempered bainite exceeds 10%, the difference in strength between the tempered martensite or tempered bainite and the remaining structure increases, and the strength characteristics become unsatisfactory, or at low temperature. Since the toughness decreases, the total volume ratio of tempered martensite and tempered bainite is set to 90% or more. Since the higher the volume ratio of tempered martensite and tempered bainite, the more desirable it is, the upper limit of the volume ratio is not particularly limited and may be 100%. In addition, there is no need to specifically limit the respective ratios of tempered martensite and tempered bainite, but it is preferable that tempered martensite is 80% or more.

On the other hand, the type of the remaining structure is not particularly limited, and structures such as ferrite, pearlite, austenite, bainite, and martensite may be mixed, but the total volume ratio thereof is set to less than 10%. There is no need to specifically limit the fraction of each structure in the residual structure, but from the viewpoint of toughness, it is preferable that the difference in hardness from tempered martensite or tempered bainite is small, so it is preferable that the residual structure is bainite.

In addition, the volume ratio of the tempered martensite and tempered bainite is set to a value at a depth of 1 mm from the surface of the steel sheet. This is because it improves the toughness of the surface layer. In addition, the volume fraction of various micro structures can be measured by the method described in the Examples described later.

[The total volume ratio of ferrite and bainite in 1/2 of the steel sheet thickness is 60% or more and 90% or less, and the volume ratio of island martensite is 10% or less]

The structure at 1/2 of the sheet thickness of a steel sheet (hereinafter also referred to as the center of the sheet thickness) has a total volume ratio of ferrite and bainite of 60% or more and 90% or less, and the volume of island martensite contained in the remainder. The rate is 10% or less. That is, if the total volume fraction of ferrite and bainite is less than 60%, the volume fraction of martensite, pearlite, and austenite other than this increases, and sufficient strength and/or toughness cannot be obtained, and mechanical properties cannot be satisfied. I can't. On the other hand, if the total volume fraction of the above structures exceeds 90%, the volume fractions of martensite, pearlite, austenite, etc. become too low, so the strength characteristics are not satisfied.

Here, the ferrite is ferrite generated during a cooling process without undergoing tempering, etc., and the bainite is bainite generated during a cooling process without undergoing tempering. In addition, defining the microstructure at the center of the plate thickness affects the strength characteristics of 1/2 of the plate thickness. In addition, there is no need to specifically limit the respective ratios of ferrite and bainite, but from the viewpoint of further improving strength characteristics, it is preferable that it contains a plurality of structures with different strengths, and it is more preferable that ferrite is 10% or more.

The remainder other than the above-mentioned ferrite and bainite may have microstructures such as pearlite or austenite, but if the remainder structure contains more than 10% of innate martensite, the toughness is greatly reduced, so the insular martensite The volume ratio of the material should be 10% or less. Preferably, it is 5% or less, and of course it may be 0%. That is, the island-like martensite in the remaining structure has higher strength and lower toughness than normal martensite, and thus becomes the starting point of failure, so the volume fraction of the structure is defined.

On the other hand, the remaining structure, which occupies 10% to 40% by volume, may contain martensite in addition to pearlite and austenite. There is no need to specifically limit the fraction of each structure in the remaining structure, but it is preferable that the remaining structure is pearlite.

In addition, the volume ratio of various micro structures can be measured by the method described in the Examples described later.

Next, the manufacturing method of the steel plate of the present invention will be described.

A steel material having the above chemical composition is heated and hot rolled to obtain a hot rolled steel sheet, and then cooled to an initial temperature of Ar ₃ transformation point or higher to obtain a steel sheet. Hereinafter, each manufacturing condition will be described in detail.

First, the manufacturing conditions of the steel material do not need to be particularly limited, but molten steel having the above-mentioned composition is melted by a known melting method such as a converter, and a slab or the like of a predetermined size is formed by a known casting method such as a continuous casting method. It is desirable to use a steel material. Additionally, there is no problem at all even if it is made into a steel material such as a slab of a predetermined size by the ingot-decomposed rolling method.

The obtained steel material is directly hot rolled without cooling, or is heated and then hot rolled. Hot rolling is performed at a temperature of Ar ₃ point or higher, and then cooling is started from a temperature of Ar ₃ point or higher, and average cooling is performed until the temperature at a depth of 1 mm from the surface of the hot rolled sheet reaches a temperature of 600°C or lower. Speed: Cool at 10 ℃/s or more (first cooling), and at the stage where the above temperature reaches 600 ℃ or less, stop cooling and leave for 10 to 600 seconds, and then continue to cool at the center of the plate thickness. Cooling (second cooling) is performed at an average cooling rate of 5 to 50°C/s at a temperature, and the cooling is completed in a temperature range where the temperature at the center of the sheet thickness is 200°C or more and 450°C or less. Additionally, if the temperature at the center of the sheet thickness at the time of the first cooling stop is less than 600°C, there are cases where recuperation does not occur and the surface layer is not tempered. Therefore, the temperature at the center of the sheet thickness is preferably 600°C or higher, and more preferably 650°C or higher.

(a) Heating temperature of steel material: above 950℃ and below 1250℃

The heating temperature of the steel material is not particularly limited, but if the heating temperature is less than 950°C, there is a risk that the heating temperature is too low, the deformation resistance increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. On the other hand, when the temperature exceeds 1250°C, there is a risk that oxidation becomes significant, oxidation loss increases, and the yield decreases. From this, it is preferable that the heating temperature is 950°C or higher and 1250°C or lower. Also, more preferably, it is 1000°C or higher and 1150°C or lower.

(b) Hot rolling temperature: Ar ₃ transformation point or higher

After heating to the above temperature, hot rolling is started, and rolling is completed at a temperature equal to or higher than the Ar ₃ transformation point. That is, when the rolling temperature is below the Ar ₃ transformation point, ferrite is generated, and the generated ferrite is affected by processing, so the toughness deteriorates. Furthermore, the load on the hot rolling mill increases. Therefore, the hot rolling temperature is set to Ar ₃ transformation point or higher. Preferably, it is Ar ₃ transformation point + 20°C or higher.

On the other hand, if the rolling temperature exceeds 950°C, there is a risk that the structure may coarsen and the toughness may deteriorate, so it is preferable to set it to 950°C or lower. More preferably, it is 930°C or lower.

Here, the Ar ₃ transformation point can be obtained, for example, by the following equation.

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

However, each element represents the content (% by mass) of the element.

(c) Cooling start temperature: Ar ₃ transformation point or higher

Next, the steel sheet after hot rolling is cooled from the Ar ₃ transformation point or higher. When the cooling start temperature is below the Ar ₃ transformation point, ferrite is generated in the surface layer of the steel sheet and coexists with a martensite structure or bainite structure with a large difference in strength, resulting in a decrease in toughness. Therefore, the cooling start temperature is set to be equal to or higher than the Ar ₃ transformation point.

(d) Speed in first cooling: Cooling speed at a depth of 1 mm from the surface of the steel sheet is 10°C/s or more.

The speed for the first cooling is 10°C/s or more. This is because tempered bainite or ferrite with a large difference in hardness from tempered bainite is generated, so low-temperature toughness is not secured. Preferably, it is 10°C/s or more. The upper limit of the cooling rate is not particularly limited, but excessive cooling increases the cooling cost, so it is preferably 200°C/s or less.

(e) First cooling stop temperature: The temperature at a depth of 1 mm from the surface of the steel sheet is 600 ℃ or less.

The stopping temperature of the first cooling is set to 600°C or lower because the structure of the surface layer is made up of martensite and/or bainite at a total of 90% or more. When the cooling stop temperature exceeds 600°C, a large amount of ferrite is generated and toughness deteriorates. Therefore, the cooling stop temperature is set to 600°C or lower. On the other hand, the lower limit of the cooling stop temperature is not limited, but is substantially 5°C or higher because it does not fall below the temperature of the cooling water. However, if the cooling stop temperature of the surface layer is too low, the central part of the subsequent sheet thickness is also cooled too much, so it is preferably 100°C or higher, and more preferably 200°C or higher.

(f) Cooling stop time: 10 seconds or more and 600 seconds or less

After the first cooling as described above, the cooling is temporarily stopped for between 10 seconds and 600 seconds. By stopping the cooling, the structure of martensite or bainite generated in the surface layer is tempered by reheating from the center of the plate thickness. If the stopping time is less than 10 seconds, the effect of tempering becomes insufficient, the toughness decreases and the strength increases excessively. On the other hand, if it exceeds 600 seconds, transformation begins in the center of the sheet thickness, and a large amount of ferrite structure is generated, which further coarsens the structure, thereby lowering strength and toughness.

(g) Second cooling rate: The cooling rate at the center of the plate thickness is 5 ℃/s or more and 50 ℃/s or less.

After the cooling stop, cooling is resumed. The cooling rate here is set to be 5°C/s or more and 50°C/s or less so that ferrite or martensite reaches a predetermined volume ratio. That is, if the cooling rate is less than 5°C/s, the volume ratio of ferrite or bainite structure becomes too large, and the strength characteristics are not satisfied. On the other hand, when the cooling rate exceeds 50°C/s, the volume ratio of martensite becomes too large, and toughness decreases.

(h) Second cooling end temperature: Cooling end temperature at the center of the sheet thickness is 200 ℃ or more and 450 ℃ or less.

The end temperature of the second cooling is set to 200°C or more and 450°C or less in order to obtain a predetermined volume ratio of ferrite and bainite structure in the center of the sheet thickness. If the cooling end temperature exceeds 450°C, the total volume ratio of ferrite and bainite at the center of the sheet thickness exceeds 90%, and the strength characteristics are not satisfied. On the other hand, when the cooling end temperature is less than 200°C, the volume ratio of island martensite becomes too large, and in addition to excessively increasing the strength, the toughness decreases.

By manufacturing a steel material having the above-described component composition according to the above-described manufacturing conditions, a steel sheet having the above-described structure can be obtained. The steel sheet obtained in this way has excellent strength characteristics and toughness. Here, excellent strength characteristics include yield strength YS (yield point YP when there is a yield point, 0.2% proof strength σ0.2 when there is no yield point): 440 MPa or less and tensile strength (TS): 490 MPa or more. Among these, yield strength YS is closely related to ammonia stress corrosion cracking, and as a structural member of liquefied gas bulk carriers, it is used to minimize the risk of ammonia stress corrosion cracking in the IMO gas code and classification rules by the International Maritime Organization. The yield point is specified to be 440 MPa or less. Therefore, if YS is 440 MPa or less, it can be said to have excellent ammonia stress corrosion cracking properties.

Basically, the higher the tensile strength (TS) of the steel sheet, the more desirable it is, but if it exceeds 620 MPa, the likelihood of problems with processability increases. Alternatively, there is a high possibility that a large amount of alloy will be added, increasing the cost. In addition, since the yield strength YS (yield point YP when there is a yield point, 0.2% proof strength σ0.2 when there is no yield point) to ensure ammonia stress corrosion cracking resistance is not compatible with 440 MPa or less, the tensile strength (TS) of the steel plate It is desirable to set it to 620 MPa or less. Additionally, the tensile strength (TS) of the steel sheet obtained in the present invention is substantially 620 MPa or less.

Example

Molten steel with the component composition shown in Table 1 was melted and used as a steel material (slab). These steel materials (slabs) were subjected to hot rolling and cooling under the conditions shown in Table 2.

For the obtained steel sheet, the tissue fraction was measured in the microstructure at a depth of 1 mm from the steel sheet surface (surface layer) and 1/2 of the sheet thickness from the steel sheet surface (center of the sheet thickness), and the tensile properties and toughness were evaluated. . Each test method is as follows.

[Measurement of microstructure fraction in the surface layer and center of plate thickness]

From each obtained steel plate, a sample was taken so that the observation surface was at a depth of 1 mm from the surface of the steel plate. The surface of the sample was mirror polished and further subjected to nital etching, and then an area of 10 mm x 10 mm was photographed using a scanning electron microscope (SEM). The captured image was analyzed using an image analysis device to determine the fraction of microstructure, and the value was taken as the volume fraction.

In addition, the microstructure at the center of the plate thickness was examined by taking samples from each obtained steel sheet so that the center of the plate thickness was the observation surface. That is, the sample was mirror polished and further subjected to nital etching, and then an area of 10 mm x 10 mm was photographed using a scanning electron microscope (SEM). The area fraction of the microstructure was obtained by analyzing the captured images using an image analysis device. When the anisotropy of the microstructure is small, the cotton fraction is equivalent to the volume ratio, so in this patent, the cotton fraction is used as the volume ratio.

In either case, when determining the fraction of micro-tissues, discrimination of each tissue was performed as follows. The steel material was mirror polished, nital etched to reveal the structure, and observed under SEM at 500 to 3000 times magnification. Ferrite is a structure that does not contain isotropically grown carbides and appears black on the inside of the mouth, and pearlite is a structure in which ferrite (black) and carbide (white) appear in a stripe shape. Bainite has a thin and elongated lath-like ferrite structure, is a structure containing carbides with an equivalent circle diameter of 0.05 ㎛ or more, and if it contains carbides in an amount of 1.0 × 10 ⁴ or more/mm2, it is called tempered bainite. It was defined as: In the tempered structure, the carbides are divided, for example, the long and thin carbides coming out between the laths of bainite are made into a plurality of round carbides, so it is easy to distinguish them by looking at the carbides. Martensite has the same thin and elongated lath-like ferrite structure as bainite, and is a structure containing carbides with an equivalent circle diameter of 0.05 ㎛ or less, and if it contains more than 1.0 × 10 ⁴ carbides/mm2, it is tempered. It was defined as martensite. Additionally, carbides appear as white dots. In addition, austenite was defined as a non-carbide structure with an equivalent circle diameter of 0.50 ㎛ or more that exists between bainite and martensite structures.

[Strength characteristics]

From the entire thickness of each steel plate, a No. 1B test piece of JIS Z 2201 was taken in a direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z 2241, yielding strength YS (yield point YP when there is a yield point, and yield point YP when there is no yield point). 0.2% yield strength (σ0.2) and tensile strength (TS) were measured. Yield strength: 440 MPa or less was evaluated as a steel plate with excellent ammonia stress corrosion cracking properties, and a tensile strength of 490 MPa or more was evaluated as a steel plate with excellent tensile strength. In addition, the yield strength YS is closely related to ammonia stress corrosion cracking, and as a structural member of a liquefied gas bulk carrier, in the IMO gas code and classification rules, the yield point is set to 440 MPa or less to minimize the risk of ammonia stress corrosion cracking. It is stipulated. Therefore, as described above, steel sheets with YS of 440 MPa or less were determined to have excellent ammonia stress corrosion cracking properties.

[tenacity]

Additionally, a V-notch test piece of JIS Z 2202 was taken from a portion cut 1 mm from the surface side of each steel plate in the rolling direction, and a Charpy impact test was performed according to JIS Z 2242 to measure vTrs. And, steel plates with vTrs of -60°C or lower were evaluated as having excellent toughness.

The evaluation results obtained in this way are listed in Table 2.

As can be seen from Tables 1 and 2, the invention examples all have a yield strength YS of 440 MPa or less and a tensile strength TS of 490 MPa or more, and a ductility brittle temperature of -60° C. or less, and have low temperature toughness and ammonia stress corrosion resistance. A steel plate with excellent cracking properties was obtained.

On the other hand, steel plate No. corresponding to the comparative example. 5, 7, 9, 11, 12, 14, 17, 18, 20, 21, 24, 25, 51, the microstructure of the surface layer and the microstructure of the center of the plate thickness are different from the invention example, and the yield strength YS and tensile The strength TS or toughness at low temperatures is inferior compared to the invention example. In addition, steel plate No. corresponding to the comparative example. In 38, the carbon content is low and the tensile strength TS is inferior to that of the invention example. Steel plate No. In No. 39, the carbon content is high, the yield strength YS is high compared to the invention example, the ammonia stress corrosion cracking resistance is inferior, and the toughness at low temperature is also inferior compared to the invention example. Steel plate No. In 40, 43, 44, 45, 49, and 50, the amount of various elements added was greater than that of the invention example, and the toughness at low temperatures was inferior to that of the invention example. Steel plate No. In 41, the amount of manganese is low and the tensile strength TS is inferior to that of the invention example. Steel plate No. In No. 42, the amount of manganese is high, the yield strength YS is high compared to the invention example, the ammonia stress corrosion cracking resistance is inferior, and the toughness at low temperature is also inferior compared to the invention example. Steel plate No. 46 and 48 have low amounts of nitrogen or titanium, and their toughness at low temperatures is inferior to that of the invention example.

Claims (4)

In mass%,
C: 0.05% or more and 0.15% or less,
Si: 0.09% or more and 0.50% or less,
Mn: 0.50% or more and 2.00% or less,
Al: 0.007% or more and 0.060% or less,
N: 0.0010% or more and 0.0100% or less,
Ti: 0.005% or more and 0.100% or less,
P: 0.020% or less,
S: 0.010% or less and
O: 0.0100% or less
Contains and has a composition of the balance Fe and inevitable impurities,
The total volume ratio of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel sheet is 90% or more, and the total volume of ferrite and bainite at 1/2 of the sheet thickness of the steel sheet. It has a microstructure in which the percentage is 60% or more and 90% or less and the volume fraction of fine martensite is 10% or less,
A steel plate whose yield strength YS is 440 MPa or less, tensile strength TS of 490 MPa or more and 620 MPa or less, and ductility brittle temperature vTrs of -60°C or less. According to claim 1,
The above component composition is further expressed in mass%,
Cu: 2.00% or less,
Ni: 2.00% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 1.00% or less,
W: 1.00% or less,
Co: 1.00% or less,
Nb: 0.100% or less,
B: 0.0100% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less and
REM: 0.0200% or less
A steel plate containing one or more selected from the group consisting of: In mass%,
C: 0.05% or more and 0.15% or less,
Si: 0.09% or more and 0.50% or less,
Mn: 0.50% or more and 2.00% or less,
Al: 0.007% or more and 0.060% or less,
N: 0.0010% or more and 0.0100% or less,
Ti: 0.005% or more and 0.100% or less,
P: 0.020% or less,
S: 0.010% or less and
O: 0.0100% or less
A steel material containing and having a component composition of the balance Fe and inevitable impurities is subjected to hot rolling at an end temperature of Ar ₃ transformation point or higher, and then cooling is started from a temperature of Ar ₃ transformation point or higher, and 1 mm from the surface of the steel sheet. Cool at an average cooling rate of 10°C/s or more until the temperature at the depth becomes 600°C or lower, stop cooling once and stop the cooling between 10 and 600 seconds, and then cool the steel sheet to a thickness of 600°C or lower. A method of manufacturing a steel sheet in which cooling is performed at an average cooling rate of 5 to 50 °C/s per half, and the cooling is terminated in a temperature range where the temperature at the center of the sheet thickness is 200 °C or more and 450 °C or less. ,
The steel sheet has a total volume ratio of tempered martensite and tempered bainite at a depth of 1 mm from the surface of the steel sheet of 90% or more, and ferrite and bainite at 1/2 the thickness of the steel sheet. It has a microstructure in which the total volume fraction of knights is 60% to 90% and the volume fraction of fine martensite is 10% or less, and the yield strength YS is 440 MPa or less, the tensile strength TS is 490 MPa or more and 620 MPa or less, and - Has a ductile brittleness temperature vTrs of 60°C or less,
The Ar ₃ transformation point can be obtained by the following manufacturing method:
Ar ₃ (℃) = 910 - 273 × C - 74 × Mn - 57 × Ni - 16 × Cr - 9 × Mo - 5 × Cu
(In the formula, each element represents the content (% by mass) of the element.) According to claim 3,
The above component composition is further expressed in mass%,
Cu: 2.00% or less,
Ni: 2.00% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
V: 1.00% or less,
W: 1.00% or less,
Co: 1.00% or less,
Nb: 0.100% or less,
B: 0.0100% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less and
REM: A method of manufacturing a steel sheet containing at least one selected from 0.0200% or less.