TW202334454A - Steel sheet and method for producing same - Google Patents

Abstract

Provided is a high-strength steel sheet having excellent low-temperature toughness and resistance to ammonia SCC, the steel sheet being provided to a storage tank, or similar, that is used to accommodate a liquefied gas in an energy transport vessel. This steel sheet has a specific component composition which contains in particular one or more of Cu: 0.01-05%, Cr: 0.01-1.0%, Sb: 0.01-0.50% and Sn: 0.01-0.50%, the content of Cu, Cr, Sb and Sn satisfying a specific relationship. Said steel sheet has: a hardness property wherein the hardness at a position at a depth of 1.0 mm from the surface of the steel sheet is Hv300 or less; and a metal structure in which, at a position equal to half the sheet thickness of the steel sheet, the volume fraction of bainite structure is 20% or more, and the total volume fraction of ferrite structure and bainite structure is 60% or more.

Description

Steel plate and manufacturing method thereof

The present invention relates to a high-strength steel plate with excellent toughness and corrosion resistance. In particular, it relates to a high-strength steel plate that is suitable for structural members such as tanks used at low temperatures and in liquid ammonia environments. It has excellent low-temperature toughness and liquid ammonia stress corrosion cracking resistance. High-strength steel plate and manufacturing method thereof.

With the increase in energy demand in recent years, the use of energy carriers to transport liquefied gas is becoming popular. For the efficient use of energy transport ships, LPG may be transported together with liquid ammonia in the tank.

Here, it is known that stress corrosion cracking (hereinafter referred to as ammonia SCC (Stress Corrosion Cracking)) caused by liquid ammonia occurs in carbon steel piping, storage tanks, tankers, pipelines, etc. that handle liquid ammonia. Therefore, for steel materials used in liquid ammonia environments, steel materials with low susceptibility to ammonia SCC should be used, or engineering measures should be taken to suppress ammonia SCC.

For example, it is known that the occurrence of ammonia SCC is related to the strength of the material. When using carbon steel, the yield strength (YS) of ammonia is controlled to 440 MPa or less to avoid stress corrosion cracking caused by ammonia. On the other hand, due to the recent increase in the size of tanks and the reduction of steel usage, there are increasing requirements for high-strength steel plates.

And because liquefied gases such as LPG and liquid ammonia are transported at low temperatures, the steel plates used in tanks for storing these liquefied gases require excellent low-temperature toughness.

As mentioned above, patent documents 1 and 2 disclose technologies that can satisfy the low-temperature toughness and strength range required for a tank for liquefied gas storage. The technology described in these documents achieves high low-temperature toughness by subjecting hot-rolled and cooled thick steel plates to several heat treatments, or by subjecting hot-rolled and water-cooled thick steel plates to several heat treatments. and established strength properties. [Prior technical literature] [Patent Document]

Patent Document 1: Japanese Patent Application Publication No. 10-140235 Patent Document 2: Japanese Patent Application Publication No. 10-168516

[Problem to be solved by the invention]

However, the methods described in the above-mentioned Patent Documents 1 and 2 require a plurality of heat treatments, and there is an economic problem in that costs related to heat treatment equipment and energy increase.

In order to solve the above problems, the object of the present invention is to provide a high-strength steel plate with excellent ammonia SCC resistance and low-temperature toughness and a manufacturing method thereof. The steel plate is used in storage tanks for containing liquefied gas in energy carriers, etc. . [Technical means to solve problems]

In order to achieve the above purpose, the inventors of the present invention conducted painstaking research on various factors related to the low-temperature toughness and strength characteristics of the steel plate using the TMCP program (Thermo Mechanical Control Process). As a result, it was found that as long as more than a predetermined amount of elements such as C, Si, Mn, and N is added to the steel plate, the fat grain iron structure and the bainite structure at the position of 1/2 of the thickness of the steel plate can be reduced. Controlling the metal structure (microstructure) of the steel plate so that the total volume ratio becomes 60% or more can effectively promote the achievement of the desired low-temperature toughness and strength characteristics.

It was further discovered that by adding elements such as Cu, Cr, Sb, and Sn in predetermined amounts or more and controlling the hardness at a depth of 1.0 mm from the surface of the steel plate to Hv300 or less, resistance to liquid ammonia environments can be obtained. SCC properties enable the elimination of costly heat treatments like previous technologies.

The present invention was developed based on the above knowledge, that is, the gist of the present invention is as follows. 1. A steel plate whose composition, in mass %, contains C: 0.010~0.200%, Si: 0.01~0.50%, Mn: 0.50~2.50%, Al: 0.060% or less, N: 0.0010~0.0100%, P : 0.020% or less, S: 0.0100% or less, O: 0.0100% or less, And further contains one or more of Cu: 0.01~0.50%, Cr: 0.01~1.00%, Sb: 0.01~0.50%, and Sn: 0.01~0.50%, The CR value calculated according to the following formula (1) is 0.70 or more, and the remainder is Fe and inevitable impurities. The hardness characteristics of the steel plate are that the hardness at a depth of 1.0 mm from the surface of the steel plate is Hv300 or less, The metal structure of the steel plate is such that the volume ratio of the toughened iron structure at 1/2 of the thickness of the steel plate is more than 20% and the total volume ratio of the fat-grained iron structure and the toughened iron structure is more than 60%. CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn]…Equation (1) Among them, [X] represents the content of X element in steel (mass %).

2. Steel plate as described in 1 above, The aforementioned component composition is calculated in mass %, and further contains one or more selected from the group consisting of Ni: 0.01~2.00%, Mo: 0.01~0.50%, and W: 0.01~1.00%.

3. Steel plates as described in the above 1 or 2, The aforementioned component composition is calculated in mass %, and further contains V: 0.01~1.00%, Ti: 0.005~0.100%, Co: 0.01~1.00%, Nb: 0.005~0.100%, B: 0.0001~0.0100%, Ca: 0.0005 ~ 0.0200%, Mg: 0.0005~0.0200% and REM: 0.0005~0.0200% or more.

A method of manufacturing a steel plate, in which a steel material having the following chemical composition is hot-rolled with a rolling end temperature equal to or above the Ar ₃ transformation point, and then cooled from a cooling start temperature above the Ar ₃ transformation point to 600°C or less Cooling at the stop temperature, the aforementioned composition, in terms of mass %, contains C: 0.010~0.200%, Si: 0.01~0.50%, Mn: 0.50~2.50%, Al: 0.060% or less, N: 0.0010~0.0100%, P: 0.020% or less, S: 0.0100% or less, O: 0.0100% or less, and further contains Cu: 0.01~0.50%, Cr: 0.01~1.00%, Sb: 0.01~0.50%, and Sn: 0.01~0.50%. 1 or 2 or more types, the CR value calculated according to the following formula (1) is 0.70 or more, and the remainder is Fe and unavoidable impurities. During the aforementioned cooling, the position will be at a depth of 1.0 mm from the surface of the steel plate. The cooling rate is set to 150℃/s or less, and the cooling rate at 1/2 of the thickness of the steel plate is set to 10℃/s or more, CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+ 3.6[Sn]...Formula (1) Where, [X] represents the content of X element in steel (mass %).

5. The manufacturing method of steel plate as described in the above 4, The composition of the aforementioned steel material further contains, in mass %, at least one selected from the group consisting of Ni: 0.01~2.00%, Mo: 0.01~0.50%, and W: 0.01~1.00%.

6. The manufacturing method of steel plate as described in the above 4 or 5, The composition of the aforementioned steel material, in terms of mass %, further contains V: 0.01~1.00%, Ti: 0.005~0.100%, Co: 0.01~1.00%, Nb: 0.005~0.100%, B: 0.0001~0.0100%, At least one of Ca: 0.0005~0.0200%, Mg: 0.0005~0.0200%, and REM: 0.0005~0.0200%. [Effects of the invention]

According to the present invention, it is possible to provide a high-strength steel plate that is excellent in low-temperature toughness, that is, impact resistance at low temperatures and ammonia SCC resistance, and is suitable for structural members such as tanks used in low-temperature and liquid ammonia environments. .

Embodiments of the present invention will be described below. In addition, the following "%" indicates the content of components (elements), unless otherwise stated, it refers to "mass %".

(1)About the ingredient composition Next, the chemical composition (chemical composition) of the steel plate will be explained.

C: 0.010~0.200% In order to improve the strength of the steel plate produced by cooling according to the present invention, C is the most effective element. In order to obtain this effect, the C content is specified to be 0.010% or more. Furthermore, from the viewpoint of reducing the content of other alloy elements and manufacturing at lower cost, the C content is preferably 0.013% or more. On the other hand, if the C content exceeds 0.200%, the toughness and weldability of the steel plate will deteriorate. Therefore, the C content is specified to be 0.200% or less. Furthermore, from the viewpoint of toughness and weldability, the C content is preferably 0.170% or less.

Si: 0.01~0.50% Si is added for deoxidation. In order to obtain this effect, the Si content is set to 0.01% or more. Furthermore, it is preferably 0.03% or more. On the other hand, if the Si content exceeds 0.50%, the toughness and weldability of the steel plate will deteriorate. Therefore, the Si content is set to 0.50% or less. Furthermore, from the viewpoint of toughness and weldability, the Si content is preferably 0.40% or less.

Mn: 0.50~2.50% Mn has the function of increasing the hardenability of steel, and is one of the important elements that must be added in order to meet the high strength as in the present invention. In order to obtain this effect, the Mn content is set to 0.50% or more. Furthermore, from the viewpoint of reducing the content of other alloy elements and manufacturing at lower cost, the Mn content is preferably 0.70% or more. On the other hand, if the Mn content exceeds 2.50%, the toughness and weldability of the steel plate will be reduced, and the alloy cost will become too high. Therefore, the Mn content is specified to be 2.50% or less. Furthermore, from the viewpoint of further suppressing decreases in toughness and weldability, the Mn content is preferably 2.30% or less.

Al: 0.060% or less Al functions as a deoxidizer and refines crystal grains. In order to obtain this effect, the Al content is preferably 0.001% or more. On the other hand, if the Al content exceeds 0.060%, oxide inclusions may increase, resulting in a decrease in cleanliness and deterioration in toughness. Therefore, the Al content is specified to be 0.060% or less. Furthermore, from the viewpoint of further preventing deterioration in toughness, the Al content is preferably 0.050% or less.

N：0.0010~0.0100% N helps to refine the structure and improve the toughness of the steel plate. In order 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, if the N content exceeds 0.0100%, the toughness will decrease. Therefore, the N content is set to 0.0100% or less. Furthermore, from the viewpoint of further suppressing decreases in toughness and weldability, the N content is preferably 0.0080% or less. And when Ti is present, N can combine with Ti and precipitate in the form of TiN.

P: 0.020% or less P segregates at the grain boundaries, causing adverse effects such as reduced toughness and weldability. Therefore, the P content should be reduced as much as possible, as long as it is below 0.020%, it is acceptable. On the other hand, the lower limit of the P content is not particularly limited, although it may be 0%, because excessive reduction will lead to an increase in refining costs. From a cost perspective, the P content is preferably 0.0005% or more.

S: 0.0100% or less S exists in steel in the form of sulfide-based inclusions such as MnS, which becomes the starting point of damage and causes adverse effects such as a decrease in the toughness of the steel plate. Therefore, the S content should be reduced as much as possible, as long as it is below 0.0100%, it is acceptable. On the other hand, the lower limit of the S content is not particularly limited, although it may be 0%, because excessive reduction will lead to an increase in refining costs. From a cost perspective, the S content is preferably 0.0005% or more.

O: 0.0100% or less O forms oxides and becomes the starting point of damage, causing adverse effects such as lowering the toughness of the steel plate, so it is limited to less than 0.0100%. The O content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited, although it may be 0%, because excessive reduction will lead to an increase in refining costs. From a cost perspective, the O content is preferably 0.0010% or more.

One or more of Cu: 0.01~0.50%, Cr: 0.01~1.00%, Sb: 0.01~0.50% and Sn: 0.01~0.50%, and the CR value calculated according to formula (1) is 0.70 above In order to improve ammonia SCC resistance, Cu, Cr, Sb and Sn are particularly important elements in the present invention. Therefore, in the present invention, one or more of them must be contained in the above-mentioned content, and the CR value calculated according to the following formula (1) must be 0.70 or more. CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn]…Equation (1) Among them, [X] represents the content of X element in steel (mass %). That is to say, Cu, Cr, Sb and Sn will quickly form protective corrosion products (Corrosion products) in a liquid ammonia environment, thereby inhibiting stress corrosion cracking. In order to obtain this effect, when Cu is added, the Cu content must be limited to 0.01% or more; when Cr is added, the Cr content must be limited to 0.01% or more; when Sb is added, the Sb content must be limited to 0.01 % or more; and when adding Sn, the Sn content must be limited to 0.01% or more. The formula for calculating the CR value is used to estimate the ammonia SCC resistance based on the content of each element. The higher the CR value, the better the ammonia SCC resistance. Furthermore, by setting the CR value to 0.70 or more, stress corrosion cracking in a liquid ammonia environment can be suppressed. On the other hand, if Cu, Cr, Sb, and Sn are added excessively, weldability and toughness will deteriorate, and this is also disadvantageous from the viewpoint of alloy cost. Therefore, the Cu content is limited to 0.50% or less, the Cr content is limited to 1.00% or less, the Sb content is limited to 0.50% or less, and the Sn content is limited to 0.50% or less. More preferably, the Cu content is 0.40% or less, the Cr content is 0.80% or less, the Sb content is 0.40% or less, and the Sn content is 0.40% or less. In addition, the upper limit of the CR value is not particularly limited. If the CR value exceeds 7.00, the effect will be saturated and excessive addition of the above elements will cause the price to rise, so it is preferably about 7.00.

In the composition of the steel sheet of the present invention, the remainder other than the above-mentioned components is Fe and inevitable impurities. However, the above-mentioned component composition may contain the elements described below as necessary.

At least one selected from Ni: 0.01~2.00%, Mo: 0.01~0.50%, and W: 0.01~1.00% Ni, Mo, and W are elements that further improve ammonia SCC resistance, and one or more of them may be contained. In order to obtain this effect, when Ni is contained, the Ni content is preferably adjusted to 0.01% or more; when Mo is contained, the Mo content is preferably adjusted to 0.01% or more; and when W is contained, the Mo content is preferably adjusted to 0.01% or more. To adjust the W content to 0.01% or more. On the other hand, if Ni is added excessively, the weldability will deteriorate and the alloy cost will increase. In addition, if Mo and W are excessively added, the weldability and toughness will be deteriorated, and this is also disadvantageous from the viewpoint of alloy cost. Therefore, it is preferable to adjust the Ni content to 2.00% or less, the Mo content to 0.50% or less, and the W content to 1.00% or less. More preferably, the Ni content is adjusted to 1.50% or less, the Mo content is adjusted to 0.40% or less, and the W content is adjusted to 0.80% or less.

V: 0.01~1.00% V has the effect of improving the strength of the steel plate and can be added arbitrarily. In order to obtain this effect, when adding V, it is preferable to set the V content to 0.01% or more. On the other hand, if the V content exceeds 1.00%, the weldability will deteriorate and the alloy cost will increase. Therefore, when adding V, it is preferable to set the V content to 1.00% or less. More preferably, the lower limit of the V content is set to 0.05% and the upper limit is set to 0.50%.

Ti: 0.005~0.100% Ti has a strong tendency to form nitrides and has the function of fixing N and reducing solid solution N. It can be added arbitrarily. In addition, Ti can improve the toughness of the base material and the welded part. In order to obtain these effects, when adding Ti, it is preferable to set the Ti content to 0.005% or more. More preferably, it is 0.007% or more. On the other hand, if the Ti content exceeds 0.100%, the toughness will be reduced. Therefore, when adding Ti, it is preferable to set the Ti content to 0.100% or less. Furthermore, it is more preferable to set the Ti content to 0.090% or less.

Co: 0.01~1.00% Co has the effect of improving the strength of the steel plate and can be added arbitrarily. In order to obtain this effect, when adding Co, it is preferable to set the Co content to 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the weldability will deteriorate and the alloy cost will increase. Therefore, when adding Co, it is preferable to set the Co content to 1.00% or less. More preferably, the lower limit of the Co content is 0.05% and the upper limit is 0.50%.

Nb: 0.005~0.100% Nb can precipitate in the form of carbonitride and reduce the particle size of former austenite, which has the effect of improving toughness. In order to obtain this effect, when Nb is added, the Nb content is set to 0.005% or more. Furthermore, it is preferably 0.007% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of NbC will precipitate, resulting in reduced toughness. Therefore, when adding Nb, it is preferable to set the Nb content to 0.100% or less. Furthermore, it is more preferably 0.060% or less.

B: 0.0001~0.0100% B has the effect of significantly improving the hardenability even when added in a trace amount. That is, the strength of the steel plate can be improved. In order to obtain this effect, when adding B, it is preferable to set the B content to 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, the weldability will decrease. Therefore, when adding B, it is preferable to set the B content to 0.0100% or less. More preferably, the lower limit of the B content is set to 0.0010% and the upper limit is set to 0.0030%.

Ca: 0.0005~0.0200% Ca combines with S and has the effect of suppressing the formation of MnS and the like extending long in the rolling direction. That is, by adding Ca, the morphology of the sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of the welded portion and the like. In order to obtain this effect, when adding Ca, it is preferable to set the Ca content to 0.0005% or more. On the other hand, if the Ca content exceeds 0.0200%, the cleanliness of the steel will decrease. Reduced clarity leads to reduced toughness. Therefore, when adding Ca, it is best to set the Ca content to 0.0200% or less. More preferably, the lower limit of the Ca content is 0.0020% and the upper limit is 0.0100%.

Mg: 0.0005~0.0200% Like Ca, Mg combines with S and has the effect of suppressing the formation of MnS and the like extending long in the rolling direction. That is, by adding Mg, the morphology of the sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of welded parts and the like. In order to obtain this effect, when Mg is added, it is preferable to set the Mg content to 0.0005% or more. On the other hand, if the Mg content exceeds 0.0200%, the cleanliness of the steel will decrease. Reduced clarity leads to reduced toughness. Therefore, when Mg is added, it is preferable to set the Mg content to 0.0200% or less. More preferably, the lower limit of the Mg content is set to 0.0020% and the upper limit is set to 0.0100%.

REM: 0.0005~0.0200% Like Ca and Mg, REM (rare earth metal) combines with S and has the effect of suppressing the formation of MnS and the like extending long in the rolling direction. That is, by adding REM, the morphology of the sulfide-based inclusions can be controlled into a spherical shape, thereby improving the toughness of the welded portion and the like. In order to obtain this effect, when REM is added, the REM content is preferably 0.0005% or more. On the other hand, if the REM content exceeds 0.0200%, the cleanliness of the steel will decrease. Reduced clarity leads to reduced toughness. Therefore, when REM is added, the REM content is preferably 0.0200% or less. More preferably, the lower limit of the REM content is set to 0.0020% and the upper limit is set to 0.0100%.

(2) About hardness characteristics and metal structure In addition to having the above-mentioned chemical composition, the steel plate of the present invention also has hardness characteristics such that the hardness at a depth of 1.0 mm from the surface of the steel plate (also referred to as the 1.0 mm position in the present invention) is Hv300 or less. Furthermore, the steel plate of the present invention has the following metal structure, that is, at the 1/2 position of the plate thickness of the aforementioned steel plate (in the present invention, it refers to the position of 1/2 depth of the plate thickness. Hereinafter, it is also simply referred to as: 1/2 position or the center of the plate thickness), the volume ratio of the toughened iron structure (hereinafter also referred to as toughened iron) is more than 20%, and the total volume of the fat-grained iron structure (hereinafter also referred to as fat-grained iron) and toughened iron The rate is over 60%. The reason why the hardness characteristics and metal structure of the steel plate are limited as mentioned above will be explained below.

[The hardness at the 1.0mm position is Hv300 or less] The hardness at the 1.0mm position is set to Hv300 or less. If there is a high hardness area on the extreme surface of the steel plate, specifically at a position 1.0 mm from the surface of the steel plate, it will promote stress corrosion cracking in a liquid ammonia environment. Therefore, in the steel plate of the present invention, excellent ammonia SCC resistance can be ensured by adjusting the hardness characteristics so that the hardness at the 1.0 mm position becomes Hv300 or less. The lower limit of the hardness at the 1.0 mm position is not particularly limited, but is preferably about Hv130. Here, the above-mentioned hardness can be calculated by measuring the Vickers hardness at a position of 0.5 mm at a plurality of locations (for example, 100 points).

[At the 1/2 position, the volume ratio of toughened iron is more than 20%, and the total volume ratio of fat iron and toughened iron is more than 60%] The structure at the 1/2 position must have a volume ratio of toughened iron of more than 20%, and a total volume ratio of fat iron and toughened iron of more than 60%. When iron is excessively produced, it can lead to a reduction in strength or toughness. Furthermore, if the total volume ratio of fat iron and toughened iron is less than 60%, the volume of other structures, namely island-shaped Asada loose iron structure, Asada loose iron structure, Plein iron structure and Worthfield iron structure The fraction will increase and sufficient strength or toughness cannot be obtained to meet the mechanical properties. In addition, the total volume ratio of fat granular iron and toughened iron may be 100%.

Here, the fat iron refers to the fat iron produced by the cooling process before tempering, and the toughened iron refers to the toughened iron produced by the cooling process before tempering. The reason why the microstructure in the center of the plate thickness is specified is that the microstructure in the center of the plate thickness will affect the strength characteristics of the center of the plate thickness, and the strength characteristics of the center of the plate thickness will affect the strength of the entire steel plate. cause impact.

The remaining part of the structure accounting for less than 40% of the volume may include, in addition to the Pleinite structure and the Waston iron structure, the Asada loose iron structure. The fraction of each structure in the remaining structure is not particularly limited, and the remaining structure is preferably a Pole iron structure. In addition, the volume fraction of various microstructures can be measured according to the method described in the Examples described later.

(3)About manufacturing conditions The manufacturing method of the present invention is to heat and hot-roll a steel material having the following composition, and then perform predetermined cooling according to the present invention. The aforementioned composition contains C: 0.010~0.200%, Si: 0.01~0.50%, Mn: 0.50~2.50%, Al: 0.060% or less, N: 0.0010~0.0100%, P: 0.020% or less, S: 0.0100% or less, O: 0.0100% or less, and further contains Cu: 0.01~0.50%, Cr: 0.01 ~1.00%, Sb: 0.01~ 0.50% and Sn: 0.01~0.50%, one or more of them, and the CR value calculated by the aforementioned formula (1) is 0.70 or more, and it is included as necessary One or more selected from Ni: 0.01~2.00%, Mo: 0.01~0.50% and W: 0.01~1.00% and/or selected from V: 0.01~1.00%, Ti: 0.005~ 0.100%, Co: 0.01~ 1.00%, Nb: 0.005~0.100%, B: 0.0001~0.0100%, Ca: 0.0005~0.0200%, Mg: 0.0005~ 0.0200% and REM: 0.0005~0.0200%, one or more of them, the remainder is Fe and unavoidable of impurities. The following explains the reasons for limiting the manufacturing conditions of steel plates. First of all, the manufacturing conditions of steel are not particularly limited. For example, it is preferable to melt molten steel having the above-described composition by a known melting method such as a converter, and to form a predetermined size by a known casting method such as a continuous casting method. Steel materials such as slabs. The ingot-block rolling method can also be used to make steel products such as flat blanks of a predetermined size.

The steel material thus obtained is directly hot-rolled without being cooled, or is heated again and then hot-rolled. This hot rolling is performed with the rolling completion temperature being equal to or higher than the temperature of the Ar ₃ transformation point (hereinafter referred to as the Ar ₃ transformation point). Immediately after hot rolling, cooling is performed from a cooling start temperature above the Ar ₃ transformation point to a cooling stop temperature of 600° C. or below under predetermined conditions.

The heating temperature of steel (temperature during hot rolling) is not particularly limited. If the heating temperature is too low, the deformation resistance increases and the load on the hot rolling mill increases, which may make hot rolling difficult. On the other hand, if the temperature exceeds 1300° C., oxidation will become severe and the oxidation loss will increase, thereby possibly lowering the yield. For this reason, the heating temperature is preferably 950°C to 1300°C.

(Hot rolling) [Rolling completion temperature: Ar ₃ transformation point or above] In the present invention, after heating to the above temperature, hot rolling is started, and the hot rolling is completed at a temperature above the Ar ₃ transformation point. If the end temperature of rolling is lower than the Ar ₃ transformation point, fat iron will be generated, and the generated iron will be affected by processing, resulting in poor toughness. Furthermore, the load on the hot rolling mill increases. Therefore, the rolling end temperature of hot rolling is set to be above the Ar ₃ transformation point. Preferably, the rolling completion temperature of hot rolling is a temperature above the Ar ₃ transformation point + 10°C. On the other hand, if the rolling end temperature exceeds 950°C, the structure may become coarsened and the toughness may deteriorate. Therefore, the rolling end temperature is preferably 950°C or lower. Here, the Ar ₃ transformation point can be calculated according to the following formula. Ar ₃ (℃)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo where each element represents the content (mass %) of the element in the steel.

(Cooling) [Cooling start temperature: Ar ₃ transformation point or above] Next, the hot-rolled steel sheet is cooled from a cooling start temperature above the Ar ₃ transformation point. If the cooling start temperature is lower than the Ar ₃ transformation point, iron particles will be excessively produced, resulting in insufficient strength. Therefore, the cooling start temperature is set to be above the Ar ₃ transformation point.

[Cooling stop temperature: below 600°C] In the present invention, after hot rolling is completed, cooling is performed under predetermined conditions to an arbitrarily set cooling stop temperature of 600°C or lower, thereby allowing the fat grain iron and toughened iron in the center of the plate thickness to reach a predetermined volume ratio. . Here, if the cooling stop temperature exceeds 600°C, the ferrite structure and the pulverized iron structure will be excessively generated, which may result in insufficient strength. Therefore, the cooling stop temperature is specified to be 600°C or lower. Although the lower limit of the cooling stop temperature is not particularly limited, if the cooling stop temperature is too low, the volume fraction of the island-like Asada loose iron structure becomes too large and the toughness decreases. Therefore, the cooling stop temperature is preferably 200°C or higher. The above-mentioned cooling stop temperature is the temperature at the 1/2 position of the steel plate.

[Cooling speed at 1.0mm position: 150℃/s or less] During the above cooling, if the cooling rate at the 1.0mm position exceeds 150°C/s, the hardness at the 1.0mm position will exceed Hv300, resulting in poor ammonia SCC resistance. Therefore, the cooling rate at the 1.0mm position is specified to be 150°C/s or less. On the other hand, although the lower limit of the cooling rate is not particularly limited, if the cooling rate is too small, the ferrite structure and the pulverized iron structure will be excessively generated, which may lead to insufficient strength and poor toughness. Therefore, from the viewpoint of more reliably preventing such a situation, it is preferable to set the cooling rate to 50° C./s or more. The cooling rate can also be controlled by controlled cooling including intermittent cooling during cooling stop periods. In addition, it is difficult to directly measure the temperature at the 1.0mm position using physical methods. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, using a process computer to perform differential calculations, the temperature distribution within the plate thickness section can be obtained in real time, especially is the temperature at the 1.0mm position.

[Cooling rate at 1/2 position: 10℃/s or more] Cooling to a cooling rate of 10°C/s or more at the 1/2 position is an indispensable procedure in order to obtain a steel plate with high strength and high toughness. By cooling at a high cooling rate, transformation strengthening can be obtained. Strength increasing effect. In order to obtain this effect, the cooling rate at the 1/2 position during cooling according to the present invention is set to 10°C/s or more. If the above-mentioned cooling rate is lower than 10°C/s, fat iron and pulverized iron will be produced excessively, and sufficient strength cannot be obtained. Therefore, the cooling rate at 1/2 of the plate thickness is set to 10°C/s or more. On the other hand, although the upper limit of the cooling rate is not particularly limited, if the cooling rate is too high, the volume fraction of island-shaped Asada loose iron becomes too high, which may lead to deterioration in toughness. Therefore, it is preferable to set the cooling rate at the 1/2 position to 80°C/s or less. The cooling rate can also be controlled by controlled cooling including intermittent cooling during cooling stop periods. It is difficult to directly measure the temperature at the 1/2 position using physical methods. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, using a data processing computer to perform differential calculations, the temperature distribution within the plate thickness section can be obtained instantly, especially 1/2 The temperature at the location.

The cooling speed at the 1.0mm position and the cooling speed at the 1/2 position can be changed by compositely adjusting the cooling start temperature, water volume, etc., respectively.

By manufacturing the steel material having the above-mentioned composition according to the above-mentioned manufacturing conditions, a steel plate having the composition, hardness characteristics and metal structure according to the present invention can be obtained. The steel plate thus obtained has excellent strength characteristics and toughness. Here, the excellent strength characteristics refer to the yield strength YS (the yield point YP when there is a yield point, and the 0.2% guaranteed stress σ0.2 when there is no yield point): 360 MPa or more and the tensile strength (TS): 490 MPa or more. Excellent toughness means vTrs is -30°C or less according to JIS Z 2241.

In addition, according to the manufacturing method of the present invention, ordinary methods can be used for items not described in this specification. Example

The steel with the composition shown in Table 1 (steel types A to AH, the remainder is Fe and inevitable impurities) is made into a flat blank by the continuous casting method, and the flat blank is used to make a thick steel plate with a thickness of 30 mm (No. 1~44). Next, hot rolling and cooling were performed sequentially according to the conditions shown in Table 2 to obtain a steel plate. For the obtained steel plate, the structure fraction of the metal structure at a position 1/2 of the thickness of the plate, the hardness at a position 1.0 mm from the surface of the steel plate, the evaluation of strength characteristics and toughness, and the evaluation of ammonia SCC resistance were carried out. Evaluation. Each test method is as follows. These results are also listed in Table 2.

[Tissue fraction of metal structure at 1/2 position] Samples were collected from each steel plate so that the 1/2 position (the plate thickness center) became the observation surface. Next, the sample was mirror-polished and further etched with nital, and then a scanning electron microscope (SEM) was used to photograph a 10 mm × 10 mm range at a magnification of 500 to 3000 times. Furthermore, the captured image is analyzed using an image analysis device to determine the area fraction of the microstructure (the tissue fraction of the metal structure). When the anisotropy of the microstructure is small, the area fraction is equivalent to the volume fraction, so in the present invention, the area fraction is regarded as the volume fraction.

In this embodiment, the determination when determining the fraction of the metal structure of the sample is performed as follows. That is, in the above-mentioned photographed image, the polygonal fat grains are identified as lath grains (F in Table 2), and the lath-shaped grains that grow into elongated lath-shaped iron grains and contain The structure of carbides with an equivalent circle diameter of 0.05 μm or more is classified as toughened iron (B in Table 2).

[Hardness at 1.0mm position] For the cross section perpendicular to the rolling direction of each steel plate, the Vickers hardness (HV10) of 100 points was measured at the 1.0 mm position in accordance with JIS Z 2244, and the maximum value was determined.

[Strength characteristics] A test piece No. 1B of JISZ 2201 was taken from the entire thickness of each steel plate in a direction perpendicular to the rolling direction and the plate thickness direction, and a tensile test was performed according to the method described in JIS Z 2241, and the yield strength YS (with When there is a yield point, it is the yield point YP. When there is no yield point, it is 0.2% guaranteed stress σ0.2) and tensile strength (TS). Furthermore, a steel plate having a yield strength of 360 MPa or more and a tensile strength of 490 MPa or more was evaluated as a steel plate having excellent strength characteristics.

[Toughness] After cutting off 1 mm from the surface side of each steel plate, a V-notch test piece of JIS Z 2202 was taken in the rolling direction, a Charpy impact test was performed according to the method of JIS Z 2242, and vTrs ( brittle transition temperature). Furthermore, steel sheets having vTrs of -30°C or less were evaluated as having excellent toughness.

[Ammonia SCC resistance] Ammonia SCC resistance is evaluated by performing a four-point bending test in a test solution and by a constant-potential anodic electrolysis promotion test in order to promote corrosion. Specifically, it is implemented according to the following procedures. Take a 5mm thick × 15mm × 115mm test piece from the surface of the steel plate, perform ultrasonic degreasing in acetone for 5 minutes, and apply a stress of 100% YS of the actual yield strength to each steel plate through four-point bending. The four-point bending test piece was placed in a test cell, filled with a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, and then controlled to +2.0 Vvs with a potentiostat. .Pt flows through the test piece and is immersed at room temperature (25°C). After 168 hours of immersion, the ammonia SCC resistance was judged to be "good" when no cracks were visible, and the ammonia SCC resistance was judged to be "poor" when cracks occurred.

As can be seen from Tables 1 and 2, all of the invention examples (No. 1 to 26) have a yield strength YS of 360 MPa or more and a tensile strength TS of 490 MPa or more. The vTrs is -30°C or less, and they can obtain excellent toughness at low temperatures and ammonia resistance. Steel plate with excellent SCC resistance.

On the other hand, although the composition of Nos. 27 to 31 is within the scope of the present invention, the manufacturing method is not within the scope of the present invention, and the desired metal structure or hardness characteristics cannot be obtained. As a result, the yield strength YS, the tensile strength TS, the toughness at low temperatures, or the ammonia SCC resistance deteriorate.

Also in Nos. 32 to 44, because the steel composition is outside the scope of the present invention, the yield strength YS, tensile strength TS, toughness at low temperatures, or ammonia SCC resistance deteriorate. In the present invention, the chemical composition of steel can be regarded as the chemical composition of the steel plate.

Claims (6)

A steel plate whose composition is measured in mass %, System contains C: 0.010~0.200%, Si: 0.01~0.50%, Mn: 0.50~2.50%, Al: 0.060% or less, N: 0.0010~0.0100%, P: 0.020% or less, S: 0.0100% or less and O: 0.0100% or less, And further contains one or more of Cu: 0.01~0.50%, Cr: 0.01~1.00%, Sb: 0.01~0.50%, and Sn: 0.01~0.50%, The CR value calculated according to the following formula (1) is 0.70 or more, and the remainder is Fe and inevitable impurities. The hardness characteristics of the steel plate are that the hardness at a depth of 1.0 mm from the surface of the steel plate is Hv300 or less, The metal structure of the steel plate is such that the volume ratio of the toughened iron structure at 1/2 of the thickness of the steel plate is more than 20% and the total volume ratio of the fat-grained iron structure and the toughened iron structure is more than 60%. CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn]…Equation (1) Among them, [X] represents the content of X element in steel (mass %). The steel plate as described in claim 1, wherein, The aforementioned ingredients are calculated in mass %, It further contains at least one selected from Ni: 0.01 to 2.00%, Mo: 0.01 to 0.50%, and W: 0.01 to 1.00%. The steel plate as described in claim 1 or 2, wherein, The aforementioned ingredients are calculated in mass %, Further containing V: 0.01~1.00%, Ti: 0.005~0.100%, Co: 0.01~1.00%, Nb: 0.005~0.100%, B: 0.0001~ 0.0100%, Ca: 0.0005~0.0200%, Mg: 0.0005~ 0.0200% and REM: one or more of 0.0005~0.0200%. A method of manufacturing a steel plate, which involves hot rolling a steel material having the following chemical composition with the rolling end temperature being equal to or higher than the Ar ₃ transformation point, and then performing cooling from the starting temperature of the Ar ₃ transformation point or higher to 600°C or lower. Cooling at the cooling stop temperature, the aforementioned components, in mass %, contain C: 0.010~0.200%, Si: 0.01~0.50%, Mn: 0.50~2.50%, Al: 0.060% or less, N: 0.0010~0.0100% , P: 0.020% or less, S: 0.0100% or less, O: 0.0100% or less, and further contains Cu: 0.01~0.50%, Cr: 0.01~1.00%, Sb: 0.01~0.50%, and Sn: 0.01~0.50% 1 or 2 or more of them, the CR value calculated according to the following formula (1) is 0.70 or more, and the remainder is Fe and unavoidable impurities. During the aforementioned cooling, the CR value will be 1.0 mm from the surface of the steel plate. The cooling rate at 1/2 of the thickness of the steel plate is set to 150°C/s or less, and the cooling rate at 1/2 of the thickness of the steel plate is set to 10°C/s or more, CR=2.3[Cu]+2.8[Cr]+7.3[Sb] +3.6[Sn]…Formula (1) Where, [X] represents the content (mass %) of the X element in the steel. The manufacturing method of steel plate as described in claim 4, wherein, The composition of the aforementioned steel materials, in mass %, It further contains at least one selected from Ni: 0.01 to 2.00%, Mo: 0.01 to 0.50%, and W: 0.01 to 1.00%. The manufacturing method of steel plate as described in claim 4 or 5, wherein, The composition of the aforementioned steel materials, in mass %, Further containing V: 0.01~1.00%, Ti: 0.005~0.100%, Co: 0.01~1.00%, Nb: 0.005~0.100%, B: 0.0001~ 0.0100%, Ca: 0.0005~0.0200%, Mg: 0.0005~ 0.0200% and REM: one or more of 0.0005~0.0200%.