TW202334455A - Steel sheet and method for producing same - Google Patents

Abstract

The present invention provides a high-strength steel sheet which has exceptional ammonia SCC resistance and low-temperature toughness, and is used for a storage tank in which a liquefied gas is stored in an energy transport ship, or the like. This steel sheet has a specific component composition and hardness characteristics such that the average hardness is Hv 210 or less at a depth of 0.5 mm from the steel sheet surface and the variation of the average hardness is Hv 50 or less, while having a metal structure which has a volume ratio of bainite structure of 90% or more at a depth of 0.5 mm from the steel sheet surface, while having a volume ratio of bainite structure of 20% or more and a total volume ratio of ferrite structure and bainite structure of 60% or more at 1/2 the sheet thickness of the steel sheet.

Description

Steel plate and its manufacturing method

The present invention relates to a high-strength steel plate that is excellent in toughness and corrosion resistance, and is particularly suitable for structural members such as tanks used at low temperatures and in a liquid ammonia environment, and has excellent low-temperature toughness and resistance to liquid ammonia stress corrosion cracking. Strength steel plate and method of making same.

In recent years, with the increase in energy demand, the transportation of liquefied gas by energy carriers has become popular. In order to efficiently use energy carriers, not only LPG but also liquid ammonia is sometimes transported in the tank.

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

For example, it is known that the generation of ammonia SCC is related to the strength of the material. When using carbon steel, the stress corrosion caused by ammonia is avoided by controlling the yield strength (YS: Yield Strength) below 440MPa. Cracked. On the other hand, in view of the recent increase in the size of tanks and the reduction in the amount of steel used, the demand for high-strength steel plates has gradually increased.

In addition, since liquefied gases such as LPG and liquid ammonia are transported at low temperatures, steel plates used in storage tanks for these liquefied gases are required to have excellent low-temperature toughness.

Patent Documents 1 and 2 disclose technologies that satisfy the low-temperature toughness and strength range required for the above-described liquefied gas storage tank. In the technology described in these documents, a high degree of heat treatment is achieved by subjecting a thick steel plate that has been hot-rolled and then cooled to heat treatment several times or a thick steel plate that has been hot-rolled and then water-cooled. Low temperature toughness and predetermined strength properties. Prior technical literature patent documents

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, so there is an economic problem in that the cost of equipment and energy consumption increases.

An object of the present invention is to solve the above problems and provide a high-strength steel plate excellent in ammonia SCC resistance and low-temperature toughness for use in storage tanks for storing liquefied gas in energy carriers, and a method for manufacturing the same. [Technical means used to solve problems]

In order to achieve the above object, the present inventors used the TMCP process and carefully studied various factors related to the low-temperature toughness and strength characteristics of the steel plate. As a result, it was found that if elements such as C, Si, Mn, and N are added to a steel plate in predetermined amounts or more, and the total volume ratio of the fat-grained iron structure and the toughened iron structure is at 1/2 of the thickness of the steel plate, it is Controlling the metal structure (microstructure) of the steel plate by more than 60% can effectively help achieve the desired low-temperature toughness and strength characteristics.

Furthermore, it was found that the microstructure can be controlled so that the volume ratio of the toughened iron structure becomes 90% or more at a depth of 0.5 mm from the surface of the steel plate, and that the microstructure can be controlled at a depth of 0.5 mm from the surface of the steel plate. By setting the average hardness to Hv210 or less at the depth and controlling the variation in the average hardness to Hv50 or less, SCC resistance in a liquid ammonia environment can be obtained, and the costly heat treatment of the conventional technology can be omitted.

That is, the present invention was developed based on the above-mentioned findings, and the gist of the present invention is explained below. 1. A steel plate, which is a steel plate with the following composition, which contains in mass %: C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less, and O: 0.0100% or less, And the remaining part is Fe and inevitable impurities, It has hardness characteristics such that at a depth of 0.5mm from the surface of the steel plate, the average hardness is Hv210 or less, and the variation in the average hardness is Hv50 or less; and The volume ratio of the Bainite structure at a depth of 0.5 mm from the surface of the steel plate is more than 90%, and the volume ratio of the Bainite structure at 1/2 of the thickness of the steel plate is It is a metal structure that contains more than 20%, and the total volume ratio of ferrite structure and toughened iron structure is more than 60%.

2. The steel plate as described in 1 above, wherein the aforementioned composition further contains: in mass %: Selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and W: 1 or more types from 0.01 to 1.00%.

3. The steel plate as described in the above 1 or 2, wherein the aforementioned composition further contains: in mass %: Selected from V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, and REM: 1 or more types from 0.0005 to 0.0200%.

4. A method of manufacturing a steel plate, which involves hot-rolling a steel raw material having the following composition with a rolling end temperature equal to or above the Ar ₃ transformation point, and then cooling from a cooling start temperature above the Ar ₃ transformation point A method of manufacturing a steel plate that is cooled once, then heated by reheating the surface and then cooled twice, wherein the composition in mass % contains: C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn : 0.50 to 2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less, and O: 0.0100% or less, and the remainder is Fe and inevitable impurities, at In the aforementioned primary cooling, at a position 0.5 mm deep from the surface of the steel plate, the cooling rate from 600°C to 400°C is set to 30 to 100°C/s, and the aforementioned surface heating by reheating, This is carried out until the reaching temperature at a depth of 0.5mm from the surface of the steel plate becomes 500°C or more. In the aforementioned secondary cooling, the cooling at 600°C or less is stopped at a position that is 1/2 of the thickness of the steel plate. The cooling rate up to the temperature is set to 10°C/s or more.

5. The manufacturing method of the steel plate as described in the aforementioned 4, wherein the composition of the aforementioned steel raw material further contains: in mass %: Selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and W: 1 or more types from 0.01 to 1.00%.

6. The manufacturing method of the steel plate as described in the aforementioned 4 or 5, wherein the composition of the aforementioned steel raw material further contains: in mass %: Selected from V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, and REM: 1 or more types from 0.0005 to 0.0200%. [Effects of the invention]

According to the present invention, low-temperature toughness can be provided with a low-cost process, that is, impact resistance at low temperatures and ammonia SCC resistance are excellent, and it is suitable for structural members such as tanks used at low temperatures and in liquid ammonia environments. It is a steel plate with high strength.

The following describes embodiments of the present invention. "%" indicating the content of the following components (elements) means "mass %" unless otherwise stated.

(1)About the ingredient composition The chemical composition (chemical composition) of the steel plate is explained below.

C: 0.010 to 0.200% C is the most effective element for increasing the strength of the steel plate produced by cooling according to the present invention. In order to obtain this effect, the C content is set to 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 set to 0.013% or more. On the other hand, when the C content exceeds 0.200%, the toughness and weldability of the steel plate will be deteriorated. 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 to 0.50% Si is added for deacidification. In order to obtain this effect, the Si content is set to 0.01% or more. A better system is set to 0.03% or more. On the other hand, when the Si content exceeds 0.50%, the toughness or weldability of the steel plate will be deteriorated. 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 to 2.50% 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 in order to satisfy 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 set to 0.70% or more. On the other hand, when the Mn content exceeds 2.50%, the toughness or weldability of the steel plate is reduced, and the alloy cost becomes 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 set to 2.30% or less.

Al: 0.060% or less Al is an element that functions as a deacidifying agent and has the function of miniaturizing crystal grains. In order to obtain this effect, it is preferable to set the Al content to 0.001% or more. On the other hand, when the Al content exceeds 0.060%, the oxide-based intermediaries increase, resulting in a decrease in cleanliness and a decrease in toughness. Therefore, the Al content is specified to be 0.060% or less. Furthermore, from the viewpoint of further preventing toughness deterioration, it is preferable to set it to 0.050% or less.

N: 0.0010 to 0.0100% N series is beneficial to the refinement of the structure and improves 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, when the N content exceeds 0.0100%, it will lead to a decrease in toughness. Therefore, the N content is specified to be 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. When Ti is present, N may bond with the Ti and precipitate as TiN.

P: 0.020% or less P segregates at grain boundaries, causing adverse effects such as a decrease in toughness or weldability. Therefore, the P content is preferably as low as possible, and it is acceptable if it is 0.020% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%. However, reduction in excess will lead to an increase in refining costs. Therefore, from a cost perspective, it is preferable to set the P content to 0.0005% or more.

S: 0.0100% or less S is an element that exists in steel in the form of sulfide-based intermediaries such as MnS. It becomes the starting point of damage and brings adverse effects such as a decrease in the toughness of the steel plate. Therefore, the S content is preferably as low as possible, and it is acceptable if it is 0.0100% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, reduction in excess will lead to an increase in refining costs. Therefore, from a cost perspective, it is preferable to set the S content to 0.0005% or more.

O: 0.0100% or less Since O is an element that forms oxides and becomes a starting point for damage, causing adverse effects such as a decrease in the toughness of the steel plate, the content 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 and may be 0%. However, reducing the excess will lead to an increase in refining costs. Therefore, from a cost perspective, it is preferable to set the O content to 0.0010% or more.

In the composition of the steel plate of the present invention, the remainder other than the above-mentioned components is Fe and inevitable impurities. However, the above ingredients may also contain the elements listed below if necessary.

Selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and W: 0.01 to 1.00 1 or more of % Cu, Ni, Cr, Sn, Sb, Mo and W are elements that improve strength or ammonia SCC resistance, and one or more of these may be contained. In order to obtain this effect, it is preferable to adjust the Cu content to 0.01% or more when Cu is contained, to adjust the Ni content to 0.01% or more when Ni is contained, and to adjust the Cr content to 0.01 when Cr is contained. % or more, when it contains Sn, adjust the Sn content to 0.01% or more, when it contains Sb, adjust the Sb content to 0.01% or more, when it contains Mo, adjust the Mo content to 0.01% or more, and when it contains In the case of W, adjust the W content to 0.01% or more. On the other hand, excessive Ni content may cause deterioration in weldability or increase in alloy cost. In addition, when Cu, Cr, Sn, Sb, Mo, and W are contained excessively, weldability or toughness deteriorates, which is disadvantageous from the viewpoint of alloy cost. Therefore, it is preferable to adjust the Cu content to 0.50% or less, the Ni content to 2.00% or less, the Cr content to 1.00% or less, the Sn content to 0.50% or less, and the Sb content to 0.50% or less. The Mo content is adjusted to 0.50% or less, and the W content is adjusted to 1.00% or less. It is particularly preferable to adjust the Cu content to less than 0.40%, the Ni content to less than 1.50%, the Cr content to less than 0.80%, the Sn content to less than 0.40%, the Sb content to less than 0.40%, and the Mo content to less than 0.40%. The content is adjusted to 0.40% or less, and the W content is adjusted to 0.80% or less.

V: 0.01 to 1.00% V is an element that 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, when the V content exceeds 1.00%, the weldability may deteriorate or the alloy cost may increase. Therefore, when adding V, it is preferable to set the V content to 1.00% or less. The lower limit of the V content of the especially good series is 0.05%, and the upper limit is 0.50%.

Ti: 0.005 to 0.100% Ti is an element that has a strong tendency to form nitrides and has the function of fixing N to reduce solid solution N, and can be added arbitrarily. In addition, Ti can improve the toughness of the base metal and welded parts. In order to obtain these effects, when adding Ti, it is preferable to set the Ti content to 0.005% or more. Furthermore, it is best to set it to above 0.007%. On the other hand, when the Ti content exceeds 0.100%, the toughness will decrease. Therefore, when adding Ti, it is preferable to set the Ti content to 0.100% or less. Furthermore, the Ti content is preferably set to 0.090% or less.

Co: 0.01 to 1.00% Co is an element that 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, when the Co content exceeds 1.00%, weldability may deteriorate or the alloy cost may increase. Therefore, when adding Co, it is preferable to set the Co content to 1.00% or less. The lower limit of the Co content of the especially good series is 0.05%, and the upper limit is 0.50%.

Nb: 0.005 to 0.100% Nb is an element that precipitates as carbonitride and reduces the particle size of old Worthfield iron, thereby improving toughness. In order to obtain this effect, when adding Nb, it is preferable to set the Nb content to 0.005% or more. Furthermore, it is best to set it to above 0.007%. 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, it is preferable to set the Nb content to 0.100% or less. Furthermore, the Nb content is preferably set to 0.060% or less.

B: 0.0001 to 0.0100% B is an element that can significantly improve the hardenability even when added in a trace amount. In other words, 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, when 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. The lower limit of the Eucalyptus B content is 0.0010% and the upper limit is 0.0030%.

Ca: 0.0005 to 0.0200% Ca is an element that bonds to S and has the effect of suppressing the formation of MnS and the like that extend excessively in the rolling direction. That is, by adding Ca to control the morphology of the sulfide-based intermediary into a spherical shape, the toughness of the welded portion and the like can be improved. 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, when the Ca content exceeds 0.0200%, the cleanliness of steel will decrease. Reduced clarity leads to reduced toughness. Therefore, when adding Ca, it is preferable to set the Ca content to 0.0200% or less. The lower limit of the Ca content of the especially good series is 0.0020%, and the upper limit is 0.0100%.

Mg: 0.0005 to 0.0200% Like Ca, Mg is an element that bonds to S and has the effect of suppressing the formation of MnS and the like that extend excessively in the rolling direction. That is, by adding Mg to control the morphology of the sulfide-based intermediary into a spherical shape, the toughness of the welded portion and the like can be improved. 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, when 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. The lower limit of the Mg content of the especially good series is 0.0020%, and the upper limit is 0.0100%.

REM: 0.0005 to 0.0200% REM (rare earth metal), like Ca or Mg, is an element that bonds to S and has the effect of suppressing the formation of MnS or the like that extends excessively in the rolling direction. That is, by adding REM to control the morphology of the sulfide-based intermediary into a spherical shape, the toughness of the welded portion and the like can be improved. In order to obtain this effect, when adding REM, the REM content is preferably 0.0005% or more. On the other hand, when 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. The lower limit of the REM content of the excellent series is 0.0020%, and the upper limit is 0.0100%.

(2) About hardness characteristics and metal structure In addition to the above-mentioned composition, the steel plate of the present invention also has an average hardness of Hv210 or less at a depth of 0.5 mm from the surface of the steel plate (also referred to as a 0.5 mm position in the present invention), and the average hardness is Hv210 or less. The change is the hardness characteristic below Hv50. Furthermore, the steel plate of the present invention has: the volume ratio of the toughened iron structure (hereinafter also referred to as toughened iron) at the position of 0.5 mm is more than 90%, and the volume ratio at the position of 1/2 of the thickness of the steel plate (in the position of 1/2 of the thickness of the steel plate) In the present invention, it means that at the position 1/2 of the depth of the plate thickness (hereinafter also referred to as the 1/2 position or the center of the plate thickness), the volume fraction of toughened iron is more than 20%, and the fat-grained iron structure (hereinafter also referred to as the 1/2 position or the center of the plate thickness) is A metal structure in which the total volume ratio of fat iron) and toughened iron is more than 60%. The following explains the reasons why the hardness characteristics and metal structure of the steel plate are limited as described above.

[At the 0.5mm position, the average hardness is Hv210 or less and the variation is Hv50 or less] The average hardness at the 0.5 mm position is set to Hv210 or less, and the variation is set to Hv50 or less. When there is a high hardness area on the extreme surface of the steel plate, specifically 0.5mm from the surface of the steel plate, it will promote stress corrosion cracking in a liquid ammonia environment. In addition, when there are local high hardness areas, stress concentration will occur when stress is imparted to the steel plate, thereby promoting stress corrosion cracking. Therefore, in the steel plate of the present invention, excellent ammonia SCC resistance can be ensured by adjusting the hardness characteristics such that the average hardness at the 0.5 mm position is Hv210 or less and the variation is Hv50 or less. The lower limit of the average hardness at the 0.5mm position is not particularly limited, but is preferably about Hv130. In addition, the lower limit of the change in average hardness can be Hv0, but industrially it is about Hv10. The above-mentioned average hardness here can be calculated by measuring the Vickers Hardness at 0.5 mm positions at a plurality of places (for example, 100 places). In addition, the variation in average hardness means the standard deviation of the measured Vickers hardness used to determine the average hardness.

[The volume ratio of toughened iron at the 0.5mm position is more than 90%] In order to satisfy the strength characteristics or ammonia SCC resistance, the structure at the 0.5 mm position must have a volume fraction of 90% or more of toughened iron. When a hard phase such as Martensite structure or MA: Martensite-Austenite structure is formed in the surface layer, the hardness of the surface layer increases, and the variation in hardness within the steel plate increases, hindering material uniformity. . That is, when the volume ratio of toughened iron is less than 90%, other structures, namely, fat grain iron, island-shaped Asada loose iron structure, Asada loose iron structure, perlite (Perlite) structure, and Vostian The volume fraction of the iron (Austenite) structure increases, and sufficient strength and/or ammonia SCC resistance cannot be obtained. Here, the toughened iron system is composed of a structure called Bainitic Ferrite or Granular Ferrite that undergoes transformation during cooling or after cooling, which is beneficial to transformation strengthening. Or the tissue after tempering. In the remaining part of the structure occupying less than 10% in terms of volume ratio, in addition to the fat grain iron, perlite structure and Worthfield iron structure, it may also contain Hemp field loose iron structure. The fraction of each tissue in the remaining tissue does not need to be particularly limited, but the remaining tissue is preferably a perlite tissue.

[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 particles are produced excessively, strength or toughness is reduced. In addition, when the total volume ratio of fat iron and toughened iron does not reach 60%, the other structures, namely, island-shaped Asada loose iron structure, Asada loose iron structure, perlite structure and Woshiten iron structure As the volume fraction increases, sufficient strength or toughness cannot be obtained and mechanical properties cannot be satisfied. The total volume ratio of the above-mentioned fertilized iron and toughened iron may be 100%.

Here, the fat iron refers to the fat iron generated during the cooling process before tempering, and the toughened iron refers to the toughened iron generated during the cooling process before tempering. In addition, the microstructure in the plate thickness center portion is specified because the microstructure in the plate thickness center portion will affect the strength characteristics of the plate thickness center portion. In addition, the strength characteristics of the plate thickness center portion will affect the strength characteristics of the plate thickness center portion. It affects the overall strength of the steel plate.

In the remaining part of the structure occupying less than 40% in terms of volume ratio, in addition to the perlite structure and the Waston iron structure, the Hemp field iron structure may also be included. The fraction of each tissue in the remaining tissue does not need to be particularly limited, but the remaining tissue is preferably a perlite tissue. The volume fraction of various microstructures can be measured by the method described in the Examples described below.

(3)About manufacturing conditions The manufacturing method of the present invention is to heat and hot-roll steel raw materials with the following composition, and then perform predetermined cooling according to the present invention. The composition contains: C: 0.010 to 0.200%, Si: 0.01 to 0.50 %, Mn: 0.50 to 2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less, and O: 0.0100% or less, and if necessary, further contains: selected from Cu : 0.01 to 0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and W: 0.01 to 1.00% 1 or more types, and/or selected from V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200 %, Mg: 0.0005 to 0.0200%, and REM: at least one of 0.0005 to 0.0200%, and the remainder is Fe and unavoidable impurities. The following explains the reasons for limiting the manufacturing conditions of steel plates. First of all, the manufacturing conditions of the raw steel material do not need to be particularly limited. For example, it is preferable to melt the molten steel having the above-mentioned composition by a commonly known melting method such as a converter, and a commonly known method such as a continuous casting method. The casting method is used to form steel raw materials such as steel blanks into predetermined sizes. There is no problem if steel raw materials such as steel blanks are formed into predetermined sizes by the block-block rolling method.

The steel material thus obtained is directly hot-rolled without cooling, or is heated again and then hot-rolled. This hot rolling is performed so that the rolling completion temperature is equal to or higher than the Ar ₃ transformation point (hereinafter simply referred to as the Ar ₃ transformation point). After hot rolling, primary cooling is performed starting from a cooling start temperature above the Ar ₃ transformation point, and then the surface is heated by reheating under predetermined conditions, and then secondary cooling is performed under predetermined conditions.

The heating temperature of the steel raw material (the temperature at which it is supplied to hot rolling) is not particularly limited. If the heating temperature is too low, the deformation resistance will increase, which will increase the load on the hot rolling mill, making it difficult to perform hot rolling. On the other hand, when the temperature reaches a temperature exceeding 1300° C., oxidation becomes significant and the oxidation loss increases, thereby raising the possibility of lowering the yield. For this reason, the heating temperature is preferably not less than 950°C and not more than 1,300°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 or above the Ar ₃ transformation point. When the rolling end temperature does not reach the Ar ₃ transformation point, fat particles are formed and hinder the material uniformity on the surface of the steel plate, increasing the variation in hardness, so the ammonia SCC resistance deteriorates. In addition, since the produced iron particles are affected by processing, the toughness deteriorates. Furthermore, the load on the hot rolling mill increases. Therefore, the rolling completion temperature during hot rolling is set to be equal to or higher than the Ar ₃ transformation point. The rolling end temperature during hot rolling is particularly preferably 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, so the rolling end temperature is preferably 950°C or lower. Here, the Ar ₃ transformation point can be obtained by the following equation. Each element system indicates the content (mass %) of the element in the steel.

(Primary cooling) [Cooling start temperature: Ar ₃ transformation point or above] Next, the hot-rolled steel sheet is subjected to primary cooling starting from a cooling start temperature above the Ar ₃ transformation point. When the cooling start temperature in primary cooling does not reach the Ar ₃ transformation point, iron particles are excessively generated, resulting in insufficient strength and deterioration in ammonia SCC resistance. Therefore, the cooling start temperature is set to be equal to or higher than the Ar ₃ transformation point.

[Cooling rate in 600℃ to 400℃ at 0.5mm position: 30 to 100℃/s] In primary cooling, when the cooling rate in the range of 600°C to 400°C at the 0.5mm position (sometimes called the primary cooling rate) exceeds 100°C/s, the average hardness at the 0.5mm position exceeds Hv210, resulting in Ammonia SCC resistance deteriorates. On the other hand, when the temperature is less than 30°C/s, fat iron or perlite is formed, which may impair the material uniformity and possibly lead to the deterioration of ammonia SCC resistance. In addition, when the temperature is less than 30°C/s, iron particles or perlite are excessively generated, which may lead to insufficient strength. Therefore, the above-mentioned primary cooling rate is specified to be 30 to 100°C/s. The above-mentioned primary cooling rate can be controlled by controlled cooling including intermittent cooling during the cooling stop period. In addition, it is difficult to physically and directly measure the temperature at a depth of 0.5mm from the surface of the steel plate. However, by using a processing computer to perform differential calculations 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, the temperature within the plate thickness section can be obtained in real time. Distribution, especially the temperature at the 0.5mm position.

(Heating of surface by reheating) [Achieved temperature at 0.5mm position: 500℃ or more] After the above-mentioned primary cooling, the cooling is temporarily stopped and the surface of the steel plate is heated by reheating. In addition, the heating of the surface by reheating is performed until the temperature reached at a depth of 0.5 mm from the surface of the steel plate becomes 500° C. or higher. The structure of Asada loose iron or toughened iron generated on the surface layer is tempered by reheating from the center side of the plate thickness when cooling is stopped. When the reaching temperature (reheating temperature) at the 0.5mm position is less than 500°C, the tempering effect is insufficient, so the hardness of the surface layer increases, material uniformity cannot be obtained, and ammonia SCC resistance deteriorates. On the other hand, the upper limit of the temperature reached at the 0.5 mm position is not particularly limited, but may be, for example, 700° C. or less.

(secondary cooling) [Cooling stop temperature at 1/2 position: 600°C or less] After the steel plate surface is heated by the above reheating, cooling is restarted, that is, secondary cooling is performed. This secondary cooling is performed until the temperature at the 1/2 position becomes 600°C or lower. In the present invention, after the hot rolling is completed, secondary cooling is performed under predetermined conditions until an arbitrarily set cooling stop temperature of 600°C or lower, whereby the fat particles and toughened iron can be made in the center of the plate thickness. The iron structure becomes a predetermined volume ratio. Here, when the cooling stop temperature exceeds 600° C., the ferrite structure or the perlite structure is excessively generated, which may lead to insufficient strength. Therefore, the cooling stop temperature is specified to be 600°C or lower. The lower limit of the cooling stop temperature is not particularly limited. If the cooling stop temperature is too low, the volume fraction of island-shaped Asada loose iron becomes too high, resulting in reduced toughness. Therefore, the cooling stop temperature is preferably set to 200°C or higher.

[Cooling rate to the cooling stop temperature below 600°C at the 1/2 position: 10°C/s or more] In addition, the cooling rate during secondary cooling is the cooling rate (sometimes called is the secondary cooling rate) and is set to 10°C/s or more. When the above-mentioned secondary cooling rate is less than 10°C/s, iron particles or perlite are excessively produced, which may result in insufficient strength. On the other hand, the upper limit of the secondary cooling rate is not particularly limited, but may be, for example, 65° C./s or less. Here, the cooling start temperature during secondary cooling (cooling start temperature at the 1/2 position) can generally be the temperature at the 1/2 position immediately after the surface is heated by reheating. The above-mentioned secondary cooling speed can be controlled by controlled cooling including intermittent cooling during the cooling stop period. In addition, the temperature at the 1/2 position is difficult to measure directly physically. However, by using a processing computer to perform differential calculations 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, the temperature within the plate thickness section can be obtained in real time. distribution, especially the temperature at the 1/2 position.

By following the above-mentioned manufacturing conditions to produce a steel material having the above-mentioned composition, a steel plate having the composition, hardness characteristics and metal structure according to the present invention can be obtained. And the obtained steel plate has excellent strength properties and toughness. The excellent strength characteristics here are yield strength YS (yield point YP when there is a yield point, and 0.2% endurance σ0.2 when there is no yield point): 360 MPa or more and tensile strength (TS): 490 MPa or more. In addition, the so-called excellent toughness means that vTrs according to JIS Z 2241 is -30°C or less.

In the manufacturing method according to the present invention, common methods can be used for items not described in this specification. [Example]

Steel with the composition shown in Table 1 (steel types A to AH, the remainder being Fe and unavoidable impurities) is formed into a steel blank by a continuous casting method, and this steel blank is used to form a thick steel plate with a thickness of 25 mm. (No.1 to 50). Then, hot rolling, primary cooling, surface heating by reheating, and secondary cooling were performed sequentially under 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 0.5 mm from the surface of the steel plate and a position 1/2 of the thickness of the plate, and the measurement of the structural fraction of the metal structure at a position 0.5 mm from the surface of the steel plate were carried out. Evaluation of hardness properties, evaluation of strength properties and toughness, and evaluation of ammonia SCC resistance. Each test method is described below. In addition, these results are also listed in Table 2.

[Tissue fraction of the metal structure at the 0.5mm position and the 1/2 position] Samples were collected so that a position 0.5 mm or a 1/2 position (the center of the plate thickness) of each steel plate became the observation surface. The sample was then mirror-polished and etched with nital, and then a scanning electron microscope (SEM: Scanning Electron Microscope) was used to photograph a 10 mm × 10 mm range at a magnification of 500 to 3000 times. Then, an image analysis device is used to analyze the captured image, thereby obtaining 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 obtaining the fraction of the metal structure of the sample is performed in the following manner. That is, in the image captured above, polygonal iron particles are identified as iron particles (F in Table 2). In addition, iron particles that grow into elongated strips and contain circular equivalents are The structure of carbides with a diameter of 0.05 μm or more is classified as toughened iron (B in Table 2).

[Hardness characteristics] For the cross section perpendicular to the rolling direction of each steel plate, the Vickers hardness (HV0.1) of 100 points was measured at the 0.5mm position in accordance with JIS Z 2244, and the average value was calculated. In addition, the standard deviation of the 100-point Vickers hardness is calculated and set as the change in the average hardness at the 0.5 mm position. Here, HV0.1 is used instead of HV10, which is usually used for hardness measurement of steel plates. This is because measuring with HV0.1 makes the indentation smaller, so the hardness information at a position closer to the surface can be obtained. Or it is because of the hardness information that is more sensitive to microstructure.

[Strength characteristics] From the entire thickness of each steel plate, collect the JIS Z 2201 No. 1B test piece in a direction that is perpendicular to the rolling direction and the plate thickness direction, and perform a tensile test according to the methods of JIS Z 2241, and then The yield strength YS (yield point YP when there is a yield point, and 0.2% endurance σ0.2 when there is no yield point) and tensile strength (TS) are measured. Then, those with a yield strength of 360 MPa or more and a tensile strength of 490 MPa or more were evaluated as steel plates with excellent strength characteristics.

[Toughness] A V-notch test piece in accordance with JIS Z 2202 was collected in the rolling direction from the area where 1 mm was removed from the surface side of each steel plate, and a Charpy impact test was performed according to the method of JIS Z 2242, and vTrs was measured. (Fracture Transition Temperature). A steel plate having vTrs of -30°C or less was evaluated as a steel plate having excellent toughness.

[Ammonia SCC resistance] Ammonia SCC resistance is evaluated by performing a 4-point bending test in a test solution and by performing a promotion test after performing constant-potential anodic electrolysis to promote corrosion. Specifically, it is implemented through the following steps: A 5 mm thick × 15 mm × 115 mm test piece was collected from the surface of the steel plate, ultrasonic degreased in acetone for 5 minutes, and then subjected to a stress load of 100% YS of the actual yield strength of each steel plate through 4-point bending. The 4-point bent test piece was placed in the test unit, and after injecting a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, a potentiostat controller was used to circulate +2.0 VvsPt in the test piece. to control and immerse at room temperature (25°C). After 168 hours of immersion, those with no cracks observed were judged to have "good" ammonia SCC resistance, and those with cracks observed were judged to have "poor" ammonia SCC resistance.

From Table 1 and Table 2, it can be seen that the invention examples (No. 1 to 31) all have a yield strength YS of 360 MPa or more and a tensile strength TS of 490 MPa or more, and vTrs is -30°C or less, thus obtaining low temperature A steel plate with excellent toughness and excellent ammonia SCC resistance.

On the other hand, although the component compositions of Nos. 32 to 39 were within the scope of the present invention, the manufacturing method was outside the scope of the present invention, so the desired metal structure and/or hardness characteristics were not obtained. As a result, any one of the yield strength YS, the tensile strength TS, the toughness at low temperatures, and the ammonia SCC resistance deteriorates.

In addition, since the chemical composition of steel Nos. 40 to 50 is outside the scope of the present invention, any one of the yield strength YS, the tensile strength TS, the toughness at low temperatures, and the ammonia SCC resistance deteriorates. In the present invention, the composition of the steel can be directly considered as the composition of the steel plate.

Claims (6)

A steel plate is a steel plate with the following composition, which is expressed in mass %: C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less, and O: 0.0100% or less, And the remaining part is Fe and inevitable impurities, It has hardness characteristics such that at a depth of 0.5mm from the surface of the steel plate, the average hardness is Hv210 or less, and the variation in the average hardness is Hv50 or less; and The volume ratio of the Bainite structure at a depth of 0.5 mm from the surface of the steel plate is more than 90%, and the volume ratio of the Bainite structure at 1/2 of the thickness of the steel plate is It is a metal structure that contains more than 20%, and the total volume ratio of ferrite structure and toughened iron structure is more than 60%. The steel plate as described in claim 1, wherein the aforementioned component composition further contains: in mass %: Selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and W: 1 or more types from 0.01 to 1.00%. The steel plate as claimed in claim 1 or claim 2, wherein the aforementioned component composition further contains: in mass %: Selected from V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, and REM: 1 or more types from 0.0005 to 0.0200%. A method of manufacturing a steel plate, which involves hot-rolling steel raw materials having the following compositions at a rolling end temperature equal to or above the Ar ₃ transformation point, and then cooling from a cooling start temperature above the Ar ₃ transformation point. A method for manufacturing a steel plate in which the surface is cooled, then heated by reheating and then cooled twice. The composition in mass % contains: C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less, and O: 0.0100% or less, and the remainder is Fe and inevitable impurities, as mentioned above During cooling, the cooling rate from 600°C to 400°C is set to 30 to 100°C/s at a depth of 0.5 mm from the surface of the steel plate. The heating of the surface by reheating is Proceed until the reaching temperature at a depth of 0.5mm from the surface of the steel plate becomes 500°C or more. In the aforementioned secondary cooling, set the cooling stop temperature to 600°C or less at a position that is 1/2 of the thickness of the steel plate. The cooling rate up to this point is set to 10°C/s or more. The manufacturing method of the steel plate as described in claim 4, wherein the composition of the aforementioned steel raw material further contains: in mass %: Selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and W: 1 or more types from 0.01 to 1.00%. The manufacturing method of the steel plate as described in claim 4 or claim 5, wherein the composition of the aforementioned steel raw material further contains: in mass %: Selected from V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, and REM: 1 or more types from 0.0005 to 0.0200%.