TWI671409B - Steel plate and method of manufacturing same - Google Patents

Abstract

The present invention provides a high-Mn steel sheet having excellent shrinkage characteristics at the center of the plate thickness. The steel sheet contains C: 0.20% or more and 0.70% or less, Si: 0.05% or more and 1.0% or less, Mn: 15% or more and 35% or less, Al: 0.1% or less, Cr: 8.0% or less, and N: 0.0010% or more and 0.0500% or less. , P: 0.03% or less and S: 0.005% or less, and the rest is composed of Fe and unavoidable impurities, the tensile strength is 600MPa or more, and the absorption energy at -196 ° C is 27J or more and the shrinkage value in the thickness direction More than 30%.

Description

Steel plate and manufacturing method thereof

The present invention relates to a structural steel which can be suitably used in a tank for a liquefied gas storage tank and the like in an extremely low temperature environment, particularly a steel plate having excellent properties at a center portion of a plate thickness and a method for manufacturing the same.

Since structures such as tanks for liquefied gas storage tanks are used at extremely low temperatures, when using hot-rolled steel plates for the structures, not only the strength of the steel plate is required, but also the toughness at extremely low temperatures is required. For example, it is necessary for hot-rolled steel plates used in LNG storage tanks to ensure excellent toughness in a lower temperature region than the boiling point of LNG, which is -164 ° C. When the low-temperature toughness of the steel plate used in the structure for an extremely low-temperature storage tank is poor, there is a risk that the safety as the structure for the ultra-low-temperature storage tank cannot be maintained, and therefore, an improvement in low-temperature toughness is strongly required for the applied steel plate.

To meet this requirement, an Austenitic stainless steel plate or an 9% Ni steel plate or a 5000-series aluminum alloy having an austenite structure that does not show brittleness at extremely low temperatures has been conventionally used. However, since the above-mentioned metal materials have a high alloy cost or manufacturing cost, there is a demand for a steel sheet that is inexpensive and excellent in extremely low-temperature toughness. Therefore, as a new steel plate that replaces the conventional steel for extremely low temperature, a lot of Mn, which is a relatively inexpensive austenite stabilization element, has been added to form an austenite structure, and high-Mn steel has been applied as a steel plate for extremely low temperature environment.

For example, Patent Document 1 discloses a Charpy impact characteristic at -196 ° C of machinability and heat-melting heat-affected zone by adding Mn 15 to 35% and Cu: 5% or less and further adding C and Cr in appropriate amounts. Improved steel.

In addition, Patent Document 2 discloses the addition of C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% and 35% or less, Ni: 0.1% or more and less than 7.0%, and Cr: 0.1% or more and not more. High Mn steel with improved low temperature toughness up to 8.0%.

Furthermore, Patent Document 3 discloses that C: 0.001 to 0.80% and Mn: 15 to 35% are added. By adding elements of Cr, Ti, Si, Al, Mg, Ca, and REM, the base material and welded portion are improved. Extremely low temperature toughness and high Mn steel. [Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Publication No. 2015-508452 Patent Document 2: Japanese Patent Publication No. 2016-84529 Patent Document 3: Japanese Patent Publication No. 2016-196703

[Questions to be Solved by the Invention]

Compared with carbon steel, high Mn steel is a high alloy, so it has a low melting point and high viscosity near the melting point. Therefore, compared with carbon steel, it has the characteristics of being prone to coarse casting defects. Therefore, if casting defects remain in the product, when tensile stress is applied to the plate thickness direction of the steel plate by a cross joint or the like, there is a possibility that the product may break and the structure may collapse.

However, the steel materials described in Patent Documents 1, 2, and 3 do not mention changes made based on the viewpoint of the manufacturing cost for achieving strength and low-temperature toughness, and the viewpoint of the safety of the structure when the above-mentioned austenitic steel is used. There is still room for review of the shrinkage characteristics of the important plate thickness center.

The present invention is made in view of this problem, and an object thereof is to provide a high-Mn steel sheet having excellent shrinkage characteristics at the center portion of the plate thickness. [Means to solve the problem]

In order to solve the above-mentioned problems, the present inventors made active research on the component composition, manufacturing method, and the like of high-Mn steel, and finally obtained the following findings. (I) By suppressing the S content to less than 0.005% based on high Mn steel, the amount of MnS can be reduced and the tensile properties in the thickness direction can be improved. (ii) Further, in the process of hot rolling, rolling is performed at a reduction ratio of 3 or more to make the casting defects harmless, and at least 2 of the final 3 times, the reduction ratio of each time is reduced. When it is set to 10% or more, the entire steel sheet can be granulated, the residual of abnormally coarse grains can be suppressed, and the tensile properties in the thickness direction can be improved.

The present invention has been completed based on the above findings and further review. The main points are as follows. 1. A steel sheet, in terms of mass%, containing C: 0.20% to 0.70%, Si: 0.05% to 1.0%, Mn: 15% to 35%, Al: 0.1% or less, Cr: 8.0% or less , N: 0.0010% or more and 0.0500% or less, P: 0.03% or less and S: 0.005% or less, and has the composition of the rest of Fe and unavoidable impurities, tensile strength of 600MPa or more and absorption energy at -196 ° C It is 27 J or more and the shrinkage value in a thickness direction is 30% or more.

Here, the shrinkage value in the thickness direction can be measured by a test according to JIS Z3199.

2. The steel sheet according to the above 1, wherein the aforementioned component composition further comprises, by mass%, one or more selected from the following: Nb: 0.003% or more and 0.030% or less, V: 0.01% or more and 0.10% or less, and Ti: 0.003% or more and 0.040% or less and B: 0.0003% or more and 0.0100% or less.

3. The steel sheet according to 1 or 2 above, wherein the aforementioned component composition further comprises, by mass%, one or more selected from the following: Cu: 0.01% or more and 0.70% or less, Ni: 0.01% or more and 0.50% or less, Sn: 0.01% to 0.30%, Sb: 0.01% to 0.30%, Mo: 0.05% to 2.0%, and W: 0.05% to 2.0%.

4. The steel sheet according to the above 1, 2 or 3, wherein the aforementioned component composition further comprises, by mass%, one or more selected from the following: Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0100% or more Below and REM: 0.0010% or more and 0.0200% or less.

5. A method for manufacturing a steel plate, which comprises manufacturing the steel plate according to any one of the foregoing 1 to 4, and heating the steel material to a temperature of 1000 ° C or more and 1300 ° C or less, and at a reduction ratio of 3 or more and at least 3 times in the end Hot rolling was performed at a rolling reduction of 10% or more per time. [Inventive effect]

According to the present invention, it is possible to provide a steel plate having excellent shrinkage characteristics at the center portion of the plate thickness. In addition, if the steel sheet of the present invention is applied to a steel structure used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, it will greatly contribute to the improvement of the safety of the structure and bring an industrially extraordinary effect. In addition, it is cheaper than existing materials, so it has the advantage of excellent economy.

Hereinafter, the steel sheet of the present invention will be specifically described. The present invention is not limited to the following embodiments. [Component composition] (1) First, the component composition of the steel sheet of the present invention and the reasons for its limitation will be described. In addition, "%" showing an ingredient composition means "mass%" as long as it is not specifically limited.

C: 0.20% or more and 0.70% or less C is effective for high strength and is an inexpensive austenite stabilizing element. It is also an important element for obtaining an austenite structure. In order to obtain this effect, it is necessary to contain C in an amount of 0.20% or more. On the other hand, if it contains more than 0.70%, it will segregate at the center of the plate thickness and promote excessive precipitation of Cr carbides and Nb, V, and Ti-based carbides, thereby lowering the low-temperature toughness and reducing the shrinkage value. Therefore, C is set to be 0.20% or more and 0.70% or less. It is preferably at least 0.25% and at most 0.60%.

Si: 0.05% or more and 1.0% or less Si is used as a deoxidizing material. It is not only necessary for steel making, but also solidified and strengthened by solid solution in steel, which has the effect of increasing the strength of steel plates. In order to obtain these effects, Si must be contained in an amount of 0.05% or more. On the other hand, when the content exceeds 1.0%, the weldability and surface properties are deteriorated. Therefore, Si is set to 0.05% to 1.0%. It is preferably at least 0.07% and at most 0.5%.

Mn: 15% to 35% Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element used to have both strength and extremely low temperature toughness. In order to obtain this effect, Mn must be contained in an amount of 15% or more. On the other hand, when the content exceeds 35%, the effect of improving the low-temperature toughness is saturated, which leads to an increase in the cost of the alloy. In addition, the weldability and cutting properties are deteriorated. In addition, it promotes segregation, causes a decrease in the extremely low temperature toughness, a deterioration in the tensile properties in the thickness direction, and causes stress corrosion cracking. Therefore, Mn is set to 15% to 35%. The range is preferably 18% to 28%.

Al: 0.1% or less Al is used as a deoxidizer and is most commonly used in the process of deoxidizing molten steel of steel plates. In addition, fixing the solid solution N in the steel to form AlN has the effect of suppressing the deterioration of toughness due to the decrease of the solid solution N. Therefore, it is preferably contained at 0.01% or more. On the other hand, when the Al content exceeds 0.1%, it diffuses into the weld metal portion during welding and deteriorates the toughness of the weld metal, so it is set to 0.1% or less. It is more preferably 0.07% or less. It is more preferably at least 0.02% and at most 0.06%.

Cr: 8.0% or less Cr is an element necessary for improving the low-temperature toughness and corrosion resistance of high-Mn steel. On the other hand, Cr may be precipitated in the form of nitrides, carbides, carbonitrides, etc. during rolling. The low-temperature toughness is lowered because the formation of these precipitates becomes the starting point of corrosion or damage, so the upper limit is set. Set it to 8.0%. The amount of Cr is preferably in the range of 1.0% to 6.0%, more preferably in the range of 1.5% to 5.5%.

N: 0.0010% or more and 0.0500% or less N is an austenite stabilizing element, and is an effective element for improving the extremely low temperature toughness. In addition, it has the effect of suppressing stress corrosion cracking by combining with Nb, V, and Ti to precipitate finely as nitrides or carbonitrides, and as a trapping site for diffusible hydrogen. In order to obtain these effects, N must be contained in an amount of 0.0010% or more. On the other hand, if the content exceeds 0.0500%, the formation of excessive nitrides or carbonitrides is promoted, which not only reduces the amount of solid-solution elements and the corrosion resistance, but also decreases the toughness. Therefore, N is set to be 0.0010% or more and 0.0500% or less. The N content is preferably 0.0020% or more and 0.0200% or less.

P: 0.03% or less When P is contained in excess of 0.03%, segregation at the grain boundary may reduce the strength of the grain boundary, and may become the starting point of failure. Therefore, 0.03% is set as an upper limit, and it is desirable to reduce it as much as possible. Therefore, P is set to 0.03% or less. The smaller the P content, the better the characteristics. Therefore, it is preferably set to 0.025% or less, and more preferably 0.020% or less. Moreover, when it is suppressed to less than 0.0005%, since considerable steelmaking cost is required, it is preferable to set it as 0.0005% or more from the viewpoint of economy.

S: 0.005% or less S forms MnS in the steel, which significantly deteriorates the low-temperature toughness and shrinkage when the steel sheet is stretched in the direction of the steel. Therefore, 0.005% is set as the upper limit, and it is desirable to reduce it as much as possible. The content is preferably 0.002% or less. In addition, when it is suppressed to less than 0.0001%, a considerable steel-making cost is required, so it is preferably set to 0.0001% or more from the viewpoint of economy.

The remainder other than the above components are Fe and inevitable impurities. Examples of unavoidable impurities include Zr and As.

In the present invention, for the purpose of further improving strength and low-temperature toughness, in addition to the above-mentioned essential elements, the following elements may be contained as necessary. Nb: 0.003% or more and 0.030% or less Nb is an effective element for improving the strength of steel plates. In order to obtain these effects, it is preferable to add Nb at 0.003% or more. On the other hand, when the content exceeds 0.030%, roughened carbonitrides may be precipitated, which may become the starting point of failure, and the tensile properties in the thickness direction may be deteriorated. In addition, the precipitate may be coarsened and the toughness of the base material may be deteriorated. Therefore, when Nb is contained, it is preferably set to 0.003% or more and 0.030% or less. It is more preferably 0.005% or more, and still more preferably 0.007% or more. Similarly, it is more preferably set to 0.025% or less, and more preferably set to 0.022% or less.

V: 0.01% or more and 0.10% or less V is an effective element for improving the strength of steel plates. In order to obtain these effects, V is preferably contained in an amount of 0.01% or more. On the other hand, when the content exceeds 0.10%, roughened carbonitrides may be precipitated and become the starting point of failure. In addition, the precipitate may be coarsened and the toughness of the base material may be deteriorated. Therefore, when V is contained, it is preferably set to 0.01% or more and 0.10% or less. It is more preferably set to be 0.02% or more, and more preferably set to be 0.03% or more. Similarly, it is more preferably 0.09% or less, and still more preferably 0.08% or less.

Ti: 0.003% or more and 0.040% or less Ti is an effective element that precipitates as a nitride or a carbonitride to improve the strength of the steel sheet. In order to obtain these effects, Ti is preferably contained in an amount of 0.003% or more. On the other hand, when the content exceeds 0.040%, the precipitates are coarsened, and the toughness of the base material may be deteriorated. In addition, roughened carbonitrides may precipitate and become the starting point of destruction. Therefore, when Ti is contained, it is preferably set to 0.003% or more and 0.040% or less. It is more preferably 0.005% or more, and still more preferably 0.007% or more. Similarly, it is more preferably 0.035% or less, and still more preferably 0.032% or less.

B: 0.0003% or more and 0.0100% or less. B is an element that enhances the austenite grain boundary strength, and is an effective element that improves the extremely low temperature toughness. In order to obtain these effects, B is preferably contained in an amount of 0.0003% or more. On the other hand, if the content exceeds 0.0100%, coarse B precipitates are formed, and toughness is reduced. Therefore, B is preferably set to a range of 0.0100% or less. It is more preferably 0.0030% or less.

The present invention may contain the following elements as necessary. Cu: 0.01% or more and 0.70% or less, Ni: 0.01% or more and 0.50% or less, Sn: 0.01% or more and 0.30% or less, Sb: 0.01% or more and 0.30% or less, Mo: 0.05% or more and 2.0% or less, and W: 0.05% or more One or more than 2.0%.

Cu, Ni, Sn, Sb, Mo, and W are elements that are added in combination with Cr to improve the corrosion resistance of high-Mn steel. This effect is noticeable in high Mn steel when any of the above elements coexist with Cr, and appears above the respective lower limit values. However, if any element is contained in excess of the above upper limit, the weldability and toughness are deteriorated, and the cost is disadvantageous.

Therefore, Cu, Ni, Sn, Sb, Mo, and W are preferably added in the above range. More preferably, the amount of Cu is 0.02% to 0.50%, the amount of Ni is 0.02% to 0.40%, the amount of Sn is 0.02% to 0.25%, the amount of Sb is 0.02% to 0.25%, and the amount of Mo is 0.05% to 1.50%. Hereinafter, the amount of W is 0.05% to 1.50%.

Furthermore, the following elements may be contained in this invention as needed. Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0100% or less, and REM: 0.0010% or more and 0.0200% or less.

Ca, Mg, and REM are elements useful for controlling the morphology of a spacer such as MnS, and may be contained as necessary. Here, the control of the form of the spacer means that the stretched sulfide-based spacer is made into a granular spacer. By controlling the morphology of the spacer, the tensile properties in the thickness direction, toughness, and resistance to sulfide stress corrosion cracking can be improved. In order to obtain these effects, it is preferable that Ca and Mg contain 0.0005% or more, and REM contain 0.0010% or more.

On the other hand, when any element is contained in a large amount, the amount of non-metallic spacers may increase, but the characteristics of the center portion of the plate thickness may decrease. Therefore, when Ca is contained, the content is preferably 0.0050% or less, when Mg is contained, the content is preferably 0.0100% or less, and when REM is contained, the content is preferably 0.0200% or less. The Ca content is more preferably 0.0010% to 0.0040%, the Mg content is 0.0010% to 0.0040%, and the REM content is 0.0020% to 0.0150%.

It is important for a steel sheet having the above composition to further reduce the shrinkage value in the thickness direction by 30% or more. That is, the reason is that when the shrinkage value in the thickness direction does not reach 30%, the cross-welded joint portion or the like is damaged, and the soundness of the structure is significantly damaged.

Next, the manufacturing method of the steel plate of this invention is demonstrated. That is, the steel sheet of the present invention can be obtained by heating a steel material having the above-mentioned component composition to a temperature of 1000 ° C. or more and 1300 ° C. or less, with a reduction ratio of 3 or more and at least 2 of the final 3 times. Manufactured by hot rolling more than 10%. In the following description, the temperature "° C" means the temperature at the center of the plate thickness.

[Heating temperature of steel: 1000 ° C or higher and 1300 ° C or lower] 加热 Heating steel to 1000 ° C or higher refers to solidifying the precipitates in the structure and uniformizing the crystal grain size. The heating temperature is set to 1000 ° C to 1300 ° C. the following. When the heating temperature is less than 900 ° C, the precipitates are not sufficiently solidified, so that desired characteristics cannot be obtained. In addition to heating at 1300 ° C or higher, in addition to the deterioration of the material due to the coarsening of the crystal grain size, too much energy is required to reduce productivity, so the upper limit of the heating temperature is 1300 ° C. The temperature is preferably 1050 ° C or higher and 1250 ° C or lower, and more preferably 1100 ° C or higher and 1250 ° C or lower.

In addition, as the steel material, in addition to continuous casting of a steel plate (slab), a material produced by a conventional method such as a block steel plate or an ingot can also be used.

[Reduction ratio during hot rolling: 3 or more] 时 When the reduction ratio during hot rolling is less than 3, it is difficult to suppress the reduction in tensile properties in the thickness direction by the pressing of casting defects. Furthermore, the recrystallization is promoted by rolling to make the graining insufficient, and the coarse austenite grains remain. As a result, properties such as strength and toughness are deteriorated. Therefore, the reduction ratio is limited to 3 or more. A better reduction ratio is 4 or more, and a better reduction ratio is 5 or more. The upper limit of the reduction ratio is not particularly limited, but it is preferably 50 or less. The reason is that when the reduction ratio exceeds 50, the anisotropy of the mechanical characteristics becomes significantly larger.

Here, the reduction ratio of hot rolling is defined by the thickness of a rolled material / the thickness of a steel sheet after rolling.

[The rolling reduction rate of at least 2 times in the last 3 times is 10% or more per time] The final 3 times of final determination of the steel material, for at least 2 times, the rolling reduction rate of each time is set to 10% In the above, firstly, the casting defects can be truly harmless, and the entire grain of the steel sheet can be achieved, thereby suppressing the residual of abnormally coarse grains. As a result, the shrinkage value of the tensile test in the thickness direction of the steel sheet can be increased, which can ensure more than 30% Shrink value.

That is, the reason is to limit the reduction ratio of at least two of the last three times, and it is possible to surely suppress the casting defect. Therefore, it is preferable to set the rolling reduction rate of each of the last three times to 10% or more. On the other hand, when the reduction ratio of at least two of the last three times is less than 10%, the casting defects remain and the shrinkage value of the center of the plate thickness decreases. The upper limit of the reduction ratio is not particularly limited, but it is preferable to set the equipment limit based on the rolling load to 30%.

[Cooling after Hot Rolling] In order to obtain necessary properties such as strength and low temperature toughness of the steel sheet, water cooling after hot rolling may be performed. [Example]

After the steels Nos. 1 to 26 shown in Table 1 were melted into a steel plate, a steel plate having a thickness of 30 to 50 mm was produced under the manufacturing conditions shown in Table 2. Next, the obtained steel plates of samples Nos. 1 to 30 were subjected to a tensile test shown below. The results of this tensile test are also shown in Table 2.

The shrinkage value of the tensile test in the thickness direction was evaluated in accordance with JIS G3199. The shape of the test piece was a type A test piece. In addition, the tensile strength of a round bar tensile test piece taken from a 1/4 depth position (hereinafter referred to as a 1 / 4t portion) from the thickness of the steel plate surface is a Charpy test piece taken from a 1 / 4t portion The Charpy absorption energy at -196 ° C was evaluated as an average of 3 tablets.

Examples of the present invention (Sample Nos. 1 to 14) consistent with the present invention confirmed that the shrinkage was satisfied to be 30% or more. On the other hand, the comparative examples (Sample Nos. 15 to 30) that deviate from the scope of the present invention are any one of tensile strength, absorbent energy, and shrinkage, which are outside the scope of the patent application of the present invention and cannot satisfy the above-mentioned target performance.

Claims (6)

A steel sheet, based on mass%, containing C: 0.20% to 0.70%, Si: 0.05% to 1.0%, Mn: 15% to 35%, Al: 0.1% or less, Cr: 8.0% or less, N : 0.0010% or more and 0.0500% or less, P: 0.03% or less and S: 0.005% or less, and has the composition of the rest being Fe and unavoidable impurities, the tensile strength is 600MPa or more, and the absorption energy at -196 ° C is 27J The above and further the shrinkage value in the thickness direction is 30% or more. For example, the steel sheet of claim 1, wherein the aforementioned component composition further comprises, by mass%, one or more selected from the following: Nb: 0.003% or more and 0.030% or less, V: 0.01% or more and 0.10% or less, and Ti: 0.003 % Or more and 0.040% or less and B: 0.0003% or more and 0.0100% or less. For example, the steel sheet of claim 1 or 2, wherein the foregoing component composition further comprises, by mass%, one or more selected from the following: Cu: 0.01% or more and 0.70% or less, Ni: 0.01% or more and 0.50% or less, Sn : 0.01% to 0.30%, Sb: 0.01% to 0.30%, Mo: 0.05% to 2.0%, and W: 0.05% to 2.0%. For example, the steel sheet of claim 1 or 2, wherein the foregoing component composition further comprises, by mass%, one or more selected from the following: Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0100% or less and REM : 0.0010% or more and 0.0200% or less. The steel sheet according to claim 3, wherein the aforementioned component composition further comprises, by mass%, one or two or more selected from the following: Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0100%, and REM: 0.0010 Above 0.0200%. A method for manufacturing a steel plate, comprising manufacturing the steel plate according to any one of claims 1 to 5, and heating the steel material to a temperature of 1000 ° C or more and 1300 ° C or less, with a reduction ratio of 3 or more and at least 2 of the final 3 times. Hot rolling is performed at a rolling reduction of 10% or more per time.