阅读说明：本技术 钢板及其制造方法 (Steel sheet and method for producing same ) 是由 木津谷茂树 中岛孝一 植田圭治 于 2018-08-28 设计创作，主要内容包括：本发明提供板厚中心部的断面收缩率特性优良的高Mn钢板。该钢板具有含有C：0.20％以上且0.70％以下、Si：0.05％以上且1.0％以下、Mn：15％以上且35％以下、Al：0.1％以下、Cr：8.0％以下、N：0.0010％以上且0.0500％以下、P：0.03％以下和S：0.005％以下且余量为Fe和不可避免的杂质的成分组成,拉伸强度为600MPa以上且-196℃下的吸收能为27J以上,并且板厚方向的断面收缩率为30％以上。(The invention provides a high Mn steel sheet having excellent reduction of area characteristic in the center portion of the sheet thickness. The steel sheet has a composition containing 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, N: 0.0010% or more and 0.0500% or less, P: 0.03% or less and S: 0.005% or less, and the balance Fe and inevitable impurities, a tensile strength of 600MPa or more, an absorption energy at-196 ℃ of 27J or more, and a reduction of area in the plate thickness direction of 30% or more.)

1. A steel sheet having a composition containing, in mass%, 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, N: 0.0010% or more and 0.0500% or less, P: 0.03% or less and S: 0.005% or less, and the balance Fe and inevitable impurities, a tensile strength of 600MPa or more, an absorption energy at-196 ℃ of 27J or more, and a reduction of area in the plate thickness direction of 30% or more.

2. The steel sheet according to claim 1, wherein the composition further contains, in mass%, a metal element selected from the group consisting of Nb: 0.003% or more and 0.030% or less, V: 0.01% or more and 0.10% or less, 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 claim 1 or 2, wherein the composition further contains, in mass%, a metal selected from the group consisting of 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% to 0.30% inclusive, Mo: 0.05% or more and 2.0% or less and W: 0.05% to 2.0% of one or more kinds of the above.

4. The steel sheet according to claim 1, 2 or 3, wherein the composition further contains, in mass%, a component selected from the group consisting of Ca: 0.0005% or more and 0.0050% or less, Mg: more than 0.0005% and less than 0.0100% and REM: 0.0010% or more and 0.0200% or less.

5. A method for producing a steel sheet according to any one of claims 1 to 4, wherein a steel material is heated to 1000 ℃ or higher and 1300 ℃ or lower, and then hot-rolled under conditions in which a reduction ratio is 3 or higher and a reduction ratio in at least two of the final three passes is 10% or higher per pass.

Technical Field

The present invention relates to a steel sheet which is suitable for structural steel used in an extremely low temperature environment, such as a tank for liquefied gas storage tanks, and which is particularly excellent in the characteristics of the center portion of the sheet thickness, and a method for producing the same.

Background

Since the use environment of a structure such as a tank for liquefied gas storage tank is extremely low temperature, in order to use a hot-rolled steel sheet for the structure, the steel sheet is required to have not only excellent strength but also excellent toughness at extremely low temperature. For example, in the case of a hot rolled steel sheet used for a storage tank for liquefied natural gas, it is necessary to ensure excellent toughness in a temperature range lower than the boiling point of liquefied natural gas, i.e., -164 ℃. If the steel plate used for the structure for the cryogenic tank has poor low-temperature toughness, the safety of the structure for the cryogenic tank may not be maintained, and therefore, it is strongly required to improve the low-temperature toughness of the steel plate used.

In response to this demand, austenitic stainless steel sheets, 9% Ni steel sheets, or 5000-series aluminum alloys, which do not exhibit brittleness at extremely low temperatures, and which have an austenitic structure, have been conventionally used. However, since the above-mentioned metal materials are expensive in alloy cost and production cost, a steel plate which is inexpensive and excellent in cryogenic temperature toughness is required. Therefore, as a new steel sheet replacing conventional steel for extremely low temperatures, studies have been made on using high Mn steel, which is formed by adding Mn as an austenite stabilizing element at a relatively low cost in a large amount and has an austenite structure, as a steel sheet for structural use in an extremely low temperature environment.

For example, patent document 1 discloses a steel material in which machinability and charpy impact characteristics at-196 ℃ in a weld heat affected zone are improved by adding 15 to 35% of Mn, 5% or less of Cu, and further adding appropriate amounts of C and Cr.

In addition, patent document 2 discloses 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%, Cr: 0.1% or more and less than 8.0% of a high Mn steel material having improved low temperature toughness.

Patent document 3 discloses a high Mn steel material in which the cryogenic toughness of the base material and the welded portion is improved by adding elements such as Cr, Ti, Si, Al, Mg, Ca, and REM to 0.001 to 0.80% of C and 15 to 35% of Mn.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 2015-508452

Patent document 2: japanese patent laid-open publication No. 2016-84529

Patent document 3: japanese patent laid-open publication No. 2016-196703

Disclosure of Invention

Problems to be solved by the invention

The high Mn steel has the following characteristics: since steel is a high alloy steel as compared with general carbon steel, the melting point is low, and since the viscosity near the melting point is high, coarse casting defects are likely to occur as compared with carbon steel. Therefore, if a casting defect remains in the product, if tensile stress acts in the plate thickness direction of the steel plate such as a cross joint, breakage may occur in the product, resulting in a structural object being broken.

However, the steel materials described in patent documents 1, 2 and 3 do not mention the reduction of area characteristic at the center of the sheet thickness, which is important from the viewpoint of manufacturing cost for achieving strength and low-temperature toughness and from the viewpoint of safety of the structure when the above-mentioned austenitic steel material is used, and there is still room for study.

In view of the above problems, an object of the present invention is to provide a high Mn steel sheet having excellent reduction of area characteristics at the center portion of the sheet thickness.

Means for solving the problems

The present inventors have conducted extensive studies on the composition of steel sheets, the production method, and the like, aiming at high Mn steels, in order to solve the above problems, and as a result, have obtained the following findings.

(i) By suppressing the S content to 0.005% or less based on the high Mn steel, the amount of MnS produced can be reduced, and the tensile properties in the sheet thickness direction can be improved.

(ii) In addition, in the finish hot rolling, casting defects can be made harmless by rolling at a reduction ratio of 3 or more, and the rolling reduction ratio in each of at least two passes out of the final three passes is set to 10% or more to achieve size stabilization of the entire steel sheet and suppress the remaining of abnormally coarse crystal grains, thereby improving the tensile properties in the sheet thickness direction.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

1. A steel sheet having a composition containing, in mass%, 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, N: 0.0010% or more and 0.0500% or less, P: 0.03% or less and S: 0.005% or less, and the balance Fe and inevitable impurities, a tensile strength of 600MPa or more, an absorption energy at-196 ℃ of 27J or more, and a reduction of area in the plate thickness direction of 30% or more.

Here, the reduction of area in the plate thickness direction can be measured by a test in accordance with JIS Z3199.

2. The steel sheet according to claim 1, wherein the composition further contains, in mass%, a chemical composition selected from the group consisting of Nb: 0.003% or more and 0.030% or less, V: 0.01% or more and 0.10% or less, 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 claim 1 or 2, wherein the composition further contains, in mass%, a metal selected from the group consisting of 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% to 0.30% inclusive, Mo: 0.05% or more and 2.0% or less and W: 0.05% to 2.0% of one or more kinds of the above.

4. The steel sheet according to claim 1, 2 or 3, wherein the composition further contains, in mass%, a component selected from the group consisting of Ca: 0.0005% or more and 0.0050% or less, Mg: more than 0.0005% and less than 0.0100% and REM: 0.0010% or more and 0.0200% or less.

5. A method for producing a steel sheet according to any one of the above 1 to 4, wherein a steel material is heated to 1000 ℃ or higher and 1300 ℃ or lower, and then hot-rolled under conditions in which a reduction ratio is 3 or higher and a reduction ratio of at least two passes in the final three passes is 10% or higher per pass.

Effects of the invention

According to the present invention, a steel sheet having excellent reduction of area characteristics at the center portion of the sheet thickness can be provided. Further, when 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, the steel sheet greatly contributes to improvement of safety of the structure, and brings a significant industrial effect. Further, since it is inexpensive as compared with conventional materials, it is also economically advantageous.

Detailed Description

The steel sheet of the present invention will be specifically described below. The present invention is not limited to the following embodiments.

[ composition of ingredients ]

First, the composition of the steel sheet of the present invention and the reasons for the limitation thereof will be described. Unless otherwise specified, "%" indicating the composition of the components means "% by mass".

C: 0.20% or more and 0.70% or less

C is an inexpensive austenite stabilizing element effective for increasing the strength, and is an important element for obtaining an austenite structure. In order to obtain this effect, C needs to be contained by 0.20% or more. On the other hand, if the content exceeds 0.70%, segregation occurs in the central portion of the sheet thickness, and excessive precipitation of Cr carbide and Nb-, V-and Ti-based carbide is promoted, so that the low-temperature toughness is lowered and the reduction of area is lowered. Therefore, C is set to 0.20% or more and 0.70% or less. Preferably, the content is set to 0.25% or more and 0.60% or less.

Si: 0.05% to 1.0%

Si is required for steel making because it functions as a deoxidizing material, and has the effect of being dissolved in steel to increase the strength of a steel sheet by solid solution strengthening. In order to obtain such an effect, Si needs to be contained by 0.05% or more. On the other hand, if the content exceeds 1.0%, weldability and surface properties deteriorate. Therefore, Si is set to 0.05% or more and 1.0% or less. Preferably, the content is set to 0.07% or more and 0.5% or less.

Mn: 15% to 35% inclusive

Mn is a relatively inexpensive austenite stabilizing element. In the present invention, Mn is an important element for achieving both strength and very low temperature toughness. In order to obtain this effect, Mn needs to be contained by 15% or more. On the other hand, if the content exceeds 35%, the effect of improving the very low temperature toughness is saturated, resulting in an increase in alloy cost. Further, weldability and cuttability are deteriorated. Further, the growth of segregation causes a decrease in the extremely low temperature toughness, a deterioration in the tensile properties in the thickness direction, and the occurrence of stress corrosion cracking. Therefore, Mn is set to 15% or more and 35% or less. Preferably, the content is set to a range of 18% to 28%.

Al: less than 0.1%

Al acts as a deoxidizer and is most commonly used in a molten steel deoxidizing process of a steel sheet. Further, it has an effect of suppressing deterioration of toughness due to reduction of the solid-solution N by fixing the solid-solution N in the steel and forming AlN. For this reason, it is preferably contained in an amount of 0.01% or more. On the other hand, when Al is contained in an amount exceeding 0.1%, Al diffuses into the weld metal portion during welding and deteriorates the toughness of the weld metal, so that it is set to 0.1% or less. Preferably, the content is set to 0.07% or less. More preferably, the content is set to 0.02% or more and 0.06% or less.

Cr: less than 8.0%

Cr is an element necessary for improving the low-temperature toughness and corrosion resistance of high Mn steel. On the other hand, Cr is precipitated in the form of nitride, carbide, carbonitride or the like during rolling, and since such precipitates are formed to become starting points of corrosion and fracture and lower the low-temperature toughness, the upper limit is set to 8.0%. The Cr content is preferably set to 1.0% or more and 6.0% or less, and more preferably set to 1.5% or more and 5.5% or less.

N: 0.0010% or more and 0.0500% or less

N is an austenite stabilizing element and is an element effective for improving the extremely low temperature toughness. In addition, Nb, V, and Ti are bonded to each other, and are finely precipitated as nitrides or carbonitrides, and serve as trap sites for diffusible hydrogen to suppress stress corrosion cracking. In order to obtain such an effect, it is necessary to contain 0.0010% or more of N. On the other hand, if the content exceeds 0.0500%, excessive formation of nitrides or carbonitrides is promoted, the amount of solid-solution elements decreases, the corrosion resistance decreases, and the toughness also decreases. Therefore, N is set to 0.0010% or more and 0.0500% or less. The N content is preferably set to 0.0020% to 0.0200%.

P: less than 0.03%

When P is contained in an amount exceeding 0.03%, segregation may occur in grain boundaries, resulting in a decrease in grain boundary strength and a fracture origin. Therefore, the upper limit is set to 0.03%, and preferably, the lower limit is set to as small as possible. Therefore, P is set to 0.03% or less. Since the characteristics are improved as P is smaller, it is preferably set to 0.025% or less, more preferably 0.020% or less. Since suppressing to less than 0.0005% requires a significant steel-making cost, it is preferably set to 0.0005% or more from the viewpoint of economy.

S: less than 0.005%

Since S forms MnS in steel and significantly deteriorates low-temperature toughness and reduction of area in drawing in the plate thickness direction, 0.005% is set as an upper limit and preferably as small as possible. Preferably, the content is set to 0.002% or less. Since suppressing to less than 0.0001% requires a significant steel-making cost, it is preferably set to 0.0001% or more from the viewpoint of economy.

The balance other than the above components being Fe and inevitable impurities. As the inevitable impurities, there are Zr, As and the like.

In the present invention, the following elements may be contained as necessary in addition to the above-described essential elements for the purpose of further improving the strength and the low-temperature toughness.

Nb: 0.003% or more and 0.030% or less

Nb is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, 0.003% or more of Nb is preferably added. On the other hand, if the content exceeds 0.030%, coarse carbonitrides precipitate to become starting points of fracture, and the tensile properties in the sheet thickness direction may deteriorate. In addition, the precipitates may coarsen to deteriorate the toughness of the base metal. Therefore, when Nb is contained, it is preferably set to 0.003% or more and 0.030% or less. More preferably, it is set to 0.005% or more, and still more preferably, 0.007% or more. Similarly, it is more preferably set to 0.025% or less, and still more preferably set to 0.022% or less.

V: 0.01% to 0.10% inclusive

V is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to contain 0.01% or more of V. On the other hand, if the content exceeds 0.10%, coarse carbonitrides precipitate and become starting points of fracture. In addition, the precipitates may coarsen to deteriorate the toughness of the base metal. Therefore, when V is contained, it is preferably set to 0.01% or more and 0.10% or less. More preferably, it is set to 0.02% or more, and still more preferably 0.03% or more. Similarly, it is more preferably set to 0.09% or less, and still more preferably set to 0.08% or less.

Ti: 0.003% or more and 0.040% or less

Ti is an element that precipitates as a nitride or carbonitride and is effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to contain 0.003% or more of Ti. On the other hand, if the content exceeds 0.040%, precipitates may coarsen and the toughness of the base material may deteriorate. In addition, coarse carbonitrides precipitate and become starting points of fracture. Therefore, when Ti is contained, it is preferably set to 0.003% or more and 0.040% or less. More preferably, it is set to 0.005% or more, and still more preferably, 0.007% or more. Similarly, it is more preferably set to 0.035% or less, and still more preferably set to 0.032% or less.

B: 0.0003% or more and 0.0100% or less

B is an element for improving austenite grain boundary strength, and is an element effective for improving very low temperature toughness. In order to obtain such an effect, it is preferable to contain 0.0003% or more of B. On the other hand, if the content exceeds 0.0100%, coarse B precipitates are formed, and the toughness is lowered. Therefore, B is preferably set to a range of 0.0100% or less. More preferably 0.0030% or less.

Further, in the present invention, the following elements may be contained 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% to 0.30% inclusive, Mo: 0.05% or more and 2.0% or less, W: 0.05% to 2.0% of one or more

Cu, Ni, Sn, Sb, Mo, and W are elements that improve the corrosion resistance of the high Mn steel by being added in combination with Cr.

This effect is remarkable in the high Mn steel when any one of the elements described above is present with Cr, and is exhibited when each of the elements is equal to or higher than the lower limit value described above. However, if any element is contained in a large amount exceeding the above upper limit, weldability and toughness deteriorate, which is also disadvantageous from the viewpoint of cost.

Therefore, Cu, Ni, Sn, Sb, Mo and W are preferably added within the above-mentioned range. More preferably, the Cu content is 0.02% to 0.50%, the Ni content is 0.02% to 0.40%, the Sn content is 0.02% to 0.25%, the Sb content is 0.02% to 0.25%, the Mo content is 0.05% to 1.50%, and the W content is 0.05% to 1.50%.

Further, in the present invention, the following elements may be contained as necessary.

Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0100% or less, REM: 0.0010% or more and 0.0200% or less

Ca. Mg and REM are elements useful for controlling the form of inclusions such as MnS, and may be contained as necessary. The term "morphology control" as used herein means that the stretched sulfide-based inclusions are changed into granular ones. By controlling the morphology of the inclusions, the tensile properties, toughness and sulfide stress corrosion cracking resistance in the sheet thickness direction can be improved. In order to obtain such effects, it is preferable that the content of Ca and Mg is 0.0005% or more and the content of REM is 0.0010% or more.

On the other hand, when any one element is contained in a large amount, the amount of non-metallic inclusions may increase, and conversely, the characteristics of the plate thickness center portion may decrease. Therefore, it is preferable to set Ca to 0.0050% or less, Mg to 0.0100% or less, and REM to 0.0200% or less. More preferably, the Ca amount is 0.0010% to 0.0040%, the Mg amount is 0.0010% to 0.0040%, and the REM amount is 0.0020% to 0.0150%.

In the steel sheet having the above composition, it is important to further reduce the reduction of area in the sheet thickness direction to 30% or more. That is, if the reduction of area in the plate thickness direction is less than 30%, for example, a cross-welded joint portion or the like is broken, and the structural strength is significantly impaired.

Next, the production conditions of the steel sheet of the present invention will be described. That is, the steel sheet of the present invention can be produced as follows: the steel sheet of the present invention is produced by heating a steel material having the above-described composition to 1000 ℃ or higher and 1300 ℃ or lower, and then hot rolling the steel material under conditions in which the reduction ratio is 3 or higher and the reduction ratio of at least two passes in the final three passes is 10% or higher per pass.

In the following description, the temperature "° c" refers to the temperature at the center of the sheet thickness.

[ heating temperature of steel material: 1000 ℃ or higher and 1300 ℃ or lower

The heating temperature of the steel material is set to 1000 ℃ to 1300 ℃ inclusive in order to dissolve precipitates in the structure and to make the crystal grain size uniform. When the heating temperature is less than 900 ℃, the precipitates are not sufficiently dissolved in a solid state, and thus desired characteristics cannot be obtained. Further, since heating at 1300 ℃ or higher causes deterioration of the material due to coarsening of the crystal grain size and requires excessive energy to lower the productivity, the upper limit of the heating temperature is set to 1300 ℃. Preferably 1050 ℃ or higher and 1250 ℃ or lower, and more preferably 1100 ℃ or higher and 1250 ℃ or lower.

As the steel material, a material produced by a conventional method such as an ingot or a billet may be used in addition to a continuous casting billet.

[ reduction ratio in hot rolling: 3 or more)

When the reduction ratio of hot rolling is less than 3, it is difficult to suppress the reduction of the tensile properties in the sheet thickness direction by the compression of the casting defect. Further, it is also insufficient to promote recrystallization by rolling to achieve grain alignment, and coarse austenite grains remain, with the result that properties such as strength and toughness deteriorate, and therefore, the reduction ratio is limited to 3 or more. The reduction ratio is preferably set to 4 or more, and more preferably set to 5 or more. The upper limit of the reduction ratio is not particularly limited, but is preferably set to 50 or less. This is because when the rolling reduction ratio exceeds 50, the anisotropy of mechanical properties significantly increases.

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

[ reduction ratios of at least two passes in the final three passes of 10% or more per pass ]

By setting the reduction ratio per one pass to 10% or more in at least two of the final three passes for finally determining the steel sheet material, firstly, casting defects can be reliably eliminated, and the steel sheet as a whole can be sized to suppress the remaining of abnormal coarse grains, and the reduction of area in the tensile test in the sheet thickness direction can be improved, and as a result, a reduction of area of 30% or more can be ensured.

That is, the reduction ratio of at least two of the final three passes is limited to reliably crimp the casting defect. Therefore, the reduction ratios in all the final three passes are preferably set to 10% or more. On the other hand, when the reduction ratio of at least two of the final three passes is less than 10%, casting defects remain, and the reduction of area at the center portion of the plate thickness decreases. The upper limit of the reduction ratio is not particularly limited, but is preferably set to 30% from the viewpoint of facility restrictions such as rolling load.

[ Cooling after Hot Rolling ]

In order to obtain the necessary properties of the steel sheet, such as strength and low-temperature toughness, water cooling or the like may be performed after hot rolling.

Examples

Steels of nos. 1 to 26 shown in table 1 were melted to prepare billets, and then steel sheets having a thickness of 30 to 50mm were prepared under the production conditions shown in table 2. The steel sheets of samples No.1 to No.30 thus obtained were subjected to a tensile test as shown below. The results of the tensile test are shown in table 2.

The reduction of area in the sheet thickness direction by the tensile test was evaluated in accordance with JIS G3199. For the test piece shape, an a-type test piece was used. Further, the tensile strength was evaluated as an average of three test pieces using a round bar tensile test piece cut from a depth position of 1/4 (hereinafter, also referred to as 1/4 t) from the surface of the steel sheet, and the charpy absorption energy at-196 ℃ was evaluated as an average of three test pieces using a charpy test piece cut from 1/4 t.

It was confirmed that: the reduction of area of the invention examples (samples No.1 to No.14) according to the invention satisfied 30% or more. On the other hand, in comparative examples (sample Nos. 15 to 30) which deviate from the scope of the present invention, one or more of the tensile strength, the absorption energy and the reduction of area are out of the range claimed in the present application, and the above-mentioned target properties cannot be satisfied.