KR102524703B1 - Steel plate and method of producing same - Google Patents

Abstract

Provided is a high Mn steel sheet having excellent cross-sectional shrinkage properties at the center of the sheet thickness.
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, the balance being Fe and unavoidable impurities, the tensile strength is 600 MPa or more, the absorbed energy at -196 ° C is 27 J or more, and further in the sheet thickness direction The cross-sectional shrinkage value is set to 30% or more.

Description

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

The present invention relates to a steel sheet suitable for use in structural steel used in a cryogenic environment, such as a tank for storing liquefied gas, and particularly excellent in sheet thickness center characteristics, and a method for manufacturing the same.

Since structures such as liquefied gas storage tanks have cryogenic temperatures, the use of hot-rolled steel sheets in these structures requires not only strength but also excellent toughness at cryogenic temperatures. For example, a hot-rolled steel sheet used for storage of liquefied natural gas needs to have excellent toughness in a temperature range lower than -164°C, which is the boiling point of liquefied natural gas. If the low-temperature toughness of a steel sheet used for a structure for cryogenic storage is inferior, there is a risk that safety as a structure for cryogenic storage may not be maintained. Therefore, there is a strong demand for improving the low-temperature toughness of the steel sheet to be applied.

In response to this demand, conventionally, an austenitic stainless steel sheet, a 9% Ni steel sheet, or a 5000 series aluminum alloy having an austenite structure that does not show brittleness at cryogenic temperatures has been used. However, since the above-mentioned metal materials have high alloying cost and manufacturing cost, there is a demand for a steel sheet that is inexpensive and has excellent cryogenic toughness. Therefore, as a new steel sheet to replace conventional steel for cryogenic use, application of high Mn steel, which has an austenite structure by adding a large amount of Mn, a relatively inexpensive austenite stabilizing element, as a structural steel sheet in a cryogenic environment has been studied.

For example, in Patent Document 1, a steel material having improved machinability and Charpy impact characteristics of the heat-affected zone at -196°C by adding 15 to 35% of Mn, 5% or less of Cu, and further adding appropriate amounts of C and Cr is has been initiated.

Further, in Patent Document 2, C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% and less than 35%, Ni: 0.1% or more and less than 7.0%, and Cr: 0.1% or more and less than 8.0% are added A high Mn steel with improved low-temperature toughness is disclosed.

Further, Patent Document 3 contains 0.001 to 0.80% of C and 15 to 35% of Mn, and improves the cryogenic toughness of the base material and weld zone by adding elements such as Cr, Ti, Si, Al, Mg, Ca, and REM. A high Mn steel is disclosed.

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

Compared to general carbon steel, high Mn steel has a low melting point because it is a high alloy, and also has a high viscosity near the melting point, so coarse casting defects are more likely to occur than carbon steel. Therefore, if casting defects remain in the product, when tensile stress acts in the thickness direction of the steel sheet, such as at a cross joint, fracture occurs in the product, which may lead to collapse of the structure.

However, the steel materials described in Patent Literatures 1, 2 and 3 are important from the point of view of manufacturing cost for achieving strength and low-temperature toughness and from the point of view of safety of the structure when using the austenitic steel material described above. It was not mentioned, so there was still room for review.

In view of these problems, an object of the present invention is to provide a high Mn steel sheet having excellent cross-sectional shrinkage characteristics at the central portion of the sheet thickness.

In order to solve the above problems, the inventors of the present invention conducted intensive research on the component composition and manufacturing method of steel sheet using high Mn steel as a target, and as a result, reached the following findings.

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

(ii) Further, in hot finish rolling, casting defects are harmless by rolling at a reduction ratio of 3 or more, and the reduction ratio per pass is set to 10% or more for at least two of the last three passes, so that the entire steel sheet is The tensile properties in the sheet thickness direction can be improved by achieving sizing and suppressing the survival of abnormally coarse grains.

This invention was made by repeating examination further on the above knowledge, and the summary is as follows.

1. 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

A steel sheet containing Fe and an unavoidable impurity as the balance, having a tensile strength of 600 MPa or more, an absorbed energy at -196°C of 27 J or more, and a cross-sectional shrinkage value in the sheet thickness direction of 30% or more.

Here, the cross-sectional shrinkage value in the sheet thickness direction can be measured by a test based on JIS Z3199.

2. The above component composition is also in mass %,

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

The steel sheet according to 1 above, containing one or two or more selected from the above.

3. The above component composition is also in mass %,

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 and 2.0% or less

The steel sheet as described in said 1 or 2 containing 1 type(s) or 2 or more types selected from.

4. The above component composition is also in mass %,

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 1, 2 or 3 above, containing one or two or more selected from the above.

5. A method for producing the steel sheet according to any one of 1 to 4 above, after heating the steel material to 1000 ° C. or more and 1300 ° C. or less, hot rolling is performed, a reduction ratio: 3 or more, and at least 2 passes among the final 3 passes A method for producing a steel sheet in which the reduction ratio of is 10% or more per pass.

According to the present invention, it is possible to provide a steel sheet having excellent cross-sectional shrinkage characteristics at the central portion of the sheet thickness. In addition, when the steel sheet of the present invention is applied to a steel structure used in a cryogenic environment, such as a liquefied gas storage tank, it greatly contributes to improving the safety of the structure and brings about a remarkable industrial effect. In addition, since it is cheaper than conventional materials, there is also an advantage of excellent economic efficiency.

Hereinafter, the steel sheet of the present invention will be described in detail. In addition, this invention is not limited to the following embodiment.

[Ingredient Composition]

First, the composition of the components of the steel sheet of the present invention and the reason for its limitation will be described. In addition, "%" indicating a component composition shall mean "mass %" unless otherwise specified.

C: 0.20% or more and 0.70% or less

C is an effective and inexpensive austenite stabilizing element for high strength, and is an important element for obtaining an austenite structure. In order to obtain the effect, C needs to contain 0.20% or more. On the other hand, when it contains more than 0.70%, it is segregated at the center of the plate thickness and promotes excessive precipitation of Cr carbides and Nb, V, Ti-based carbides, so that low-temperature toughness and cross-sectional shrinkage value decrease. For this reason, C is made into 0.20 % or more and 0.70 % or less. Preferably it is 0.25% or more and 0.60% or less.

Si: 0.05% or more and 1.0% or less

Si acts as a deoxidizer and is not only necessary for steelmaking, but also has an effect of increasing the strength of the steel sheet by solid solution strengthening in the steel. In order to obtain such an effect, Si needs to contain 0.05% or more. On the other hand, when it contains exceeding 1.0 %, weldability and surface properties will deteriorate. For this reason, Si is made into 0.05 % or more and 1.0 % or less. Preferably it is 0.07% or more and 0.5% or less.

Mn: 15% or more and 35% or less

Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and cryogenic toughness. In order to obtain the effect, Mn needs to contain 15% or more. On the other hand, when it contains more than 35%, the effect of improving cryogenic toughness is saturated, resulting in an increase in alloy cost. Moreover, weldability and cutability deteriorate. Furthermore, segregation is encouraged, resulting in deterioration of cryogenic toughness, deterioration of tensile properties in the thickness direction, and occurrence of stress corrosion cracking. For this reason, Mn is 15% or more and 35% or less. It is preferably in the range of 18% or more and 28% or less.

Al: 0.1% or less

Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of steel sheet. In addition, it has an effect of fixing solid solution N in steel to form AlN and suppressing toughness deterioration due to reduction of solid solution N. For that purpose, it is preferable to contain it at 0.01% or more. On the other hand, when Al is contained in excess of 0.1%, it diffuses into the weld metal portion during welding and deteriorates the toughness of the weld metal. Therefore, the content is set to 0.1% or less. Preferably it is 0.07% or less. More preferably, it is 0.02% or more and 0.06% or less.

Cr: 8.0% or less

Cr is an element necessary for improving low-temperature toughness and corrosion resistance of high-Mn steel. On the other hand, Cr may precipitate in the form of nitrides, carbides, carbonitrides, etc. during rolling, and the formation of such precipitates serves as a starting point for corrosion or fracture and lowers low-temperature toughness. Therefore, the upper limit is set to 8.0%. The Cr amount is preferably within a range of 1.0% or more and 6.0% or less, more preferably 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 cryogenic toughness. In addition, it bonds with Nb, V and Ti, precipitates finely as a nitride or carbonitride, and has an effect of suppressing stress corrosion cracking as a trap site for diffusible hydrogen. In order to obtain such an effect, it is necessary to contain N at 0.0010% or more. On the other hand, when it contains more than 0.0500%, the formation of excessive nitrides or carbonitrides is promoted, and the amount of dissolved elements decreases, not only corrosion resistance decreases, but also toughness. For this reason, N is 0.0010% or more and 0.0500% or less. Preferably, the amount of N is 0.0020% or more and 0.0200% or less.

P: 0.03% or less

When P is contained in an amount exceeding 0.03%, it may segregate at grain boundaries to reduce grain boundary strength and become a starting point of fracture. Therefore, it is preferable to make 0.03% the upper limit and reduce it as much as possible. Therefore, P is 0.03% or less. Since characteristics improve as the amount of P decreases, it is preferably 0.025% or less, and more preferably 0.020% or less. In addition, since a great steelmaking cost is required to suppress it to less than 0.0005%, it is preferable to set it as 0.0005% or more from an economic point of view.

S: 0.005% or less

Since S forms MnS in steel and remarkably deteriorates low-temperature toughness and cross-sectional shrinkage during stretching in the sheet thickness direction, it is preferable to make 0.005% the upper limit and reduce it as much as possible. Preferably it is 0.002% or less. In addition, since a great steelmaking cost is required to suppress it to less than 0.0001%, it is preferable to set it as 0.0001% or more from an economic point of view.

The balance other than the above components is Fe and unavoidable impurities. Inevitable impurities include Zr and As.

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

Nb: 0.003% or more and 0.030% or less

Nb is an element effective in improving the strength of a steel sheet. In order to obtain such an effect, it is preferable to add 0.003% or more of Nb. On the other hand, when the content exceeds 0.030%, coarse carbonitrides are precipitated, which may become a starting point of fracture and deteriorate tensile properties in the sheet thickness direction. In addition, precipitates may coarsen and deteriorate base metal toughness. For this reason, when it contains Nb, it is preferable to set it as 0.003 % or more and 0.030 % or less. More preferably, it is 0.005% or more, and furthermore, 0.007% or more. Similarly, it is more preferably 0.025% or less, and more preferably 0.022% or less.

V: 0.01% or more and 0.10% or less

V is an element effective in improving the strength of a steel sheet. In order to obtain such an effect, it is preferable to contain V at 0.01% or more. On the other hand, when it contains more than 0.10%, coarse carbonitrides may precipitate and become the starting point of destruction. In addition, precipitates may coarsen and deteriorate base metal toughness. For this reason, when containing V, it is preferable to set it as 0.01 % or more and 0.10 % or less. More preferably, it is 0.02% or more, and furthermore, it is 0.03% or more. Similarly, it is more preferably 0.09% or less, and more preferably 0.08% or less.

Ti: 0.003% or more and 0.040% or less

Ti precipitates as a nitride or carbonitride and is an effective element for improving the strength of a steel sheet. In order to obtain such an effect, it is preferable to contain Ti at 0.003% or more. On the other hand, when it contains more than 0.040%, precipitates may coarsen and deteriorate base metal toughness. Coarse carbonitrides may also precipitate and become the starting point of destruction. For this reason, when it contains Ti, it is preferable to set it as 0.003 % or more and 0.040 % or less. More preferably, it is 0.005% or more, and furthermore, 0.007% or more. Similarly, it is more preferably 0.035% or less, and more preferably 0.032% or less.

B: 0.0003% or more and 0.0100% or less

B is an element that increases austenite grain boundary strength and is an element effective for improving cryogenic toughness. In order to obtain such an effect, it is preferable to contain B at 0.0003% or more. On the other hand, when it contains exceeding 0.0100%, coarse B precipitates are formed and toughness falls. For this reason, it is preferable to set B as the range of 0.0100% or less. More preferably, it is 0.0030% or less.

In addition, in this invention, the following elements can be contained as needed.

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, W: 0.05% or more 2.0% or less 1 type or 2 or more types

Cu, Ni, Sn, Sb, Mo, and W are elements that improve the corrosion resistance of high Mn steel by complex addition with Cr.

This effect is manifested in the case where any of the above elements coexists with Cr in high Mn steel, and is manifested above the lower limit, respectively. However, when any element is contained in excess of the above upper limit, weldability and toughness are deteriorated, which is also disadvantageous from the viewpoint of cost.

Therefore, Cu, Ni, Sn, Sb, Mo and W are preferably added within the above ranges. More preferably, the amount of Cu is 0.02% or more and 0.50% or less, the amount of Ni is 0.02% or more and 0.40% or less, the amount of Sn is 0.02% or more and 0.25% or less, the amount of Sb is 0.02% or more and 0.25% or less, and the amount of Mo is 0.05% or more. 1.50% or less, and the amount of W is 0.05% or more and 1.50% or less.

Furthermore, 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, one or more of these

Ca, Mg, and REM are elements useful for morphological control of inclusions such as MnS, and may be contained as necessary. Here, controlling the shape of inclusions refers to making stretched sulfide-based inclusions into granular inclusions. Through control of the shape of this inclusion, it is possible to improve tensile properties in the plate thickness direction, toughness, and sulfide stress corrosion cracking resistance. In order to obtain such an effect, it is preferable to contain 0.0005% or more of Ca and Mg, and 0.0010% or more of REM.

On the other hand, when a large amount of any element is contained, the amount of non-metallic inclusions increases, and the characteristics of the central portion of the plate thickness may rather deteriorate. For this reason, when containing Ca, it is 0.0050% or less, when containing Mg, 0.0100% or less, and when containing REM, it is preferable to set it as 0.0200% or less. More preferably, the amount of Ca is 0.0010% or more and 0.0040% or less, the amount of Mg is 0.0010% or more and 0.0040% or less, and the amount of REM is 0.0020% or more and 0.0150% or less.

It is further important that the steel sheet having the above component composition has a cross-sectional shrinkage value of 30% or more in the sheet thickness direction. That is, when the cross-sectional shrinkage value in the plate thickness direction is less than 30%, for example, fracture occurs at the cross-welded joint, and the integrity of the structure is significantly impaired.

Next, the manufacturing conditions of the steel plate of this invention are demonstrated. That is, in the steel sheet of the present invention, after heating the steel material having the above component composition to 1000 ° C. or more and 1300 ° C. or less, hot rolling is performed, the reduction ratio: 3 or more, and the reduction ratio of at least 2 passes among the final 3 passes per pass. It can be manufactured by carrying out at 10% or more.

In the following description, the temperature "°C" means the temperature at the center of the plate thickness.

[Heating temperature of steel material: 1000 ° C or more and 1300 ° C or less]

Heating the steel material to 1000 ° C or higher is to dissolve the precipitates in the structure and to uniformize the crystal grain size, etc., and the heating temperature is set to 1000 ° C or more and 1300 ° C or less. When the heating temperature is less than 900°C, the desired properties are not obtained because the precipitates are not sufficiently dissolved. In addition, since heating at 1300°C or higher requires excessive energy and reduces productivity due to material deterioration due to coarsening of the crystal grain size, the upper limit of the heating temperature is set to 1300°C. It is preferably 1050°C or more and 1250°C or less, more preferably 1100°C or more and 1250°C or less.

In addition, as a steel material, in addition to a continuous casting slab, a material manufactured by a conventional method such as an ingot slab or a bloom may be used.

[Reduction ratio in hot rolling: 3 or more]

If the reduction ratio in hot rolling is less than 3, it becomes difficult to suppress the decrease in tensile properties in the sheet thickness direction due to compression of casting defects. In addition, it is not sufficient to promote recrystallization by rolling to achieve size, and as a result of coarse austenite grains remaining, properties such as strength and toughness deteriorate, so the reduction ratio is limited to 3 or more. Preferably, the reduction ratio is 4 or more, more preferably the reduction ratio is 5 or more. The upper limit of the reduction ratio is not particularly limited, but is preferably 50 or less. This is because, when the reduction ratio exceeds 50, the anisotropy of the mechanical properties becomes remarkably large.

Here, the reduction ratio in hot rolling is defined as the plate thickness of the raw material to be rolled/the plate thickness of the steel plate after rolling.

[The reduction ratio of at least two of the last three passes is 10% or more per pass]

Among the final three passes that finally determine the material of the steel sheet, for at least two passes, by setting the reduction ratio per pass to 10% or more, first, casting defects are reliably harmless, and the overall steel sheet is sized. The survival of abnormal coarse grains can be suppressed and the cross-sectional shrinkage value in the tensile test in the sheet thickness direction is improved. As a result, the cross-sectional shrinkage value of 30% or more can be secured.

That is, regulating the reduction ratio of at least two passes among the final three passes is to reliably compress casting defects. Therefore, it is preferable to set the reduction ratio of all the passes of the final three passes to 10% or more. On the other hand, if the reduction ratio of at least two of the final three passes is less than 10%, casting defects remain and the cross-sectional shrinkage value at the central portion of the plate thickness decreases. The upper limit of the reduction ratio does not need to be particularly set, but it is preferably set to 30% because there are equipment restrictions such as rolling load and the like.

[Cooling after hot rolling]

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

Example

Steels No. 1 to 26 shown in Table 1 were smelted and made into slabs, and then steel plates with a plate thickness of 30 to 50 mm were obtained under the manufacturing conditions shown in Table 2. The steel sheets of sample Nos. 1 to 30 obtained in this way were subjected to a tensile test shown below. The results of this tensile test are also listed in Table 2.

The cross-sectional shrinkage value by the tensile test in the plate thickness direction was evaluated based on JIS G3199. As for the shape of the test piece, a Type A test piece was used. In addition, the tensile strength was measured using a round bar tensile test piece taken from the steel sheet surface at a depth of 1/4 of the plate thickness (hereinafter referred to as the 1/4 t portion), using a Charpy test piece taken from the 1/4 t portion. The Charpy absorbed energy at -196°C was evaluated as an average of three values.

It was confirmed that the examples of the present invention (sample Nos. 1 to 14) conforming to the present invention satisfied the cross-sectional shrinkage of 30% or more. On the other hand, Comparative Examples (Sample Nos. 15 to 30) outside the scope of the present invention cannot satisfy the above-described target performance because any one or more of tensile strength, absorbed energy, and cross-sectional shrinkage is outside the scope of the claims of the present application.

Claims (9)

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: 0.70% or more and 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
A steel sheet containing Fe and an unavoidable impurity as the balance, having a tensile strength of 600 MPa or more, an absorbed energy at -196°C of 27 J or more, and a cross-sectional shrinkage value in the sheet thickness direction of 35% or more. According to claim 1,
The component composition is also in mass %,
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
A steel sheet containing one or two or more selected from According to claim 1,
The component composition is also in mass %,
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 and 2.0% or less
A steel sheet containing one or two or more selected from According to claim 2,
The component composition is also in mass %,
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 and 2.0% or less
A steel sheet containing one or two or more selected from According to claim 1,
The component composition is also in mass %,
Ca: 0.0005% or more and 0.0050% or less; and
REM: 0.0010% or more and 0.0200% or less
A steel sheet containing one or two selected from According to claim 2,
The component composition is also in mass %,
Ca: 0.0005% or more and 0.0050% or less; and
REM: 0.0010% or more and 0.0200% or less
A steel sheet containing one or two selected from According to claim 3,
The component composition is also in mass %,
Ca: 0.0005% or more and 0.0050% or less; and
REM: 0.0010% or more and 0.0200% or less
A steel sheet containing one or two selected from According to claim 4,
The component composition is also in mass %,
Ca: 0.0005% or more and 0.0050% or less; and
REM: 0.0010% or more and 0.0200% or less
A steel sheet containing one or two selected from A method for producing the steel sheet according to any one of claims 1 to 8, after heating the steel material to 1000 ° C. or more and 1300 ° C. or less, hot rolling is performed, the reduction ratio: 3 or more, and at least in the last 3 passes. A method for manufacturing a steel sheet in which the reduction ratio of two passes is 10% or more per pass.