JP6566166B1 - Steel sheet and manufacturing method thereof - Google Patents

Abstract

Provided is a high Mn steel plate having excellent drawing characteristics at the center of the plate thickness. 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, the composition of the remaining Fe and inevitable impurities, the tensile strength is 600 MPa or more and The absorbed energy at −196 ° C. is 27 J or more, and the drawing value in the plate thickness direction is 30% or more.

Description

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

  Structures such as tanks for liquefied gas storage tanks are used at extremely low temperatures, so in order to use hot-rolled steel sheets for these structures, not only the strength of the steel sheets but also the toughness at cryogenic temperatures is excellent. Is required. For example, it is necessary to ensure excellent toughness in a temperature range lower than −164 ° C., which is the boiling point of liquefied natural gas, in a hot-rolled steel sheet used for a liquefied natural gas storage tank. If the low-temperature toughness of the steel sheet used for the cryogenic storage tank structure is inferior, the safety as the cryogenic storage tank structure may not be maintained, so there is a strong demand for improving the low-temperature toughness of the applied steel sheet. is there.

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

  For example, in Patent Document 1, by adding Mn in an amount of 15 to 35%, Cu: 5% or less, and appropriate amounts of C and Cr, the machinability and Charpy impact characteristics at −196 ° C. of the heat and heat affected zone are described. A steel material with improved is disclosed.

  In Patent Document 2, 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 7.0 %, And Cr: 0.1% or more and less than 8.0% is added, and a high Mn steel material with improved low temperature toughness is disclosed.

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

JP-T-2015-508452 JP-A-2006-84529 Japanese Patent Laid-Open No. 2006-196703

  High Mn steel is a high alloy compared to general carbon steel, and therefore has a low melting point and a high viscosity in the vicinity of the melting point, and therefore has a feature that coarse casting defects are likely to occur compared to carbon steel. Therefore, if casting defects remain in the product, the product may break when the tensile stress acts in the thickness direction of the steel plate such as a cruciform joint, leading to the collapse of the structure.

  However, the steel materials described in Patent Documents 1, 2, and 3 are important from the viewpoint of manufacturing costs for achieving strength and low temperature toughness, and from the viewpoint of the safety of the structure when using the austenitic steel materials described above. There was no mention of the aperture characteristics in the center, and there was still room for study.

  The present invention has been made in view of the above problems, and an object thereof is to provide a high Mn steel plate having excellent drawing characteristics at the center of the plate thickness.

In order to solve the above-mentioned problems, the present inventors have conducted intensive research on the component composition and manufacturing method of steel sheet for high Mn steel, and have obtained the following knowledge.
(I) By suppressing the S content to 0.005% or less based on high Mn steel, the amount of MnS produced can be reduced, and the tensile properties in the plate thickness direction can be improved.
(Ii) Further, in hot finish rolling, the casting defect is rendered harmless by rolling at a reduction ratio of 3 or more, and at least 2 passes among the final 3 passes, the reduction rate per pass is 10% or more. The tensile properties in the plate thickness direction can be improved by reducing the size of the particles and suppressing the remaining of coarse particles.

The present invention has been made by further studying the above knowledge, and the gist thereof is as follows.
1. % By 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, the composition of the remaining Fe and unavoidable impurities, a tensile strength of 600 MPa or more and an absorbed energy at -196 ° C of 27 J or more, and a further plate A steel sheet having a drawing value in the thickness direction of 30% or more.

  Here, the aperture value in the thickness direction can be measured by a test based on JIS Z3199.

2. The component composition is further mass%,
Nb: 0.003% to 0.030%,
V: 0.01% or more and 0.10% or less,
The steel plate according to 1 above, containing one or more selected from Ti: 0.003% to 0.040% and B: 0.0003% to 0.0100%.

3. The component composition is further 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,
The steel plate according to 1 or 2 above, containing one or more selected from Mo: 0.05% to 2.0% and W: 0.05% to 2.0%.

4). The component composition is further mass%,
Ca: 0.0005% or more and 0.0050% or less,
The steel plate according to 1, 2 or 3, which contains one or more selected from Mg: 0.0005% to 0.0100% and REM: 0.0010% to 0.0200%.

5. 5. The method for producing a steel plate according to any one of 1 to 4, wherein after the steel material is heated to 1000 ° C. or higher and 1300 ° C. or lower, the hot rolling is performed at a reduction ratio of 3 or more and the final three passes. A method for producing a steel sheet, wherein the rolling reduction of at least two passes is 10% or more per pass.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate which is excellent in the drawing characteristic of plate | board thickness center part can be provided. If the steel plate of the present invention is applied to a steel structure used in a cryogenic environment, such as a tank for a liquefied gas storage tank, it greatly contributes to improving the safety of the structure and brings about a remarkable industrial effect. It will be. Moreover, since it is cheaper than existing materials, there is also an advantage that it is excellent in economic efficiency.

Hereinafter, the steel plate of the present invention will be specifically described. In addition, this invention is not limited to the following embodiment.
[Ingredient composition]
First, the component composition of the steel plate of this invention and the reason for limitation are demonstrated. “%” Representing the component composition means “% by mass” unless otherwise specified.

C: 0.20% or more and 0.70% or less C is effective for increasing the strength, is an inexpensive austenite stabilizing element, and is an important element for obtaining an austenite structure. In order to acquire the effect, C needs to contain 0.20% or more. On the other hand, if it exceeds 0.70%, it segregates at the center of the plate thickness and promotes excessive precipitation of Cr carbide, Nb, V, and Ti-based carbides, so that low temperature toughness is lowered and drawing value is lowered. . For this reason, C is made 0.20% or more and 0.70% or less. Preferably, the content is 0.25% or more and 0.60% or less.

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

Mn: 15% to 35% 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 acquire the effect, Mn needs to contain 15% or more. On the other hand, when the content exceeds 35%, the effect of improving the cryogenic toughness is saturated, leading to an increase in alloy cost. In addition, the weldability and cutability are deteriorated. Furthermore, segregation is promoted, causing a reduction in cryogenic toughness, deterioration in tensile properties in the thickness direction, and occurrence of stress corrosion cracking. For this reason, Mn shall be 15% or more and 35% or less. Preferably, the range is 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 sheets. Moreover, it has the effect of fixing solid solution N in steel and forming AlN to suppress toughness deterioration due to reduction of solid solution N. For that purpose, it is preferable to contain 0.01% or more. On the other hand, if the Al content exceeds 0.1%, it diffuses into the weld metal part during welding and deteriorates the toughness of the weld metal, so the content is made 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 lowers the low temperature toughness as a starting point of corrosion and fracture. Is set to 8.0%. Preferably, the Cr amount is in the range of 1.0% to 6.0%, more preferably 1.5% to 5.5%.

N: 0.0010% or more and 0.0500% or less N is an austenite stabilizing element and is an element effective for improving the cryogenic toughness. Moreover, it combines with Nb, V, and Ti, precipitates finely as nitride or carbonitride, and has the effect of suppressing stress corrosion cracking as a diffusible hydrogen trap site. In order to acquire such an effect, it is necessary to contain N at 0.0010% or more. On the other hand, if the content exceeds 0.0500%, the formation of excess nitride or carbonitride is promoted, the amount of solid solution elements is reduced and corrosion resistance is lowered, and the toughness is also lowered. For this reason, N is made into 0.0010% or more and 0.0500% or less. Preferably, the N amount 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 the grain boundary to lower the grain boundary strength, and may become a fracture starting point. Therefore, it is desirable to make 0.03% as the upper limit and reduce it as much as possible. Therefore, P is set to 0.03% or less. The smaller the P, the better the characteristics. Therefore, it is preferably 0.025% or less, more preferably 0.020% or less. In addition, in order to suppress to less than 0.0005%, since a great steelmaking cost is required, it is preferable to set it as 0.0005% or more from an economical viewpoint.

S: 0.005% or less S is desirable to reduce S as much as possible, with 0.005% as the upper limit, because M forms MnS in the steel and significantly deteriorates the low temperature toughness and drawing during tensile in the thickness direction. Preferably, it is 0.002% or less. In addition, in order to control to less than 0.0001%, since a great steelmaking cost is required, it is preferable to set it as 0.0001% or more from an economical viewpoint.

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

In the present invention, for the purpose of further improving the strength and the low temperature toughness, the following elements can be contained as required in addition to the above essential elements.
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, Nb is preferably added at 0.003% or more. On the other hand, if the content exceeds 0.030%, coarse carbonitrides precipitate, which may become a starting point of fracture and deteriorate the tensile properties in the thickness direction. Further, the precipitates may become coarse and the base material toughness may be deteriorated. 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, further 0.007% or more. Similarly, it is more preferably 0.025% or less, and further preferably 0.022% or less.

V: 0.01% or more and 0.10% or less 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 V at 0.01% or more. On the other hand, if the content exceeds 0.10%, coarse carbonitride precipitates and may become a starting point of fracture. Further, the precipitates may become coarse and the base material toughness may be deteriorated. For this reason, when it contains 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 further 0.03% or more. Similarly, it is more preferably 0.09% or less, further 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 in improving the strength of the steel sheet. In order to obtain such an effect, Ti is preferably contained at 0.003% or more. On the other hand, if the content exceeds 0.040%, the precipitates become coarse and the base material toughness may be deteriorated. In addition, coarse carbonitrides may precipitate and become the starting point of fracture. 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, further 0.007% or more. Similarly, it is more preferably 0.035% or less, and further 0.032% or less.

B: 0.0003% or more and 0.0100% or less B is an element that increases the austenite grain boundary strength, and is an element effective for improving the cryogenic toughness. In order to acquire such an effect, it is preferable to contain B at 0.0003% or more. On the other hand, if the content exceeds 0.0100%, coarse B precipitates are generated, and the toughness decreases. For this reason, B is preferably in the range of 0.0100% or less. More preferably, it is 0.0030% or less.

Furthermore, in this invention, the following elements can be contained as needed.
Cu: 0.01% to 0.70%, Ni: 0.01% to 0.50%, Sn: 0.01% to 0.30%, Sb: 0.01% to 0.30% Hereinafter, Mo: 0.05% to 2.0%, W: 0.05% to 2.0%, one or two or more

Cu, Ni, Sn, Sb, Mo, and W are elements that improve the corrosion resistance of high-Mn steel by being compounded with Cr.
This effect is manifested in high-Mn steel when any of the above elements coexists with Cr, and is manifested at the above lower limit value or more. However, if any of these elements is contained in excess of the above upper limit, weldability and toughness are deteriorated, which is disadvantageous from the viewpoint of cost.

  Therefore, Cu, Ni, Sn, Sb, Mo and W are preferably added in the above-described 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%, and the Sb content is 0. 0.02% or more and 0.25% or less, Mo amount is 0.05% or more and 1.50% or less, and W amount is 0.05% or more and 1.50% or less.

Furthermore, in this invention, the following elements can be contained as needed.
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 can be contained as necessary. Here, the form control of inclusions means that the expanded sulfide inclusions are made into granular inclusions. Through this form control of inclusions, tensile properties in the thickness direction, toughness, and resistance to sulfide stress corrosion cracking can be improved. In order to acquire 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 on the contrary, the characteristics at the center portion of the plate thickness may deteriorate. For this reason, when Ca is contained, it is preferably 0.0050% or less, when Mg is contained, 0.0100% or less, and when REM is contained, it is preferably 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%.

  It is important that the steel sheet having the above component composition has a drawing value in the thickness direction of 30% or more. That is, when the drawing value in the plate thickness direction is less than 30%, for example, the cross welded joint or the like is broken, and the soundness of the structure is significantly impaired.

Next, manufacturing conditions for the steel sheet of the present invention will be described. That is, in the steel sheet of the present invention, after the steel material having the above-described component composition is heated to 1000 ° C. or higher and 1300 ° C. or lower, hot rolling is performed at a reduction ratio of 3 or more and at least 2 passes of the final 3 passes. Manufacture can be performed by performing the rolling reduction at 10% or more per pass.
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 higher and 1300 ° C or lower]
The reason why the steel material is heated to 1000 ° C. or higher is to dissolve precipitates in the structure and make the crystal grain size uniform, and the heating temperature is 1000 ° C. to 1300 ° C. When the heating temperature is less than 900 ° C., the desired properties cannot be obtained because the precipitate is not sufficiently dissolved. In addition, heating at 1300 ° C. or higher requires excessive energy in addition to material deterioration due to the coarsening of the crystal grain size, and the productivity is lowered. Therefore, the upper limit of the heating temperature is set to 1300 ° C. Preferably it is 1050 degreeC or more and 1250 degrees C or less, More preferably, it is the range of 1100 degreeC or more and 1250 degrees C or less.

  In addition, as a steel material, the raw material manufactured by regular methods, such as an ingot slab and a bloom other than a continuous casting slab, can be used.

[Reduction ratio in hot rolling: 3 or more]
When the rolling reduction ratio of the hot rolling is less than 3, it is difficult to suppress a decrease in tensile properties in the thickness direction due to pressure bonding of casting defects. Furthermore, recrystallization is promoted by rolling, and it becomes insufficient for grain size reduction. As a result of remaining coarse austenite grains, properties such as strength and toughness deteriorate, so the rolling ratio is limited to 3 or more. To do. Preferably, the reduction ratio is 4 or more, more preferably, the reduction ratio is 5 or more. The upper limit of the rolling ratio is not particularly limited, but is preferably 50 or less. This is because when the rolling ratio exceeds 50, the anisotropy of the mechanical characteristics is remarkably increased.

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

[The reduction ratio of at least two of the last three passes is 10% or more per pass]
Of at least two passes among the final three passes that ultimately determine the material of the steel plate, the reduction rate per pass is set to 10% or more to first make the casting defects harmless, and further to the entire steel plate. By adjusting the grain size, it becomes possible to suppress the remaining of abnormally coarse grains, and as a result of improvement of the drawing value in the tensile test in the thickness direction, a drawing value of 30% or more can be secured.

  That is, the reason why the rolling reduction rate of at least two passes among the final three passes is regulated is to ensure that the casting defects are crimped. Therefore, it is preferable that the reduction ratios of all the last three passes be 10% or more. On the other hand, if the rolling reduction ratio of at least two passes among the final three passes is less than 10%, casting defects remain and the drawing value at the center of the plate thickness decreases. The upper limit of the rolling reduction is not particularly required, but is preferably 30% due to equipment constraints such as rolling load.

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

  No. shown in Table 1. 1 to 26 steel was melted to form a slab, and then a steel sheet having a thickness of 30 to 50 mm was prepared according to the manufacturing conditions shown in Table 2. Sample No. obtained in this way. 1 to 30 steel plates were subjected to the following tensile tests. The results of this tensile test are also shown in Table 2.

  The drawing value by the tensile test in the plate thickness direction was evaluated according to JIS G3199. As the test piece shape, a Type A test piece was used. Also, using a round bar tensile test piece taken from a depth position of 1/4 of the plate thickness from the steel sheet surface (hereinafter referred to as 1/4 t part), the tensile strength was used, and a Charpy test piece taken from the 1/4 t part was used. The Charpy absorbed energy at −196 ° C. was evaluated as an average value of three.

  It was confirmed that the examples of the present invention (sample Nos. 1 to 14) consistent with the present invention satisfied the aperture of 30% or more. On the other hand, in Comparative Examples (Sample Nos. 15 to 30) that are out of the scope of the present invention, any one or more of tensile strength, absorbed energy, and drawing are outside the scope of claims of the present application, and satisfy the above target performance. Can not.

Claims (5)

% By 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% to 8.0%,
N: 0.0010% or more and 0.0500% or less,
P: 0.03% or less and S: 0.005% or less, having a component composition of the balance Fe and inevitable impurities, a tensile strength of 600 MPa or more, and an absorbed energy at −196 ° C. of 27 J or more, and further a plate A steel sheet having a drawing value in the thickness direction of 30% or more. The component composition is further mass%,
Nb: 0.003% to 0.030%,
V: 0.01% or more and 0.10% or less,
The steel plate according to claim 1, containing one or more selected from Ti: 0.003% to 0.040% and B: 0.0003% to 0.0100%. The component composition is further 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,
The steel plate according to claim 1 or 2, containing one or more selected from Mo: 0.05% to 2.0% and W: 0.05% to 2.0%. The component composition is further mass%,
Ca: 0.0005% or more and 0.0050% or less,
The steel plate according to claim 1, 2 or 3, which contains one or more selected from Mg: 0.0005% to 0.0100% and REM: 0.0010% to 0.0200%.   A method for producing a steel sheet according to any one of claims 1 to 4, wherein the steel material is heated to 1000 ° C or higher and 1300 ° C or lower, and then hot rolling is performed at a reduction ratio of 3 or more and a final 3 passes. A method for producing a steel sheet, wherein the rolling reduction of at least two passes is 10% or more per pass.