JP5845837B2 - High-strength thin steel sheet with excellent rigidity and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength thin steel sheet excellent in rigidity suitable for use mainly in automobile bodies and a method for producing the same. The high-strength thin steel sheet according to the present invention is suitable for application to structural members having a column thickness sensitivity index close to 1, such as a center pillar, a side sill, a side frame and a cross member of an automobile, or a cross-sectional shape close to that. It has a tensile strength of 1180 MPa or more and is excellent in ductility.

  In recent years, in response to growing interest in global environmental issues, the reduction of vehicle body weight in automobiles is an extremely important issue, such as regulations on exhaust gas in automobiles. Therefore, the weight of the vehicle body has been reduced by reducing the plate thickness by increasing the strength of the steel plate, but recently, as a result of the remarkable progress in increasing the strength of the steel plate, the plate thickness is less than 1.6 mm. The use of new steel plates is increasing, and some parts use steel plates with a tensile strength of 1180 MPa or more. Improvement is becoming essential. The problem of reduced part rigidity due to the thinning of the steel sheet has become apparent with steel sheets having a tensile strength of 590 MPa or more.

In general, to increase the rigidity of a part, change the shape of the part, or, for parts that have been spot welded, change the welding conditions such as increasing the number of welding points or switching to laser welding. Is valid.
However, when used as an automobile part, it is not easy to change the part shape in a limited space in the automobile, and there is a problem that the cost increases even if the welding conditions are changed.

Therefore, in order to increase the rigidity of the component without changing the component shape and welding conditions, it is effective to increase the Young's modulus of the member used for the component.
The Young's modulus is strongly governed by the texture, and in the case of steel with a body-centered cubic lattice, the <111> direction, which is the closest dense direction of atoms, is the highest, and conversely, the <100> direction, where the atomic density is low, is the smallest. It is known. It is well known that the Young's modulus of ordinary iron with a small anisotropy in the crystal orientation is about 210 GPa, but if the anisotropy is given to the crystal orientation and the atomic density in a specific direction can be increased. The Young's modulus in that direction can be increased.

Conventionally, regarding the Young's modulus of a steel sheet, various studies have been made to increase the Young's modulus in a specific direction by controlling the texture.
For example, Patent Document 1 uses a steel in which Nb or Ti is added to an extremely low carbon steel, and the rolling reduction in the temperature range of Ar ₃ to (Ar ₃ + 150 ° C.) is 85% or more in the hot rolling process. By promoting the ferrite transformation from unrecrystallized austenite, {311} <011> and {332} <113> orientation ferrite is developed in the hot-rolled sheet stage, and then cold rolling, A technique for increasing the Young's modulus in the direction perpendicular to the rolling direction by crystal annealing with {211} <011> as the main orientation is disclosed.

Patent Document 2 discloses that Nb, Mo, and B are added to low carbon steel having a C content of 0.02 to 0.15%, and the rolling reduction in the temperature range of Ar _{3 to} 950 ° C. is 50% or more. A method for producing a hot-rolled steel sheet with an increased Young's modulus by developing the {211} <011> orientation is disclosed.

  Further, Patent Documents 3 and 4 use a steel obtained by adding Nb to low carbon steel, specify the amount of C that is not fixed as carbonitride, and reduce the total reduction amount at 950 ° C. or lower in the hot rolling process by 30%. As described above, by promoting the ferrite transformation from the unrecrystallized austenite, the ferrite of {113} <110> orientation is developed at the hot-rolled sheet stage, and then {112} <110 by the cold rolling and recrystallization annealing. A technique for increasing the Young's modulus in the direction perpendicular to the rolling direction with> as the main orientation is disclosed.

Japanese Patent Laid-Open No. 5-255804 JP-A-8-311541 JP 2006-183131 A JP 2005-314792 A

However, the above-described prior art has the following problems.
That is, although the technique disclosed in Patent Document 1 uses an ultra-low carbon steel having a C content of 0.01% or less and controls the texture, the Young's modulus of the steel sheet is increased, but the resulting tensile strength is 450 at most. However, there was a limit to further increase the strength by applying this technology.

  The technique disclosed in Patent Document 2 is a hot-rolled steel sheet, so that it is not possible to use texture control by cold working, and it is difficult to achieve higher Young's modulus, There is a problem that it is difficult to stably produce a high-strength steel sheet having a thickness of less than 2.0 mm by low-temperature finish rolling.

The technique disclosed in Patent Document 3 increases the tensile strength by increasing the alloy addition amount and increasing the martensite fraction, but the total elongation is low, and the strength-elongation balance (TS × El ) Also decreases, it was difficult to improve the workability along with the increase in strength.
In addition, the techniques disclosed in Patent Documents 3 and 4 increase the Young's modulus by setting the total reduction amount at 950 ° C. or lower to 30% or higher in the hot rolling process. There was a problem that it was difficult to ensure a total reduction amount of 30% or more due to the high load.

  As described above, the conventional technology has a high Young's modulus for hot-rolled steel sheets and soft steel sheets with a large thickness, those with a strength level up to 980 MPa, and ductility even for high-strength materials. As a result, it is difficult to manufacture and has a high Young's modulus while maintaining high ductility in high-strength steel sheets with a thickness of 1.6 mm or less and TS of 1180 MPa or more. It was difficult to do.

  The present invention solves the above problems, and is advantageous in that it has a high strength thin steel sheet having a high tensile strength in the direction perpendicular to the rolling of 1180 MPa or more and an excellent rigidity satisfying a Young's modulus in the direction perpendicular to the rolling of 230 GPa or more. It aims at proposing together with various manufacturing methods.

Now, the Young's modulus of steel depends greatly on the texture, and in the case of plain steel with a body-centered cubic lattice, it is high in the <111> direction, which is the close-packed direction of atoms, and conversely, the atomic density is small <100> Since the (112) [1-10] orientation is developed, the <111> direction is aligned in the direction perpendicular to the rolling direction of the steel sheet, so that the Young's modulus in this direction can be increased.
In addition, there are various methods for strengthening steel. For example, DP steel obtained by strengthening a soft ferrite phase with a hard martensite phase is generally known to have good ductility. In ultra-high strength steel, the volume ratio of the martensite phase tends to increase as a whole, which not only reduces ductility but is also effective in increasing the Young's modulus in the direction perpendicular to the rolling (112) [1-10] orientation It was difficult to develop.

Therefore, in order to solve the above problems, the inventors examined the Young's modulus in the direction perpendicular to the rolling direction in the high-strength thin steel sheet having a TS of 1180 MPa or more.Solution strengthening, refinement strengthening, precipitation By using strengthening, it becomes possible to keep the volume ratio of martensite low even at an ultra-high strength of TS of 1180 MPa or higher, and by increasing the integration of ferrite into (112) [1-10] It was found that both high strength and high rigidity can be achieved while maintaining high ductility.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.12 to 0.20%, Si: 0.5 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, and Ti : Containing 0.02 to 0.20% and satisfying the relationship shown in the following formulas (1) and (2), the balance has a composition consisting of Fe and inevitable impurities,
By area ratio, ferrite phase: 50% or more, martensite phase: 35-45%, and the total of ferrite phase and martensite phase is 95% or more, and the average grain size of ferrite is 4.0 μm or less. It has a structure with an average particle size of 3.0 μm or less,
Tensile strength (TS) in the direction perpendicular to rolling is 1180 MPa or more, Young's modulus is 230 GPa or more, and the strength-elongation balance (TS × EL) expressed by the product of tensile strength (TS) and total elongation (EL) is High-strength thin steel sheet with excellent rigidity, characterized by 15000 MPa ·% or more.
Record
0.11 ≦ [% C] − (12 / 47.9) × [% Ti ^* ] ≦ 0.15 --- (1)
Here, Ti ^* = [% Ti] − (47.9 / 14) × [% N] − (47.9 / 332.1) × [% S] --- (2)
[% M] is the content of M element (mass%)

2. In addition to the above composition, the steel sheet further contains, in mass%, Nb: 0.02 to 0.10%, and satisfies the relationship of the following formula (3) instead of the formula (1): 1. A high-strength thin steel sheet having excellent rigidity as described in 1.
Record
0.11 ≦ [% C] − (12 / 92.9) × [% Nb] − (12 / 47.9) × [% Ti ^* ] ≦ 0.15 --- (3)
Here, [% M] is the content of M element (mass%)

3. In addition to the above composition, the steel sheet further comprises, in mass%, Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Cu: 0.1 to 2.0%, and B: 0.0005 to 0.0030%. 3. The high-strength thin steel sheet having excellent rigidity according to 1 or 2 above, comprising one or more selected from among them.

4). In mass%, C: 0.12 to 0.20%, Si: 0.5 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, and Ti : 0.02 to 0.20% steel, and the contents of C, N, S and Ti satisfy the relationship shown in the following formulas (1) and (2), with the balance being the composition of Fe and inevitable impurities In the hot rolling process, after the finish rolling is finished at 850 to 950 ° C., the material is wound at 650 ° C. or lower, pickled, and then cold rolled at a reduction rate of 60% or higher, and then in the annealing process. , (Ac ₁ -100 ° C.) to Ac ₁ average heating rate: after heating to a soaking temperature of 780 to 880 ° C. at a rate of 15 ° C./s or more and holding at that soaking time for 150 s or less By cooling to 350 ° C. or lower with an average cooling rate of at least 350 ° C. as 5 to 50 ° C./s ,
By area ratio, ferrite phase: 50% or more, martensite phase: 35-45%, and the total of ferrite phase and martensite phase is 95% or more, and the average grain size of ferrite is 4.0 μm or less. It has a structure with an average grain size of 3.0μm or less, tensile strength (TS) in the direction perpendicular to rolling is 1180 MPa or more, Young's modulus is 230 GPa or more, and tensile strength (TS) and total elongation (EL) A method for producing a high strength thin steel sheet having excellent rigidity, characterized in that the strength-elongation balance (TS × EL) expressed by the product is a thin steel sheet having a strength of 15000 MPa ·% or more .
Record
0.11 ≦ [% C] − (12 / 47.9) × [% Ti *] ≦ 0.15 --- (1)
Here, Ti * = [% Ti] − (47.9 / 14) × [% N] − (47.9 / 332.1) × [% S] --- (2)
[% M] is the content of M element (mass%)

5. In addition to the composition, the steel material further contains Nb: 0.02 to 0.10% by mass%, and satisfies the relationship of the following formula (3) instead of the formula (1). 5. A method for producing a high-strength thin steel sheet having excellent rigidity as described in 4 above.
Record
0.11 ≦ [% C] − (12 / 92.9) × [% Nb] − (12 / 47.9) × [% Ti ^* ] ≦ 0.15 --- (3)

6). In addition to the above composition, the steel material further includes, in mass%, Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Cu: 0.1 to 2.0%, and B: 0.0005 to 0.0030% The method for producing a high-strength thin steel sheet having excellent rigidity as described in 4 or 5 above, comprising one or more selected from among the above.

  According to the present invention, it is possible to obtain a high-strength steel sheet having a tensile strength of 1180 MPa or more, a Young's modulus in the direction perpendicular to the rolling of 230 GPa or more, more preferably 235 GPa or more, and satisfying TS × El of 15000 MPa ·% or more. it can.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel sheet is limited to the above range in the present invention will be described. In addition, although the unit of element content in the component composition of the steel sheet is “mass%”, hereinafter, it is simply indicated by “%” unless otherwise specified.

C: 0.12-0.20%
C is an element that stabilizes austenite, and in the cooling process during annealing after cold rolling, it enhances hardenability and greatly promotes the generation of low-temperature transformation phase, which can greatly contribute to high strength. it can. Here, in order to obtain a high strength of 1180 MPa or more, the C content needs to be 0.12% or more. On the other hand, if the C content is more than 0.20%, the fraction of the hard low-temperature transformation phase becomes large, and not only the steel is extremely strengthened but also the workability is deteriorated. In addition, the inclusion of a large amount of C suppresses recrystallization in an orientation advantageous for increasing the Young's modulus in the annealing process after cold rolling. Furthermore, the inclusion of a large amount of C causes deterioration of weldability. For this reason, C content needs to be 0.20% or less. Preferably it is 0.18% or less. More preferably, it is 0.16% or less.

Si: 0.5-1.5%
Si is one of the important elements in the present invention. Since Si raises the Ar ₃ transformation point in hot rolling, recrystallization of processed austenite is promoted when rolling immediately above Ar ₃ , so that a large amount of Si exceeding 1.5% is contained. In this case, it becomes impossible to obtain a crystal orientation necessary for increasing the Young's modulus. In addition, the addition of a large amount of Si not only deteriorates the weldability of the steel sheet, but also promotes the formation of firelite on the slab surface during heating in the hot rolling process, and promotes the generation of a so-called red scale surface pattern. To do. Furthermore, when used as a cold-rolled steel sheet, the Si oxide formed on the surface deteriorates the chemical conversion property, and when used as a hot-dip galvanized steel sheet, the Si oxide generated on the surface is not present. Induces plating. For this reason, Si content needs to be 1.5% or less. In the case of a steel sheet or hot dip galvanized steel sheet that requires surface properties, the Si content is preferably 1.2% or less.
On the other hand, Si is an element that stabilizes ferrite. In the cooling process after soaking in the two-phase region in the annealing process after cold rolling, the ferrite transformation is promoted and C is concentrated in the austenite. Austenite can be stabilized and the generation of a low temperature transformation phase can be promoted. Furthermore, Si can increase the strength of steel by solid solution strengthening. In order to obtain such an effect, the Si content needs to be 0.5% or more. Preferably it is 0.7% or more.

Mn: 1.0-3.0%
Mn is also one of the important elements in the present invention. Mn is an austenite stabilizing element that lowers the Ac ₁ transformation point in the heating process in the annealing process after cold rolling, promotes the austenite transformation from unrecrystallized ferrite, and forms in the cooling process after soaking. With respect to the orientation of the low-temperature transformation phase, an orientation that is advantageous for improving the Young's modulus can be developed, and a decrease in Young's modulus associated with the generation of the low-temperature transformation phase can be suppressed.
In addition, Mn can greatly contribute to high strength by enhancing the hardenability and greatly promoting the generation of the low temperature transformation phase in the cooling process after soaking in the annealing process. Furthermore, by acting as a solid solution strengthening element, it contributes to high strength of steel. In order to obtain such an effect, the Mn content needs to be 1.0% or more.
On the other hand, the inclusion of a large amount of Mn exceeding 3.0% remarkably suppresses the formation of ferrite during cooling after annealing, and the inclusion of a large amount of Mn also deteriorates the weldability of the steel sheet. Therefore, the Mn content is 3.0% or less, more preferably 2.5% or less.

P: 0.05% or less P not only segregates at the grain boundaries to lower the ductility and toughness of the steel sheet, but also deteriorates the weldability. Moreover, when using it as an alloying hot-dip galvanized steel plate, the problem which delays alloying speed arises. Therefore, the P content is set to 0.05% or less.

S: 0.01% or less S significantly reduces the ductility in hot rolling, induces hot cracking, and significantly deteriorates the surface properties. In addition, S is desirably reduced as much as possible because it forms coarse MnS as an impurity element, thereby reducing ductility and hole expansibility. Since these problems become significant when the S content exceeds 0.01%, the S content is set to 0.01% or less. In addition, from the viewpoint of particularly improving the hole expansibility, the S amount is preferably 0.005% or less.

Al: 0.5% or less
Al is a ferrite stabilizing element, and greatly increases the Ac ₃ point during annealing. Therefore, by suppressing the austenite transformation from unrecrystallized ferrite, when ferrite forms from austenite during cooling, Young This will impede the development of a favorable rate. For this reason, the Al content is set to 0.5% or less. Preferably it is 0.1% or less. On the other hand, since Al is useful as a deoxidizing element for steel, the Al content is preferably 0.01% or more.

N: 0.01% or less When N is contained in a large amount, there is a risk of surface flaws accompanied by slab cracking during hot rolling. Therefore, the N amount needs to be 0.01% or less.

Ti: 0.02-0.20%
Ti is the most important element in the present invention. That is, Ti promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite in the heating process in the annealing process, and improves Young's modulus with respect to ferrite generated in the cooling process after annealing. It is possible to develop a dominant orientation. Ti fine precipitates contribute to an increase in strength, and also have an advantageous effect on the refinement of ferrite and martensite. In order to obtain such an effect, the Ti content needs to be 0.02% or more. Preferably it is 0.04% or more.
On the other hand, even if a large amount of Ti is added, the carbonitride cannot be completely dissolved at the time of reheating in the normal hot rolling process, and coarse carbonitride remains. And the recrystallization suppression effect is inhibited. In addition, when the hot rolling is started after continuous casting without going through the process of reheating after once cooling the slab from continuous casting, the effect of increasing the strength when the added amount of Ti exceeds 0.20% Further, the contribution to the recrystallization suppressing effect is small, and the alloy cost is also increased. Therefore, the Ti content needs to be 0.20% or less.

The basic composition of the present invention has been described above. However, in the present invention, it is not sufficient to simply satisfy the above basic composition. The contents of C, N, S and Ti are expressed by the following formulas (1) and (2 It is necessary to satisfy the relationship shown in the formula.
Record
0.11 ≦ [% C] − (12 / 47.9) × [% Ti ^* ] ≦ 0.15 --- (1)
Here, Ti ^* = [% Ti] − (47.9 / 14) × [% N] − (47.9 / 332.1) × [% S] --- (2)
[% M] is the content of M element (mass%)
The above relational expression regulates the amount of C that is not fixed as carbide, but if this amount of C exceeds 0.15%, the martensite fraction increases and the Young's modulus decreases. And ductility is also reduced. Therefore, the amount of C that is not fixed as a carbide calculated by the formula (1) needs to be 0.15% or less. On the other hand, if the amount of C not fixed as carbide is less than 0.11%, the amount of C in austenite decreases during annealing in the two-phase region after cold rolling, and consequently the martensite phase generated after cooling decreases. It is difficult to increase the strength. For this reason, the amount of C which is not fixed as a carbide needs to be 0.11% or more. Preferably it is 0.12% or more.

Moreover, in this invention, the element described below can be contained suitably.
Nb: 0.02 to 0.10%
Nb, like Ti, is an important element in the present invention. In the heating process in the annealing process after cold rolling, by suppressing the recrystallization of the processed ferrite, the austenite transformation from the unrecrystallized ferrite is promoted, and the coarsening of the austenite grains is suppressed, and after annealing soaking With respect to the ferrite generated during the cooling process, it is possible to develop an orientation that is advantageous for improving the Young's modulus. Furthermore, the fine carbon nitride of Nb contributes effectively to an increase in strength. Furthermore, it works advantageously for miniaturization of ferrite and martensite. In order to have such an effect, the Nb content needs to be 0.02% or more.
On the other hand, even when a large amount of Nb is added, the carbonitride cannot be completely dissolved at the time of reheating in the normal hot rolling process, and coarse carbonitride remains, so in the hot rolling process The effect of suppressing recrystallization of processed austenite and the effect of suppressing recrystallization of processed ferrite in the annealing process after cold rolling cannot be obtained. In addition, even if the hot rolling is started after continuous casting without going through the process of reheating after once cooling the slab from continuous casting, the recrystallization suppression is as much as the amount of Nb added exceeds 0.10%. The contribution to the effect is small, and the alloy cost is also increased. Therefore, the Nb content needs to be 0.10% or less. Preferably it is 0.08% or less.

When Nb is contained in addition to Ti, it is necessary to satisfy the relational expression shown in the following expression (3) instead of the above expression (1).
Record
0.11 ≦ [% C] − (12 / 92.9) × [% Nb] − (12 / 47.9) × [% Ti ^* ] ≦ 0.15 --- (3)
Since Nb reduces the amount of C not fixed as carbide by forming carbide, in order to make the amount of C not fixed as carbide 0.11 to 0.15%, when Nb is added, [% C] − (12 /92.9)×[%Nb]-(12/47.9)×[%Ti ^*] it is necessary to value the of to from 0.11 to 0.15%. Preferably it is 0.11 to 0.13%.

Cr: 0.1-1.0%
Cr is an element that improves hardenability by suppressing the formation of cementite, and has the effect of greatly promoting the formation of martensite phase in the cooling process after soaking in the annealing process. In order to acquire this effect, it is preferable to contain Cr 0.1% or more. On the other hand, addition of a large amount of Cr not only saturates the effect, but also increases the alloy cost, so Cr is preferably added at 1.0% or less. Further, when used as a hot dip galvanized steel sheet, the Cr content is preferably 0.5% or less because the Cr oxide formed on the surface induces non-plating.

Ni: 0.1-1.0%
Ni is an element that enhances hardenability and can promote the formation of a martensite phase in the cooling process after soaking in the annealing process. Ni also contributes effectively to increasing the strength of steel as a solid solution strengthening element. Furthermore, in the case of Cu-added steel, surface defects are induced by cracks associated with a decrease in hot ductility during hot rolling, but the occurrence of surface defects can be suppressed by incorporating Ni together. . In order to obtain such an action, the Ni content is preferably 0.1% or more. On the other hand, addition of a large amount of Ni hinders the formation of ferrite necessary for increasing the Young's modulus in the cooling process after soaking, and increases the alloy cost. Therefore, Ni is preferably contained at 1.0% or less.

Mo: 0.1-1.0%
Mo is an element that enhances hardenability, and can contribute to high strength by promoting the formation of martensite phase in the cooling process after soaking in the annealing process. In order to obtain this effect, the Mo content is preferably 0.1% or more. On the other hand, the addition of a large amount of Mo not only saturates the effect, but also increases the alloy cost. Therefore, Mo is preferably contained at 1.0% or less. More preferably, it is 0.5% or less.

Cu: 0.1-2.0%
Cu is an element that enhances hardenability, and contributes to high strength by promoting the formation of martensite phase in the cooling process after soaking in the annealing process. In order to obtain this effect, the Cu content is preferably 0.1% or more. On the other hand, excessive Cu addition reduces hot ductility and induces surface defects accompanying cracks during hot rolling, so the Cu content is preferably 2.0% or less.

B: 0.0005-0.0030%
B is an element that enhances hardenability by suppressing transformation from austenite to ferrite, and contributes to high strength by promoting the formation of martensite in the cooling process after soaking in the annealing process. In order to obtain this effect, the B content is preferably 0.0005% or more. On the other hand, excessive addition of B significantly inhibits the formation of ferrite during cooling after soaking, and lowers the Young's modulus. Therefore, it is preferably contained at 0.0030% or less.

Next, the reason for limiting the structure of the present invention will be described.
The steel sheet of the present invention has a ferrite phase as a main phase, has a ferrite phase with an area ratio of 50% or more, and includes a martensite phase at 35 to 45%.
Since the ferrite phase is effective for the development of a texture that is advantageous for improving the Young's modulus, it is necessary to make the area ratio 50% or more. Moreover, since the strength and the strength-elongation balance are improved by including the martensite phase, it is necessary to include a martensite phase having an area ratio of 35% or more. On the other hand, if the area ratio of the martensite phase exceeds 45%, the Young's modulus in the direction perpendicular to the rolling direction cannot be secured, so the area ratio of the martensite phase needs to be 45% or less. Further, in order to improve the strength-elongation balance, the total of the area ratio of the ferrite phase and the area ratio of the martensite phase needs to be 95% or more.
Examples of phases other than the ferrite phase and the martensite phase include pearlite, bainite, and cementite. However, these phases may be included if they are 5% or less. Preferably it is 3% or less, more preferably 1% or less.

  Further, when the average particle diameter of ferrite exceeds 4.0 μm, the strength is lowered, so that an increase in the fraction of martensite phase and an increase in additive elements are required, leading to a decrease in Young's modulus and an increase in production cost. Therefore, the average grain size of ferrite needs to be 4.0 μm or less. In particular, in order to stably satisfy the tensile strength of 1180 MPa or more, the thickness is preferably 3.5 μm or less. In addition, when the average particle size of martensite exceeds 3.0 μm, the connection of voids tends to proceed when subjected to processing and deformation, and as a result, the ductility of the steel sheet decreases, so it is necessary to make it 3.0 μm or less. . More preferably, it is 2.5 μm or less

  The area ratio of the ferrite phase and martensite phase was measured by scanning electron microscope (SEM) observation after taking a nital corrosion of the cross section of the steel sheet, and taking three photos of the 25μm × 30μm region. Then, the area of the ferrite phase and the martensite phase was measured. The average particle size was determined by dividing the sum of the areas of the ferrite phase and martensite phase in the field of view from the SEM photograph by the number of the phases, and obtaining the average area, which was a value of the 1/2 power.

  With the above composition and structure, the tensile strength (TS) in the direction perpendicular to the rolling direction is 1180 MPa or more, the Young's modulus is 230 GPa or more, and the strength-elongation balance (TS × EL) is 15000 MPa ·% or more. An excellent high strength thin steel sheet can be obtained.

Next, the suitable manufacturing method of this invention steel plate is demonstrated.
In manufacturing the steel sheet of the present invention, first, steel of chemical composition according to the above-described composition is melted according to the target strength level. As a melting method, a normal converter method, an electric furnace method, or the like can be applied as appropriate. The molten steel is cast into a slab and then heated as it is or after being cooled, and is hot-rolled at a finishing temperature of 850 to 950 ° C. Next, it is wound at 650 ° C. or lower, pickled, and then cold-rolled at a rolling reduction of 60% or more. After that, in the annealing process, the temperature range from (Ac ₁ -100 ° C.) to Ac ₁ is heated at an average rate of temperature increase of 15 ° C./s or more, and a soaking temperature of 780 to 880 ° C. for 150 s or less. After being held, the average cooling rate up to at least 350 ° C. is set to 5 to 50 ° C./s and then cooled to 350 ° C. or lower.

[Finish temperature: 850-950 ° C]
By setting the finishing temperature to 950 ° C or lower, the transformation from non-recrystallized austenite to ferrite progressed, and a fine ferrite structure was obtained. Further, accumulation in the (112) [1-10] orientation was achieved by cold rolling and annealing. The degree can be increased. On the other hand, if the finishing temperature is lower than 850 ° C, the risk of falling below the Ar ₃ transformation point increases, and as a result of the processing structure mixed with the hot-rolled structure, accumulation in the (112) [1-10] orientation is hindered after cold rolling annealing. It is done. In addition, there are manufacturing difficulties such as a significant increase in rolling load due to an increase in deformation resistance. Accordingly, the finishing temperature needs to be in the range of 850 to 950 ° C.

[Winding temperature: 650 ° C or less]
When the coiling temperature after finish rolling exceeds 650 ° C., Ti and Nb carbonitrides become coarse, and in the heating stage in the annealing process after cold rolling, the effect of suppressing recrystallization of ferrite and austenite The coiling temperature is preferably 650 ° C. or lower because the effect of suppressing grain coarsening is reduced. On the other hand, when the coiling temperature is lower than 400 ° C, a lot of hard low-temperature transformation phase is generated, the deformation in the subsequent cold rolling becomes non-uniform, and the accumulation in the direction advantageous for Young's modulus is hindered. The texture after annealing does not develop and it is difficult to improve the Young's modulus. Furthermore, since the load in cold rolling after winding increases, the winding temperature is preferably 400 ° C. or higher.

[Cold rolling ratio: 60% or more]
After the winding described above, after pickling, it is subjected to cold rolling at a reduction rate of 60% or more. By this cold rolling, the (112) [1-10] orientation effective for improving the Young's modulus is accumulated. That is, by developing the (112) [1-10] orientation by cold rolling, the number of ferrite grains having the (112) [1-10] orientation is increased and the Young's modulus is increased even in the structure after the subsequent annealing process. To do. In order to obtain such an effect, the rolling rate during cold rolling needs to be 60% or more. More preferably, it is 65% or more. On the other hand, if the cold rolling rate increases, the rolling load increases and manufacturing becomes difficult, so the upper limit of the rolling rate is preferably 85%.

[Average heating rate from (Ac ₁ −100 ° C.) to Ac ₁ : 15 ° C./s or more]
In order to increase the Young's modulus of the steel sheet after annealing, recrystallization of ferrite with (112) [1-10] orientation developed by cold rolling is suppressed in the annealing process, and transformed from processed ferrite to austenite. For this purpose, a temperature increase rate of 15 ° C./s or more on average is required.
Here, Ac _1, based on the content of C, Si, Mn, Al, Ni, Cr, Cu, Mo, Ti, Nb and B, represented by mass%, Ac ₁ determined from the following equation (4) Transformation temperature.
Ac ₁ = 750.8−26.6 [% C] +17.6 [% Si] −11.6 [% Mn] −169.4 [% Al] −23.0 [% Ni] +24.1 [% Cr]
−22.9 [% Cu] +22.5 [% Mo] −5.7 [% Ti] +232.6 [% Nb] −894.7 [% B] --- (4)
Here, [% M] is the content of M element (mass%)

[Soaking temperature: 780-880 ° C, Soaking time: 150s or less]
A sufficient amount of ferrite transforms to austenite during soaking in the annealing process, and retransforms into ferrite during cooling, thereby developing a texture and improving Young's modulus. Further, when the soaking temperature is low, the rolling structure remains and the elongation decreases. For these reasons, the soaking temperature needs to be 780 ° C. or higher. On the other hand, if the soaking temperature is too high, the austenite grains become coarse, and it becomes difficult for ferrite retransformed after annealing to accumulate in the (112) [1-10] orientation. For this reason, the soaking temperature needs to be 880 ° C. or less. In addition, since the austenite grains are coarsened by holding in this temperature range for a long time, the soaking time needs to be 150 s or less.

[Average cooling rate from soaking temperature to at least 350 ° C: 5-50 ° C / s]
In the production method of the present invention, it is important to control the cooling conditions after the soaking described above. That is, a texture that is advantageous for improving the Young's modulus develops by generating ferrite during cooling after soaking. Therefore, 50% or more of ferrite is generated during this cooling. For this purpose, the upper limit of the cooling rate needs to be 50 ° C./s. On the other hand, when cooling is too slow, martensite is not generated, so the cooling rate needs to be 5 ° C./s or more. Preferably it is 10 degrees C / s or more.
When the cooling stop temperature is high, martensite is not generated, but bainite and pearlite are generated, leading to a decrease in strength and an increase in YS / TS ratio. Alternatively, even if martensite is generated, the hardness of martensite is reduced by tempering during cooling, so that not only the contribution to strength improvement is reduced, but also a good TS-El balance cannot be obtained. For this reason, it is necessary to cool at least to 350 ° C. at a predetermined cooling rate. Furthermore, in order to improve the TS-El balance, it is preferable to perform cooling at a predetermined cooling rate to at least 300 ° C.

Thereafter, a process of passing the overaging zone may be performed. Moreover, when manufacturing as a hot dip galvanized steel plate, you may let it pass in hot dip zinc, and when manufacturing as an alloying hot dip galvanized steel plate, you may perform an alloying process.
Note that temper rolling may be performed to adjust the shape of the steel sheet, and if the rolling reduction is 0.8% or less, there is no significant change in Young's modulus and tensile properties. Preferably, it is 0.6% or less.

Next, examples of the present invention will be described. In addition, this invention is not limited only to these Examples.
Example 1
First, steel A having the composition shown in Table 1 was melted in a vacuum melting furnace, hot-rolled, pickled, cold-rolled, and then annealed to produce a cold-rolled steel sheet. At that time, heating conditions prior to hot rolling: 1 hour at 1250 ° C, finishing temperature of hot rolling: 880 ° C, plate thickness after hot rolling: 4.4 mm, winding conditions: furnace after holding at 600 ° C for 1 hour winding corresponding processing of cooling, the reduction ratio of cold rolling: 68%, thickness after cold rolling: 1.4 mm, average heating rate from (Ac ₁ -100 ℃) to Ac _1: 20 ℃ / s, Soaking temperature: Holding time at 830 ° C .: 60 s, average cooling rate up to 300 ° C .: 15 ° C./s, then cooling to room temperature: air cooling was the basic condition. Table 2 shows these basic conditions.
Furthermore, among the basic conditions of the cooling rate of the reduction ratio of cold rolling, from (Ac ₁ -100 ℃) in the annealing step heating rate of up to Ac _1, the soaking temperature, until quench stop temperature and quenching stop temperature Was changed as shown in Table 3.

After the above annealing, a 10 mm x 50 mm test piece was cut out from a direction perpendicular to the rolling direction of the steel sheet, and a Young's modulus (Ec) according to the American Society to Testing Materials standard (C1259) using a transverse vibration type resonance frequency measuring device. ) Was measured. Further, a JIS No. 5 tensile test piece was cut out from a direction perpendicular to the rolling direction from a cold rolled steel sheet subjected to 0.5% temper rolling, and tensile properties (tensile strength TS and elongation El) were measured.
The area ratio (α) of the ferrite phase, the area ratio (M) of the martensite phase, and the average crystal grain size of each phase were determined by the method described above.
The obtained results are shown in Tables 2 and 3.

As shown in Table 2, the cold-rolled steel sheet (steel plate: A1) produced according to the basic conditions was TS: 1224 MPa, El: 13.2%, TS × El: 16157 MPa ·%, Ec: 239 GPa, ferrite area ratio: 63 %, Area ratio of martensite: 37%, ferrite particle size: 3.2 μm, martensite particle size: 2.3 μm, a good strength-elongation balance and a high Young's modulus.
Even when the rolling reduction and annealing conditions of cold rolling are changed, if these conditions satisfy the scope of the present invention (steel plates: A3, A4, A7), TS is 1180 MPa or more, TS × El Excellent values of 15000 MPa ·% or more and E of 230 GPa or more were obtained.

Example 2
Further, steels B to M having the components shown in Table 4 were melted in a vacuum melting furnace, and hot rolling, pickling, cold rolling and annealing were sequentially performed under the conditions shown in Table 5. Note that the thickness of each hot-rolled sheet is 4.4 mm.
The cold rolled steel sheet thus obtained was examined in the same manner as in Example 1.
The obtained results are also shown in Table 5.

As shown in Table 5, all the steel plates (steel plates: B to F, K to M) obtained according to the present invention were excellent in that TS was 1180 MPa or more, TS × El was 15000 MPa ·% or more, and Ec was 230 GPa or more. The value is obtained.
On the other hand, all of the comparative examples (steel plates: G to J) in which the component composition deviated from the appropriate range of the present invention had a tensile strength (TS), a strength-elongation balance (TS × El), and a Young's modulus (Ec). At least one of the characteristics is inferior.

  According to the present invention, it is possible to provide a thin steel sheet having a high strength such as a tensile strength of 1180 MPa or higher and a Young's modulus of 230 GPa or higher.

Claims (6)

In mass%, C: 0.12 to 0.20%, Si: 0.5 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, and Ti : Containing 0.02 to 0.20% and satisfying the relationship shown in the following formulas (1) and (2), the balance has a composition consisting of Fe and inevitable impurities,
By area ratio, ferrite phase: 50% or more, martensite phase: 35-45%, and the total of ferrite phase and martensite phase is 95% or more, and the average grain size of ferrite is 4.0 μm or less. It has a structure with an average particle size of 3.0 μm or less,
Tensile strength (TS) in the direction perpendicular to rolling is 1180 MPa or more, Young's modulus is 230 GPa or more, and the strength-elongation balance (TS × EL) expressed by the product of tensile strength (TS) and total elongation (EL) is High-strength thin steel sheet with excellent rigidity, characterized by 15000 MPa ·% or more.
Record
0.11 ≦ [% C] − (12 / 47.9) × [% Ti *] ≦ 0.15 --- (1)
Here, Ti * = [% Ti] − (47.9 / 14) × [% N] − (47.9 / 332.1) × [% S] --- (2)
[% M] is the content of M element (mass%) In addition to the composition, the steel sheet further contains Nb: 0.02 to 0.10% in mass%, and satisfies the relationship of the following formula (3) instead of the formula (1). Item 2. A high-strength thin steel sheet having excellent rigidity according to Item 1.
Record
0.11 ≦ [% C] − (12 / 92.9) × [% Nb] − (12 / 47.9) × [% Ti *] ≦ 0.15 --- (3)   In addition to the above composition, the steel sheet further comprises, in mass%, Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Cu: 0.1 to 2.0%, and B: 0.0005 to 0.0030%. The high-strength thin steel sheet having excellent rigidity according to claim 1 or 2, comprising one or more selected from among them. In mass%, C: 0.12 to 0.20%, Si: 0.5 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, and Ti : 0.02 to 0.20% steel, and the contents of C, N, S and Ti satisfy the relationship shown in the following formulas (1) and (2), with the balance being the composition of Fe and inevitable impurities In the hot rolling process, after the finish rolling is finished at 850 to 950 ° C., the material is wound at 650 ° C. or lower, pickled, and then cold rolled at a reduction rate of 60% or higher, and then in the annealing process. , (Ac ₁ -100 ° C.) to Ac ₁ average heating rate: after heating to a soaking temperature of 780 to 880 ° C. at a rate of 15 ° C./s or more and holding at that soaking time for 150 s or less By cooling to 350 ° C. or lower with an average cooling rate of at least 350 ° C. as 5 to 50 ° C./s ,
By area ratio, ferrite phase: 50% or more, martensite phase: 35-45%, and the total of ferrite phase and martensite phase is 95% or more, and the average grain size of ferrite is 4.0 μm or less. It has a structure with an average grain size of 3.0μm or less, tensile strength (TS) in the direction perpendicular to rolling is 1180 MPa or more, Young's modulus is 230 GPa or more, and tensile strength (TS) and total elongation (EL) A method for producing a high strength thin steel sheet having excellent rigidity, characterized in that the strength-elongation balance (TS × EL) expressed by the product is a thin steel sheet having a strength of 15000 MPa ·% or more .
Record
0.11 ≦ [% C] − (12 / 47.9) × [% Ti *] ≦ 0.15 --- (1)
Here, Ti * = [% Ti] − (47.9 / 14) × [% N] − (47.9 / 332.1) × [% S] --- (2)
[% M] is the content of M element (mass%) In addition to the composition, the steel material further contains Nb: 0.02 to 0.10% by mass%, and satisfies the relationship of the following formula (3) instead of the formula (1). The manufacturing method of the high intensity | strength thin steel plate excellent in the rigidity of Claim 4.
Record
0.11 ≦ [% C] − (12 / 92.9) × [% Nb] − (12 / 47.9) × [% Ti *] ≦ 0.15 --- (3)   In addition to the above composition, the steel material further includes, in mass%, Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Cu: 0.1 to 2.0%, and B: 0.0005 to 0.0030% The manufacturing method of the high strength thin steel plate excellent in rigidity of Claim 4 or 5 characterized by containing 1 type, or 2 or more types selected from these.