JP2013087331A - Thin steel sheet with excellent rigidity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin steel sheet with excellent rigidity, which, even as a thin steel sheet with a sheet thickness of 1.6 mm or less, has Young's modulus in the direction perpendicular to the rolling direction of 230 GPa or more, and also achieves average Young's modulus of 220 GPa or more after subjected to soft nitriding treatment after press molding.SOLUTION: The thin steel sheet has a composition containing, in mass, 0.008% or less of C, 0.5-1.0% of Si, 1.0-3.0% of Mn, 0.05% or less of P, 0.01% or less of S, 0.5% or less of Al, 0.01% or less of N, and 0.02-0.10% of Ti, with the balance comprising Fe and unavoidable impurities, and satisfying the concentration of solid solution Ti, Ti, defined by the following formula (1): Ti=[%Ti]-(47.9/14)×[%N]-(47.9/32.1)×[%S]-(47.9/12)×[%C] to be in the range of -0.01 to 0.05%; has a structure of, in area%, 90% or more of a ferrite phase and 10% or less (including 0%) of a martensite phase; further has Young's modulus in the direction perpendicular to the rolling direction of 230 GPa or more, a hardness of the surface layer of the steel sheet, Hv, of 300 or less after subjected to soft nitriding treatment, and average Young's modulus, Edefined by the following formula (2): E=(E+2E+E)/4 of 220 GPa or more.

Description

  The present invention relates to a thin steel sheet excellent in rigidity, which is suitable mainly for use in an automobile body, and is characterized in that the Young's modulus is advantageously improved by performing gas soft nitriding after press forming. It is suitable for a structural member having a columnar shape having a rigid plate thickness sensitivity index close to 1, or a cross-sectional shape close thereto, such as a pillar, a rocker, a side frame, and a cross member.

  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. In order to reduce weight by further increasing the strength, it is indispensable to simultaneously improve the reduction in component rigidity due to the reduction in thickness.

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.

  Patent Document 5 proposes a technique for increasing the rigidity of a steel material by applying powder metallurgy and dispersing high-rigidity compound particles in iron or an iron alloy.

On the other hand, surface hardening treatment is known as a technique for hardening a processed steel sheet. This surface hardening treatment is a treatment for improving the wear resistance and fatigue characteristics by hardening the steel surface and at the same time generating a residual stress on the steel surface. As typical surface hardening treatment methods currently in practical use, carburizing treatment and nitriding treatment can be mentioned.
Nitriding is a technique for obtaining a high hardness diffusion layer on the steel surface by diffusing active nitrogen, and the nitriding mechanism is activated nitrogen N generated by decomposition of NH ₃ gas by reaction of <2NH ₃ → 2N + 3H ₂ >. Is a technique for obtaining a high-hardness diffusion layer (nitriding layer) by diffusing steel on the steel surface. Such nitriding treatment is characterized in that the treatment temperature is as low as 500 to 550 ° C. in order to diffuse and permeate nitrogen below the A ₁ point. Therefore, no phase transformation due to heating does not occur, and the steel is not distorted unlike carburizing treatment. However, the treatment time was as long as 50 to 100 hours, and it was necessary to remove the brittle compound layer formed on the surface even after the treatment.

Therefore, a method called gas soft nitriding has been developed. This gas soft nitriding treatment is mainly performed by adding ₃₀ to 50 vol% of NH ₃ gas in a carburizing gas (specifically, a carburizing gas such as a rapid heating modified gas or a pyrolysis gas of an organic solvent) or a nitrogen gas atmosphere. In this method, nitrogen and carbon are simultaneously penetrated and diffused by heating and holding at a temperature of 550 to 600 ° C. for 1 to 5 hours to form carbonitrides on the surface. In addition to the carburizing gas, a method using a mixed atmosphere of N ₂ —NH ₃ —CO ₂ gas has been developed. By such treatment, a compound layer containing an ε (Fe _2-3 N) phase mainly composed of Fe and a mixed phase of Fe ₃ C is formed in the vicinity of the surface (surface layer), and a diffusion layer is formed in the inside thereof. As a result, a ν ′ (Fe ₄ N) phase is formed, and the surface hardness is increased. Hereinafter, the above-described gas soft nitriding treatment is simply referred to as “soft nitriding treatment”.

  As steel used by performing such soft nitriding treatment, for example, Patent Literature 6 and Patent Literature 7 disclose steel plates for soft nitriding treatment having good press formability.

Japanese Patent Laid-Open No. 5-255804 JP-A-8-311541 JP 2006-183131 A JP 2005-314792 A Japanese Unexamined Patent Publication No. 7-252609 Japanese Patent No. 3153108 Japanese Patent No. 3840939

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 amount of alloy addition and increasing the martensite fraction, but the presented Young's modulus is only in the direction perpendicular to the rolling direction, and in the other direction. No consideration is given to Young's modulus.
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.

  Since the technique disclosed in Patent Document 5 basically applies the powder metallurgy method, there is a problem that the cost is high and mass production is difficult due to the complexity of the process.

  The technique disclosed in Patent Document 6 is not only costly because V addition is essential, but there is no particular consideration of controlling the texture when manufacturing a steel sheet before soft nitriding, and the strength of the base plate is also low. It is unknown.

  The technique disclosed in Patent Document 7 has a low strength of 440 MPa or less, and therefore there is a problem in increasing the strength by applying this technique.

As described above, the conventional technology is intended for high-thickness hot-rolled steel sheets, soft steel sheets, and steel sheets having a strength of about 440 to 450 MPa, or has difficulty in manufacturability. The Young's modulus in the other direction is unclear because it is accompanied, or the Young's modulus in the direction perpendicular to the rolling direction is unknown, so in such a thin steel plate with a plate thickness of 1.6 mm or less using such conventional technology It was difficult to achieve the Young's modulus in the direction perpendicular to the rolling direction (C direction) ≧ 230 GPa and the average Young's modulus ≧ 220 GPa.
That is, a steel sheet with anisotropy in the crystal orientation and an increased Young's modulus only in the direction perpendicular to the rolling direction (C direction) was able to be obtained by the prior art. The Young's modulus in the 45 ° direction (D direction) decreases. As a result, the bending rigidity is inferior although excellent bending rigidity can be obtained by collecting the longitudinal direction of the automobile part so as to be parallel to the direction perpendicular to the rolling direction. In order to secure torsional rigidity while having excellent bending rigidity, it is necessary to satisfy Young's modulus ≧ 230 GPa in the direction perpendicular to the rolling direction (C direction) and to satisfy the average Young's modulus ≧ 220 GPa expressed by the following formula: There is.
E _AVE = (E _L + 2E _D + E _C ) / 4

  The present invention was developed in view of the above situation, and even in a thin steel sheet having a plate thickness of 1.6 mm or less, the Young's modulus in the direction perpendicular to the rolling direction is 230 GPa or more, and after performing soft nitriding after press forming The object is to propose a thin steel sheet with excellent rigidity and an average Young's modulus of 220 GPa or more.

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.
However, if the degree of integration in the (112) [1-10] orientation is too high, the anisotropy of Young's modulus increases, and in particular, the Young's modulus in the 45 ° direction (D direction) tends to decrease from the rolling direction.

Thus, as a result of intensive studies to solve the above problems, the inventors have determined that the solid solution Ti concentration defined by the total Ti addition amount excluding nitride, carbide, sulfide and its complex ( By controlling Ti ^* ) within a certain range, not only is the press formability excellent, but the Young's modulus in the direction perpendicular to the rolling can be increased to 230 GPa or more, and the average Young's modulus after soft nitriding after press forming can be increased. The knowledge that it can be increased to 220 GPa or more was obtained.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) By mass%, C: 0.008% or less, Si: 0.5 to 1.0%, 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 0.10%, the solid solution Ti concentration Ti ^* defined by the following formula (1) satisfies the range of -0.01 to 0.05%, and the balance has a composition composed of Fe and inevitable impurities. And
In area ratio, it has a structure of ferrite phase: 90% or more, martensite phase: 10% or less (including 0%),
Young's modulus in the direction perpendicular to rolling is 230 GPa or more,
A thin steel sheet having excellent rigidity, characterized in that the hardness of the steel sheet surface after nitriding is Hv300 or less and the average Young's modulus E _AVE defined by the following formula (2) is 220 GPa or more.
Record
Ti ^* = [% Ti] − (47.9 / 14) × [% N] − (47.9 / 332.1) × [% S]
-(47.9 / 12) x [% C] --- (1)
Here, [% M] is the content of M element (mass%)
_After E _AVE = ( _After E _L + 2E _{D After} + E _C ) / 4 --- (2)
Here, _after EL: Young's modulus in the rolling direction after soft nitriding
_After ED: Young's modulus in 45 ° direction from rolling direction after soft nitriding
_After EC: Young's modulus in the direction perpendicular to the rolling after soft nitriding

(2) The thin steel plate having excellent rigidity according to (1), wherein the steel plate further contains Nb: 0.02 to 0.2% by mass% in addition to the composition.

(3) The thin steel sheet having excellent rigidity according to (1) or (2), wherein the steel sheet further contains Cr: 0.1 to 0.5% in addition to the composition, in terms of mass%.

(4) In addition to the composition, the steel sheet further contains one or two kinds selected from Ni: 0.1 to 1.0% and Cu: 0.2 to 2.0% in terms of mass% ( The thin steel plate excellent in rigidity as described in any one of 1) to (3).

  According to the present invention, a thin steel sheet having a tensile strength of 440 MPa or more, a Young's modulus in the direction perpendicular to the rolling of 230 GPa or more, and performing a soft nitriding treatment after press forming provides an average Young's modulus of 220 GPa or more. be able to.

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: 0.008% or less C is an element that greatly affects the texture control of steel, and when the amount of so-called solid solution C that is not fixed to Ti and Nb increases, the Young's modulus of the steel sheet tends to deteriorate. Therefore, if the C content is large, the amount of Ti and Nb added must be increased accordingly, resulting in an increase in manufacturing cost. Further, in the annealing process after cold rolling, since there is a tendency to suppress recrystallization in an orientation advantageous for increasing the Young's modulus, the content is made 0.008% or less. Preferably it is less than 0.005%. In addition, if it is less than 0.001%, the load for high purity of steel increases, and the manufacturing cost increases dramatically, which is not economical. Therefore, the content is preferably 0.001% or more.

Si: 0.5-1.0%
Since Si raises the Ar ₃ transformation point in hot rolling, in order to promote recrystallization of processed austenite during rolling immediately above Ar ₃ , when a large amount of Si exceeding 1.0% is contained, The crystal orientation necessary for increasing the Young's modulus cannot be obtained. 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 was limited to 1.0% or less. In the case of a steel sheet or hot dip galvanized steel sheet that requires surface properties, the Si content is preferably 0.8% or less.
On the other hand, 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 0.6% or more.

Mn: 1.0-3.0%
Mn is an austenite stabilizing element, which lowers the Ac ₁ transformation point in the temperature rising process in the annealing process after cold rolling, promotes austenite transformation from unrecrystallized ferrite, and in the cooling process after soaking With respect to the orientation of the generated ferrite, it is possible to develop an orientation that is advantageous for improving the Young's modulus.
Mn also acts as a solid solution strengthening element, thereby contributing to an increase in strength of steel. In order to obtain such an effect, the Mn content needs to be 1.0% or more. Preferably it is 1.2% or more. On the other hand, a large Mn content exceeding 3.0% remarkably inhibits the formation of ferrite during cooling after annealing, and a large Mn content also deteriorates the weldability of the steel sheet. Therefore, the Mn content is set to 3.0% or less. Preferably it is 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. On the other hand, since Al is useful as a deoxidizing element for steel, the Al content is preferably 0.01% or more. In addition, Al is an element that has a strong affinity for nitrogen and makes the surface layer of the nitride layer very hard. Therefore, when Al is added to improve nitriding properties, it is preferably made 0.10% or more.

N: 0.01% or less If N is contained in a large amount, there is a risk of surface flaws accompanied by slab cracking during hot rolling, and the press formability also deteriorates. Therefore, the N amount is 0.01% or less. There is a need to. In order to secure the press formability, it is better to have a small amount, but to reduce it to less than 0.0002%, the manufacturing cost is drastically improved and it is not economical, so the lower limit is preferably set to 0.0002%. .

Ti: 0.02-0.10%
Ti is the most important element in the present invention. Namely, Ti promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite in the temperature rising process in the annealing process, and the Young's modulus of ferrite generated in the cooling process after annealing is An azimuth that is superior to improvement can be developed. 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, the effect of increasing the strength when the amount of Ti added exceeds 0.10% even when the hot rolling is started after continuous casting without going through the process of reheating after once cooling the slab from continuous casting. Further, the contribution to the recrystallization suppressing effect is small, and the alloy cost is also increased. Therefore, the Ti content is set to 0.10% 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, and Ti ^{* represented} by the following formula (1) is controlled in the range of −0.01 to 0.05%. It is important to. This Ti ^* is a solid solution Ti concentration that is not fixed by nitrogen, sulfur, or carbon during the production of the steel sheet, and it is important to strictly control this Ti ^* . If this Ti ^* is less than -0.01%, the Young's modulus is not sufficiently increased by soft nitriding. On the other hand, if it exceeds 0.05%, the hardness of the surface layer enters into a brittle region of Hv: 300 or more after soft nitriding, resulting in deterioration of toughness. Invite.
Ti ^* = [% Ti] − (47.9 / 14) × [% N] − (47.9 / 332.1) × [% S]
-(47.9 / 12) x [% C] --- (1)
Here, [% M] is the content of M element (mass%)

Moreover, in this invention, the element described below can be contained suitably.
Nb: 0.02-0.2%
Nb promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite in the temperature rising process in the annealing process after cold rolling, and also suppresses coarsening of austenite grains, Contributes to the development of orientation that is advantageous for improving the Young's modulus of ferrite produced in the cooling process after annealing. Furthermore, the fine carbon nitride of Nb contributes effectively to an increase in strength. 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 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 recrystallization suppression is as much as the amount of Nb added exceeds 0.2%. The contribution to the effect is small, and the alloy cost is also increased. Therefore, the Nb content needs to be 0.2% or less. Preferably it is 0.15% or less.

Cr: 0.1-0.5%
Cr is a component having an effect of increasing the surface hardness by forming a nitride by soft nitriding treatment, and in order to obtain such an effect, the content of 0.1% or more is necessary. Preferably it is 0.2% or more. On the other hand, the addition of a large amount of Cr may excessively harden the steel surface after soft nitriding and deteriorate the toughness. When used as a hot dip galvanized steel sheet, the oxide of Cr formed on the surface is not effective. In order to induce plating, the Cr content needs to be 0.5% or less.

Ni: 0.1-1.0%
Ni, as a solid solution strengthening element, contributes to increasing the strength of steel. In addition, 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 containing Ni in a composite manner. . In order to obtain such an action, the Ni content needs to be 0.1% or more. On the other hand, the addition of a large amount of Ni inhibits the formation of ferrite necessary for increasing the Young's modulus in the cooling process after soaking, and increases the alloy cost, so Ni needs to be contained at 1.0% or less.

Cu: 0.2-2.0%
Cu, as a solid solution strengthening element, contributes to increasing the strength of steel. In order to obtain such an effect, the Cu content needs to be 0.2% 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 needs to be 2.0% 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 structure including a ferrite phase as a main phase, having a ferrite phase of 90% or more by area ratio, and containing a martensite phase at 10% or less.
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 90% or more. In addition, the martensite phase may include a martensite phase with an area ratio of 10% or less because the strength and the strength-elongation balance are improved. Preferably it is 3% or less. It may be 0%.
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 3% or less, and there is no problem. Preferably it is 1% 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. You can ask for it.

With the above composition and structure, the tensile strength TS is 440 MPa or more, the Young's modulus in the direction perpendicular to the rolling is 230 GPa or more, and the hardness of the surface layer after performing soft nitriding after press molding is Hv: 300 or less. In addition, a thin steel sheet excellent in press formability satisfying 220 GPa or more _{after the} average Young's modulus E _AVE defined by the following formula (2) can be obtained.
_After E _AVE = ( _After E _L + 2E _{D After} + E _C ) / 4 --- (2)
Here, _after EL: Young's modulus in the rolling direction after soft nitriding
_After ED: Young's modulus in 45 ° direction from rolling direction after soft nitriding
_After EC: Young's modulus in the direction perpendicular to the rolling after soft nitriding

And it can extract | collect the outstanding component rigidity by extract | collecting so that the longitudinal direction in a motor vehicle component may become parallel to a rolling right angle direction. In order to increase the bending rigidity of the part, it is desirable that the Young's modulus in the direction perpendicular to the rolling direction is high, and it is necessary to set it to 230 GPa or more. Preferably it is 235 GPa or more. However, when only Young's modulus in the direction perpendicular to the rolling direction is increased preferentially, the Young's modulus in the direction of 45 ° from the rolling direction decreases, which increases the anisotropy and decreases the torsional rigidity. Is preferably 240 GPa. In order to secure torsional rigidity while having excellent bending rigidity, it is difficult to achieve by only crystal orientation control, and it is possible only by raising the bottom by soft nitriding. Therefore, the average Young's modulus _{after AVE after} soft nitriding is set to 220 GPa or more.

Further, the hardness of the steel sheet surface layer after the nitrocarburizing treatment is the Vickers hardness measured at a depth of 50 μm from the steel sheet surface. In particular, if the hardness of the surface layer exceeds Hv: 300, high toughness cannot be obtained after the part, so Hv: 300 or less is required.
Here, the characteristics of the steel sheet after the soft nitriding treatment should be evaluated by the steel sheet characteristics after performing the gas soft nitriding treatment at 580 ° C. for 2 hours in an atmosphere gas of NH ₃ : RX = 50: 50. Can do.

  In addition, the thin steel plate of this invention is used as a structural component for motor vehicles, after mainly performing a forming process and a soft nitriding process. Therefore, in addition to being excellent in press moldability, Young's modulus, and Young's modulus after soft nitriding, the material itself has high strength, preferably tensile strength is in the range of 440 to 680 MPa, more preferably in the range of 460 to 660 MPa. It is desirable to be. This is because if the tensile strength is less than 440 MPa, the strength required for the parts and the like cannot be obtained, while if it exceeds 660 MPa, the press formability is reduced. There are several indexes of press formability. In the present invention, an average r value of 1.5 or more is preferable as a steel sheet having excellent press formability. More preferably, the average r value is 1.55 or more.

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.
By setting the finishing temperature to 950 ° C or lower, the transformation from non-recrystallized austenite to ferrite proceeds, and a fine ferrite structure is obtained. Further, accumulation in the (112) [1-10] orientation is achieved by cold rolling and annealing. The degree can be increased. On the other hand, when the finishing temperature is lower than 850 ° C., the risk of falling below the Ar ₃ transformation point increases, and as a result of mixing the hot rolled structure with the processed 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.

Hot rolling is completed under the above conditions, and then wound at 650 ° C. or lower.
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.

  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, even in the structure after the subsequent annealing process, the ferrite grains having the (112) [1-10] orientation are increased and the Young's modulus is increased. 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%.

Then, annealing is performed. At this time, at least a temperature range of 550 ° C. to 700 ° C. is heated at an average temperature increase rate of 15 ° C./s or more. The soaking temperature is in the range of 780 to 880 ° C., and the holding time at this soaking temperature is 150 s or less. After soaking, it is cooled to at least 400 ° C. under the condition of average cooling rate: 3 to 50 ° C./s.
In order to increase the Young's modulus of the steel sheet after annealing, recrystallization of (112) [1-10] -oriented ferrite developed by cold rolling during the annealing temperature rise process is suppressed, and the transformation from processed ferrite to austenite For this purpose, a temperature increase rate of 15 ° C./s or more is required in the temperature range of at least 550 ° C. to 700 ° C. The heating rate is not particularly limited, but it is preferable that the average heating rate is 30 ° C./s or less because rapid heating equipment and the like are required to obtain a large heating rate and the manufacturing cost increases. . By transforming a sufficient amount of ferrite into austenite during annealing and retransforming into ferrite during cooling, a texture develops and Young's modulus is improved. Further, when the annealing temperature is low, the rolling structure remains and the elongation decreases. For this reason, 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 the ferrite retransformed after annealing to accumulate in the (112) [1-10] orientation, so the soaking temperature is 880 ° C. It is necessary to do the following. 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.

By forming ferrite during cooling after soaking, a texture that is advantageous for improving the Young's modulus develops. For this purpose, it is necessary to generate 80% or more of ferrite. Here, if the cooling rate at the time of cooling is too high, it becomes difficult to obtain the required amount of ferrite, so the cooling rate needs to be 50 ° C./s or less. On the other hand, when cooling is too slow or when the cooling stop temperature is high, bainite and pearlite are generated, leading to an increase in the YS / TS ratio and a decrease in ductility. For this reason, the cooling rate is preferably 3 ° C./s or more, and it is necessary to perform controlled cooling at a predetermined cooling rate up to at least 400 ° 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 a hot dip galvanizing bath, and when manufacturing as an alloyed hot dip galvanized steel plate, you may perform an alloying process.

Next, examples of the present invention will be described. In addition, this invention is not limited only to these Examples.
First, steels A to M having the component compositions shown in Table 1 were melted in a vacuum melting furnace, hot-rolled, pickled, cold-rolled, and then annealed to produce cold-rolled steel sheets. At that time, heating condition prior to hot rolling: 1 hour at 1250 ° C., finishing temperature of hot rolling: 880 to 920 ° C., thickness after hot rolling: 4.0 mm, winding condition: held at 600 ° C. for 1 hour Winding-equivalent treatment for subsequent furnace cooling, cold rolling reduction: 60 to 70%, plate thickness after cold rolling: 1.0 to 2.0 mm, temperature increase rate in the temperature range of 550 to 700 ° C .: 18 ° C./s The holding time at the annealing temperature was 60 s, the cooling rate to 400 ° C. was 15 ° C./s, and then air-cooled to room temperature. Table 2 shows the manufacturing conditions.

After the above annealing, a test piece of 10 mm × 50 mm is cut out from the rolling direction (L direction) of the steel sheet, the 45 ° direction (D direction) from the rolling direction and the direction (C direction) perpendicular to the rolling direction, and a transverse vibration type resonance frequency measuring device. Was used to measure Young's modulus (E _L , E _D , E _C ) according to the American Society to Testing Materials standard (C1259). In addition, a JIS No. 5 tensile test piece was cut out from a cold rolled steel sheet subjected to 0.5% temper rolling from a direction perpendicular to the rolling direction, and the tensile properties (tensile strength TS and elongation El) were measured. JIS No. 5 tensile specimens were collected from the direction, D direction, and C direction, and the width strain and plate thickness strain of each specimen were measured when 10% uniaxial tensile strain was applied, and these measured values were used. The average r value (average plastic strain ratio) was calculated in accordance with JIS Z 2254, and this was used as the r value.
The area ratio of the ferrite phase and the area ratio of the martensite phase were determined by the methods described above. The measurement results for each phase are also shown in Table 2.
Further, the annealed plate was subjected to gas soft nitriding treatment at 580 ° C. for 2 hours in an atmospheric gas in which ammonia gas (NH ₃ ) and RX gas (RX) were in a volume ratio of NH ₃ : RX = 50: 50. . Nitrocarburizing treatment after the Young's modulus _(after E L, _after E D, _after E C) is the rolling direction of the steel sheet, test piece was cut out from 45 ° direction and a perpendicular direction to the rolling direction, it was measured by the method described above. The hardness after the nitrocarburizing treatment was measured at 5 points at a position of 50 μm from the surface layer under a load of 50 g, and the average value was used.
The results of the measurement are shown in Table 3.

As shown in Table 3, all of the invention examples (steel plates: A1, A2, B1, C1, D1, L1, M1) manufactured by the method satisfying the composition of the present invention and satisfying the conditions of the present invention, It can be seen that after annealing, it has moderate strength, excellent Young's modulus and r value (press formability), and is excellent in Young's modulus after soft nitriding.
On the other hand, all of the comparative examples (steel plates: A3, E1, F1, G1, H1, I1, J1, K1) in which the component composition and manufacturing conditions of the present invention are not included are the steel plate strength, r value, Young's modulus, At least one of the surface layer hardness and the Young's modulus after the soft nitriding treatment is inferior.

  According to the present invention, it is possible to provide a highly rigid thin steel sheet having a high tensile strength of 440 MPa or more, a Young's modulus in the direction perpendicular to the rolling of 230 GPa or more, and an average Young's modulus after soft nitriding of 220 GPa or more.

Claims (4)

In mass%, C: 0.008% or less, Si: 0.5 to 1.0%, 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 0.10%, and the solid solution Ti concentration Ti ^* defined by the following formula (1) satisfies the range of -0.01 to 0.05%, and the balance has a composition consisting of Fe and inevitable impurities,
In area ratio, it has a structure of ferrite phase: 90% or more, martensite phase: 10% or less (including 0%),
Young's modulus in the direction perpendicular to rolling is 230 GPa or more,
A thin steel sheet having excellent rigidity, characterized in that the hardness of the steel sheet surface after nitriding is Hv300 or less and the average Young's modulus E _AVE defined by the following formula (2) is 220 GPa or more.
Record
Ti ^* = [% Ti] − (47.9 / 14) × [% N] − (47.9 / 332.1) × [% S]
-(47.9 / 12) x [% C] --- (1)
Here, [% M] is the content of M element (mass%)
_After E _AVE = ( _After E _L + 2E _{D After} + E _C ) / 4 --- (2)
Here, _after EL: Young's modulus in the rolling direction after soft nitriding
_After ED: Young's modulus in 45 ° direction from rolling direction after soft nitriding
_After EC: Young's modulus in the direction perpendicular to the rolling after soft nitriding   The thin steel sheet having excellent rigidity according to claim 1, wherein the steel sheet further contains Nb: 0.02 to 0.2% by mass% in addition to the composition.   3. The thin steel sheet having excellent rigidity according to claim 1, wherein the steel sheet further contains Cr: 0.1 to 0.5% by mass% in addition to the composition.   The said steel plate contains the 1 type (s) or 2 types selected from Ni: 0.1-1.0% and Cu: 0.2-2.0% by mass% further in addition to the said composition. The thin steel plate excellent in the rigidity as described in any of the above.