JP5233142B2 - High-stiffness and high-strength steel sheet excellent in hole expansibility and method for producing the same - Google Patents

Description

  The present invention relates to a high-rigidity and high-strength steel sheet having a high Young's modulus and strength, which is suitable mainly for structural parts of automobile bodies, and a method for producing the steel sheet stably and efficiently.

In recent years, in response to increasing interest in global environmental problems, automobiles are demanded to improve exhaust gas regulations and fuel efficiency. In order to realize these requirements, weight reduction of the vehicle body in an automobile is an extremely important issue. For this purpose, it is an effective method to reduce the plate thickness by increasing the strength of the steel plate to reduce the weight of the vehicle body.
Recently, as a result of remarkable progress in increasing the strength of steel sheets, it has become possible to manufacture thin steel sheets with a tensile strength of 590 MPa or more and a thickness of less than 2.0 mm. There is. On the other hand, since the component rigidity is determined by the plate thickness and the Young's modulus if the cross-sectional shape is the same, the Young's modulus needs to be improved in order to achieve both weight reduction and component rigidity.

Here, as a steel sheet having both high tensile strength and high Young's modulus, Patent Document 1 discloses that a thin steel sheet having a structure including a ferrite phase as a main phase and a martensite phase having an area ratio of 1% or more as a second phase. Is disclosed.
JP 2006-183131 A

  By the way, as the characteristics required for steel sheets used for structural parts of automobiles, in addition to the above-described strength and rigidity, press forming is often used for forming steel sheets, so it is also important to have excellent hole expansibility. is there. That is, the hole expandability is also called stretch flangeability and is an index of formability for stretch flange processing. Specifically, it is evaluated by the product TS × λ of the tensile strength TS (MPa) and the hole expansion rate λ described later. When TS × λ is 23000 MPa ·% or more, sufficient formability is satisfied for stretch flange processing.

  This hole expansibility tends to decrease especially with increasing strength, but it is important that the specified hole expansion performance is ensured even with high strength. Need to balance.

  However, in the past, it has been difficult to provide excellent hole expandability in addition to high strength and high rigidity. Therefore, an object of the present invention is to propose a technique that satisfies the various characteristics at the same time.

  Now, the Young's modulus of steel depends greatly on the texture. In the case of plain steel with a body-centered cubic lattice, the atomic density is high in the <111> direction, and conversely, the atomic density is small <100>. If the (112) [1-10] orientation is developed, the <111> direction is aligned in the direction perpendicular to the rolling direction of the steel sheet, and the Young's modulus in this direction can be increased.

Therefore, various studies were made from this point of view. In the annealing process after cold rolling, the ferrite-austenite two-phase region was gradually heated during heating to the austenite single-phase region, and then again ferrite during the cooling. It has been found that the ferrite orientation can be accumulated in a direction advantageous to the Young's modulus by transformation.

  On the other hand, the hole expandability is controlled by controlling the heating conditions up to the soaking temperature range in the annealing process, so that a fine second phase having a grain size of the same or less than that of ferrite is uniformly dispersed in fine ferrite. Thus, the present inventors have found that the second phase formed in a band shape can be suppressed to suppress the formation of the second phase, leading to the present invention.

This invention is made | formed based on the above knowledge, The summary structure is as follows.
(1) By mass% C: 0.05 to 0.15%,
Si: 1.5% or less,
Mn: 1.5-3.0%
P: 0.05% or less,
S: 0.01% or less,
Al: 0.5% or less,
N: 0.01% or less,
Nb: 0.02-0.15% and
Ti: 0.01-0.15%
And the balance has a composition composed of iron and inevitable impurities, the ferrite phase area ratio is 50% or more, and the (112) [1-10] orientation ODF analysis strength is 6.0 or more. Furthermore, the tensile strength TS is 590 MPa or more, the product of the tensile strength TS (MPa) and the hole expansion ratio λ (%) TS × λ ≧ 23000 MPa ·%, and the Young in the direction perpendicular to the rolling direction A high-rigidity, high-strength steel sheet with excellent hole expansibility, characterized by a rate of 235 GPa or higher.

(2) In said (1), as said component composition, in mass%, V: 0.01-0.20% and W: 0.01-0.20%
A high-rigidity and high-strength steel sheet excellent in hole expansibility, containing either one or two of the above.

(3) In the above (1) or (2), as the component composition ,
Cu : 0.1-2.0 %
High rigidity and high strength steel sheet excellent in hole expandability containing.

(4) By mass% C: 0.05 to 0.15%,
Si: 1.5% or less,
Mn: 1.5-3.0%
P: 0.05% or less,
S: 0.01% or less,
Al: 0.5% or less,
N: 0.01% or less,
Nb: 0.02-0.15% and
Ti: 0.01-0.15%
The remainder is steel slab consisting of iron and unavoidable impurities, hot-rolled at a finishing temperature of 950-800 ° C, wound at 550 ° C or higher, and reduced by 40-75% after pickling In the annealing process in which the soaking temperature is 780 to 860 ° C, the average heating rate from room temperature to 750 ° C is 1.0 ° C / s or more and from 750 ° C to soaking. The average heating rate up to the heat temperature is 0.10 to 10 ° C./s, and the average heating rate from room temperature to the soaking temperature is 1.0 ° C./s or more, and then maintained at the soaking temperature for 150 s or less. A method for producing a high-rigidity, high-strength steel sheet excellent in hole expansibility, characterized by cooling at an average cooling rate up to 600 ° C at 3 to 50 ° C / s.

(5) In said (4), the said steel slab is further mass%. V: 0.01-0.20% and W: 0.01-0.20%
A method for producing a high-rigidity and high-strength steel sheet excellent in hole expansibility, containing either one or two of the above.

(6) In the above (4) or (5), the steel slab is further in mass% .
Cu : 0.1-2.0 %
A method for producing a high-rigidity and high-strength steel sheet excellent in hole expansibility containing bismuth.

(7) In any one of the above (4) to (6), the cooling after the soaking is further excellent in the hole expansion property for cooling from 600 ° C. to 350 ° C. at an average cooling rate of 3 to 50 ° C./s. Manufacturing method of high-rigidity and high-strength steel sheet.

  According to the present invention, a steel plate that is suitable for automobile structural parts and has high strength and high rigidity and excellent hole expansibility, specifically, a tensile strength TS of 590 MPa or more, a Young's modulus of 235 GPa or more and A steel plate with TS × λ ≧ 23000 MPa ·% can be stably provided.

Next, the high-rigidity and high-strength steel sheet of the present invention will be described in order from the component composition. In addition, unless otherwise indicated, "%" display regarding a component composition means the mass% altogether.
C: 0.05-0.15%
C is an element that stabilizes austenite. In the cooling process during annealing after cold rolling, it enhances hardenability and greatly enhances the generation of low-temperature transformation phases such as martensite phase, thereby increasing the strength. Greatly contributes. In order to obtain such an effect, the C content needs to be 0.05% or more.
On the other hand, when the amount of C is increased, the fraction of martensite phase and the like is increased, the steel is extremely strengthened, the workability is deteriorated, and the ferrite phase is decreased, so that the Young's modulus is decreased. Therefore, the C content needs to be 0.15% or less.

Si: 1.5% or less
When Si is contained in an amount exceeding 1.5%, the weldability of the steel sheet is deteriorated, and at the time of hot rolling, the formation of a surface pattern called a so-called red scale is promoted by promoting the formation of firelite on the slab surface. Contributes to the occurrence. 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 rigid plate, the Si oxide generated on the surface is not plated. To trigger. Accordingly, the Si content needs to be 1.5% or less, and is preferably 0.5% or less in the case of a steel plate or hot dip galvanized rigid plate that requires surface properties.

Mn: 1.5-3.0%
Mn is one of the important elements of the present invention. Mn, an austenite stabilizing element, lowers the Ac1 transformation point in the temperature rising process in the annealing process after cold rolling, and promotes austenite transformation from unrecrystallized ferrite, thereby cooling 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. Further, Mn can greatly contribute to the increase in 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. And it can contribute to the strengthening of steel by acting as a solid solution strengthening element. In order to obtain such an effect, the Mn content needs to be 1.5% or more. On the other hand, the addition of a large amount of Mn significantly suppresses the formation of ferrite necessary for increasing the Young's modulus during cooling after soaking, and increases the martensite phase, thereby increasing the steel strength and processing. Will deteriorate. Furthermore, the addition of a large amount of Mn also deteriorates the weldability of the steel sheet. Therefore, the Mn content needs to be 3.0% or less.

P: 0.05% or less P segregates at the grain boundary, lowers the ductility and toughness of the steel sheet, and degrades the weldability. Further, when used as an alloyed hot-dip galvanized steel sheet, the alloying rate is delayed by P. Therefore, the P content needs to be 0.05% or less. On the other hand, P is an element effective for increasing strength as a solid solution strengthening element, and also has an action of promoting C concentration in austenite as a ferrite stabilizing element. Furthermore, steel added with Si also has an effect of suppressing the occurrence of red scale. In order to obtain such an action, the P content is preferably 0.01% or more.

S: 0.01% or less S significantly reduces the ductility in hot, induces hot cracking, and significantly deteriorates the surface properties. Further, S not only contributes to the strength, but also reduces the ductility and hole expansibility by forming coarse MnS as an impurity element, so it is desirable to reduce it as much as possible. Since these problems become significant when the S content exceeds 0.01%, the S content needs to be 0.01% or less. Furthermore, from the viewpoint of particularly improving the hole expandability, it is preferably 0.005% or less.

Al: 0.5% or less
Al is an element that contributes to the stabilization of ferrite, and greatly increases the _Ac3 point during annealing. Therefore, by suppressing the austenite transformation from unrecrystallized ferrite, ferrite is generated from austenite during cooling. In addition, the development of an orientation advantageous for Young's modulus is hindered. Therefore, the content needs to be 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.

N: 0.01% or less If N is contained in a large amount, slab cracking may occur during hot rolling, and surface defects may occur. Therefore, the N amount needs to be 0.01% or less.

Nb: 0.02 to 0.15%
Nb is the most important element in the present invention. In the heating process in the annealing process after cold rolling, by suppressing recrystallization of the processed ferrite, it promotes austenite transformation from unrecrystallized ferrite, further suppresses coarsening of austenite grains, 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 can also contribute to the strength increase. In order to have such an effect, the Nb content needs to be 0.02% or more. Preferably, it is 0.03% or more.

  On the other hand, even when a large amount of Nb is added, the carbonitride cannot completely dissolve at the time of reheating in the normal hot rolling process, and coarse carbonitride remains, so the hot rolling process It is not possible to obtain the effect of suppressing the recrystallization of processed austenite and the effect of suppressing the recrystallization of processed ferrite in the annealing process after cold rolling. In addition, after the continuous casting, the slab is once cooled and then reheated, and after the continuous casting, when hot rolling is started as it is, the recrystallization of the Nb content exceeding 0.15% The contribution of the suppression effect is small, and the alloy cost is also increased. Therefore, the Nb content needs to be 0.15% or less.

Ti: 0.01-0.15%
Ti, like Nb, is the most important element in the present invention. Ti can contribute to an increase in strength by forming fine carbonitrides. Further, in the annealing process, it is possible to contribute to a higher Young's modulus by suppressing recrystallization of the processed ferrite or suppressing austenite grain coarsening. In order to have such an effect, the Ti content needs to be 0.10% or more.

On the other hand, even when a large amount of Ti is added, the carbonitride cannot completely dissolve at the time of reheating in the normal hot rolling process, and coarse carbonitride remains, so that the strength increasing effect and A recrystallization inhibiting effect cannot be obtained. In addition, when the hot rolling is started after continuous casting without cooling the slab once after continuous casting and after the continuous casting, the effect of increasing the strength when the amount of Ti added exceeds 0.15% In addition, the contribution of the recrystallization suppressing effect is small, and the alloy cost is also increased. Therefore, the Ti content needs to be 0.15% or less.
The balance other than the components described above is Fe and inevitable impurities.

In addition to the above chemical components, in order to increase the strength using fine carbonitrides, one or two of V: 0.01 to 0.20%, W: 0.01 to 0.20%, and hardenability In order to increase, one or more of Cr: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.1-1.0%, Cu: 0.1-2.0% and B: 0.0005-0.0030% are added. be able to.
V: 0.01-0.20%
V contributes to an increase in strength by forming fine carbonitrides. In order to have such an effect, the V content is preferably 0.01% or more. On the other hand, even if a large amount of V is added, the effect of increasing the strength exceeding 0.20% is small, and the alloy cost is also increased. Therefore, the V content is preferably 0.20% or less.

W: 0.01-0.20%
W, like V, contributes to the strength increase by forming fine carbonitrides. In order to have such an effect, the W content is preferably 0.01% or more. On the other hand, even if a large amount of W is added, the effect of increasing the strength exceeding 0.20% is small, and the alloy cost is also increased. Therefore, the W content is preferably 0.20% or less.

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

Ni: 0.1-1.0%
Ni is an element that enhances hardenability in the cooling process after soaking in the annealing process, and greatly contributes to high strength by greatly promoting the generation of low-temperature transformation such as martensite phase. Ni, like Mn, is an austenite stabilizing element, and lowers the Ac ₁ transformation point and promotes austenite transformation from unrecrystallized ferrite in the temperature rising process in the annealing process after cold rolling, With respect to the orientation of the ferrite generated in the cooling process after soaking, an orientation that is advantageous for improving the Young's modulus can also be developed. And it can contribute to the strengthening of steel by acting as a solid solution strengthening element. 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. it can. In order to obtain such an action, the Ni content is preferably 0.1% or more. On the other hand, the inclusion of a large amount of Ni suppresses the formation of ferrite necessary for increasing the Young's modulus during cooling after soaking, and increases the low-temperature transformation phase, so that the steel becomes extremely strong, Workability will deteriorate. Furthermore, since the alloy cost also increases, the Ni content is preferably 1.0% or less.

Mo: 0.1-1.0%
Mo is an element that enhances hardenability by reducing the mobility of the interface, and in the cooling process in the annealing process after cold rolling, it greatly enhances the generation of martensite phase, etc. Can greatly contribute to the development. In order to obtain such an action, it is preferable to contain 0.1% or more of Mo. On the other hand, the addition of a large amount of Mo not only saturates the effect but also increases the alloy cost, so the Mo content is preferably 0.5% or less.

Cu: 0.1-2.0%
Cu is an element that enhances hardenability, and greatly contributes to high strength by greatly promoting the formation of a low-temperature transformation phase in the cooling process of the annealing process after cold rolling. In order to obtain this effect, the Cu content is preferably 0.1% or more. On the other hand, excessive Cu content reduces hot ductility, induces surface defects due to cracking during hot rolling, and also saturates the quenching effect by Cu, so the Cu content is 2.0% or less. It is preferable that

B: 0.0005-0.0030%
B is an element that suppresses the transformation from austenite to ferrite and enhances the hardenability. In the cooling process in the annealing process after cold rolling, it greatly enhances the formation of martensite phase and the like, thereby increasing the strength. Greatly contributes to In order to acquire this effect, it is preferable to contain B 0.0005% or more. On the other hand, excessive addition of B significantly suppresses the formation of ferrite during cooling after annealing soaking and lowers the Young's modulus. Therefore, the B content is preferably 0.0030% or less.

Furthermore, it is important that the cold-rolled steel sheet of the present invention has a structure containing 50% or more of the ferrite phase by area ratio.
[Ferrite fraction> 50%]
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.
Further, from the viewpoint of improving hole expandability, it is preferable to have a structure in which the second phase is uniformly dispersed in fine ferrite. In other words, the refined ferrite is the main phase, and the second phase consisting of one or more low-temperature transformation phases such as martensite and bainite other than ferrite is about the same as ferrite at an area ratio of 5% or more. Alternatively, it is preferable to have a uniformly dispersed structure with a particle size smaller than that.

  The area ratio of the ferrite phase was determined by performing scanning electron microscope (SEM) observation after taking a nital corrosion of the cross section of the steel sheet, taking three photographs of 30 × 30 μm area, and processing these photographs for image processing of the ferrite phase. It was determined by measuring the area.

[Texture: (112) [1-10] orientation ODF analysis strength ≧ 6.0]
(112) Since the Young's modulus in the direction perpendicular to the rolling direction can be improved by developing a texture in the [1-10] orientation, (112) [1-10] ODF analysis strength of orientation needs to be 6.0 or more.

Here, the ODF analysis strength of (112) [1-10] orientation was reduced to 1/4 plate thickness by chemical polishing to eliminate the influence of mechanical grinding and processing distortion, and then by Schulz method ( 110), (200), (211) Pole diagrams are obtained, ODF analysis is performed, and the analysis strength is when φ ₁ = 0 °, Φ = 35 °, and φ ₂ = 45 °.

  By following the above composition and structure, the tensile strength TS is 590 MPa or more, the product of the tensile strength TS and the hole expansion ratio λ × λ ≧ 23000 MPa ·%, and the Young's modulus perpendicular to the rolling direction This is a high-rigidity and high-strength steel sheet with excellent hole expansibility that is 235GPa or higher.

Next, the manufacturing conditions of the present invention will be described.
First, a steel having a chemical composition according to the above-described composition is melted in accordance with a target strength level. As a melting method, a normal converter method, an electric furnace method, or the like can be appropriately applied. The melted steel is cast into a slab and then, as it is or once cooled, is heated and subjected to hot rolling at a finishing temperature of 950 to 800 ° C.

[Finish temperature: 800 ~ 950 ℃]
In the present invention, it is not particularly necessary to develop the texture in the hot rolling process. By setting the finishing temperature to 950 ° C. or lower, transformation from non-recrystallized austenite to ferrite proceeds, and a fine ferrite structure is obtained. Further, cold rolling and annealing lead to the (112) [1-10] orientation. Accumulation can be promoted. On the other hand, if the finishing temperature is below 800 ° 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. Therefore, the lower limit of the finishing temperature is set to 800 ° C.

After hot rolling is finished under the above-mentioned finishing conditions, it is wound at 550 ° C. or higher.
[Winding temperature: 550 ° C or higher]
When the coiling temperature after finish rolling is lower than 550 ° C., hard bainite and martensite are generated in addition to ferrite. In this case, deformation due to cold rolling becomes non-uniform, and accumulation in an orientation advantageous for Young's modulus is hindered. As a result, the texture after annealing does not develop and the Young's modulus does not improve. Therefore, the winding temperature needs to be 550 ° C. or higher. Note that if the coiling temperature is too high, the ferrite grains become coarse, preventing orientation accumulation in cold rolling, and the effect of suppressing the recrystallization of ferrite during annealing due to coarsening of Nb and Ti carbonitrides. And, since the effect of suppressing the coarsening of austenite grains becomes small, it is preferable to be 700 ℃ or less

The steel sheet rewound after the winding is subjected to pickling and then subjected to cold rolling at a rolling reduction of 40 to 75%.
[Cold rolling ratio: 40-75%]
Cold rolling is performed after the hot rolling step to accumulate the (112) [1-10] orientation effective for improving Young's modulus. That is, by developing the (112) [1-10] orientation by cold rolling, the ferrite grains having the (112) [1-10] orientation are increased in the structure after the subsequent annealing process, 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 40% or more. On the other hand, when the cold rolling rate increases, the rolling load increases and manufacturing becomes difficult. Therefore, the rolling rate is preferably 75% or less. Furthermore, from the viewpoint of improving the Young's modulus, the cold rolling rate is preferably 50% or more.

  Next, when heating to the soaking temperature in the annealing process where the soaking temperature is 780 to 860 ° C., the average heating rate from room temperature to 750 ° C. is 1.0 ° C./s or more, and the average heating rate from 750 ° C. to the soaking temperature Is set to 0.10 to 10 ° C./s, and the average heating rate from room temperature to the soaking temperature is 1.0 ° C./s or more, then held at the soaking temperature for 150 s or less, and after the soaking, the average cooling rate to 600 ° C. Is cooled to 600 ° C. at 3 to 50 ° C./s. Hereinafter, the average heating rate is simply referred to as a heating rate.

[Heating rate up to 750 ° C: 1.0 ° C / s or more]
In order to increase the Young's modulus of the steel sheet after annealing, the recrystallization of ferrite with (112) [1-10] orientation developed by cold rolling during the annealing temperature rise process is suppressed, and the transformation from processed ferrite to austenite For this purpose, a heating rate of 1.0 ° C./s or more is required. Furthermore, by transforming from non-recrystallized ferrite to austenite, increasing the nucleation site of austenite and making the austenite grain size fine, after transformation into ferrite and second phase during cooling, the fine ferrite is converted into a base material. Thus, the second phase is uniformly dispersed, and the hole expandability can be improved. In order to obtain this effect, the heating rate up to 750 ° C. is set to 1.0 ° C./s or more.

[Heating rate from 750 ° C to soaking temperature: 0.10 to 10 ° C / s, heating rate from room temperature to annealing temperature: 1.0 ° C / s or more]
When the austenite transformed from ferrite with (112) [1-10] orientation is preferentially grown by gradually heating between the two phases of ferrite and austenite, the ferrite transforms from austenite during cooling after soaking. High Young's modulus can be obtained by more effectively generating ferrite having the (112) [1-10] orientation. Furthermore, by gradually heating between the two phases of ferrite and austenite, the austenite grain size can be made uniform, so that a structure in which the second phase is uniformly dispersed after cooling can be obtained, and the hole expandability is improved. I can do it. For this effect, the heating rate from 750 ° C. to the soaking temperature needs to be 10 ° C./s or less.

  On the other hand, when the heating rate is too low, since the recrystallization of ferrite proceeds prior to the austenite transformation, the Young's modulus decreases. Further, since the austenite grain size becomes coarse, the second phase cannot be uniformly dispersed in the annealed structure, and the hole expandability deteriorates. Therefore, it is necessary to set the heating rate from 750 ° C. to the soaking temperature to 0.10 ° C./s or more and the heating rate from room temperature to the soaking temperature to 1.0 ° C./s or more.

  Here, the relationship between the heating rate from 750 ° C. to the soaking temperature described above, Young's modulus, and hole expandability (TS × λ) is shown in FIG. 1 and FIG. 2 respectively. That is, as shown in FIG. 1, the Young's modulus can be increased to 235 GPa or more by setting the heating rate (heating rate 2) from 750 ° C. to the soaking temperature in the range of 0.10 to 10 ° C./s. Similarly, as shown in FIG. 2, the product TS × λ can be increased to 23000 MPa ·% or more by setting the heating rate 2 in the range of 0.10 to 10 ° C./s.

[Soaking temperature: 780-860 ° C, Soaking time: 150 s or less]
A sufficient amount of ferrite transforms to austenite during annealing and retransforms into ferrite during cooling, thereby developing a texture and improving Young's modulus. When the annealing temperature is low, the rolled structure remains and the elongation is lowered, and the second phase is dispersed in a band shape and the hole expandability is lowered. For these reasons, the soaking temperature needs to be 780 ° C. or higher. On the other hand, if the soaking temperature is too high, austenite grains become coarse, and it becomes difficult for ferrite retransformed during annealing to accumulate in the (112) [1-10] orientation. For this reason, the soaking temperature needs to be 860 ° C. or less. Further, since the austenite grains are coarsened by holding for a long time in this temperature range, the soaking time needs to be 150 s or less.

[Average cooling rate from soaking temperature to 600 ° C: 3-50 ° C / s]
After the soaking, a texture that is advantageous for improving the Young's modulus develops by generating ferrite during cooling, so it is necessary to generate 50% or more of ferrite. Therefore, the upper limit of the cooling rate needs to be 50 ° C./s. On the other hand, when cooling is too slow, austenite is coarsened before ferrite is formed and the Young's modulus is lowered, so the cooling rate needs to be 3 ° C./s or more.

  Furthermore, from the viewpoint of improving the strength / ductility balance while increasing the Young's modulus, it is desirable to generate ferrite during cooling to 600 ° C. after soaking, and to convert the phases other than ferrite into martensite. From this point, it is more preferable to cool from 600 ° C. to 350 ° C. or less at 3 to 50 ° C./s.

  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 galvanized steel, and when manufacturing as an alloying hot dip galvanized steel plate, you may perform an alloying process. Furthermore, after cooling to room temperature once, it may be reheated to a ferrite single phase region or a ferrite + austenite two phase region and passed through molten zinc, and thereafter an alloying treatment may be performed.

Next, examples of the present invention will be described. In addition, this invention is not limited only to these Examples.
First, steel A having the components shown in Table 1 was melted in a vacuum melting furnace, and hot rolled, cold rolled, and annealed to produce a cold rolled steel sheet. At this time, heating conditions prior to hot rolling: 1 hour at 1250 ° C., finishing temperature of hot rolling: 880 ° C., plate thickness after hot rolling: 2.7 mm, winding conditions: furnace after holding at 600 ° C. for 1 hour Treatment equivalent to winding at a coiling temperature of 600 ° C, cold rolling reduction: 55%, sheet thickness after cold rolling: 1.2 mm, average heating rate from room temperature (20 ° C) to 750 ° C: 3 ° C / s, average heating rate from 750 ° C to 830 ° C: 0.5 ° C / s, retention time at 830 ° C: 60s, average cooling rate to 600 ° C: 15 ° C / s, then cooling to room temperature The basic condition was air cooling.
In addition to the above basic conditions, the heating rate up to 750 ° C (heating rate 1), the heating rate from 750 ° C to 830 ° C (heating rate 2), and the heating rate from room temperature to soaking temperature (average heating rate) are shown. As shown in FIG. 3, the soaking temperature and soaking time were changed as shown in Table 4, and the cooling rate and the controlled cooling stop temperature were changed as shown in Table 5, respectively. In other words, the basic conditions are the above except for the changed conditions.

After the above annealing, a 10 mm x 60 mm test piece was cut out from a direction perpendicular to the rolling direction of the steel sheet, and a Young's modulus (C1259) was used according to the American Society to Testing Materials standard (C1259) using a transverse vibration type resonance frequency measuring device. E) 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.
Further, according to the above-described method, the area ratio of the ferrite phase and the ODF analysis strength of the (112) [1-10] orientation of the plate surface at the 1/4 plate thickness of the steel plate were obtained.

Furthermore, the hole expansion test was conducted according to the Japan Iron and Steel Federation standard (JFST1001-1996). After the above annealing, a test piece of 100 mm × 100 mm was cut out from a direction perpendicular to the rolling direction of the steel sheet, and an initial hole with a diameter d ₀ (= 10 mm) was punched out in the center of the test piece. insert the conical punch ° enlarged hole, determine the hole diameter d _f when cracks passing through the plate thickness occurs, the following equation λ (%) = {(d f -d 0) / d 0} × 100
Was used to determine the hole expansion rate λ (%).

As shown in Table 2, the cold-rolled steel sheets produced according to the basic conditions are TS: 645 MPa, El: 25%, E: 238 GPa, ferrite area ratio: 91%, ODF analysis strength: 9.5, λ: 45%, TS × λ: 29475 MPa ·%, which is a cold-rolled steel sheet having high strength and high Young's modulus with excellent hole expansibility.
Table 3 shows the influence of heating rate 1 and heating rate 2. When both heating rates are within the range of the present invention, E is 235 GPa or more.

  Next, Table 4 shows the effect of soaking temperature and soaking time. When these conditions are within the scope of the present invention, TS is 590 MPa or more and E is 235 GPa or more. Table 5 shows the influence of the average cooling rate up to the control cooling stop temperature and the control cooling stop temperature. When these conditions are within the scope of the present invention, TS is 590 MPa or more and E is 235 GPa or more. In particular, a steel sheet having a low cooling stop temperature has a high El and an excellent balance between strength and ductility.

  Further, as shown in Tables 3 to 5, when the conditions are within the range of the present invention, TS × λ: 2300 MPa ·% or more, and excellent in strength and hole expansibility.

  Steel B to Q having the components shown in Table 6 were melted in a vacuum melting furnace and subjected to hot rolling, cold rolling, and annealing under the above basic conditions to produce steel plates 1 to 16. The cold rolled steel sheet thus obtained was examined in the same manner as described above.

  The results are shown in Table 7. In the steel plates 1 to 14 satisfying the components according to the present invention, excellent strength, Young's modulus is 235 GPa or more, and hole expansibility is also good. On the other hand, in the steel plate 15 in which the amounts of Nb and Ti are outside the scope of the present invention, TS is as low as 543 MPa and E is as low as 221 GPa. Further, in the steel plate 16 having a remarkably high Mn content, E is as low as 213 GPa, and the ferrite area ratio is also 36%, which is lower than the scope of the invention.

It is a figure which shows the influence of the heating rate 2 which acts on a Young's modulus. It is a figure which shows the influence of the heating rate 2 which acts on a hole expansion rate.

Claims (7)

% By mass C: 0.05 to 0.15%,
Si: 1.5% or less,
Mn: 1.5-3.0%
P: 0.05% or less,
S: 0.01% or less,
Al: 0.5% or less,
N: 0.01% or less,
Nb: 0.02-0.15% and
Ti: 0.01-0.15%
And the balance has a composition composed of iron and inevitable impurities, the ferrite phase area ratio is 50% or more, and the (112) [1-10] orientation ODF analysis strength is 6.0 or more. Furthermore, the tensile strength TS is 590 MPa or more, the product of the tensile strength TS (MPa) and the hole expansion ratio λ (%) TS × λ ≧ 23000 MPa ·%, and the Young in the direction perpendicular to the rolling direction A high-rigidity, high-strength steel sheet with excellent hole expansibility, characterized by a rate of 235 GPa or higher. The component composition according to claim 1, further comprising, in mass%, V: 0.01 to 0.20% and W: 0.01 to 0.20%.
A high-rigidity and high-strength steel sheet excellent in hole expansibility, containing either one or two of the above. According to claim 1 or 2, wherein the chemical composition further contains, by mass%
Cu : 0.1-2.0 %
High rigidity and high strength steel sheet excellent in hole expandability containing. % By mass C: 0.05 to 0.15%,
Si: 1.5% or less,
Mn: 1.5-3.0%
P: 0.05% or less,
S: 0.01% or less,
Al: 0.5% or less,
N: 0.01% or less,
Nb: 0.02-0.15% and
Ti: 0.01-0.15%
The remainder is steel slab consisting of iron and unavoidable impurities, hot-rolled at a finishing temperature of 950-800 ° C, wound at 550 ° C or higher, and reduced by 40-75% after pickling In the annealing process in which the soaking temperature is 780 to 860 ° C, the average heating rate from room temperature to 750 ° C is 1.0 ° C / s or more and from 750 ° C to soaking. The average heating rate up to the heat temperature is 0.10 to 10 ° C./s, and the average heating rate from room temperature to the soaking temperature is 1.0 ° C./s or more, and then maintained at the soaking temperature for 150 s or less. A method for producing a high-rigidity, high-strength steel sheet excellent in hole expansibility, characterized by cooling at an average cooling rate up to 600 ° C at 3 to 50 ° C / s. 5. The steel slab according to claim 4, further comprising, in mass%, V: 0.01 to 0.20% and W: 0.01 to 0.20%.
A method for producing a high-rigidity and high-strength steel sheet excellent in hole expansibility, containing either one or two of the above. The steel slab according to claim 4 or 5, further comprising mass% .
Cu : 0.1-2.0 %
A method for producing a high-rigidity and high-strength steel sheet excellent in hole expansibility containing bismuth.   7. The high-rigidity and high-strength steel sheet having excellent hole expansibility, wherein the cooling after soaking is further performed at an average cooling rate of 3 to 50 ° C./s from 600 ° C. to 350 ° C. Production method.

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