JP5157215B2 - High rigidity and high strength steel plate with excellent workability - 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 cold-rolled steel sheet having both high tensile strength and high Young's modulus, Patent Document 1 has 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. It is disclosed.
JP 2006-183131 A

  By the way, as characteristics required for steel plates used for structural parts of automobiles, in addition to the strength and rigidity described above, it is also important to have excellent workability when processing into parts, and the total elongation El is secured according to the strength. It is necessary to. That is, the workability can be determined by using the product TS x El of the tensile strength TS and total elongation El as an index. If this product TS x El is 16800 MPa ·% or more, the necessary processing is performed according to the strength. Satisfies sex.

  The technique disclosed in Patent Document 1 increases the tensile strength by increasing the martensite fraction by increasing the alloy addition amount, but the total elongation decreases and the product TS × El also decreases. For this reason, it has been difficult to improve the workability along with the increase in strength. Therefore, an object of the present invention is to propose a technique that satisfies the various characteristics at the same time.

  The Young's modulus of steel greatly depends 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 in the <100> direction where the atomic density is small. Therefore, 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.

  Although there are various steel strengthening methods, it is known that DP steel obtained by strengthening a soft ferrite phase with a hard martensite phase generally has good ductility.

  Various studies have been conducted from this point of view, and the orientation of the ferrite is controlled by controlling the soaking temperature and time in the annealing process, generating ferrite at a certain rate or more during cooling after soaking, and making the balance martensite. It has been found that an excellent balance between strength and ductility can be obtained by setting the cooling stop temperature after soaking to 350 ° C. or lower.

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 component composition consisting essentially of iron and inevitable impurities, the ferrite phase area ratio is 50% or more, the martensite phase area ratio is 5% or more, and (112) [1- 10] It has a structure with ODF analysis strength of 5 or more in orientation, tensile strength (TS) is 590MPa or more, yield strength (YS) to tensile strength ratio YS / TS is 0.70 or less, tensile strength and A high-rigidity, high-strength steel sheet with excellent workability, characterized in that the product TS x El of total elongation (El) is 16800 MPa ·% or more and the Young's modulus in the direction perpendicular to the rolling direction is 230 GPa or more.

(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 workability containing any one or two of the above.

In the above (1) or (2), as the component composition,
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%
Any 1 type or 2 types or more of these may be contained .

( 3 )% 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 steel slab having a component composition consisting of iron and inevitable impurities is subjected to hot rolling at a finishing temperature of 800 to 950 ° C., and then wound at 550 ° C. or higher, and 40 to 75 after pickling. %, And then heated to a soaking temperature of 780 to 860 ° C. at an average heating rate of 1 ° C./s or more, and the holding time at the soaking temperature is set to 150 s or less. A method for producing a high-rigidity, high-strength steel sheet with excellent workability, characterized by cooling to 350 ° C or lower after heating at an average cooling rate of at least 350 ° C at 3 to 50 ° C / s.

( 4 ) In the above ( 3 ), the steel slab is further contained in mass% by V: 0.01 to 0.20% and W: 0.01 to 0.20%.
The manufacturing method of the high-rigidity high-strength steel plate excellent in workability containing any 1 type or 2 types of these.

In the above ( 3 ) or ( 4 ), the steel slab is further in mass%.
Cr: 0.1-1.0%
Ni: 0.1-1.0%
Mo: 0.1-1.0%,
Cu: 0.1-2.0%
B: 0.0005-0.0030%
Any 1 type or 2 types or more of these may be contained.

  According to the present invention, a steel plate that is suitable for automobile structural parts and has high strength and high rigidity and excellent workability, specifically, tensile strength TS is 590 MPa or more, Young's modulus is 235 GPa or more, and TS It is possible to stably provide a steel plate with El ≧ 16800 MPa.

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 to 0.15%,
C is an element that stabilizes austenite, and contributes to high strength by increasing the hardenability and greatly promoting the formation of martensite phase in the cooling process during annealing after cold rolling. Further, when the amount of C is increased, the martensite generation temperature decreases, and the strain generated during the generation of martensite increases, so that yielding easily occurs and the ratio YS / TS can be decreased. 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 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 steel sheet, the Si oxide generated on the surface is not plated. Trigger. Accordingly, the Si content needs to be 1.5% or less, and in the case of a steel sheet or hot dip galvanized steel sheet that requires surface properties, it is preferably 0.5% or less.

Mn: 1.5-3.0%
Mn is one of the important elements of the present invention. Mn, an austenite stabilizing element, 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 produced in, an orientation that is advantageous for improving the Young's modulus can be developed. 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 also contribute to the strengthening of steel by acting as a solid solution strengthening 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
Since Al is a ferrite stabilizing element and greatly increases the Ac ₃ point during annealing, it suppresses the austenite transformation from unrecrystallized ferrite, so that when ferrite forms from austenite during cooling, Young This will impede the development of a favorable rate. For this reason, Al 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 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.

Nb: 0.02 to 0.15%
Nb is an 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 continuous casting, even when hot rolling is started as it is, recrystallization for the amount of Nb addition 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 an 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 0.01% 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 carbonitride, one or two of V: 0.01 to 0.20% and W: 0.01 to 0.20% are to be improved in hardenability. One or more of 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: 0.003 to 0.0030% can be added. .
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 contributes to an increase in strength 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 acquire such an 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. Furthermore, when used as a hot dip galvanized steel sheet, the Cr content 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 can greatly contribute to high strength by greatly promoting the formation of the 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. 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 addition of Ni can suppress the occurrence of surface defects. it can. In order to obtain such an action, the Ni content is preferably 0.1% or more. On the other hand, the addition 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, thereby increasing the strength of the steel and processing. 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, the formation of martensite phase is greatly promoted to increase the strength. Can greatly contribute. 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 generation of a low-temperature transformation phase in the cooling process in 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 addition decreases hot ductility, induces surface defects accompanying cracking during hot rolling, and also saturates the quenching effect by Cu, so the Cu content is 2.0% or less. It is preferable to do.

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, the strength is increased by greatly promoting the formation of the martensite phase. Greatly contributes. In order to acquire this effect, it is preferable to add 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, it is preferable to add B in an amount of 0.0030% or less.

Furthermore, it is important that the steel sheet of the present invention has a structure containing 50% or more of the ferrite phase and 5% or more of the martensite phase.
Since the ferrite phase is effective for the development of a texture that is advantageous for improving the Young's modulus, the area ratio needs to be 50% or more. Moreover, since the strength and the strength-elongation balance are improved by containing the martensite phase, it is necessary to include a martensite phase of 5% or more in area ratio. Furthermore, in order to improve the strength-elongation balance, it is more preferable that all phases other than the ferrite phase are composed of a martensite phase. Examples of phases other than the ferrite phase and martensite phase include pearlite, bainite, and cementite.

  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 photographs of 30 × 30μm area. Thus, the areas of the ferrite phase and the martensite phase were measured.

[Texture: (112) [1-10] orientation ODF analysis strength ≧ 5]
(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] Orientation ODF analysis strength needs to be 5 or more.

Here, the ODF analysis strength of (112) [1-10] orientation was reduced to 1/4 plate thickness by chemical polishing and chemical polishing in order to remove the influence of mechanical distortion and then by Schulz method ( 110), (200), (211) Obtain pole figures, perform ODF analysis, and analysis strength when φ ₁ = 0 °, Φ = 35 °, φ ₂ = 45 °

  By following the above composition and structure, tensile strength TS is 590 MPa or more, yield strength (YS) to tensile strength ratio YS / TS is 0.70 or less, product of tensile strength and total elongation (El) TS × El Is a high-stiffness and high-strength steel sheet with excellent workability, with a Young's modulus in the direction perpendicular to the rolling direction of 230 GPa or more.

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 molten 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 800 to 950 ° 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 (112) [1-10] orientations effective for improving Young's modulus. That is, by developing the (112) [1-10] orientation by cold rolling, the structure after the subsequent annealing process increases the number of ferrite grains having the (112) [1-10] orientation and increases the Young's modulus. 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, heating is performed at an average heating rate of 1 ° C./s or higher to a soaking temperature of 780 to 860 ° C., and the holding time at the soaking temperature is set to 150 s or less. After soaking, the average cooling to at least 350 ° C. Cool to 350 ° C or lower at a speed of 3-50 ° C / s. Hereinafter, the average heating rate is simply referred to as a heating rate.

[Heating rate to soaking temperature: 1 ° 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 was suppressed during the heating process of annealing, and the transformation from processed ferrite to austenite For this purpose, a heating rate of 1 ° C./s or more is required. The heating temperature is an average heating temperature from room temperature.

[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. Further, when the annealing 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, 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 350 ° C: 3-50 ° C / s]
In the production method of the present invention, it is important to control the cooling conditions after the soaking. Generating ferrite at the time of cooling after soaking develops a texture that is advantageous for improving the Young's modulus. Therefore, 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, martensite is not generated, so the cooling rate needs to be 3 ° 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 the ratio YS / TS. Or, even if martensite is generated, the hardness of martensite is reduced by rewinding during cooling, so that not only the contribution to strength improvement is reduced, but also a good balance between TS and El 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 balance between TS and El, it is preferable to perform cooling at a predetermined cooling rate to at least 300 ° C.

  FIG. 1 shows the relationship between the cooling stop temperature and the product TS × El, which is an index of workability, based on the evaluation result in Example 1 described later. As shown in the figure, by setting the cooling stop temperature to 350 ° C. or lower, further 300 ° C. or lower, TS and El are improved with a good balance.

  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 Cold rolling equivalent processing, cold rolling reduction ratio: 55%, sheet thickness after cold rolling: 1.2 mm, heating rate up to 830 ° C: 3 ° C / s, holding time at 830 ° C: 60s, 300 Cooling rate to ° C .: 15 ° C./s, and then cooling to room temperature was based on air cooling.
Furthermore, in addition to the above basic conditions, the cooling rate after soaking and the controlled cooling stop temperature (rapid cooling temperature) were changed as shown in Table 3. That is, the above conditions are the same as the conditions changed here.
In this example, the cooling after soaking is a constant rate up to the control cooling stop temperature. When cooling to 350 ° C. or less, the average cooling rate from the soaking temperature to 350 ° C. is up to the control cooling stop temperature. Is equal to the cooling rate.

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.
In addition, according to the method described above, the ferrite phase area ratio (α) and martensite phase area ratio (M), and the ODF analysis of the (112) [1-10] orientation of the plate surface at 1/4 the thickness of the steel plate The strength was determined.

  As shown in Table 2, the cold-rolled steel sheets produced according to the basic conditions are TS: 660 MPa, YS: 390 MPa, YS / TS: 0.59, El: 28%, TS × El: 18480 MPa ·%, E: 234 GPa, ferrite The area ratio was 91%, the martensite area ratio was 9%, and the ODF analysis strength was 8.7. The strength-elongation balance was good and the Young's modulus was high.

  Table 3 shows the influence of the cooling rate after soaking and the controlled cooling stop temperature. When these conditions satisfy the scope of the present invention, TS is 590 MPa or more, TS × El is 16800 MPa ·% or more, and E is 230 GPa or more.

  Steels B to Q having the components shown in Table 4 were melted in a vacuum melting furnace, and hot rolled, cold rolled, and annealed under the above basic conditions to produce cold rolled steel plates 1 to 16. The cold rolled steel sheet thus obtained was examined in the same manner as described above.

The survey results are shown in Table 5 . In steel plates 1 to 14, the Young's modulus is greater than or equal to 230GPa exhibit high strength. 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. In the steel plate 17 whose Mn content is lower than the claimed range, the martensite phase fraction is as low as 0%, TS and TS × El are low, and YS / TS is high.

It is a figure which shows the influence of the control cooling stop temperature which acts on TSxEl.

Claims (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%
And the balance has a composition composed of iron and inevitable impurities, the ferrite phase area ratio is 50% or more, the martensite phase area ratio is 5% or more, and the (112) [1-10] orientation The structure has an ODF analysis strength of 5 or more. Furthermore, the tensile strength (TS) is 590 MPa or more, the yield strength (YS) to tensile strength ratio YS / TS is 0.70 or less, and the tensile strength and total elongation ( A high-stiffness, high-strength steel sheet with excellent workability, characterized in that the product TS x El is 16800 MPa ·% or more and the Young's modulus in the direction perpendicular to the rolling direction is 230 GPa or more. 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 workability containing any one or two of the above. In 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 steel slab having a component composition consisting of iron and inevitable impurities is subjected to hot rolling at a finishing temperature of 800 to 950 ° C., and then wound at 550 ° C. or higher, and 40 to 75 after pickling. %, And then heated to a soaking temperature of 780 to 860 ° C. at an average heating rate of 1 ° C./s or more, and the holding time at the soaking temperature is set to 150 s or less. A method for producing a high-rigidity, high-strength steel sheet with excellent workability , characterized by cooling to 350 ° C or lower after heating at an average cooling rate of at least 350 ° C at 3 to 50 ° C / s . In Claim 3, the said steel slab is further in the mass%.
V: 0.01-0.20% and
W: 0.01-0.20%
The manufacturing method of the high-rigidity high-strength steel plate excellent in workability containing any 1 type or 2 types of these .

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