JP4867257B2 - High-strength thin steel sheet with excellent rigidity and manufacturing method thereof - Google Patents

Description

The present invention relates to a high-strength thin steel sheet having excellent rigidity suitable for structural parts such as automobile center pillars, rockers, side frames, and cross members, and a method for manufacturing the same .

  In recent years, in response to growing interest in global environmental issues, there has been a demand for reducing the weight of automobile bodies in automobiles. In order to reduce the weight of the vehicle body, it is an effective method to reduce the thickness (thinning) by increasing the strength of the steel plate. Recently, as a result of remarkable progress in increasing the strength of steel sheets, there is a movement to actively apply thin steel sheets with a tensile strength TS of 590 MPa or more and a thickness of less than 2.0 mm. However, a reduction in the rigidity of the vehicle body due to the thinning has become a problem, and it has become necessary to improve the rigidity of structural parts of automobiles. If the cross-sectional shape is the same, the rigidity of the structural part is determined by the plate thickness and Young's modulus of the steel sheet. Therefore, a Young's modulus of 225 GPa or more is required to achieve both weight reduction and structural part rigidity. Furthermore, as the strength of the steel plate increases, a greater stress is applied to the steel plate, so a high yield ratio YR (= YS / TS, YS: yield strength) of 0.65 or more is also required.

  As a method for producing a cold-rolled steel sheet having a high Young's modulus of 225 GPa or more, for example, in mass%, C: 0.0003 to 0.010%, Mn: 1.2 to 2.5%, Al: 0.005 to 0.10%, and Nb: 0.005 to 0.10% , Ti: including at least one of 0.005 to 0.10%, slab of the composition consisting of the balance Fe and inevitable impurities, hot-rolled, cold-rolled at a reduction rate of 30% or more, and then recrystallization annealing Has disclosed a method for producing a cold-rolled steel sheet having excellent formability and rigidity (Patent Document 1).

  As a cold-rolled steel sheet having a TS of 590 MPa or more, for example, in mass%, C: 0.070 to 0.200%, Si: 0.30% or less, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.025% or less, Al : 0.002-0.100%, N: 0.0120% or less, slab consisting of remaining Fe and unavoidable impurities is hot-rolled, and after cold rolling, from 500 ° C to 730-830 ° C annealing temperature at 300-2000 ° C / s Disclosed is a method for manufacturing cold-rolled steel sheets with high strength and excellent bake hardenability by rapid heating, staying for 2 seconds or less, and rapidly cooling at 100 to 500 ° C / s to form a fine and uniform composite structure (Patent Document 2).

  As a cold-rolled steel sheet having a high yield ratio YR, for example, in mass%, C: 0.03-0.15%, Si: 0.05% or less, Mn: 0.5-1.2%, P: 0.030% or less, Nb: 0.005-0.045%, Al: Steel with 0.10% or less, balance Fe and unavoidable impurities is hot-rolled, cold-rolled at a reduction rate of 50% or more, and then heated at a rate of 5 ° C / s or more, preferably 10 ° C / s or more during annealing. And is maintained at 20 to 60 s in the temperature range of 720 to 780 ° C., and then cooled at a cooling rate of preferably 20 ° C./s or more, and thereby a fine and uniform ferrite structure having an average crystal grain size of 20 μm or less For example, a high-tensile cold-rolled steel sheet and hot-dip galvanized steel sheet excellent in stretch flange characteristics and a method for producing them are disclosed (Patent Document 3).

Non-Patent Document 1 below relates to an ADC method for ODF analysis described in [Best Mode for Carrying Out the Invention] described later.
Japanese Patent Laid-Open No. 5-255804 JP 7-34136 A JP-A-4-350 Phys. Status Solid (b), 134 (1986) 447

  However, in the production method described in Patent Document 1, it is difficult to obtain a TS having a C content of 0.0003 to 0.010% as small as 590 MPa or more. Moreover, in the manufacturing method described in Patent Document 2, since rapid cooling of 100 to 500 ° C./s is required, temperature control is difficult, and it is difficult to stably manufacture a steel plate. Furthermore, in the high-tensile cold-rolled steel sheet and hot-dip galvanized steel sheet described in Patent Document 3, alloy components such as Mn are reduced, and it is difficult to stably obtain TS of 590 MPa or more.

The present invention solves the above problems, TS is 590 MPa or more, YR is 0.65 or more, preferably 0.70 or more, more preferably 0.75 or more, and high-strength thin steel sheet having excellent rigidity and Young's modulus of 225 GPa or more and It aims at providing the manufacturing method .

  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. It is known to be low. Therefore, in the steel process consisting of rolling and heat treatment, if the {112} <110> orientation is developed, the <111> direction is aligned in a direction perpendicular to the rolling direction of the steel sheet, and the Young's modulus in this direction can be increased. it can.

  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 can generally obtain good ductility.

  In view of the above, the present inventors have made various studies in order to improve the Young's modulus of a high-strength thin steel sheet reinforced with a hard martensite phase, and the steel obtained by adding Mn to a low-carbon steel. After hot rolling, the slab develops a crystal orientation that is advantageous for improving Young's modulus during cold rolling, and transforms from non-recrystallized ferrite to austenite by rapid heating during subsequent annealing. By accumulating the orientation in an orientation advantageous for Young's modulus and making the austenite grains finer, it is possible to refine the ferrite produced during cooling to achieve higher strength, higher Young's modulus, and higher yield ratio. I found out that I can do it.

  The present invention has been made based on these findings, and in mass%, C: 0.05 to 0.20%, Si: 1.5% or less, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.01% or less Al: 1.5% or less, N: 0.01% or less, having a composition composed of the balance Fe and inevitable impurities, the average grain size of the ferrite phase is 5 μm or less, and the ferrite phase is 50% in area ratio The average ODF analysis strength f in the (113) [1-10] to (223) [1-10] orientations of the plate surface at 1/4 thickness of the steel plate is 4 or more. A high-strength thin steel sheet having excellent rigidity characterized by being provided. Here, [1-10] represents the direction of (1, -1,0).

In the high strength thin steel sheet of the present invention, furthermore, by mass%, Nb: 0.005~0.1%, Ti : 0.005~0.2%, V: 0.01~0.2%, Cr: 0.05~1.0%, Ni: 0.05~1.0%, It can contain at least one element selected from Mo: 0.05 to 1.0%, B: 0.0005 to 0.0030%, Cu: 0.1 to 2.0%, and W: 0.1 to 2.0%.

The high-strength thin steel sheet of the present invention, for example, casts steel having the above composition into a slab, directly or after being cooled and reheated, and then hot-rolled at a finish rolling finish temperature not lower than the Ar ₃ transformation point, 500 ° C. After coiling at the above coiling temperature, pickling, cold rolling at a rolling reduction in the range of 50 to 85%, and then annealing from room temperature to 750 ° C at 30 ° C / s or more After heating at an average heating rate of 750 ° C to 900 ° C for a time v (s) that satisfies the following formula (1) at an annealing temperature in the range of 750 to 900 ° C, the temperature range of 800 to 600 ° C is 3 to 30 ° C. It can be produced by a method for producing a high strength thin steel sheet having excellent rigidity, characterized by cooling at an average cooling rate of / s.

Here, F (w) is the temperature (° C.) at an arbitrary time w (s) within the time v (s) in which the steel sheet stays in the temperature range of 750 to 900 ° C. after the temperature reaches 750 ° C. Represents.

  According to the present invention, it is possible to produce a high-strength thin steel sheet excellent in rigidity and having a TS of 590 MPa or more, a YR of 0.65 or more, and a Young's modulus of 225 GPa or more, which are suitable for automobile structural parts.

  Below, the high-strength thin steel sheet and its manufacturing method which are this invention are demonstrated in detail.

1) Component (“%” below represents “% by mass”)
C: 0.05-0.20%
C is an element that stabilizes austenite, and enhances hardenability in the cooling process during annealing after cold rolling, promotes the formation of a low temperature transformation phase, and greatly contributes to high strength. In order to obtain such an effect, the C content needs to be 0.05% or more. On the other hand, if the amount of C exceeds 0.20%, the martensite phase increases, the steel sheet becomes extremely strong, its workability deteriorates, and weldability deteriorates. Therefore, the C amount needs to be 0.20% or less.

Si: 1.5% or less
When Si is contained in excess of 1.5%, it deteriorates the weldability of the steel sheet or promotes the formation of firelite on the slab surface during heating before hot rolling. Promotes the occurrence of defects. Furthermore, when used as a cold-rolled steel sheet, the Si oxide produced on the surface deteriorates the chemical conversion property, and when used as a hot-dip galvanized steel sheet, the Si oxide produced on the surface is not plated. To trigger. Therefore, the Si amount needs to be 1.5% or less. In the case of a cold-rolled steel sheet and a hot-dip galvanized steel sheet that place importance on surface properties, the Si content is preferably 0.5% or less. On the other hand, Si is an element that stabilizes ferrite.It improves Young's modulus by promoting ferrite transformation in the cooling process during annealing, and stabilizes austenite by concentrating C in austenite. Since the production | generation of a transformation phase can be accelerated | stimulated, the intensity | strength of steel can be raised as needed. In order to obtain such an effect, the Si content is desirably 0.2% or more.

Mn: 1.0-2.5%
Mn is one of the important elements in the present invention. Mn, an austenite stabilizing element, reduces the Ac ₁ transformation point in the heating process during annealing, promotes austenite transformation from unrecrystallized ferrite, and is advantageous for improving Young's modulus in the cooling process during annealing. Can be developed. In addition, Mn not only greatly contributes to increasing the strength by enhancing the hardenability and promoting the formation of a low temperature transformation phase in the cooling process during annealing, but also contributes to increasing the strength as a solid solution strengthening element. In order to obtain such an effect, the Mn content needs to be 1.0% or more, preferably 1.5% or more. On the other hand, if the Mn content exceeds 2.5%, the formation of ferrite necessary for increasing the Young's modulus during the cooling process during annealing is remarkably suppressed, and the steel is extremely strengthened due to an increase in the low-temperature transformation phase. Deteriorates. Such a large amount of Mn also deteriorates the weldability of the steel sheet. Therefore, the amount of Mn needs to be 2.5% or less.

P: 0.05% or less
When P is contained in an amount exceeding 0.05%, it segregates at the grain boundaries to lower the ductility and toughness of the steel sheet and deteriorate the weldability. In addition, when the high-strength cold-rolled steel sheet of the present invention is used as an alloyed hot-dip galvanized steel sheet, P retards the alloying rate. Therefore, the P content needs to be 0.05% or less. On the other hand, P is an element effective for increasing the strength as a solid solution strengthening element, promotes the concentration of C in austenite as a ferrite stabilizing element, and generates red scale in steel added with Si. It also has the effect of suppressing the above. Therefore, the P content is preferably 0.01% or more.

S: 0.01% or less
When S is contained in an amount exceeding 0.01%, the hot ductility is remarkably lowered to induce hot cracking, and the surface properties of the steel sheet are remarkably deteriorated. Moreover, it not only contributes to the strength, but also precipitates as coarse MnS, reducing the ductility such as hole expandability. Therefore, the S amount needs to be 0.01% or less. Note that the smaller the amount of S, the better, but 0.005% or less is more preferable from the viewpoint of further improving the hole expandability.

Al: 1.5% or less
Al is a ferrite stabilizing element, and if contained in excess of 1.5%, the Ar ₃ transformation point is greatly increased, the austenite single phase region disappears, and hot rolling cannot be completed in the austenite region. Therefore, the Al amount needs to be 1.5% or less. On the other hand, since Al is useful as a deoxidizing element for steel, the Al content is preferably 0.005% or more. Furthermore, Al, which is a ferrite-forming element, promotes ferrite formation during the cooling process during annealing, stabilizes austenite by concentrating C in austenite, and promotes the formation of low-temperature transformation phase. Accordingly, the strength of the steel can be increased. In order to obtain such an effect, the Al content is more preferably 0.2% or more.

N: 0.01% or less
If N is contained in a large amount exceeding 0.01%, it may induce slab cracking during hot rolling and cause surface flaws on the steel sheet. Therefore, the N content needs to be 0.01% or less.

  The balance is preferably Fe and inevitable impurities, but even if it contains other trace elements, the effect of the present invention is not impaired. Examples of other trace elements include Ca and REM, and these elements contribute to improving the stretch flangeability of the steel sheet by controlling the form of sulfide inclusions. Therefore, although not particularly limited, in order to obtain this effect, it is preferable to include one or more of Ca and REM and to make the total of these contents 0.001% or more. Further, since the effect is saturated when the total content of Ca and REM exceeds 0.01%, the total of these contents is preferably 0.01% or less, and more preferably 0.005% or less. Examples of the impurity element include Sb, Sn, Zn, Co, etc., and the allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less.

  In addition to the above component elements, for the following reasons, in mass%, Nb: 0.005-0.1%, Ti: 0.005-0.2%, V: 0.01-0.2%, Cr: 0.05-1.0%, Ni: 0.05-1.0%, It is preferable to contain at least one element selected from Mo: 0.05 to 1.0%, B: 0.0005 to 0.0030%, Cu: 0.1 to 2.0%, and W: 0.1 to 2.0%.

Nb: 0.005-0.1%
Nb promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite during the heating process during annealing, and further develops an oriented orientation that is advantageous for improving Young's modulus during the cooling process during annealing. Can be generated. Further, the fine carbon nitride of Nb can contribute to an increase in strength. In order to have such an effect, the Nb amount needs to be 0.005% or more. On the other hand, when a large amount of Nb exceeding 0.1% is added, carbonitrides cannot be completely dissolved during normal slab reheating, and coarse carbonitrides remain, so that processed austenite during hot rolling It is not possible to obtain a recrystallization suppressing effect or a recrystallization suppressing effect of the processed ferrite during annealing. In addition, even if the continuously cast slab is once cooled and then subjected to hot rolling without being subjected to reheating, the contribution of the recrystallization suppression effect for the amount of Nb exceeding 0.1% is It is small, and the cost of the alloy is also increased. Therefore, the Nb amount needs to be 0.1% or less.

Ti: 0.005-0.2%
Ti promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite during the heating process during annealing, and further develops an orientation that is advantageous for improving Young's modulus during the cooling process during annealing. Can be generated. Further, fine carbonitride of Ti can contribute to an increase in strength. In order to have such an effect, the Ti amount needs to be 0.005% or more. On the other hand, if the amount of Ti exceeds 0.2%, carbonitrides cannot be completely dissolved during normal slab reheating, and coarse carbonitrides remain, which has the effect of increasing strength and suppressing recrystallization. I can't get it. In addition, even when the continuously cast slab is cooled and then hot-rolled without being reheated, the contribution to the recrystallization suppression effect is small even when the Ti content exceeds 0.2%. In addition, an increase in alloy cost is also caused. Therefore, the Ti amount needs to be 0.2% or less.

V: 0.01-0.2%
V promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite during the heating process during annealing, and further develops an orientation that is advantageous for improving the Young's modulus during the cooling process during annealing. Can be generated. In addition, fine carbonitrides of V can also contribute to an increase in strength. In order to have such an action, the V amount needs to be 0.01% or more. On the other hand, even if the amount of V exceeds 0.2%, the effect of increasing the strength and the effect of suppressing recrystallization are small, resulting in an increase in alloy cost. Therefore, the V amount needs to be 0.2% or less.

Cr: 0.05-1.0%
Cr is an element that suppresses the formation of cementite and enhances the hardenability, and promotes the formation of a low-temperature transformation phase in the cooling process during annealing and greatly contributes to the increase in strength. In order to obtain such an effect, the Cr amount needs to be 0.05% or more. On the other hand, when the Cr content exceeds 1.0%, the effect is not only saturated but also the alloy cost is increased. Therefore, the Cr amount needs to be 1.0% or less. In addition, when hot dip galvanizing is applied to the steel sheet of the present invention, the Cr oxide generated on the surface induces non-plating, so the Cr content is preferably 0.5% or less.

Ni: 0.05-1.0%
Ni is an element that enhances hardenability by stabilizing austenite and greatly contributes to high strength by promoting the formation of a low-temperature transformation phase in the cooling process during annealing. Ni, an austenite stabilizing element, lowers the Ac ₁ transformation point in the heating process during annealing, promotes austenite transformation from unrecrystallized ferrite, and forms a low-temperature transformation phase formed in the cooling process after soaking. It is possible to develop an orientation that is advantageous for improving the Young's modulus with respect to the orientation, and to suppress a decrease in Young's modulus that accompanies the generation of a low temperature transformation phase. In addition, in the case of Cu-added steel, surface defects of the steel sheet are likely to be induced by cracks due to a decrease in hot ductility during hot rolling, but suppressing the occurrence of this surface defect by adding Ni in combination. Can do. In order to obtain such an action, Ni needs to be 0.05% or more. On the other hand, the addition of a large amount of Ni exceeding 1.0% significantly suppresses the formation of ferrite necessary for high Young's modulus during the cooling process during annealing, and increases the strength of the steel by increasing the low-temperature transformation phase. In addition, the workability is degraded. Furthermore, the alloy cost is increased. Therefore, Ni needs to be 1.0% or less.

Mo: 0.05-1.0%
Mo is an element that increases the hardenability by reducing the mobility of the interface, and promotes the formation of a low-temperature transformation phase in the cooling process during annealing, and greatly contributes to an increase in strength. In order to obtain such an action, the Mo amount needs to be 0.05% or more. On the other hand, if the amount of Mo exceeds 1.0%, not only the effect is saturated, but also the alloy cost increases. Therefore, the Mo amount needs to be 1.0% or less.

B: 0.0005-0.0030%
B is an element that suppresses the transformation from austenite to ferrite and enhances the hardenability, and promotes the formation of a low-temperature transformation phase in the cooling process during annealing, and greatly contributes to an increase in strength. In order to obtain such an effect, the B content needs to be 0.0005% or more. On the other hand, addition of B exceeding 0.0030% remarkably suppresses the formation of ferrite in the cooling process during annealing, and lowers the Young's modulus. Therefore, the B content needs to be 0.0030% or less.

Cu: 0.1-2.0%
Cu is an element that enhances hardenability and greatly contributes to high strength by promoting the formation of low-temperature transformation phase in the cooling process during annealing. In order to obtain this effect, Cu needs to be 0.1% or more. On the other hand, addition of Cu exceeding 2.0% reduces the hot ductility, induces surface defects of the steel sheet accompanying cracks during hot rolling, and also saturates the Cu quenching effect. Therefore, the amount of Cu needs to be 2.0% or less. In addition, when adding Cu, in order to prevent the crack at the time of hot rolling as mentioned above, it is preferable to also add Ni.

W: 0.1-2.0%
W promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite during the heating process during annealing, and ferrite with an orientation that is advantageous for improving Young's modulus during the cooling process during annealing. Can be generated. Further, fine carbonitrides of W can also contribute to an increase in strength. In order to have such an effect, the W amount needs to be 0.1% or more. On the other hand, the addition of W exceeding 2.0% contributes little to the strength increasing effect and the recrystallization suppressing effect, and also causes an increase in alloy cost. Therefore, the amount of W needs to be 2.0% or less.

2) Microstructure If the average grain size of ferrite grains exceeds 5 μm, it will not be possible to achieve high strength and high yield ratio, so it is necessary to make the grain size 5 μm or less. Further, since the ferrite phase has an effect on the development of a texture that is advantageous for improving the rigidity, the area ratio needs to be 50% or more.

  Here, the area ratio of the ferrite phase and the average grain size of the ferrite grains were measured by performing SEM observation after the steel plate section was subjected to Nital corrosion, taking three 30 × 30 μm region images, and performing image processing.

3) Texture
By developing a texture in the (113) [1-10] to (223) [1-10] orientation, it is possible to improve the Young's modulus in the 90 ° direction with respect to the rolling direction. The average ODF (Orientation Distribution Function) analysis strength f in the (113) [1-10] to (223) [1-10] orientations of the plate surface at / 4 plate thickness needs to be 4 or more.

Here, the average ODF analysis strength f in the (113) [1-10] to (223) [1-10] orientations was reduced to 1/4 plate thickness by chemical polishing in order to remove the influence of processing strain. After that, (110), (200), (211) pole figure is obtained by Schulz method, ODF analysis is performed by ADC method described in Non-Patent Document 1, and at φ ₁ = 0 °, φ ₂ = 45 °, It is the average value of analysis intensity when Φ is 25 °, 30 °, 35 °, 45 °.

  In addition to hot-rolled steel sheets and cold-rolled steel sheets, steel sheets subjected to surface treatment such as hot-dip galvanized materials including alloying and electrogalvanized materials are also included in the thin steel plates targeted by the present invention.

4) Manufacturing method The high-strength thin steel sheet of the present invention is obtained by, for example, casting a steel having the above-described composition into a slab and cooling it as it is, or after reheating and hot rolling. It is made into a steel plate, wound up, pickled, cold-rolled into a cold-rolled steel plate, and manufactured by annealing. The details will be described below.

4-1) Finish rolling end temperature If the finish rolling temperature of finish rolling during hot rolling (the temperature immediately after finishing finish rolling) is less than the Ar ₃ transformation point, the structure after hot rolling becomes the processed structure, and the Young's modulus Since an unfavorable orientation develops, the finish rolling finish temperature needs to be higher than the Ar ₃ transformation point. When the finish rolling finish temperature exceeds 950 ° C., the structure after hot rolling becomes coarse, and it may be impossible to develop an orientation advantageous for Young's modulus during cold rolling. Therefore, the finish rolling finish temperature is preferably 950 ° C. or lower.

4-2) Winding temperature When winding the steel sheet after hot rolling, if the winding temperature is 500 ° C or less, a hard phase such as bainite and martensite is generated, and in the subsequent cold rolling (113) Development of [1-10] to (223) [1-10] orientation is impeded. Therefore, the coiling temperature needs to be 500 ° C. or higher.

  The hot-rolled steel sheet after winding needs to be pickled before cold rolling in order to remove scale. In addition, what is necessary is just to perform pickling conditions on normal conditions.

4-3) Reduction ratio during cold rolling When cold rolling a hot-rolled steel sheet after pickling, optimizing the reduction ratio is effective in improving Young's modulus (113) [1-10 ] To (223) [1-10] direction. In order to develop such an orientation, the rolling reduction needs to be 50 to 85%.

4-4) Average heating rate during annealing The average heating rate during annealing is an important condition in the present invention. In addition to not only improving the Young's modulus after cooling by suppressing recovery and recrystallization, but transforming from unrecrystallized processed ferrite directly to austenite, the austenite grains can be refined, so ferrite is generated during cooling. Thus, the Young's modulus can be further improved. As shown in FIG. 1, by setting the heating rate to 30 ° C./s or more, a Young's modulus of 225 GPa or more can be obtained. Moreover, since it can be made into a structure mainly composed of finer ferrite grains, TS and YS can be increased. In order to obtain such an effect, the average heating rate in the temperature range from room temperature to 750 ° C. needs to be 30 ° C./s or more, preferably 50 ° C./s or more, more preferably 100 ° C./s or more. . On the other hand, if the average heating rate exceeds 2000 ° C./s, the temperature control becomes difficult, so the average heating rate is preferably 2000 ° C./s or less. In addition, in a heating method such as radiant tube heating used in a normal continuous annealing furnace, the average heating rate is about 10 ° C / s, and in order to obtain an average heating rate of 30 ° C / s or more, electromagnetic induction heating is used. It is preferable to use an apparatus or direct heating heating.

4-5) Annealing temperature In order to promote transformation from non-recrystallized ferrite to austenite, the annealing temperature must be 750 ° C or higher. On the other hand, if the annealing temperature exceeds 900 ° C., the austenite grains become coarse even if the residence time is short, so the annealing temperature needs to be 900 ° C. or less.

4-6) Residence time at annealing temperature Even if the annealing temperature is within the range of the present invention, if retained in that temperature range for a long time, the austenite grains become coarse and not only the Young's modulus after cooling decreases. TS and YS decrease because a fine ferrite structure cannot be obtained. For this reason, it is necessary to retain the time v (s) within the annealing temperature range of 750 to 900 ° C. so as to satisfy the above formula (1). Here, the formula (1) is an experimental formula for obtaining the residence time conditions for suppressing the coarsening of the austenite grains and preventing the Young's modulus, YS, and TS from decreasing.

4-7) Cooling conditions during annealing Cooling after heating during annealing is performed in order to obtain fine ferrite grains from the fine austenite during cooling. It is necessary to cool at a cooling rate. On the other hand, if the average cooling rate in this temperature range exceeds 30 ° C./s, ferrite is not sufficiently generated during cooling, and the Young's modulus cannot be improved. For this reason, the average cooling rate in this temperature range needs to be 30 ° C./s or less.

  Note that cooling in the temperature range of less than 600 ° C is not particularly required, but in the case of cold-rolled steel sheet, it can be over-aged, and in the case of hot-dip galvanized steel sheet, It can also be dipped in. Furthermore, in the case of an alloyed hot-dip galvanized steel sheet, reheating at 500 ° C. or higher can be performed for alloying treatment.

  After annealing, the shape can be corrected and the rigidity can be further improved by rotating the crystal by processing, so that temper rolling can be performed at an elongation of 0.3% or more.

Steel A having the components shown in Table 1 was melted in a laboratory in a vacuum melting furnace and subjected to hot rolling, cold rolling, and annealing to produce a cold rolled steel sheet. Here, the Ar ₃ transformation point in Table 1 was determined by measuring the center thickness of the steel sheet during hot rolling in a laboratory and examining the inflection point during cooling. At this time, reheating prior to hot rolling: 1 hour at 1250 ° C, finish rolling finish temperature of hot rolling: 860 ° C, plate thickness after hot rolling: 3.5mm, winding condition: held at 600 ° C for 1 hour Post-furnace cooling equivalent to winding (winding temperature: 600 ° C), cold rolling reduction: 71%, plate thickness after cold rolling: 1.0 mm, average heating rate from room temperature to 750 ° C during annealing : 50 ° C / s, average heating rate from 750 ° C to 820 ° C: 10 ° C / s, annealing temperature: 820 ° C, holding time: 10s, average cooling rate of 800-600 ° C: 10 ° C / s, then to room temperature Air cooling was the basic condition. At this time, the left side of the above equation (1) was 0.12. In addition to these basic conditions, the average heating rate during annealing is as shown in Table 2, the average heating rate from 750 ° C to the annealing temperature, annealing temperature, holding time, the left side of the above equation (1) as shown in Table 3, and The average cooling rate in the temperature range of 800 to 600 ° C. was changed as shown in Table 4. In other words, the basic conditions are the above except for the changed conditions.

  Then, from the produced cold-rolled steel sheet, a test piece of 10 × 60 mm was cut from the direction perpendicular to the rolling direction, and a transverse vibration type resonance frequency measuring device was used, and the Young Society according to the American Society for Testing Materials standard (C1259). The rate E (GPa) was measured. Further, a JIS No. 5 tensile test piece was cut out from a cold rolled steel sheet subjected to temper rolling of 0.5% from a direction perpendicular to the rolling direction, and tensile properties (TS, YS and elongation El) were measured. In addition, by the above method, the area ratio and average grain size of the ferrite phase, and the average of the (113) [1-10] to (223) [1-10] orientations of the plate surface at 1/4 of the steel plate thickness The ODF analysis strength f was obtained.

  Cold rolled steel sheet produced under basic conditions is TS: 620 MPa, YS: 455 MPa, YR: 0.73, El: 26%, E: 234 GPa, ferrite phase area ratio: 88%, ferrite phase average grain size: 3.4 μm, ODF analysis strength f: 10.2, high strength, high yield ratio and high Young's modulus steel plate.

  Table 2 shows the influence of the average heating rate during annealing. When the average heating rate is 30 ° C./s or more according to the present invention, YR is 0.65 or more and E is 225 GPa or more.

  Table 3 shows the influence of the heating rate, annealing temperature, holding time, and left side of the above equation (1) from 750 ° C. to annealing temperature. When these conditions are within the scope of the present invention, TS is 590 MPa or more, YR is 0.65 or more, and E is 225 GPa or more.

  Table 4 shows the influence of the average cooling rate in the temperature range of 800-600 ° C. When the average cooling rate is 3 to 30 ° C./s according to the present invention, TS is 590 MPa or more, YR is 0.65 or more, and E is 225 GPa or more.

Steels B to N having the components shown in Table 5 were melted in a laboratory in a vacuum melting furnace and subjected to hot rolling, cold rolling, and annealing under the basic conditions of Example 1 to cold rolled steel sheets 1 to 13 Was made. Then, the same investigation as in Example 1 was performed. The finish rolling finishing temperature of hot rolling was not less than the Ar ₃ transformation point in all the steels.

  The results are shown in Table 6. In the steel plates 1 to 11 that satisfy the component conditions of the present invention, TS is 590 MPa or more, YR is 0.65 or more, and E is 225 GPa or more. On the other hand, in the steel plate 12 whose Mn amount is outside the scope of the present invention and the steel plate 13 whose C amount is outside the scope of the present invention, YR is less than 0.65, E is less than 225 GPa, and the average grain size of the ferrite phase exceeds 5 μm. . Further, in the steel plate 13 having a remarkably low amount of C, TS is 420 MPa and high strength cannot be obtained.

It is a figure which shows the relationship between an average heating rate and a Young's modulus.

Claims (6)

In mass%, C: 0.05-0.20%, Si: 1.5% or less, Mn: 1.0-2.5%, P: 0.05% or less, S: 0.01% or less, Al: 1.5% or less, N: 0.01% or less , Having a composition composed of the remaining Fe and inevitable impurities, having an average grain size of the ferrite phase of 5 μm or less, and having a microstructure in which the ferrite phase is present in an area ratio of 50% or more, and 1/4 of the steel plate A high-strength thin steel sheet with excellent rigidity, characterized in that the average ODF analysis strength f in the (113) [1-10] to (223) [1-10] orientation of the plate surface in the plate thickness is 4 or more;
Here, [1-10] represents the direction of (1, -1,0).   Furthermore, it contains at least one element selected from Nb: 0.005 to 0.1%, Ti: 0.005 to 0.2%, and V: 0.01 to 0.2% by mass%. High strength thin steel plate with excellent rigidity.   Furthermore, it is characterized by containing at least one element selected from Cr: 0.05-1.0%, Ni: 0.05-1.0%, Mo: 0.05-1.0%, B: 0.0005-0.0030% by mass%. The high-strength thin steel sheet having excellent rigidity according to claim 1 or 2.   4. The high-strength thin steel sheet having excellent rigidity according to any one of claims 1 to 3, further comprising Cu: 0.1 to 2.0% by mass.   5. The high-strength thin steel sheet having excellent rigidity according to any one of claims 1 to 4, further comprising W: 0.1 to 2.0% by mass%. A steel having the composition according to any one of claims 1 to 5 is cast into a slab, and directly or once cooled and reheated, Ar _Three After hot rolling at the finish rolling finish temperature above the transformation point, winding at a coiling temperature of 500 ° C. or higher, pickling, and cold rolling at a rolling reduction in the range of 50 to 85%, When performing annealing, heat from room temperature to 750 ° C at an average heating rate of 30 ° C / s or more, and maintain v (s) for a time that satisfies the following formula (1) at an annealing temperature in the range of 750 to 900 ° C. 6. After cooling, the temperature range of 800 to 600 ° C. is cooled at an average cooling rate of 3 to 30 ° C./s. Manufacturing method of high strength steel sheet;

Here, F (w) is the temperature (° C.) at an arbitrary time w (s) within the time v (s) in which the steel sheet stays in the temperature range of 750 to 900 ° C. after the temperature reaches 750 ° C. Represents.

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