JP2004346430A - Method for producing high-tension steel plate for working - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high tension steel plate for working having high ductility and ≤200 GPa Young's modulus and ≥380 MPa yield stress after press-forming. <P>SOLUTION: To a steel blank containing by mass% ≤0.20% C, 0.005-1.5% Si, 0.05-3.5% Mn, 0.005-0.15% P, 0.005-0.2% Al and ≤0.020% N, a hot-rolling is applied in which the rolling-reduction ratio in the temperature range of Ar<SB>3</SB>transformation point to (Ar<SB>3</SB>transformation point-100°C) is ≥50% and the rolling-reduction ratio at the finish pass is ≤15% and desirably, the rolling-reduction ratio of the passes except the finish pass, is ≤30%/pass, and the number of the rolling passes is ≥5 passes, and the finish rolling temperature is ≥(Ar<SB>3</SB>transformation point-100°C). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a steel sheet that is mainly subjected to processing such as press forming and is suitable for use in an automobile body or the like, and particularly to a method for producing a high-tensile steel sheet having a low Young's modulus. The steel sheet in the present invention includes a steel strip.

  2. Description of the Related Art In recent years, from the viewpoint of global environmental protection, there has been an active movement to regulate carbon dioxide emission, and attention has been focused on improving fuel efficiency by reducing the weight of automobiles. In order to reduce the weight of automobiles, it is an effective means to reduce the thickness of a steel sheet, which accounts for a large proportion of the vehicle body, and the steel sheet used tends to be thinned. Recently, high-strength steel sheets having a tensile strength of 340 MPa or more have been developed and widely used for the purpose of thinning steel sheets. However, when the steel sheet is increased in strength and the steel sheet used is reduced in thickness in this way, the rigidity of the vehicle body is generally inevitably reduced, which has been a technical barrier in thinning.

  For this reason, recently, in designing a vehicle body, it has been studied to allow a certain degree of strain to occur within an elastic range depending on parts. Under such design guidelines, it is desirable that a steel sheet having the same yield stress has a large strain before plastic deformation occurs, that is, a steel sheet having a small Young's modulus.

  In addition, when the steel sheet used is strengthened and thinned, plastic deformation of the member easily occurs due to a load such as an impact during the transporting process until press assembly after the member is pressed, and defects such as dents are generated. There was a problem. In addition, the flanges and the like joined by spot welding or the like at the time of assembly tend to be greatly deformed due to impact or the like, and there has been a problem that a defect occurs in the assembly process. If the steel sheet has a low Young's modulus and a high yield stress, it is possible to absorb a load due to such an impact or the like in elastic deformation and prevent plastic deformation.

  However, a method of lowering the Young's modulus of a steel sheet is hardly known until now. Conventional knowledge about the Young's modulus of a steel sheet is, as disclosed in, for example, Patent Document 1, merely trying to obtain a high Young's modulus, and there is no attempt to lower the Young's modulus. In addition, cast iron is known as an iron-based material having a low Young's modulus. However, there is a problem that it is industrially difficult to manufacture a plate shape and it is difficult to stably obtain a low Young's modulus.

Also, for example, as disclosed in Patent Document 2, it is known that the iron single crystal has a low Young's modulus in the <100> direction, and Patent Document 3 discloses that ND // <100> ( It has been proposed to perform finish rolling below the Ar ₃ transformation point in order to develop a crystallographic texture in which the <100> direction is parallel to the normal direction of the rolling surface. However, in practice, it is not yet well known how to optimize the hot rolling conditions of a low Young's modulus steel sheet even if the finish rolling is performed simply at the Ar ₃ transformation point or lower. It is difficult at present.

  A method of adding ND // <100> texture by adding an alloy element such as Si may be considered, but it is necessary to add a large amount of an alloy element such as Si, and the workability is degraded and It is difficult to provide sufficient moldability. A steel sheet for processing is required to maintain elongation of 30% or more, preferably 40% or more, depending on strength. Conventionally, a high-tensile steel sheet having both such high ductility and low Young's modulus is required. Could not be produced stably.

The main phase is an austenitic phase, for example, an austenitic stainless steel has a low Young's modulus, but the cost is high as a working steel sheet.
JP-A-4-432216 JP-A-56-139619 JP-A-62-284016

  An object of the present invention is to provide a method for producing a high-tensile steel sheet for processing, which has high ductility and a low Young's modulus, and has a yield stress after press forming of 380 MPa or more, adapted to recent design guidelines. . High-strength steel sheets that have a low Young's modulus and a yield stress after press forming of 380 MPa or more are elastically deformed to prevent impacts and other shocks received during the transfer process from press forming to forming member assembly, preventing plastic deformation. It is possible to prevent the occurrence of defects during assembly. Specifically, the low Young's modulus aims at a Young's modulus E of 200 GPa or less at room temperature.

The present inventors have conducted intensive experiments and studies to achieve the above object. As a result, a steel sheet composition to which a solid solution strengthening element and / or a precipitation strengthening element are added, and further, in a temperature range from the Ar ₃ transformation point to (Ar ₃ transformation point −100 ° C.), accumulation of strain during rolling is avoided. Meanwhile, it was found that by performing hot rolling applying a reduction of a predetermined amount or more, a high-tensile steel sheet having high ductility and a low Young's modulus, and having a yield stress after press forming of 380 MPa or more can be obtained. .

  First, the results of basic experiments performed by the present inventors will be described.

  A steel material having a composition of 0.002% C-0.30% Si-0.5% Mn-0.002% P-0.01% S-0.05% Al-0.003% N-0.01% Ti-0.0003% B by mass% was heated to 1080 ° C in a laboratory. Then, as a rough rolling, and as a finish rolling, rolling was performed at a total reduction of 40, 50, 60% in three passes or at a total reduction of 50% in five passes at each temperature of 650 to 950 ° C. During the finish rolling, the temperature was appropriately kept in the furnace, and the rolling temperature in each pass was kept constant. The rolling reduction for each pass is 20-15-10% for a total rolling reduction of 40%, 30-20-10% for a total rolling reduction of 50%, and 30-30-15 for a total rolling reduction of 60%. %, And 15-15-15-10-10% for a total reduction of 50% (5 passes).

The Young's modulus of the obtained hot rolled sheet was measured by a longitudinal vibration resonance method (room temperature: 18 ° C.). The Young's modulus E is given by the following equation (1)
E = (E ₀ + 2E ₄₅ + E ₉₀ ) / 4 (1)
Here, E ₀ , E ₄₅ and E ₉₀ are Young's moduli (GPa) in the rolling direction, 45 ° in the rolling direction and 90 ° in the rolling direction, respectively.
The average Young's modulus defined by The result is shown in FIG.

From FIG. 1, it can be seen that the Young's modulus E sharply decreases by hot rolling at a rolling reduction of 50% or more in a temperature range of Ar ₃ transformation point or lower, preferably Ar ₃ transformation point to (Ar ₃ transformation point −100 ° C.). You can see that At the same rolling reduction, the Young's modulus of the five-pass rolling with a large number of passes is stably reduced over a wide range of the rolling temperature.

  The present invention has been completed based on the above findings.

That is, in the present invention, C: 0.20% or less, Si: 0.005 to 1.5%, Mn: 0.05 to 3.5%, P: 0.005 to 0.15%, S: 0.02% or less, Al: 0.005 to 0.2%, N: the composition of the steel material containing 0.020% or less, Ar ₃ transformation point - reduction ratio in the temperature range of (Ar ₃ transformation point -100 ° C.) of 50% or more, reduction ratio of the final pass is 15% or less, And hot rolling at a finish rolling temperature of (Ar ₃ transformation point −100 ° C.) or higher. A method for producing a high-tensile hot-rolled steel sheet for processing, wherein the hot rolling is performed at an Ar ₃ transformation point. In the following, it is preferable to perform hot rolling in which the rolling reduction except for the final pass is 30% / pass or less and the number of rolling passes is 5 or more. In the present invention, in addition to the above composition, one or more selected from among Nb: 0.003 to 0.20%, Ti: 0.003 to 0.20%, V: 0.003 to 0.20% by mass%, Or one or two or more selected from Cu: 0.005 to 0.20%, Ni: 0.005 to 0.20%, Cr: 0.005 to 0.20%, Mo: 0.005 to 0.20%, or a combination thereof. And B: 0.0005 to 0.005% by mass in addition to the above, or B: 0.0005 to 0.005% by mass in addition to the above composition. Good.

In the present invention, C: 0.20% or less, Si: 0.005 to 1.5%, Mn: 0.05 to 3.5%, P: 0.005 to 0.15%, S: 0.02% or less, Al: 0.005 to 0.2%, N: the composition of the steel material containing 0.020% or less, Ar ₃ transformation point - reduction ratio in the temperature range of (Ar ₃ transformation point -100 ° C.) of 50% or more, reduction ratio of the final pass is 15% or less, A method for producing a high-tensile cold-rolled steel sheet for processing, which comprises performing hot rolling at a finish rolling temperature of (Ar ₃ transformation point −100 ° C.) or higher, followed by cold rolling and recrystallization annealing. Yes, the hot rolling is preferably hot rolling in which the rolling reduction is 30% / pass or less and the number of rolling passes is 5 or more, excluding the final pass, below the Ar ₃ transformation point. In addition to the above composition, B: 0.0005 to 0.005% by mass may be further contained. In the present invention, in addition to the above composition, one or more selected from among Nb: 0.003 to 0.20%, Ti: 0.003 to 0.20%, V: 0.003 to 0.20% by mass%, Or one or two or more selected from Cu: 0.005 to 0.20%, Ni: 0.005 to 0.20%, Cr: 0.005 to 0.20%, Mo: 0.005 to 0.20%, or a combination thereof. Is preferred.

  In the present invention, it goes without saying that hot-rolled sheet annealing may be performed for the purpose of softening and improving workability after hot rolling. In addition, normal temper rolling as a surface adjustment of the obtained steel sheet and temper rolling up to 10% for the purpose of improving YR do not impair the purpose of the present invention.

  According to the present invention, a high tensile strength steel sheet having a low Young's modulus, a high yield stress after press forming, a large elastic deformation energy until plastic deformation occurs, and a high tensile strength for processing useful for an automobile body can be manufactured. Steel plate can be provided at low cost, and it has a remarkable industrial effect. Furthermore, the low Young's modulus high-tensile steel sheet of the present invention has an effect that even when a pebble collides, the external force can be absorbed without plastic deformation. Further, by further lowering the Young's modulus, the effect of lowering the resonance frequency and widening the vibration isolation range can be expected.

  The steel sheet of the present invention includes a hot-rolled steel sheet, a cold-rolled steel sheet, and a surface-treated steel sheet using these cold-rolled steel sheets as base plates. The hot-rolled steel sheet includes a hot-rolled annealed steel sheet annealed after hot rolling, and the cold-rolled steel sheet includes a cold-rolled annealed steel sheet annealed after the cold rolling, and a temper rolling after the cold rolling annealing. Cold-rolled annealed tempered steel sheet. The hot-rolled steel sheets also include those subjected to temper rolling after hot-rolling annealing. Furthermore, in the steel sheet of the present invention, the presence or absence of the oxide scale layer on the surface does not matter.

  The steel sheet of the present invention has a ferrite phase as a mother phase in order to provide excellent workability. When a structure other than ferrite is used as a matrix, it is difficult to easily secure high ductility such that uniform elongation is 30% or more. As the second phase, pearlite, bainite, and martensite may be contained in an area ratio of 10% or less. If the second phase exceeds 10%, the ductility is significantly deteriorated. In addition, since YR tends to decrease as the amount of martensite increases, the amount of martensite is set to less than 5% of the whole. The area ratio of the structure is determined by cross-sectional observation.

Further, the steel sheet of the present invention has the following formula (1)
E = (E ₀ + 2E ₄₅ + E ₉₀ ) / 4 (1)
(Where E ₀ , E ₄₅ and E ₉₀ are rolling directions, Young's modulus (GPa) in the direction of 45 ° in the rolling direction and 90 ° in the rolling direction, respectively), and E is 200 GPa or less at room temperature. Here, room temperature means 0 to 30 ° C., and 10 to 25 ° C. is preferable as the measurement temperature of the Young's modulus.

  If E defined by the equation (1), which represents the average of the Young's modulus, exceeds 200 GPa, it becomes difficult to absorb shocks and the like applied until the completion of assembly by elastic deformation, and plastic deformation occurs in the member. E is preferably 180 GPa or less. When the Young's modulus decreases, the resonance frequency of the steel sheet decreases, and the vibration isolation range of the vehicle body increases.

In addition, the steel plate of the present invention has the following formula (2)
_{YR = (YS 0 + 2YS 45} + YS 90) / (TS 0 + 2TS 45 + TS 90) ... (2)
(However, YS ₀ , YS ₄₅ , and YS ₉₀ are the yield stress (MPa) in the rolling direction, 45 ° in the rolling direction, and 90 ° in the rolling direction, respectively, and TS ₀ , TS ₄₅ , and TS ₉₀ are the rolling directions in the rolling direction, respectively. The tensile strength (MPa) in the direction of 45 ° in the rolling direction and 90 ° in the rolling direction is preferably 0.8 or more.

  By setting the YR to 0.8 or more, there is an effect that the risk of deformation during transportation of the formed panel can be significantly reduced. To increase YR, in addition to suppressing the generation of martensite, add solid solution C, solid solution N, P, Si, Mn, etc., which are solid solution strengthening elements, or finely precipitate carbides, nitrides, and the like. It is effective. Further, temper rolling may be performed.

The steel sheet of the present invention has a yield stress after press forming of 380 MPa or more. Here, “after press forming” refers to a process in which the strain is approximately 10% or more, but it is preferable that the yield stress before the process is approximately 250 MPa or more. The elastic deformation energy until plastic deformation occurs is expressed by (yield stress) ² / (2E). Therefore, in order to increase the energy that can be absorbed by elastic deformation, it is effective to lower E or increase the yield stress. By increasing the yield stress after press molding to 380 MPa or more, It is possible to prevent plastic deformation due to a load such as an impact received in a transportation process up to assembling. Further, when used as an automobile outer panel, such a steel sheet having a low Young's modulus, a high yield stress, and a high YR can absorb the external force only by elastic deformation even if a pebble collides, causing dents, etc. There is a great advantage that no flaw is left.

  In addition, if the yield stress after press forming, that is, after performing a process that causes a strain of 10% or more, is less than 380 MPa, the strength of the component alone becomes insufficient. In order to further exert the above effects, it is preferable that YS after processing is 400 MPa or more.

  Next, the composition limitation of the steel sheet having the above-described characteristics will be described.

C: 0.20% or less C is an important element for securing the strength of the steel sheet. If the C content exceeds 0.20%, weldability and ductility deteriorate, and formability deteriorates. For this reason, C is limited to 0.20% or less. The content is preferably 0.004 to 0.10% from the viewpoint of ductility and weldability.

Si: 0.005 to 1.5%
Si is an element effective for strengthening the steel sheet while minimizing the reduction in ductility of the steel sheet. This effect is observed at 0.005% or more. However, if the addition exceeds 1.5%, the strength of the steel sheet is significantly increased, so that the load in the steel sheet manufacturing process such as an increase in hot deformation resistance is large and hinders the manufacture. For this reason, Si is limited to the range of 0.005 to 1.5%. In addition, from the viewpoint of high strength, the content is preferably 0.10% or more, and more preferably 0.5% or more in order to achieve high strength mainly by increasing the amount of Si.

Mn: 0.05-3.5%
Mn is an effective element for increasing the strength of the steel sheet, and is effective for refining the structure of the steel sheet and forming a low-temperature transformation structure. Such an effect is observed when the addition is 0.05% or more. However, when the addition exceeds 3.5%, the Ar ₃ transformation point becomes too low, and the rolling load increases, so that the rolling in the ferrite region becomes difficult. For this reason, Mn was limited to 0.05 to 3.5%. In order to minimize the decrease in ductility mainly by strengthening with Mn, and to increase the strength to a high strength at which the yield stress after press molding is 380 MPa or more, Mn is added at 0.5% or more, preferably 0.8% or more. It is desirable to do.

P: 0.005 to 0.15%
P is an element effective for solid solution strengthening of a steel sheet, but 0.005% or more of P must be added for this effect to be recognized. On the other hand, if added in excess of 0.15%, the ductility of the steel sheet is significantly reduced. Therefore, P is limited to the range of 0.005 to 0.15%. In order to minimize the decrease in ductility mainly by strengthening with P and to increase the strength so that the yield stress after press molding is 380 MPa or more, P is 0.02% or more, more preferably 0.04% or more. It is preferred that

S: 0.02% or less S is preferably reduced as much as possible in order to reduce the ductility of the steel sheet. From the viewpoint of ensuring ductility, it is allowable up to 0.02%. When a particularly high ductility is required, the content is preferably 0.008% or less.

Al: 0.005 to 0.2%
Al acts as a deoxidizing element, and the addition of 0.005% or more can sufficiently reduce the amount of oxides in steel. If the addition exceeds 0.2%, alumina clusters are formed and surface defects occur frequently, and the hot ductility decreases. Therefore, Al is limited to the range of 0.005 to 0.2%. From the viewpoint of surface properties, the content is preferably in the range of 0.005 to 0.15%. Note that Al may be substantially not added using another deoxidizing element such as Ti or Ca. In the case where reinforcement by solid solution N is mainly used, the amount of Al is preferably set to 0.02% or less in order to reduce the amount of solid solution N fixed as AlN.

N: 0.02% or less N is an element that forms a solid solution in steel and increases the strength of the steel sheet. However, since it deteriorates the aging resistance, it must be added within the range that does not deteriorate the aging resistance to increase the strength. Can be. However, excessive addition causes blowholes on the steel sheet surface, so N is limited to 0.02% or less. For applications requiring ductility, N is preferably set to 0.007% or less. In the case where reinforcement by solid solution N is mainly used, N is preferably 0.005% or more.

Nb: 0.003 to 0.20%, Ti: 0.003 to 0.20%, V: 0.003 to 0.20%, one or more selected from the group
Nb, Ti, and V are effective elements that form carbides or nitrides and precipitate finely in the matrix to increase the strength of the steel sheet and also to uniformly and refine the steel sheet structure. One or more selected from the above can be added as necessary. Nb, Ti, and V are all effective when added in an amount of 0.003% or more. However, when any of the elements is added in excess of 0.20%, the effects are saturated, and an effect corresponding to the added amount cannot be expected. Therefore, Nb, Ti, and V are each limited to the range of 0.003 to 0.20%. In the case of adding in combination, the total amount of Nb, Ti, and V is preferably limited to 0.20% or less. If the total amount of Nb, Ti, and V exceeds 0.20%, the effect tends to be saturated, which is not preferable.

Cu: 0.005 to 0.20%, Ni: 0.005 to 0.20%, Cr: 0.005 to 0.20%, Mo: 0.005 to 0.20% One or more selected from
Cu, Ni, Cr, and Mo are elements that increase the strength (yield stress) of the steel sheet by solid solution strengthening, and one or more of these elements can be added as necessary. The effects of Cu, Ni, Cr, and Mo can be recognized when added in amounts of 0.005% or more, respectively. However, when added in amounts of more than 0.20%, the steel sheet is significantly hardened and the formability is deteriorated. Therefore, Cu, Ni, Cr, and Mo are each preferably in the range of 0.005 to 0.20%. In addition, in the case of adding as a composite, the addition exceeding 0.20% in total amount remarkably reduces ductility and deteriorates moldability. For this reason, the total amount of each element is preferably limited to 0.20% or less.

B: 0.0005-0.01%
B can be added as necessary in order to improve the brittleness in secondary processing when the amount of solute C is significantly reduced. If B is less than 0.0005%, the above effects cannot be expected. On the other hand, if the addition exceeds 0.01%, workability deteriorates. For this reason, B is preferably set in the range of 0.0005 to 0.01%. The content is more preferably 0.0005 to 0.005% from the viewpoint of improving the surface properties of steel.

  The balance is Fe and inevitable impurities. As unavoidable impurities, for example, 0.01% or less can be tolerated for Sn mainly mixed from scrap.

  As described above, in order to increase the strength of the steel sheet by solid solution strengthening of Si, Mn, and P other than C and N, at least one of Si: 0.1% or more, Mn: 0.5% or more, and P: 0.02% or more. Preferably, the composition satisfies at least two or more than one of Si: 0.5% or more, Mn: 0.8% or more, and P: 0.04% or more.

  With the above composition, a high tensile strength steel sheet having a yield stress after press forming of 380 MPa or more is obtained.

  Next, a method for manufacturing a steel sheet having the above-described characteristics will be described.

Hot rolling is performed on a steel material having the above composition range to obtain a hot-rolled steel sheet. The hot rolling in the present invention is a ferrite region rolling, and preferentially forms a ND // <100> texture effective for reducing the Young's modulus as a rolling texture of the ferrite phase. For this purpose, the reduction rate of the final pass is set to 15% or less in the temperature range from the Ar ₃ transformation point to the (Ar ₃ transformation point-100 ° C.), and the rolling end temperature is set to the (Ar ₃ transformation point). (-100 ° C) or higher. In the ferrite zone rolling, ND // <100>, ND // <211>, ND // <111> are developed as rolling textures, but under these rolling conditions, any texture can be recrystallized. Since no strain is accumulated, it is considered that ND // <100> in which the rotation of the crystal is fast erodes the crystal grains of other textures, and ND // <100> texture is formed preferentially. If the rolling conditions deviate from the above range and the accumulation of strain increases, the progress of recrystallization promotes the formation of the ND // <111> texture, and the accumulated strain hinders the rotation of the crystal. Therefore, the formation of the ND // <100> texture is weakened, and a low Young's modulus cannot be achieved.

If the temperature range that regulates the rolling reduction exceeds the Ar ₃ transformation point, rolling in the ferrite region does not occur, and the ND // <100> texture is not formed due to the randomization of the structure due to the transformation. On the other hand, when rolling is performed at a temperature lower than (Ar ₃ transformation point −100 ° C.), strain accumulates. As a result, ND // <111>, in which strain is easily accumulated, is preferentially recrystallized and grown. The number of rolling passes below the Ar ₃ transformation point is desirably 3 or more.

If the rolling reduction in this temperature range is less than 50%, the rotation of the crystal is small and the ND // <100> texture is not formed. The final pass most affects the finally accumulated distortion, so the amount of reduction in this pass is regulated. If the rolling reduction of the final pass exceeds 15%, the amount of strain accumulation increases. If the rolling end temperature is lower than the (Ar ₃ transformation point −100 ° C.), the crystal grains in the ND // <111> orientation tend to recrystallize and grow, and the formation of the ND // <100> texture is not promoted. .

In the rolling at the Ar ₃ transformation point or lower, it is preferable to perform hot rolling in which the rolling reduction excluding the final pass is 30% / pass or less and the number of rolling passes is 5 or more. As a result, the Young's modulus E becomes 180 GPa or less. Outside of this condition, strain accumulation increases, the formation of ND // <111> texture is promoted, and the formation of ND // <100> texture is weakened.

  By using the above hot rolling conditions, a texture effective for lowering the Young's modulus can be effectively formed. In order to precipitate carbides of Ti, Nb and V, it is preferable to control the cooling pattern after hot rolling. Specifically, although it differs depending on the steel composition and the target material (strength, ductility, etc.), it is rapidly cooled at a cooling rate of at least 30 ° C./s from 600 to 750 ° C. immediately after rolling (pre-cooling). It is preferable to perform twill cooling (for example, air cooling) for 3 to 20 seconds. It is not necessary to limit the cooling rate from the twill cooling to the winding, but rapid cooling at about 20 to 80 ° C./s is preferable.

  Next, if the ND // <100> texture is fully developed with hot rolling, it is confirmed that the ND // <100> texture is maintained even after annealing this hot-rolled steel sheet. did. After that, the ND // <100> texture is maintained even after the cold rolling-annealing, hot rolled sheet annealing-cold rolling-recrystallization annealing steps, and the steel sheet remains at a low Young's modulus. Furthermore, since temper rolling hardly affects the Young's modulus, by subjecting the steel sheet to temper rolling, the yield stress of the steel sheet can be increased while keeping the Young's modulus low.

The hot-rolled sheet annealing is performed in a box-type annealing furnace or a continuous annealing furnace at a temperature not higher than the Ac ₁ transformation point, preferably at 400 to 750 ° C (box-type annealing) and 400 to 850 ° C (continuous annealing). It can be performed as needed.

  Further, it is desirable that the cold rolling is performed at a rolling reduction of 40 to 95% from the viewpoint of a low Young's modulus. If the rolling reduction is less than 40%, the structure is remarkably coarsened, and there is a risk of roughening. If it exceeds 95%, cold rolling becomes extremely difficult.

Annealing after cold rolling is performed in a box-type annealing furnace or a continuous annealing furnace at an Ac ₁ transformation point or lower, preferably at a temperature of 650 to 750 ° C (box-type annealing) and 400 to 850 ° C (continuous annealing). It can be carried out.

  The cold-rolled annealed sheet may be subjected to temper rolling at a rolling reduction of 10% or less. Thereby, the yield stress of the steel sheet increases, and the YR increases.

  Molten steel having the chemical composition shown in Table 1 was melted in a converter and cast into a slab by a continuous casting method. After heating these slabs to 1200 ° C., hot rolled steel sheets having a thickness of 1.4 mm were formed under the hot rolling conditions shown in Table 2. A test piece was sampled from the obtained hot-rolled steel sheet, and its Young's modulus was measured. The Young's modulus was measured by a longitudinal vibration resonance method in each of the rolling direction, the rolling direction at 45 ° and the rolling direction at 90 °, and the average E defined by the above equation (1) was obtained. At the time of measurement, the room temperature was 20 ° C.

Furthermore, the obtained hot rolled steel sheet
(B) hot rolled sheet annealing at 700 ° C for 4 hours,
(B) Cold rolling (rolling reduction 50%)-recrystallization annealing at -750 ° C x 30 sec.
(C) Each process of hot rolled sheet annealing at 450 ° C. × 4 h, cold rolling (rolling reduction 50%), recrystallization annealing at 750 ° C. × 30 sec, and temper rolling was performed. In (b) and (c), there was almost no difference in material, but the load of cold rolling was greatly reduced. A test piece was sampled from the steel sheet that passed through each of these steps, and the Young's modulus was measured in the same manner as the hot-rolled sheet. In addition, the structure of each steel sheet was observed.

  Further, test pieces were taken from these steel sheets, and tensile properties (yield stress, tensile strength, elongation) were measured. Furthermore, tensile deformation giving 10% strain was performed, and the yield stress after deformation was measured. Further, with respect to each steel plate, the steel plate was constrained in a cylindrical shape of 300 mmφ, and the amount of impact deformation (concave amount) generated when several levels of weight were colliding was measured. Compare the ratio of impact deformation (concave amount) of each steel plate to the amount of impact deformation (concave amount) of hot rolled sheet of steel plate No. 1 with 1.0 as the amount of impact deformation (concave amount) of hot rolled sheet of steel plate No. 1. did.

  Tables 2 to 5 show these results.

  Each steel sheet had a structure having ferrite as a matrix. The area ratio of the parent phase was 90% or more. The second phase was pearlite, bainite, and martensite.

  Each of the examples of the present invention is a high tensile strength steel sheet having a low Young's modulus E of 200 GPa or less and a yield stress after deformation of 10% of 380 MPa or more. Furthermore, it can be seen that each of the examples of the present invention has characteristics in which plastic deformation is less likely to occur under an impact load than the comparative examples of the same YS and YR.

4 is a graph showing the effect of the rolling temperature and the rolling reduction of hot finish rolling on the Young's modulus of a hot-rolled sheet.

Claims (2)

In mass%,
C: 0.20% or less, Si: 0.005 to 1.5%,
Mn: 0.05-3.5%, P: 0.005-0.15%,
S: 0.02% or less, Al: 0.005 to 0.2%,
N: the composition of the steel material containing 0.020% or less, Ar ₃ transformation point - reduction ratio in the temperature range of (Ar ₃ transformation point -100 ° C.) of 50% or more, reduction ratio of the final pass is 15% or less, A method for producing a high-tensile hot-rolled steel sheet for working, wherein hot rolling is performed at a finish rolling temperature of (Ar ₃ transformation point −100 ° C.) or higher. In mass%,
C: 0.20% or less, Si: 0.005 to 1.5%,
Mn: 0.05-3.5%, P: 0.005-0.15%,
S: 0.02% or less, Al: 0.005 to 0.2%,
N: the composition of the steel material containing 0.020% or less, Ar ₃ transformation point - reduction ratio in the temperature range of (Ar ₃ transformation point -100 ° C.) of 50% or more, reduction ratio of the final pass is 15% or less, A method for producing a high-tensile cold-rolled steel sheet for processing, comprising: performing hot rolling at a finish rolling temperature of (Ar ₃ transformation point −100 ° C.) or higher, followed by cold rolling and recrystallization annealing.

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