JPH10259448A - High strength steel sheet excellent in static absorbed energy and impact resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength steel sheet excellent in absorbed energy and impact resistance at the time of deforming at low strain rate without deteriorating its press formability and to provide a producing method suitable therefor. SOLUTION: This steel sheet has components contg., by mass, 0.05 to 0.20% C, <=2.0% Si, 0.3 to 3.0% Mn, <=0.1% P, <=0.1% Al, the balance Fe and inevitable impurities and has a two phase structure composed of bainitic phases, and the balance substantial ferritic phases, in which the volume ratio of the bainitic phases is regulated to be 20 to 60%, and the ratio of the hardness Hv(B) of the bainitic phases to the hardness Hv(F) of the ferritic phases, i.e., Hv(B)/Hv(F) is regulated to be 1.5 to 2.5. The components may further contain one or more kinds among <=1.0% Mo, 2.5% Cr and <=0.002% B and/or one or more kinds among Ti, Nb, Zr and V by <=0.4% in total, <=2.5% Cu, <=1.5% Ni, and <=0.02% Ca.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention requires formability at the time of press working and is typified by a collision at the time of running of a car, such as a steel plate for a part constituting a reinforcing member of a cabin of a car. The present invention relates to a high-strength steel sheet suitable for a member requiring an excellent protective action against such an impact, that is, impact resistance, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, weight reduction of a vehicle body has been realized mainly by reducing the thickness of a steel plate by increasing the strength of the steel plate. As a conventional method for increasing the strength of a steel sheet for automobiles, for example, Japanese Unexamined Patent Publication No.
Japanese Patent No. 41849 discloses a method in which a solid solution strengthening element such as P and Si is added in addition to carbide forming elements such as Nb and Ti at low C. Also, JP-A-60-5252
No. 8 discloses a method of obtaining a high-strength steel sheet having excellent ductility by precipitating a martensite phase by high-temperature annealing and rapid cooling.

[0003] In these techniques, press formability is taken into consideration, but characteristics at a high strain rate, which is a problem in the event of an automobile collision, are not taken into consideration. It was only designed based on it. That is,
Conventionally, the strength of a steel sheet is determined based on the static tensile strength at a low strain rate, and in the case of an impact state such as a collision accident, that is, the steel sheet undergoes plastic deformation when deformation proceeds at a high strain rate. The absorbed energy absorbed by the steel sheet was not considered much, and the contribution of static strength to the absorbed energy of the steel sheet during high-speed deformation was fixed.

According to the study of the present inventors, the contribution of the static strength to the absorbed energy of the steel sheet during high-speed deformation is not always constant. It was found that the absorption energy of the steel sheet was not improved. FIG. 1 shows that the inventors obtained various steel plates at the time of static tension (strain rate = 0.01 s).
^-1 ) absorbed energy and dynamic tension (strain rate = 8)
00s ^-1 ) is a summary of the absorbed energy,
It can be seen that the absorbed energy during dynamic tension does not increase as much as the absorbed energy during static tension increases when the static strength increases. As shown in FIG. 2, the absorbed energy was calculated as an absorbed energy per unit volume up to a strain amount of 5% based on a stress-strain curve obtained by a tensile test.

In the study of the present inventors, when evaluating the impact resistance of a steel sheet, the crush absorption energy obtained in an actual member impact crush test was obtained by a high-speed tensile test of the steel sheet. It has been found that the correlation with the work hardening property near the yield stress based on the stress-strain curve is very high, and a typical value of this property is a unit volume at which the strain amount at the time of tension is up to about 5%. The energy absorbed per hit is used.

[0006] Therefore, when the weight is reduced by reducing the thickness of the sheet, the impact strength is not improved as expected by the impact deformation when the static strength is simply evaluated. Insufficient impact resistance.

[0007] Incidentally, it is necessary to minimize the deformation of members constituting the periphery of the cabin of an automobile in order to secure a living space for the occupant in the event of a frontal collision of the automobile. At the time of a frontal collision, a member such as a front side member at the front of the vehicle body is largely deformed and absorbs impact, so that the deformation speed of the deformation of the member is low. Therefore, it is desirable that the steel sheets constituting these members have high yield strength and absorbed energy at the time of low strain rate deformation as long as the press workability is not significantly reduced. On the other hand, at the time of a side collision of an automobile, these members around the vehicle body are also directly deformed and deformed at a high strain rate.
Therefore, the steel sheet constituting such a member has high strength and not only absorbed energy at low strain rate deformation (static absorption energy) but also absorbed energy at high strain rate deformation (dynamic absorption energy). It is important to be high.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has a high strength having excellent impact resistance, that is, excellent absorption energy at low strain rate deformation, that is, static absorption energy, without deteriorating press formability. An object of the present invention is to provide a steel sheet and a preferable method for manufacturing the same.

As a steel sheet having excellent impact resistance, for example, Japanese Patent Application Laid-Open No. 52-86919 discloses a method of adjusting the injection operation of molten steel in a casting process and adding an alloy element to molten steel in a mold. A high-strength steel sheet having two inner and outer layers having a specific component composition has been disclosed. The feature of this steel sheet is to control the oxide composition of the welded part and improve the fluidity of the molten metal generated in the welded part. In order to improve the adhesive strength of the nugget and to improve the energy absorption at the time of impact, it is not intended to improve the impact resistance of the steel sheet itself. Is economically disadvantageous.

[0010]

The high-strength steel sheet of the present invention comprises:
It is a two-phase steel sheet having a steel component described below, the bainite phase and the balance substantially consisting of a ferrite phase. The volume ratio of the bainite phase is 20 to 60%, and the hardness Hv of the bainite phase is obtained.
(H) / Hv (B) / Hv
(F) is set to 1.5 to 2.5. here,
The phrase "consisting substantially of a ferrite phase" means that the ferrite phase contains only or a precipitated carbide of a carbide-forming element such as Ti or Nb added as necessary in the ferrite phase. Further, the term “bainite” in the present invention means that bainite containing bainitic ferrite or carbide is included.

Although the reason for the improvement of the impact resistance based on the two-phase structure and the hardness ratio of the steel sheet of the present invention is not always clear, it is necessary to control the bainite volume ratio to an appropriate range and to obtain the bainite phase and the ferrite phase. By properly controlling the hardness ratio, the yield stress can be improved without deteriorating the press workability during static deformation. on the other hand,
At the time of dynamic deformation, the interface between the ferrite phase and the bainite phase hinders the dislocation motion, and the yield stress greatly increases, so that not only static absorption energy but also dynamic absorption energy can be increased.

The reason for limiting the volume fraction of the bainite phase is that when the bainite amount is less than 20%, not only the dynamic absorption energy but also the static absorption energy is reduced and the tensile strength is also reduced. On the other hand, if it exceeds 60%, only the static absorption energy is increased, and the impact resistance is lowered, and at the same time, the hardness is too high and the press formability is deteriorated. Therefore, the lower limit of the bainite amount is set to 20% and the upper limit is set to 60%. In addition,
The amount of bainite can be adjusted by adjusting the C content or adjusting the secondary cooling start temperature and cooling rate during hot rolling or annealing.

The reason for limiting the ratio Hv (B) / Hv (F) between the hardness Hv (B) of the bainite phase and the hardness Hv (F) of the ferrite phase is that Hv (B) / Hv (F) is It contributes to the improvement of impact resistance. If it is less than 1.5, the static absorption energy increases, but the dynamic absorption energy cannot be improved.
Further, although the dynamic absorption energy increases with an increase in the hardness ratio, workability deteriorates, and in particular, stretch flangeability decreases. Therefore, the lower limit of Hv (B) / Hv (F) is 1.5 and the upper limit is 2.5. Preferably 1.7 to 2.
It is better to set to 1.

Incidentally, baking coating is often applied to automotive parts after press molding. It is known that when such a strain aging treatment is applied, if solid solution C remains in the ferrite phase, bake hardenability (BH property), that is, yield stress increases after the aging treatment.

According to the study by the present inventors, in the case of the steel sheet of the present invention, the required amount of solute C remains in the ferrite, so that the yield stress due to bake hardening is significantly higher at high speed deformation than at low speed deformation. I understand. The invention described in claim 4 has been made based on such knowledge, and not only defines the amount of bainite in the two-phase structure and the hardness ratio between the bainite phase and the ferrite phase, but also defines the solid solution in the ferrite phase. By controlling the amount of C to 10 to 30 ppm, the aging treatment at the time of baking coating after press forming such as an automobile member without generating a yield point elongation due to the introduction of movable dislocation near the bainite phase, It is possible to improve absorption energy and impact resistance. If the amount of solute C is less than 10 ppm, the effect of improving the absorbed energy is small, and if it exceeds 30 ppm, elongation at the yield point occurs, and press formability decreases.

The steel sheet of the present invention has a two-phase structure consisting of a predetermined amount of a bainite phase and the remainder substantially a ferrite phase, and the hardness ratio of the two phases or the amount of solute C in the ferrite is adjusted to the predetermined value. By limiting the chemical components (% by mass) as described below, the tensile strength and the static absorption energy are excellent, and the dynamic absorption energy, that is, the impact resistance, is effectively reduced without deteriorating the press formability. Can be improved.

C: 0.05 to 0.20%, Si: 2.
0% or less, Mn: 0.3 to 3.0%, P: 0.1% or less, Al: 0.1% or less, the balance being Fe and unavoidable impurities. Alternatively, in addition to the above basic components, [Mo: 1.0% or less, Cr: 2.5% or less, B: 0.
002% or less] and / or [T
i, Nb, Zr, V: 0.4% or less in total; Cu: 2.
5% or less, Ni: 1.5% or less, Ca: 0.02% or less].

Hereinafter, the reasons for limiting the components will be described. C: 0.05 to 0.20% The workability is improved as the content of C is smaller, but 0.05 to 0.20%.
%, The volume ratio of the bainite phase decreases, making it difficult to secure sufficient strength, and making it impossible to obtain high impact absorption energy. On the other hand, if it is added in excess of 0.20%, the spot weldability, the press formability, and particularly the elongation francy, will decrease. Therefore, the lower limit of the addition amount is 0.05% and the upper limit is 0.20%.

Si: 2.0% or less Si solid-solution strengthens ferrite, is effective in increasing the strength of a steel sheet, and also improves the ductility of the steel sheet. Therefore, it may be added according to the required strength of the steel sheet.
%, Surface flaws are likely to occur, so the upper limit is made 2.0%.

Mn: 0.3 to 3.0% Mn strengthens the steel sheet and improves the hardenability.
However, if it is less than 0.3%, the effect is too small, while if it exceeds 3.0%, the press formability is deteriorated and the spot weldability is also reduced. Therefore, the upper limit is set to 3.0%.

P: 0.1% or less Since P enhances the strength of the steel sheet by solid solution strengthening, it may be added according to the required strength of the steel sheet. However, 0.1
%, The secondary work embrittlement becomes remarkable due to the decrease in the grain boundary strength, and also the impact resistance decreases. Therefore, the upper limit is set to 0.1%.

Al: 0.1% or less Al is added as a deoxidizing element. If added in a large amount, C-based inclusions increase to cause surface flaws and reduce workability. The upper limit is set to 0.1%.

S and N as inevitable impurities deteriorate the material properties when added in large amounts. That is, if a large amount of S is added, the stretch flangeability decreases, so the upper limit is preferably set to 0.01% as a preferable range. Further, when N is added in a large amount, the aging property at normal temperature is lowered and the yield point elongation is generated. Therefore, the upper limit is preferably set to 0.01% as a preferable range.

In addition to the above basic components, in order to improve hardenability, Mo: 1.0% or less, Cr: 2.5% or less,
B: One or more of 0.002% or less can be added.

Mo: 1.0% or less Mo is effective for improving the hardenability and for strengthening the steel sheet by strengthening the precipitation and strengthening the structure. However, 1.0%
Is added, the effect is not only saturated, but also the ductility is reduced, so the upper limit is made 1.0%. Preferably, addition of 0.05% or more is good.

Cr: 2.5% or less Cr is effective in improving the hardenability and strengthening the steel sheet by solid solution strengthening. However, when added in large amounts,
The effect is not only saturated but also lowers the ductility, so the upper limit is made 2.5%. Preferably, addition of 0.05% or more is good.

B: 0.002% or less B is effective for improving the hardenability and for preventing the secondary working embrittlement due to the increase in the strength of the steel sheet and the strengthening of the grain boundary. However, the addition of a large amount lowers the ductility. So its upper limit is 0.0
02%. Preferably, 0.0003% or more is added.

Further, one or more of the following elements can be added without inhibiting the effects of the present invention. Ti, Nb, Zr, V: 0.4% or less in total Ti, Nb, Zr, V are effective for strengthening the steel sheet by precipitation strengthening. However, an excessive addition not only saturates the effect but also reduces the ductility, so the upper limit is made 0.4% in total.

Cu: 2.5% or less, Ni: 1.5% or less Cu and Ni are effective for increasing the strength of a steel sheet by solid solution strengthening and precipitation strengthening, and also contribute to improvement of corrosion resistance. However, the addition of a large amount lowers the ductility, so the upper limit is Cu.
The added amount is 2.5%, and Ni is 1.5%. In addition, although the single addition of Cu may cause surface flaws in the steel sheet depending on the addition amount, it is improved by adding Ni in combination.

Ca: 0.02% or less Ca has the effect of improving the form of inclusions in steel, and is effective in improving the workability and toughness of steel sheets. However, if added in a large amount, the amount of inclusions increases and conversely decreases the cold workability and toughness of the steel sheet, so the upper limit is made 0.02%.

Next, the manufacturing method of the present invention will be described. In the production method of the present invention, the steel having the above components is hot-rolled, and then cold-rolled.
After annealing at a soaking temperature of 920 ° C., 500
50 ° C / s to the secondary cooling start temperature within the range of ~ 850 ° C
After cooling below, at least up to 400 ° C. is cooled at a cooling rate of 10 ° C./s or more and 200 ° C./s or less,
It is further characterized in that it is kept in a temperature range of not higher than 600 ° C. and not lower than 250 ° C. and then cooled. In cooling after maintaining the temperature at 600 to 250 ° C, the temperature is reduced to 5 ° C / 100 ° C or less.
By cooling at s or more, a predetermined amount of solid solution C can be secured.

In the present invention, hot rolling and cold rolling may be carried out in a conventional manner, but it is preferable that the hot rolling finish temperature is not less than ₃ points and the winding temperature is not more than 600 ° C.
The rolling reduction of the cold rolling is preferably 30% or more in order to recrystallize by annealing after cold rolling. Annealing in this case is not particularly limited, but continuous annealing is preferable from the viewpoint of productivity and quality stability.

The soaking temperature during annealing is preferably 760 to 920 ° C. If the temperature is lower than 760 ° C., a sufficient amount of the bainite phase cannot be obtained. On the other hand, if the temperature exceeds 920 ° C., the crystal grains become coarse, and the press formability, particularly the stretch flangeability, deteriorates.

After annealing, cooling is performed at a rate of 50 ° C./s or less from the soaking temperature to the secondary cooling start temperature in the range of 500 to 850 ° C. Thereby, the distribution state of the bainite phase can be made uniform. When the temperature is higher than 850 ° C., the distribution state of the bainite phase is not uniform, and as a result, the ductility is reduced. On the other hand, when the temperature is lower than 500 ° C., precipitation of carbides is promoted, and the bainite phase decreases. If the cooling rate at that time exceeds 50 ° C./s, it is difficult to adjust the temperature to the cooling start temperature, and the reaction for precipitating a predetermined amount of ferrite is non-equilibrium, so that the bainite phase becomes unstable. Easy, and the variation in the material is large.

Cooling from the secondary cooling start temperature is 400 ° C.
Is cooled at a cooling rate of 10 ° C./s or more and 200 ° C./s or less. It is necessary to actively cool to at least 400 ° C., and if the predetermined cooling is stopped before the temperature exceeds 400 ° C., the bainite phase becomes soft and the balance between strength and elongation is reduced. The cooling stop temperature may be any temperature higher than the martensite transformation temperature, but since the temperature fluctuates depending on the components, the cooling is quickly performed after reaching 400 ° C. in order to prevent the formation of martensite as much as possible. It is desirable to stop.

The cooling rate from the secondary cooling start temperature requires rapid cooling to form bainite without reducing the volume ratio of the second phase at the start of cooling and without generating pearlite. If it is less than 10 ° C./s, it is difficult to form a bainite phase, while if it exceeds 200 ° C./s, the amount of solid solution C in ferrite becomes excessive, yield point elongation occurs, and press formability decreases. So
The upper limit is set to 200 ° C./s. Preferably, 25-60 ° C
/ S, and this cooling rate is obtained by mist cooling, roll quenching, or the like.

After the secondary cooling is stopped, the temperature is kept in a temperature range from 600 ° C. to 250 ° C. Austenite stabilized by this treatment (tempering treatment) can be efficiently transformed into bainite. At a temperature higher than 600 ° C., the bainite phase becomes soft, and the strength-elongation balance is reduced. As a result, the static absorption energy is reduced and the dynamic absorption energy is also reduced. On the other hand, if the temperature is lower than 250 ° C., the untransformed austenite is transformed into martensite before transforming into bainite, so that the amount of martensite increases and the stretch flangeability is adversely affected. The holding at the predetermined temperature may be performed simply by keeping the temperature depending on the secondary cooling stop temperature, or the temperature may be held by reheating to the predetermined holding temperature. Although the holding time is not particularly limited, it is usually 5 seconds or longer, and preferably 180 seconds or shorter from the viewpoint of productivity.

After holding at a holding temperature, the mixture is cooled to 100 ° C. or less at a rate of 5 ° C./s or more, so that an appropriate solid solution C (1
0 to 30 ppm). Even when the cooling rate is 5 ° C./s or more, if the lower limit temperature of cooling exceeds 100 ° C., or if the cooling rate is less than 5 ° C./s, the amount of solid solution C decreases, and an appropriate amount of solid solution C cannot be secured.

The steel sheet of the present invention exerts its effect not only on a hot-rolled steel sheet but also on a cold-rolled steel sheet, and is also suitable as an original sheet for various types of coated steel sheets such as hot-dip galvanizing. In addition, the production method of the present invention relates to a method for producing a cold-rolled steel sheet. However, it is needless to say that the production method can also be used as a method for producing an original sheet of various types of plated steel sheets such as hot dip galvanization.

[0040]

【Example】

[Example A] After hot rolling and cold rolling were performed according to a conventional method using steel slabs having the components shown in Tables 1 and 2,
Annealing was performed to obtain a steel plate having a thickness of 1.2 mm. Annealing is 76
After soaking at 0 to 920 ° C, 80 ° C /
s or slower (primary cooling).
Secondary cooling was performed at a temperature of at least 300C / s. Also, 2
After the next cooling, a tempering treatment was performed in which the temperature was maintained at a temperature in the range of 150 to 750 ° C. Further, in order to examine the bake hardenability, some steel sheets were subjected to a BH treatment (aging treatment at 170 ° C. × 20 minutes after imparting 2% prestrain) after annealing.

[0041]

[Table 1]

[0042]

[Table 2]

With respect to the obtained steel sheet, the volume ratio% of the bainite phase, the hardness Hv (B) of the bainite phase and the hardness Hv (F) of the ferrite phase were measured, and the hardness ratio Hv (B) / Hv (F) was measured. ) Was calculated. The volume fraction of the bainite phase was measured from an SEM photograph of the structure at a position 1/4 of the plate thickness from the surface in a section in the plate thickness direction perpendicular to the rolling direction. The hardness of each phase was measured with a Vickers hardness meter at a load of 5 gf.

Further, JIS No. 7 test pieces were sampled from these steel sheets and subjected to tensile tests at a strain rate of 0.01 s ⁻¹ (static test) and 800 s ⁻¹ (dynamic test), and the static tensile strength and Based on the stress-strain curve obtained from each test, the static absorption energy and the dynamic absorption energy per unit volume of the strain amount = 5% were obtained, and whether or not this value exceeded the conventional level shown in FIG. In this manner, the absorbed energy and the impact resistance during low strain rate deformation were evaluated.

Further, with respect to the steel sheet of steel type A, hole expanding characteristics were examined in order to examine press workability. The hole expansion characteristic is determined by inserting a conical punch having a vertex angle of 60 ° into a punched hole having a diameter of 10 mm and expanding the hole to obtain a diameter D in a hole (limit hole) in which a crack has occurred at the edge of the hole. The value was evaluated. λ value (%) = {(D-10) / 10} × 100

In addition, steel type F which was subjected to bake hardening
For the steel sheet No. 3, the amount of solid solution C in ferrite was determined from an internal friction test, and the yield point elongation was determined from a static tensile test.
The test results are shown in Tables 3 and 4.

[0047]

[Table 3]

[0048]

[Table 4]

Looking at sample Nos. C1 to C5 in Table 3, C1 has an insufficient bainite volume ratio, and the absorbed energy is static and dynamic at the conventional level. On the other hand, in the case of the invention example satisfying 20 to 60% of the range of the present invention,
In C2 to C5, the strength increases according to the amount of bainite,
It can be seen that the absorption energy is improved in both static and dynamic, and that excellent characteristics can be obtained.

Regarding press formability, it is desired that stretch flangeability is 90% or more.
Looking at No. 5, No. A2 in which the hardness ratio was controlled within the range of the present invention.
In A4, the absorption energy is improved both in static and dynamic, and it can be seen that good press moldability is obtained.

Looking at the influence of the amount of added Mn from the sample Nos. D1 to D4, the volume fraction of the bainite phase is low in No. D1 because the Mn content is low, and the absorption energy is low in both static and dynamic. You can see that there is.

Next, the influence of the amount of solid solution C in the ferrite from the sample Nos. F1 to F5 was examined. No significant decrease in impact resistance was observed depending on the amount of solid solution C. Since the amount of C is too small, the effect of improving the impact resistance before and after the BH treatment is small. Further, in No. F5, since the amount of solid solution C is excessive, the yield point elongation is large, and the press formability is reduced. No. 3 in which the amount of solid solution C was controlled to 10 to 30 ppm.
From F2 to F4, it can be seen that the absorbed energy is further increased with almost no yield point elongation.

Sample Nos. H1 to H3, I1 to I2,
J, K1-K2, L1-L2, M1-M2, N1-N
2, From P1 to P2 and Q1 to Q5, it can be seen that addition of an auxiliary element within the scope of claim 4 does not hinder improvement in impact resistance.

Example B A billet having the components shown in Table 5
After heating to 00 to 1250 ° C, the finishing temperature is raised to 850 to 9
Hot rolling was completed at a sheet thickness of 4.0 mm at a temperature of 00 ° C., wound at a winding temperature of 550 to 600 ° C., cold-rolled to a sheet thickness of 1.2 mm, and then annealed.

[0055]

[Table 5]

The annealing conditions are as shown in Tables 6 and 7. After soaking at 740 to 980 ° C. (soaking temperature: ST) for 90 seconds, the temperature was increased from 8 to 550 to 880 ° C. (secondary cooling start temperature: TQ). Cool at 70 ° C / s or less (cooling rate from soaking temperature to TQ: CR1).
After cooling at a temperature of s to 280 ° C / s (secondary cooling rate: CR2), it is reheated to a temperature of 680 ° C or less (holding temperature: QT) except for a part, and held at that temperature for 60 to 300s. 2 to 10 ° C / s (cooling rate from reheating temperature: C
The mixture was cooled to 100 ° C. or lower in R3). In addition, BH treatment (170 ° C. ×
Aging treatment for 20 minutes).

From the steel sheet after annealing, the volume ratio% of bainite phase (VB) and the hardness ratio of bainite phase to ferrite phase (Hv (B) / Hv (F)) in the same manner as in [Example A]. , The amount of solute C, the mechanical properties in a static tensile test, the static absorption energy (Es) and the dynamic absorption energy (Ed). The results are shown in Tables 6 and 7.

[0058]

[Table 6]

[0059]

[Table 7]

As shown in Table 6, the samples according to the examples have high strength, good press formability, and static and dynamic absorption energies. Although sample No. E3 had sufficient impact resistance before the BH treatment, the cooling rate from the reheating temperature was slow, so the amount of dissolved C was insufficient, and the absorbed energy after the BH treatment was low. The effect of improving is slightly lacking.

On the other hand, sample Nos. A1, A3 and A
In 4, C2 and C4, the holding temperature is not appropriate, so that the press workability or the improvement in absorbed energy is reduced. In addition, No. B1 has a low secondary cooling start temperature and an increased bainite volume ratio, so that press workability is reduced.

Further, in No. G2, since the secondary cooling rate was high, martensite was formed, a desired bainite volume fraction was not obtained, and the static absorption energy was reduced. Further, in No. G3, the primary cooling rate from the annealing and soaking temperature was high, so that the desired bainite volume fraction was not obtained, and the static / dynamic absorption energy was reduced. On the other hand, since No. G4 has a high soaking temperature, the crystal grains are coarsened and the press workability is reduced.

In Samples D and F, since the steel composition is out of the range of the present invention, the bainite volume ratio is low, and the static / dynamic absorption energy is low. Further, in No. M, since the amount of P was too high, the dynamic absorption energy was low, and the impact resistance was low. On the other hand, it can be seen from Sample Nos. N to Y that the addition of a predetermined amount of an auxiliary element such as Mo, Cr, Ti, or Nb does not hinder the improvement of the impact resistance.

The manufacturing conditions of the steel sheet of the present invention are not limited to the manufacturing conditions of Examples A and B. In addition, the steel sheet of the present invention is required to have formability at the time of press working and is excellent in protection against impacts typified by collisions during driving of automobiles, such as automobile parts, especially parts for reinforcing a passenger compartment. It is suitable as a material steel sheet having an effect, and it is possible to reduce the weight without lowering the collision safety. However, it is needless to say that the steel sheet is not limited to such applications, and various types of steel sheets for which impact resistance is required. It is suitable as.

[0065]

According to the high-strength steel sheet of the present invention, the steel sheet has a specific steel component and has a two-phase structure of a specific amount of bainite phase and the remainder substantially a ferrite phase. Is specified to a predetermined value, without impairing press formability,
It has high strength at the time of low-speed strain rate deformation, and can obtain large static energy and dynamic absorption energy. Further, the amount of solid solution C in the ferrite phase is 10 to 30 pp.
By setting the value of m, the impact resistance can be remarkably improved with almost no yield point elongation.
Further, the production method of the present invention is excellent as an industrial production method of the high-strength steel sheet of the present invention.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between static tensile strength and conventional levels of static tensile absorbed energy (under a strain rate of 0.01 s ⁻¹ ) and dynamic tensile absorbed energy (under a strain rate of 800 s ⁻¹ ).

FIG. 2 is a graph showing a relationship between a stress-strain curve and absorbed energy per unit volume up to a strain amount of 5%.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Mitsuru Kitamura 1 Kanazawacho, Kakogawa-shi, Hyogo Kobe Steel, Ltd. Inside the Kakogawa Works

Claims (6)

[Claims] 1. Mass%: C: 0.05 to 0.20% Si: 2.0% or less Mn: 0.3 to 3.0% P: 0.1% or less Al: 0.1% or less And the balance consists of Fe and unavoidable impurities, has a two-phase structure consisting of a bainite phase at a volume ratio of 20 to 60% and a balance substantially of a ferrite phase, and has a hardness of the bainite phase and a hardness of the ferrite phase. A high strength steel sheet excellent in static absorption energy and impact resistance, having a ratio of 1.5 to 2.5. 2. In addition to the components described in claim 1, Mo: 1.0% or less, Cr: 2.5% or less, B:
The high-strength steel sheet excellent in static absorption energy and impact resistance according to claim 1, containing at least one of 0.002% or less. 3. In addition to the components described in claim 1 or 2,
Further, Ti, Nb, Zr, V: 0.4% or less in total,
Cu: 2.5% or less, Ni: 1.5% or less, Ca: 0.
The high-strength steel sheet excellent in static absorption energy and impact resistance according to claim 1 or 2 containing at least one kind of not more than 02%. 4. The amount of solid solution C in the ferrite phase is 10 to 30.
The high-strength steel sheet excellent in static absorption energy and impact resistance according to any one of claims 1 to 3, which is ppm. 5. A steel having the composition according to claim 1, which is hot-rolled, then cold-rolled, and has a secondary temperature in the range of 500 to 850 ° C. from the soaking temperature. After cooling to the cooling start temperature at a cooling rate of 50 ° C./s or less, 4
Static absorption energy and impact resistance characterized by cooling to 00 ° C at a cooling rate of 10 ° C / s or more and 200 ° C / s or less, and further cooling at a temperature range of 600 ° C or less and 250 ° C or more. For manufacturing high-strength steel sheets with excellent heat resistance. 6. A high strength excellent in static absorption energy and impact resistance according to claim 5, wherein after holding at a temperature range of 600 ° C. or lower and 250 ° C. or higher, cooling is performed at a temperature of 5 ° C./s or lower to 100 ° C. or lower. Steel plate manufacturing method.