JP6237365B2 - High strength steel plate with excellent formability and impact properties - Google Patents

Description

  The present invention relates to a high-strength steel sheet excellent in collision characteristics suitable for manufacturing automobile members.

  Conventionally, in order to increase the fuel efficiency of automobiles and reduce the amount of carbon dioxide emissions, it has been promoted to reduce the weight of automobiles, and high-strength steel sheets capable of reducing the plate thickness have been used as automobile steel sheets. . Moreover, in order to ensure a passenger | crew's safety, the high strength steel plate is used. Therefore, the high-strength steel sheet for automobiles is required to have excellent crash characteristics that can ensure the safety of passengers as well as excellent formability (see, for example, Patent Documents 1 to 3).

  DP steel sheet having a two-phase structure of ferrite and martensite has strength and formability and has excellent energy absorption characteristics, and is therefore used as a steel sheet for automobiles (for example, see Patent Document 4). ). TRIP steel sheets (transformation-induced plastic steel sheets) are used as automobile steel sheets because they have better energy absorption characteristics than DP steel sheets (see, for example, Patent Document 5).

  On the other hand, these steel plates have the following problems. Usually, an automobile body is manufactured by a process of forming a steel plate → assembly by welding → painting.

  Therefore, the impact performance as an automobile member or body is governed by the strength characteristics after molding and painting, but since distortion is not uniformly applied in the part at the time of molding, the inside of the part is localized. When viewed from the viewpoint, the strain-aged portion is hardened by the paint baking while the strain-free portion has a smaller strain age-hardening amount.

  In the case of DP steel, there is a large difference in yield strength after paint baking between the place where distortion is applied and the place where little distortion is applied, and bending may occur in soft parts where distortion is not added. For this reason, stable reaction force characteristics may not be obtained.

  On the other hand, for TRIP steel, there is a case where the difference in yield strength after the paint baking process between a place where distortion is applied and a place where little distortion is applied is large, and it is required to achieve stable impact characteristics. It was.

  In addition, in steel sheets mainly composed of martensite and bainite without containing soft ferrite, the yield strength of the material is originally high, so that the local strength difference in the part after forming-paint baking is reduced, but elongation is increased. Since it is low, there exists a subject that a moldability is inferior.

JP 2009-185355 A JP 2011-111672 A JP 2012-251239 A Japanese Patent Laid-Open No. 11-080878 Japanese Patent Laid-Open No. 11-080879

  In view of the above-mentioned problems, the present invention has a high yield strength that is excellent in formability and impact characteristics, and has a predetermined ductility increase due to paint baking even when parts are not subjected to distortion, and has good ductility. It is an object to provide a steel plate.

  The present inventors first studied diligently about a method for increasing the yield strength even in a place where no strain is applied by a heat treatment equivalent to paint baking.

  As a result, it was found that by increasing the dislocation density in the ferrite and bainite in the steel sheet, the yield strength can be increased by aging without pre-strain. Furthermore, it has been found that the amount of hardening due to aging can be further increased by containing a certain amount of residual γ and further reducing the grain size of ferrite and bainite.

  On the other hand, when the dislocation density of the ferrite and bainite structure was increased, the elongation of the steel sheet was lowered and the formability was lowered. Therefore, the present inventors have examined conditions that can achieve both formability and age-hardening properties. As a result, it has been found that the dislocation density in ferrite and bainite is within an appropriate range, and further, the formability and age-hardening property can be compatible by optimizing the fraction in ferrite and bainite.

  Next, the present inventors conducted extensive studies on a method for optimizing the amount of dislocations in ferrite and bainite.

  As a result of examining the method of introducing dislocation into ferrite or bainite using the volume expansion at the time of transformation from γ phase to martensite, when the martensite formation temperature and the amount of martensite are optimized, the average in ferrite and bainite It was found that the dislocation density can be optimized. Further, it has been found that the control of the dislocation amount in bainite can also be achieved by optimizing the temperature at which bainite is formed.

  Furthermore, the present inventors diligently studied a method for controlling the dislocation density in ferrite according to the condition of temper rolling performed after annealing. As a result, it has been found that the dislocation density in ferrite and bainite can be further optimized by controlling the elongation of temper rolling and the linear load / tension ratio in temper rolling.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) Component composition is mass%, C: 0.05-0.40%, Si: 0.05-3.0%, Mn: 1.5-4.0%, Al: 1.5% Hereinafter, N: 0.02% or less, P: 0.2% or less, S: 0.01% or less, the total amount of Nb and Ti: 0.005 to 0.2%, and the balance: Fe and inevitable Consisting of impurities,
Structure is volume fraction, total amount of ferrite and bainite: 2-60%, martensite: 10-90%, ratio of ferrite fraction to martensite fraction: 0.03 ≦ f _F / f _M ≦ 1 , Residual austenite: 15% or less, and the remaining structure,
The average dislocation density in ferrite and bainite is 3 × 10 ¹² to 1 × 10 ¹⁴ m / m ³ , respectively.
The average particle size of ferrite and bainite is 5 μm or less,
A tensile strength above 980MPa, (YS _BH5 -YS _BH0) is /TS≦0.27, high strength steel sheet excellent in formability and crashworthiness, which is a TS × uEl ≧ 7000.

  (2) The component composition further includes, in mass%, one or two of V and Ta in a total amount of 0.3% or less. High strength steel plate.

  (3) The component composition further includes 1.0% or less in total amount of one or more of Cr, Mo, Ni, Cu, and Sn in mass%. ) Or a high-strength steel sheet having excellent impact characteristics as described in (2).

  (4) The high-strength steel sheet having excellent collision characteristics according to any one of (1) to (3), wherein the component composition further includes, by mass%, B: 0.01% or less. .

  (5) The component composition further includes one or more of Ca: 0.005% or less, Ce: 0.005% or less, and La: 0.005% or less in mass%. A high-strength steel sheet having excellent collision characteristics according to any one of (1) to (4).

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate for motor vehicles provided with the outstanding moldability and collision characteristic can be provided.

The high-strength steel sheet (hereinafter referred to as “the present invention steel sheet”) having excellent formability and impact characteristics according to the present invention.
(a) Component composition is mass%, C: 0.05-0.40%, Si: 0.05-3.0%, Mn: 1.5-4.0%, Al: 1.5% Hereinafter, N: 0.02% or less, P: 0.2% or less, S: 0.01% or less, the total amount of Nb and Ti: 0.005 to 0.2%, and the balance: Fe and inevitable Consisting of impurities,
(b) The structure is a volume fraction, and the total amount of ferrite and bainite: 2 to 60%, martensite: 10 to 90%, ratio of ferrite fraction to martensite fraction: 0.03 ≦ f _F / f _M ≦ 1, residual austenite: 15% or less, and the remaining structure,
(c) The average dislocation density in ferrite and bainite is 3 × 10 ¹² to 1 × 10 ¹⁴ m / m ³ , respectively.
(d) The average particle size of ferrite and bainite is 5 μm or less,
(e) tensile strength is greater than or equal to 980MPa, is a (YS _BH5 -YS _BH0) /TS≦0.27, a TS × uEl ≧ 7000,
It is characterized by that.

Here, YS _BH5 is yield strength after 20% aging treatment at 170 ° C. after adding 5% tensile pre- _strain , YS _BH0 is aging at 170 ° C. for 20 minutes without adding _pre-strain. Yield strength after processing.

  First, the reason for limitation of the component composition which becomes the basis of this invention steel plate is demonstrated. In addition,% concerning a component composition means the mass%.

C: 0.05 to 0.40%
C is an element mainly used for controlling the metal structure of steel. If it is less than 0.05%, it becomes difficult to maintain a tensile strength of 980 MPa or more, so 0.05% or more. Preferably it is 0.08% or more.

  On the other hand, if it exceeds 0.40%, it becomes difficult to secure an appropriate dislocation density in the ferrite while securing a predetermined martensite amount and retained austenite amount, so it is made 0.40% or less. From the viewpoint of weldability, it is preferably 0.35% or less.

Si: 0.05-3.0%
Si is an element that affects the formation of iron carbide, and is used to control strain age hardening characteristics and further to control the metal structure. If it is less than 0.05%, the amount of solid solution C decreases, and it may not be possible to ensure an increase in yield strength after baking, so 0.05% or more. Preferably it is 0.10% or more. On the other hand, if it exceeds 3.0%, a predetermined metal structure cannot be obtained. From the viewpoint of manufacturability, it is preferably 2.0% or less.

Mn: 1.5-4.0%
Mn is an element used for controlling the γ → α transformation and obtaining a predetermined metal structure. If it is less than 1.5%, a tensile strength of 980 MPa or more may not be obtained. Preferably it is 2.0% or more.

  On the other hand, if it exceeds 4.0%, the ductility of the steel sheet is lowered, and it becomes difficult to secure ferrite and bainite having a predetermined dislocation density. Preferably it is 3.5% or less.

Al: 1.5% or less Al is an element used for controlling the γ → α transformation and obtaining a predetermined metal structure. Moreover, it is an element which contributes to ensuring of strain age hardening by suppressing the formation of carbides. However, if it exceeds 1.5%, the martensite fraction cannot be optimized. Preferably it is 1.0% or less.

  The lower limit is not particularly limited, but if it is added as a deoxidizing element in the refining process, usually about 0.002% remains in the steel, so 0.002% on the practical steel plate is a substantial lower limit.

N: 0.02% or less N is an element that contributes to an increase in yield strength after baking by leaving solid solution N remaining. However, if it exceeds 0.02%, a large amount of nitride precipitates and the ductility of the steel sheet decreases, so the content is made 0.02% or less. Preferably it is 0.01% or less.

  The lower limit is not particularly limited, but a reduction to 0.001% or less causes an increase in manufacturing cost, so 0.001% is a substantial lower limit.

P: 0.2% or less P is an element that contributes to improvement in strength and affects strain age hardening. However, if it exceeds 0.2%, the P compound is precipitated and the ductility of the steel sheet is lowered, so the content is made 0.2% or less. From the viewpoint of weldability, it is preferably 0.07% or less.

  The lower limit is not particularly limited, but a reduction to 0.001% or less causes an increase in manufacturing cost, so 0.001% is a substantial lower limit.

S: 0.01% or less S is an impurity element. If it exceeds 0.01%, a large amount of sulfide precipitates to lower the ductility of the steel sheet, so 0.01% is made the upper limit. Preferably it is 0.003% or less. The lower limit is not particularly limited, but a reduction to 0.0002% or less causes an increase in manufacturing cost, so 0.0002% is a substantial lower limit.

Total amount of Nb and Ti: 0.005 to 0.2%
Nb and Ti are elements used for controlling the crystal grains and for obtaining precipitation strengthening. Nb and Ti are elements that form (Ti, Nb) carbonitride and contribute to optimization of the amount of dissolved C and the amount of dissolved N after annealing.

  If the total amount of Nb and Ti is less than 0.005%, the effect of refining the crystal grain size of ferrite and bainite cannot be obtained, and the difference in yield strength after paint baking in the unstrained part and the strain-added part is within a predetermined range. Therefore, the total amount of Nb and Ti is 0.005% or more. Preferably it is 0.010% or more. On the other hand, if the total amount of Nb and Ti exceeds 0.2%, a large amount of carbonitride precipitates and ductility decreases, so the total amount is set to 0.2% or less. Preferably it is 0.1% or less.

  In the above component composition, the balance is Fe and inevitable impurities.

  The steel sheet of the present invention has a total amount of (a) one or two of V and Ta within a range not impairing the properties of the steel sheet of the present invention, and (b) Cr, One or more of Mo, Ni, Cu, and Sn in a total amount of 1.0% or less, (c) B: 0.005% or less, and (d) Ca: 0.005% or less, One group or two or more groups of Ce: 0.005% or less and La: 0.005% or less may be contained.

The total amount of one or two of V and Ta: 0.3% or less V and Ta form carbides, nitrides, or carbonitrides, and contribute to the refinement of ferrite and bainite. It is an element that contributes to improving the strength of steel sheets. However, if the total amount exceeds 0.3%, a large amount of carbonitride precipitates and the ductility of the steel sheet decreases, so V and Ta are set to 0.3% or less in total amount. From the viewpoint of manufacturability, it is preferably 0.1% or less. Although a minimum is not specifically limited, In order to acquire the addition effect, 0.01% or more is preferable.

Total amount of one or more of Cr, Mo, Ni, Cu, and Sn : 1.0% or less Cr, Mo, Ni, Cu, and Sn have a predetermined metal structure in the same manner as Mn. It is an element used for obtaining. However, if it exceeds 1.0%, a predetermined metal structure cannot be obtained. From the viewpoint of manufacturability, it is preferably 0.5% or less. Although a minimum is not specifically limited, In order to acquire the addition effect, 0.1% or more is preferable.

B: 0.01% or less B is an element used to enhance the hardenability of the steel sheet and to control the metal structure. However, if it exceeds 0.01%, a large amount of boride precipitates and the ductility of the steel sheet decreases, so the content is made 0.01% or less. Preferably it is 0.003% or less. Although a minimum is not specifically limited, In order to acquire an addition effect, 0.0003% or more is preferable.

One or more of Ca: 0.005% or less, Ce: 0.005% or less, and La: 0.005% or less. Ca, Ce, and La control the form of oxides and sulfides. It is an element that makes the action. In any case, if it exceeds 0.005%, the effect of addition is saturated and the moldability is lowered, so the content is made 0.005% or less. Preferably it is 0.003% or less. Although a minimum is not specifically limited, In order to acquire the addition effect, 0.001% or more is preferable.

  Next, the reason for limiting the structure of the steel sheet of the present invention will be described. % Related to tissue means volume fraction.

Total amount of ferrite and bainite: 2-60%
Ferrite and bainite are structures that improve the formability of the steel sheet. Here, the ferrite may be any of polygonal ferrite (αp), pseudo-polygonal ferrite (αq), and granular bainitic ferrite (αB) (Reference 1: “Stay Bainite Photo Album— 1 ”Japan Steel Association (1992) p.4).

  Here, bainite includes lower bainite and upper bainite, and bainitic ferrite (α ° B) is classified into bainite (Reference 1: “Steel Bainite Photobook-1” Japan Steel Association. (1992) p.4).

If the total amount of ferrite and bainite is less than 2%, the formability deteriorates, so the total amount is 2% or more. Preferably it is 5% or more. On the other hand, if the total amount of ferrite and bainite is more than 60%, since is possible to ensure (YS _BH5 -YS _BH0) /TS≦0.27 difficult, the total amount is 60% or less. Preferably it is 40% or less.

Martensite: 10-90%
Martensite is a metal structure necessary for securing a tensile strength of 980 MPa or more and controlling the dislocation density in ferrite within an appropriate range.

When martensite is below 10%, it may be kept more tensile strength 980MPa becomes difficult, and the dislocation density in the ferrite becomes less proper _{range, (YS BH5 -YS BH0) /} TS ≦ Since it becomes difficult to obtain 0.27, martensite is made 10% or more. Preferably it is 15% or more.

  On the other hand, if the martensite exceeds 90%, the dislocation density in the ferrite or bainite exceeds the appropriate range and the ductility of the steel sheet decreases, so the martensite is 90% or less. Preferably it is 85% or less. The martensite may be either martensite or tempered martensite as it is quenched, but 80% or more of the martensite is preferably tempered martensite.

Ratio of ferrite fraction to martensite fraction: 0.03 ≦ f _F / f _M ≦ 1.0
The ratio of the ferrite fraction and the martensite fraction is an important parameter for controlling the dislocation density in the ferrite within an appropriate range. Ratio of ferrite fraction to martensite fraction: When f _F / f _M is less than 0.03, the dislocation density in the ferrite increases and the ductility of the steel sheet decreases, so that f _F / f _M is 0 0.03 or more. Preferably it is 0.05% or more.

On the other hand, if f _F / f _M exceeds 1.0, the dislocation density in the ferrite becomes less proper range, since it is difficult to obtain _{(YS BH5 -YS BH0) /TS≦0.27,} f _F / f _M is 1.0 or less. Preferably it is 0.8% or less.

Residual austenite: 15% or less Residual austenite is a metal structure that is effective in improving moldability and impact energy absorption characteristics. Moreover, it is a metal structure which can expect the effect which raises the strain age hardening amount of a ferrite structure and a bainite structure at the time of a paint baking process.

  However, if it exceeds 15%, it becomes difficult to ensure an appropriate dislocation density in the ferrite, and the steel sheet after forming becomes brittle, so the content is made 15% or less. Preferably it is 12% or less. Although a minimum is not specifically limited, If it contains 2% or more, the effect which raises the strain age hardening amount can be anticipated.

  The remaining structure other than the above structure is not particularly limited, but pearlite is preferably within 2%.

Average dislocation density in ferrite and bainite: 3 × 10 ¹² to 1 × 10 ¹⁴ m / m ^{3 respectively}
The dislocation density in ferrite and bainite correlates with the amount of increase in yield strength due to paint baking in the portion where no distortion is caused by molding. As each average dislocation density is less than ^{3 × 10 12 m / m 3} , (YS BH5 -YS BH0) since /TS≦0.27 be ensured ^{difficult, 3 × 10 12 m / m} 3 That's it. Preferably, it is 6 × 10 ¹² m / m ³ or more.

On the other hand, if each of the average dislocation density exceeds ^{1 × 10 14 m / m 3} , with the moldability decreases, because it tends to increase the _{(YS BH5 -YS BH0) / TS} ≦, 1 × 10 14 m / M ³ or less. Preferably, it is 8 × 10 ¹³ m / m ³ or less.

Average grain size of ferrite and bainite : 5 μm or less The average crystal grain size of ferrite and bainite affects the increase in yield strength during aging at 170 ° C. for 20 minutes in the unstrained part.

If the average crystal grain size exceeds 5 [mu] m, the yield strength increases amount of a portion where the distortion is not applied is reduced, (YS _BH5 -YS _BH0) since /TS≦0.27 to meet the difficulty, of ferrite and bainite The average crystal grain size is 5 μm or less. Preferably it is 3 micrometers or less.

  The area ratio of ferrite, bainite, pearlite, and martensite can be measured by a point count method or image analysis using a structure photograph taken with an optical microscope or a scanning electron microscope (SEM). Discrimination between granular bainitic ferrite (αB) and bainitic ferrite (α ° B) is performed by observing the structure with an SEM and a transmission electron microscope (TEM) and referring to Reference Document 1. The fraction of retained austenite is measured by the X-ray diffraction method.

  Next, mechanical properties of the steel sheet of the present invention will be described.

Tensile strength: 980 MPa or more When the tensile strength is less than 980 MPa, the advantage of reducing the weight of the member by increasing the strength of the member cannot be obtained. Therefore, the tensile strength is 980 MPa or more.

(YS _BH5 -YS _BH0) /TS≦0.27
This parameter represents the difference between the yield strength after paint baking of the pre-strained part and the yield strength after paint baking of the non-prestrained part with respect to the maximum tensile strength. It means that the smaller this value is, the smaller the difference in strength in the member after molding and painting. In the case of a skeleton member of an automobile, since a molding strain of 5% or more is introduced in the bent portion or the drawn portion, the pre-strain amount is calculated using a tensile pre-strain of 5%.

When (YS _BH5 -YS _BH0) / TS is greater than 0.27, when the member is subjected to a collision deformation, buckling or deformation to occur locally from soft portion, appropriate reaction force characteristic and energy absorption amount is obtained It may disappear. _{Therefore, (YS BH5 -YS BH0) /} TS is 0.27 or less. Preferably it is 0.18 or less.

TS × uEl ≧ 7000
uEl is a uniform elongation amount in a tensile test, and correlates with stretch formability, stretch flange formability and draw formability. If the product of TS and uEl is less than 7000, cracks often occur due to molding or collision, and it becomes impossible to contribute to weight reduction of automobile members. For this reason, TS × uEl is set to 7000 or more. Preferably it is 8000 or more.

  Next, the manufacturing method of this invention steel plate is demonstrated.

  A slab having a predetermined component composition is manufactured and hot-rolled. The manufacturing method of the slab used for hot rolling is not particularly limited, and may be manufactured by, for example, a continuous casting method, a block method, or a thin slab caster. A process such as continuous casting-direct rolling in which hot rolling is performed immediately after casting can also be employed.

The heating temperature of the hot-rolled slab is preferably 1100 ° C. or higher because it is necessary to redissolve carbonitride precipitated during casting. Rough rolling and descaling performed after slab heating may be performed in accordance with conventional methods, and are not particularly limited. Although there are no particular limitations on the rolling reduction, time between passes, and rolling temperature in finish rolling, the finish rolling temperature is preferably Ar ₃ temperature or higher.

  After the finish rolling, the steel sheet is cooled and subsequently wound up. When the coiling temperature exceeds 680 ° C., the grain size of the ferrite and bainite after annealing becomes coarse and the increase in yield strength after the coating baking process tends to decrease, so the coiling temperature is 680 ° C. or less. Is preferred.

  After winding, it is cooled and subsequently pickled. Following the pickling, cold rolling is performed, but annealing may be performed after pickling and cold rolling may be performed. The annealing performed before cold rolling may be either a continuous annealing furnace or a batch annealing furnace, but when the annealing temperature exceeds 680 ° C., the grain size of ferrite and bainite becomes coarse, and the yield after the coating baking process. Since the amount of increase in strength tends to decrease, the annealing temperature is preferably 680 ° C. or less.

  Regarding the conditions for cold rolling, the number of rolling passes does not have to be specified in particular, and may be in accordance with a conventional method. The rolling reduction is not particularly limited, but if it is less than 30%, the ferrite and bainite grains are coarsened and the yield strength after the coating baking process tends to decrease, so the rolling reduction Is preferably 30% or more.

Moreover, it is preferable that the annealing performed after cold rolling is performed within the range of (Ac ₃ -60) ° C. to 900 ° C. at the highest attained temperature. This is because when the annealing temperature is less than (Ac ₃ -60) ° C., a predetermined metal structure cannot be obtained, and solid solution C or solid solution N cannot be controlled to an appropriate amount, and the yield strength after baking is applied. because it may increase the is not obtained, the annealing temperature is set to (Ac ₃ -60) ℃ or higher. Preferably (Ac ₃ -40) ℃ or higher.

  On the other hand, if the annealing temperature exceeds 900 ° C., the crystal grain size of ferrite and bainite becomes coarse, so the annealing temperature is set to 900 ° C. Preferably it is 870 degrees C or less.

  In order to obtain a suitable crystal grain size, it is preferable that the holding time within the annealing temperature range is 3 to 200 seconds. More preferably, it is 10 to 180 seconds.

  Cooling is performed after annealing at the highest temperature. The cooling conditions are not particularly limited, and may be appropriately selected so that a predetermined metal structure can be obtained. In addition, in order to make the dislocation density in bainite into an appropriate range, the cooling rate between 700-550 degreeC has preferable 4-50 degreeC / s.

  Note that the steel sheet may be plated. Plating may be performed on a continuous annealing / plating line, or may be performed on equipment dedicated to plating different from the annealing line. The composition of the plating is not particularly limited, and any of hot dipping, alloying hot dipping, and electroplating may be used.

After forming a cold-rolled steel sheet, a galvanized steel sheet, and an galvannealed steel sheet, temper rolling is performed. In temper rolling, B / A satisfies 2-120 when the line load during temper rolling is A (N / m) and the tension applied to the steel plate during temper rolling is B (N / m ² ). It is preferable that the rolling conditions be 0.1 to 0.8%.

  Here, B / A is a parameter that affects the uniformity of dislocation density in the steel sheet. If this B / A is less than 2, dislocations are not introduced into the ferrite to the center of the plate thickness of the steel sheet, and the yield strength after baking of the unstrained part does not increase. Therefore, B / A is preferably 2 or more. . More preferably, it is 10 or more.

  On the other hand, if B / A exceeds 120, the non-uniformity of dislocation density in the steel sheet surface increases, and the yield strength may not increase during paint baking, so B / A is preferably 120 or less. More preferably, it is 100 or less.

  If the temper rolling ratio is less than 0.1%, the amount of dislocations introduced into the ferrite becomes insufficient, and the yield strength after baking of the unstrained portion does not increase, so the temper rolling ratio is 0.1%. The above is preferable. More preferably, it is 0.2% or more. On the other hand, if the temper rolling ratio exceeds 0.8%, the formability of the steel sheet may deteriorate, so 0.8% or less is preferable. More preferably, it is 0.6% or less.

  Thus, by optimizing the metal structure and further setting the temper rolling conditions, a predetermined amount of dislocations can be introduced into the ferrite and bainite. As a result, the difference in yield strength between the unstrained part and the strain-added part after the paint baking process can be reduced.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
Steel having the composition shown in Table 1 was melted to produce a steel slab, and this steel slab was heated to 1200 to 1250 ° C., subjected to rough rolling, and subsequently subjected to finish rolling. The blank in Table 1 means that the analysis value was less than the detection limit.

  After finish rolling, cooling was performed, and a winding process was performed at 550 to 700 ° C. After the hot rolling was completed, pickling treatment was performed, the scale was removed, and then cold rolling was performed at a cold rolling rate of 25 to 70% so that the thickness became 1.2 mm. Some of the samples were annealed at 550 ° C. after pickling and then cold-rolled.

  The annealing was performed at an annealing temperature of 780 to 900 ° C. for 60 seconds, and then cooling was performed so that the average cooling rate between 700 to 550 ° C. was 20 ° C./s. After annealing, temper rolling was performed under the conditions of an elongation of 0.3% and B / A = 80.

  Among these steel sheets, “GI” was applied when hot dip galvanizing treatment was performed during and after continuous annealing, “GA” was applied when hot galvanizing was alloyed, and electrogalvanization was applied after cold rolling annealing. In this case, “EG” is shown in Table 2 (below). “CR” is a cold-rolled steel sheet.

  The following evaluation was performed about the obtained steel plate. Tensile test pieces in accordance with JIS Z 2201 were taken with the perpendicular direction of rolling as the longitudinal direction, and tensile tests were conducted in accordance with JIS Z 2241 to evaluate mechanical properties.

  Tensile tests were as follows: (1) raw material, (2) aging treatment at 170 ° C for 20 minutes without addition of prestrain, (3) aging for 20 minutes at 170 ° C after adding 5% tensile prestrain. This was performed on three kinds of the treated ones, and the yield strength (YS), the maximum tensile strength (TS), and the uniform elongation (uEl) were measured.

  The metal structure, its fraction, and the crystal grain size were measured by a point count method or image analysis using a structure photograph taken by SEM and TEM for a ¼ thickness portion of the steel sheet. The crystal grain size of ferrite and bainite was measured as an average nominal grain size of 50 or more crystal grains, each of which is a region surrounded by a grain boundary having an inclination of 15 ° or more.

  The dislocation density ρ in ferrite and bainite is obtained by cutting out a thin film sample for a transmission electron microscope (TEM) from a ¼ thickness portion from the steel sheet surface layer, and then observing the image with a transmission electron microscope, ρ = 2N The dislocation density was calculated from / (Lt). Here, L is the total length of the line arbitrarily drawn on the TEM photograph, N is the number of intersections of the line with the dislocation line, and t is the thickness of the thin film sample. The value of t may be obtained accurately, but in general, a value of 0.1 μm may be used simply. In the image observation, ferrite and bainite were observed at five or more locations, and the average value was calculated.

  The above measurement results and evaluation results are shown in Table 2.

As is apparent from the results shown in Table 2, in the case of an invention example (invention example in the remarks column of Tables 1 and 2) in which steel having a component composition within the range of the present invention was manufactured under appropriate conditions, the tensile strength was 980 MPa or more, in (YS _BH5 -YS _BH0) /TS≦0.27, a TS × uEl ≧ 7000. As a result, it is clear that the invention examples are excellent in moldability and collision characteristics.

(Example 2)
Steel No. For A, the steel sheet having a metal structure of f _{F + B} : 30%, f _M = 68%, f _A = 2%, f _F / f _M = 0.22, And the test was performed by changing the ratio of the line load and tension at the time of rolling. The results are shown in Table 3.

By dislocation density of the ferrite and the bainite is _{changed, (YS BH5 -YS BH0) /} TS, and, TS × UEL changes.

  As described above, according to the present invention, it is possible to provide an automotive steel plate having excellent formability and collision characteristics. Thus, the present invention has high applicability in the steel plate manufacturing and processing industries.

Claims (5)

Component composition is mass%, C: 0.05-0.40%, Si: 0.05-3.0%, Mn: 1.5-4.0%, Al: 1.5% or less, N : 0.02% or less, P: 0.2% or less, S: 0.01% or less, the total amount of Nb and Ti: 0.005 to 0.2%, and the balance: Fe and inevitable impurities ,
Structure is volume fraction, total amount of ferrite and bainite: 2-60%, martensite: 10-90%, ratio of ferrite fraction to martensite fraction: 0.03 ≦ f _F / f _M ≦ 1 , Residual austenite: 15% or less, and the remaining structure,
The average dislocation density in ferrite and bainite is 3 × 10 ¹² to 1 × 10 ¹⁴ m / m ³ , respectively.
The average particle size of ferrite and bainite is 5 μm or less,
A tensile strength above 980MPa, (YS _BH5 -YS _BH0) is /TS≦0.27, high strength steel sheet excellent in formability and crashworthiness, which is a TS × uEl ≧ 7000.   The high-strength steel sheet with excellent impact characteristics according to claim 1, wherein the component composition further includes 0.3% or less of one or two of V and Ta in a total amount by mass%.   The component composition further comprises 1.0% or less of a total amount of one or more of Cr, Mo, Ni, Cu, and Sn in mass%. High-strength steel sheet with excellent impact characteristics as described.   The high-strength steel sheet having excellent collision characteristics according to any one of claims 1 to 3, wherein the component composition further includes, in mass%, B: 0.01% or less.   The component composition further includes one or more of Ca: 0.005% or less, Ce: 0.005% or less, and La: 0.005% or less in mass%. The high-strength steel plate excellent in the collision characteristic of any one of Claims 1-4.