EP2465961B1

EP2465961B1 - High-strength steel sheets and processes for production of the same

Info

Publication number: EP2465961B1
Application number: EP11193464.2A
Authority: EP
Inventors: Kenji Saito; Tomokazu Masuda; Masaaki Miura; Yoichi Mukai; Shushi Ikeda
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2006-07-14
Filing date: 2007-07-13
Publication date: 2013-12-04
Anticipated expiration: 2027-07-13
Also published as: EP2465962B1; EP2465962A1; EP2053140B1; KR101082680B1; CN101460647B; WO2008007785A1; CN101460647A; EP2053140A1; KR20090018166A; EP2465961A1; US20090277547A1; EP2053140A4

Description

The present invention relates to a high strength steel sheet for which high press formability is required, typically including steel sheets for automobiles, particularly to a high strength steel sheet with both elongation and stretch-flanging performance and a method for manufacturing the same.

[Background Art]

High strength steel sheets, which are generally used by being press-molded, are used in industrial product such as automobiles, electric devices and industrial machines. Since high strength steel sheets are used for the purpose of lightening industrial products, they need not only have high strength, but also have the ability to form various configurations of the products. Accordingly, it is required for high strength steel sheets to have excellent press formability. To meet this requirement, high-strength steel sheets having excellent elongation and stretch-flanging performance, which are necessary for improving press formability, are required.
Examples of known steels having such characteristics include dual phase steel (DP steel) whose metal structure is composed of a ferrite phase and a martensite phase, as described in Patent document 1. Since this DP steel can ensure ductility (elongation) due to its soft ferrite and strength due to its rigid martensite, it has both strength and elongation (in particular, uniform elongation) . However, because of the coexistence of soft ferrite and rigid martensite, distortion (stress) is concentrated at the interface of the two phases when deformed, and therefore the interface is likely to serve as the starting point of rupture, thereby disadvantageously preventing ensuring stretch-flanging performance (local elongation).
Examples of steel sheets which expectedly have ductility (especially, uniform elongation) higher than those of DP steels include TRIP steels utilizing the TRIP (Transformation Induced Plasticity) phenomenon, as described in Patent document 2. This TRIP steel is a steel sheet in which uniform elongation is increased by transforming retained austenite into martensite during deformation (working-induced transformation). However, since martensite which has been transformed from retained austenite in the TRIP steel is extremely hard, it likely serves as the starting point of rupture, lowering the stretch-flanging performance of the steel sheet.
Other methods of improving the stretch-flanging performance of the high strength steel sheets include that in which the metal structure is single-phase structure and localization of process distortion is suppressed by homogenizing the in the metal structure, and that in which a difference in strength between a soft phase having a multi-phase metal structure and a hard phase is reduced.
Since martensite single-phase structure steel sheet has a uniform structure, it is known as a steel sheet which has both strength and stretch-flanging performance. However, the martensite single-phase structure steel sheet disadvantageously has low ductility, and insufficient elongation.
Patent document 3 discloses a high-stretch-strength cold-rolled steel sheet in which martensite single-phase structure is achieved by justifying the composition and heat treatment conditions of the steel sheet, and tensile strength is 880 to 1170 MPa. That is, the high-stretch-strength cold-rolled steel sheet of Patent document 3 is produced by heating and retaining a steel sheet having a predetermined composition range at 850°C, which is normally reachable temperature industrially, to transform the steel sheet into austenite, and then rendering it a martensite single-phase structure. A steel sheet of a martensite single-phase structure produced by this invention has a tensile strength of 880 to 1170 MPa, and thus has excellent stretch-flanging performance. However, it has elongation EL (%) lower than 8% and thus has low ductility. In the high strength steel sheet of the invention of Patent document 3, if ductility is improved, press formability can be further improved.
Moreover, Patent document 4 discloses a method for manufacturing a high tensile strength steel sheet, in which a steel sheet in which the ratio by volume of a low-temperature transformation phase comprising a martensite phase and others and a retained austenite phase is 90% or higher of the entire metal structure is heated and retained to produce a two phase region: a ferrite phase and an austenite phase, a metal structure comprising a fine ferrite phase which has succeeded the laths of the low-temperature transformation phase and the austenite phase is provided, and finally the steel sheet is given such a metal structure that comprises ferrite and the low-temperature transformation phase finely dispersed in the form of laths.
However, since the steel sheet produced by the steelmaking method disclosed in Patent document 4 has a relatively high cooling stop temperature in the steelmaking process, a large amount of bainite is deposited, while a large amount of retained austenite also remains therein, and therefore the steel sheet has excellent ductility, but has insufficient stretch-flanging performance. By the steelmaking method of Patent document 4, a steel sheet which is excellent in both elongation and stretch-flanging performance cannot be produced.
[Patent document 1] Japanese Unexamined Patent Application Publication (JP-A) No. S55-122820
[Patent document 2] JP-A-S60-43425
[Patent document 3] Japanese Patent No. 3729108
[Patent document 4] JP-A-2005-272954
JP 2005-213603 discloses a cold rolled steel plate which comprises 0.03 to 0.2% C, ≤2% Si, 0.5 to 3% Mn, ≤0.1% P, ≤0.01% S, 0.01 to 0.1% Sol Al, ≤0.005% N and the balance Fe, and has two phases of ferrite and martensite with ≤2.0 µm grain size of the ferrite. The steel plate is manufactured by hot rolling a steel having the above composition at a temperature equal to or higher than the Ar₃ point for finish rolling, cooling from the finish temperature to ≤550°C at ≥70°C/s cooling rate, winding up at ≤500°C, heat treating the steel at a temperature in a range from 600°C to the Ac₁ point, subjecting the steel to acid pickling and cold rolling, then keeping the steel at a temperature in a range from the Ac₁ point to the Ac₃ point for 10 seconds,; rapidly cooling to 100°C at a cooling rate of ≥100°C/s and tempering at 300 to 500°C for recrystallization annealing/tempering treatment.
JP 2001-92768 discloses a steel plate having a composition containing 0.05 to 0.20% C, 0.3 to 1.8% Si, 1.0 to 3.0% Mn and ≤0.005% S, which is subjected to a first stage in which the same is subjected to primary heating treatment in the temperature range of (the Ac₃ transformation point -50 °C) to (the Ac₃ transformation point + 100 °C) and is thereafter rapidly cooled to the M_s point or less, a second stage in which the same is subjected to secondary heating treatment in a two phase region and is rapidly cooled to 500 °C or less and a third stage in which the same is subjected to hot dip galvanizing treatment and is rapidly cooled to 300 °C in succession. In this way, a composite structure containing a low temperature transformed phase inclusive of, by volume, ferrite of 30% or more, tempered martensite of 20% or more, retained austenite of 2% or more and, preferably, martensite of 2 to 5%, and in which the average crystal grain size of ferrite and tempered martensite is 10 µm or less, preferably, the average grain size is 5 µm or less, is obtained.

[Disclosure of the Invention]

[Problem to be Solved by the Invention]

As mentioned above, since DP steel sheets, TRIP steel sheets, and martensite single-phase structure steel sheets have their advantages and disadvantages, a steel sheet which has high strength and excellent elongation and stretch-flanging performance at the same time is required. The present invention has been made to solve such a problem, and an object thereof is to provide a high strength steel sheet excellent in both elongation and stretch-flanging performance and a method for manufacturing the same.
Another object of the present invention is to provide a high strength steel sheet having a tensile strength of 780 MPa or higher, in which elongation and stretch-flanging performance are both improved, and a method for manufacturing the same.

[Means for Solving the Problem]

The high strength steel sheet of the present invention is constituted of, in percent by mass, C: 0.05 to 0.3%, Si: 0.01 to 3%, Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%, at least an element selected from Ti, Nb, V and Zr in an amount of 0.01 to 1% in total, and optionally, Ni and/or Cu in an amount of 1% or lower in total, Cr: 2% or less, Mo: 1% or less, B: 0.0001 to 0.005%, Ca and/or REM in an amount of 0.003% or lower in total, and the remainder comprising iron and inevitable impurities, has a space factor of martensite phase, wherein the structure which is a main part of the metal structure is the martensite phase and a ferrite phase; the space factor of the martensite phase is 70 to 95%; the space factor of the ferrite phase is 5 to 25%; the total sum of the martensite phase and the ferrite phase is 80% or higher, and the mean grain size of the martensite phase is 10 µm or smaller in terms of the equivalent of a circle diameter, and wherein the structure is allowed as the rest of the metal structure containing bainite, pearlite and retained austenite phase; and has a tensile strength of 590MPa or higher.
To this end, the inventors of the present invention have studied various structures that can ensure high strength and improve elongation, especially stretch-flanging performance at the same time.
The term "equivalent of a circle diameter" means the diameter of an anticipated circle having the same area as the grains of tempered martensite, and is determined by subjecting a structure picture to image analysis. Moreover, the term "space factor" means the percentage by volume, and is determined by corroding a structure observation test piece with nital, observing the test piece with an optical microscope (1000 times), and by subjecting the observed structure picture to image analysis. Moreover, annealed bainite is observed as a body centered cubic structure in terms of a crystal structure.
Herein, Ac₃ point is a temperature at which a two-phase region comprising an austenite phase and a ferrite phase transforms into an austenite single-phase region that is stable at high temperatures in a temperature raising step.
In the high strength steel sheet of the present invention, the structure which is the main part of the metal structure is a martensite phase and a ferrite phase; the space factor of the martensite phase is 70 to 95% (meaning "% by volume", and so on); the space factor of the ferrite phase is 5 to 25%; the total sum of the martensite phase and the ferrite phase is 80% or higher, and the mean grain size of the martensite phase is 10 µm or smaller in terms of the equivalent of a circle diameter and wherein the structure is allowed as the rest of the metal structure containing bainite, pearlite and retained austenite phase.
The method for manufacturing the high strength steel sheet according to the present invention is for manufacturing a high strength steel sheet of the present invention by using, as a material steel sheet, a steel sheet in which the total space factor of the martensite phase and/or bainite phase in the entire metal structure is 90% or higher and the grain size of the former austenite is 20 µm or smaller in terms of the equivalent of a circle diameter, heating and retaining the steel sheet at a temperature of (Ac₃ point -100°C) or higher but not higher than Ac₃ point for 1 to 2400 seconds, then cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10°C/sec. or higher, and subsequently conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 550°C for 60 to 1200 seconds.
The high strength steel sheet according to the present invention may comprise, in addition to the above-mentioned basic components, any of the element groups (a) to (e) described below, or one or more elements selected from a plurality of groups within a range defined for each element group.

(a) an element selected from Ti, Nb, V and Zr: 0.01 to 1% by mass in total
(b) Ni and/or Cu: 1% by mass or less in total
(c) Cr: 2% by mass or less and/or Mo: 1% by mass or less
(d) 0.0001 to 0.005% by mass of B
(e) Ca and/or REM: 0.003% by mass or less in total

[Effect of the Invention]

According to the present invention, it is also possible to achieve a high strength steel sheet which has excellent elongation and stretch-flanging performance et the same time by designing the steel sheet especially for a dual phase steel sheet mainly composed of a ferrite phase and martensite, ensuring high strength of the steel sheet as a whole, and appropriately controlling the space factors of especially the ferrite phase and martensite and the mean grain sizes of the same.

[Best Mode for Carrying out the Invention]

(1)

The best mode for carrying out the invention will be described below in detail.

(3)

Still another embodiment of the present invention will be described in detail below.
The inventors of the present invention studied the requirements for obtaining both strength and elongation, which are the features of a dual phase steel sheet (DP steel sheet), and also stretch-flanging performance by presupposingly using this DP steel sheet comprising the ferrite phase and martensite, from various angles. As a result, the inventors of the present invention found that a very fine ferrite + martensite dual structure can be obtained by subjecting a steel sheet having a fine lath-shaped structure (martensite and/or bainite) as a material steel sheet (that is, as an initial structure) to annealing (hereinafter referred to as "dual-phase range annealing") in a dual-phase range (ferrite +austenite range) . Moreover, the inventors of the present invention also found that a steel sheet having such a structure has good elongation and stretch-flanging performance.
In a steel sheet having the fine lath-shaped structure (martensite and/or bainite) as mentioned above, ferrite produced by the dual-phase range annealing is finely dispersed, and the growth of austenite during the dual-phase range annealing is suppressed by its pinning effect. Accordingly, its structure after hardening becomes a very fine ferrite + martensite structure. In addition, crystal grain micronizing elements such as Ti, Nb, V and Zr are added to the steel sheet as chemical components, whereby further micronization of the structure can be achieved. The thus-obtained dual phase steel sheet is imparted further improved elongation and stretch-flanging performance.
The high strength steel sheet of the present invention is a dual phase steel sheet which is mainly composed of a ferrite phase and martensite. In order to achieve the above object, it is necessary that the space factors of these phases to the entire structure are adjusted appropriately. That is, in the high strength steel sheet of the present invention, the space factors of the ferrite phase and martensite are 5 to 25% and 70 to 95%, respectively.
When the space factor of the ferrite phase is 5% or lower, good elongation cannot be ensured, and the pinning effect which suppresses the growth of austenite becomes weak. When the space factor is higher than 25%, the stretch-flanging performance is deteriorated. A preferable space factor of the ferrite phase is 7% or higher.
When the space factor of martensite is lower than 70%, the stretch-flanging performance is lowered. When the space factor is higher than 95%, elongation is lowered. A preferable space factor of the martensite phase is not higher than 85%.
It should be noted that the term "space factor" mentioned above means the ratio (% by volume) of each phase constituting a metal structure in the steel material to the entire structure, and the space factors of the ferrite phase and martensite can be determined by corroding a steel material with nital, observing the material with an optical microscope (1000 times), and then subjecting the material to image analysis.
In the high strength steel sheet of the present invention, it is preferable that the mean grain size of the above ferrite phase is 3 µm or smaller in terms of the equivalent of a circle diameter, and that the mean grain size of the martensite phase is 6 µm or smaller in terms of the equivalent of a circle diameter. If these sizes are increased, elongation and stretch-flanging performance are lowered. The "mean grain sizes" of these phases are determined, for example, by measuring the grain sizes of twenty grains by observing the structure using an optical microscope and FE/SEM-EBSP, and averaging the measurements.
The dual phase steel sheet according to the present invention is composed of a ferrite phase and martensite as its main structure, but it is not necessarily 100% composed of these phases, and it is also allowed that at least the total sum is 80% or higher, in terms of space factor, due to the intention that it is merely the main part, and that bainite, pearlite, retained austenite and the like are contained. However, the less these structures, the better, from the standpoint of not reducing stretch-flanging performance.
In the steel sheet of the present invention, the structure is controlled in the above-mentioned manner, whereby good elongation and stretch-flanging performance are exhibited. A preferable composition of constituents considering the strength (590 MPa or higher as tensile strength TS) and other points is as follows: C: 0.05 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%; at least one element selected from the group consisting of Ti, Nb, V and Zr: 0.01 to 1% in total; and iron and inevitable impurities as the remainder. The reason for the definition of these preferable ranges is as follow:

[C: 0.05 to 0.3%]

C is an important element in producing martensite to increase the strength of the steel sheet. To perform such an effect, the amount of C contained is preferably 0.05% or higher. From the perspective of increasing strength, the higher the amount of C contained, the better. However, if the amount of C is excessively high, a large amount of retained austenite which deteriorates stretch-flanging performance is produced, and weldability is also adversely affected. Therefore, the amount is preferably 0.3% or lower. A more preferable lower limit of the amount of C contained is 0.07%, and a more preferable upper limit is 0.25%.

[Si: 0.01 to 3%]

Si is an element which effectively acts as a deoxidizing element when steel is melted, and effectively increases strength without deteriorating the ductility of steel. It also acts to suppress deposition of coarse carbide which deteriorates stretch-flanging performance. In order to perform these effects effectively, it is preferably contained in an amount of 0.01% or higher. However, since the effect of adding Si is saturated in an amount of about 3%, A preferable upper limit is set to 3%. A more preferable lower limit of the amount of Si contained is 0.1%, and a more preferable upper limit is 2.5%.

[Mn: 0.5 to 3.0%]

Mn is an element useful in increasing the hardening characteristics of the steel sheet to ensure high strength. To perform such an effect, it is preferably contained in an amount of 0.5% or higher. However, when the amount of Mn contained is excessively high, ductility is lowered and therefore processability is adversely affected. For this reason, the upper limit is set to 3.0%. A more preferable amount of Mn contained is 0.7% or higher but not higher than 2.5% or lower.

[Al: 0.01 to 0.1%]

Al is an element having a deoxidation effect, and when Al deoxidation is performed, it needs to be added in an amount of 0.01% or higher. However, when the amount of Al contained is excessively high, the above effect is saturated, and it also becomes a source of non-metallic mediators to deteriorate physical properties and surface properties. Therefore, the upper limit is set to 0.1%. a more preferable amount of Al contained is 0.03% or higher but not higher than 0.08%.
[One or more members selected from the group consisting of Ti, Nb, V and Zr: 0.01 to 1% in total]
These elements have the action to form precipitates such as carbide, nitride and carbonitride together with C and N to contribute to increased strength, and micronize crystal grains during hot rolling to increase elongation and stretch-flanging performance. Such effects are effectively performed when they are added in an amount of 0.01% or higher in total (of one or more members). A more preferable amount of these elements contained is 0.03% or higher, however, when the amount is excessively high, elongation and stretch-flanging performance are deteriorated rather than improved. Therefore, the amount is to be limited to 1% or lower, and more preferably 0.7% or lower.
Preferable basic components in the dual phase steel sheet of the present invention are as stated above, and the remainder is iron and inevitable impurities. Examples of inevitable impurities include steel raw materials, and P, S, N, 0 and others which can get into steel during the manufacturing process of the materials, among others.
It is also effective to add to the steel sheet of the present invention, as needed, the following substances: (a) Ni and/or Cu: 1% or lower (not including 0%) in total; (b) Cr: 2% or lower (not including 0%) and/or Mo: 1% or lower (not including 0%); (c) B: 0.0001 to 0.005%; and (d) Ca and/or REM: 0.003% or lower (not including 0%) in total, among others. The characteristics of the steel sheet are further improved depending on the types of the components contained. The reason for setting the range of these elements when contained is as follows.

[Ni and/or Cu: 1% or lower (not including 0%) in total]

These elements are effective in maintaining the balance of strength and ductility high and realizing high strength at the same time, such effects increase as their amount contained is increased, but if they are contained in an amount higher than 1% in total (of one or more members), the above effects may be saturated and cracks may be produced during hot rolling. A more preferable lower limit of the amount of these elements contained is 0.05%, and a more preferable upper limit is 0.7%.

[Cr: 2% or lower (not including 0%) and/or Mo: 1% or lower (not including 0%)]

Both Cr and Mo are elements effective in stabilizing the austenite phase, and facilitating the generation of the low-temperature transformation phase in the course of cooling. Although their effects increase as their amount contained is increased, if they are contained in an excessively high amount, ductility is deteriorated. Therefore, the amount of Cr is to be limited to 2% or lower (more preferably 1.5% or lower), and the amount of Mo is to be limited to 1% or lower (more preferably 0.7% or lower).

[B: 0.0001 to 0.005%]

B is an element effective in improving hardening characteristics, and increasing the strength of the steel sheet when added in a minute amount. To exhibit such an effect, it is preferably contained in an amount of 0.0001% or higher. However, when the amount of B contained is excessively high and is higher than 0.005%, crystal grain boundaries may be embrittled and cracks may occur during rolling.

[Ca and/or REM: 0.003% or lower (not including 0%) in total]

Ca and REM (rare earth element) are elements effective in controlling the form of sulfide in the steel and improving processability. The higher their amount contained, the higher the effects. However, when they are contained in an excessively high amount, the above-mentioned effect is saturated. Therefore, the amount is to be 0.003% or lower.
The method for producing the high strength steel sheet having a structure as mentioned above will be described now. In order to produce a high strength steel sheet as mentioned above, it is necessary to conduct a predetermined heat treatment by using a steel sheet in which the space factor of martensite and/or bainite (hereinafter these two phases may be referred to as "low-temperature transformation phases") in total to the entire structure is 90% or higher and the former the grain size of austenite is 20 µm or smaller in terms of the equivalent of a circle diameter.
The material steel sheet used in the present invention has a space factor of the low-temperature transformation phase of 90% or higher. This low-temperature transformation phase may be constituted only by martensite or bainite. When the space factor of the low-temperature transformation phase is lower than 90%, and the material steel sheet is heated to a 2-phase range of the ferrite phase and austenite phase (dual-phase range annealing) in the annealing step (final annealing step) described later, a coarse ferrite phase and an austenite phase are produced. Therefore, the fine ferrite phase and martensite mentioned above cannot be obtained in the final structure. As a result, stretch-flanging performance cannot be improved.
A material steel sheet having space factor of the low-temperature transformation phase of 90% or higher can be produced by the following steps: First, a steel slab adjusted to meet the composition of chemical constituents as mentioned above is hot-rolled in such a manner that the finishing rolling temperature is higher than Ac₃ point. Second, the material steel sheet is cooled to a temperature lower than a martensite transformation start temperature, Ms point (temperature at which the austenite phase starts to transform into martensite), at an average cooling rate of 10°C/sec. or higher, and is then wound up, giving a material steel sheet having a space factor of martensite of 90% or higher. Moreover, a material steel sheet which is mainly composed of bainite and has a space factor of the low-temperature transformation phase of 90% or higher is obtained by cooling the material steel sheet to a bainite transformation temperature after the hot rolling, at an average cooling rate of 10°C/sec. or higher and winding up the same. When the finishing rolling temperature is Ac₃ point or lower or the cooling rate after the hot rolling is 10°C/sec. or lower, a ferrite phase is likely to be produced during cooling after the hot rolling, and therefore the space factor of the low-temperature transformation phase after the hot rolling is not 90% or higher.
In the above hot rolling step, from the perspective of micronization of the structure, it is preferable to appropriately adjust a predetermined heating temperature, and the time for retaining at the heating temperature (retaining time). In the present invention, the grain size of austenite is micronized by utilizing the pinning effect by finely depositing a micro-alloy (Ti, Nb, V, Zr, etc.). In order to do so, it is necessary to re-solutionize the deposition of the coarse micro-alloy produced prior to the hot rolling step. Accordingly, the heating temperature and its retaining time is preferably 1000°C or higher, and 600 seconds or longer, respectively, to perform the solutionization effect of the micro-alloy (Ti, Nb, V, Zr and the like) . When the heating temperature is 1400°C or higher and its retaining time is longer than 1000 seconds, the grain size of austenite becomes undesirably coarse.
The material steel sheet used in the present invention needs to have the grain size of the former austenite of 20 µm or smaller. This is from the perspective of improvement of elongation and stretch-flanging performance due to the micronization of the structure. That is, by subjecting a basis steel sheet having a grain size of the former austenite of 20 µm or smaller to the final annealing step and tempering step, the final structure becomes finer than in the case where the grain size is larger than 20 µm, and elongation and stretch-flanging performance are significantly improved.
Even a steel sheet produced from a steel slab adjusted so as to meet the chemical components as mentioned above under conditions which do not meet the hot rolling and cooling rate conditions mentioned above can be imparted a space factor of the low-temperature transformation phase of 90% or higher by subjecting to the following preliminary annealing (Experiments No.5, 6 in Table 14 described later).
This preliminary annealing is a treatment in which the above steel sheet is retained in a temperature range of Ac₃ point or higher for 5 seconds or longer, and is then cooled at an average cooling rate of 10°C/sec. or higher to a temperature of Ms point or lower or to the bainite transformation temperature range and retained. When the retaining temperature of the steel sheet is lower than Ac₃ point, a ferrite phase is likely to be produced, and a space factor of the low-temperature transformation phase of 90% or higher is not attained. Moreover, even in the case where the steel sheet is retained in a temperature range of Ac₃ point or higher, if the retaining time is shorter than 5 seconds, transformation of the metal structure into austenite is insufficient, and a space factor of 90% or higher is not attained.
By subjecting the material steel sheet whose structure and grain size of the former austenite have been adjusted in the manner described above to a heat treatment (final annealing step and tempering step) as described below, a high strength steel sheet whose space factors and grain sizes of the ferrite phase and martensite are appropriately adjusted can be obtained. At this time, the case where not only the preliminary annealing step but also acid cleaning, a cold rolling step and other processes are carried out between the hot rolling step and the heat treatment step described below also falls within the scope of the present invention. The actions and effects under the heat treatment conditions at this time are as follow:
First, a material steel sheet is subjected to a heat treatment in which it is heated to and retained at a temperature range of (Ac₃ point -100°C) or higher but not higher than Ac₃ point for 1 second or longer but not longer than 2400 seconds, and is then cooled at a cooling rate of 10°C/sec or higher to Ms point or lower (cooling stop temperature). By performing such an annealing step, a steel sheet having the structure (the space factor of ferrite: 5 to 30%, the space factor of martensite: 50 to 95%) mentioned above is obtained. Moreover, the mean crystal grain diameters of the ferrite phase and martensite in the high strength steel sheet which is finally obtained are determined by the sizes of the crystal grains of the ferrite phase and austenite produced when the material steel sheet is heated to and retained at a temperature range of (Ac₃ point -100°C) or higher but not higher than Ac₃ point. That is, in order to obtain a fine dual phase steel sheet in which the mean grain size of the ferrite phase is 3 µm or smaller and the mean grain size of martensite is 6 µm or smaller, it is necessary to heat the material steel sheet to a temperature range of (Ac₃ point -100°C) or higher but not higher than Ac₃ point and retain at the same.
In this annealing step, when the material steel sheet is heated and retained at a temperature range higher than Ac₃ point in which the austenite single phase is stable, crystal grains of austenite grow and combine with each other to become coarse, and the pinning effect of fine ferrite is not produced, whereby a fine dual phase steel sheet cannot be obtained. As a result, the stretch-flanging performance of the high strength steel sheet is lowered.
The "pinning effect" mentioned above is as follows: the basis steel sheet has a Structure form mainly composed of a highly micronized lath-shaped low-temperature transformation phase due to the micronization effect of the micro-alloy. When such a steel sheet is heated to the high temperature side of the dual-phase range, a finely dispersed ferrite phase having a low space factor is produced. The term "ferrite phase" used in this invention denotes annealed martensite or annealed bainite produced when martensite or bainite is annealed at a high temperature (dual-phase range). Since such a ferrite phase suppresses the growth and combination of the austenite phase, the final structure obtained in the following hardening and tempering steps becomes a structure mainly composed of a very fine ferrite phase and martensite. When the material steel sheet is heated and retained at a temperature lower than (Ac₃ point -100°C), transformation into austenite does not proceed sufficiently, and the space factor of martensite after the heat treatment becomes lower than 50%, thereby lowering the stretch-flanging performance of the steel sheet.
In this annealing step, if the heating and retaining time is shorter than 1 second, the austenite phase is not sufficiently produced, and therefore a space factor of martensite of 50% or higher cannot be attained after this annealing step. When the heating and retaining time is longer than 2400 seconds, the crystal grains of austenite produced become coarse, and therefore the fine dual structure mentioned above cannot be obtained. From such a perspective, the heating and retaining time in the final annealing needs to be in the range of 1 second or longer but not longer than 2400 seconds. It is preferably 5 seconds or longer but shorter than 1200 seconds.
When the cooling rate after heating and retaining is 10°C/sec. or lower or the cooling stop temperature is higher than Ms point, generation of bainite, retained austenite phase and pearlite, generation of more ferrite phase than necessary, and deposition of a cementite phase are caused, and structures other than martensite are formed in large amounts. Therefore, the space factor of martensite is lowered, and the space factor and mean crystal grain size of the ferrite phase are excessively increased, leading to lowered elongation and stretch-flanging performance. The higher the cooling rate at this time, and the lower the cooling stop temperature, the higher the space factor of martensite is likely to be. However, since the temperature and time of the above dual-phase range annealing are suitably controlled, the space factor becomes no greater than 95%.
After the annealing step as mentioned above is performed, it is necessary to carry out tempering (reheating treatment) in which the material steel sheet is retained at a temperature range of 300 to 550°C for 60 seconds or longer but not longer than 1200 seconds. In the steel sheet which has undergone the annealing step as mentioned above, fine (ferrite phase +martensite) is formed in its metal structure. However, martensite in an annealed state is very hard, which lowers elongation. Moreover, since martensite is hard, a difference in hardness between martensite and soft ferrite is large, which leads to lowered stretch-flanging performance. In order to obtain excellent elongation and stretch-flanging performance, the hardness of martensite needs to be reduced than in an annealed state, which is why it is subjected to the tempering step.
When the retaining temperature in this tempering step is lower than 300°C, softening of martensite is insufficient, and therefore elongation and stretch-flanging performance of the steel sheet are lowered. In contrast, when the retaining temperature is higher than 550°C, a coarse cementite phase is deposited, whereby the stretch-flanging performance of the steel sheet is lowered.
When the retaining time of the tempering step is shorter than 60 seconds, softening of martensite is insufficient, and therefore elongation and stretch-flangingperformance of the steel sheet are lowered. In contrast, when the retaining time is longer than 1200 seconds, martensite is too softened so that ensuring strength is made difficult, and the stretch-flanging performance of the steel sheet is lowered by the deposition of cementite. This retaining time is preferably 90 seconds or longer but not longer than 900 seconds, and more preferably 120 seconds or longer but not longer than 600 seconds.
By subjecting the material steel sheet as mentioned above to annealing (final annealing) and tempering as mentioned above, a steel sheet in which the space factors and grain sizes of the ferrite phase and martensite are suitably adjusted can be obtained, and a tensile strength as high as 590 MPa and excellent elongation and stretch-flanging performance are achieved. Such a high strength steel sheet can be used as a steel sheet with excellent press formability as a material for various steel products typically including automobiles.

(Example 1)

Now, the present invention will be described more specifically by referring to Examples. The present invention is not restricted in itself by the following Examples. Therefore, it is possible to carry out the invention by properly modifying the Examples within the above described or later describe spirit of the invention, and such modifications are all to be included in the technical scope of the present invention.
steel slabs having compositions of chemical constituents shown in Tables 10 and 11 below were prepared, and material steel sheets were prepared from the steel slabs under the hot rolling conditions and preliminary annealing conditions shown in Tables 12 and 13 below. Tables 10 and 11 also show the Ac₃ point (Ac₃ transformation point) and martensite transformation start temperature, Ms point, for each steel type determined by equations (1) and (2). ${Ac}_{3} ({}^{°}C) = 910 - 203 \cdot \sqrt [C] - 15.2 \cdot [Ni] + 44.7 \cdot [Si] + 104 \cdot [V] + 31.5 \cdot [Mo] + 13.1 \cdot [W] - 330 \cdot [Mn] + 11 \cdot [Cr] + 20 \cdot [Cu] - 720 \cdot [P] - 400 [Al] - 120 \cdot [As] - 400 \cdot [Ti]$
$Ms ({}^{°}C) = 550 - 361 \cdot [C] - 39 \cdot [Mn] - 35 \cdot [V] - 20 \cdot [Cr] - 17 \cdot [Ni] - 10 \cdot [Cu] - 5 \cdot [Mo] - 5 [W] + 15 \cdot [Co] + 30 \cdot [Al]$
However, [C], [Ni], [Sil], [v], [Mo], [W], [Mn], [Cr], [Cu], [P], [Al], [As], [Ti] and [Co] represent the amounts contained of C, Ni, Si, V, Mo, W, Mn, Cr, Cu, P, Al, As, Ti and Co (% by mass), respectively.

[Table 10]

Steel type	Composition of chemical components* (% by mass)											Transformation point
Steel type	C	Si	Mn	P	S	Al	Ti	Nb	V	Zr	Others	Ac₃(°C)	Ms (°C)
A	0.12	1.23	1.58	0.010	0.001	0.045	-	-	-	-	-	872	446
B	0.25	1.57	2.24	0.013	0.002	0.041	-	-	-	-	-	837	374
C	0.01	0.87	1.53	0.012	0.002	0.042	-	-	0.015	-	-	909	487
D	0.08	1.87	2.21	0,016	0.001	0.037	-	-	0.020	-	-	898	435
E	0.25	1.51	2.05	0.020	0.002	0.031	-	-	0.016	-	-	843	380
F	0.35	1.49	1.98	0.012	0.002	0.032	-	-	0.012	-	-	820	347
G	0.18	0.05	2.03	0.009	0.001	0.029	0.022	-	-	-	-	797	407
H	0.16	2.63	1.20	0.009	0.002	0.033	0.037	-	-	-	-	945	446
I	0.21	3.54	2.08	0.011	0.001	0.038	-	0.011	-	-	-	936	394
J	0.13	1.51	0.38	0.009	0.003	0.039	-	0.015	-	-	-	915	489
K	0.12	1.49	0.62	0.009	0.001	0.031	-	0.021	-	-	-	906	483
L	0.22	1.21	2.78	0.006	0.002	0.031	-	-	-	0,021	-	802	363
M	0.20	1.50	3.49	0.013	0.001	0.033	-	-	-	0.022	-	804	343
N	0,17	1.35	2.02	0.015	0.002	0.005	-	-	0.018	-	-	840	409
O	0.19	1.32	1.99	0.011	0.003	0.087	-	-	0.017	-	-	865	406
P	0.18	1.43	2.04	0.012	0.001	0.162	-	-	0.012	-	-	901	410
Q	0.17	1.32	1.92	0.012	0.002	0.040	0.005	-	-	-	-	854	415
R	0.18	1.28	2.12	0.014	0.002	0.038	-	0.001	0.001	-	-	843	403

* Remainder: Iron and inevitable impurities other than P and S

[Table 11]

Steel type	Composition of chemical components* (% by mass)											Transformation point
Steel type	C	Si	Mn	P	S	Al	Ti	Nb	V	Zr	Others	AC₃ (°C)	Ms (°C)
A1	0.17	1.40	2.05	0.009	0.002	0.028	0.015	-	-	-	-	851	410
B1	0.17	1.20	2.20	0.010	0.001	0.031	-	0.018	-	-	-	833	404
C1	0.19	1.45	2.10	0.009	0.001	0.031	-	-	0.021	-	-	844	400
D1	0.20	1.37	2.04	0.015	0.003	0.027	-	-	-	0.015	-	841	399
E1	0.17	1.35	2.00	0.011	0.002	0.035	0.016	0.012	-	-	-	855	412
F1	0.18	1.37	2.13	0.010	0.002	0.037	0.011	-	0.022	-	-	850	402
G1	0.19	1.34	2.06	0.010	0.002	0.031	0.005	0.015	0.004	-	-	841	402
H1	0.16	1.26	1.78	0.014	0.002	0.035	-	-	0.150	-	-	872	419
I1	0.17	1.34	1.99	0.009	0.002	0.028	-	-	0.014	-	Ni:0.2	845	411
J1	0.16	1.41	1.90	0.010	0.001	0.031	-	-	0.023	-	Cu:0.1	857	418
K1	0.17	1.32	2.18	0.012	0.002	0.040	-	-	0.025	-	Cr:0.35	843	397
L1	0.16	1.26	2.00	0.015	0.003	0.042	-	-	0.019	0.03	Mo:0.1	858	414
M1	0.19	1.32	1.95	0.016	0.003	0.045	-	-	0.025	-	B:0.0002	854	406
N1	0.16	1.39	1.95	0.013	0.002	0.039	-	-	0.017	-	Ca+REM:0.001	859	417
O1	0.19	1.33	1.89	0.009	0.003	0.039	-	1.135	-	-	-	846	409
P1	0.21	1.35	2.08	0.011	0.003	0.037	0.154	-	0.404	0.551	-	941	380

* Remainder: Iron and inevitable impurities other than P and S

[Table 12]

Experiment No.	Steel type	Hot-rolling conditions				Preliminary annealing conditions
		Heating temperature	Retaining time	Hot finishing temperature	Winding temperature	Heating temperature	Retaining time	Cooling rate	Cooling stop temperature
		(°C)	(sec.)	(°C)	(°C)	(°C)	(sec.)	(°C/sec.)	(°C)
1	A	1300	1800	930	550	850	120	500	20
2	B	1250	1800	950	550	880	240	300	20
3	C	1300	2400	850	600	900	90	300	20
4	D	1300	1800	850	550	930	240	200	20
5	E	1200	1800	880	250	-	-	-	-
6	F	1200	1200	850	300	-	-	-	-
7	G	1300	2400	800	500	900	60	200	20
8	H	1250	1800	850	600	930	120	300	20
9	I	1300	1800	900	550	930	120	100	50
10	J	1300	1800	900	500	910	60	100	20
11	K	1200	2400	900	550	930	120	500	20
12	L	1300	1800	850	550	930	360	300	20
13	M	1200	1800	850	500	850	120	100	20
14	N	1300	1800	880	500	860	120	300	20
15	O	1300	2400	900	500	880	10	300	20
16	P	1300	1800	900	550	930	180	100	50
17	Q	1250	1800	850	550	880	120	300	20
18	R	1250	1800	850	550	930	180	300	20

[Table 13]

Experiment No.	Steel type	Hot-rolling conditions				Preliminary annealing conditions
		Heating temperature	Retaining time	Hot finishing temperature	Winding temperature	Heating temperature	Retaining time	Cooling rate	Cooling stop temperature
		(°C)	(sec.)	(°C)	(°C)	(°C)	(sec.)	(°C/sec.)	(°C)
19	A1	1300	1800	850	550	880	120	300	20
20	B1	1200	1800	850	550	850	60	300	20
21	C1	1300	1800	850	550	850	0	300	20
22	D1	1300	1800	850	550	880	120	300	20
23	E1	1300	1800	900	550	870	30	300	20
24	F1	1300	1800	900	550	860	90	300	20
25	G1	1300	1800	900	550	850	60	300	20
26	H1	1350	1800	900	550	890	120	300	20
27	I1	1250	1800	850	550	860	120	300	20
28	J1	1300	1800	870	550	875	120	300	20
29	K1	1200	1800	850	550	870	10	300	20
30	L1	1300	2400	870	550	860	120	300	20
31	M1	1300	1200	850	550	900	120	300	20
32	N1	1300	1200	850	550	870	120	300	20
33	O1	1200	1800	850	550	930	240	200	20
34	P1	1300	1200	950	550	950	120	200	20
35	B1	1300	1800	850	550	900	120	300	20
36	C1	1300	1800	850	550	880	120	300	20

The material steel sheets obtained were subjected to the final annealing and reheating (tempering) under the conditions shown in Tables 14 and 15 below to prepare test steel sheets, and the structures (space factor of ferrite α, mean grain size of ferrite α, space factor of martensite M, and mean grain size of martensite M) and mechanical characteristics (tensile strength TS, elongation EL, hole expansion rate λ) of the test steel sheets were determined by the methods described below. Tables 14 and 15 below also show the structures [phase constitution, space factor of low-temperature transformation phase, grain size of former austenite (γ)] of the test steel sheets before the final annealing.

[Method for measuring structures of test steel sheets]

The space factors of ferrite α and martensite M were determined by subjecting the structure pictures of the test steel sheets after being corroded with nital to image analysis. The mean grain sizes of ferrite α and martensite M were measured by structure analysis using FE/SEM-EBSP, and the measurements were converted into "the equivalent of a circle diameter" described above to determine their mean value.

[Method for measuring mechanical characteristics of test steel sheets]

(a) Tensile test: A universal tensile tester manufactured by Instron was used to determine tensile strength (TS) and elongation (total elongation rate: EL) by using JIS No. 5 tensile test pieces.
(b) Hole expansion test: 20-ton hole expansion tester manufactured by Tokyo Koki was used to determine hole expansion rates (λ) according to Japan Iron and Steel Federation standard (JFST1001-1996), and stretch-flanging performance was evaluated.

[Table 14]

Experiment No.	Steel type	Structure before final annealing			Final annealing conditions				Tempering conditions
Experiment No.	Steel type	Phase constitution*	Space factor of low-temperature transformation phase (% by volume)	Grain size of former γ (µm)	Heating temperature (°C)	Retaining time (sec.)	Cooling rate (°C/sec.)	Cooling stop temperature (°C)	Heating temperature (°C)	Retaining time (sec.)
1	A	M	100	28	850	120	500	20	400	180
2	B	M	100	29	810	120	300	20	400	120
3	C	M	100	18	850	240	100	20	500	180
4	D	M	100	11	870	120	200	20	500	180
5	E	M	93	13	815	90	300	20	520	120
6	F	M	97	17	810	240	100	20	350	180
7	G	M	100	12	750	120	200	100	400	120
8	H	α + M	95	11	910	360	300	50	500	180
9	I	α + M	95	16	870	120	100	20	350	120
10	J	α + M	95	14	900	90	100	20	450	180
11	K	M	100	12	850	180	500	20	520	180
12	L	M	100	12	770	120	300	20	500	180
13	M	M	100	13	795	120	100	20	400	180
14	N	M	100	10	820	120	300	20	500	180
15	O	M	100	14	850	120	300	20	500	180
16	P	M	100	9	880	120	100	50	400	120
17	Q	M	100	24	830	120	300	20	500	180
18	R	M	100	29	830	120	300	20	400	120

* M: Martensite, α: Ferrite, γ: Austenite

[Table 15]

Experiment No.	Steel type	Structure before final annealing			Final annealing conditions				Tempering conditions
Experiment No.	Steel type	Phase constitution*	Space factor of low-temperature transformation phase (% by volume)	Grain size of former γ (µm)	Heating temperature (°C)	Retaining time (sec.)	Cooling rate (°C/sec.)	Cooling stop temperature (°C)	Heating temperature (°C)	Retaining time (sec.)
19	A1	M	100	12	830	180	300	20	500	180
20	B1	M	100	9	825	120	300	20	500	240
21	C1	M	100	7	800	120	300	20	500	180
22	D1	M	100	12	810	120	300	20	500	180
23	E1	M	100	10	790	180	300	20	520	180
24	F1	M	100	11	810	180	300	20	500	180
25	G1	M	100	9	810	180	300	20	500	180
26	H1	M	100	10	840	240	300	20	500	120
27	I1	M	100	12	825	120	300	20	500	120
28	J1	M	100	11	830	120	300	20	500	180
29	K1	M	100	8	810	120	300	20	500	180
30	L1	M	100	9	850	60	300	20	500	180
31	M1	M	100	13	820	120	300	20	500	180
32	N1	M	100	10	830	120	300	20	500	180
33	O1	M	100	16	830	180	200	50	500	180
34	P1	M	100	14	900	120	200	50	500	180
35	B1	M	100	15	730	120	300	20	500	120
36	C1	M	100	13	860	120	300	20	400	120

* M: Martensite, α: Ferrite, γ: Austenite

The measurement results of the structures (space factor of ferrite α, mean grain size of ferrite α, space factor of martensite M, mean grain size of M), and mechanical characteristics (tensile strength TS, elongation EL, hole expansion rate λ) of the test steel sheets are shown in Tables 16 and 17 below. As for "evaluation" of the mechanical characteristics, tensile strength (TS) of 590 MPa or higher, elongation (EL) of 10% or higher, and hole expansion rate (λ) of 80% or higher were rated excellent characteristics. The samples which were excellent in all three characteristics were rated o; those which were excellent in two characteristics out of three were rated Δ; and those which were excellent in only one characteristic out of three were rated ×. Only ○ was rated a pass.

[Table 16]

Experiment No.	Steel type	Structure of steel plate				Mechanical characteristics			Evaluation
		Space factor of α	Mean grain size of α	Space factor of M	Mean grain size of M	Tensile strength TS	Elongation EL	λ
		(% by volume)	(µm)	(% by volume)	(µm)	(MPa)	(%)	(%)
1	A	12	4.8	88	4.3	609	28.5	67.3	Δ
2	B	13	4.9	87	4.5	1341	7.9	71.4	×
3	C	19	2.9	81	2.5	548	32.1	82.5	Δ
4	D	12	2.1	88	2.2	993	12.1	113.0	○
5	E	13	2.3	85	2.0	1107	11.9	108.0	○
6	F	11	2.7	89	2.1	1398	6.1	54.2	×
7	G	16	2.8	84	2.4	776	17.9	107.3	○
8	H	14	2.3	86	2.3	1024	13.2	105.0	○
9	I	28	3.0	72	2.3	1419	5.8	27.4	×
10	J	35	3.8	65	2.8	587	29.0	65.9	×
11	K	25	2.8	75	2.4	603	28.3	86.3	○
12	L	14	2.5	86	2.1	1117	12.1	100.3	○
13	M	12	2.5	88	2.2	1311	7.9	59.9	×
14	N	11	1.6	89	1.7	1024	14.2	125.0	○
15	O	10	1.3	90	1.5	1018	15.4	128.3	○
18	P	13	2.5	87	2.0	1032	10.1	67.3	Δ
17	Q	12	4.0	88	3.9	1098	12.1	72.9	Δ
18	R	11	4.7	89	4.1	1015	10.8	69.5	Δ

* M: Martensite, α: Ferrite

[Table 17]

Experi-ment No.	Steel type	Structure of steel plate				Mechanical characteristics			Evaluation
		Space factor of α	Mean grain size of α	Space factor of M	Mean grain size of M	Tensile strength TS	Elongation EL	λ
		(% by volume)	(µm)	(% by volume)	(µm)	(MPa)	(%)	(%)
19	A1	12	2.0	88	1.9	1022	11.9	112.0	○
20	B1	9	1.6	91	1.5	995	13.2	117.2	○
21	C1	26	1.8	74	1.9	978	14.2	119.9	○
22	D1	12	2.1	88	2.0	1017	12.5	112.7	○
23	E1	22	2.2	78	1.9	716	16.2	97.3	○
24	F1	16	2.0	84	2.1	1012	14.9	118.6	○
25	G1	13	1.7	87	1.9	1023	13.6	124.1	○
26	H1	8	1.4	92	1.3	1100	13.8	118.8	○
27	I1	10	1.7	90	1.9	1025	14.3	123.6	○
28	J1	11	1.8	89	1.8	1098	13.7	121.8	○
29	K1	12	1.9	88	1.9	1167	11.6	112.1	○
30	L1	7	1.1	93	1.3	1228	10.9	98.5	○
31	M1	12	2.0	88	1.9	1145	11.0	103.5	○
32	N1	12	1.8	88	1.8	1097	11.7	105.2	○
33	O1	13	2.7	87	2.1	1212	8.4	22.9	×
34	P1	16	2.2	84	1.9	1329	5.3	19.1	×
35	B1	78	13.2	22	3.5	578	16.3	34.4	×
36	C1	0	-	100	3.8	1383	5.9	78.5	×

* M: Martensite, α: Ferrite

It is possible to consider as follows from these results: Since the samples of Experiments No.4, 5, 7,8,11,12,14,15, 19 to 20 and 22 to 32 all meet the requirements defined in the present invention, they are all provided with excellent characteristics.
In contrast, the test pieces of No.1 to 3, 6, 9, 10, 13, 16 to 18 and 33 to 36 are not provided with satisfactory characteristics as the followings because at least one requirement of their composition of chemical constituents and manufacturing conditions falls outside the scope defined in the present invention.
Since the test pieces of Experiments No. 1,2 do not contain Ti, Nb, V, Zr and the like, the grain size of the former γ in the material steel sheet (steel sheet before the final annealing) became coarse, and the desired elongation and stretch-flanging performance could not be obtained.
The test piece of Experiment No.3 has low tensile strength TS since the amount of C contained does not fall with in the preferable range defined in the present invention. The test piece of Experiment No. 6 has strength higher than necessary because the amount of C contained is higher than the preferable range defined in the present invention, so that ductility is lowered and elongation characteristics are deteriorated.
In the test piece of Experiment No. 9, the amount of Si contained is higher than the preferable range defined in the present invention, and therefore its ductility is lowered, and elongation and stretch-flanging performance are deteriorated.
In the test piece of Experiment No. 10, the amount of Mn contained does not fall with in the preferable range defined in the present invention, and therefore the space factor of ferrite is increased, deteriorating tensile strength and stretch-flanging performance.
In the test piece of Experiment No. 13, the amount of Mn contained is higher than the preferable range defined in the present invention, and therefore its ductility is lowered, deteriorating elongation and stretch-flanging performance.
In the test piece of No. 16, flaws on the surface of the steel material are increased because the amount of Al is higher than the preferable range defined in the present invention, whereby ductility of flawed material is lowered, and stretch-flanging performance is deteriorated.
In the test pieces of Experiments Nos. 17 and 18, the amounts of Ti, Nb, V, Zr and the like contained are low. Therefore, micronization has not been sufficiently produced, and desired stretch-flanging performance has not been obtained.
In the test pieces of Experiments Nos.33 and 34, the amounts of Ti, Nb, V, Zr and the like contained are too high. Therefore, coarse carbide remains even under predetermined heat treatment conditions, and elongation and stretch-flanging performance are deteriorated.
In the test piece of Experiment No. 35, the heating temperature in the final annealing is much below the range defined in the present invention. Therefore, the space factor and mean grain size of ferrite, the space factor and mean grain size of martensite in the final structure fall outside the range defined in the present invention, and desired tensile strength and stretch-flanging performance have not been obtained.
The test piece of Experiment No. 36, the heating temperature in the final annealing is much above the range defined in the present invention. Therefore, the final structure became a single-phase structure of martensite, and the space factor of ferrite and the space factor and mean grain size of martensite fall outside the range defined in the present invention. Accordingly, desired elongation and stretch-flanging performance have not been obtained.

[Industrial Applicability]

The high strength steel sheet according to the present invention has excellent elongation and stretch-flanging performance at the same time, and thus has excellent press formability. Therefore, the high strength steel sheet according to the present invention can be processed by press molding to be used for various industrial products such as automobiles, especially for industrial products where weight reduction is necessary.

Claims

A high strength steel sheet which comprises, in percent by mass, C: 0.05 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%; at least an element selected from Ti, Nb, V and Zr in an amount of 0.01 to 1% in total, and optionally, Ni and/or Cu in an amount of 1% or lower in total, Cr: 2% or less, Mo: 1% or less, B: 0.0001 to 0.005%, Ca and/or REM in an amount of 0.003% or lower in total, and the remainder comprising iron and inevitable impurities, the high strength steel sheet having a space factor of a martensite phase, wherein the structure which is a main part of the metal structure is the martensite phase and a ferrite phase; the space factor of the martensite phase is 70 to 95%; the space factor of the ferrite phase is 5 to 25%; the total sum of the martensite phase and the ferrite phase is 80% or higher, and the mean grain size of the martensite phase is 10 µm or smaller in terms of the equivalent of a circle diameter, and wherein the structure is allowed as the rest of the metal structure containing bainite, pearlite and retained austenite phase; and a tensile strength of 590 MPa or higher.
A method for manufacturing a high strength steel sheet according to claim 1, the method comprising providing a total space factor of the martensite phase and/or bainite phase in the entire metal structure is 90% or higher; using a steel sheet having a grain size of the former austenite of 20 µm or smaller in terms of the equivalent of a circle diameter as a material steel sheet; heating and retaining the steel sheet at a temperature of (Ac₃ point -100°C) or higher but not higher than Ac₃ point for 1 to 2400 seconds; then cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10°C/sec. or higher; and subsequently conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 550°C for 60 to 1200 seconds.