JP5017751B2 - Highly ductile hot-rolled steel sheet excellent in press formability and strain age hardening characteristics and method for producing the same - Google Patents

Description

【００６５】
【表２】

【００６６】
【表３】

【００６７】
本発明例は、いずれも、高い伸びElと高い強度延性バランス（ＴＳ×El）を有し、さらに大きな穴拡げ率λを示して、伸びフランジ成形性に優れている。 また、 本発明例は、いずれも、極めて大きなΔＴＳを示し、歪時効硬化特性に優れた鋼板となっている。これに対し、本発明の範囲を外れる比較例では、伸びElが低いか、あるいは穴拡げ率λが小さいか、ΔＴＳが小さく、プレス成形性、歪時効硬化特性が低下した鋼板となっている。
【００６８】
(実施例２）
表４に示す組成の溶鋼を転炉で溶製し、連続鋳造法でスラブとした。ついで、これら鋼スラブを加熱し、表５に示す条件で熱間圧延して、板厚2.0mmの熱延鋼帯（熱延板）にし、さらに圧下率：1.0％の調質圧延を施した。
得られた熱延鋼帯（熱延板）について、実施例１と同様の方法で、微視組織、引張特性、歪時効硬化特性、穴拡げ率を求めた。なお、プレス成形性は、伸びＥｌ（延性）、ＴＳ×Ｅｌバランスおよび穴拡げ率λから評価した。
【００６９】
得られた結果を表６に示す。
【００７０】
【表４】

【００７１】
【表５】

【００７２】
【表６】

【００７３】
本発明例は、いずれも、高い伸びElと高い強度延性バランス（ＴＳ×El）を有し、プレス成形性に優れるとともに、極めて大きなΔＴＳを示し、歪時効硬化特性に優れた鋼板となっている。これに対し、本発明の範囲を外れる比較例では、伸びElが低いか、あるいはΔＴＳが小さく、プレス成形性、歪時効硬化特性が低下した鋼板となっている。
【００７４】
【発明の効果】
本発明によれば、優れたプレス成形性を維持しつつ、プレス成形後の熱処理により引張強さが顕著に上昇する熱延鋼板を、安定して製造することが可能となり、産業上格段の効果を奏する。本発明の熱延鋼板を自動車部品に適用した場合、プレス成形が容易で、かつ完成後の部品特性を安定して高くでき、自動車車体の軽量化に十分に寄与できるという効果もある。
【図面の簡単な説明】
【図１】予変形−熱処理後のΔＴＳと鋼板組織の関係におよぼすCu含有量の影響を示すグラフである。
【図２】予変形−熱処理後のΔＴＳと熱処理温度の関係におよぼすCu含有量の影響を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention mainly relates to hot-rolled steel sheets for automobiles, and in particular, press formability such as ductility, stretch flangeability, drawability, etc. is good, and the tensile strength is remarkably increased by heat treatment after press forming, The present invention relates to a high ductility hot-rolled steel sheet having extremely large strain age hardening characteristics and a method for producing the same.
[0002]
[Prior art]
In recent years, in connection with exhaust gas regulations due to global environmental conservation issues, the reduction of vehicle body weight has become a very important issue. Recently, in order to reduce the weight of the vehicle body, it has been studied to increase the strength of a steel plate for automobiles and reduce the thickness of the steel plate.
Since many automotive body parts made of steel sheets are formed by press working, the hot-rolled steel sheets used are required to have excellent press formability. In order to obtain a steel sheet having excellent press formability, it is important to secure high ductility first. In addition, stretch flange molding is often used, and it is necessary to have a high hole expansion rate. However, generally, when the strength of a steel plate is increased, the ductility is lowered, the hole expansion rate is lowered, and the press formability tends to be lowered. For this reason, conventionally, a high-strength hot-rolled steel sheet having high ductility and excellent press formability has been desired.
[0003]
Recently, in order to protect an occupant in the event of a collision, importance is placed on the safety of the automobile body. For this reason, an improvement in impact resistance that is a measure of safety in the event of a collision is required. The higher the strength of the finished vehicle, the more advantageous for improving the impact resistance. Therefore, when forming automotive parts, hot-rolled steel sheets that have low strength, high ductility, excellent press formability, and high strength and excellent impact resistance are most strongly desired when finished products are produced. It was.
[0004]
In response to such a demand, a steel sheet having both press formability and high strength has been developed. This steel sheet is a paint bake hardening type steel sheet in which the yield stress increases when a coating baking process including holding at a high temperature of 100 to 200 ° C. is performed after press working. In this steel sheet, the amount of C (solid C amount) that is finally present in a solid solution state is controlled within an appropriate range, and it is soft during press forming, ensures shape freezing and ductility, and is baked after press forming. At the time of processing, the remaining solid solution C adheres to the dislocations introduced during press forming, thereby preventing the dislocations from moving and increasing the yield stress. However, in the paint bake hardened automotive steel sheet, although the yield stress can be increased, the tensile strength cannot be increased.
[0005]
Japanese Examined Patent Publication No. 5-24979 includes C: 0.08 to 0.20%, Mn: 1.5 to 3.5%, and has a composition composed of the remaining Fe and inevitable impurities, and the structure has a ferrite content of 5% or less. A bake hardenable high-tensile cold-rolled steel sheet composed of uniform bainite or bainite partially containing martensite is disclosed. The cold-rolled steel sheet described in Japanese Patent Publication No. 5-24979 is rapidly cooled in a temperature range of 400 to 200 ° C. in the cooling process after continuous annealing, and then gradually cooled, whereby the structure is made mainly of conventional ferrite. From a structure | tissue, it is going to obtain the high bake hardening amount which was not conventionally as a structure | tissue mainly composed of bainite.
[0006]
However, in the steel sheet described in Japanese Patent Publication No. 5-24979, the yield stress increases after baking, and a high bake hardening amount that has not been obtained in the past can be obtained, but it is still difficult to increase the tensile strength, There was a problem that improvement in impact characteristics could not be expected.
Several hot-rolled steel sheets that are subjected to heat treatment after press forming to increase not only yield stress but also tensile strength have been proposed.
[0007]
For example, Japanese Patent Publication No. 8-23048 discloses steel containing C: 0.02 to 0.13%, Si: 2.0% or less, Mn: 0.6 to 2.5%, sol.Al: 0.10% or less, and N: 0.0080 to 0.0250%. , Reheat to 1100 ° C or higher, perform hot rolling to finish finish rolling at 850-950 ° C, then cool and wind to a temperature of less than 150 ° C at a cooling rate of 15 ° C / s, ferrite and martensite There has been proposed a method for producing a hot-rolled steel sheet having a composite structure mainly composed of. However, the steel sheet manufactured by the technique described in Japanese Patent Publication No. 8-23048 is wound at an extremely low coiling temperature of less than 150 ° C, although the tensile strength increases with yield stress due to strain age hardening, There was a problem that the fluctuation of mechanical characteristics was large. In addition, there is a large variation in the amount of increase in yield stress after press molding-paint baking, and there is also a problem that the hole expansion rate (λ) is low, stretch flangeability is lowered, and press formability is insufficient.
[0008]
Japanese Patent No. 2802513 proposes a method for producing a hot-dip galvanized steel sheet using a hot-rolled sheet as a plating original sheet. In this method, a steel slab containing C: 0.05% or less, Mn: 0.05-0.5%, Al: 0.1% or less, and Cu: 0.8-2.0% is hot-rolled at a coiling temperature of 530 ° C. or less. Subsequently, after heating to a temperature of 530 ° C. or lower to reduce the surface of the steel sheet, hot dip galvanization is performed, whereby significant hardening by heat treatment after forming is obtained. However, in the steel sheet produced by this method, in order to obtain significant hardening by post-forming heat treatment, the heat treatment temperature needs to be 500 ° C. or higher, and the heat treatment temperature is high, leaving a practical problem.
[0009]
Japanese Patent Application Laid-Open No. 10-310824 proposes a method for producing an alloyed hot-dip galvanized steel sheet in which a hot rolled plate or a cold rolled plate is used as a plating original plate and an increase in strength can be expected by heat treatment after forming. This method includes C: 0.01 to 0.08%, and Si, Mn, P, S, Al, and N are used in appropriate amounts, and one or more of Cr, W, and Mo are added in a total amount of 0.05 to 3.0. % Hot-rolled steel, or cold-rolled or temper-rolled and annealed, and then hot-dip galvanized, and then heat-alloyed. This steel sheet is said to have an increased tensile strength by heating in a temperature range of 200 to 450 ° C. after forming. However, since the obtained steel sheet has a microstructure of ferrite single phase, ferrite + pearlite, or ferrite + bainite structure, high ductility and low yield strength cannot be obtained and press formability is deteriorated. there were.
[0010]
Japanese Patent Application Laid-Open No. 11-199975 includes C: 0.03 to 0.20%, and Cu, 0.2 to 2.0% and B: 0.0002 to 0.002 with appropriate amounts of Si, Mn, P, S and Al. %, And the microstructure is a composite structure with ferrite as the main phase and martensite as the second phase, and the presence of Cu in the ferrite phase is 2 nm or less in a solid solution state and / or a precipitated state. A hot-rolled steel sheet for processing having excellent characteristics has been proposed. The steel sheet described in Japanese Patent Application Laid-Open No. 11-199975 improves the fatigue limit ratio only when Cu and B are added in combination and the presence state of Cu is as fine as 2 nm or less. And for that, Ar _Three Hot finish rolling is completed at the transformation point or higher, and Ar during the cooling process _Three ~ Ar ₁ It is essential that the air is cooled for 1 to 10 seconds in the temperature range up to the transformation point, then cooled at a cooling rate of 20 ° C./s or more, and wound at a temperature of 350 ° C. or less. Thus, when the coiling temperature is set to a low temperature of 350 ° C. or less, there is a problem that the shape of the hot-rolled steel sheet is greatly disturbed and it is difficult to produce it stably industrially.
[0011]
[Problems to be solved by the invention]
As described above, the present invention has been made in view of the fact that there has never been a technique for industrially and stably producing a steel sheet that satisfies these characteristics, despite extremely strong demands. Strain age hardening that solves the above-mentioned problems advantageously, has excellent press formability, suitable as a steel sheet for automobiles, and has an extremely large tensile strength by heat treatment at a relatively low temperature after press forming. An object of the present invention is to propose a high-ductility high-tensile hot-rolled steel sheet having excellent characteristics and a production method capable of stably producing the high-ductility high-tensile hot-rolled steel sheet.
[0012]
[Means for Solving the Problems]
In order to achieve the above-described problems, the present inventors have conducted intensive research on the influence of steel sheet structure and alloy elements on strain age hardening characteristics. As a result, the steel sheet structure is a composite structure of ferrite and a phase containing residual austenite, and in addition, the C content is set to a low carbon region to a medium carbon region, and Cu, Mo in place of Cu, or Cr in an appropriate range. By adding one or more of W, pre-strain amount: 5% or more, after pre-deformation treatment and heat treatment at a relatively low temperature of 150 ° C to 350 ° C, increase yield stress In addition, it has been found that a steel sheet having a high strain age hardening in which the tensile strength is remarkably increased can be obtained. Further, the steel sheet has been found to be a steel sheet having excellent ductility and a high hole expansion ratio in addition to such high strain age hardening characteristics, and having excellent press formability.
[0013]
First, basic experimental results performed by the present inventors will be described.
In mass%, C: 0.10%, Si: 1.4%, Mn: 1.5%, P: 0.01%, S: 0.005%, Al: 0.04%, N: 0.002%, Cu 0.3%, 1.3% The sheet bar having the changed composition is heated to 1250 ° C., soaked, and then subjected to three-pass rolling so that the finish rolling finish temperature is 850 ° C. to a sheet thickness of 2.0 mm. Various changes were made to form a hot-rolled sheet having a composite structure in which the structure is a single phase of ferrite and the second phase is a phase containing ferrite as a main phase and containing retained austenite (hereinafter also referred to as a composite structure of ferrite and retained austenite). .
[0014]
About the obtained hot-rolled sheet, the tensile test was implemented and the tensile characteristic was investigated. Further, the specimens taken from these hot-rolled sheets were subjected to a pre-deformation treatment with a tensile pre-strain amount of 5%, followed by a heat treatment of 50 to 350 ° C. × 20 min. Age hardening properties were evaluated.
The strain age hardening property was evaluated by the increase in tensile strength ΔTS before and after the heat treatment. ΔTS is the tensile strength TS after heat treatment _HT And the difference in tensile strength TS when heat treatment is not applied {= (tensile strength TS after heat treatment _HT )-(Tensile strength TS before pre-deformation treatment)}. In addition, the tensile test was implemented using the JIS No. 5 tensile test piece extract | collected in the rolling direction.
[0015]
FIG. 1 shows the effect of Cu content on the relationship between ΔTS and steel sheet structure. ΔTS was obtained by performing a pre-deformation treatment with a tensile pre-strain amount of 5% and then performing a heat treatment at 250 ° C. × 20 min. FIG. 1 shows that when the Cu content is 1.3% by mass, a high strain age hardening characteristic of ΔTS: 80 MPa or more can be obtained by making the steel sheet structure a composite structure of ferrite and retained austenite. When the Cu content is 0.3% by mass, ΔTS: less than 80 MPa regardless of the steel sheet structure, and high strain age hardening characteristics cannot be obtained.
[0016]
Thus, a hot rolled steel sheet having high strain age hardening characteristics can be obtained by setting the Cu content to an appropriate range, and making the structure a composite structure having ferrite as a main phase and the second phase as a phase containing residual austenite. It can be seen that it can be manufactured.
FIG. 2 shows the influence of the Cu content on the relationship between ΔTS and the heat treatment temperature after the pre-deformation treatment. The microstructure of the steel sheet was a composite structure in which ferrite was the main phase and the second phase was a phase containing residual austenite, and the structural fraction of residual austenite was 8% in terms of volume ratio relative to the entire structure.
[0017]
From FIG. 2, ΔTS increases as the heat treatment temperature increases, but the amount of increase greatly depends on the Cu content. It can be seen that when the Cu content is 1.3% by mass, a high strain age hardening property of ΔTS: 80 MPa or more can be obtained at a heat treatment temperature of 150 ° C. or higher. When the Cu content is 0.3% by mass, ΔTS: less than 80 MPa at any heat treatment temperature, and high strain age hardening characteristics cannot be obtained.
[0018]
Further, a hole expansion test was performed on a hot rolled sheet containing a ferrite single phase structure or a composite structure of ferrite + residual austenite and containing 0.3% by mass and 1.3% by mass of Cu to obtain a hole expansion rate λ. In the hole expansion test, punch holes are formed in the specimen by punching with a 10mmφ punch, and then a conical punch with an apex angle of 60 degrees is used so that the burr is on the outside and cracks that penetrate the plate thickness occur. The hole expansion rate λ was obtained. The hole expansion rate λ is λ (%) = {(dd ₀ ) / D ₀ } It calculated | required in * 100. Where d ₀ : Initial hole diameter, d: Inner hole diameter when cracking occurs.
[0019]
In the case of a hot-rolled sheet having a Cu content of 1.3% by mass and having a composite structure of ferrite + residual austenite, the hole expansion ratio is about 140%, and even when the structure is a ferrite single phase. The hole expansion rate was about 140%. On the other hand, when the Cu content is 0.3%, the hole expansion ratio is 120% when the structure is a single ferrite phase. However, when the structure has a composite structure of ferrite and retained austenite, the hole expansion ratio is It was as low as about 80%.
[0020]
As described above, in the hot-rolled steel sheet having a composite structure of ferrite and retained austenite, it has been clarified that when the Cu content is increased, the hole expansion rate is increased and the hole expansion formability is improved. The detailed mechanism by which the hole-expanding formability is enhanced by the inclusion of Cu has not been clarified until now, but the hardness difference between ferrite, retained austenite and strain-induced martensite due to the inclusion of Cu is not clear. It may be because it has become smaller.
[0021]
Further, in the hot-rolled steel sheet of the present invention, pre-deformation with a strain amount greater than 2%, which is a pre-strain amount when measuring the increase in deformation stress before and after normal heat treatment, and relatively low such as 150 ° C. or more and 350 ° C. or less. Due to the heat treatment in the temperature range, ultrafine Cu is precipitated in the steel sheet. According to the study by the present inventors, it is considered that high strain age hardening characteristics in which the tensile strength is remarkably increased in addition to the increase in yield stress are obtained by the precipitation of this ultrafine Cu. Such precipitation of ultrafine Cu by heat treatment in a low temperature region was not observed at all in the extremely low carbon steel or low carbon steel reported so far. The reason why ultrafine Cu is precipitated by heat treatment in a low temperature range has not been clarified until now, but it is rapidly cooled after the end of hot rolling and isothermically maintained in a temperature range of 620 to 780 ° C. During the gradual cooling treatment from the temperature range, a large amount of Cu is distributed to the γ phase, which is taken over after cooling and becomes a state in which Cu is supersaturated in the residual austenite. It is considered that the retained austenite is strain-induced transformed into martensite by pre-straining of 5% or more, and Cu is precipitated very finely in the martensite subjected to strain-induced transformation by subsequent low-temperature heat treatment.
[0022]
As a result of further intensive studies based on the above new findings, the present inventors have found that the above-described phenomenon can also occur with respect to a steel composition not containing Cu.
A steel structure with a composition containing one or more of Mo, Cr, and W is a composite structure in which the ferrite phase is the main phase and the phase containing residual austenite is the second phase. It was found that by performing heat treatment in the region, ultrafine carbides were strain-induced precipitated in the strain-induced martensite and the tensile strength was increased. It has also been found that this strain-induced low-temperature fine precipitation becomes more prominent by adding one or more of Nb, Ti and V in addition to one or more of Mo, Cr and W. . The present invention has been completed by further study based on the above findings. The gist of the present invention is as follows.
[0023]
(1) By mass%, C: 0.05-0.20%, Si: 1.0-3.0%, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.30% or less, N: 0.02% or less Cu: 0.5 to 3.0%, the balance has a composition consisting of Fe and inevitable impurities, the structure, 75% or more by volume ratio With the ferrite phase as the main phase, 8-11% Of residual austenite phase The remainder consists of bainite phase and martensite phase A high-ductility hot-rolled steel sheet having a composite structure having a phase as a second phase, excellent in press formability and excellent in strain aging hardening characteristics such that ΔTS: 80 MPa or more.
(2) In (1), in addition to the above-mentioned composition, in mass%, the following group A to group C
Group A: Ni: 2.0% or less
Group B: 2.0% or less of one or two of Cr and Mo in total
Group C: 0.2% or less total of one or more of Nb, Ti and V
A high-ductility hot-rolled steel sheet having excellent press formability and excellent strain aging hardening characteristics of ΔTS: 80 MPa or more, characterized by containing one group or two or more groups.
(3) By mass%, C: 0.05-0.20%, Si: 1.0-3.0%, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.30% or less, N: 0.02% or less And further containing W: 0.05-2.0%, the balance is composed of Fe and inevitable impurities, 75% or more by volume ratio With the ferrite phase as the main phase, 8-11% Of residual austenite phase The remainder consists of bainite phase and martensite phase A high-ductility hot-rolled steel sheet having a composite structure having a phase as a second phase, excellent in press formability and excellent in strain aging hardening characteristics such as ΔTS: 80 MPa or more.
(4) In (3), in addition to the above composition, in addition to mass, one or two of Mo: 0.05 to 2.0% and Cr: 0.05 to 2.0% are added in total of W, Mo, and Cr. A high ductility hot-rolled steel sheet characterized by containing 2.0% or less.
(5) In (3) or (4), in addition to the above composition, the composition further contains one or more of Nb, Ti, and V in an amount of 2.0% or less in total in 2.0% or less. A high ductility hot-rolled steel sheet with excellent press-formability and excellent strain age hardening characteristics such that ΔTS: 80 MPa or more.
(6) In mass%
C: 0.05 to 0.20%, Si: 1.0 to 3.0%,
Mn: 3.0% or less, P: 0.10% or less,
S: 0.02% or less, Al: 0.30% or less,
N: 0.02% or less, Cu: 0.5-3.0%
The steel slab having a composition composed of the remaining Fe and inevitable impurities is hot-rolled into a hot-rolled sheet having a predetermined thickness, and the hot rolling is performed at a finish rolling finishing temperature of 780 to 980 ° C. A certain hot rolling is performed, and after the finish rolling is finished, the temperature is cooled to a temperature range of 620 to 780 ° C. at a cooling rate of 60 ° C./s or more within 0.5 seconds, and the isothermal holding treatment or cooling rate for 1 to 10 seconds in the temperature range : After performing a slow cooling treatment of 20 ° C./s or less, and then cooling again to 300-500 ° C. at a cooling rate of 60 ° C./s or more, and winding up on a coil, excellent press formability, And the manufacturing method of the high ductility hot-rolled steel plate excellent in the strain age hardening characteristic which becomes (DELTA) TS: 80MPa or more.
(7) In (6), in addition to the above-mentioned composition, in mass%, the following groups A to C
Group A: Ni: 2.0% or less,
Group B: 2.0% or less of one or two of Cr and Mo in total
Group C: 0.2% or less total of one or more of Nb, Ti and V
The manufacturing method of the high ductility hot-rolled steel plate characterized by including 1 group or 2 groups or more of them.
(8) In (6), the steel slab is, in mass%, C: 0.05 to 0.20%, Si: 1.0 to 3.0%, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al : 0.30% or less, N: 0.02% or less, and W: 0.05 to 2.0%, with the balance being a steel slab having a composition composed of Fe and inevitable impurities, excellent press formability And the manufacturing method of the high ductility hot-rolled steel plate excellent in the strain age hardening characteristic which becomes (DELTA) TS: 80MPa or more.
(9) In (8), in addition to the above composition, in addition to mass, one or two of Mo: 0.05 to 2.0% and Cr: 0.05 to 2.0% are combined with W, Mo, and Cr. A method for producing a high-ductility hot-rolled steel sheet having excellent press formability, characterized by containing 2.0% or less, and having excellent strain age hardening characteristics such that ΔTS: 80 MPa or more.
(10) In (8) or (9), in addition to the above composition, the composition further contains one or more of Nb, Ti, and V in a mass% of 2.0% or less in total. A method for producing a high-ductility hot-rolled steel sheet having excellent press formability and ΔTS: strain age hardening characteristics of 80 MPa or more.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
The hot-rolled steel sheet of the present invention is a high-tensile hot-rolled steel sheet having a tensile strength of TS: 440 MPa or more, and the second phase is a phase whose structure includes a ferrite phase as a main phase and a volume ratio of 1% or more of retained austenite phase. It has a composite structure, excellent in press moldability, and has a high tensile aging strength due to heat treatment at a relatively low temperature after press molding, and excellent in strain aging hardening characteristics of ΔTS: 80 MPa or higher. It is a ductile hot rolled steel sheet.
[0025]
The term “highly ductile hot-rolled steel sheet” as used in the present invention means that the balance between strength (TS) and ductility (El) (TS × El) has a tensile property of 19000 MPa% or more.
First, the structure of the hot-rolled steel sheet of the present invention will be described.
The hot-rolled steel sheet of the present invention has a composite structure in which the structure has a ferrite phase as a main phase and a phase containing a retained austenite phase of 1% or more by volume with respect to the entire structure as a second phase.
[0026]
By setting it as such a composite structure, it becomes a steel plate which has high ductility (El) and high intensity | strength ductility balance (TS * El), and has the outstanding press formability.
The ferrite phase as the main phase is preferably 50% or more by volume. If the ferrite phase is less than 50%, it is difficult to ensure high ductility, and press formability deteriorates. Further, when a better ductility is required, the volume fraction of the ferrite phase is preferably 80% or more. In order to take advantage of the composite structure, the ferrite phase is preferably 98% or less.
[0027]
Moreover, as a 2nd phase, in this invention, it is necessary to set it as the phase which contains a residual austenite phase 1% or more with respect to the whole structure | tissue by volume ratio. When the retained austenite phase is less than 1%, high ductility (El) cannot be obtained. In order to obtain higher ductility, the retained austenite phase is preferably contained at 2% or more, more preferably 3% or more. In addition, the second phase is mixed with a residual austenite phase with a volume ratio of 1% or more alone or with a residual austenite phase with a volume ratio of 1% or more and any other pearlite phase, bainite phase, or martensite phase. There is no particular limitation as well.
[0028]
A hot-rolled steel sheet having the above-described structure is a steel sheet having high ductility, excellent press formability, and excellent strain age hardening characteristics.
In the present invention, “excellent strain age hardening property” or “excellent strain age hardening property” means holding at a temperature in the range of 150 to 350 ° C. after pre-deformation treatment with a tensile plastic strain amount of 5% or more. Time: When heat treatment for 30 s or longer, the amount of increase in tensile strength before and after this heat treatment ΔTS {= (tensile strength TS after heat treatment _HT )-(Tensile strength TS before pre-deformation treatment)} is 80 MPa or more. Desirably, ΔTS is 100 MPa or more. This heat treatment also increases the yield stress, and ΔYS {= (yield stress YS after heat treatment YS _HT )-(Yield stress YS before pre-deformation treatment)} is obviously 80 MPa or more.
[0029]
The amount of pre-strain (pre-deformation) is an important factor when defining strain age hardening characteristics. The present inventors investigated the influence of the amount of pre-strain on the subsequent strain age hardening characteristics assuming a deformation mode to which the steel sheet for automobiles is applied. As a result, except for extremely deep drawing, it can be organized by the amount of uniaxial equivalent strain (tensile strain), and in actual parts, the amount of uniaxial equivalent strain exceeds about 5%, and the strength of the parts Was found to correspond well with the strength obtained after 5% pre-strain strain aging treatment. Therefore, in the present invention, the pre-strain (deformation) of the heat treatment is set to a tensile plastic strain of 5% or more.
[0030]
Conventional baking treatment conditions of 170 ° C x 20 min have been adopted as a standard. However, as in the present invention, when using ultrafine Cu or fine carbide precipitation strengthening, the heat treatment temperature should be 150 ° C or higher. Necessary. On the other hand, when the temperature exceeds 350 ° C., the effect is saturated and, conversely, it tends to soften slightly. In addition, when heated to a temperature exceeding 350 ° C., generation of thermal distortion, temper color, etc. becomes remarkable. Therefore, in the present invention, the heat treatment temperature for strain age hardening is set to 150 to 350 ° C. The holding time at the heat treatment temperature is 30 s or longer. Regarding the holding time of the heat treatment, if the holding time is about 30 seconds or more at 150 to 350 ° C., almost sufficient strain age hardening is achieved. When it is desired to obtain a larger and stable strain age hardening, it is desirably 60 s or more, and more preferably 300 s or more.
[0031]
The heating method in the heat treatment after the pre-deformation is not particularly limited, but, for example, induction heating, non-oxidizing flame, heating by laser, plasma, etc. are all suitable in addition to the atmosphere heating by a furnace as in the ordinary paint baking process. It is. In addition, so-called warm pressing in which the temperature of the steel sheet is increased and pressed is also an extremely effective method in the present invention.
[0032]
Next, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. The mass% is simply written as%.
C: 0.05-0.20%
C is an element that increases the strength of the steel sheet and further promotes the formation of a composite structure of ferrite and retained austenite. In the present invention, it is necessary to contain 0.05% or more from the viewpoint of forming the composite structure. On the other hand, if the content exceeds 0.20%, the fraction of carbides in the steel increases, and ductility and further press formability decrease. Furthermore, as a more important problem, when the C content exceeds 0.20%, spot weldability, arc weldability, and the like are significantly reduced. For this reason, in the present invention, C is limited to 0.05 to 0.20%. From the viewpoint of moldability, it is preferably 0.18% or less.
[0033]
Si: 1.0-3.0%
Si is a useful strengthening element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel sheet, and is an element necessary for obtaining a retained austenite phase, and it must be contained at 1.0% or more. There is. More preferably 1.2% or more. On the other hand, if the Si content exceeds 3.0%, the press formability is deteriorated and the surface properties are deteriorated. For this reason, Si was limited to 1.0 to 3.0%.
[0034]
Mn: 3.0% or less
Mn has an effect of strengthening steel and is an effective element for preventing hot cracking due to S, and is preferably contained according to the amount of S contained. Such an effect becomes remarkable when the content is 0.5% or more. On the other hand, if it exceeds 3.0%, press formability and weldability deteriorate. For this reason, in the present invention, Mn is limited to 3.0% or less. In addition, More preferably, it is 1.0% or more.
[0035]
P: 0.10% or less
P has an effect of strengthening steel and can be contained in a necessary amount according to a desired strength, but it is preferably contained in an amount of 0.005% or more from the viewpoint of increasing the strength. On the other hand, when it contains exceeding 0.10%, press moldability will deteriorate. For this reason, P was limited to 0.10% or less. When more excellent press formability is required, the content is preferably 0.08% or less.
[0036]
S: 0.02% or less
S is an element present in the steel sheet as an inclusion, and is an element that causes deterioration of the ductility and formability of the steel sheet, particularly stretch flangeability, and is preferably reduced as much as possible. In the present invention, the upper limit of S is 0.02%. When more excellent stretch flange formability is required, S is preferably 0.010% or less.
[0037]
Al: 0.30% or less
Al is added as a deoxidizing element for steel, is an element useful for improving the cleanliness of steel, and is also an element effective for the formation of retained austenite. Such an effect becomes remarkable when the content is 0.01% or more. However, even if the content exceeds 0.30%, a further effect cannot be obtained, and conversely, press formability deteriorates. For this reason, Al was limited to 0.30% or less. In addition, Preferably it is 0.10% or less. Further, in the present invention, it does not exclude a melting method by a deoxidation method other than Al deoxidation, for example, Ti deoxidation or Si deoxidation may be performed, and a steel plate by these deoxidation methods is also of the present invention. Included in the range. At that time, even if Ca, REM or the like is added to the molten steel, the characteristics of the steel sheet of the present invention are not hindered at all.
[0038]
N: 0.02% or less
N is an element that increases the strength of the steel sheet by solid solution strengthening or strain age hardening. To obtain these effects, N is preferably contained in an amount of 0.0010% or more. Nitride increases in the inside, and the ductility of the steel sheet and further the press formability deteriorate significantly. For this reason, N was limited to 0.02% or less. When further improvement in press formability is required, the content is preferably 0.01% or less, and more preferably less than 0.0050%.
[0039]
Cu: 0.5-3.0%
Cu is an element that remarkably increases the strain age hardening (predeformation—increase in strength after heat treatment) of the steel sheet, and is the most important element in the present invention. If the Cu content is less than 0.5%, even if the predeformation-heat treatment conditions are changed, an increase in tensile strength of ΔTS: 80 MPa or more cannot be obtained. On the other hand, if the content exceeds 3.0%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous, causes deterioration of press formability, and further deteriorates the surface properties of the steel sheet. For this reason, Cu was limited to 0.5 to 3.0%. In order to achieve both a larger ΔTS and excellent press formability, Cu is preferably in the range of 1.0 to 2.5%.
[0040]
Moreover, in this invention, in addition to the above-mentioned composition containing Cu, it is the mass%, and the following A group-C group
Group A: Ni: 2.0% or less
Group B: 2.0% or less of one or two of Cr and Mo in total
Group C: 0.2% or less total of one or more of Nb, Ti and V
It is preferable to contain 1 group or 2 groups or more.
[0041]
Group A: Ni: 2.0% or less
Group A: Ni is effective in preventing surface defects generated on the steel sheet surface when Cu is added, and can be contained as required. When contained, the content depends on the Cu content, and is preferably about half of the Cu content, that is, about 30 to 80% of the Cu content (%). Even if the content exceeds 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous, and conversely, press formability deteriorates. Therefore, Ni is preferably limited to 2.0% or less.
[0042]
Group B: 2.0% or less of one or two of Cr and Mo in total
Group B: Cr and Mo, like Mn, both have the effect of strengthening the steel sheet, and can contain one or two types as necessary. Such an effect becomes remarkable by containing Cr 0.1% or more and Mo 0.1% or more. For this reason, it is preferable to contain one or two of Cr: 0.1% or more and Mo: 0.1% or more. On the other hand, if one or two of Cr and Mo are contained in total exceeding 2.0%, press formability is lowered. For this reason, it is preferable to limit one or two of Cr and Mo to 2.0% or less in total.
[0043]
Group C: 0.2% or less total of one or more of Nb, Ti and V
Group C: Nb, Ti, and V are all carbide-forming elements, and effectively act to increase the strength by fine dispersion of carbides. Therefore, they can be selected and contained as necessary. Such effects can be advantageously obtained at Nb: 0.01% or more, Ti: 0.01% or more, and V: 0.01% or more, respectively. However, if one or more of Nb, Ti, and V are contained in a total exceeding 2.0%, the press formability deteriorates. For this reason, Nb, Ti, and V are preferably limited to 2.0% or less in total.
[0044]
Moreover, in this invention, it replaces with above-mentioned Cu, or containing 1 group or 2 groups or more of said A group-C group further. , W : 0.05-2.0% Or even Mo: 0.05-2.0%, Cr: 0.05-2.0% 1 or 2 of Seeds of W, Mo, Cr The total content may be 2.0% or less, or further one or more of Nb, Ti, and V may be contained in a total of 2.0% or less.
W : 0.05-2.0% Or even Mo: 0.05-2.0%, Cr: 0.05-2.0% 1 or 2 of Seeds of W, Mo, Cr 2.0% or less in total
W or even Mo, Cr Is an element that remarkably increases the strain age hardening (predeformation—increase in strength after heat treatment) of the steel sheet, and is the most important element in the present invention. That is, the present inventors have made a steel sheet structure a composite structure in which ferrite is a main phase and a phase containing residual austenite is a second phase, and W Or even Mo, Cr By containing one or two of the above, when a prestrain of 5% or more is added and low-temperature heat treatment is performed, the retained austenite is strain-induced transformed into martensite, and the strain-induced transformed martensite is contained in the martensite. Strain-induced low-temperature precipitation of fine carbide occurs, and an increase in tensile strength of ΔTS: 80 MPa or more is obtained. In addition, these W, or even Mo, Cr 1 or 2 of Seed When the content is less than 0.05%, even if the steel sheet structure and the predeformation-heat treatment conditions are changed, an increase in tensile strength of ΔTS: 80 MPa or more cannot be obtained. on the other hand, W, or even Mo, Cr 1 or 2 of Seed When the content exceeds 2.0%, the effect is saturated, an effect commensurate with the content cannot be expected, and it is economically disadvantageous, and press formability is deteriorated. For this reason, it is preferable to limit Mo, Cr, and W to 0.05 to 2.0%, respectively. In addition, from the viewpoint of press formability, when combined and contained, it is more preferable to limit the total content of Mo, Cr, and W to 2.0% or less.
[0045]
2.0% or less of one or more of Nb, Ti and V in total
Nb, Ti, and V are all carbide-forming elements and can be contained as necessary. In addition to one or more of Mo, Cr, and W, one or more of Nb, Ti, and V are contained, and a phase containing residual austenite with a ferrite phase as a main phase. By using the composite structure as the second phase, fine carbides are formed in the strain-induced transformed martensite, strain-induced low-temperature precipitation is induced, and an increase in tensile strength of ΔTS: 80 MPa or more is obtained. In order to obtain such an effect, Nb, Ti, and V are preferably Nb: 0.01% or more, Ti: 0.01% or more, and V: 0.01% or more, respectively, and one or two or more as necessary. Can be selected and contained. However, if one or more of Nb, Ti, and V are contained in a total exceeding 2.0%, the press formability deteriorates. For this reason, it is preferable to limit one or more of Nb, Ti, and V to 2.0% or less in total.
[0046]
In addition to the above components, one or two of Ca: 0.1% or less and REM: 0.1% or less may be contained. Both Ca and REM are elements that contribute to the improvement of stretch flangeability through the form control of inclusions. However, if the content exceeds Ca: 0.1% and REM: 0.1%, the cleanliness is lowered and ductility is lowered instead.
The balance other than the above components consists of Fe and inevitable impurities. Inevitable impurities include Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less, Zr: 0.1% or less, and B: 0.1% or less.
[0047]
Below, the manufacturing method of the hot rolled sheet steel of this invention is demonstrated.
The hot-rolled steel sheet of the present invention uses a steel slab having a composition within the above-described range as a raw material, and hot-rolls the raw material to obtain a hot-rolled sheet having a predetermined thickness.
The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab casting method. In addition to the conventional method in which a steel slab is manufactured and then cooled to room temperature and then reheated, it is not cooled and inserted into a heating furnace as it is, or rolled immediately after a slight heat retention. Energy saving processes such as direct rolling and direct rolling can be applied without problems.
[0048]
The heating temperature SRT of the material (steel slab) is not particularly limited, but is preferably 900 ° C. or higher.
Slab heating temperature: 900 ℃ or more
When the material has a composition containing Cu, the slab heating temperature is preferably low in order to prevent surface defects caused by Cu. However, if the heating temperature is less than 900 ° C., the rolling load increases and the risk of trouble occurring during hot rolling increases. Note that the slab heating temperature is desirably 1300 ° C. or less because of an increase in scale loss accompanying an increase in oxidized weight.
[0049]
Needless to say, using a so-called sheet bar heater that heats the sheet bar from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling is of course effective.
The heated steel slab is then hot rolled into a hot rolled sheet. The hot rolling is preferably hot rolling with a finish rolling finish temperature FDT of 780 to 980 ° C.
[0050]
In the present invention, the finish rolling conditions are particularly important, and the finish rolling finish temperature FDT is preferably 780 to 980 ° C.
If the FDT is less than 780 ° C., the processed structure remains in the steel sheet and the ductility is deteriorated. On the other hand, if the FDT exceeds 980 ° C., the structure becomes coarse and the formability is deteriorated due to the delay of ferrite transformation. For this reason, it is preferable that FDT shall be 780-980 degreeC.
[0051]
Although forced cooling treatment is performed after finish rolling, this forced cooling condition is particularly important in the present invention. In the present invention, it is preferable to forcibly cool to 620 to 780 ° C. at a cooling rate of 50 ° C./s or less within 2 s after finish rolling. When the cooling start time exceeds 2 s, the structure becomes coarse, the ferrite transformation is delayed, and the press formability is lowered. For this reason, it is preferable to limit the cooling start time after finishing rolling to within 2 s.
[0052]
Moreover, if the cooling rate after finish rolling is less than 50 ° C./s, ferrite transformation starts during forced cooling, and the ferrite transformation in the subsequent isothermal holding treatment or slow cooling treatment is delayed, resulting in press formability. Leading to a decline. Therefore, the cooling rate is preferably limited to 50 ° C./s or more. However, when the cooling rate exceeds 300 ° C./s, there is a concern that the shape of the steel sheet is deteriorated. Therefore, the upper limit of the cooling rate is preferably 300 ° C./s.
[0053]
It is preferable to cool to the nose vicinity of the pro-eutectoid ferrite region in the temperature range of 620 to 780 ° C. by the forced cooling described above. When the cooling stop temperature of forced cooling is less than 620 ° C, there is a problem that pearlite is generated without forming pro-eutectoid ferrite. There is a problem that decreases. More preferably, the cooling stop temperature of forced cooling is 650 to 750 ° C.
[0054]
After forced cooling to the nose of the pro-eutectoid ferrite region in the temperature range of 620 to 780 ° C., isothermal holding treatment for 1 to 10 s or slow cooling treatment at a cooling rate of 20 ° C./s or less is performed in the temperature range. preferable.
A desired amount of pro-eutectoid ferrite can be obtained by the isothermal holding treatment for a short time in the temperature range (620 to 780 ° C.) or the slow cooling treatment for a short time in the temperature range described above.
[0055]
The isothermal holding treatment or slow cooling treatment is more preferably performed in a temperature range of 620 ° C. or higher and 750 ° C. or lower in order to concentrate carbon to austenite accompanying the ferrite transformation.
Further, if the holding time of the isothermal holding treatment or the time required for the slow cooling treatment is less than 1 s, the concentration of carbon to austenite is insufficient, while if it exceeds 10 s, pearlite transformation occurs.
[0056]
Further, when the cooling rate of the slow cooling treatment exceeds 20 ° C./s, there is a problem that the carbon concentration to austenite is insufficient.
After the isothermal holding treatment or the slow cooling treatment, it is preferably cooled again to 300 to 500 ° C. at a cooling rate of 50 ° C./s or more, that is, wound around a coil at a winding temperature CT of 300 to 500 ° C.
[0057]
After the isothermal holding treatment or the slow cooling treatment, cooling is performed to 300 to 500 ° C., and the cooling rate at this time is preferably 50 ° C./s or more. This is because if the cooling rate at this time is less than 50 ° C./s, pearlite transformation occurs and ductility is lowered. In addition, More preferably, it is 50-200 degrees C / s.
When the coiling temperature CT is less than 300 ° C., the second phase becomes martensite, whereas when the coiling temperature CT exceeds 500 ° C., the second phase becomes pearlite. For this reason, the winding temperature CT is preferably set to 300 to 500 ° C.
[0058]
In the present invention, part or all of the finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
[0059]
After hot rolling, temper rolling of 10% or less may be performed for shape correction, surface roughness adjustment and the like.
The hot-rolled steel sheet of the present invention can be applied not only for processing but also as a surface treatment original sheet. Examples of the surface treatment include galvanization (including alloy system), tin plating, enamel and the like.
[0060]
The hot-rolled steel sheet of the present invention may be subjected to special treatment after annealing or galvanization to improve chemical conversion property, weldability, press formability, corrosion resistance, and the like.
[0061]
【Example】
Example 1
Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method. Next, these steel slabs were heated and hot-rolled under the conditions shown in Table 2 to form a hot-rolled steel strip (hot-rolled sheet) having a thickness of 2.0 mm, and further subjected to temper rolling with a reduction ratio of 1.0%. .
[0062]
About the obtained hot-rolled steel strip (hot-rolled sheet), the microstructure, tensile characteristics, strain age hardening characteristics, and hole expansion ratio were determined. The press formability was evaluated from the elongation El (ductility), the TS × El balance, and the hole expansion ratio λ. The test method was as follows.
(1) Microscopic organization
A test piece was collected from the obtained hot-rolled sheet, and the microstructure was observed with respect to a cross section (C cross section) perpendicular to the rolling direction of the steel sheet using an optical microscope or a scanning electron microscope. For the ferrite phase, bainite phase, and martensite phase in the steel sheet, the structure fraction of each structure was obtained by an image analyzer using a cross-sectional structure photograph at a magnification of 1000 times, and the volume ratio of the corresponding phase was obtained. Moreover, about the retained austenite phase, the steel plate was grind | polished to the center plane of a plate | board thickness direction, and it calculated | required by the diffraction X-ray intensity measurement in a plate | board thickness center plane. MoKα rays are used as incident X-rays, and {200}, {220}, {311} in the retained austenite phase with respect to the diffracted X-ray intensities on the {110}, {200}, and {211} surfaces of the ferrite phase The diffracted X-ray intensity ratio of each surface was determined, and the volume fraction of retained austenite was determined from the average value.
(2) Tensile properties
A JIS No. 5 tensile test piece was sampled in the rolling direction from the obtained hot-rolled sheet and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to obtain yield strength YS, tensile strength TS, and elongation El.
(3) Strain age hardening characteristics
From the obtained hot-rolled sheet, a JIS No. 5 tensile specimen was taken in the rolling direction, subjected to 5% plastic deformation as a pre-deformation (tensile pre-strain), and then subjected to a heat treatment at 250 ° C. for 20 min. Tensile properties after the heat treatment (yield stress YS) _TH , Tensile strength TS _HT ), ΔYS = YS _TH -YS, ΔTS = TS _HT -TS was calculated. YS _TH , TS _HT Is the yield stress and tensile strength after pre-deformation-heat treatment, and YS and TS are the yield stress and tensile strength of the hot-rolled sheet.
(4) Hole expansion rate
The test specimen collected from the hot-rolled sheet was punched with a 10mmφ punch in accordance with the Japan Iron and Steel Federation standard (JFS T 1001-1996) to form a punch hole. The hole expansion test was performed until the cracks penetrating the plate thickness were generated with the burr on the outside, and the hole expansion ratio λ was obtained. The hole expansion rate λ is λ (%) = {(d−d ₀ ) / D ₀ } It calculated | required in * 100. D ₀ : Initial hole diameter (punch diameter), d: inner hole diameter when cracking occurs.
[0063]
These results are shown in Table 3.
[0064]
[Table 1]

[0065]
[Table 2]

[0066]
[Table 3]

[0067]
Each of the examples of the present invention has a high elongation El and a high strength ductility balance (TS × El), exhibits a larger hole expansion ratio λ, and is excellent in stretch flange formability. In addition, each of the examples of the present invention shows a very large ΔTS and is a steel plate excellent in strain age hardening characteristics. On the other hand, in the comparative example outside the scope of the present invention, the steel sheet has a low elongation El, a small hole expansion ratio λ, a small ΔTS, and a reduced press formability and strain age hardening characteristics.
[0068]
(Example 2)
Molten steel having the composition shown in Table 4 was melted in a converter and made into a slab by a continuous casting method. Subsequently, these steel slabs were heated and hot-rolled under the conditions shown in Table 5 to form hot-rolled steel strips (hot-rolled sheets) with a thickness of 2.0 mm, and further subjected to temper rolling with a reduction ratio of 1.0%. .
About the obtained hot-rolled steel strip (hot-rolled sheet), the microstructure, tensile properties, strain age hardening properties, and hole expansion rate were determined in the same manner as in Example 1. The press formability was evaluated from the elongation El (ductility), the TS × El balance, and the hole expansion ratio λ.
[0069]
The results obtained are shown in Table 6.
[0070]
[Table 4]

[0071]
[Table 5]

[0072]
[Table 6]

[0073]
Each of the examples of the present invention has a high elongation El and a high strength ductility balance (TS × El), is excellent in press formability, exhibits a very large ΔTS, and has excellent strain age hardening characteristics. . On the other hand, in the comparative example outside the scope of the present invention, the elongation El is low or ΔTS is small, and the steel sheet has low press formability and strain age hardening characteristics.
[0074]
【Effect of the invention】
According to the present invention, while maintaining excellent press formability, it becomes possible to stably produce a hot-rolled steel sheet whose tensile strength is remarkably increased by heat treatment after press forming. Play. When the hot-rolled steel sheet of the present invention is applied to automobile parts, press forming is easy, and the characteristics of the parts after completion can be stably increased, and there is an effect that it can sufficiently contribute to weight reduction of the automobile body.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of Cu content on the relationship between ΔTS after pre-deformation and heat treatment and steel sheet structure.
FIG. 2 is a graph showing the effect of Cu content on the relationship between ΔTS after pre-deformation and heat treatment and the heat treatment temperature.

Claims (10)

In mass% C: 0.05 to 0.20%, Si: 1.0 to 3.0%,
Mn: 3.0% or less, P: 0.10% or less,
S: 0.02% or less, Al: 0.30% or less,
N: 0.02% or less, Cu: 0.5-3.0%
It includes having the balance consisting of Fe and unavoidable impurities, tissue, and the main phase 75% or more of ferrite phase by volume, see containing 8-11% residual austenite phase by volume, the balance Is characterized by having a composite structure in which a phase composed of a bainite phase and a martensite phase is the second phase. steel sheet. The high ductility hot-rolled steel sheet according to claim 1, further comprising one group or two or more groups of the following groups A to C in mass% in addition to the composition.
Group A: Ni: 2.0% or less,
Group B: 2.0% or less of one or two of Cr and Mo in total
Group C: 0.2% or less total of one or more of Nb, Ti and V In mass% C: 0.05-0.20%, Si: 1.0-3.0%,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.30% or less,
N: 0.02% or less, W: 0.05 to 2.0% further, the balance is composed of Fe and inevitable impurities, the structure is a ferrite phase whose volume ratio is 75% or more as a main phase, look containing 8-11% residual austenite phase by volume, the balance being having a phase consisting bainite phase and martensite phase composite structure as a second phase, excellent press formability, and ΔTS : High ductility hot-rolled steel sheet with excellent strain age hardening characteristics of 80MPa or more.   In addition to the above composition, the composition further comprises one or two of Mo: 0.05 to 2.0% and Cr: 0.05 to 2.0% in terms of mass%, and the total of W, Mo, and Cr is 2.0% or less. The high ductility hot-rolled steel sheet according to claim 3.   The high ductility hot rolling according to claim 3 or 4, further comprising one or more of Nb, Ti, and V in a total of 2.0% or less in addition to the composition. steel sheet. In mass% C: 0.05-0.20%, Si: 1.0-3.0%,
Mn: 3.0% or less, P: 0.10% or less,
S: 0.02% or less, Al: 0.30% or less,
N: 0.02% or less, Cu: 0.5-3.0%
The steel slab having a composition composed of the remaining Fe and inevitable impurities is hot-rolled into a hot-rolled sheet having a predetermined thickness, and the hot rolling is performed at a finish rolling finishing temperature of 780 to 980 ° C. A certain hot rolling is performed, and after the finish rolling is finished, the temperature is cooled to a temperature range of 620 to 780 ° C. at a cooling rate of 60 ° C./s or more within 0.5 seconds, and the isothermal holding treatment or cooling rate for 1 to 10 seconds in the temperature range : After performing a slow cooling treatment of 20 ° C./s or less, and then cooling again to 300-500 ° C. at a cooling rate of 60 ° C./s or more, and winding up on a coil, excellent press formability, And the manufacturing method of the high ductility hot-rolled steel plate excellent in the strain age hardening characteristic which becomes (DELTA) TS: 80MPa or more. The method for producing a high-ductility hot-rolled steel sheet according to claim 6, further comprising one group or two or more groups of the following groups A to C in mass% in addition to the composition.
Group A: Ni: 2.0% or less,
Group B: 2.0% or less of one or two of Cr and Mo in total
Group C: 0.2% or less total of one or more of Nb, Ti and V The steel slab in mass% C: 0.05 to 0.20%, Si: 1.0 to 3.0%,
Mn: 3.0% or less, P: 0.10% or less,
S: 0.02% or less, Al: 0.30% or less,
The high ductility hot rolling according to claim 6, wherein the steel slab contains N: 0.02% or less, further contains W: 0.05-2.0%, and the balance is composed of Fe and inevitable impurities. A method of manufacturing a steel sheet.   In addition to the above composition, the composition further comprises one or two of Mo: 0.05 to 2.0% and Cr: 0.05 to 2.0% in terms of mass%, and the total of W, Mo, and Cr is 2.0% or less. The high ductility hot-rolled steel sheet according to claim 8.   The high ductility hot-rolling according to claim 8 or 9, further comprising 2.0% or less of one or more of Nb, Ti, and V in total in addition to the composition. A method of manufacturing a steel sheet.

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