JP2004315959A - Steel sheet superior in strain age hardening characteristics, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet superior in strain age hardening characteristics, which has superior press formability and acquires the tensile strength greatly enhanced by a heat treatment at a comparatively low temperature after being press-formed, and to provide a manufacturing method therefor. <P>SOLUTION: The steel sheet includes 0.01-0.15% C, 2.0% or less Si, 3.0% or less Mn, one or two elements of Nb and Mo, and P, S, Al and N, which are adjusted to a correct amount. The manufacturing method comprises preparing a steel slab having the above composition as a material; hot-rolling it, then cooling it to 600°C or lower at a cooling rate of 20°C/s or higher and winding it up, to control the average particle diameter of precipitates having particle sizes of 80 nm or less to 32 nm or less in the hot-rolled plate; subsequently cold-rolling it; and annealing it at (Ac<SB>3</SB>transformation temperature - 110°C) or higher. Thereby, the steel sheet acquires superior press formability and such superior strain age hardening characteristics as to obtain ΔTS of 150 MPa or higher. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention mainly relates to a steel sheet for automobiles, in particular, has good press formability such as bending workability, stretch flangeability, drawability, and the tensile strength is significantly increased by heat treatment after press forming. The present invention relates to a steel sheet having extremely large strain age hardening characteristics and a method for producing the same. The steel sheet according to the present invention may be a so-called cold-rolled steel sheet manufactured by performing cold rolling and annealing, or a cold-rolled steel sheet that has been annealed after cold rolling and further subjected to galvanizing or electroplating. It shall also include rolled steel sheets.

  The term "excellent strain aging hardening property" as used in the present invention, that is, "excellent in strain aging hardening property" means having a strain aging hardening property of ΔTS: 150 MPa or more, preferably ΔTS: 170 MPa or more. In the present invention, ΔTS is the tensile strength before and after heat treatment when a pre-deformation treatment with a plastic strain amount of 5% or more and a heat treatment at a temperature in the range of 150 to 350 ° C. and a holding time of 30 s or more are performed. The amount of increase {= (tensile strength after heat treatment) − (tensile strength before pre-deformation treatment)}.

  2. Description of the Related Art In recent years, reduction of the weight of a vehicle body has become a very important issue in connection with emission gas regulations due to global environmental conservation issues. Recently, in order to reduce the weight of a vehicle body, it has been studied to increase the strength of a steel plate for an automobile and reduce the thickness of the steel plate.

  Since many automotive body parts made of steel plates are formed by press working, the steel plates used are required to have excellent press formability. However, in general, when the strength of a steel sheet is increased, the yield stress increases, the shape freezing property deteriorates, and the ductility tends to decrease, and the press formability tends to decrease.

  Recently, safety of an automobile body has been emphasized in order to protect an occupant at the time of a collision, and therefore, an improvement in impact resistance, which is a measure of safety at the time of a collision, has been required. The higher the strength of the completed vehicle, the better the impact resistance is. Therefore, when forming automotive parts, steel sheets having low strength, high ductility and excellent press formability, and when finished, high strength and excellent impact resistance were the most strongly desired. .

  In response to such a demand, a steel sheet that achieves both press formability and high strength has been developed. This steel sheet is a paint bake hardening steel sheet whose yield stress increases when subjected to paint baking treatment including holding at a high temperature of 100 to 200 ° C. after press working. In this steel sheet, the amount of C finally dissolved in a solid solution state (the amount of solid solution C) is controlled to an appropriate range, and is soft at the time of press forming, secures shape freezing and ductility, and paint baking performed after press forming. During processing, the remaining solid solution C adheres to dislocations introduced during press forming, hinders the movement of the dislocations, and increases the yield stress. However, in this paint-bake hardening type steel sheet, although the yield stress can be increased, it cannot be increased by the tensile strength.

  Further, Patent Document 1 discloses that a uniform bainite or iron alloy containing C: 0.08 to 0.20%, Mn: 1.5 to 3.5%, the balance being Fe and an unavoidable impurity, and having a ferrite content of 5% or less. A bake-hardenable high-tensile cold-rolled thin steel sheet composed of bainite containing part martensite is disclosed. The cold-rolled thin steel sheet described in Patent Literature 1 rapidly cools the temperature range of 400 to 200 ° C. in a cooling process after continuous annealing, and then gradually cools it, thereby changing the structure from the conventional ferrite-based structure to bainite. The main structure is to obtain a high bake hardening amount, which has never existed before.

  However, in the cold-rolled thin steel sheet described in Patent Document 1, although the yield strength increases after baking paint, a higher bake hardening amount than before can be obtained, but it is still not possible to increase the tensile strength. There is a problem that improvement in impact resistance cannot be expected.

  Further, although a steel sheet which is subjected to heat treatment after press forming to increase not only the yield stress but also the tensile strength is a hot rolled steel sheet, some steel sheets have been proposed.

  For example, Patent Document 2 discloses a 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%, at 1100 ° C or more. Re-heated, hot-rolled to finish the finish rolling at 850-950 ° C, then cooled at a cooling rate of 15 ° C / s or more to a temperature of less than 150 ° C and wound, mainly composed of ferrite and martensite. A method for producing a hot-rolled steel sheet having a composite structure has been proposed. However, in the steel sheet manufactured by the technique described in Patent Document 2, although the tensile strength increases with the yield stress due to strain age hardening, the steel sheet is wound at an extremely low winding temperature of less than 150 ° C. There is a problem that the fluctuation is large and the variation in the amount of increase in the yield stress and the like due to the press forming-heat treatment is large.

  Patent Document 3 proposes a method for producing a hot-dip galvanized steel sheet using a hot-rolled sheet as a plating base sheet. In this method, a steel slab containing C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less, and Cu: 0.8 to 2.0% is hot-rolled at a winding temperature of 530 ° C or less. Subsequently, the steel sheet is heated to a temperature of 530 ° C. or lower to reduce the surface of the steel sheet, and then hot-dip galvanized, whereby remarkable hardening by heat treatment after forming is obtained. However, in the steel sheet manufactured by this method, the heat treatment temperature must be 500 ° C. or higher in order to obtain a significant hardening by heat treatment after forming, and the heat treatment temperature is high, and there is a practical problem.

  Further, Patent Document 4 proposes a method for producing an alloyed hot-dip galvanized steel sheet in which a hot-rolled sheet or a cold-rolled sheet is used as an original plate for plating and a strength increase can be expected by heat treatment after forming. This method contains C: 0.01 to 0.08%, and after making appropriate amounts of Si, Mn, P, S, Al and N, one or two or more of Cr, W and Mo in a total amount of 0.05 to 3.0. % Hot-rolled steel, or further temper rolling or cold-rolled, annealed, hot-dip galvanized, and then heat-alloyed. It is said that this steel sheet can be increased in tensile strength by being heated in a temperature range of 200 to 450 ° C. after forming. However, the amount of increase in tensile strength is about 140 MPa at the maximum, which is insufficient for recent demands of users.

  Patent Document 5 proposes a method of manufacturing a press-formed body, and uses a steel sheet in which Si, Al, and P are adjusted to be in a range of {Si + 1.4Al + 6.3P}: 0.2 to 3%. It is stated that a heat treatment at a low temperature after press molding can increase the tensile strength up to about 400 MPa. However, the technique described in Patent Literature 5 requires a large amount of Si, Al, and P to be contained, and thus has a problem that press formability and chemical conversion properties are reduced.

  Patent Document 6 discloses that a composition containing C: 0.15% or less, Mn: 3.0% or less, Cu: 0.5 to 3.0%, and containing appropriate amounts of Si, P, S, Al, and N, and a structure as a main phase. A cold-rolled steel sheet which is excellent in press formability and strain age hardening characteristics as a composite structure of a ferrite phase and a second phase containing a martensite phase having an area ratio of 2% or more is disclosed. It is stated that the tensile strength of this Cu-containing steel sheet increases by 80 MPa or more by heating in a temperature range of 150 to 350 ° C. after forming. However, in the technology described in Patent Document 6, Cu content is essential, and Cu content is not preferable from the viewpoint of steel material recycling. Patent Document 6 also discloses a steel sheet containing one or more selected from Mo, Cr, and W instead of Cu. In this case, the amount of increase in the tensile strength of the steel sheet is disclosed. Is about 140MPa at the maximum, which is insufficient for recent user demands.

  Further, Patent Document 7 discloses a composition containing C: 0.20% or less, Mn: 3.0% or less, Cu: 0.5 to 3.0%, and a composition containing appropriate amounts of Si, P, S, Al, and N, or instead of Cu. A composition containing one or more selected from Mo, Cr, and W; a ferrite phase having a structure as a main phase; and a second phase containing a residual austenite phase having a volume fraction of 1% or more. A high-ductility cold-rolled steel sheet having a composite structure and excellent in press formability and strain age hardening characteristics is disclosed. It is stated that the tensile strength of this steel sheet is increased by 80 MPa or more by heating in a temperature range of 150 to 350 ° C. after forming.

However, the technique described in Patent Document 7 is not preferable from the viewpoint of recycling steel materials when Cu is contained. Furthermore, in the technique described in Patent Document 7, it is necessary to contain a large amount of Si in order to leave untransformed austenite, and there is a case where there is a problem in plating property and chemical conversion treatment property, and in order to perform surface modification. And the production cost increases.
Japanese Patent Publication No. 5-24979 Japanese Patent Publication No. 8-23048 Patent No. 2802513 JP-A-10-310824 JP 2000-178684 A JP 2001-348645 A JP 2003-13176 A

  As described above, the present invention has been made in view of the fact that there has never been a technology for industrially stably producing a steel sheet satisfying these characteristics despite extremely strong demands. Strain aging, which advantageously solves the above-mentioned problems and has excellent press formability suitable for use as a steel sheet for automobiles, and has a very large increase in tensile strength by heat treatment at a relatively low temperature after press forming. An object of the present invention is to propose a steel sheet having excellent hardening characteristics and a manufacturing method capable of stably producing the high ductility steel sheet.

The present inventors have conducted intensive studies on the effects of alloying elements on the strain age hardening characteristics in order to achieve the above-mentioned object. As a result, C was set to a low carbon region, and a composition containing one or two of Nb and Mo in an appropriate range, and the size of the precipitate was adjusted to an average particle diameter of 32 nm or less, preferably 30 nm or less. Using the hot rolled sheet as a base plate, annealing is performed in a temperature range of cold rolling (also referred to as cold rolling) and a temperature range of (Ac ₃ transformation point −110 ° C.) or more, preferably (Ac ₃ transformation point −100 ° C.) or more. Pre-strain: 5% or more of pre-deformation treatment and heat treatment at a relatively low temperature of 150 to 350 ° C. are applied to a cold-rolled steel sheet (also referred to as a cold-rolled annealed sheet). As a result, it has been found that a steel sheet having a high strain age hardening characteristic in which the tensile strength is significantly increased is obtained.

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

  In mass%, C: 0.08%, Si: 0.25%, Mn: 1.9%, P: 0.01%, S: 0.001%, Al: 0.04%, N: 0.002%, Mo: 0.19%, Nb: 0.05% After heating to 1250 ° C and soaking, the sheet bar was subjected to three-pass rolling so that the exit temperature on the finish rolling was 900 ° C to form a hot-rolled sheet having a thickness of 4.0 mm. Cooling was performed at a cooling rate, and a heat retention process of maintaining the coil at various temperatures for 1 hour was performed as a process corresponding to coil winding. The structure of the obtained hot-rolled sheet was observed with a transmission electron microscope, and the size (average particle size) of the precipitate was measured. The average particle size of the precipitates was determined for each hot-rolled sheet by observing 10 or more fields at a magnification of 100,000 and using an image analyzer to determine the area of each precipitate in each field of view. The average particle size of the precipitate in each visual field was determined as the obtained particle size of each precipitate, and the measured average value in all the visual fields was defined as the average value of each hot-rolled sheet. In determining the average particle size of the precipitates, except for coarse precipitates exceeding 80 nm in circle equivalent diameter in each field of view, for precipitates having a particle size of 80 nm or less, the average particle size of the precipitates as described above. I asked. The precipitates having a circle equivalent diameter exceeding 80 nm were excluded because these precipitates were relatively stable precipitates, and it was considered that they could not be expected to dissolve during annealing after cold rolling. The precipitates having a particle size of 80 nm or less were analyzed using EDX (Energy Dispersive X-ray Spectroscopy), and as a result, carbon, Nb and / or Mo were detected. Is presumed to be a carbide containing one or two of the following.

  Subsequently, the hot-rolled sheet was subjected to 60% cold rolling to obtain a 1.2-mm-thick cold-rolled sheet. Next, these cold rolled sheets were annealed under various conditions.

  About the obtained cold rolled sheet, the tensile test was implemented and the tensile characteristic was investigated. Furthermore, the strain aging hardening characteristics of these cold rolled sheets were investigated.

First, test specimens were taken from these cold-rolled sheets, subjected to a pre-deformation treatment with a tensile pre-strain of 5%, and then subjected to a heat treatment at 250 ° C. for 20 min. I asked. The strain age hardening property was evaluated by the increase in tensile strength ΔTS before and after the heat treatment. ΔTS, the difference between the tensile strength TS _HT after heat treatment, the tensile strength TS when not subjected to pre-deformation and heat treatment (= (tensile strength TS _HT after heat treatment) - (pre-deformation processing Previous tensile strength TS)). The tensile test was performed using a JIS No. 5 tensile test piece.

The obtained results are shown in FIG. 1 in the relationship between ΔTS and the average particle size of precipitates of the hot-rolled sheet. FIG. 1 shows that after hot rolling was performed on a hot rolled sheet having a different cooling rate and a processing temperature equivalent to winding after hot rolling, the steel sheet was annealed at 800 ° C. immediately below the Ac ₃ transformation point (810 ° C.) for 90 seconds. This is the result of the case. From FIG. 1, by adjusting the size of the precipitate of the hot-rolled sheet to 32 nm or less in average particle size, a steel sheet having a high strain age hardening property of ΔTS of 150 MPa or more was obtained. It can be seen that by adjusting the size of the product to an average particle size of 30 nm or less, a steel sheet having a high strain age hardening property of ΔTS of 170 MPa or more can be obtained.

FIG. 2 shows the relationship between ΔTS and the annealing temperature. FIG. 2 shows that the hot rolled sheet having a cooling rate of 50 ° C./s, a processing temperature equivalent to winding at 400 ° C., and an average grain size of precipitates of 17 nm was subjected to cold rolling at various temperatures. Is a result when annealing is performed for 90 seconds. From FIG. 2, by setting the annealing temperature to 700 ° C. or higher which is (Ac ₃ transformation point−110 ° C.), a steel sheet having a high strain aging hardening property with ΔTS of 150 MPa or higher can be obtained. _It can be seen that a steel sheet having a high strain aging hardening property with a ΔTS of 170 MPa or more can be obtained by setting the temperature to 710 ° C. or more, which is ₃ transformation point −100 ° C.). The Ac ₃ transformation point was determined by measuring a thermal expansion-temperature curve during heating at a heating rate of 5 ° C./s.

As a result of various examinations based on the above results, C was set to a low carbon region, and a composition containing one or two of Nb and Mo in an appropriate range, and the size of the precipitate was set to an average particle size of 32 nm Using the hot-rolled sheet adjusted as follows as a cold-rolled sheet, a steel sheet obtained by performing cold rolling and annealing in a temperature range of (Ac ₃ transformation point -110 ° C.) or more is subjected to a pre-strain of 5% or more. It has been found that an extremely large increase in tensile strength can be obtained by deformation and heat treatment at a relatively low temperature of 150 to 350 ° C. The reason why an extremely large increase in tensile strength can be obtained by performing the predeformation and the heat treatment at a relatively low temperature as described above has not been clarified so far, but the present inventors have as follows. I think.

When cold rolling is performed on a hot-rolled sheet with fine precipitates having an average particle size of 32 nm or less as a cold-rolled sheet base plate, the interface energy and strain energy are high and unstable fine precipitates are introduced by cold rolling. The interaction with the high-density dislocations makes the precipitates more unstable, and becomes easier to dissolve in the ferrite phase as solid solution C by annealing in the next step. The precipitate is considered to be a carbide containing one or two of Nb and Mo. Further, by performing the annealing treatment in a temperature range of (Ac ₃ transformation point -110 ° C.) or more, fine precipitates generated at the stage of hot-rolled sheet during annealing can be sufficiently dissolved. Therefore, when the steel sheet obtained through the above-described steps is subjected to a pre-strain: a pre-strain of 5% or more and a heat treatment at a relatively low temperature, the steel sheet contains one or two of Nb and Mo. It is considered that fine carbides are precipitated by strain induction. It is considered that due to the strain-induced precipitation of the ultrafine carbides, high strain age hardening characteristics in which the tensile strength significantly increases with the yield stress are obtained.

  In addition, the present inventors, in addition to one or two of Nb and Mo as a carbide-forming element, further contain one or two of Ti and V, thereby increasing the tensile strength. Was found to further increase.

The present invention has been completed by further study based on the above findings. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.01 to 0.15%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less Nb: 0.01 to 0.2%, Mo: 0.05 to 2.0%, one or two selected from the group consisting of Fe and unavoidable impurities, and ΔTS: 150 MPa or more. A steel sheet having excellent strain aging hardening characteristics characterized by having strain aging hardening characteristics.
(2) (1), having said composition, hot-rolled sheet and the precipitate average particle diameter determined is less than 32nm for particle size 80nm following precipitates, cold-rolled, and then (Ac ₃ transformation point A steel sheet characterized by being annealed in a temperature range of −110 ° C. or higher.
(3) The steel sheet according to (1) or (2), further comprising, in addition to the above composition, 0.4% or less in total of one or two of Ti and V in mass%.
(4) hot rolling a steel slab and then cooling it to form a rolled hot rolled sheet; cold rolling the hot rolled sheet to form a cold rolled sheet; An annealing step of annealing a sheet to form a cold-rolled annealed sheet, wherein the steel slab is, by mass%, C: 0.01 to 0.15%, Si: 2.0% or less, Mn: 3.0%. In the following, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, Nb: 0.01 to 0.2%, Mo: 0.05 to 2.0% Or a steel slab containing two kinds, the balance being Fe and unavoidable impurities, and cooling and winding in the hot rolling step are cooled to 600 ° C. or less at a cooling rate of 20 ° C./s or more. Winding is performed by cooling and winding, and the annealing temperature in the annealing is set to a temperature of (Ac ₃ transformation point−110 ° C.) or more. Method for producing steel sheets with excellent strain age hardening characteristics.
(5) The method for producing a steel sheet according to (4), further comprising, in addition to the above composition, one or two of Ti and V in total of 0.4% or less by mass%.
(6) The method for producing a steel sheet according to (4) or (5), wherein the slab heating temperature in the hot rolling is 900 ° C. or more and the finish-rolling exit temperature is 700 ° C. or more.
(7) The method for producing a steel sheet according to any one of (4) to (6), wherein the cooling rate after the annealing is 1 ° C./s or more.

  Advantageous Effects of Invention According to the present invention, it is possible to stably produce a steel sheet whose tensile strength is remarkably increased by heat treatment after press forming, while maintaining excellent press formability, and has an industrially remarkable effect. . When the steel sheet of the present invention is applied to automobile parts, there is also an effect that press forming is easy, the component properties after completion can be stably increased, and it is possible to sufficiently contribute to weight reduction of an automobile body.

  The steel sheet of the present invention is a high-tensile steel sheet having a tensile strength TS of 440 MPa or more, has excellent press-formability, and has a remarkably increased tensile strength due to heat treatment at a relatively low temperature after press-forming. It is a steel sheet having excellent strain aging hardening characteristics of 150 MPa or more, preferably ΔTS: 170 MPa or more.

  In the present invention, "excellent in strain age hardening characteristics" means, as described above, that after a pre-deformation treatment with a tensile plastic strain amount of 5% or more, a holding time of 30 seconds or more at a temperature of 150 to 350 ° C. When the heat treatment is performed, the increase in the tensile strength before and after the heat treatment ΔTS ｛= (tensile strength after heat treatment) − (tensile strength before pre-deformation treatment)｝ is 150 MPa or more. Preferably, ΔTS is 170 MPa or more, and more preferably, 200 MPa or more.

  When defining the strain age hardening characteristics, the amount of prestrain (prestrain) is an important factor. The present inventors have investigated the effect of the amount of prestrain on the subsequent strain age hardening characteristics, assuming a deformation mode in which a steel sheet for automobiles is applied. As a result, except for extremely deep drawing, it can be arranged by the amount of strain (tensile strain) equivalent to one axis, and in an actual part, the amount of strain equivalent to one axis is generally more than 5%. Clearly corresponded to the strength obtained after the pre-strain 5% strain aging treatment. For these reasons, in the present invention, the pre-strain (deformation) before the heat treatment was set to a tensile plastic strain of 5% or more.

  The conventional paint baking treatment condition is 170 ° C x 20 min as standard, but when utilizing precipitation strengthening of ultra-fine carbides as in the present invention, a heat treatment temperature of 150 ° C or higher is required. Become. On the other hand, when the temperature exceeds 350 ° C., the effect saturates and, on the contrary, it tends to soften slightly. Further, when heated to a temperature exceeding 350 ° C., heat distortion and generation of a temper color become remarkable. For this reason, in the present invention, the heat treatment temperature for strain age hardening was set to 150 to 350 ° C. The holding time at the heat treatment temperature is 30 s or more. As for 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 can be achieved. In order to obtain a larger and more stable strain aging hardening, the holding time is desirably at least 60 s, more preferably at least 300 s.

  The heating method in the heat treatment after the pre-deformation treatment (pre-strain treatment) is not particularly limited, but may be, for example, induction heating, non-oxidizing flame, laser, plasma, etc., in addition to the atmosphere heating by a furnace, as in the ordinary coating baking treatment. Heating and the like are all suitable. Also, a so-called warm press in which the temperature of the steel sheet is raised and pressed is also an extremely effective method in the present invention.

  Next, the reasons for limiting the composition of the steel sheet of the present invention will be described. In addition, mass% is simply described as%.

C: 0.01 to 0.15%
C is an element that increases the strength of the steel sheet and affects the increase in strength after pre-deformation-heat treatment. In the present invention, C must be contained at 0.01% or more. In addition, more preferably, it is 0.02% or more. On the other hand, when the content exceeds 0.15%, the fraction of carbide in the steel increases, and the ductility and further the press formability decrease. Further, as a more important problem, when the C content exceeds 0.15%, spot weldability, arc weldability, and the like are significantly reduced. For this reason, in the present invention, the C content is limited to 0.01 to 0.15%. In addition, from the viewpoint of moldability, the content is preferably 0.10% or less.

Si: 2.0% or less
Si is a useful strengthening element capable of increasing the strength of a steel sheet without significantly reducing the ductility of the steel sheet, and is preferably contained at 0.01% or more. However, when the content exceeds 2.0%, the press formability is deteriorated and the surface properties are deteriorated. For this reason, Si is limited to 2.0% or less. The content is preferably 1.0% or less, and more preferably 0.02 to 0.8%.

Mn: 3.0% or less
Mn has an effect of strengthening steel and is an effective element for preventing hot cracking due to S. It is preferable that Mn is contained according to the amount of S contained. Such effects become remarkable when the content is 0.5% or more. In addition, Mn has an effect of lowering the Ar ₃ transformation point. As the content increases, the Ar ₃ transformation point decreases, so that ferrite transformation occurs at a low temperature during annealing cooling, and the dislocation density in the ferrite after annealing. Will be higher. The increase in the dislocation density after annealing has the same effect as the increase in the amount of prestrain, and promotes the strain-induced precipitation of fine carbides, so that a greater increase in strength after prestrain aging can be obtained. Such effects become remarkable when the content is 1.0% or more. It is more preferably at least 1.5%. On the other hand, if the content exceeds 3.0%, press formability and weldability deteriorate. Therefore, in the present invention, Mn is limited to 3.0% or less.

P: 0.1% or less P is an element having an effect of strengthening steel, and is preferably contained in an amount of 0.005% or more according to a desired strength. On the other hand, if it is contained excessively, press formability deteriorates. Therefore, P is limited to 0.1% or less. When more excellent press formability is required, the content is preferably 0.05% or less.

S: 0.02% or less S is an element that exists as an inclusion in the steel sheet and causes deterioration of the ductility, formability, and particularly stretch flangeability of the steel sheet. It is preferable to reduce the content as much as possible, but it is reduced to 0.02% or less. In this case, the upper limit of S is set to 0.02% because the influence is not so great. When excellent stretch flange formability is required, S is preferably set to 0.010% or less.

Al: 0.1% or less
Al is an element added as a deoxidizing element of steel and is useful for improving the cleanliness of steel, and is preferably contained at 0.01% or more. However, if the content exceeds 0.1%, no further deoxidizing effect can be obtained, and conversely, the press formability deteriorates. For this reason, Al was limited to 0.1% or less. Preferably it is 0.08% or less. Note that, in the present invention, a melting method by a deoxidizing method other than Al deoxidizing is not excluded, and for example, Ti deoxidizing or Si deoxidizing may be performed, and a steel sheet by these deoxidizing methods is also included in the present invention. Included in the range. At this time, even if Ca or REM is added to the molten steel, the characteristics of the steel sheet of the present invention are not hindered at all. Of course, a steel sheet containing Ca, REM or the like is also included in the scope of the present invention.

N: 0.02% or less N is an element that increases the strength of a steel sheet by solid solution strengthening and strain age hardening, and it is preferable to contain 0.001% or more, but if it exceeds 0.02%, nitrides are contained in the steel sheet. Increases, thereby significantly deteriorating the ductility and further the press formability of the steel sheet. For this reason, N was limited to 0.02% or less. When the press formability is required to be further improved, the content is preferably 0.01% or less, more preferably 0.008% or less.

Nb: 0.01 to 0.2%, Mo: one or two selected from 0.05 to 2.0%
Both Nb and Mo are elements that significantly increase strain age hardening (pre-deformation-increase in strength after heat treatment) of a steel sheet, and are one of the most important elements in the present invention. In the present invention, one or two selected from Nb and Mo are contained, and the structure of the hot-rolled sheet is 32 nm or less, preferably a structure in which ultrafine precipitates of 30 nm or less are precipitated, and cold-rolled. By performing annealing, ultra-fine carbides containing one or two of Nb and Mo are strain-induced precipitate during pre-deformation-heat treatment, and the tensile strength is ΔTS: 150 MPa or more, preferably ΔTS: 170 MPa or more. Is obtained. When Nb is less than 0.01% and Mo is less than 0.05%, even if the pre-deformation-heat treatment conditions and the size of the hot-rolled sheet precipitate are changed, an increase in the tensile strength of ΔTS: 150 MPa or more cannot be obtained. On the other hand, a content exceeding 0.2% of Nb and 2.0% of Mo saturates the effect, cannot provide an effect corresponding to the content, is economically disadvantageous, and causes deterioration of press formability. Therefore, Nb is limited to 0.01-0.2% and Mo is limited to 0.05-2.0%. Preferably, Nb is 0.015 to 0.1% and Mo is 0.1 to 1.0%.

  In the present invention, it is preferable that one or two of Ti and V be further contained in a total of 0.4% or less in addition to the above basic components.

One or two of Ti and V: 0.4% or less in total
Both Ti and V are carbide forming elements and effectively act to increase the strength by utilizing strain age hardening, so that they can be selectively contained as necessary. In addition, such an effect is remarkable when Ti alone is 0.01% or more and V: 0.01% or more when combined, and when combined, the total content is 0.01% or more. However, when one or two of Ti and V are contained in total exceeding 0.4%, press formability deteriorates. Therefore, it is preferable that the total content of Ti and V is limited to 0.4% or less.

  In addition to the above-mentioned components, there is no particular limitation, but there is no problem even if B: 0.1% or less, Zr: 0.1% or less, Ca: 0.1% or less, REM: 0.1% or less.

  The balance other than the above components is Fe and unavoidable impurities. As inevitable impurities, for example, Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, and Co: 0.1% or less can be tolerated.

  Next, a method for producing a cold-rolled steel sheet according to the present invention will be described.

  The cold-rolled steel sheet of the present invention comprises a steel slab having a composition in the above-described range as a raw material, the raw material is hot-rolled, and then cooled and taken up as a hot-rolled steel sheet. The cold-rolled sheet is subjected to cold rolling to obtain a cold-rolled sheet, and an annealing step of annealing the cold-rolled sheet to form a cold-rolled annealed sheet is sequentially performed.

  The steel slab to be used is preferably a steel slab obtained by a continuous casting method in order to prevent macro segregation of components from molten steel smelted by a known smelting method such as a converter. It may be manufactured by a continuous 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 inserted directly into a heating furnace without cooling, or after being slightly heated, and then immediately rolled. Energy saving processes such as direct rolling and direct rolling can be applied without any problem.

  In the hot rolling step, the above-described steel slab is heated and subjected to hot rolling, and then cooled to take up a hot rolled sheet. In the hot rolling step, the slab heating temperature is preferably 900 ° C. or more, and the finish-rolling exit temperature is preferably 700 ° C. or more. In the present invention, the cooling and winding in the hot rolling step are preferably cooling at a cooling rate of 20 ° C./s or more to 600 ° C. or less, and a cooling and winding step.

Slab heating temperature: 900 ° C. or more If the slab heating temperature is less than 900 ° C., the rolling load increases, and the risk of occurrence of trouble during hot rolling increases. Therefore, the slab heating temperature is preferably 900 ° C. or more. Note that the slab heating temperature is desirably 1300 ° C. or lower because of an increase in scale loss accompanying an increase in oxidation weight.

  From the viewpoint of lowering the slab heating temperature and preventing trouble during hot rolling, it is needless to say that utilizing a so-called sheet bar heater for heating the sheet bar is an effective method. .

Finishing roll exit side temperature: 700 ° C. or more By setting the finish rolling exit side temperature to 700 ° C. or more, a uniform hot rolled base plate structure that can obtain excellent formability after cold rolling and annealing can be obtained. On the other hand, if the finish-rolling exit temperature is less than 700 ° C., the structure of the hot-rolled base plate becomes uneven, and the rolling load during hot rolling increases, increasing the risk of trouble during hot rolling. . For this reason, the finish-rolling exit temperature in the hot rolling step is preferably set to 700 ° C. or higher.

Cooling rate: 20 ° C./s or more Cooling after hot rolling is preferably performed at an average cooling rate from the finish-rolling exit temperature to the winding temperature of 20 ° C./s or more. If the cooling rate is less than 20 ° C./s on average, the precipitates become coarse, and the hot-rolled sheet does not become a hot-rolled sheet having a structure in which precipitates having an average grain size of 32 nm or less, preferably 30 nm or less are precipitated, and the pre-deformation is not performed -During the heat treatment, a sufficient increase in strength may not be obtained due to precipitation of ultrafine carbides containing one or two of Nb and Mo. For this reason, it is preferable that the average cooling rate from the finish-rolling discharge side temperature to the winding temperature be 20 ° C./s or more. Note that the temperature is more preferably 30 ° C./s or more.

Winding temperature: 600 ° C or less Cool at the above cooling rate to 600 ° C or less and wind. At this time, when the winding temperature, which is the end point of cooling, exceeds 600 ° C., the precipitates are coarsened, and the hot-rolled sheet has a structure in which precipitates having an average particle size of 32 nm or less, preferably 30 nm or less are precipitated. In some cases, a sufficient increase in strength may not be obtained due to precipitation of ultrafine carbide containing one or two of Nb and Mo during the pre-deformation-heat treatment. For this reason, the winding temperature is preferably set to 600 ° C. or lower. The temperature is more preferably 500 ° C. or lower. If the winding temperature is less than 200 ° C., the shape of the steel sheet is remarkably disturbed, and the risk of causing trouble in actual use of the steel sheet increases.

  In the hot rolling step of the present invention, part or all of the finish rolling may be performed by lubrication rolling in order to reduce the rolling load during hot rolling. Performing the lubricating rolling is also effective from the viewpoint of uniformizing the shape of the steel sheet and uniforming the material. In addition, it is preferable that the wear coefficient at the time of lubrication rolling be in the range of 0.25 to 0.10. Further, it is preferable to adopt a continuous rolling process in which successive sheet bars are joined and finish rolling is continuously performed. Applying the continuous rolling process is also desirable from the viewpoint of operational stability of hot rolling.

  The hot-rolled sheet obtained by the above-described hot-rolling step becomes a hot-rolled sheet having a structure in which precipitates having an average particle size of 32 nm or less, preferably 30 nm or less are deposited. The average particle size is the average particle size of the precipitate obtained as described above for the precipitate having a particle size of 80 nm or less. By making the precipitate of the hot-rolled sheet an average grain size of 32 nm or less, preferably 30 nm or less, the cold-rolled annealed sheet subjected to the cold-rolling step-annealing step is subjected to pre-deformation-after heat treatment. It shows a remarkable increase in strength due to precipitation of ultrafine carbide containing one or two of Nb and Mo. If the average grain size of the precipitates exceeds 32 nm, the effect of increasing the strength of ΔTS: 150 MPa or more after the pre-deformation-heat treatment cannot be obtained. In order to obtain a remarkable strength increase effect of ΔTS: 170 MPa or more, it is preferable that the average particle size of the precipitate does not exceed 30 nm and does not become coarse. When the precipitates become finer, the interfacial energy and the strain energy become higher, so that the precipitates become unstable.Moreover, the interaction with the high-density dislocations introduced in the subsequent cold rolling step makes the precipitates more unstable, resulting in annealing. It is considered that the precipitates are dissolved in the process, and as a result, the subsequent pre-deformation-heat treatment causes strain-induced precipitation as ultrafine carbides, and the strength is remarkably increased.

  Next, the hot-rolled sheet having such a structure is subjected to a cold-rolling step. In the cold rolling step, the hot rolled sheet is subjected to cold rolling to obtain a cold rolled sheet. The cold rolling conditions are not particularly limited as long as a cold rolled sheet having a desired size and shape can be obtained, but the rolling reduction during cold rolling is preferably 40% or more. When the rolling reduction is less than 40%, recrystallization is unlikely to occur uniformly during the subsequent annealing.

  Next, an annealing process is performed on the cold-rolled sheet to form a cold-rolled annealed sheet.

Annealing is preferably performed in either a continuous annealing line or a continuous hot-dip galvanizing line. In the present invention, the annealing is preferably performed in a temperature range of (Ac ₃ transformation point−110 ° C.) or higher. If the annealing temperature is lower than (Ac ₃ transformation point-110 ° C.), a large strength increase effect of ΔTS: 150 MPa or more cannot be obtained after pre-deformation-heat treatment. In order to obtain a remarkable strength increasing effect of ΔTS: 170 MPa or more, the annealing temperature is preferably (Ac ₃ transformation point −100 ° C.) or more. Annealing temperature is preferably a as high as possible in terms of dissolving the precipitate of the hot-rolled sheet (Ac ₃ transformation point -110 ° C.) or higher, exceeds (Ac ₃ transformation point +100 ° C.), grain It is preferable that the temperature be not more than (Ac ₃ transformation point + 100 ° C.), because it becomes coarse and the press formability tends to deteriorate. Note that the Ac ₃ transformation point is determined by measuring a thermal expansion-temperature curve during heating at a heating rate of 5 ° C./s.

  Further, the cooling rate after annealing is preferably 1 ° C./s or more. If the cooling rate after annealing is less than 1 ° C./s, carbides are likely to precipitate during cooling, and a sufficient increase in strength due to precipitation of ultrafine carbides during pre-deformation-heat treatment may not be obtained. Preferably, the cooling stop temperature after annealing is 400 ° C. or less.

  Further, the cold-rolled annealed sheet after the annealing step may be subjected to temper rolling at an elongation of 10% or less for the purpose of adjusting the shape, the surface roughness and the like.

  The steel sheet of the present invention can be applied not only as a steel sheet for processing but also as an original sheet of a surface-treated steel sheet for processing. Examples of the surface treatment include zinc plating (including alloys), tin plating, and enamel. Further, in the case of a hot-dip galvanized steel sheet, after performing the above-described annealing in a continuous hot-dip galvanizing line, it is preferable to perform a plating process by cooling to a plating bath temperature, or to further perform an alloying process. .

  Further, after the galvanization, the cold-rolled steel sheet of the present invention may be subjected to a special treatment for improving the chemical conversion property, weldability, press formability, corrosion resistance and the like.

Molten steel having the composition shown in Table 1 was smelted in a converter and made into a steel slab by a continuous casting method. These steel slabs were hot-rolled steel sheet having a thickness of 4.0mm (the hot-rolled sheet) by hot rolling process conditions shown in Table 2. Further, test pieces were taken from the resulting hot-rolled steel plate (hot-rolled sheet) was investigated hot rolled sheet structure. Subsequently, these hot-rolled steel sheets (hot-rolled sheets) were pickled, and then cold-rolled sheets having a thickness of 1.6 mm were obtained by a cold-rolling process under the conditions shown in Table 2. Next, these cold-rolled sheets were subjected to an annealing step under the conditions shown in Table 2 in a continuous annealing line to obtain cold-rolled steel sheets (cold-rolled annealed sheets). The obtained cold-rolled steel sheet (cold-rolled annealed sheet) was further subjected to temper rolling at an elongation of 0.8%.

Test specimens were collected from the obtained cold-rolled steel sheet (cold-rolled annealed sheet), and tensile properties and strain age hardening properties were investigated. The Ac ₃ transformation point was determined from a thermal expansion-temperature curve measured at a heating rate of 5 ° C./s.

(1) Tensile properties From the obtained cold-rolled steel sheet (cold-rolled annealed sheet), a JIS No. 5 tensile test piece was sampled in the rolling direction, and a tensile test was performed in accordance with the provisions of JIS Z 2241, and the yield strength YS , Tensile strength TS, elongation El, and yield ratio YR.

(2) Strain aging hardening characteristics A JIS No. 5 test piece was sampled in the rolling direction from the obtained steel sheet (cold rolled annealed sheet) and subjected to 5% plastic deformation as pre-deformation (tensile pre-strain). After a heat treatment of 20 ° C. × 20 min, a tensile test was carried out, a tensile strength TS _HT after the heat treatment was determined, and ΔTS = TS _HT −TS was calculated. Note that TS _HT is the tensile strength after pre-deformation-heat treatment, and TS is the tensile strength of a steel sheet (a cold-rolled annealed sheet).

(3) hot rolled sheet structure resulting hot-rolled steel plate were taken test specimens from (hot-rolled sheet), the rolling direction cross-section (L cross section), magnification using a transmission electron microscope: 10 thousand times with 10 fields Observed and imaged, using an image analyzer, determine the area of each precipitate, determine the equivalent circle diameter from this area and determine the particle size of each precipitate, and for each field of view the precipitate with a particle size of 80 nm or less Of the hot rolled sheet was determined as the average particle diameter of the hot rolled sheet.

  The steel sheet No. 24 was cooled from the annealing temperature to 460 ° C at a rate of 20 ° C / s, hot-dip galvanized, and then alloyed at 520 ° C to obtain an alloyed hot-dip galvanized steel sheet. Was evaluated.

  Table 3 shows the results.

  Each of the examples of the present invention shows an extremely large ΔTS and is a steel sheet excellent in strain age hardening characteristics. On the other hand, in Comparative Examples outside the scope of the present invention, the steel sheet has a small ΔTS and a reduced strain age hardening property.

4 is a graph showing the relationship between ΔTS after pre-deformation and heat treatment and the average grain size of precipitates in a hot-rolled sheet. It is a graph which shows the relationship between (DELTA) TS after pre-deformation-heat treatment and annealing temperature.

Claims (7)

In mass%,
C: 0.01 to 0.15%, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
N: not more than 0.02%, Nb: 0.01 to 0.2%, Mo: 0.05 to 2.0%, one or two selected from the group consisting of Fe and unavoidable impurities. , ΔTS: a steel sheet having excellent strain age hardening characteristics, having a strain age hardening characteristic of 150 MPa or more. A hot-rolled sheet having the above-described composition and having a precipitate average particle diameter of 32 nm or less determined for precipitates having a particle diameter of 80 nm or less is cold-rolled, and then subjected to a temperature range of (Ac ₃ transformation point −110 ° C.) or higher. The steel sheet according to claim 1, wherein the steel sheet is annealed.   3. The steel sheet according to claim 1, further comprising, by mass%, one or two of Ti and V in a total of 0.4% or less in addition to the composition. After hot rolling a steel slab, it is cooled and rolled into a hot rolled sheet, a hot rolling step is performed by cold rolling the hot rolled sheet to form a cold rolled sheet, and annealing is performed on the cold rolled sheet. Performing an annealing step of applying a cold-rolled annealed sheet to the steel slab.
C: 0.01 to 0.15%, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
N: 0.02% or less, Nb: 0.01 to 0.2%, Mo: 0.05 to 2.0%, one or two selected from the group consisting of Fe and unavoidable impurities. The slab is cooled and taken up in the hot rolling step at a cooling rate of 20 ° C./s or more to 600 ° C. or less and taken up. The annealing temperature in the annealing is (Ac ₃ transformation point− A method for producing a steel sheet having excellent strain aging hardening characteristics, wherein the temperature is set to 110 ° C. or higher.   5. The method for producing a steel sheet according to claim 4, wherein one or two of Ti and V are further contained in a total of 0.4% or less by mass% in addition to the composition.   6. The method for producing a steel sheet according to claim 4, wherein a slab heating temperature of the hot rolling is 900 ° C. or more and a finish rolling exit temperature is 700 ° C. or more.   The method for producing a steel sheet according to claim 4, wherein a cooling rate after the annealing is 1 ° C./s or more.

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