JP5034296B2 - Hot-rolled steel sheet with excellent strain age hardening characteristics and method for producing the same - Google Patents

Description

  The present invention mainly relates to hot-rolled steel sheets for automobiles, in particular, press formability such as bending workability and stretch flange workability is good, and the strain aging significantly increases tensile strength by heat treatment after press forming. The present invention relates to a hot-rolled steel sheet having excellent hardening characteristics, a hot-rolled steel sheet having excellent fatigue characteristics, and a method for producing such a hot-rolled steel sheet.

  In the present invention, “excellent in strain age hardening characteristics” means having strain age hardening characteristics where ΔTS is 100 MPa or more. ΔTS is the amount of increase in tensile strength before and after pre-deformation and heat treatment when pre-deformation treatment with a plastic strain amount of 2% or more followed by heat treatment at a temperature in the range of 150 to 200 ° C. for a holding time of 30 s or more. {= (Tensile strength after heat treatment) − (Tensile strength before pre-deformation treatment)}.

  In recent years, in connection with exhaust gas regulations from the viewpoint of conservation of the global environment, the reduction of vehicle body weight has become a very important issue. For this reason, it has been studied to increase the strength of a steel plate used for an automobile body to reduce the thickness of the steel plate and reduce the weight of the vehicle body.

  Since car body structural parts for automobiles to which such high-strength steel plates are applied are mainly used for press forming and hole expansion molding, the steel plate that is the material may have high hole expandability in addition to press formability. Necessary.

  However, in general, when the strength of the steel sheet is increased, the elongation is reduced, so that the press formability is lowered, and the yield strength is also increased. In addition, in a high-strength steel sheet mainly composed of a martensite structure, if the emphasis is placed on press formability and the elongation is increased, the hole expandability decreases, and if the hole expandability is increased, the elongation decreases. As described above, it is difficult to achieve both press formability and hole expandability of a vehicle body structural component by simply increasing the strength of the steel plate.

  On the other hand, in addition to environmental protection problems, recently, in order to protect passengers in the event of a collision, the safety of the automobile body has been emphasized, and 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, hot-rolled steel sheets that have low strength when molding automobile parts, are excellent in press formability and hole expandability, and have high strength and excellent impact resistance properties when they are finished products have been most strongly desired.

  As a conventional technique for such a demand, a coating baking process including a constant temperature holding of 100 to 200 ° C. after press working was developed for the purpose of obtaining a steel sheet having excellent press formability while being a high-strength steel sheet. It is a paint bake hardened steel sheet that yield strength increases when applied. This steel sheet has a structure with ferrite as the main phase, and finally the amount of C present in the solid solution state (solid solution C amount) is controlled within an appropriate range, and is soft at the time of press forming and is baked after press forming. At the time of processing, the remaining solid solution C adheres to the dislocations introduced at the time of press forming and hinders the movement of dislocations. As a result, the yield stress is increased by strain age hardening. However, with the paint bake hardened steel sheet that uses strain age hardening obtained by this technology, although the yield stress can be increased, the tensile strength cannot be increased, and the effect on impact resistance is sufficient. That's not true.

  Patent Document 1 includes C: 0.08 to 0.2%, Mn: 1.5 to 3.5%, has a component composition composed of the remaining Fe and inevitable impurities, and has a structure of 5% or less. A high-tensile hot-rolled steel sheet that is excellent in strain age hardening and aging resistance is disclosed by adopting a composite structure including a ferrite phase and bainite or partly martensite. The strain age hardenability of the hot-rolled steel sheet described in Patent Document 1 is that the yield stress increases after baking and a high amount of hardening that did not exist before that is obtained, and also excellent in age resistance. However, since the bainite phase contains carbide, it cannot be raised by the tensile strength, and the effect of improving impact resistance is insufficient.

  Patent Document 2 includes C: 0.02 to 0.13%, Si: 2% or less, Mn: 0.6 to 2.5%, and has a component composition composed of the balance Fe and inevitable impurities. However, a high-tensile hot-rolled steel sheet excellent in strain age hardening and aging resistance has been disclosed by adopting a composite structure mainly composed of ferrite and martensite. The strain aging property of the hot-rolled steel sheet described in Patent Document 2 is the same as that of Patent Document 1, but the yield stress increases after baking, and a high bake hardening amount is obtained and the aging resistance is excellent. It cannot be raised, and the effect of improving the impact resistance is still insufficient. Moreover, it is a composite structure composed of martensite and ferrite having a large hardness difference, and is inferior in hole expansibility.

  Furthermore, Patent Document 3 proposes a method for producing an alloyed hot-dip galvanized steel sheet in which the hot rolled sheet or the hot rolled sheet is used as a plated steel sheet original plate and the strength can be increased by heat treatment after forming. This technology includes C: 0.01 to 0.08%, and after adding Si, Mn, P, S, Al, and N to appropriate amounts, one or more of Cr, W, and Mo are combined. After hot rolling the steel containing 0.05 to 3.0%, or cold rolling or temper rolling in addition to that, annealing, hot dip galvanizing, and then heat alloying treatment It is to give. In Patent Document 3, an increase in tensile strength is obtained by heating the steel sheet thus obtained 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. is there.

On the other hand, since stress is repeatedly applied depending on the parts constituting the automobile body, such parts are required to have excellent fatigue characteristics in addition to the above characteristics. In particular, when the plate thickness is reduced by increasing the strength, the demand is great.
As a technique for improving the fatigue characteristics, Patent Document 4 includes C: 0.03 to 0.20%, and after setting Si, Mn, P, S, and Al to appropriate amounts, Cu: 0.0. 2 to 2.0% and B: 0.0002 to 0.002%, and the microstructure is a composite structure having ferrite as the main phase and martensite as the second phase, and the presence state of Cu in the ferrite phase Has been proposed for a hot-rolled steel sheet for processing excellent in fatigue properties, in which a solid solution state and / or a precipitation state of 2 nm or less is proposed. However, although the steel sheet described in Patent Document 4 takes fatigue characteristics into consideration, it does not describe that it has pressability, hole expansibility, and impact resistance. Moreover, since addition of Cu is required, there is also a problem that it is difficult to scrap and recycle.
JP-A-62-74051 Japanese Patent Laid-Open No. 4-74824 Japanese Patent Laid-Open No. 10-310824 JP-A-11-199975

  As described above, hot-rolled steel sheets with low strength when molding automotive parts, excellent press formability and hole expandability, and high strength and excellent impact resistance when finished products, and in addition to this Although there is a strong demand for hot-rolled steel sheets having excellent fatigue properties, there is still no technology for industrially producing steel plates that satisfy these characteristics.

  The present invention has been made in view of such circumstances, and has excellent press formability and hole expansibility suitable as a steel sheet for automobiles, and is heat-treated at the same level as the conventional baking coating temperature after press forming. A hot-rolled steel sheet with excellent strain-age hardening characteristics, which has a significantly increased tensile strength, and a hot-rolled steel sheet with significantly improved fatigue characteristics in addition to strain-age hardening characteristics. It aims at providing the manufacturing method which can be produced stably.

  In order to achieve the above-mentioned problems, the present inventors have conducted extensive studies on the influence of the steel sheet structure and alloy elements on the strain age hardening characteristics.

  In this study, in order to measure the tensile strength due to strain aging hardening, the tensile strength after strain aging treatment (corresponding to the tensile strength after heat treatment) TS 'and the tensile strength without strain aging treatment Strength (equivalent to the tensile strength before the pre-deformation treatment) TS was evaluated using a difference ΔTS.

  FIG. 1 shows the relationship between the aging heat treatment temperature, the tensile strength (TS) and the tensile strength after strain aging (TS ′) of each hot-rolled steel sheet in which the hot rolling conditions and the C content are changed. In this case, the pre-strain amount was set to 3%. In FIG. 1, the structure in the case where the hot rolling finishing temperature FT = 900 ° C. and the amount of C is 0.25 mass% is a martensite single phase structure. On the other hand, when FT = 900 ° C. and the amount of C is 0.10% by mass, and when FT = 750 ° C. and the amount of C is 0.15% by mass, the structure forms are both a composite structure composed of martensite and ferrite. The amount of ferrite is about the same, but when FT = 750 ° C. and the amount of C is 0.15% by mass, the amount of solid solution C is reduced by precipitation treatment.

  As is apparent from FIG. 1, the strength after strain aging decreases in the martensite single-phase structure, whereas the tensile strength of 200 MPa or more by strain aging heat treatment at 200 ° C. in the composite structure steel plate composed of martensite and ferrite. An increase (ΔTS) is obtained. Further, when FT = 900 ° C. where the amount of solid solution C is high without precipitation treatment and the amount of C is 0.10% by mass, even higher strain age hardening can be obtained even if the amount of ferrite is almost the same.

  As described above, it has been found that a high strain age hardening can be obtained by a structure containing martensite as a main phase and ferrite as a second phase.

  As a result of further research based on such new knowledge, in order to obtain such high strain age hardening, in the structure containing martensite as the main phase and ferrite as the second phase, the amount of dissolved C is reduced. It has been found that it is necessary to make it 0.01% by mass or more, and at the same time, it is necessary to define the ferrite fraction and the ferrite average particle diameter. FIG. 2 shows the results of a detailed investigation on the influence of the ferrite fraction, ferrite average particle size, and solid solution C content on ΔTS. First, when the average ferrite particle diameter is 20 μm or less and the solid solution C amount is 0.01% by mass or more, ΔTS of 100 MPa or more is obtained when the ferrite fraction is in the range of 1 to 30%. Furthermore, when the ferrite average particle diameter is 5 μm or less and the solid solution C amount is 0.01% by mass or more, a large ΔTS of 150 MPa or more is obtained when the ferrite fraction is in the range of 3 to 25%. On the other hand, even if the amount of solute C is 0.01% by mass or more, when the average ferrite particle diameter exceeds 20 μm, only ΔTS of about 50 to 70 MPa can be obtained regardless of the ferrite fraction. Furthermore, a steel plate having an average ferrite grain size of 20 μm or less (for example, 5 μm or less in the example of FIG. 2) and a solid solution C content of 0.01% by mass or more is subjected to heat treatment at 350 ° C. × 20 min to form iron carbide. When the amount of solute C is less than 0.01% by mass, ΔTS is greatly reduced to 50 MPa or less. That is, in order to obtain high strain age hardening, the martensite phase is the main phase, the area ratio and average particle size of the ferrite as the second phase are appropriately adjusted, and a solid solution C of 0.01% by mass or more is further obtained. It is necessary to secure the quantity. Further, if martensite is not tempered, ferrite having an average particle diameter of 20 μm or less is included in the range of 1% or more and 30% or less by area ratio, so that the amount of dissolved C is 0.01% by mass or more. It can be.

  Thus, since solute C plays an important role in the present invention, tempering treatment at a high temperature exceeding 350 ° C. performed for improving toughness in martensitic steel or the like forms carbides, It is necessary not to do this because the dissolved C decreases. The tempering referred to in the present invention means the high-temperature or long-time heat treatment actively performed as described above. Self-tempering during cooling, which is difficult to avoid in manufacturing, and low-temperature short-time tempering are the distortions. It does not impair any curing properties and is not included in the tempering of the present invention.

  The mechanism of strain age hardening of the present invention is not completely clear, but the present inventors are based on the interaction between C atoms and dislocations as in the conventional bake hardened (BH) steel sheet. The mechanism is considered as follows.

  In other words, the structure of the steel sheet according to the present invention has martensite as the main phase and surrounds soft ferrite. Therefore, when deformed with pre-strain, hard martensite does not deform and stress is applied to the soft ferrite. As a result of the concentration, a large amount of distortion is introduced and hardened. Furthermore, the martensite is tempered by the subsequent aging heat treatment, so that carbon (C) existing in supersaturation in the martensite diffuses and precipitates through dislocations and strains in the ferrite. As a result, dislocations in the ferrite are pinned to C precipitates, which further increases TS (tensile strength). Here, when there is no pre-strain, since the dislocation / strain in the ferrite is small, C cannot be diffused, and it is considered that the effect of increasing the strength does not occur. Although the details of the precipitation form of C that contributes to this strengthening are not clear, it is presumed to be a metastable iron carbide because it age-hardens in a temperature range of 200 ° C. or less.

  In addition, the present inventors have repeatedly studied the structure and fatigue characteristics of the steel sheet after strain aging treatment. In this study, hardness (Hv) measurement after strain aging treatment was performed in order to measure changes in steel sheet structure due to strain age hardening. Fatigue properties were evaluated by a tensile fatigue test. The tensile fatigue test was performed using a steel plate that had been subjected to strain aging treatment, and was evaluated by a fatigue limit ratio (FL / TS) that is a ratio between the fatigue endurance limit (FL) and the tensile strength (TS) before the strain aging treatment. . FIG. 3 shows the influence of the hardness ratio Hv (α) / Hv (M) between the hardness Hv (α) of ferrite after strain aging treatment and the hardness Hv (M) of martensite on fatigue properties. As shown in this figure, in the steel having a high ferrite fraction, the hardness ratio Hv (α) / Hv (M) between ferrite and martensite after strain aging treatment is less than 0.6, and the fatigue limit obtained at this time The ratio (FL / TS) is also as low as about 0.7. On the other hand, in steel with a low ferrite fraction, this composite structure steel is subjected to strain aging heat treatment at 200 ° C., so that the hardness ratio Hv (α) / Hv (M) between ferrite and martensite exceeds a high value exceeding 0.6. In addition to the above, it was found that the fatigue limit ratio (FL / TS) obtained at this time was markedly improved to 0.8 or more.

  The present invention has been completed by further studies based on the above findings, and provides the following (1) to (8).

  (1) In mass%, C: 0.01 to 0.2%, 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, the balance is Fe and inevitable impurities, the martensite phase not tempered is the main phase, and the ferrite phase is 1 in area ratio as the second phase. A hot-rolled steel sheet excellent in strain age hardening characteristics, characterized in that it is contained in a range of from 30% to 30% and the average grain size of the ferrite phase is 20 μm or less.

  (2) In mass%, C: 0.01 to 0.2%, 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, consisting of the balance Fe and inevitable impurities, with the martensite phase as the main phase and the ferrite phase as the second phase in an area ratio of 1% or more and 30% A hot-rolled steel sheet having excellent strain age hardening characteristics, characterized in that the ferrite is contained in the following range, the average particle size of the ferrite is 20 μm or less, and the amount of dissolved C is 0.01% by mass or more.

  (3) Strain aging characterized in that in (1) or (2) above, one or more of Nb, Ti, V, and Mo are further contained by 0.2% or less in total by mass%. Hot-rolled steel sheet with excellent hardening characteristics.

  (4) By mass%, C: 0.01 to 0.2%, 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, the balance is Fe and inevitable impurities, the martensite phase not tempered is the main phase, and the ferrite phase is 1 in area ratio as the second phase. % Of the ferrite phase is 15 μm or less, and the hardness Hv (M) of the martensite phase and the hardness Hv (α) of the ferrite phase after strain aging treatment are included. A hot-rolled steel sheet having excellent fatigue characteristics and strain age hardening characteristics, wherein Hv (α) / Hv (M) ≧ 0.6.

  (5) By mass%, C: 0.01 to 0.2%, 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, consisting of the balance Fe and inevitable impurities, with the martensite phase as the main phase and the ferrite phase as the second phase in an area ratio of 1% or more and 30% The ferrite phase has an average particle size of 15 μm or less, a solid solution C content of 0.01% by mass or more, and a hardness Hv (M) of the martensite phase after strain aging treatment. And a ferrite phase hardness Hv (α) of Hv (α) / Hv (M) ≧ 0.6, a hot-rolled steel sheet having excellent fatigue characteristics and strain age hardening characteristics.

  (6) In the above (4) or (5), the fatigue properties further comprising 0.2% or less in total of one or more of Nb, Ti, V, and Mo in mass%. And hot rolled steel sheet with excellent strain age hardening characteristics.

(7) In mass%, C: 0.01 to 0.2%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, A steel slab containing Al: 0.1% or less, N: 0.02% or less, and the balance Fe and inevitable impurities is subjected to hot rolling with a finish rolling finish temperature not lower than the Ar ₃ transformation point, and finished. After rolling, the steel sheet is cooled at an average cooling rate of 20 ° C./sec or higher to the martensite transformation temperature (Ms point) or lower, wound at a temperature of 300 ° C. or lower, and then not subjected to tempering heat treatment at 350 ° C. or higher. A method for producing a hot-rolled steel sheet having excellent strain age hardening characteristics.

  (8) In the above (7), the steel slab further contains 0.2% or less of one or more of Nb, Ti, V, and Mo in total in mass%. A method for producing a hot-rolled steel sheet having excellent age-hardening characteristics.

  In addition, aging property becomes a problem in the steel plate which shows strain age hardening property. This is a phenomenon in which the steel sheet is stored at room temperature for a long period of time, resulting in a rise in strength and the like. With respect to the steel sheet according to the present invention, when a tensile test was performed after heat treatment (200 ° C., 20 min) without pre-deformation (0%) for the purpose of investigating this aging property, the increase in strength (TS, YP) It was not recognized and it was confirmed that it also has high aging resistance.

  According to the present invention, the martensite phase is the main phase and the second phase has a microstructure that includes a predetermined ferrite, so that it has excellent press formability, thus maintaining the productivity by press forming and press forming. Later, it is possible to obtain a hot-rolled steel sheet having excellent strain age hardening characteristics in which the tensile strength is greatly increased by a heat treatment at the same level as the conventional baking coating temperature. Moreover, it becomes possible to manufacture such a hot-rolled steel sheet stably. In addition to the above characteristics, the fatigue limit ratio is dramatically improved by satisfying the relationship of the hardness ratio between the hardness of the martensite phase and the hardness of the ferrite phase after strain aging treatment. An excellent hot-rolled steel sheet can be obtained.

Hereinafter, the present invention will be specifically described.
The present invention is intended for high-tensile hot-rolled steel sheets having a tensile strength TS of 450 MPa or more, particularly 600 MPa or more, and the tensile strength is remarkably increased by heat treatment at a relatively low temperature after press forming, and the strength changes. It is a steel plate excellent in strain age hardening characteristics in which ΔTS is 100 MPa or more, and in addition to this, a steel plate excellent in fatigue characteristics, and has a specific structure and a specific composition.

First, the structure of the steel plate will be described.
The structure of the steel sheet in the present invention is a composite structure including a martensite phase which is not tempered as a main phase and a ferrite phase having an area ratio of 1% or more and 30% or less and an average grain size of 20 μm or less as a second phase. Have.

  The reason why the average particle diameter of ferrite is set to 20 μm or less is that a large amount of dislocations that become C precipitation sites can be introduced into the ferrite during pre-deformation. In particular, when the average particle size is 5 μm or less, remarkable strain age hardening can be obtained. The ferrite phase area ratio is set to 1% or more and 30% or less because when the ferrite phase area ratio is less than 1%, the tempering softening of martensite is large, and when it exceeds 30%, strain age hardening occurs. This is because even if the effective amount of solute C is 0.01% by mass or more, a high strength increasing effect (ΔTS) cannot be obtained.

  In addition to martensite as the main phase and ferrite as the second phase, the steel sheet of the present invention may contain residual austenite, bainite, and pearlite in a fraction (area ratio) less than the second phase as the third phase. Good. From the viewpoint of obtaining a higher strength increasing effect, the third phase is preferably a fraction of 1/2 or less of the second phase.

  In order to improve the fatigue characteristics in addition to the strain age hardening characteristics, the average particle diameter of the ferrite phase as the second phase is set to 15 μm or less. In order to refine the ferrite average grain size, it is effective to increase the average cooling rate after the end of hot rolling, which will be described later, and to shorten the cooling start time after the end of hot rolling. It is restricted by the specifications, and an excessively large cooling rate and a start of cooling in a short time have a large equipment burden. Therefore, it is preferable that the average particle diameter of the ferrite phase is 0.5 μm or more in practice.

  In order to improve the fatigue characteristics, it is effective that the difference between the hardness of the martensite phase after the strain aging treatment and the hardness of the ferrite phase is small. When strain is applied to a steel sheet having a structure with martensite as the main phase and ferrite as the second phase, soft ferrite causes greater work hardening compared to martensite, and further, heat treatment at 200 ° C. or lower is applied. Ferrite becomes harder. This hardening becomes more prominent as the ferrite particle size becomes smaller, and the fatigue characteristics are remarkably improved by setting the average particle size to 15 μm or less.

  Next, the reason for limiting the component composition of the hot rolled steel sheet according to the present invention will be described. In the following,% means mass%.

C: 0.01 to 0.2%
C is an element that increases the strength of the steel sheet and further promotes the formation of a composite structure of martensite and ferrite. However, if it is less than 0.01%, it is difficult to form a desired martensite-ferrite composite structure, and in order to obtain the high strain age-hardening property of the present invention, 0.01% or more of solid solution C A quantity is needed. On the other hand, when the amount of C exceeds 0.2%, the fraction of martensite and the third phase increases, and the fraction of ferrite is remarkably lowered, so that the ductility is lowered. When the ferrite fraction is low, the dislocations in the ferrite in which the solid solution C acts (adheres) become insufficient, and the strain age-hardening property of the present invention is lowered. Therefore, the C content is set to 0.01 to 0.2%. In addition, from a viewpoint of making spot weldability favorable, 0.15% or less is preferable.

Si: 2.0% or less Si is a useful reinforcing element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel sheet, and has the effect of promoting the formation of ferrite. In order to promote the formation of ferrite, it is preferable to add 0.005% or more. However, if the content exceeds 2.0%, ferrite is excessively formed, and press formability is deteriorated and strength is increased. And the surface properties are deteriorated. For this reason, Si content shall be 2.0% or less.

Mn: 3.0% or less Mn has an action of strengthening steel and further has an action of promoting the formation of a composite structure of martensite and ferrite. Moreover, it is an element effective in preventing the hot crack by S, and it is preferable to make it contain according to the amount of S to contain. Since such an effect becomes remarkable at 0.5% or more, the Mn content is preferably 0.5% or more. On the other hand, if it exceeds 3.0%, press formability and weldability deteriorate, and the formation of ferrite is suppressed. For this reason, Mn content shall be 3.0% or less. From the viewpoint of ferrite formation, 2.0% or less is preferable.

P: 0.1% or less P has an effect of strengthening steel and can be contained in a necessary amount according to desired strength. When utilizing this strengthening, it is preferable to set it as 0.005% or more, but when it contains excessively, press formability will deteriorate. Therefore, the P content is 0.1% or less.

S: 0.02% or less S is an element which exists as an inclusion in a steel sheet and causes deterioration of ductility and formability of the steel sheet, particularly stretch flangeability, and is preferably reduced as much as possible. In the present invention, the content of S is set to 0.02% or less because the adverse effect is not so much reduced when the content is reduced to less than or equal to%. When more excellent stretch flange formability is required, the content is preferably 0.01% or less. From the viewpoint of steelmaking costs for desulfurization, S is preferably 0.001% or more.

Al: 0.1% or less Al is an element which is added as a deoxidizing element for steel and is useful for improving the cleanliness of the steel. The deoxidizing effect cannot be obtained, and conversely the press formability deteriorates. For this reason, Al content shall be 0.1% or less. In addition, in order to acquire the effect as a deoxidation element, it is preferable to add Al 0.01% or more.

N: 0.02% or less N is an element that increases the strength of the steel sheet in the same manner as C by solid solution strengthening and strain age hardening, but if it exceeds 0.02%, nitride increases in the steel sheet. As a result, the ductility and further press formability of the steel sheet are significantly deteriorated. For this reason, N content shall be 0.02% or less. In addition, when improvement of press formability is requested | required more, it is suitable to set it as 0.01% or less. Note that N is an element that is easily mixed from the atmosphere. From the viewpoint of manufacturability, N is preferably 0.002% or more.

One or more of Nb, Ti, V, and Mo: 0.2% or less in total Nb, Ti, and V are all carbide-forming elements and effectively act to increase the strength by fine dispersion of carbides. Therefore, it can be selected and contained as necessary. In addition, Mo is one of the strengthening elements and has an effect of improving the hardenability, so that it can be contained as necessary. When these elements are used for strengthening, in order to obtain a sufficient effect, it is preferable to contain 0.005% or more in total. However, if the total content exceeds 0.2%, problems such as deterioration of press formability and deterioration of chemical conversion property occur. Furthermore, since these elements are carbide forming elements, the amount of solid solution C required to obtain the strength increasing effect of the present invention is reduced, and improvement of ΔTS is hindered. For this reason, when these are included, one or more of Nb, Ti, V, and Mo are made 0.2% or less in total.

  In addition to the elements described above, one or two of Ca: 0.1% or less and REM: 0.1% or less may be contained. These are all elements that contribute to the improvement of stretch flangeability through the form control of inclusions. However, if each of these exceeds 0.1%, the cleanliness of the steel is lowered and ductility is reduced. Moreover, you may contain 1 type or 2 types in B: 0.1% or less and Zr: 0.1% or less from a viewpoint of martensite formation.

  In addition to the above elements and the remaining Fe, various impurity elements and trace addition elements essential in the manufacturing process are inevitably mixed in the manufacturing process. Such inevitable impurities are particularly effective for the effects of the present invention. It does not affect and is allowed. Examples of inevitable impurities include Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, and Co: 0.1% or less.

  A hot-rolled steel sheet having such a structure and composition has excellent press formability and excellent strain age hardening characteristics.

  In the present invention, “excellent in strain age hardening characteristics” means, as described above, after a pre-deformation treatment with a plastic strain amount of 2% or more, for example 3%, at a temperature in the range of 150 to 200 ° C. for a holding time of 30 s. When the above heat treatment is performed, it means that the amount of increase in tensile strength ΔTS {= (tensile strength after heat treatment) − (tensile strength before pre-deformation treatment)} before and after this heat treatment is 100 MPa or more. Desirably, ΔTS is 150 MPa or more. The yield stress also increases by this heat treatment, and the yield stress increase amount ΔYS {= (yield stress after heat treatment) − (yield stress before pre-deformation treatment)} before and after the heat treatment becomes 100 MPa or more.

  In the conventional coating bake hardening test method, 170 ° C. and 20 min are adopted as the heat treatment conditions. In the present invention, the heat treatment temperature is 150 ° C. or more and 200 ° C. or less. Sufficient effect.

  In addition, although Al was described as a deoxidation element for the reason of limitation of each component, in this invention, it does not exclude the melting method by deoxidation methods other than Al, for example, Ti deoxidation or Si deoxidation is performed. In that case, Ca or REM may be added to the molten steel.

  In order to improve fatigue properties in addition to the strain age hardening properties, in addition to the above requirements, the hardness Hv (M) of the martensite phase and the hardness Hv (α) of the ferrite phase after the strain aging treatment are Hv (α ) / Hv (M) ≧ 0.6. That is, when the hardness Hv (M) of the martensite phase and the hardness Hv (α) of the ferrite phase are Hv (α) / Hv (M) <0.6, the hardness difference between the martensite and the ferrite is large, so the repeated fatigue test Occasionally, fatigue cracks are generated from the interface between martensite and ferrite, and the generated cracks propagate, resulting in poor fatigue characteristics. On the other hand, when the hardness Hv (M) of the martensite phase and the hardness Hv (α) of the ferrite phase are Hv (α) / Hv (M) ≧ 0.6, the occurrence of cracks during the fatigue test is suppressed and Since the propagation of cracks is also suppressed, fatigue characteristics are improved.

Next, the manufacturing method of the hot rolled steel sheet of the present invention will be described.
The hot-rolled steel sheet of the present invention is obtained by using a steel slab having a component composition within the above-described range as a raw material, hot-rolling the raw material under predetermined conditions, and winding it to obtain a hot-rolled steel sheet having the above structure.

  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. After manufacturing the steel slab, in addition to the conventional method of cooling to room temperature and then reheating it, without cooling it, it is charged in a heating furnace as it is, or after a little heat retention Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately, can be applied without any problem.

Although it is not necessary to specifically limit the heating temperature of the steel slab, if it 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.
Then, although processes, such as hot rolling, cooling, and winding, are passed, these processes are prescribed | regulated as follows.

Finishing temperature of hot rolling: Ar ₃ transformation point or more By setting finishing rolling finish temperature FT to Ar ₃ transformation point or more, a uniform hot rolled base metal structure can be obtained, and martensite which is a requirement of the present invention A composite structure with ferrite can be easily obtained. If the finish rolling end temperature is less than the Ar ₃ transformation point, the rolling load during hot rolling becomes high, and the risk of occurrence of trouble during hot rolling increases. Furthermore, since ferrite is formed during rolling and the fraction exceeds the range of the present invention, the effect of increasing the strength intended by the present invention cannot be obtained.

Cooling condition: after finishing rolling, cooling to the martensite transformation temperature (Ms point) or lower at an average cooling rate of 20 ° C./sec or higher After finishing rolling, cooling to the Ms point or lower causes the untransformed austenite to martensite To metamorphosis. When not cooled below the Ms point temperature, it transforms into pearlite or bainite, and the martensite that is a requirement of the present invention cannot be obtained. Therefore, the cooling stop temperature after finish rolling is set to the Ms point or lower. Further, the fraction of martensite, ferrite, and the like and the ferrite particle size change depending on the cooling rate, and the average cooling rate of less than 20 ° C./sec does not become the target fraction or the ferrite particle size. A cooling rate shall be 20 degrees C / sec or more. From the viewpoint of securing the amount of dissolved C, a more preferable average cooling rate is 50 ° C./sec or more, and more preferably 100 ° C./sec or more. By producing the steel composition of the present invention under the above-described cooling conditions, the desired morphology of the ferrite fraction and grain size can be obtained.

  In order to improve fatigue properties in addition to strain age hardening properties, after finish rolling, cooling is performed at an average cooling rate of 40 ° C./sec or more to a martensite transformation temperature (Ms point) or less. In order to improve fatigue properties, it is effective to reduce the hardness difference between martensite and ferrite after strain aging treatment, and to reduce the hardness difference by refining the grain size of ferrite and decreasing the fraction. Is possible. Depending on the cooling rate, the grain size and fraction of ferrite change, and at an average cooling rate of less than 40 ° C./sec, the hardness difference after strain aging is large and the fatigue properties are poor. Therefore, the average cooling rate is set to 40 ° C./sec or more in order to obtain the ferrite grain size and fraction within the range of the present invention which are excellent in fatigue characteristics. In order to stably obtain excellent fatigue characteristics, the average cooling rate is preferably 50 ° C./sec or more, and in order to obtain even higher fatigue characteristics, 100 ° C./sec or more is preferable.

  Although the time from the end of finish rolling to the start of cooling is not particularly specified, if the time to the start of cooling is too short, the ferrite fraction and average grain size exceed the range of the present invention and the martensite single phase. Therefore, it is desirable to start cooling after exceeding 0.3 sec after the end of hot rolling in order to obtain the structure form defined in the present invention and obtain a high strength increasing effect. In addition, for the purpose of reducing the base material strength (tensile strength) before the strain aging treatment, the cooling is not started immediately after finish rolling, but the ferrite fraction is increased and softened by cooling after a time of 1 sec or more. Therefore, it is effective. On the other hand, if too much time is left, the temperature of the steel sheet decreases and the temperature becomes the single phase of ferrite, and martensite cannot be obtained. Therefore, it is desirable to start cooling before that. In order to improve the fatigue characteristics, it is desirable to start cooling within 3 seconds after finish rolling so as to refine the ferrite grain size and reduce the ferrite fraction.

Winding temperature: 300 ° C. or lower The winding temperature CT is important for obtaining the tissue of the present invention. When the coiling temperature is higher than 300 ° C., untransformed austenite is transformed into pearlite or bainite, and martensite is not formed. Therefore, the structure does not have martensite as the main phase, which is a requirement of the present invention. A more preferable range of the coiling temperature is 200 ° C. or less from the viewpoint of suppressing the formation of carbides and securing the amount of dissolved C.

No tempering heat treatment at 350 ° C. or higher The tempering treatment at a high temperature of 350 ° C. or higher, which is performed to improve toughness in martensitic steel, etc., is not performed because it forms carbides and decreases the solid solution C. It is necessary.

  The hot-rolled steel sheet of the present invention can be applied not only for processing but also as a surface treatment original sheet. As the surface treatment, an electroplating or the like that does not involve high-temperature heat treatment is possible. Further, the hot-rolled steel sheet of the present invention may be subjected to a special treatment after plating to improve chemical conversion property, weldability, press formability, and corrosion resistance.

(First embodiment)
First, a first example in which the strain age hardening characteristics are examined will be described.

  After melting the molten steel having the composition shown in Table 1 to form steel slabs, these steel slabs were heated and hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel strip having a thickness of 3.0 mm (hot rolled) Board). About the obtained hot-rolled steel strip (hot-rolled sheet), the microstructure, the amount of solute C, tensile properties, and strain age hardening properties were determined.

(1) Microscopic structure A test piece is taken from the obtained steel strip, and a microscopic structure is imaged using an optical microscope or a scanning electron microscope with respect to a cross section (L cross section) parallel to the rolling direction. Was used to determine the ferrite structure fraction of the second phase. The ferrite phase particle diameter was calculated from the area of the ferrite phase obtained by image analysis and the number of crystal grains, and the diameter obtained by circular approximation was used as the average particle diameter.

(2) Amount of solute C After collecting test specimens from the obtained hot-rolled steel sheet, the amount of C in steel and the amount of precipitated C are determined by wet analysis, and the difference between the amount of C in steel and the amount of precipitated C is calculated. The amount of solute C was taken. Note that the amount of precipitated C may be determined from the size and density of the carbide by observation using a microscopic tissue sample.

(3) Tensile properties JIS No. 5 half tensile test specimens were collected from the obtained steel strip and subjected to a tensile test in accordance with the provisions of JIS Z 2241. Yield stress YS, tensile strength TS, elongation (total elongation T .EL, local elongation L.EL).

(4) Strain age hardening characteristics From the obtained steel strip (hot-rolled steel sheet), a JIS No. 5 half tensile test piece was taken in the rolling direction and given 3% plastic deformation as pre-deformation (tensile pre-strain). After heat treatment at 150 to 200 ° C. for 20 minutes, a tensile test was performed to determine the tensile property strength TS ′ after the heat treatment, and ΔTS = TS′−TS was calculated. YS and TS are the yield stress and tensile strength of the steel strip (hot rolled steel plate).
These results are also shown in Table 2.

  As shown in Table 2, sample symbols A, D, E, H, K, L, N, O, S, T, and U, which are examples of the present invention, all exhibit extremely large ΔTS, and strain age hardening characteristics It was confirmed that the steel sheet was excellent in On the other hand, sample symbols G, I, and P that are out of the component range of the present invention have a martensite single-phase structure, and thus have a small ΔTS. In addition, the sample symbol C in which Si is excessive has a high ferrite fraction, and ΔTS is also low. Furthermore, since the sample symbol M in which Ti is excessive has a solid solution C amount of less than 0.01% by mass, ΔTS is also a small value.

  Further, even if the composition is within the range of the present invention, in the sample symbol F where the hot-rolling finishing temperature is low and the ferrite is formed, the ferrite fraction is removed and the ferrite becomes the main phase, and the winding temperature is In the sample symbol J, which is off, the ferrite fraction is satisfied, but the solid solution C amount is out of the range, and ΔTS is a small value. Further, when the average cooling rate is low, the sample symbol B has a high ferrite fraction, and the sample symbols Q and R satisfy the ferrite fraction, but the average grain size deviates, so that ΔTS is also a small value. Thus, in the comparative examples outside the scope of the present invention, all are steel plates having a small ΔTS.

  Further, regarding the formability of the steel of the present invention, the total elongation (T.EL) is the same as that of the martensitic steel sheet, and the local elongation (L.EL) serving as an index of hole expansibility is the present invention. In the examples, both are 10% or more. This value has a value equal to or higher than that of a conventional material having the same strength level, and it can be seen that the hole expandability is equal to or superior to that of the conventional material.

(Second embodiment)
Next, a second embodiment will be described. Here, in addition to strain age hardening characteristics, attention is also paid to fatigue characteristics.

  After melting the molten steel having the composition shown in Table 3 into steel slabs, these steel slabs were heated and hot-rolled under the conditions shown in Table 4 to obtain a hot-rolled steel strip having a thickness of 3.0 mm (hot rolled) Board). About the obtained hot-rolled steel strip (hot-rolled sheet), the microstructure, the amount of solute C, tensile properties, strain age hardening properties, and fatigue properties were determined. (1) Microstructure, (2) solute C content, (3) tensile properties, and (4) strain age hardening properties were determined in the same manner as in the first example. The fatigue characteristics were determined as follows.

(5) Fatigue properties From the obtained steel strip (hot-rolled annealed plate), a JIS No. 5 tensile specimen was taken in the rolling direction and subjected to 1.5% plastic deformation as pre-deformation (tensile pre-strain). After heat treatment at 200 ° C. for 20 minutes, a tensile fatigue test was performed, the fatigue limit after strain aging treatment: FL was obtained, and the fatigue limit ratio: FL / TS (TS is the tension without strain aging treatment of the steel strip) Strength) was calculated.
These results are also shown in Table 4.

  As shown in Table 4, sample symbols a, c, d, f, g, i, and j, which are examples of the present invention, all exhibit extremely large ΔTS, and are steel plates having excellent strain age hardening characteristics. It was confirmed.

  On the other hand, in the sample symbol h where Ti deviates from the component range of the present invention, the steel sheet has a small ΔTS because it has a martensite single phase structure. In addition, in the sample symbol k where Mn is outside the component range of the present invention, a steel sheet having a small ΔTS is obtained because it has a martensite single phase structure although the average cooling rate after hot rolling is small and it is difficult to obtain a martensite phase. It has become.

  Further, even if the composition is within the range of the present invention, in the sample symbol b where the average cooling rate after hot rolling is small, the ferrite fraction is removed to become the ferrite main phase, and in the sample symbol e where the coiling temperature is removed. Although the ferrite fraction is satisfied, the amount of dissolved C is out of the range, and ΔTS is a small value. Thus, in the comparative examples outside the scope of the present invention, all are steel plates having a small ΔTS.

  Further, as shown in Table 4 for the fatigue characteristics, the sample symbols a, c, d, f, g, i, and j, which are examples of the present invention, all show high FL / TS of 0.8 or more, and fatigue. It was confirmed that the steel sheet had excellent properties. On the other hand, in the sample symbol b, since the ferrite fraction and the average particle diameter are outside the scope of the present invention, Hv (α) / Hv (M) ≦ 0.5, and the fatigue limit ratio FL / TS is 0. It can be seen that the fatigue properties are inferior to 8 or less and the inventive example.

  In sample symbol e, the ferrite fraction and average particle diameter are within the range of the present invention, but the amount of dissolved C is outside the range of the present invention, and Hv (α) / Hv (M) ≦ 0.5. Therefore, it can be seen that the fatigue limit ratio FL / TS is 0.8 or less, and the fatigue characteristics are inferior to those of the examples of the present invention.

  On the other hand, sample symbol h and Ti where Mn is outside the component range of the present invention in Ti and sample symbol k where Mn is out of the component range of the present invention have a martensitic single phase structure that does not contain soft ferrite, and thus have excellent fatigue properties. However, as described above, the steel sheet has a small strain age hardening characteristic (ΔTS).

  As described above, the sample symbols a, c, d, f, g, i, and j, which are examples of the present invention, all exhibit extremely large ΔTS and FL / TS, and are excellent in strain age hardening characteristics and fatigue characteristics. It was confirmed that

(Third embodiment)
Next, a third embodiment will be described. Here, attention is focused on the cooling start time after the end of hot rolling and low temperature tempering, which are manufacturing conditions, and the influence of the third phase (bainite) on the strength increasing effect (ΔTS) and fatigue characteristics.

By mass%, C: 0.1%, Si: 0.01%, Mn: 2.2%, P: 0.012%, S: 0.005%, Al: 0.045%, N: 0.00. After melting molten steel having a composition of 003% and the balance Fe and inevitable impurities into a steel slab, the steel slab was heated to 1250 ° C. and hot rolled at a finish rolling finish temperature of 800 ° C. A hot-rolled steel strip (hot-rolled sheet) having a thickness of 2.0 mm was used. Here, the Ar ₃ transformation point of this steel is 701 ° C., that is, the finish rolling end temperature is Ar ₃ transformation point + about 100 ° C. The cooling stop temperature and the coiling temperature are 180 ° C. (Ms point is 429 ° C.). Obtained. Sample symbol 3E was subjected to low-temperature tempering treatment under the conditions shown in Table 5 after coil winding. Sample symbol 3F was intentionally decooled for a short time in the bainite nose region (about 500 ° C.) to generate a small amount of bainite as shown in Table 5.

  The results of the microstructure and solute C content obtained in the same manner as in the first and second examples are shown together in Table 5, and the results of tensile properties, strain age hardening properties, and fatigue properties are shown in Table 6.

  As shown in Table 5 and Table 6, any of sample symbols 3A to 3F corresponds to the examples of the present invention, and has good strain age hardening characteristics and press formability.

  As shown in sample symbols 3A to 3C, it is confirmed that the longer the time until the start of cooling, the higher the ferrite phase becomes, and the more effective it is for softening (low TS). Similarly, from 3A to 3C, it can be seen that ΔTS, which is the object of the present invention, increases significantly when the cooling start time is short and the ferrite phase has a low fraction, and the average particle size becomes fine. When paying attention to the sample symbol 3C, the time until the start of cooling is relatively long as 5 seconds, but a high ΔTS is obtained because the average cooling rate is sufficiently large (150 ° C./sec). In addition, although the sample symbol 3F has a relatively low average cooling rate, a sufficiently high ΔTS is obtained because the time to start cooling is 3 sec or less.

  Further, when comparing the sample symbols 3A and 3D that are close to the start of cooling, 3D having a larger average cooling rate has a finer ferrite phase and a lower fraction, and ΔTS is higher. Such a tendency of increasing ΔTS is particularly prominent when the average grain size of the ferrite phase is 10 μm or less.

  Sample symbol 3E was tempered at 200 ° C. for 20 minutes at a low temperature in a short time. However, strain age hardening characteristics and fatigue characteristics were not deteriorated, and low temperature short time tempering was strain age hardening characteristics. And fatigue properties were not deteriorated. Furthermore, it can be seen from the sample symbol 3F that the strain age hardening characteristics and fatigue characteristics of the present invention are not deteriorated even in a structure including a third phase (bainite phase) other than the martensite phase and ferrite phase.

  According to the present invention, it has excellent press formability, excellent strain age hardening characteristics, and in addition to this, fatigue characteristics are also greatly improved. Therefore, it is suitable as a material for automobile parts, and the weight of an automobile body is reduced. Can contribute enough.

The figure which shows the relationship between the aging heat processing temperature, the tensile strength (TS) of each hot-rolled steel plate which changed hot rolling conditions, and C amount, and the tensile strength (TS ') after strain aging. The figure which shows the result of investigating in detail about the influence on (DELTA) TS of a ferrite fraction, a ferrite average particle diameter, and solid solution C amount. The figure which shows the relationship of the hardness ratio Hv ((alpha)) / Hv (M) of the hardness Hv ((alpha)) of the ferrite after the strain aging treatment and the hardness Hv (M) of a martensite which acts on a fatigue characteristic.

Claims (8)

  In mass%, C: 0.01 to 0.2%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0 0.1% or less, N: 0.02% or less, the balance being Fe and inevitable impurities, the martensite phase not tempered as the main phase, and the ferrite phase as the second phase in the area ratio of 1% or more 30 %, And the average particle size of the ferrite phase is 20 μm or less.   In mass%, C: 0.01 to 0.2%, 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, the balance being Fe and inevitable impurities, martensite phase as main phase, ferrite phase as second phase in the range of 1% or more and 30% or less in area ratio A hot rolled steel sheet having excellent strain age hardening characteristics, wherein the ferrite phase has an average grain size of 20 μm or less and a solid solution C content of 0.01% by mass or more.   The strain age hardening property according to claim 1 or 2, further comprising 0.2% or less in total of one or more of Nb, Ti, V, and Mo in mass%. Excellent hot-rolled steel sheet. In mass%, C: 0.01 to 0.2%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0 0.1% or less, N: 0.02% or less, the balance being Fe and inevitable impurities, the martensite phase not tempered as the main phase, and the ferrite phase as the second phase in the area ratio of 1% or more 30 %, And the ferrite phase has an average particle size of 15 μm or less, and the hardness Hv (M) of the martensite phase and the hardness Hv (α) of the ferrite phase after strain aging treatment are Hv (α ) / Hv (M) ≧ 0.6
Hot rolled steel sheet with excellent fatigue characteristics and strain age hardening characteristics. In mass%, C: 0.01 to 0.2%, 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, the balance being Fe and inevitable impurities, martensite phase as main phase, ferrite phase as second phase in the range of 1% or more and 30% or less in area ratio And the ferrite grain has an average particle size of 15 μm or less, a solid solution C content of 0.01% by mass or more, and the hardness Hv (M) of the martensite phase after strain aging treatment and the ferrite phase Hardness Hv (α) of Hv (α) / Hv (M) ≧ 0.6
A hot-rolled steel sheet with excellent fatigue characteristics and strain age hardening characteristics.   The fatigue property and strain aging according to claim 4 or 5, further comprising one or more of Nb, Ti, V, and Mo in a mass% of 0.2% or less in total. Hot-rolled steel sheet with excellent hardening characteristics. In mass%, C: 0.01 to 0.2%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0 0.1% or less, N: 0.02% or less, and the steel slab composed of the remaining Fe and inevitable impurities is subjected to hot rolling at which the finish rolling finish temperature is equal to or higher than the Ar ₃ transformation point, and after finish rolling is finished. The strain is characterized by being cooled at an average cooling rate of 20 ° C./sec or higher to a martensite transformation temperature (Ms point) or lower, wound at a temperature of 300 ° C. or lower, and then not subjected to tempering heat treatment at 350 ° C. or higher. A method for producing a hot-rolled steel sheet having excellent age-hardening characteristics.   8. The strain age hardening characteristic according to claim 7, wherein the steel slab further contains one or more of Nb, Ti, V, and Mo in a mass% of 0.2% or less in total. The manufacturing method of the hot-rolled steel plate excellent in.

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