JP5392223B2 - 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, and in particular, press formability such as bending workability, stretch flange workability and drawability is good, and the tensile strength is remarkably increased by heat treatment after press forming. The present invention relates to a hot-rolled steel sheet having extremely large strain age hardening characteristics and a method for producing the same. The extremely large strain age hardening characteristic referred to in the present invention, that is, “excellent strain age hardening characteristic” means that it has a strain age hardening characteristic of ΔTS: 80 MPa or more. In the present invention, ΔTS is the amount of increase in 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 with a holding time of 30 seconds or more at a temperature in the range of 150 to 350 ° C. {= (Tensile strength after heat treatment) − (Tensile strength before pre-deformation treatment)}.

In recent years, in connection with exhaust gas regulations due to global environmental conservation issues, the reduction of vehicle body weight has become a very important issue. Recently, in order to reduce the weight of the vehicle body, it has been studied to increase the strength of a steel plate for automobiles and reduce the thickness of the steel plate.
Since many automotive body parts made of steel sheets are formed by press working, the hot-rolled steel sheets used are required to have excellent press formability. In order to obtain a steel sheet having excellent press formability, it is important to secure low yield strength and high ductility. In addition, stretch flange molding is often used, and it is necessary to have a high hole expansion rate. However, generally, when the strength of a steel plate is increased, the yield strength increases and the shape freezing property deteriorates, the ductility decreases, the hole expansion rate decreases, and the press formability tends to decrease. For this reason, conventionally, a high-strength hot-rolled steel sheet having high ductility and excellent press formability has been desired.

  Recently, in order to protect an occupant in the event of a collision, importance is placed on the safety of the automobile body. For this reason, an improvement in impact resistance that is a measure of safety in the event of a collision is required. The higher the strength of the finished vehicle, the more advantageous for improving the impact resistance. Therefore, when forming automotive parts, hot-rolled steel sheets that have low strength, high ductility, excellent press formability, and high strength and excellent impact resistance are most strongly desired when finished products are produced. It was.

  In response to such a demand, a steel sheet having both press formability and high strength has been developed. This steel sheet is a paint bake hardening type steel sheet whose yield stress increases when it is subjected to a paint baking process including holding at a high temperature of usually 100 to 200 ° C. after press working. In this steel sheet, the amount of C remaining in the final solid solution state (solid solution C amount) is controlled within an appropriate range, soft at the time of press forming, ensuring shape freezing and ductility, and baking after press forming. At the time of processing, the remaining solid solution C adheres to the dislocations introduced during press forming, thereby preventing the dislocations from moving and increasing the yield stress. However, in this paint bake hardened automotive steel sheet, although the yield stress can be increased, the tensile strength cannot increase it.

  Patent Document 1 discloses a uniform bainite having a component composition including C: 0.08 to 0.20%, Mn: 1.5 to 3.5% and the balance Fe and unavoidable impurities and having a ferrite content of 5% or less. A bake-hardening high-tensile cold-rolled thin steel plate composed of bainite containing part martensite is disclosed. The cold-rolled steel sheet described in Patent Document 1 rapidly cools a temperature range of 400 to 200 ° C. in the cooling process after continuous annealing, and then gradually cools the structure to change the structure from a conventional ferrite-based structure to a bainite-based structure. As a structure of this, an attempt is made to obtain a high bake hardening amount which has not been conventionally obtained.

However, in the steel sheet described in Patent Document 1, although the yield strength increases after painting and a high bake hardening amount that has not been obtained in the past can be obtained, the tensile strength cannot still be increased, and the impact resistance characteristics There was a problem that improvement could not be expected.
Several hot-rolled steel sheets that are subjected to heat treatment after press forming to increase not only yield stress but also tensile strength have been proposed.

  For example, in Patent Document 2, 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, N: 0.0080 to 0.0250%, 1100 ° C. or more To 850 to 950 ℃, and finish rolling at 850 to 950 ℃, then cool down to a temperature of less than 150 ℃ at a cooling rate of 15 ℃ / s and wind up, mainly ferrite and martensite A method for producing a hot-rolled steel sheet having a composite structure has been proposed. However, in the steel plate manufactured by the technique described in Patent Document 2, although tensile strength increases with yield stress due to strain age hardening, it is wound at an extremely low winding temperature of less than 150 ° C. There was a problem of large fluctuations. In addition, there is a large variation in the amount of increase in yield stress after press molding-paint baking, and there is also a problem that the hole expansion rate (λ) is low, stretch flangeability is lowered, and press formability is insufficient.

  Patent Document 3 proposes a method for producing a hot dip galvanized steel sheet using a hot rolled sheet as a plating original sheet. In this method, a steel slab containing C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less, and Cu: 0.8 to 2.0% is hot-rolled at a coiling temperature of 530 ° C. or less. Subsequently, after heating to a temperature of 530 ° C. or lower to reduce the surface of the steel sheet, hot-dip galvanization is performed, whereby significant hardening by heat treatment after forming is obtained. However, in the steel sheet produced by this method, in order to obtain significant hardening by post-forming heat treatment, the heat treatment temperature needs to be 500 ° C. or higher, and the heat treatment temperature is high, leaving a problem in practical use.

  Patent Document 4 proposes a method for producing an alloyed hot-dip galvanized steel sheet in which a hot rolled plate or a cold rolled plate is used as a plating original plate and an increase in strength can be expected by heat treatment after forming. This method includes C: 0.01 to 0.08%, and Si, Mn, P, S, Al, and N are used in appropriate amounts, and one or more of Cr, W, and Mo are added in a total amount of 0.05 to 3.0. % Hot-rolled steel, or further cold-rolled or temper-rolled and annealed, and then hot-dip galvanized and then heat-alloyed. The steel sheet is said to have an increased tensile strength by heating in a temperature range of 200 to 450 ° C. after forming. However, since the obtained steel sheet has a microstructure of ferrite single phase, ferrite + pearlite, or ferrite + bainite structure, high ductility and low yield strength cannot be obtained, and press formability is deteriorated. was there.

Patent Document 5 includes C: 0.03 to 0.20%, Si, Mn, P, S, and Al, with appropriate amounts, Cu: 0.2 to 2.0% and B: 0.0002 to 0.002%, The microstructure is a composite structure in which ferrite is the main phase and martensite is the second phase, and the presence of Cu in the ferrite phase is set to a solid solution state and / or a precipitation state of 2 nm or less, and has excellent fatigue characteristics. Hot-rolled steel sheets for processing have been proposed. The steel sheet described in Patent Document 5 is that the fatigue limit ratio is remarkably improved only when Cu and B are added in combination and the existence state of Cu is extremely fine, 2 nm or less. In addition, for that purpose, the hot finish rolling is finished at the Ar ₃ transformation point or higher, air-cooled for ₁ to 10 seconds in the temperature range from the Ar _{3 to} Ar ₁ transformation point in the cooling process, and then cooled at 20 ° C./s or higher. It is essential to cool at a speed and wind up at a temperature of 350 ° C or lower. Thus, when the coiling temperature is as low as 350 ° C. or less, there is a problem that the shape of the hot-rolled steel sheet is easily disturbed and cannot be manufactured industrially stably.

Japanese Patent Publication No. 5-24979 Japanese Patent Publication No. 8-23048 Japanese Patent No.2802513 Japanese Patent Laid-Open No. 10-310824 JP-A-11-199975

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

  In order to achieve the above-described problems, the present inventors have conducted intensive research on the influence of steel sheet structure and alloy elements on strain age hardening characteristics. As a result, the C content is set to a low carbon range, Cu within the appropriate range, or one or more of Mo, Cr, W instead of Cu is contained, and in addition, the steel sheet structure is ferrite. In addition to the increase in yield stress, the tensile strength is remarkable after pre-deformation treatment with a pre-strain amount of 5% or more and heat treatment at a relatively low temperature of 150 to 350 ° C. It was found that a high strain age hardening can be obtained. In addition to such high strain age hardening characteristics, the present inventors have found that the steel sheet has excellent ductility, low yield strength, and high hole expansion ratio, and is excellent in press formability.

First, basic experimental results performed by the present inventors will be described.
Containing 0.04%, Si: 0.82%, Mn: 1.6%, P: 0.01%, S: 0.005%, Al: 0.04%, N: 0.002%, Cu 0.3%, 1.3% The sheet bar having the changed composition was heated to 1150 ° C, soaked, and then subjected to three-pass rolling so that the finish rolling finish temperature was 850 ° C to a thickness of 2.0 mm. The structure was changed to a hot rolled sheet having a composite structure of ferrite + martensite from a single ferrite phase.

About these hot-rolled sheets, the tensile test was implemented and the tensile characteristic was investigated. Furthermore, the specimens taken from these hot-rolled sheets were pre-deformed with a tensile pre-strain amount of 5%, and then subjected to a heat treatment of 50 to 350 ° C x 20 min. Age hardening properties were evaluated.
The strain age hardening property was evaluated by the increase in tensile strength ΔTS before and after the heat treatment. ΔTS is a tensile strength TS _HT after heat treatment, the difference between the tensile strength TS when not subjected to heat treatment (= (tensile strength TS HT after heat _{treatment) - (tensile} strength before pre-deformation treatment TS)). The tensile test was carried out using JIS No. 5 tensile test pieces.

  FIG. 1 shows the influence of the Cu content on the relationship between ΔTS and the steel sheet (hot rolled sheet) structure. ΔTS was obtained by subjecting the test piece to a pre-deformation treatment with a tensile pre-strain amount of 5% and then a heat treatment at 250 ° C. × 20 min. FIG. 1 shows that when the Cu content is 1.3% by mass, a high strain age hardening characteristic of ΔTS: 80 MPa or more can be obtained by making the steel sheet structure a composite structure of ferrite and martensite. When the Cu content is 0.3 mass%, ΔTS: less than 80 MPa, and even if the steel sheet structure is a composite structure of ferrite and martensite, high strain age hardening characteristics cannot be obtained.

Thus, it turns out that it is possible to manufacture the hot-rolled steel plate which has a high strain age hardening characteristic by making Cu content into an appropriate range, and setting it as the composite structure of a ferrite + martensite.
FIG. 2 shows the influence of the Cu content on the relationship between ΔTS and the heat treatment temperature after the pre-deformation treatment. The hot-rolled sheet used was cooled to 700 ° C. at a cooling rate of 20 ° C./s after completion of hot rolling, then air-cooled for 5 s, then cooled to 450 ° C. at a cooling rate of 30 ° C./s, Thereafter, a coil winding process equivalent to 450 ° C. × 1 h was performed. The microstructure of the hot-rolled sheet thus obtained was a composite structure of ferrite as the main phase and martensite having an area ratio of 8%. ΔTS was obtained by subjecting these hot-rolled sheets to a pre-deformation treatment followed by a heat treatment.

  From FIG. 2, ΔTS increases as the heat treatment temperature increases, but the amount of increase greatly depends on the Cu content. It can be seen that when the Cu content is 1.3% by mass, high strain age hardening characteristics can be obtained with a heat treatment temperature of 150 ° C. or higher and ΔTS: 80 MPa or higher. When the Cu content is 0.3% by mass, ΔTS: less than 80 MPa, and high strain age hardening characteristics cannot be obtained at any heat treatment temperature.

In addition, for steel sheets having a Cu content of 0.3% by mass and 1.3% by mass, the cooling rate after hot rolling was changed variously, the structure was changed from ferrite + martensite to a ferrite single phase, and the yield ratio YR (= (yield strength YS / Tensile strength (TS) × 100%) was prepared as a material (hot rolled sheet) with 50 to 90%. About these materials (hot-rolled sheet), the hole expansion test was implemented and the hole expansion rate ((lambda)) was calculated | required. In the hole expansion test, a punch hole is formed in the specimen by punching with a 10mmφ punch, and then a conical punch with a vertex angle of 60 ° is used, and a crack that penetrates the plate thickness is generated with the beam facing outward. Hole expansion was performed until the hole expansion rate λ was obtained. The hole expansion ratio λ was obtained by λ (%) = {(d−d ₀ ) / d ₀ } × 100. D ₀ : initial hole diameter, d: inner hole diameter when cracking occurs.

These results are summarized in the relationship between the hole expansion ratio λ and the yield ratio YR, and the influence of the Cu content on the relationship between the hole expansion ratio λ and the yield ratio YR is shown in FIG.
From FIG. 3, in the steel sheet with Cu: 0.3% by mass, the composite structure of ferrite (α) + martensite becomes YR less than 70%, and λ decreases with a decrease in YR, but Cu: 1.3% by mass. It can be seen that the steel sheet has a composite structure of ferrite (α) + martensite and maintains a high λ value even when YR is lowered. On the other hand, in a steel sheet having a Cu content of 0.3% by mass, low YR and high λ cannot be obtained simultaneously.

Thus, it is possible to produce a steel sheet that satisfies both the low yield ratio and the high hole expansion rate by setting the Cu content within the appropriate range and making it a composite structure of ferrite (α) + martensite. I understand.
In the hot-rolled steel sheet of the present invention, pre-deformation with a strain amount higher than 2%, which is a pre-strain amount when measuring the increase in deformation stress before and after normal heat treatment, and in a relatively low temperature range of 150 ° C. to 350 ° C. By this heat treatment, ultrafine Cu is precipitated in the steel sheet. According to the study by the present inventors, it is considered that high strain age hardening characteristics in which the tensile strength is remarkably increased in addition to the increase in yield stress are obtained by the precipitation of this ultrafine Cu. Such precipitation of ultrafine Cu by heat treatment in a relatively low temperature region was not observed at all in the extremely low carbon steel or low carbon steel reported so far. The reason why ultrafine Cu is precipitated by heat treatment in a relatively low temperature region has not been clarified until now, but during the retention in the two-phase region of ferrite (α) + austenite (γ), Cu is distributed in a large amount in the γ phase, and it is taken over after cooling, and Cu is in a supersaturated solid solution in martensite, resulting in ultrafine precipitation by adding 5% pre-strain and low-temperature heat treatment. It is thought that.

In addition, the detailed mechanism by which the hole expansion rate of steel sheets with a Cu-added microstructure with a composite structure of ferrite and martensite is high has not been clarified so far. It is thought that this is because the difference in hardness between the two is small.
Based on the above-described novel findings, the present inventors have conducted further intensive studies, and as a result, have found that the phenomenon described above also occurs in a steel sheet not containing Cu. Instead of Cu, containing one or more of Mo, Cr, W, and making the structure a composite structure of ferrite + martensite, applying pre-strain and performing heat treatment at low temperature, It was found that very fine carbides are strain-induced precipitated in martensite and the tensile strength is increased. This strain-induced fine precipitation during low-temperature heating is more prominent by containing one or more of Nb, V and Ti in addition to one or more of Mo, Cr and W. I also found out.

The present invention has been completed by further study based on the above-described findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.15% or less, 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, Cu : A composition containing 0.5 to 3.0%, the balance being composed of Fe and inevitable impurities, the structure is composed of a ferrite phase with an area ratio of 80% or more as a main phase and a martensite phase with an area ratio of 2% or more alone second and having a composite structure of the phase, excellent press formability, and the .DELTA.TS: excellent hot rolled steel sheet strain age hardening characteristics be more than 80 MPa.

(2) In (1), in addition to the above composition, the following groups A to C: Group A: Ni: 2.0% or less,
Group B: One or two of Cr and Mo in total 2.0% or less,
Group C: Excellent in press formability, characterized by containing one or more of Nb, Ti, and V selected from a total of 0.2% or less of one or more of Nb, Ti, and V, In addition, a hot-rolled steel sheet having excellent strain age hardening characteristics such that ΔTS: 80 MPa or more.

(3) In mass%, C: 0.15% or less, 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 Furthermore, it contains one or more selected from Mo: 0.05-2.0%, Cr: 0.05-2.0%, W: 0.05-2.0% in total, 2.0% or less in total, the balance being Fe and inevitable It has a composition consisting of impurities, and the structure has a composite structure of a ferrite phase with an area ratio of 80% or more as a main phase and a martensite phase alone with an area ratio of 2% or more. A hot-rolled steel sheet with excellent press-formability and excellent strain age hardening properties such that ΔTS: 80 MPa or more.

  (4) In (3), in addition to the above-mentioned composition, in addition to the above composition, one or more of Nb, Ti, and V are contained in a total of 2.0% or less in terms of press formability. A hot-rolled steel sheet with excellent strain age-hardening properties that is excellent and ΔTS: 80 MPa or more.

(5) In mass%, C: 0.15% or less, 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 Further, it contains one or more selected from Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0%, W: 0.05 to 2.0% in total of 2.0% or less, or further Nb, Ti, A steel slab containing a total of 2.0% or less of one or more of V, preferably having the composition of the balance Fe and inevitable impurities, is hot-rolled into a hot-rolled sheet having a predetermined thickness. after around, the hot rolling, finish rolling temperature FDT is the hot rolling is Ar ₃ transformation point or higher, the completion of the finish rolling, 5 ° C. / s or more cooling rate (Ar ₃ transformation point) ~ (Ar ₁ After cooling to the temperature range of the transformation point), air cooling or slow cooling for 1 to 20 seconds in the temperature range, and then cooling again at a cooling rate of 5 ° C / s or more. Characterized in that the winding at 550 ° C. temperature below excellent press formability, and .DELTA.TS: preparation of hot-rolled steel sheet having excellent strain age hardening characteristics become more than 80 MPa.

  According to the present invention, while maintaining excellent press formability, it becomes possible to stably produce a hot-rolled steel sheet whose tensile strength is remarkably increased by heat treatment after press forming. Play. When the hot-rolled steel sheet of the present invention is applied to automobile parts, there is an effect that press forming is easy and the characteristics of the parts after completion can be stably increased, which can sufficiently contribute to weight reduction of the automobile body.

It is a graph which shows the influence of Cu content which has on the relationship between (DELTA) TS after a pre-deformation and heat processing, and a steel plate (hot rolled sheet) structure | tissue. It is a graph which shows the influence of Cu content on the relationship between (DELTA) TS after a pre-deformation and heat processing, and heat processing temperature. It is a graph which shows the influence of Cu content on the relationship between (lambda) and YR.

  The hot-rolled steel sheet of the present invention is a high-tensile hot-rolled steel sheet having a tensile strength TS: 440 MPa or more, excellent in press formability, and has a remarkable increase in tensile strength due to heat treatment at a relatively low temperature after press forming. In addition, the steel sheet is excellent in strain age hardening characteristics such that ΔTS: 80 MPa or more.

  In the present invention, “excellent in strain age hardening characteristics” means, as described above, after a pre-deformation treatment with a tensile plastic strain amount of 5% or more, at a temperature in the range of 150 to 350 ° C. and a holding time of 30 seconds or more. This means that when heat treatment is performed, 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 80 MPa or more. Desirably, ΔTS is 100 MPa or more. It goes without saying that the yield stress is increased by this heat treatment, and ΔYS: 80 MPa or more is obtained. ΔYS means an increase in yield strength before and after heat treatment, and is defined as ΔYS = {(yield strength after heat treatment) − (yield strength before heat treatment)}.

  The amount of pre-strain (pre-deformation) is an important factor when defining strain age hardening characteristics. The present inventors investigated the influence of the amount of pre-strain on the subsequent strain age hardening characteristics assuming a deformation mode to which the steel sheet for automobiles is applied. As a result, except for extremely deep drawing, it can be organized by the amount of uniaxial equivalent strain (tensile strain), and in actual parts, the amount of uniaxial equivalent strain exceeds about 5%, and the strength of the parts Was found to correspond well with the strength obtained after 5% pre-strain strain aging treatment. For these reasons, in the present invention, the pre-strain (deformation) of the strain aging treatment is a tensile plastic strain of 5% or more.

  Conventional baking treatment conditions of 170 ° C x 20 min have been adopted as standard. However, when using ultrafine Cu precipitation strengthening as in the present invention, a heat treatment temperature of 150 ° C or higher is required. . On the other hand, when the temperature exceeds 350 ° C., the effect is saturated, and conversely, it tends to soften somewhat. In addition, when heated to a temperature exceeding 350 ° C., the occurrence of thermal strain and temper color becomes remarkable. Therefore, in the present invention, the heat treatment temperature for strain age hardening is set to 150 to 350 ° C. The holding time at the heat treatment temperature is 30 s or longer. With respect to the heat treatment holding time, if it is kept at about 150 to 350 ° C. for about 30 seconds or more, almost sufficient strain age hardening is achieved. In order to obtain a larger and more stable strain age hardening, the holding time is desirably 60 seconds or longer, and more preferably 300 seconds or longer.

  The heating method in the heat treatment is not particularly limited, but, for example, induction heating, non-oxidizing flame, laser, plasma, and the like are all suitable in addition to the atmosphere heating by the furnace as in the ordinary paint baking process. In addition, so-called warm pressing in which the temperature of the steel sheet is increased and pressed is also an extremely effective method in the present invention.

First, the structure of the steel sheet of the present invention will be described.
The hot-rolled steel sheet of the present invention has a composite structure of a ferrite phase having an area ratio of 80% or more and a martensite phase- only second phase having an area ratio of 2% or more with respect to the entire structure.
In order to obtain a steel sheet having low yield strength YS and high ductility (El) and having excellent press formability, in the present invention, the structure of the steel sheet is a second containing a ferrite phase as a main phase and martensite. It is necessary to have a complex structure with the phase. The ferrite as the main phase is preferably 50% or more in terms of area ratio. If the ferrite content is less than 50%, it is difficult to ensure high ductility, and press formability deteriorates. Further, when a better ductility is required, the area ratio of the ferrite phase is preferably 80% or more. In order to take advantage of the composite structure, the ferrite phase is preferably 98% or less.

Moreover, as a 2nd phase, in this invention, it is necessary to contain 2% or more of martensite with respect to the whole structure | tissue by area ratio. If the martensite is less than 2%, low YS and high El cannot be satisfied simultaneously. The second phase is a martensite phase alone with an area ratio of 2% or more .

A hot-rolled steel sheet having the above-described structure is a steel sheet having low yield strength, high ductility, excellent press formability, and excellent strain age hardening characteristics.
Next, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. The mass% is simply written as%.
C: 0.15% or less C is an element that increases the strength of the steel sheet and further promotes the formation of a composite structure of ferrite and martensite. In the present invention, C is preferably contained in an amount of 0.01% or more in order to form a composite structure. . On the other hand, if the content exceeds 0.15%, the fraction of carbides in the steel increases, and ductility and further press formability decrease. Furthermore, as a more important problem, when the C content exceeds 0.15%, spot weldability, arc weldability, and the like are significantly reduced. For this reason, in the present invention, C is limited to 0.15% or less. From the viewpoint of moldability, the content is preferably 0.10% or less.

Si: 2.0% or less
Si is a useful strengthening element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel sheet, and also promotes the transformation of ferrite and the formation of martensite by concentration of C in untransformed austenite. It is an effective element for promotion. However, if the Si content exceeds 2.0%, the press formability is deteriorated and the surface properties are deteriorated. For this reason, Si was limited to 2.0% or less. In addition, it is preferable to contain 0.1% or more from the viewpoint of martensite formation.

Mn: 3.0% or less
Mn has an effect of strengthening steel and further has an effect of promoting the formation of a composite structure of ferrite and martensite. Moreover, it is an effective element which prevents the hot crack by S, and it is preferable to contain according to the amount of S to contain. Such an effect becomes remarkable when the content is 0.5% or more. On the other hand, if it exceeds 3.0%, press formability and weldability deteriorate. Therefore, in the present invention, Mn is limited to 3.0% or less. More preferably, it is 1.0% or more.

P: 0.10% or less P has an effect of strengthening steel and can be contained in a necessary amount according to a desired strength, but if it is contained excessively, press formability deteriorates. For this reason, P was limited to 0.10% or less. When more excellent press formability is required, the content is preferably 0.08% or less.

S: 0.02% or less S is an element that exists as an inclusion in a steel sheet and causes deterioration of the ductility and formability of the steel sheet, especially stretch flangeability, and is preferably reduced as much as possible, but is reduced to 0.02% or less. Then, since there is not much adverse effect, in the present invention, the upper limit of S is 0.02%. When excellent stretch flange formability is required, S is preferably 0.010% or less.

Al: 0.10% or less
Al is added as a deoxidizing element for steel and is an element useful for improving the cleanliness of steel. However, even if it exceeds 0.10%, a further deoxidizing effect cannot be obtained. Press formability deteriorates. For this reason, Al was limited to 0.10% or less. In addition, Preferably it is 0.01% or more. Further, in the present invention, it does not exclude a melting method by a deoxidation method other than Al deoxidation, for example, Ti deoxidation or Si deoxidation may be performed, and a steel plate by these deoxidation methods is also of the present invention. Included in the range.

N: 0.02% or less N is an element that increases the strength of the steel sheet by solid solution strengthening or strain age hardening. However, if it exceeds 0.02%, nitride increases in the steel sheet, thereby increasing the ductility of the steel sheet. Furthermore, the press formability is significantly deteriorated. For this reason, N was limited to 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.

Cu: 0.5-3.0%
Cu is an element that remarkably increases the strain age hardening (predeformation—increase in strength after heat treatment) of the steel sheet, and is one of the most important elements in the present invention. If the Cu content is less than 0.5%, even if the predeformation-heat treatment conditions are changed, an increase in tensile strength of ΔTS: 80 MPa or more cannot be obtained. For this reason, in this invention, Cu needs to contain 0.5% or more. On the other hand, if the content exceeds 3.0%, the effect is saturated, an effect commensurate with the content cannot be expected, which is economically disadvantageous, and press formability is deteriorated, and the surface properties of the steel sheet are further deteriorated. For this reason, Cu was limited to 0.5 to 3.0%. In order to achieve both a larger ΔTS and excellent press formability, Cu is preferably in the range of 1.0 to 2.5%.

Moreover, in this invention, in addition to the above-mentioned composition containing Cu, the following A group-C group by mass% further
Group A: Ni: 2.0% or less
Group B: One or two of Cr and Mo in total 2.0% or less
Group C: It is preferable that one or two or more of Nb, Ti, and V are included in one or more of 0.2% or less in total.

Group A: Ni: 2.0% or less Group A: Ni is an element effective for preventing surface defects generated on the steel sheet surface when Cu is added, and can be contained as necessary. When contained, the content depends on the Cu content, and is preferably about half of the Cu content. Even if the content exceeds 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous, and conversely, press formability deteriorates. For these reasons, Ni is preferably limited to 2.0% or less.

Group B: 2.0% or less of one or two of Cr and Mo in total B Group: Cr and Mo, like Mn, have the effect of promoting the formation of a ferrite + martensite composite structure. And can be contained if necessary. If one or two of Cr and Mo are contained in excess of 2.0% in total, press formability is lowered. For this reason, it is preferable to limit one or two of Group B: Cr and Mo to 2.0% or less in total.

Group C: One or more of Nb, Ti, and V totaling 0.2% or less in total. Group C: Nb, Ti, and V are all carbide-forming elements, and high strength is achieved by fine dispersion of carbides. Since it works effectively, it can be selected and contained as necessary. However, if one or more of Nb, Ti, and V are contained in a total exceeding 0.2%, press formability deteriorates. For this reason, Nb, Ti, and V are preferably limited to 0.2% in total.

  In the present invention, Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0%, W: in place of the above-described Cu, or the inclusion of one or more of the above-described groups A to C Contains one or more selected from 0.05 to 2.0% in total of 2.0% or less, or further contains one or more of Nb, Ti and V in total of 2.0% or less Also good.

Mo: 0.05-2.0%, Cr: 0.05-2.0%, W: 0.05-2.0%, one or two or more selected from a total of 2.0% or less
Mo, Cr, and W are all elements that significantly increase the strain age hardening of the steel sheet, and are the most important elements in the present invention, and can be selected and contained. By containing one or more of these Mo, Cr, and W, and further forming a composite structure of ferrite and martensite, fine carbides undergo strain-induced fine precipitation during pre-deformation and heat treatment, and ΔTS: 80 MPa or more An increase in tensile strength is obtained. If the content of these elements is less than 0.05%, even if the predeformation-heat treatment conditions and the steel sheet structure are changed, an increase in tensile strength of ΔTS: 80 MPa or more cannot be obtained. On the other hand, even if the content of these elements exceeds 2.0%, the effects described above are saturated, an effect commensurate with the content cannot be expected, and it is economically disadvantageous, and the press formability is deteriorated. For this reason, Mo, Cr, and W are limited to the ranges of Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0%, and W: 0.05 to 2.0%. In addition, from the viewpoint of press formability, the total content of Mo, Cr, and W is limited to 2.0% or less when combined and contained.

2.0% or less of one or more of Nb, Ti and V in total
Nb, Ti, and V are all carbide-forming elements and can be selected and contained as necessary. By containing one or more of these Nb, Ti, and V, and further forming a composite structure of ferrite and martensite, fine carbides are strain-induced finely precipitated during pre-deformation and heat treatment, and ΔTS: 80 MPa or more An increase in tensile strength is obtained. However, if one or more of Nb, Ti, and V are contained in a total exceeding 2.0%, the press formability deteriorates. For this reason, Nb, Ti, and V are preferably limited to 2.0% or less in total.

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. Both Ca and REM are elements that contribute to the improvement of ductility through shape control of inclusions. However, if the content exceeds Ca: 0.1% and REM: 0.1%, the cleanliness is lowered and ductility is lowered.
Moreover, from a viewpoint of martensite formation, you may contain 1 type (s) or 2 or more types in B: 0.1% or less and Zr: 0.1% or less.

The balance other than the above components is composed of Fe and inevitable impurities. As unavoidable impurities, Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less are acceptable.
A hot-rolled steel sheet having the above-described composition and structure is a steel sheet having low yield strength, high ductility, excellent press formability, and excellent strain age hardening characteristics.

Below, the manufacturing method of the hot rolled sheet steel of this invention is demonstrated.
The hot-rolled steel sheet of the present invention uses a steel slab having a composition within the above-described range as a raw material, and hot-rolls the raw material to obtain a hot-rolled sheet having a predetermined thickness.
The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab 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 not cooled and inserted into a heating furnace as it is, or rolled immediately after a slight heat retention. Energy saving processes such as direct rolling and direct rolling can be applied without problems.

The heating temperature SRT of the material (steel slab) is not particularly limited, but is preferably 900 ° C. or higher.
Slab heating temperature: 900 ° C. or higher In the case of a composition containing Cu, the slab heating temperature is preferably low in order to prevent surface defects caused by Cu. However, if the heating temperature is less than 900 ° C., the rolling load increases and the risk of trouble 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.

In addition, it goes without saying that using a so-called sheet bar heater for heating the sheet bar from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling is an effective method. .
The heated slab is then subjected to hot rolling, and the hot rolling is preferably hot rolling in which the finish rolling finish temperature FDT is equal to or higher than the Ar3 transformation point.

Finishing rolling end temperature: Ar ₃ transformation point or more By setting finishing rolling finishing temperature FDT to Ar ₃ transformation point or more, a uniform hot-rolled base metal structure can be obtained, and ferrite and martensite are formed by cooling after hot rolling. The composite tissue of is obtained. Thereby, the excellent press formability is ensured. On the other hand, if the finish rolling finish temperature is less than the Ar ₃ transformation point, the hot-rolled base metal structure becomes non-uniform, and the processed structure remains and press formability deteriorates. Furthermore, 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. For this reason, it is preferable that the hot rolling FDT is not less than the Ar ₃ transformation point.

After finishing rolling, it is then preferably cooled to a temperature range of (Ar ₃ transformation point) to (Ar ₁ transformation point) at a cooling rate of 5 ° C./s or more.
By performing such cooling after hot rolling, the ferrite transformation can be promoted by subsequent cooling treatment. When the cooling rate is less than 5 ° C./s, the ferrite transformation is not promoted by the subsequent cooling treatment, and the press formability deteriorates.

Next, it is preferable to air cool or gradually cool for ₁ to 20 seconds in the temperature range of (Ar ₃ transformation point) to (Ar ₁ transformation point). By air cooling or slow cooling in the temperature range of (Ar ₃ transformation point) to (Ar ₁ transformation point), transformation from austenite to ferrite is promoted, and C is concentrated in untransformed austenite. Transformation into martensite forms a composite structure of ferrite and martensite. When air cooling or slow cooling in the temperature range of (Ar ₃ transformation point) to (Ar ₁ transformation point) is less than 1 s, the transformation amount from austenite to ferrite is small, and therefore the amount of C enriched in untransformed austenite is also small. , The amount of martensite formed is reduced. On the other hand, if it exceeds 20 s, austenite is transformed into pearlite, and a composite structure of ferrite and martensite cannot be obtained.

After air cooling or gradual cooling treatment, it is cooled again at a cooling rate of 5 ° C./s or more and wound at a winding temperature of 550 ° C. or less.
By cooling at a cooling rate of 5 ° C./s or more, untransformed austenite is transformed into martensite. As a result, the structure becomes a composite structure of ferrite and martensite. However, when the cooling rate is less than 5 ° C./s or the coiling temperature is higher than 550 ° C., untransformed austenite is transformed into pearlite or bainite, and martensite is not formed. More preferably, the cooling rate is 10 ° C./s or more, and more preferably 100 ° C./s or less from the viewpoint of hot-rolled plate shape. The coiling temperature is less than 500 ° C, more preferably 350 ° C or more from the viewpoint of the shape of the hot-rolled sheet. When the coiling temperature is less than 350 ° C., the shape of the steel sheet is significantly disturbed, increasing the risk of causing problems in actual use.

  In the hot rolling in the present invention, part or all of the finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the wear coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.

After hot rolling, temper rolling of 10% or less may be performed for shape correction, surface roughness adjustment and the like.
The hot-rolled steel sheet of the present invention can be applied not only for processing but also as a surface treatment original sheet. Examples of the surface treatment include galvanization (including alloy system), tin plating, enamel and the like.

  The hot-rolled steel sheet of the present invention may be subjected to special treatment after annealing or galvanization to improve chemical conversion properties, weldability, press formability, corrosion resistance, and the like.

Example 1
Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method. These steel slabs were heated and hot-rolled under the conditions shown in Table 2 to form hot-rolled steel strips (hot-rolled plates) with a thickness of 2.0 mm, and further subjected to temper rolling with a reduction ratio of 1.0%.

About the obtained hot-rolled steel strip (hot-rolled sheet), the microstructure, tensile characteristics, strain age hardening characteristics, and hole expansion ratio were determined. The press formability was evaluated from the elongation El (ductility), the yield strength, and the hole expansion rate.
(1) Microscopic structure A test piece is taken from the obtained steel strip, and a microscopic structure is imaged with respect to a cross section (C cross section) perpendicular to the rolling direction using an optical microscope or a scanning electron microscope. Was used to determine the structure fraction of the main phase ferrite and the type and structure fraction of the second phase.
(2) Tensile properties JIS No. 5 tensile test specimens were collected from the obtained steel strip (hot-rolled sheet) and subjected to a tensile test in accordance with the provisions of JIS Z 2241. Yield strength YS, tensile strength TS The elongation El and the yield ratio YR were determined.

(3) Strain age hardening characteristics JIS No. 5 test piece was taken in the rolling direction from the obtained steel strip (hot-rolled sheet), subjected to 5% plastic deformation as pre-deformation (tensile pre-strain), and then 250 ℃ × after heat treatment of 20min, a tensile test was performed to obtain the tensile properties after the heat treatment (yield stress _{YS HT,} tensile strength _{TS HT),} calculated ΔYS ₌ YS HT -YS, a ΔTS _{= TS} HT -TS did. It should be _noted, YS _HT, the _{TS HT} pre-deformation - is the yield stress after the heat treatment, the tensile strength, YS, TS is the yield stress of the steel strip (hot-rolled sheet), a tensile strength.
(4) Hole expansion rate After punching a test piece taken from the obtained steel strip (hot-rolled sheet) with a 10mmφ punch to form a hole, a conical punch with an apex angle of 60 ° is used, and the beam is on the outside In this way, hole expansion was performed until a crack penetrating the plate thickness occurred, and the hole expansion ratio λ was obtained. The hole expansion ratio λ was obtained by λ (%) = {(d−d ₀ ) / d ₀ } × 100. D ₀ : initial hole diameter, d: inner hole diameter when cracking occurs.
These results are shown in Table 3.

本発明例は、いずれも、低い降伏強さＹＳと高い伸びＥｌと、低い降伏比ＹＲと、さらに大きな穴拡げ率λを示して、伸びフランジ成形性を含むプレス成形性に優れるとともに、大きなΔＹＳと極めて大きなΔＴＳを示し、歪時効硬化特性に優れた熱延鋼板となっている。これに対し、本発明の範囲を外れる比較例では、降伏強さＹＳが高いか、伸びＥｌが低いか、あるいは穴拡げ率λが小さいか、ΔＴＳが小さく、プレス成形性、歪時効硬化特性が低下した熱延鋼板となっている。
（実施例２）
表４に示す組成の溶鋼を転炉で溶製し、連続鋳造法で鋼スラブとした。これら鋼スラブを加熱し、表５に示す条件で熱間圧延して板厚 2.0mmの熱延鋼帯（熱延板）にし、さらに圧下率： 1.0％の調質圧延を施した。

Each of the examples of the present invention shows a low yield strength YS, a high elongation El, a low yield ratio YR, and a larger hole expansion ratio λ, which is excellent in press formability including stretch flange formability and large ΔYS. It shows a very large ΔTS and is a hot-rolled steel sheet having excellent strain age hardening characteristics. On the other hand, in the comparative example outside the scope of the present invention, the yield strength YS is high, the elongation El is low, the hole expansion ratio λ is small, ΔTS is small, press formability, and strain age hardening characteristics are obtained. It is a lowered hot-rolled steel sheet.
(Example 2)
Molten steel having the composition shown in Table 4 was melted in a converter and formed into a steel slab by a continuous casting method. These steel slabs were heated and hot-rolled under the conditions shown in Table 5 to form hot-rolled steel strips (hot-rolled sheets) having a sheet thickness of 2.0 mm, and further subjected to temper rolling at a reduction ratio of 1.0%.

For the obtained hot-rolled steel strip (hot-rolled sheet), the microstructure, tensile properties, strain age hardening properties, and hole expansion rate were determined in the same manner as in Example 1.
These results are shown in Table 6.

本発明例は、いずれも、低い降伏強さＹＳと高い伸びＥｌと、低い降伏比ＹＲと、さらに大きな穴拡げ率λを示して、伸びフランジ成形性を含むプレス成形性に優れるとともに、極めて大きなΔＹＳと極めて大きなΔＴＳを示し、歪時効硬化特性に優れた熱延鋼板となっている。これに対し、本発明の範囲を外れる比較例では、降伏強さＹＳが高いか、伸びＥｌが低いか、あるいは穴拡げ率λが小さいか、ΔＴＳが小さく、プレス成形性、歪時効硬化特性が低下した熱延鋼板となっている。

Each of the examples of the present invention shows a low yield strength YS, a high elongation El, a low yield ratio YR, a larger hole expansion ratio λ, and is excellent in press formability including stretch flange formability and extremely large. ΔYS and an extremely large ΔTS are exhibited, and the hot-rolled steel sheet has excellent strain age hardening characteristics. On the other hand, in the comparative example outside the scope of the present invention, the yield strength YS is high, the elongation El is low, the hole expansion ratio λ is small, ΔTS is small, press formability, and strain age hardening characteristics are obtained. It is a lowered hot-rolled steel sheet.

Claims (5)

% By mass
C: 0.15% or less, 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, Cu: 0.5-3.0%
And the balance is Fe and inevitable impurities, and the structure has a ferrite phase with an area ratio of 80% or more as a main phase and a martensite phase alone second area with an area ratio of 2% or more. A hot-rolled steel sheet having excellent press formability and having excellent strain age hardening characteristics such that ΔTS: 80 MPa or more. The hot-rolled steel sheet according to claim 1, further comprising one group or two or more groups selected from the following groups A to C in mass% in addition to the composition.
Group A: Ni: 2.0% or less,
Group B: 2.0% or less of one or two of Cr and Mo in total
Group C: 0.2% or less total of one or more of Nb, Ti and V % By mass
C: 0.15% or less, 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, and Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0%, W: 0.05 to 2.0%, or a total of 2.0% or less The balance is composed of Fe and inevitable impurities, and the structure is composed of a ferrite phase having an area ratio of 80% or more as the main phase and a martensite phase alone second area having an area ratio of 2% or more. A hot-rolled steel sheet that has excellent press formability and has excellent strain age hardening characteristics of ΔTS: 80 MPa or more.   The hot-rolled steel sheet according to claim 3, further comprising 2.0% or less of one or more of Nb, Ti, and V in total in addition to the composition. % By mass
C: 0.15% or less, 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, and further, Mo: 0.05% to 2.0%, Cr: 0.05% to 2.0%, W: 0.05% to 2.0%, or a total of 2.0% or less When hot rolling is performed on a steel slab having a composition to obtain a hot rolled sheet having a predetermined thickness, the hot rolling is hot rolling with a finish rolling finish temperature FDT equal to or higher than the Ar ₃ transformation point, and finish rolling is finished. Then, after cooling to a temperature range of (Ar ₃ transformation point) to (Ar ₁ transformation point) at a cooling rate of 5 ° C./s or higher, air cooling or slow cooling for 1 to 20 seconds in the temperature range, and then again 5 ° C. / A method for producing a hot-rolled steel sheet having excellent press formability and excellent strain age hardening characteristics of ΔTS: 80 MPa or more, characterized by cooling at a cooling rate of s or more and winding at a temperature of 550 ° C. or less. .

Images