JP5780210B2 - High-strength hot-rolled steel sheet excellent in elongation and hole-expandability and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in elongation and hole expandability and a method for producing the same.

  In recent years, global environmental awareness has increased, and in the automobile field, reduction of carbon dioxide emissions and improvement of fuel efficiency are strongly demanded. For these problems, the weight reduction of the vehicle body is extremely effective, and the weight reduction by applying a high-strength steel sheet is being promoted. Currently, many hot-rolled steel sheets having a tensile strength of 440 MPa are used for undercar parts of automobiles, but application of higher-strength steel sheets is desired in order to cope with weight reduction of the vehicle body.

  Many automobile undercarriage members have complicated shapes in order to ensure high rigidity. Therefore, since a plurality of processes such as burring, stretch flange process, and stretch process are performed in press forming, the hot-rolled steel sheet as a raw material is required to have workability corresponding to these. In general, burring workability and stretch flange workability are correlated with the hole expansion rate measured in the hole expansion test, and many studies have been made to increase the hole expansion rate.

  Dual phase steel (hereinafter referred to as DP steel) composed of ferrite and martensite is excellent in elongation while having high strength, but has low hole expandability. This is because, since the strength difference between ferrite and martensite is large, a large strain and stress concentration are generated in the ferrite in the vicinity of martensite, and cracks occur. Based on this knowledge, hot-rolled steel sheets with increased hole expansion rate by reducing the difference in strength between structures have been developed.

  Patent Document 1 proposes a steel sheet that has bainite or bainitic ferrite as a main phase to ensure strength and greatly improve hole expansibility. By using single structure steel, the strain and stress concentration as described above do not occur, and a high hole expansion rate can be obtained. However, by using a single-structure steel of bainite or bainitic ferrite, the elongation is greatly deteriorated, and it has not been possible to achieve both elongation and hole expandability.

  In recent years, steel sheets that use ferrite having excellent elongation as the structure of single-structure steel and have increased strength using carbides such as Ti and Mo have been proposed (for example, Patent Documents 2 to 4). However, the steel sheet proposed in Patent Document 2 contains a large amount of Mo, and the steel sheet proposed in Patent Document 3 contains a large amount of V. Furthermore, the steel sheet proposed in Patent Document 4 needs to be cooled during rolling in order to refine crystal grains. Therefore, there exists a problem that an alloy cost and manufacturing cost become high. Also in this steel sheet, the elongation has deteriorated due to the fact that the ferrite itself is greatly strengthened. Although the elongation of bainite and bainitic ferrite single-strength steel exceeded the elongation, the balance between elongation and hole expansion was not always sufficient.

  Further, Patent Document 5 proposes a composite structure steel plate in which martensite in DP steel is bainite and the hole expandability is improved by reducing the difference in strength between the microstructures with ferrite. However, as a result of increasing the area ratio of the bainite structure in order to ensure the strength, the elongation deteriorated and the stretch-hole expanding property balance was not sufficient. Further, in Patent Document 6, in order to provide hole expandability and formability, in addition to quenching and tempering of martensite after quenching, the amount of solid solution C in the ferrite before quenching is controlled. High-strength steel sheets with excellent ductility and formability that are excellent in formability and hole formability, both of which are tempered martensite and have good strength and hole expansibility, have been disclosed. It is hoped that

JP 2003-193190 A JP 2003-089848 A JP 2007-063668 A JP 2004-143518 A JP 2004-204326 A JP 2007-302918 A

  An object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent elongation and hole-expandability and a method for producing the same without containing an expensive element.

  The present inventors have conducted a detailed investigation on the relationship between the structure of a DP steel having high strength and high elongation, and the relationship between elongation and hole expansibility, and a method for improving both elongation and hole expansibility with respect to conventional steel types. Was examined. As a result, the inventors have found a technique for improving the hole expandability while maintaining high elongation of DP steel by controlling the dispersion state of martensite. In other words, even in DP structures such as ferrite and martensite, where the strength difference is large and hole expansibility is generally low, the area ratio and average diameter of martensite are controlled and the formula (1) is satisfied. It was clarified that the hole expandability can be improved while maintaining a high elongation.

  This invention is made | formed based on such knowledge, The summary is as follows.

(1) In mass%,
C: 0.036 to 0.10%,
Mn: 0.5 to 2.5%
P: 0.04% or less,
S: 0.01% or less,
N: 0.01% or less,
Including
And the total amount of addition of Si and Al is: 0.41 to 2.5%,
The balance consists of Fe and unavoidable impurities, the metal structure includes ferrite with an area ratio of 80% or more and 3 to 13.1% martensite, and pearlite is 2.7% or less. 1/4 martensite number density of more than the equivalent circle diameter of 3μm in thickness is at 4.9 pieces / 10000 ² or less, still meets the following formula (1), uniform elongation is 11.5% or more, the hole expanding ratio is A high-strength hot-rolled steel sheet excellent in elongation and hole-expandability, characterized by being 83% or more and a tensile strength of 630 MPa or more .
R / D _M ² ≧ 1.07 Formula (1)
Here, R: average martensite interval (μm) defined by the following formula (2), D _M : martensite average diameter (μm)
... Formula (2)
Here, V _M : Martensite area ratio (%), D _M : Martensite average diameter (μm)

... Formula (2)
Here, V _M : Martensite area ratio (%), D _M : Martensite average diameter (μm)

(2) Further, Nb: 0.06% or less, Ti: 0.20% or less, any one or two of the above, (1) high strength excellent in elongation and hole expansibility Hot rolled steel sheet.

(3) Furthermore, in mass%,
V: 0.02~ 0.15%,
W: 0.1-0.5%
Mo: 0.05 to 0.12 %
A high-strength hot-rolled steel sheet excellent in elongation and hole-expanding property as described in (1) or (2) above, comprising one or more of the above.

(4) Further, by mass%, Cr: 1.0% or less, Cu: 1.2% or less, Ni: 0.6% or less, B: 0.005% or less A high-strength hot-rolled steel sheet excellent in elongation and hole-expandability according to any one of (1) to (3) above.

  (5) The above-mentioned (1) to (4), characterized by further containing any one or two of REM: 0.0005 to 0.01% and Ca: 0.0005 to 0.01% by mass%. ) A high-strength hot-rolled steel sheet excellent in elongation and hole-expandability as described in any of the above.

(6) After heating the slab having the chemical component according to any one of (1) to (5) above to 1150 to 1300 ° C, among all the rolling passes of rough rolling, the final four passes or more are set to 1000 to Rolling is performed in a temperature range of 1050 ° C., and rolling is performed so that the total reduction ratio in this temperature range is 30% or more, finish rolling is started within 60 seconds from the end of rough rolling, and finish rolling finish temperature is 850 to Rolled at 950 ° C., cooled to a range of 600 to 750 ° C. at a cooling rate of 50 ° C./s or more, air cooled for 5 to 10 seconds, then cooled at a cooling rate of 31 ° C./sec or more, and 400 ° C. or less. Is a mixed structure in which the metal structure contains ferrite with an area ratio of 80% or more and 3 to 13.1% martensite and pearlite is 2.7% or less, and is a circle at a thickness of 1/4 of the plate thickness. Maltese with an equivalent diameter of 3 μm or more And the site number density of 4.9 pieces / 10000 ² or less, still meets the following formula (1), uniform elongation is 11.5% or more, the hole expanding ratio is 83% or more, so that the tensile strength is more than 630MPa A method for producing a high-strength hot-rolled steel sheet excellent in elongation and hole-expandability, characterized in that
R / D _M ² ≧ 1.07 Formula (1)
Here, R: average martensite interval (μm) defined by the following formula (2), D _M : martensite average diameter (μm)
... Formula (2)
Here, V _M : Martensite area ratio (%), D _M : Martensite average diameter (μm)

  According to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet excellent in both elongation and hole-expandability without containing an expensive element, and the industrial contribution is extremely remarkable.

It is a diagram showing the relationship between martensite average diameter (μm) D _M and martensite area fraction V _{M (%).} The values in parentheses indicate λ values. The average is a diagram showing the relationship between divided by R / D _M ² and the hole expanding ratio by the square of the martensite intervals R martensite average diameter D _M (%). Is a diagram showing the relationship between martensite number above the circle equivalent diameter 3μm in 1/4 the thickness of the plate thickness density _{N M} (pieces / 10000 ²⁾ and hole expansion ratio (λ,%). When the number density of martensite having a circle equivalent diameter of 3 μm or more (pieces / 10000 μm ² ) is 5 or more, the hole expandability is low. Note that only data with R / D _M ² exceeding 1.0 is shown in this graph.

  DP steel is a steel sheet in which hard martensite is dispersed in soft ferrite, and realizes high elongation while having high strength. However, at the time of deformation, strain and stress concentration due to the strength difference between ferrite and martensite occurs, and voids that cause ductile fracture are likely to be generated, so the hole expandability is very low. However, no detailed investigation on void formation behavior has been conducted, and the relationship between the microstructure and ductile fracture of DP steel has not always been clear.

  Therefore, the present inventors conducted a detailed investigation on the relationship between the structure and void generation behavior and the relationship between the void generation behavior and hole expansibility in DP steel having various structural configurations. As a result, it has been clarified that the martensite dispersion state, which is a hard second phase structure, has a great influence on the hole expandability of DP steel. Furthermore, by setting the value obtained by dividing the average martensite interval obtained by the formula (1) by the square of the average martensite diameter to 1.0 or more, even in a structure having a large inter-structure strength difference such as DP steel It has been found that high hole expandability can be obtained.

  In the hole expansion process, cracks are generated and propagated by ductile fracture, which is the process of void formation, growth and connection. In a structure with a large strength difference such as DP steel, high strain and stress concentration occur due to hard martensite, so voids are easily formed, and the hole expandability is low.

  However, a detailed investigation of the relationship between microstructure and void formation behavior and the relationship between void formation behavior and hole expansibility revealed that void formation, growth, and connection were delayed depending on the dispersion state of martensite, the hard second phase. As a result, it was revealed that high hole expansibility may be obtained.

  Specifically, it has been clarified that void formation is delayed by reducing the martensite size. This is thought to be due to the fact that martensite becomes smaller and the strain and stress concentration areas formed in the vicinity thereof become narrower. Moreover, when the space | interval between the martensites which changes with the number density and average diameter of a martensite is large, the distance between the voids formed from a martensite as the starting point also expanded simultaneously, and it discovered that it became difficult to connect.

  Based on the above findings, a DP structure having high hole expansibility was examined. As a result, as shown in FIG. 1, it was found that high hole expansibility can be obtained by controlling the area ratio and size of martensite within a certain range.

Further average martensite R / D _M ² is the distance R value satisfying divided by the following formula (1) by the square of the martensite average diameter D _M as shown in FIG. 2, hole expansion ratio (%) and a clear By having a correlation and setting R / D _M ² to 1.0 or more, high hole-expandability can be obtained even with a DP structure, and a hot-rolled steel sheet having excellent elongation and hole-expandability can be obtained. I understood it. In the present invention, R / D _M ² is limited to 1.07 or more based on the embodiment .
R / D _M ² ≧ 1.07 (1)
Here, R: average martensite interval (μm) defined by the following formula (2), D _M : martensite average diameter (μm)

... Formula (2)
Here, V _M : Martensite area ratio (%), D _M : Martensite average diameter (μm)

  Equation (1) represents the difficulty of void formation and connection, and the average martensite spacing R obtained by equation (2) is divided by the square of the average diameter of martensite from the area ratio and average diameter of martensite. It has a shape. The average means an arithmetic average. As the distance between martensites increases, voids generated from martensite become harder to be connected, and void generation and connection are suppressed due to the refinement of martensite.

Although the reason why the connection of voids is suppressed by the refinement of martensite is not clear, it is considered that the growth of voids is greatly delayed. If the martensite is small, the size of the void generated starting from the martensite also becomes finer. The generated void grows and becomes connected, but it is considered that the void surface area / void volume ratio increases as the void size becomes finer, that is, the growth of the void is delayed due to the increased surface tension.

However, as shown in FIG. 3, it was also found that even if the formula (1) is satisfied, if coarse martensite is present, local destruction proceeds and the hole expandability is lowered. In order to prevent this, the martensite number density of the circle equivalent diameter of 3 μm or more at the 1/4 thickness position of the plate thickness needs to be 5 pieces / 10000 μm ² or less.

  Hereinafter, the steel plate component of the present invention will be described in detail. In addition, "%" about a component means the mass%.

(C: 0.036 to 0.10%)
C is an important element that generates martensite and contributes to strengthening. In order to generate martensite, it is necessary to make it 0.03% or more. However, if it exceeds 0.10%, the area ratio of martensite increases and the hole expandability decreases, so 0.10% is made the upper limit. A preferable range is 0.04 to 0.07%. In the present invention, the lower limit of C is limited to 0.036% from the examples.

(Mn: 0.5-2.5%)
Mn is an important element related to ferrite strengthening and hardenability. In order to improve the hardenability and generate martensite, it is necessary to add 0.5% or more. However, if adding over 2.5%, ferrite cannot be generated sufficiently, so 2.5% is made the upper limit. A preferable range is 0.8 to 2.0%, more preferably 1.0 to 1.5%.

(P: 0.04% or less)
P is an impurity element, and if it exceeds 0.040%, embrittlement of the weld becomes significant, so the upper limit is made 0.04% or less. Although the lower limit of P is not particularly defined, it is economically disadvantageous to make it less than 0.0001%.

(S: 0.01% or less)
S is an impurity element and adversely affects weldability, manufacturability during casting and hot rolling, so the upper limit is made 0.01% or less. Moreover, when S is contained excessively, coarse MnS is formed and the hole expandability is lowered. Therefore, in order to improve the hole expandability, the content is preferably reduced. Although the lower limit of S is not particularly defined, it is preferable to set this value as the lower limit because it is economically disadvantageous to make it less than 0.0001%.

(N: 0.01% or less)
N is an impurity element, and if the content exceeds 0.01%, coarse nitrides are formed and the bendability and hole expansibility are deteriorated, so the upper limit is made 0.01% or less. Moreover, since it will cause the blowhole generation | occurrence | production at the time of welding when content of N increases, it is preferable to reduce. The lower limit is desirably less, and is not particularly determined. However, in order to reduce the N content to less than 0.0005%, the manufacturing cost increases, so it is preferable that the lower limit be 0.0005% or more.

(Si + Al: 0.41 to 2.5%)
Si and Al are important elements involved in strengthening ferrite and producing ferrite. However, if the total amount of Si and Al exceeds 2.5%, the effect is saturated and the cost increases, so this is the upper limit. In order to generate ferrite, it is necessary to add 0.1% or more in total, so Si + Al: 0.1 to 2.5%, but the preferred range is 0.5 to 1.5 %, And more preferably 0.8 to 1.3%. In the present invention, the lower limit of the total amount of Si and Al added is limited to 0.41% from the examples.

  The steel component used in the steel sheet of the present invention contains the above elements and consists of the remainder Fe and inevitable impurities, but the following elements may be further added.

(Nb: 0.06% or less, Ti: 0.20% or less)
Nb and Ti are elements related to precipitation strengthening of ferrite. If Nb is added in excess of 0.06%, ferrite transformation is significantly delayed and elongation deteriorates, so 0.06% is made the upper limit. If Ti is added in excess of 0.2%, the ferrite is excessively strengthened and high elongation cannot be obtained, so 0.2% is made the upper limit. In order to strengthen the ferrite, Nb: 0.005% or more and Ti: 0.02% or more are preferably added. The preferred ranges are Nb: 0.01 to 0.03% and Ti: 0.06 to 0.16%, respectively, more preferably Nb: 0.015 to 0.025%, Ti: 0.08 to 0.00. 14%.

(V: 0.02~ 0.15%, W : 0.1~0.5%, Mo: either one or two or more of from 0.05 to 0.12%)
V, Mo and W are elements contributing to strengthening and may be added. In order to obtain the effect of increasing the strength, it is necessary to add V: 0.02% or more, Mo: 0.05% or more, and W: 0.1% or more, respectively. However, if too much is added, the formability may deteriorate, so V: 0.2%, Mo: 0.4%, W: 0.5% are the upper limits . The upper limit is limited to 0.15%, and the upper limit of Mo is limited to 0.12%.

(Cr: 1.0% or less, Cu: 1.2% or less, Ni: 0.6% or less, B: 0.005% or less)
Furthermore, you may add any 1 type or 2 types or more of Cr, Cu, Ni, and B for high intensity | strength. In order to obtain the effect of increasing the strength, it is necessary to add Cr: 0.01% or more, Cu: 0.1% or more, Ni: 0.05% or more, and B: 0.0001% or more. However, if too much is added, the formability may be deteriorated, so Cr: 1.0%, Cu: 1.2%, Ni: 0.6%, B: 0.005% are the upper limits.

(REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, either one or two)
Furthermore, in order to control the form of inclusions, one or two of REM: 0.0005 to 0.01% and Ca: 0.0005 to 0.01% may be added. Ca and REM are effective elements for controlling the form of oxides and sulfides, and preferable lower limit values are Ca: 0.0005% and REM: 0.0005%. On the other hand, if added excessively, moldability may be impaired, so the preferable upper limit is 0.01%. In the present invention, REM refers to La and lanthanoid series elements, which are often added by misch metal, and contain a series of elements such as La and Ce. Metal La or Ce may be added.

Hereinafter, the structure of the present invention will be described in detail.
Ferrite is the most important structure for securing elongation. If the ferrite area ratio is less than 80%, the high elongation of the conventional DP steel cannot be realized. However, if the ferrite area ratio exceeds 97%, martensite decreases, so it is not possible to utilize the strengthening by martensite. If the strength is ensured by increasing the precipitation strengthening amount by other methods, for example, uniform elongation is achieved. It will decline.

Martensite is an important organization for securing strength and elongation. When the area ratio of martensite is less than 3%, it is difficult to ensure uniform elongation. Further, if it exceeds 15%, the hole expandability deteriorates, so 15% is set as the upper limit, but in the present invention, 13.1% is set as the upper limit in accordance with the embodiment. In addition, if there is coarse martensite, destruction proceeds locally and the hole expandability deteriorates. In order to suppress this, the number density of martensite having an average diameter of 3 μm or more needs to be 5/10000 μm ² or less. In the present invention, the number density of martensite having an average diameter of 3 μm or more is limited to 4.9 / 10000 μm ² or less based on the examples .

Since pearlite deteriorates hole expandability, it is better not to use it. However, if the area ratio is 3% or less, there is no significant influence, so this is allowed as the upper limit, but based on the examples, the upper limit is 2.7%.

Bainite may exist as another structure. Bainite is a structure that contributes to increasing the strength. However, if the strength is increased by utilizing the bainite in a large amount, the area ratio of ferrite becomes lower, and high elongation cannot be achieved. Therefore, bainite is not essential in the present invention, and the area ratio may be 0%. The hot-rolled steel sheet of the present invention has a tensile strength of 630 Mpa or more, preferably 740 Mpa or more. Further, the hot-rolled steel sheet of the present invention has a uniform elongation (u-El) of 11.5% or more and a hole expansion ratio (λ) of 83% or more.

  Hereinafter, the manufacturing method of the hot-rolled steel sheet of this invention is demonstrated.

  First, steel is melted by a conventional method, and a steel piece is manufactured by casting. Casting is preferably continuous casting from the viewpoint of productivity.

  The slab having the components of the present invention is heated before rolling. When the heating temperature is less than 1150 ° C., the rolling load during rough rolling is remarkably increased, which hinders productivity. Although the upper limit is not particularly defined, it is not preferable in view of manufacturing cost to exceed 1300 ° C. The cast slab may be directly fed and rolled while still hot. In order to obtain the effect of strengthening by precipitation strengthening, it is necessary to form a solution of elements such as Nb and Ti, and thus heating to 1200 ° C. or higher is preferable.

  After heating, rough rolling is performed. Of all the rolling passes of rough rolling, the final four passes or more are performed within the range of 1000 to 1050 ° C., and the total rolling reduction in this temperature range is set to 30% or more, and within 60 seconds from the final pass of rough rolling Start finish rolling. In order not to generate coarse martensite, it is important to refine austenite. For this purpose, it is effective to recrystallize repeatedly before finish rolling, but if the temperature exceeds 1050 ° C., the grain growth after recrystallization is remarkably fast, so the fine grain effect is small. If the temperature is lower than 1000 ° C., the next reduction is performed without completely recrystallizing, and the grain sizes in the non-recrystallized portion and the recrystallized portion become non-uniform, and as a result, the martensite number density with an average diameter of 3 μm or more increases. If the total rolling reduction is less than 30%, it cannot be sufficiently miniaturized. However, even when rolling at 30% or more, if the number of rolling passes is less than 4, the austenite grain size becomes non-uniform, resulting in the formation of coarse martensite. Moreover, even if such rough rolling is performed, if a long time elapses before the start of finish rolling, the grains become coarse, and thus it is necessary to start finish rolling within 60 seconds.

  The finish rolling finish temperature is 850 to 950 ° C. When the temperature exceeds 950 ° C., the recrystallized γ grain size becomes coarse, and the ferrite nucleation sites decrease, so that the ferrite transformation is significantly delayed. On the other hand, if it is less than 850 ° C., the rolling load increases, so this is the lower limit.

  After finish rolling, primary cooling is performed, air cooling is performed, and then secondary cooling is performed and winding is performed. The primary cooling rate is 50 ° C./s or more. If the cooling rate is low, the ferrite grain size becomes coarse. Martensite is obtained by transformation of the remaining austenite that has undergone ferrite transformation. When the ferrite grain size is coarsened, the remaining martensite is also coarsened. Although the upper limit is not particularly defined, a cooling rate exceeding 100 ° C./s is not preferable because the equipment cost becomes excessive.

  The primary cooling stop temperature is 600 to 750 ° C. If it is less than 600 ° C., the ferrite transformation cannot be sufficiently advanced during air cooling. On the other hand, when the temperature exceeds 750 ° C., ferrite transformation proceeds excessively, pearlite transformation occurs during subsequent cooling, and hole expansibility also deteriorates.

  Air cooling time is 5 to 10 seconds. If it is less than 5 seconds, the ferrite transformation cannot sufficiently proceed. Further, when air cooling is performed for more than 10 seconds, pearlite transformation occurs and the hole expandability deteriorates. Therefore, the air cooling time is set to 5 to 10 seconds.

The secondary cooling rate is 31 ° C./s or more. If it is less than 31 ° C./s, the bainite transformation proceeds excessively during cooling, and the area ratio of ferrite cannot be obtained sufficiently, so that the uniform elongation deteriorates. Although the upper limit is not particularly defined, a cooling rate exceeding 100 ° C./s is not preferable because the equipment cost becomes excessive.

  The winding temperature is from room temperature to 400 ° C. If it exceeds 400 ° C., the bainite transformation proceeds excessively, and sufficient martensite cannot be obtained, so that uniform elongation cannot be ensured. A preferred range is 250 ° C. or lower, more preferably 100 ° C. or lower, and may be room temperature.

  Steel pieces obtained by melting and casting steel having chemical components shown in Table 1 were rolled under the conditions shown in Table 2.

  A sample was collected from the obtained steel plate, and the metal structure at a thickness of 1/4 was observed using an optical microscope. As adjustment of the sample, the plate thickness cross section in the rolling direction was polished as an observation surface and etched with a Nital reagent and a repeller reagent. The area ratio of ferrite and the area ratio of pearlite were determined by image analysis from an optical micrograph at a magnification of 500 times etched with a Nital reagent. Further, the area ratio and average diameter of martensite were determined by image analysis from an optical micrograph having a magnification of 500 times etched with a repeller reagent. The average diameter is the number average of equivalent circle diameters of each martensite grain. The area ratio of bainite was determined as the balance of ferrite, pearlite, and martensite.

Tensile strength (TS) was evaluated in accordance with JIS Z 2241 using a JIS Z 2201 No. 5 test piece taken in a direction perpendicular to the rolling direction from a 1/4 position in the sheet width direction. Elongation (EL) was measured along with tensile strength (TS). The hole expansion test was evaluated according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. Table 3 shows the structure and mechanical properties of the steel sheet. In Table 3, _{V F} is ferrite, _{V B} is bainite, _{V P} pearlite, _{V M} is the respective area ratio% of martensite. D _M is the average martensite diameter (μm), and N _M is the martensite number density per 10000 μm ^{2 with a} circle equivalent diameter of 3 μm or more at ¼ thickness of the plate thickness.

The results will be described. Steel No. C to G2, H, J, K, M, N, P, R to T, V to W2, X, Z to AB, AD to AG, AJ are examples of the present invention, steel components, production conditions and The structure satisfies the requirements of the present invention, and is excellent in both elongation and hole expansibility. On the other hand, the following is a comparative example, and the effect could not be obtained for the following reason. Steel No. AH and AI are reference examples.

  Steel No. In A, the amount of Mn added is high, and the ferrite transformation did not proceed sufficiently. Therefore, the ferrite fraction is less than 80% and the uniform elongation is low.

  Steel No. In B, the amount of Nb added is high, and the ferrite transformation did not proceed sufficiently, so the ferrite fraction is less than 80% and the uniform elongation is low.

  Steel No. Since G3 has an air cooling time that is too long, pearlite is generated beyond the appropriate range, and the hole expandability is low.

  Steel No. Since G4 has too high FT, the ferrite transformation does not proceed sufficiently, the ferrite fraction is less than 80%, and the uniform elongation is low.

  Steel No. Since the air cooling time of G5 is too short, the ferrite transformation does not proceed sufficiently, the ferrite fraction is less than 80%, and the uniform elongation is low.

  Steel No. Since the primary cooling rate of G6 is slow, the average diameter of martensite is large, and as a result, Formula 1 is not satisfied, so that the hole expandability is low.

  Steel No. Since G7 and L have a small number of rolling passes at 1000 to 1050 ° C., the number density of coarse martensite is high and the hole expandability is low.

  Steel No. Since G8 has a small rolling reduction at 1000 to 1050 ° C., the average diameter of martensite is large, and as a result, Formula 1 is not satisfied, so that the hole expandability is low.

Steel No. Since G9 takes a long time from the end of rough rolling to the start of finish rolling, austenite is coarsened, the average diameter of martensite is large, and as a result, R / D _M ² is small, and the hole expandability is low.

  Steel No. As for I, since C is added more than the upper limit of the specified range and the martensite area ratio is high, the hole expandability is low.

  Steel No. O has a low uniform elongation because the total addition amount of Si + Al has not reached the lower limit of the specified range, and the ferrite transformation has not progressed sufficiently.

  Steel No. Since Q has a slow primary cooling rate, the average diameter of martensite is large, and as a result, Equation 1 is not satisfied.

  Steel No. U is added with more Ti than the upper limit of the specified range, and the uniform elongation is low because ferrite is strengthened excessively.

  Steel No. Since the air cooling temperature of W3 is too high, pearlite is generated, and the hole expandability is low.

  Steel No. Since W4 has too high CT, martensite can hardly be generated and uniform elongation is low.

  Steel No. Since W5 has an air cooling temperature that is too low, the ferrite transformation does not proceed sufficiently, the ferrite fraction is less than 80%, and the uniform elongation is low.

  Steel No. Since W6 has a low secondary cooling rate, bainite is generated, the ferrite fraction is less than 80%, and the uniform elongation is low.

  Steel No. In Y, the amount of C added does not reach the lower limit of the specified range, the area ratio of martensite is less than 3%, and the uniform elongation is low.

  Steel No. AC has a low uniform elongation because the amount of Mn does not reach the lower limit of the specified range and martensite is not generated.

Claims (6)

% By mass
C: 0.036 to 0.10%,
Mn: 0.5 to 2.5%
P: 0.04% or less,
S: 0.01% or less,
N: 0.01% or less,
Including
And the total amount of addition of Si and Al is: 0.41 to 2.5%,
The balance consists of Fe and unavoidable impurities, the metal structure includes ferrite with an area ratio of 80% or more and 3 to 13.1% martensite, and pearlite is 2.7% or less. 1/4 martensite number density of more than the equivalent circle diameter of 3μm in thickness is at 4.9 pieces / 10000 ² or less, still meets the following formula (1), uniform elongation is 11.5% or more, the hole expanding ratio is A high-strength hot-rolled steel sheet excellent in elongation and hole-expandability, characterized by being 83% or more and a tensile strength of 630 MPa or more .
R / D _M ² ≧ 1.07 Formula (1)
Here, R: average martensite interval (μm) defined by the following formula (2), D _M : martensite average diameter (μm)
... Formula (2)
Here, V _M : Martensite area ratio (%), D _M : Martensite average diameter (μm) In addition,
Nb: 0.06% or less,
The high-strength hot-rolled steel sheet excellent in elongation and hole-expandability according to claim 1, comprising Ti: 0.20% or less. In addition,
V: 0.02-0.15%,
W: 0.1-0.5%
Mo: 0.05 to 0.12%
The high-strength hot-rolled steel sheet excellent in elongation and hole-expandability according to claim 1 or 2, characterized in that any one or two or more of these are included. In addition,
Cr: 1.0% or less,
Cu: 1.2% or less,
Ni: 0.6% or less,
B: Any one type or two types or more of 0.005% or less are included, The high-strength hot-rolled steel sheet excellent in elongation and hole-expandability according to any one of claims 1 to 3. In addition,
REM: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%
The high-strength hot-rolled steel sheet excellent in elongation and hole expansibility according to any one of claims 1 to 4, characterized in that any one or two of the above are included. After heating the slab having the chemical component according to any one of claims 1 to 5 to 1150 to 1300 ° C, among all the rolling passes of rough rolling, the final four passes or more are performed in a temperature range of 1000 to 1050 ° C. And rolling so that the total reduction ratio in this temperature range is 30% or more, finish rolling is started within 60 seconds from the end of the rough rolling, and the finish rolling finish temperature is 850 to 950 ° C. , Cooled at a cooling rate of 50 ° C./s or more in the range of 600 to 750 ° C., air cooled for 5 to 10 seconds, then cooled at a cooling rate of 31 ° C./sec or more, wound up at 400 ° C. or less, and metal structure Is a mixed structure containing ferrite with an area ratio of 80% or more and 3 to 13.1% martensite and pearlite of 2.7% or less, and a martens having an equivalent circle diameter of 3 μm or more at a quarter thickness. Number of sites Degree is at 4.9 pieces / 10000 ² or less, still meets the following formula (1), uniform elongation is 11.5% or more, the hole expanding ratio is 83% or more, the tensile strength is to be a more 630MPa A method for producing a high-strength hot-rolled steel sheet having excellent elongation and hole-expanding characteristics.
R / D _M ² ≧ 1.07 Formula (1)
Here, R: average martensite interval (μm) defined by the following formula (2), D _M : martensite average diameter (μm)
... Formula (2)
Here, V _M : Martensite area ratio (%), D _M : Martensite average diameter (μm)