JP5110965B2 - High strength hot-rolled steel sheet with excellent stretch flangeability and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of, for example, a 980 MPa class or more and excellent in stretch flangeability and a method for producing the same, and the hot-rolled steel sheet includes, for example, automobile members, bumpers, pillars, etc. It is useful as a reinforcing material.

  Recently, a hot-rolled steel sheet having a high tensile strength of 980 MPa or more and an improved stretch flangeability from the viewpoint of forming processability has been developed. Means for obtaining high stretch flangeability include a method in which the metal structure is a single phase structure such as ferrite and bainitic ferrite, and a structure in which the strength difference between the two phases is suppressed as much as possible while using a composite structure of ferrite and bainite. In order to obtain high strength while securing such a structure, a technique using precipitation strengthening by microalloy is known. For example, Patent Documents 1 to 5 attempt to increase the strength by adding Ti, which is a precipitation strengthening element.

  Patent Document 6 discloses a technique for achieving both high strength and workability by using a martensite-based metal structure by quenching immediately after hot rolling rather than using precipitation strengthening of alloy elements. ing.

However, some problems remain with these methods. That is, in the method using precipitation strengthening by addition of microalloy disclosed in Patent Documents 1 to 5 among the above, Ti and the like are added in a large amount, so the slab heating temperature before hot rolling must be increased. , There are difficulties in thermal energy. In addition, when Ti or the like is added to increase the strength by precipitation strengthening, the yield ratio becomes remarkably high, and the shape freezing property after press working becomes poor.
JP 2000-109951 A JP 2002-180189 A JP 2003-89848 A JP 2004-204326 A JP 2004-285420 A JP 2003-105446 A

  The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is not to use precipitation strengthening but to ensure a high strength by using a martensite-based metal structure. Another object of the present invention is to provide a hot-rolled steel sheet and a useful manufacturing method thereof that can simultaneously satisfy a high level of stretch flange characteristics.

The high-strength hot-rolled steel sheet of the present invention that was able to solve the above-mentioned problems is that the chemical component is C: 0.03 to 0.10% (in the case of a chemical component, it represents mass%, the same applies hereinafter),
Si: 0.2-2.0%,
Mn: 0.5 to 2.5%
Al: 0.02 to 0.10%,
Cr: 0.2 to 1.5%
Mo: 0.1 to 0.5%,
In which the balance is made of steel made of iron and inevitable impurities, and 80% by area or more of the longitudinal cross-sectional structure is a martensitic structure.

  The steel according to the present invention contains, as other components, B: 0.0050% or less (not including 0%), or Ni: 2.0% or less (not including 0%) or Ca: It may contain 0.0050% or less (excluding 0%). The strength level of the high-strength hot-rolled steel sheet according to the present invention preferably has a tensile strength of 980 MPa or more.

Further, the production method of the present invention is an invention that is positioned as a useful production method of a high-strength hot-rolled steel sheet having the above-mentioned characteristics, and the gist thereof is that after heating a steel slab that satisfies the above-mentioned compositional requirements, When hot rolling is performed, after hot rolling is finished at a finishing temperature not lower than the Ar ₃ transformation point, cooling is performed at an average cooling rate of 30 ° C./sec or higher, and winding is performed at a temperature lower than 400 ° C. is doing.

  According to the present invention, rather than using precipitation strengthening elements such as Ti, Nb, and V, the main phase is a martensite structure that is stronger than ferrite and bainite structures, and the contents of mainly C and Mo in the steel material. Is controlled to an appropriate range, and a high-strength hot-rolled steel sheet having particularly good stretch flangeability and excellent formability can be provided at a relatively low cost while securing a high strength of 980 MPa or higher in tensile strength.

  Under the above-mentioned problems, the present inventors have achieved a high strength by making the metal structure a martensite-based structure, and at the same time, workability [stretch flangeability (λ) and elongation characteristics (total elongation )], Especially in order to improve the stretch flange characteristics. As a result, in order to improve stretch flangeability in a steel having a martensite-based metal structure, the first requirement is to keep the C content lower than that of conventional martensite steel, and an appropriate amount in the steel. It is known that if Mo is contained, the structure is mainly martensite, but also has excellent stretch flange characteristics, and the present invention has been conceived.

  Hereinafter, the specific configuration of the present invention will be clarified following the reasons for determining the chemical composition and the metal structure of the steel material.

  First, the reason for determining the chemical composition of the steel material will be described.

C: 0.03% or more and 0.10% or less C is an element indispensable for enhancing hardenability and generating martensite which is a low-temperature transformation phase to promote high strength. In order to secure the amount of martensite structure specified in (1) and to obtain a tensile strength of 980 MPa class or higher, at least 0.03% or more of C is required, preferably 0.04% or more. However, if the C content exceeds 0.10%, the strength is improved, but the stretch flangeability, which is another object of the present invention, sharply decreases and the workability deteriorates. Preferably it is good to restrain to 0.08% or less.

Si: 0.2% or more and 2.0% or less Si is an element that increases the strength by solid solution in steel, and is added according to the required strength. Moreover, since it also has the effect of suppressing the formation of cementite that deteriorates stretch flangeability, at least 0.2% or more of Si is necessary, and preferably 0.5% or more is contained. However, if it exceeds 2.0%, surface defects are liable to occur and the pickling property and paintability are deteriorated, so the upper limit is made 2.0%.

Mn: 0.5% or more, 2.5% or less Mn has the effect of enhancing the hardenability and facilitating the generation of a low-temperature transformation product, and in order to ensure a tensile strength of 980 MPa class or more, It must be contained at least 0.5% or more. However, the effect saturates at about 2.5%, and not only the further effect is not exhibited, but also a band-like structure is formed by solidification segregation, and the workability and delayed fracture resistance are deteriorated. Should be kept below 5%. A more preferable content of Mn is 1.0% or more and 2.0% or less.

Al: 0.02% or more and 0.10% or less In addition to acting as a deoxidizer, Al is indispensable for fixing N (nitrogen) inevitably mixed in steel and improving workability. It is an element and needs to be contained at least 0.02% or more. However, if the Al content becomes too large, it becomes a non-metallic inclusion source and deteriorates the surface properties, so the upper limit is made 0.10%. A more preferable Al content is 0.03% or more and 0.05% or less.

Cr: 0.2% or more, 1.5% or less Cr has the effect of enhancing the hardenability and suppressing the formation of ferrite and bainite structures during the cooling after hot rolling to promote the formation of martensite structures. In order to effectively exhibit these actions, it is necessary to contain 0.2% or more. However, the effect is saturated at about 1.5%, and if it is added more than that, the chemical conversion processability deteriorates, so 1.5% is made the upper limit. The more preferable content of Cr is 0.5% or more and 1.0% or less.

Mo: 0.1% or more and 0.5% or less Mo is the most important element in the present invention, and is an element indispensable for obtaining a martensite-based structure having high stretch flangeability. For example, a martensite-based metal structure can be obtained by other elements that improve the hardenability, such as C, Mn, and Cr. However, elements other than Mo cannot achieve both high strength and excellent stretch flangeability, and it is essential to add an appropriate amount of Mo to achieve the purpose.

  The reason why a martensitic structure with high stretch flangeability can be obtained by using Mo has not yet been clarified yet, but the steel of the present invention has a high Ms point due to its low C content, Steel sheets that have undergone martensitic transformation at a relatively high temperature during cooling after hot rolling are considered to be in a self-tempered state during cooling to the coiling temperature, producing precipitates and deteriorating stretch flangeability. , Mo is considered to suppress the formation of precipitates in the self-tempered state and suppress the deterioration of stretch flangeability.

  In order to exhibit such an effect effectively, Mo must be contained in an amount of 0.1% or more. However, since the effect is saturated by the addition of about 0.5%, the addition of more than that is economically wasteful. A more preferable content of Mo is 0.2% or more and 0.4% or less.

  The essential constituent elements of the steel used in the present invention are as described above, and the balance is substantially Fe. “Substantially” means that inevitable contamination of elements inevitably mixed, such as P (phosphorus), S (sulfur), N (nitrogen), and O (oxygen) is allowed. In order to suppress obstacles due to the inclusion of the above elements, for example, P is 0.015% or less, S is 0.01% or less, N is 0.006% or less, and O is 0.003% or less. Good.

  By the way, P has the effect of increasing the strength by solid solution in steel, but if it is too much, it segregates at the grain boundaries and lowers the impact properties and cold workability of the parts, so keep it as small as possible. It is good to keep it at most 0.015% or less, preferably 0.010% or less.

  Since S becomes a source of non-metallic inclusions such as MnS and deteriorates workability, it is preferable to suppress S as much as possible. Such adverse effects of S can be suppressed to some extent by adding an appropriate amount of Ca as will be described later, and controlling the form of S-based inclusions, which is a factor that hinders workability, but even in this case, deterioration of workability due to S is prevented. In order to suppress it reliably, S content should be suppressed to 0.01% or less.

  In addition, N and O are inevitably contained in a small amount from the atmosphere gas at the time of melting, but both of these become a generation source of non-metallic inclusions and adversely affect workability. Therefore, N is preferably 0.004% or less, more preferably 0.003% or less, and even more preferably 0.001% or less. O is preferably 0.003% or less, more preferably 0.002% or less, and even more preferably 0.001% or less.

  In addition to the above elements, the steel material used in the present invention is effective to contain the following selective elements in order to give further additional characteristics as desired. Is also included in the technical scope of the present invention.

B: 0.005% or less B, like Cr and Mo, enhances hardenability, suppresses ferrite or bainite transformation, and promotes martensitic transformation. Such an effect is exhibited in a very small amount of about several ppm. However, since the effect is saturated at about 0.005%, it is better to keep it below that in consideration of economy.

Ni: 2.0% or less Ni has little effect on strength and bainite transformation, but has an effect of suppressing delayed fracture, so an appropriate amount is added especially when improvement in delayed fracture resistance is required. It is good. However, since the effect is saturated at about 2.0%, it should be suppressed to 2.0% or less in consideration of economy.

Ca: 0.0050% or less Ca has the effect of controlling the form of S-based inclusions that may inevitably be mixed and reducing adverse effects on processability, but the effect is too large. Saturates and causes only an increase in cost, so it should be suppressed to 0.0050% or less in view of economy.

  The steel sheet of the present invention is composed of the above-mentioned chemical composition, and in particular, by containing an appropriate amount of Mo in a state where the C content is suppressed, and other appropriate amounts of Si and Cr, precipitation strengthening of Ti, Nb, V, and the like can be achieved. Without using it, it is a martensite-based metal structure having a strength higher than that of ferrite or bainite, and a hot-rolled steel sheet with enhanced stretch flangeability while ensuring a tensile strength of 980 MPa level or higher. In order to obtain the strength and stretch flangeability intended by the present invention, the martensite area ratio in the total metal structure must be at least 80% or more, preferably 90% or more, more preferably about It is desirable that 100% has a martensite structure.

  The type of structure other than martensite allowed to be mixed in an amount of less than 20% by area is not particularly limited, and examples thereof include ferrite structure, bainite structure, retained austenite structure, etc., including one or more of these. It does not matter. In addition, the metallographic structure corrodes the longitudinal section of the steel plate with a repeller, and after performing a C vapor deposition process using a vacuum vapor deposition apparatus “JEE-4X” manufactured by JEOL Ltd., EPMA “JJA-8100” manufactured by JEOL Ltd. Was obtained by an image analysis apparatus “LUZEX-F” manufactured by NIRECO on the basis of a tissue photograph (magnification: 1000 times) observed using the above.

  Next, a method for producing a hot-rolled steel sheet according to the present invention will be described based on the conditions for obtaining the metal structure.

  The hot-rolled steel sheet of the present invention is manufactured by heating (soaking) a steel slab that satisfies the above-mentioned component composition, followed by hot rolling, cooling, and winding. (SRT) may be at a temperature necessary for the additive element to be dissolved, and generally, a temperature of 1000 ° C. or higher and 1250 ° C. or lower is generally employed. Considering production costs and the like, the upper limit of SRT is preferably about 1200 ° C. That is, in the case of Ti, Nb, V, etc., which are ordinary precipitation strengthening elements, high temperature heating of 1200 to 1300 ° C. or higher is required to make a solid solution in the austenite structure. The additive element can be sufficiently dissolved at a heating temperature of not lower than 1200 ° C. and not higher than 1200 ° C. The soaking time is not particularly limited, and the above-mentioned additive elements are dissolved in about 120 minutes, but it is better to be about 200 to 240 minutes for the sake of certainty, and heating for 300 minutes or more only reduces productivity. It is useless.

In hot rolling after soaking, the finishing temperature (FDT) is set to the Ar ₃ transformation point or higher, and then cooled to the coiling temperature at an average cooling rate of 30 ° C./sec or higher, and at a temperature of less than 400 ° C. Take up. Incidentally, when the finishing temperature of hot rolling becomes less than the Ar ₃ transformation point, the amount of ferrite produced increases, and this ferrite phase remains processed, so that not only the ductility is deteriorated but also the in-plane anisotropy is reduced. It is because it raises and material deterioration occurs. A more preferable finishing temperature of the hot rolling is “Ar ₃ transformation point + 30 ° C. or higher”. Although the upper limit of the finishing temperature is not specified, “Ar ₃ transformation point + about 80 ° C.” is considered as the upper limit when comprehensively considering thermal efficiency and productivity.

In the present specification, the Ar ₃ transformation point is calculated by the following formula.
Ar ₃ =
910-203√ [C] +44.7 [Si] +31.5 [Mo] -30 [Mn] -11 [Cr] +700 [P] +400 [Al] -15.2 [Ni] +104 [V] -20 [ Cu] +400 [Ti]
In the formula, [] means the content (% by mass) of each element.

  The average cooling rate after hot rolling is 30 ° C./sec or more, more preferably 50 ° C./sec or more, in order to suppress the formation of low-temperature transformation products such as polygonal ferrite and bainite as much as possible to increase the martensite structure fraction. Should be adopted. Incidentally, if the average cooling rate after hot rolling is less than 30 ° C./sec, the amount of polygonal ferrite and bainite produced in the cooling process cannot be ignored, and the martensite area ratio intended by the present invention (at least 80 area% or more) ) Cannot be secured.

  The austenite remaining after cooling after finishing the hot rolling is transformed into martensite. In the above-mentioned component system defined in the present invention, the coiling temperature after cooling is less than 400 ° C, more preferably 350 ° C or less. Thus, austenite can be transformed into martensite, and as a result, a desired martensite area ratio can be secured. If the temperature at this time is 400 ° C. or higher, 20% or more of austenite is transformed into bainite, and if the temperature is higher than that, ferrite transformation or pearlite transformation occurs, and it becomes impossible to secure a martensite area ratio of 80% or more.

  The high-strength hot-rolled steel sheet of the present invention uses a steel material having a specified chemical component as described above, and appropriately controls the finishing temperature during hot rolling, the subsequent cooling rate, and the coiling temperature. Thus, a high-strength hot-rolled steel sheet having high strength of 980 MPa class or more, particularly good stretch flangeability and excellent formability can be provided at low cost.

  Hereinafter, the present invention will be described more specifically with reference to experimental examples.However, the present invention is not limited by the following experimental examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

Experimental Example A steel material having the composition shown in Table 1 was cast after being melted by a vacuum melting method, and hot rolling was performed by performing hot rolling under the conditions shown in Tables 2 to 4 using the obtained slabs. A steel plate was obtained. In the steel materials of Table 1, the amount of O is approximately 0.001 to 0.002%. While confirming the metal structure of the obtained hot-rolled steel sheet by the following method, the mechanical characteristics were measured by the following method, and the results are shown in Tables 2 to 4. Tables 2 to 4 show the steel type No. of Table 1 used. Is also written.

[Metal structure]
Structural identification method: The longitudinal section of the steel sheet is repeller-corroded so that the martensite phase and other phases can be visually distinguished. Next, after performing a C deposition process using a vacuum deposition apparatus “JEE-4X” manufactured by JEOL Ltd., the structure was observed (magnification: 1000 times) using EPMA “JXA-8100” manufactured by JEOL Ltd. Based on the image analysis apparatus “LUZEX-F” manufactured by NIRECO, the martensite area ratio was determined. In the column of “structure fraction” in Tables 2 to 4 to be described later, M is martensite, and “others” is ferrite, bainite, and retained austenite.

  [Tensile strength (TS: MPa), yield strength (YS: MPa), elongation (El:%), yield ratio (YR = YS / TS)]: JIS No. 5 tensile test piece was prepared from each test steel plate. The tensile tester “AG-100” manufactured by Shimadzu Corporation was used. In this example, a tensile strength of 980 MPa or more was regarded as acceptable (◯), and a tensile strength of less than 980 MPa was regarded as unacceptable (x).

[Hole expansion ratio (λ;%)]: A punched hole having a diameter of 10 mm is formed as an initial hole diameter (d ₁ ) in each test steel sheet, and the punched hole is expanded using a conical punch having a vertex angle of 60 °. The crack generated in the punched hole portion measures the diameter (d ₂₎ when passed through the plate thickness, rate hole expansion by the following equation; Request (lambda%). This value is generally adopted as an evaluation index of stretch flangeability, which is one of the moldability. In this example, the stretch flangeability was generally 70% or more as acceptable (O), and generally less than 70% as unacceptable (X).
Hole expansion rate (λ;%) = (d ₂ −d ₁ ) × 100 / d ₁

  From these tables, we can think as follows.

  Experiment No. 2 in Table 2 Nos. 1 to 7 satisfy the components in the steel of the present invention and contain steel elements Nos. 1 is an example in which the influence of the coiling temperature (CT) is mainly examined. From Table 2, the experiment No. 1 was wound up in the temperature range (less than 400 ° C.) defined in the present invention. As for 1-4, as for all, it turns out that desired intensity | strength is ensured and the stretch flange characteristic is also favorable. In contrast, no. In Nos. 5 to 7, since the coiling temperature exceeds the specified range of the present invention, the martensite texture is less than 80% and the hole expansion rate (λ) is low.

  No. in Table 2 Nos. 8 and 9 satisfy the components in the steel of the present invention and include steel type Nos. 2 is mainly an example in which the coiling temperature (CT) is changed within the scope of the present invention, and both are excellent in both strength and stretch flangeability. Similarly, no. Nos. 10 to 12 satisfy the components in the steel of the present invention and contain the preferable selection elements of steel types No. 1 in Table 1. 3 is mainly an example in which the coiling temperature (CT) is changed within the scope of the present invention, and both are excellent in both strength and stretch flangeability.

  Experiment No. 2 in Table 2 Nos. 13 and 14 satisfy the components in the steel of the present invention and contain steel type Nos. 4 is an example in which the influence of the average cooling rate (CR) after the end of hot rolling was mainly examined, and No. 4 satisfying the cooling condition (CR: 30 ° C./s or more) defined in the present invention. No. 13 is a No. 13 that does not satisfy the above cooling conditions. Since a larger amount of martensite than 14 can be obtained, the desired tensile strength and hole expansion rate (λ) are ensured. Similarly, in Experiment No. 2 of Table 2. Nos. 19 and 20 satisfy the components in the steel of the present invention and contain steel type Nos. 7 is an example in which the influence of the average cooling rate (CR) after the hot rolling is finished is mainly examined, and No. 7 satisfying the cooling condition defined in the present invention. No. 19 is a No. 19 that does not satisfy the above cooling conditions. Since a larger amount of martensite than 20 can be obtained, desired tensile strength and hole expansion ratio (λ) are ensured.

  Experiment No. 2 in Table 2 Nos. 15 to 17 satisfy the steel components of the present invention and contain steel elements Nos. 5 is mainly an example in which the influence of the heating temperature (SRT) of the slab was examined. If the heating temperature is 1000 ° C. or higher, a steel plate having the strength and stretch flangeability intended by the present invention can be obtained. You can see that

  No. in Table 2 No. 18 is a steel type No. 18 in Table 1 that satisfies the components in the steel of the present invention and contains preferred selective elements. 6 was produced under the conditions specified in the present invention, and a steel sheet excellent in strength and stretch flangeability was obtained.

Experiment No. 1 in Table 3 Nos. 21 to 23 are steel types No. 1 in Table 1 that satisfy the components in steel of the present invention. 8 is an example in which the influence of hot rolling finishing temperature (FDT) is mainly examined, and No. 8 in which FDT satisfies the Ar ₃ transformation point or higher. In Nos. 21 and 22, since a predetermined martensite amount is ensured, a steel sheet satisfying the desired strength and stretch flangeability is obtained, whereas the FDT is lower than the Ar ₃ transformation point. In 23, a predetermined amount of martensite could not be obtained, and the strength and stretch flangeability deteriorated.

  Experiment No. 1 in Table 3 No. 24 is a steel type No. 24 in Table 1 that satisfies the components in the steel of the present invention. 9 is an example of the steel sheet manufactured under the conditions specified in the present invention, and a steel sheet excellent in strength and stretch flangeability was obtained.

  Experiment No. 1 in Table 3 25, and no. Nos. 26 to 28 are steel types No. 1 in Table 1 whose C amount is less than the range of the present invention. 10 and no. 11 is an example in which the amount of martensite is insufficient and both the tensile strength and the hole expansion rate (λ) are low.

  Experiment No. 1 in Table 3 No. 29 is a steel grade No. in Table 1 whose C amount exceeds the specified range of the present invention. Since 12 was used, the hole expansion rate (λ) was low. Experiment No. 1 in Table 3 30 to 32 are steel types No. 1 in Table 1 that do not contain Mo. In this example, the hole expansion rate (λ) is low. Experiment No. 1 in Table 3 No. 33 is a steel type No. in Table 1 in which the C amount exceeds the specified range of the present invention and Mo is not added. 14 is used, the hole expansion rate (λ) is low.

  Experiment No. 1 in Table 3 Nos. 34 and 35 are steel types No. 1 in Table 1 whose Si content is below the specified range of the present invention. 15 was used, the formation of cementite could not be sufficiently suppressed, and the hole expansion rate (λ) was low.

  Experiment No. 1 in Table 3 No. 36 is a steel type No. 36 in Table 1 in which both the Si content and the Mn content are less than the specified range of the present invention. Since No. 16 was used, the amount of martensite was insufficient, and both the tensile strength and the hole expansion rate (λ) were low. Experiment No. 1 in Table 3 Nos. 37 to 39 are steel types No. 1 in Table 1 whose Cr content is less than the specified range of the present invention. 17 was used, the amount of martensite was insufficient, and both the tensile strength and the hole expansion rate (λ) were low.

  Experiment No. 4 in Table 4 40 to 45 are steel types No. 1 in Table 1, respectively. 18-23 (both of which contain the components in the steel of the present invention or preferred selection components) and are manufactured under the conditions specified in the present invention, and a steel sheet excellent in strength and stretch flangeability is obtained. It was.

  Further, FIGS. 1 to 5 show, based on the above experimental data, the martensite amount, the C content, the Mo content, the coiling temperature (CT), and the hot rolling that affect the tensile strength (TS) and the stretch flange characteristic (λ). It is the graph which arranged and showed the influence of later average cooling rate (CR). In each figure, even if the items on the horizontal axis satisfy the requirements of the present invention (thick solid line region), those where the chemical composition of steel and the hot rolling conditions deviate from the requirements of the present invention are indicated by x Yes. In each figure, for reference, the effect of the amount of martensite on the product (TS × λ) of tensile strength (TS) and stretch flange characteristic (λ) is also shown. The larger the value of “TS × λ”, the better the balance between tensile strength and stretch flange characteristics.

  Specifically, FIG. 1 is a graph showing the effect of the amount of martensite on tensile strength and stretch flange characteristics, and the x mark in the graph indicates that the chemical composition and hot rolling conditions are outside the scope of the present invention. Even if the amount of martensite exceeds 80% in area ratio, the hole expansion ratio (λ) is low.

  FIG. 2 is a graph showing the influence of the C content on the tensile strength and stretch flange characteristics. In the graph, the X mark indicates that chemical components other than C and hot rolling conditions are outside the requirements of the present invention. Therefore, even if the C content is within the specified range of the present invention, the hole expansion rate (λ) is low.

  FIG. 3 is a graph showing the influence of the Mo content on the tensile strength and stretch flange characteristics. In the graph, the X mark indicates that chemical components other than Mo and hot rolling conditions are outside the requirements of the present invention. Therefore, even if the Mo content is within the specified range of the present invention, the hole expansion ratio (λ) is low.

  Fig. 4 is a graph showing the influence of the coiling temperature (CT) on the tensile strength and stretch flange characteristics. The x marks in the graph indicate the hot rolling conditions other than the chemical composition and coiling temperature (CT). Since it deviates from the requirement of the invention, the hole expansion rate (λ) is low even when the coiling temperature (CT) is within the specified range.

  FIG. 5 is a graph showing the influence of the average cooling rate (CR) after hot rolling on the tensile strength and stretch flange characteristics, and the x marks in the graph indicate heat other than chemical components and average cooling rate (CR). Since the extending condition is out of the specified requirement of the present invention, the hole expansion rate (λ) is low even if the average cooling rate (CR) is within the specified range.

  In each of the above figures, when the data of the ○ mark excluding the X mark is seen, in any graph, high strength and stretch flange characteristics can be obtained within the specified range of the present invention, and the strength and stretch flange characteristics can be obtained. It can be confirmed that product balance is obtained.

  Note that FIG. 3 and 31 are diagrams showing a microstructure observed by a transmission electron microscope, even when the coiling temperature (CT) is the same (300 ° C.), No. 3 to which an appropriate amount of Mo was added. No precipitate was generated in No. 3, while No. No Mo addition. In No. 31, a large number of precipitates are formed, which seems to have an adverse effect on stretch flangeability (λ).

It is a graph which shows the influence of the amount of martensite which affects the tensile strength and the stretch flange characteristic in an Example. It is a graph which shows the influence of C content which has on the tensile strength and the stretch flange characteristic in an Example. It is a graph which shows the influence of Mo content which has on the tensile strength and stretch flange characteristic in an Example. It is a graph which shows the influence of coiling temperature (CT) which has on the tensile strength and stretch flange characteristic in an Example. It is a graph which shows the influence of the average cooling rate (CR) after hot rolling which has on the tensile strength and the stretch flange characteristic in an Example. Experiment No. in Example It is a figure which shows the microstructure of the result of having observed about 3 and 31 with the transmission electron microscope.

Claims (6)

C: 0.03 to 0.10% (in the case of chemical components, represents mass%, the same shall apply hereinafter),
Si: 0.2-2.0%,
Mn: 0.5 to 2.5%
Al: 0.02 to 0.10%,
Cr: 0.2 to 1.5%
Mo: 0.1 to 0.5%,
A high-strength hot-rolled steel sheet excellent in stretch flangeability, characterized in that the balance is made of steel consisting of iron and inevitable impurities, and 80% by area or more of the longitudinal cross-sectional structure is a martensite structure.   The high-strength hot-rolled steel sheet according to claim 1, wherein the steel contains B: 0.0050% or less (not including 0%) as another component.   The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein the steel further contains Ni: 2.0% or less (not including 0%) as another element.   The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein the steel further contains Ca: 0.0050% or less (not including 0%) as another element.   The high-strength hot-rolled steel sheet according to any one of claims 1 to 4, having a tensile strength of 980 MPa or more. When hot rolling is performed after heating the steel slab satisfying the component composition according to any one of claims 1 to 4, after hot rolling is finished at a finishing temperature not lower than the Ar ₃ transformation point, 30 The steel sheet is cooled at an average cooling rate of at least ° C / sec and wound at a temperature of less than 400 ° C to obtain a high-strength hot-rolled steel sheet according to any one of claims 1 to 5, and has excellent stretch flangeability A high strength hot-rolled steel sheet.