JP2013133524A - High-tensile hot-rolled steel sheet excellent in blanking property and stretch flange formability, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-tensile hot-rolled steel sheet with a high tensile strength of 780-900 MPa excellent in blanking property and stretch flange formability, and a manufacturing method therefor.SOLUTION: Molten steel with a composition containing, by mass%, 0.03-0.06% C, 0.0020-0.0100% N, and 0.135-0.195% Ti with Si, Mn, and Al being adjusted to an appropriate amount is made into a steel raw material by a continuous casting method at a casting speed of 5 m/min or lower. The steel raw material is heated to a temperature of 1,250°C or higher, and rough rolling is started at a temperature of 1,200°C or higher. Hot rolling is finished at 900°C or higher. The steel raw material is cooled and is made into a wound hot rolled steel sheet at a winding temperature of 550°C or higher. The high-tensile hot-rolled steel sheet has a structure in which Ti carbide with a mean diameter of less than 6 nm is dispersed and precipitated in the ferrite crystal grain, the ferrite crystal grain occupies 95% or more of the metal structure in the area rate, TiN with a mean size of 20 nm or more is dispersed in the metal structure; and has a high tensile strength TS of 780-900 MPa and excellent blanking property and stretch flange formability.

Description

  The present invention is suitable as a structural member for parts such as automobiles and other transportation machinery, switchboards and buildings, and has a high tensile strength TS with a high strength of 780 to 900 MPa. The present invention relates to a hot-rolled steel sheet and a method for manufacturing the hot-rolled steel sheet, and particularly to improvement of punchability.

In recent years, improving the fuel efficiency of automobiles has always been an important issue in the automobile industry in order to reduce carbon dioxide CO ₂ emissions from the viewpoint of protecting the global environment. It is effective to reduce the weight of an automobile body to improve the fuel efficiency of the automobile, but it is necessary to reduce the weight of the vehicle body while maintaining the strength of the automobile body. If the strength of the steel sheet used for automobile parts is increased and the material is made thinner, the weight of the car body can be reduced without reducing the strength of the car body. For example, increasing the strength of steel sheets for automobile undercarriage parts leads to a significant reduction in weight of the automobile body, and is an extremely effective means for improving the fuel efficiency of automobiles. For these reasons, recently, there is a strong demand for increasing the strength of these component materials. In addition, with respect to other structural members, there is a strong demand for higher strength in order to reduce weight.

  As the strength of steel sheets increases, workability, particularly stretch flangeability, deteriorates, and various techniques have been proposed for increasing strength and improving stretch flangeability. Among them, the technology using a structure in which the metal structure is substantially a single ferrite phase and fine carbides are precipitated in the ferrite phase grains is used to combine high strength and excellent stretch flangeability. It is known to be a useful technique.

  As a high-tensile steel plate having a substantially single-phase ferrite phase metal structure and excellent stretch flangeability, for example, Patent Document 1 discloses an ultrafine ferrite structure high-strength hot-rolled steel plate excellent in stretch flangeability. Has been proposed. The technique described in Patent Document 1 is wt%, C: 0.01 to 0.10%, Si: 1.5% or less, Mn: more than 1.0 to 2.5%, P: 0.15% or less, S: 0.008% or less, Al: 0.01 ~ 0.08%, total of one or two of Ti and Nb: composition containing 0.10 ~ 0.60%, ferrite content is 95% or more by area ratio, and the average grain size of ferrite is 2.0 ~ 0.0μm, It is a high-strength hot-rolled steel sheet with an ultrafine ferrite structure that has a structure free from martensite and retained austenite, has a tensile strength of 490 MPa or more, and is excellent in stretch flangeability. According to the technique described in Patent Document 1, by setting the Mn content to more than 1.0% to 2.5%, the strength of the steel sheet is improved and fine ferrite grains are obtained, thereby achieving high strength. Further, in the technique described in Patent Document 1, fatigue strength is also improved.

Further, in Patent Document 2, in mass%, C: 0.01 to 0.1%, S: 0.03% or less, N: 0.005% or less, Ti: 0.05 to 0.5%, Si: 0.01 to 2%, Mn: 0.05 to 2 %, P: 0.1% or less, Al: 0.005 to 1.0%, and (Ti-48 / 12C-48 / 14N-48 / 32S) containing Ti in a range satisfying 0% or more, and in steel with an average size of 10 ¹ to 10 ³ nm of precipitates containing 5nm or more Ti particle, the minimum interval is less than 10 ¹ nm ultra 10 ⁴ nm, the tensile strength of more than 640MPa at the burring workability fatigue properties An excellent hot-rolled steel sheet has been proposed.

Patent Document 3 contains, in mass%, C: 0.02 to 0.08%, Si: 0.01 to 1.5%, Mn: 0.1 to 1.5%, Ti: 0.03 to 0.06%, P: 0.1% or less, S: 0.005% or less, Al: 0.5% or less, N: 0.009% or less, further, the total content of Nb, Mo, V is limited to 0.01% or less, Ti / C: 0.375 to 1.6 An alloy-saving high-strength hot-rolled steel sheet with an average diameter of TiC precipitates in crystal grains of 0.8 to 3 nm, an average number density of 1 × 10 ¹⁷ / cm ³ and a tensile strength of 540 to 650 MPa has been proposed. ing.

JP 2000-328186 A JP 2002-161340 A JP 2011-26690 A

  Many automobile parts made of steel plates are formed by pressing. Prior to pressing, the steel sheet is punched into a shape suitable for forming called a blank. When punching, it is important that the properties of the punched end face are not disturbed. As shown in FIG. 1, the punched end surface is divided into a sheared surface and a fracture surface, and “the punched end surface is disturbed” means that the properties of the fractured surface are particularly rough. And, “the property of the fractured surface becomes rough” means that a large opening occurs in the fractured surface, the sheared surface part and the fractured surface part are separated so as to be separated, or the fractured surface is abnormally different from the sheared surface. When it is tilted. The presence of an opening at the fracture surface or an opening where the shearing surface part and the fracture surface part peel off may cause cracks during subsequent pressing, and deteriorate the appearance and corrosion resistance of the part. Let In addition, the fact that the fracture surface is inclined with respect to the shearing surface deteriorates the dimensional accuracy of the blank and causes the dimensional accuracy of the final product (part) to be reduced.

None of Patent Document 1, Patent Document 2, and Patent Document 3 have a description of punchability, and the punchability has not been studied in detail so far.
From the viewpoint of punchability, in the technique described in Patent Document 1, since the Mn content of the steel sheet is high, there are many places where Mn segregates in the steel sheet (Mn segregation part). Cracks are likely to occur from the segregated part. Therefore, it is difficult to say that the properties of the punched end face fracture surface in the steel sheet manufactured by the technique described in Patent Document 1 are good.

  In the technique described in Patent Document 2, a predetermined amount of Ti is contained, and Ti carbide is formed to reduce the solid solution C. However, according to the study by the inventors, when an excessive amount of Ti is contained with respect to C, Ti carbide tends to be coarsened, and it is difficult to stably ensure high strength of, for example, a tensile strength of 780 MPa or higher. Further, since C is completely fixed by Ti, coarse cementite is not generated, and microvoids are hardly generated at the time of punching. For this reason, one microvoid tends to grow greatly on the punched end face, and the microvoid grows in the direction parallel to the plate surface in the vicinity of the boundary between the shear plane and the fracture surface, and an opening is likely to be formed. In addition, the angle of inclination of the fracture surface with respect to the shear surface also increases, and there is a problem that the dimensional accuracy of the blank is lowered. Therefore, it is difficult to say that the hot-rolled steel sheet manufactured by the technique described in Patent Document 2 is a hot-rolled steel sheet having excellent punchability.

In the technique described in Patent Document 3, the strength of the hot-rolled steel sheet obtained as shown in the examples is at most about 540 MPa class tensile strength, which is higher than 780 MPa class tensile strength. There is a problem that it is difficult to ensure the strength stably.
The present invention advantageously solves the problems of the prior art, and has a high tensile strength TS: 780 MPa or more and 900 MPa or less, which is suitable as a material for automobile parts, and has excellent punchability and excellent stretch flange processing. It aims at providing the high-tensile-strength hot-rolled steel plate which combines the property, and its manufacturing method.

In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting stretch flange workability and punchability.
When a material (steel plate) is punched into a predetermined shape with a punch, first, a number of microvoids are generated in the material (steel plate) in the vicinity of the tip of the punch that is recessed into the material (steel plate). And they connect and unite, and grow into a big crack. The grown crack penetrates the material (steel plate) in the thickness direction, and punching is completed. At this time, the starting point of the void is mainly coarse inclusions and precipitates dispersed in the material (steel plate). However, if inclusions and precipitates are dispersed in the material (steel plate), the precipitates and inclusions are the starting point, and stretch flangeability is deteriorated. For these reasons, it has been conventionally considered that stretch flange workability and punchability are incompatible.

  Therefore, the present inventors diligently studied further in order to achieve both the punching performance and the stretch flange workability after increasing the strength. As a result, fine carbide containing fine Ti of less than 6 nm is dispersed in the ferrite crystal grains, and this ferrite crystal grain accounts for 95% or more in terms of the area ratio with respect to the total metal structure, and Ti nitride, which is also relatively coarse I came up with the idea of dispersing Ti nitride in a metallographic structure consisting of ferrite grains. Ti nitride is harder than ferrite grains and becomes the starting point for microvoid formation at the time of punching. In addition, if Ti nitride is relatively coarser than 20 nm, the dispersion of Ti nitride becomes sparse. In particular, it has been found that there is no significant adverse effect on stretch flangeability.

  In order to improve stretch flange workability, first, it is necessary to occupy the metal structure observed at about 500 to 5000 times with an optical microscope or a scanning electron microscope with ferrite crystal grains having a low dislocation density. If fine carbide containing Ti having a size of less than 6 nm is densely dispersed and precipitated in the ferrite crystal grains, the desired high strength can be secured. Furthermore, it is possible to improve punchability while maintaining desired high strength and excellent stretch flangeability by sparsely dispersing TiN of 20 nm or more in the metal structure consisting of ferrite crystal grains. I found it.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.03-0.06%, Si: 0.1% or less, Mn: 0.6% or less, P: 0.025% or less, S: 0.02% or less, N: 0.0020-0.0100%, Al: 0.1% or less , Ti: 0.135 to 0.195%, the composition comprising the balance Fe and inevitable impurities, and fine carbide containing Ti having an average diameter of less than 6 nm are dispersed in the ferrite crystal grains, and the ferrite crystal grains are all metals. The structure occupies an area of 95% or more of the structure, and has a structure in which TiN having an average size of 20 nm or more is dispersed in the metal structure made of the ferrite crystal grains, and has excellent punchability and excellent stretch flange A high-tensile hot-rolled steel sheet with a tensile strength of 780 to 900 MPa, characterized by having workability.
(2) In (1), in addition to the said composition, B: 0.0035% or less is further contained in the mass%, The high-tensile-strength hot-rolled steel plate characterized by the above-mentioned.
(3) In (1) or (2), in addition to the above-mentioned composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta are also contained in mass%. , REM, V, Cs, Zr, Hf, containing a total of 1% or less, high tensile hot-rolled steel sheet,
(4) The high-tensile hot-rolled steel sheet according to any one of (1) to (3), wherein the steel sheet surface has a plating film.
(5) By mass%, C: 0.03-0.06%, Si: 0.1% or less, Mn: 0.6% or less, P: 0.025% or less, S: 0.02% or less, N: 0.0020-0.0100%, Al: 0.1% or less , Ti: 0.135 to 0.195% containing molten steel having the balance Fe and unavoidable impurities is used as a steel material by a continuous casting method with a casting speed of 5 m / min or less. After heating to 1250 ° C or higher, hot rolling consisting of rough rolling at a rolling start temperature of 1200 ° C or higher and finish rolling at a rolling end temperature of 900 ° C or higher is performed, followed by cooling and winding temperature: 550 A method for producing a high-tensile hot-rolled steel sheet having a tensile strength of 780 to 900 MPa, excellent in punchability and stretch flangeability, characterized by winding at a temperature of ℃ or higher.
(6) In (5), in addition to the said composition, B: 0.0035% or less is further contained in the mass%, The manufacturing method of the high-tensile hot-rolled steel plate characterized by the above-mentioned.
(7) In (5) or (6), in addition to the above-mentioned composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta are further contained in mass%. , REM, V, Cs, Zr, Hf, or a total of 1% or less.

  According to the present invention, the tensile strength TS: high strength of 780 to 900 MPa, which is suitable as a structural member such as parts for transportation machinery such as automobiles, switchboards and buildings, etc., and excellent punchability High-strength hot-rolled steel sheet that combines excellent stretch-flange formability with excellent industrial advantages.

It is explanatory drawing which shows the property of a punching end surface, and the measuring method of opening part length.

Hereinafter, the present invention will be described in detail.
The hot-rolled steel sheet of the present invention is in mass%, C: 0.03-0.06%, Si: 0.1% or less, Mn: 0.6% or less, P: 0.025% or less, S: 0.02% or less, N: 0.0020-0.0100%, Al : 0.1% or less, Ti: 0.135 to 0.195% is contained, and the composition is composed of the remaining Fe and inevitable impurities. First, the reason for limiting the composition of the hot-rolled steel sheet of the present invention will be described. Hereinafter, mass% is expressed as% unless otherwise specified.

C: 0.03-0.06%
C has the effect of forming fine carbides and increasing the strength of the steel sheet. In order to ensure a high strength of 780 MPa or more, which is a desired tensile strength, a content of 0.03% or more is required. If it is less than 0.03%, fine carbides for obtaining a tensile strength of 780 MPa class cannot be secured. On the other hand, if the content exceeds 0.06%, the strength increases excessively and pearlite is easily formed. Since pearlite serves as a starting point for generating voids during stretch flange processing, the formation of pearlite is a factor that reduces stretch flange processability. For this reason, C was limited to the range of 0.03-0.06%. In addition, Preferably it is 0.04-0.06%, More preferably, it is 0.04-0.05%.

Si: 0.1% or less In the present invention, since ferrite grains are strengthened with fine carbides containing Ti, it is not necessary to contain Si. If Si is contained in an amount exceeding 0.1%, an opening along the segregation is likely to occur at the time of punching due to segregation, and the occurrence of burrs becomes significant. For this reason, Si was limited to 0.1% or less. In addition, Preferably it is 0.05% or less.

Mn: 0.6% or less
When Mn exceeds 0.6%, segregation of Mn tends to occur. In the Mn segregation portion (micro segregation), an opening is likely to occur along the segregation during punching, and therefore punchability is reduced. For these reasons, Mn is limited to 0.6% or less. In addition, Preferably it is 0.5% or less.
P: 0.025% or less When P is contained in a large amount exceeding 0.025%, segregation becomes prominent, opening tends to occur along the segregation, and punchability decreases. In the present invention, it is preferable to reduce as much as possible from the viewpoint of suppressing segregation. Therefore, P is limited to 0.025% or less. In addition, Preferably it is 0.020% or less.

S: 0.02% or less In the present invention containing Mn and Ti, S combines with Ti to form TiS and Mn to form MnS. These sulfides precipitate at the ferrite grain boundaries and reduce stretch flangeability. For this reason, S was limited to 0.02% or less. In addition, Preferably it is 0.005% or less, More preferably, it is 0.001% or less.

N: 0.0020-0.0100%
N is an important element in the present invention, and combines with Ti to form hard TiN, which becomes a starting point for microvoid formation at the time of punching, reduces the roughness of the punched fracture surface, and improves punchability. If the N content is less than 0.0020%, the TiN content decreases, the starting point of microvoid formation at the time of punching decreases, the punched fracture surface becomes rough, and the punchability decreases. On the other hand, when the content exceeds 0.0100%, TiN aggregates and becomes coarse, so that stretch flangeability is lowered and punchability is also lowered. For this reason, N was limited to the range of 0.0020 to 0.0100%.

Al: 0.1% or less
Al is an element that acts as a deoxidizer. In order to acquire such an effect, it is desirable to contain 0.001% or more. On the other hand, if the content exceeds 0.1%, the deoxidized product is aggregated and coarsened, so that both stretch flangeability and punchability are reduced. For this reason, Al was limited to 0.1% or less.

Ti: 0.135 to 0.195%
Ti is the most important element in the present invention. Ti forms fine carbides and contributes to increasing the strength of steel sheets while maintaining excellent stretch flangeability. In order to obtain such an effect, the content of 0.135% or more is required. If the Ti content is less than 0.135%, the desired high strength cannot be secured. On the other hand, if it is contained in a large amount exceeding 0.195%, a carbide containing coarse Ti is likely to be produced, and a desired tensile strength cannot be obtained. For this reason, Ti was limited to the range of 0.135 to 0.195%. In addition, Preferably it is 0.145 to 0.175%.

The above-mentioned components are basic components, but as a selective element in addition to the basic composition, B: 0.0035% or less, and / or Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, One or two or more of W, Nb, Pb, Ta, REM, V, Cs, Zr, and Hf can be selected and contained as required in total.
B: 0.0035% or less B is an element that segregates at austenite grain boundaries, delays ferrite transformation after rolling, and refines Ti carbide. In order to acquire such an effect, it is desirable to contain 0.0010% or more. On the other hand, when the content exceeds _{0.0035%, Fe 23 (CB)} 6 precipitates, punching is lowered. For this reason, when it contains, it is preferable to limit B to 0.0035% or less.
In addition to the above components, the present invention includes Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, and Hf. 1 type or 2 types or more may be contained, but when it is contained, the total content is preferably 1% or less. More preferably, the total content is 0.5% or less.

The balance other than the components described above consists of Fe and inevitable impurities.
In addition to the above composition, the hot-rolled steel sheet of the present invention includes ferrite crystal grains in which fine carbides containing Ti having an average diameter of less than 6 nm are dispersed, and a metal structure in which the ferrite crystal grains occupy 95% or more in area ratio. In addition, the metal structure composed of the ferrite crystal grains has a structure in which TiN having an average size of 20 nm or more is dispersed.

Next, the reason for limiting the structure of the hot-rolled steel sheet of the present invention will be described.
Metal structure in which ferrite crystal grains occupy 95% or more in area ratio In order to ensure excellent stretch flangeability, it is effective to configure the metal structure with ferrite crystal grains having excellent ductility with low dislocation density. The phrase “consisting of ferrite crystal grains” as used herein can ensure desired characteristics not only when 100% is occupied by ferrite crystal grains but also when substantially 100% is occupied by ferrite crystal grains. “Substantially occupied by ferrite crystal grains” means a structure in which ferrite crystal grains are 95% or more in terms of the area ratio with respect to the entire metal structure. The area ratio is preferably 97% or more. The metal structure in this case refers to a structure that can be observed when observed with an optical microscope or a scanning electron microscope at a magnification of 500 to 5000 times.

In the hot-rolled steel sheet of the present invention, examples of the structure other than the ferrite crystal grains include cementite, pearlite, bainite phase, martensite phase, retained austenite phase, and the like. Less than about, preferably less than about 3% is acceptable.
Average diameter of fine carbides containing Ti: less than 6nm
The carbide containing Ti has a strong tendency to be a fine carbide having an extremely small average diameter, and the strength of the hot-rolled steel sheet can be increased by dispersing and precipitating such fine carbide in the ferrite crystal grains.

  From the viewpoint of increasing the strength, it is important to reduce the particle size of the fine carbide containing Ti. In order to ensure the desired high strength (tensile strength: 780 MPa or more) in the present invention, it is sufficient that the average diameter of fine carbides containing Ti is 10 nm or less. However, due to the relatively coarse TiN dispersed and deposited to ensure excellent punchability, a slight decrease in stretch flangeability is inevitable, so Ti is included to compensate for this decrease in stretch flangeability. The average diameter of the fine carbide was set to less than 6 nm. This is to keep the adverse effects on stretch flangeability of fine carbides (TiC, etc.) containing Ti as small as possible. By limiting the average diameter of fine carbides containing Ti (such as TiC) to be small, it is possible to sufficiently prevent the stretch flangeability from being lowered by TiN. Note that when the average diameter of the fine carbide containing Ti is 3 nm or less, the stretch flangeability is significantly improved. If the average diameter of the fine carbide containing Ti is less than 0.6 nm, the dislocation proceeds by bypassing the carbide containing Ti, and the desired high strength cannot be secured. For this reason, it is preferable that the average diameter of the carbide containing Ti is 0.6 nm or more.

Average size of TiN: 20nm or more
Dispersing TiN is extremely important in the present invention. In the present invention, Ti is contained and fine carbides containing Ti are dispersed and precipitated to achieve high strength of the steel sheet. Therefore, TiN is also dispersed and precipitated by containing Ti. Conventionally, TiN has been attempted to suppress dispersion precipitation as a precipitate that reduces stretch flangeability. However, in the present invention, TiN is actively used to improve punchability. In order to improve punchability, the average size of TiN was limited to 20 nm or more. The relatively coarse TiN of 20 nm or more acts as a starting point for microvoids during punching. On the other hand, when the particle size exceeds 1 μm, TiN agglomerates, the number of TiNs decreases, the number of microvoids at the time of punching decreases, the punched fracture surface becomes rough, and the punchability decreases. For this reason, the average size of TiN dispersed and precipitated was limited to 20 nm or more, preferably 1 μm or less. More preferably, it is 500 nm or less.

  Moreover, this invention hot-rolled steel plate may form a plating film on the surface. By forming a plating film on the surface of the steel sheet, the corrosion resistance of the hot-rolled steel sheet is improved, and a hot-rolled steel sheet suitable as a material for parts exposed to severe corrosive environments, for example, parts for automobile undercarriage parts. Examples of the plating film include a hot dip galvanized film, an alloyed hot dip galvanized film, and an electroplated film.

Next, the preferable manufacturing method of this invention hot-rolled steel plate is demonstrated.
The steel material having the above composition is subjected to hot rolling consisting of rough rolling and finish rolling, then cooled and wound to obtain a hot rolled steel sheet.
In the production method of the present invention, a steel material (slab) made by continuous casting obtained by melting molten steel having the above composition and casting it by a continuous casting method is used as a starting material.

In the present invention, the method for melting molten steel is not particularly limited, and any conventional melting method such as a converter or an electric furnace can be applied. The molten steel having the above composition is cast into a steel material (slab or the like) having a predetermined shape using a continuous casting method.
In the present invention, in order to improve punchability, it is necessary to adjust the size of TiN to be equal to or larger than a predetermined average size. Since TiN precipitates mainly at a high temperature of 1200 ° C or higher, the present invention adjusts the drawing speed (casting speed) of the steel material (slab) immediately after casting, that is, the casting speed during continuous casting, so that TiN has a desired size. Adjust to. In the present invention, the casting speed during continuous casting is limited to 5 m / min or less. When the casting speed exceeds 5 m / min, the average size of TiN becomes as fine as less than 20 nm, so that the punchability improvement effect cannot be obtained. For this reason, the casting speed during continuous casting is limited to 5 m / min or less. In addition, Preferably it is about 0.5-1.5 m / min.

  The obtained continuous cast steel material (slab) is then subjected to hot rolling. In performing hot rolling, the steel material is reheated to a temperature of 1250 ° C. or higher in a heating furnace. Since the size of TiN is controlled by precipitating and growing at a high temperature of 1200 ° C or higher, if it is reheated to a high temperature of 1250 ° C or higher, TiN can be stably grown to a specified value (20nm or higher). Can be adjusted. If the heating temperature is less than 1250 ° C, fine TiN may be present, and a sufficient punching effect may not be obtained. When heated to a high temperature exceeding 1350 ° C., TiN becomes abnormally coarse and stretch flangeability deteriorates. For this reason, the heating temperature of the steel material is limited to 1250 ° C. or higher, preferably 1350 ° C. or lower.

  The heated steel material is then subjected to hot rolling comprising rough rolling and finish rolling. In the present invention, hot rolling starts at a temperature of 1200 ° C. or higher. That is, the start of rough rolling is 1200 ° C. or higher. If the rolling start temperature is lower than 1200 ° C., fine TiN is likely to occur due to strain-induced precipitation by rolling, and the punchability is lowered. For this reason, the hot rolling (rough rolling) start temperature is limited to 1200 ° C. or higher.

The conditions for rough rolling are not particularly limited as long as a rough rolling bar having a predetermined shape can be secured, and conditions other than the rolling start temperature are not particularly limited.
After the rough rolling, finish rolling is performed.
Finish rolling is rolling with a finish rolling finishing temperature of 900 ° C. or higher. When the finish rolling finish temperature is less than 900 ° C., the ferrite grains tend to extend in the rolling direction, so that an opening is likely to be formed on the fracture surface after punching. For this reason, the finish rolling finish temperature was limited to 900 ° C. or higher.
After finishing rolling, it is cooled and wound at a winding temperature of 550 ° C or higher.

The cooling after finishing rolling is not particularly limited, but is preferably 50 ° C./s or more at an average cooling rate up to 750 ° C. from the viewpoint of strength. If the cooling rate is below 50 ° C./s on average, it becomes difficult to substantially occupy the metal structure with ferrite crystal grains. For this reason, the average cooling rate is preferably 50 ° C./s or more.
When the coiling temperature is less than 550 ° C., the structure exhibits a metal structure in which a bainite phase is mixed in a band shape. When such a band-like bainite phase is mixed, an opening is formed on the punched surface, and the punchability is lowered. For this reason, the coiling temperature was limited to 550 ° C. or higher. In addition, Preferably it is 600 degreeC or more. The upper limit of the coiling temperature is preferably 750 ° C. or less from the viewpoint of strength.

  In addition, you may form a plating film in the steel plate surface with a plating process with respect to the hot-rolled steel plate manufactured as mentioned above. As a plating process, for example, even if a hot dip galvanizing process is performed to form a hot dip galvanized film, or after the hot dip galvanizing process, an alloying process is further performed to form an alloyed hot dip galvanized film on the surface of the steel sheet. May be. Also, an electroplating treatment may be performed to form an electroplating film.

  Hereinafter, the present invention will be further described in detail based on examples.

The molten steel having the composition shown in Table 1 was melted in a converter and then continuously cast at the casting speed (drawing speed) shown in Table 2 to obtain a slab (steel material) having a thickness of 270 mm. Only steel 1O had a wall thickness of 50 mm. Next, these slabs were heated to the heating temperature shown in Table 2, and then rough rolled at the rolling start temperature shown in Table 2, and then finish rolling was finished at the finish rolling end temperature shown in Table 2. Table 2 Cool at the indicated cooling rate. After cooling, it was wound at the winding temperature shown in Table 2 to obtain a hot-rolled steel sheet (steel strip) having a thickness of 2.0 mm. For some hot-rolled steel plates (steel plates No. 8, No. 9, No. 31, No. 32, No. 33), after pickling to remove the scale of the surface layer, hot dip galvanizing treatment ( Dip in a 480 ° C galvanizing bath (0.1% Al-Zn bath) to form a hot-dip galvanized film with a coating weight of 48g / m ² and further alloying at 520 ° C for alloying and melting A galvanized steel sheet was obtained.

Test pieces were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, and hole expansion test. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained hot-rolled steel sheet, a cross section (L cross section) parallel to the rolling direction is mechanically polished and corroded with a nital solution, and then a scanning electron microscope (magnification: The tissue was observed and imaged at 3000x. Using the obtained structure photograph, the type of structure other than the ferrite phase and the ferrite phase, and the structure fraction (area ratio) thereof were determined by an image analysis apparatus.

  Moreover, the thin film for transmission electron microscope observation was produced from the obtained hot-rolled steel plate, and it observed with the transmission electron microscope, and calculated | required the average diameter of the fine carbide | carbonized_material containing Ti, and the average size of TiN. Note that fine carbides containing Ti were imaged by observing 30 or more fields at 340,000 times. By image analysis using the obtained structure photograph, the diameter of the fine carbide containing a total of 300 or more Ti was obtained by circular approximation, and those values were arithmetically averaged to include Ti in the steel sheet (test piece). The average diameter of fine carbides was used.

In addition, the size of TiN was observed and imaged by 10 times or more at 1000 times, and the obtained tissue photograph was used to obtain the size of 30 or more TiN in total by image analysis. Since TiN is a cube or a rectangular parallelepiped, its size is the average of the length of one side in a cube and the longest and shortest sides in a rectangular parallelepiped, and the value is obtained as the size of each TiN. Arithmetic average was taken as the average TiN size in the steel sheet (test piece).
(2) Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece (GL: 50mm) with the direction perpendicular to the rolling direction as the tensile direction was sampled, and the tensile test was performed in accordance with the provisions of JIS Z 2241. And tensile strength TS was determined.
(3) Punchability test A test piece (size: 50 mm x 50 mm) was sampled from the obtained hot-rolled steel sheet and punched with a 30 mmφ cylindrical punch. The clearance was 25%. After punching, the punched fracture surface of the test piece was observed, and the opening length of the fracture surface was measured. The opening length was calculated as follows (see FIG. 1).

Draw a fan that connects both ends of the opening and the center of the punched hole, and measure the center angle θ _i (rad.). Total angle θ obtained

And the product rθ of the fan-shaped radius r (15 mm) was obtained as the length of the opening. The case where the opening length was 10 or less was evaluated as having good punching workability.
(4) Hole Expansion Test A hole expansion test piece (size: 130 mm × 130 mm) was taken from the obtained steel sheet, and a hole with an initial diameter do (10 mmφ) was formed by punching the hole expansion test piece. Using these test pieces, a hole expansion test was performed. In the test, a conical punch having a vertex angle of 60 ° is inserted into a hole formed by punching, the hole is expanded, and the diameter d of the hole when the crack penetrates the steel plate (test piece) is measured. The hole expansion rate λ (%) defined by the following formula was calculated.

Hole expansion rate λ (%) = {(d−do) / do} × 100
The case where the hole expansion ratio λ was 100% or more was evaluated as excellent in stretch flangeability.
The obtained results are shown in Table 3.

  Each of the examples of the present invention is a high-tensile hot-rolled steel sheet that has a high tensile strength TS: 780 MPa or more, a good stretch flangeability, and a good punching property. On the other hand, in the comparative example that is out of the scope of the present invention, the desired high strength cannot be ensured, the punchability is lowered, or the stretch flange workability is lowered.

Claims (7)

% By mass
C: 0.03-0.06%, Si: 0.1% or less,
Mn: 0.6% or less, P: 0.025% or less,
S: 0.02% or less, N: 0.0020 to 0.0100%,
Al: 0.1% or less, Ti: 0.135-0.195%
And a fine carbide containing Ti having an average diameter of less than 6 nm and a composition comprising the balance Fe and inevitable impurities is dispersed in the ferrite crystal grains, and the ferrite crystal grains have an area of 95% or more of the total metal structure And having a structure in which TiN having an average size of 20 nm or more is dispersed in a metal structure composed of the ferrite crystal grains, and having both excellent punchability and excellent stretch flangeability A high-tensile hot-rolled steel sheet with a characteristic tensile strength of 780 to 900 MPa.   The high-tensile hot-rolled steel sheet according to claim 1, further comprising B: 0.0035% or less by mass% in addition to the composition.   In addition to the above composition, any one of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, and Hf can be used. The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein the total content is 1% or less.   The high-tensile hot-rolled steel sheet according to any one of claims 1 to 3, further comprising a plating film on the surface of the steel sheet. % By mass
C: 0.03-0.06%, Si: 0.1% or less,
Mn: 0.6% or less, P: 0.025% or less,
S: 0.02% or less, N: 0.0020 to 0.0100%,
Al: 0.1% or less, Ti: 0.135-0.195%
After the molten steel having a composition comprising the balance Fe and inevitable impurities is made into a steel material by a continuous casting method with a casting speed of 5 m / min or less, the steel material is heated to a heating temperature of 1250 ° C. or higher. The rolling start temperature is 1200 ° C or higher, and the rolling end temperature is 900 ° C or higher and finish rolling is applied, followed by cooling and winding at a winding temperature of 550 ° C or higher. A method for producing a high-tensile hot-rolled steel sheet, which is characterized by a tensile strength of 780 to 900 MPa and excellent in punchability and stretch flangeability.   The method for producing a high-tensile hot-rolled steel sheet according to claim 5, further comprising B: 0.0035% or less by mass% in addition to the composition.   In addition to the above composition, any one of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Pb, Ta, REM, V, Cs, Zr, and Hf can be used. The method for producing a high-tensile hot-rolled steel sheet according to claim 5 or 6, wherein one or more types are contained in total of 1% or less.

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