JP4888255B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a hot-rolled steel sheet having a high tensile strength of 900 MPa or more and excellent stretch flangeability and its production.

  A so-called hot-rolled steel sheet manufactured by continuous hot rolling is widely used as a relatively inexpensive structural material in various industrial equipment including automobiles. In particular, in automobile parts that are required to be reduced in weight from the viewpoint of reducing fuel consumption, application of high-strength hot-rolled steel sheets that can maintain strength even when the weight is reduced is increasing. Recently, due to the increasing awareness of environmental issues, further weight reduction of the vehicle body is required. Therefore, hot-rolled steel sheets used by press forming are required to have both excellent strength and ductility, and particularly 870 MPa or more. In the ultra high strength steel sheet, a steel sheet that is excellent in stretch flange workability in addition to ductility is desired.

  Patent Document 1 discloses a steel sheet having a high strength of 900 MPa or more and excellent stretch flangeability. In order to obtain high strength, it is necessary to have a bainite-based structure, and furthermore, when the bainite ratio is low. It is disclosed that stretch flangeability is reduced. Therefore, according to the method described in this document, it is necessary to use a bainite-based structure in order to achieve both strength and stretch flangeability. However, a steel sheet having such a structure has a problem that it is inferior in ductility compared to a steel sheet having a ferrite-based structure that is more ductile than bainite.

Patent Document 2 discloses a steel sheet having a high strength of 900 MPa or more and excellent stretch flangeability and ductility. However, this steel sheet requires a large amount of Si and Al to obtain high ductility due to residual γ. For this reason, there are problems such as deterioration of surface properties, reduction of cleanliness in steel, and deterioration of weldability.
JP 2000-282175 A JP 2003-171736 A

  The present invention provides a high-strength hot-rolled steel sheet having excellent stretch flangeability in a high-strength region having a tensile strength of 900 to 1200 MPa, and having good chemical conversion treatment in a preferred embodiment, and a method for producing the same. For the purpose.

The present inventors diligently studied to solve the above problems. As a result, the following knowledge was obtained.
(A) By making the structure mainly composed of ferrite with high formability, it is possible to ensure the high ductility that was not obtained in the structure mainly composed of bainite and to remarkably exert the strength improving effect by precipitation strengthening.

  (B) When strengthening this ferrite with Ti, Nb, V, considering the balance between these effective precipitation strengthening elements and N and C and the content, and making the remaining structure other than ferrite bainite, good Stretch flangeability can also be realized.

(C) By reducing the difference in hardness between ferrite as the main phase and bainite as the remaining structure, stretch flangeability can be further enhanced.
(D) By optimizing the contents of Si, Al, and V, it is possible to ensure good chemical conversion properties while maintaining the above mechanical characteristics.

  (E) In order to obtain a steel sheet having the above mechanical properties efficiently and stably, the temperature history that determines the properties of ferrite, which plays an important role in the balance between strength and formability, is closely controlled, and in austenite By using the difference in solubility product of precipitation strengthening elements in ferrite and advancing ferrite transformation at an appropriate temperature range, it avoids coarsening of precipitates and insufficient precipitation, and exhibits a remarkable precipitation strengthening effect. It is important to appropriately control the temperature history determined by the C and Ti content balance in all processes from the start of hot rolling to winding.

This invention is based on said new knowledge, The summary is as follows.
(1) By mass%, C: 0.05% or more and less than 0.20%, Si: less than 1.0%, Mn: 0.7% or more and 2.0% or less, Al: more than 0.1% Less than 0%, Ti: 0.05% or more and 0.3% or less, Nb: 0.1% or less, V: 0.05% or more and 1.0% or less, P: 0.1% or less, S: 0.0. It contains not more than 01% and N: not more than 0.01%, has a balance of Fe and impurities, has a chemical composition satisfying the following formula (1), contains 50% by area or more of ferrite, and the balance is bainite The tensile strength TS is 900 MPa or more, and the product of the tensile strength TS (MPa) and the hole expansion ratio HER (%) is a TS × HER value of 45000 (MPa ·%) or more. Ri, the average value H of the Vickers hardness of the bainite and the average value Hv _alpha Vickers hardness of the ferrite hot-rolled _steel sheet _β and is characterized by satisfying the following formula (3).

Here, C, Ti, N, Nb, and V in the formula indicate the content (unit: mass%) of each element in the steel.

  (2) The hot rolled steel sheet according to claim 1, wherein the chemical composition satisfies the following formula (2).

  Here, Si, Al and V in the formula indicate the content (unit: mass%) of each element in the steel.

  (3) The chemical composition is characterized by containing one or more selected from the group consisting of Ca, Mg, Nd and B in place of a part of Fe in a total amount of 0.1% by mass or less. The hot-rolled steel sheet according to claim 1 or 2.

  (4) The chemical composition contains 0.6% by mass or less in total of one or two selected from Cr and Mo instead of a part of Fe. Hot rolled steel sheet according to crab.

(5) After the steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 is set to 1250 ° C. or higher, hot rolling is performed, and (Ae ₃ points + 100 ° C.) to (Ae ₃ points) Complete the hot rolling in the temperature range of −50 ° C.
The obtained hot-rolled steel sheet is subjected to primary cooling which starts cooling within 3 seconds after completion of the hot rolling and cools to a temperature range of 750 to 550 ° C. at an average cooling rate of 30 ° C./second or more,
Secondary cooling is performed by cooling for 3 to 20 seconds at an average cooling rate of 30 ° C./second or less from the cooling stop temperature of the primary cooling,
While performing the third cooling to cool from the cooling stop temperature of the secondary cooling to a temperature range of 550 ° C. or less at an average cooling rate of 30 ° C./second or more, winding up,
In the process from the start of the hot rolling to the winding, the time during which the plate thickness center temperature of the steel sheet is T _pef (° C.) or less (Ae ₃ points−50 ° C.) defined by the following formula (4) is 180. A method for producing a hot-rolled steel sheet, characterized in that it is not more than 2 seconds.

  Here, C and Ti in a formula show content (unit: mass%) of each element in steel.

  According to the present invention, a hot-rolled steel sheet having excellent strength and stretch flangeability even at a high strength of 900 MPa or more is obtained. If such a steel plate is used, for example, as a steel material for automobiles, it contributes to a significant reduction in vehicle weight.

  The best mode of the present invention, the range of manufacturing conditions, and the reasons for setting them will be described below. In the present specification, “%” representing the chemical composition is “% by mass” unless otherwise specified.

1. Chemical composition The chemical composition of the steel according to the present embodiment will be described.
C: 0.05% or more and less than 0.20% C is an element contributing to strength improvement, and is contained in an amount of 0.05% or more in order to obtain a tensile strength of 900 MPa or more. On the other hand, if the C content is excessive, ferrite transformation after hot rolling is delayed, the ferrite content is reduced, and ductility is deteriorated. Therefore, the C content is less than 0.20%. It is preferable to make it less than 0.16%. In order to obtain an ultrahigh strength of 950 MPa or more, the C content is preferably 0.08% or more, and more preferably 0.10% or more.

Si: Less than 1.0% Si is generally contained as an impurity. However, Si is also a useful solid solution strengthening element that raises the strength without deteriorating the ductility relatively, so it may be actively contained. However, if it is contained excessively, the transformation temperature rises, and the equipment load in finish hot rolling increases. In addition, retained austenite is easily formed in the steel structure, and stretch flangeability is lowered. For this reason, Si content shall be less than 1.0%. From the viewpoint of suppressing scale wrinkles and ensuring chemical conversion properties, the Si content is preferably 0.5% or less, and more preferably 0.3% or less.

Mn: 0.7% or more and 2.0% or less Mn has an effect of improving strength. The steel plate according to this embodiment has a Mn content of 0.7% or more in order to ensure a tensile strength of 900 MPa or more. In addition, the transformation temperature from austenite to ferrite is lowered to lower the finishing temperature in hot rolling. For this reason, when Mn content is increased, refinement | miniaturization of a ferrite crystal grain is accelerated | stimulated and stretch flangeability is improved. However, if excessively contained, ferrite transformation after hot rolling is delayed, and the volume fraction of ferrite is reduced. Therefore, the Mn content is set to 2.0% or less. From the viewpoint of avoiding the incorporation of martensite and martensite-austenite constituent (hereinafter referred to as “MA”) that deteriorates stretch flangeability into the steel structure, it is preferably 1.5% or less. Moreover, it is preferable to set it as 0.9% or more from a viewpoint of suppressing the formation of the pearlite which reduces stretch flangeability similarly.

Al: more than 0.1% and less than 1.0% Al is an element effective for forming ferrite in the steel structure, so it is contained in excess of 0.1%. It is preferable to be 0.2% or more. On the other hand, when it contains excessively, the increase of the operation load accompanying the raise of transformation temperature and the fall of the cleanliness of steel will be caused. For this reason, Al content shall be less than 1.0%. It is preferable to be less than 0.5%.

Ti: 0.05% or more and 0.3% or less Ti has an effect of strengthening ferrite. For this reason, Ti content shall be 0.05% or more. It is preferable to set it as 0.08% or more. On the other hand, if contained excessively, coarse carbonitrides are formed in the steel, and the stretch flangeability is deteriorated conversely. For this reason, Ti content is made into 0.3% or less. It is preferable to be 0.2% or less.

Nb: 0.1% or less Nb has the effect of strengthening ferrite and refining the structure. In order to reliably obtain these effects, the Nb content is preferably set to 0.005% or more. On the other hand, when it contains excessively, the hot deformation resistance of steel will rise and it will cause operational load. For this reason, Nb content shall be 0.1% or less. It is preferable to make it 0.05% or less.

V: 0.05% or more and 1.0% or less V is an important element in the steel sheet according to the present embodiment. Since V has an action of strengthening ferrite, the V content is set to 0.05% or more. Furthermore, since it also has the effect | action which accelerates | stimulates ferrite transformation, it is preferable to make V content into 0.1% or more, and it is more preferable if it is 0.2% or more. On the other hand, even if contained excessively, the effect due to the above action is saturated and the cost is increased. Therefore, the V content is set to 1.0% or less. Moreover, since there exists a tendency for chemical conversion property to be impaired when V content increases, it is preferable to set it as 0.5% or less.

P: 0.1% or less P is an element contained as an impurity. However, since P is an element effective for strengthening steel, it may be positively contained. However, since the grain boundary segregation tendency is strong and has an effect of deteriorating stretch flangeability, the P content is set to 0.1% or less. It is preferably 0.05% or less, and more preferably 0.02% or less.

S: 0.01% or less S is an impurity element that forms sulfide inclusions and lowers workability. For this reason, S content shall be 0.01% or less. When it is desired to further improve the workability, the content is preferably 0.008% or less, and more preferably 0.003% or less.

N: 0.01% or less N is an impurity element that forms nitrides with Ti, Nb, and the like, thereby reducing workability. For this reason, N content shall be 0.01% or less. It is preferable to set it to 0.006% or less.

Further, the following elements may be contained as optional components.
One or more selected from the group consisting of Ca, Mg, Nd, and B is 0.1% or less in total. Ca, Mg, Nd, and B are oxides and nitrides that precipitate when the molten steel solidifies. It has the effect | action which refines and improves the soundness of slab. For this reason, you may contain 1 type, or 2 or more types chosen from the group which consists of Ca, Mg, Nd, and B. However, even if these elements are contained excessively, not only the effect due to the above action is saturated, but also the cost is increased, and the cleanliness of the steel is lowered. For this reason, the total content of Ca, Mg, Nd and B is set to 0.1% or less. In order to obtain the above action more reliably, the total content of Ca, Mg, Nd and B is preferably set to 0.0004% or more.

  Further, since Nd also has an effect of improving stretch flangeability by acting with P and S in steel, it is preferably contained so as to satisfy the following formula (5). When the value on the middle side is lower than the value on the left side, that is, less than 0.5, it becomes difficult to obtain the effect of improving the stretch flangeability by Nd. On the other hand, when the value on the middle side exceeds the value on the right side, that is, exceeds 2.0, an oxide is formed and the cleanliness is easily impaired. Therefore, it is more preferable to contain it so as to satisfy the following formula (6).

  Here, Nd, P, and S in the formula indicate the content (unit: mass%) of each element in the steel.

One or two selected from Cr and Mo are 0.6% or less in total. Cr and Mo have the effect of improving hardenability and refining the structure of bainite. For this reason, you may contain 1 type or 2 types chosen from Cr and Mo. However, all of these elements have an action of reducing chemical conversion properties. For this reason, the sum total of content of these elements shall be 0.6% or less. In order to more reliably obtain the hardenability improving effect and the bainite structure refinement effect, the total content is preferably 0.2% or more.

2. Chemical composition balance regulation (1) Balance relating to mechanical properties In order to achieve excellent mechanical properties, the steel sheet according to the present embodiment is a balance of Ti, Nb, V precipitation strengthening elements and the content of N and C. The following formula (1) is satisfied.

  When the value of the middle side in the above formula is lower than the value of the left side, that is, less than 0.60, the balance between strength and stretch flangeability decreases. The cause of this is not clear, but in a high strength region of 900 MPa or more, it is presumed that the decrease in grain boundary strength due to the depletion of C dissolved in the crystal grain boundaries reduces the stretch flangeability.

  On the other hand, when the value of the middle side in the above formula exceeds the value of the right side, that is, exceeds 2.50, the balance between strength and stretch flangeability decreases. This is presumed to be due to the formation of cementite, martensite, MA, and the like, and the second phase being excessively cured.

Preferably, the left side of the above formula is 0.80 and the right side is 2.00, and more preferably, the left side of the above formula is 0.90 and the right side is 1.80.
(2) Balance regarding chemical conversion property The steel plate which concerns on this embodiment satisfy | fills following formula (2) as a balance of content of Si, Al, and V regarding chemical conversion property.

  When the contents of Si, Al, and V satisfy the above formula, it is possible to obtain even better mechanical properties and secure excellent chemical conversion properties.

  Si, Al, and V are all elements that promote the formation of ferrite, and inclusion thereof makes it easy to obtain the amount of ferrite defined in the present invention. For this reason, the total content of Si, Al and V is set to more than 0.3%. Preferably, it is 0.4% or more.

  On the other hand, Si, Al, and V further reduce the chemical conversion treatment property by the combined addition. For this reason, the total content of Si, Al and V is set to 1.5% or less. Preferably it is 1.0% or less.

3. Steel structure (1) Area ratio of ferrite, etc. The steel sheet according to the present embodiment contains at least 50% or more ferrite by area ratio in order to obtain good ductility and strength. As the ferrite area ratio when the structure changes in the plate thickness direction, the average value of the ferrite area ratios at the 1/4 t position and 1/2 t position of the total plate thickness is adopted. In order to further improve the balance between ductility and stretch flangeability, it is preferable to contain 60% by area or more of ferrite.

  The remaining structure other than the ferrite is bainite. Moreover, it is effective to make this bainite into a lower bainite or a fine upper bainite based on new knowledge obtained by conducting a detailed study on the relationship between the properties of bainite and stretch flangeability.

  That is, the bainite that is the remaining structure is preferably the lower bainite, and in the case of so-called upper bainite that has a lath-like structure and carbide precipitation is observed between the laths, the lath width is 2.0 μm or less. It is preferable.

  This is because when the bainite lath is coarse or the carbides in the bainite are coarse, cracks occur in these parts during processing that requires stretch flangeability, such as burring. This is presumed to be a starting point.

In addition, although martensite, MA, and retained austenite may be mixed in the bainite, the effect of the present invention is not impaired if the total area ratio thereof is less than 5% of the entire structure. Preferably it is 3% or less, More preferably, it is 1% or less.
(2) Hardness difference between ferrite and bainite As a preferred embodiment, the steel plate according to the present embodiment has an average value Hv _α of ferrite Vickers hardness and an average value Hv _β of Vickers hardness of bainite expressed by the following formula (3): Fulfill.

  That is, stretch flangeability is improved by reducing the difference in hardness between ferrite and bainite as much as possible. The right side is more preferably 0.40, and particularly preferably 0.25.

4). Manufacturing conditions The hot-rolled steel sheet according to the present embodiment has the above-mentioned chemical composition characteristics and steel structure characteristics, and has a mechanical strength of a tensile strength TS of 900 MPa or more, and a tensile strength TS (MPa ) And the hole expansion ratio HER (%), the manufacturing method is not particularly limited as long as the TS × HER value is 45000 (MPa ·%) or more. However, when the following manufacturing method is adopted, it is possible to efficiently and stably obtain the hot-rolled steel sheet according to the present embodiment.

(1) Heating before hot rolling It is preferable to subject the steel ingot or steel slab having the above chemical composition to 1250 ° C. or higher and subject it to hot rolling to dissolve coarse carbonitride. Coarse carbonitrides inhibit the stretch flangeability and consume elements that form fine carbonitrides that contribute to strength improvement. Therefore, by heating to 1250 ° C. or higher, deterioration of mechanical properties such as strength reduction and stretch flangeability is avoided.

  In addition, if the temperature of the steel ingot obtained by continuous casting or the steel slab after the partial rolling is 1250 ° C. or higher, additional heating may not be performed. On the other hand, when the steel ingot or steel piece once lower than 1250 ° C. is heated and then subjected to hot rolling, the heating time is preferably set to 1 hour or more. The upper limit of the temperature of the steel ingot or steel slab is not particularly limited, but it is preferably 1400 ° C. or lower from the viewpoint of durability of the heat resistant wall in the furnace of the heating furnace and yield reduction due to scale loss.

(2) Hot rolling completion temperature The steel ingot or steel slab having the above temperature range is subjected to hot rolling, and the hot rolling completion temperature is (Ae ₃ points + 100 ° C.) to (Ae ₃ points−50 ° C.). It is preferable that the temperature range.

If the hot rolling completion temperature exceeds (Ae ₃ points + 100 ° C.), the frequency of ferrite nucleation decreases, and it becomes difficult to obtain a sufficient amount of ferrite in the subsequent controlled cooling process. On the other hand, if the hot rolling completion temperature is less than (Ae ₃ points-50 ° C.), processed ferrite is formed and formability is deteriorated, and rolling is caused by the difference in hot deformation resistance between austenite and ferrite. It becomes unstable and the shape accuracy of the steel sheet decreases. Therefore, the hot rolling completion temperature is set to (Ae ₃ points + 100 ° C.) to (Ae ₃ points−50 ° C.). (Ae ₃ points + 100 ° C.) to (Ae ₃ points−30 ° C.) is preferable.

(3) Cooling and winding after hot rolling When hot rolling is completed in the above temperature range, it is preferable to perform winding in three stages as follows. In addition, when performing the following cooling, the cooling means may employ any of water cooling, contact heat removal by a sheet feeding device such as a roll, and cooling by air blowing. A plurality of these cooling means may be combined.

(I) Primary cooling First, as primary cooling, cooling is started within 3 seconds after completion of hot rolling, and is cooled to a temperature range of 750 to 550 ° C. at an average cooling rate of 30 ° C./second or more. It is preferable.

  In this primary cooling, if the cooling start time is more than 3 seconds after hot rolling, or the average cooling rate is less than 30 ° C./second, coarse pearlite is likely to be formed, and stretch flangeability is deteriorated. There is. A preferable average cooling rate is 50 ° C./second or more. When the crystal grain size is refined to further improve the characteristics, the time from hot rolling to the start of cooling is preferably within 1.5 seconds.

  On the other hand, if the cooling stop temperature in the primary cooling is higher than 750 ° C., it is difficult to secure a sufficient amount of ferrite even if control cooling is performed thereafter. On the other hand, if the cooling stop temperature in the primary cooling is less than 550 ° C., ferrite transformation is difficult to occur, so that a sufficient amount of ferrite cannot be ensured, and ductility deteriorates or strength decreases. From the viewpoint of stably securing a sufficient amount of ferrite, it is particularly preferable to set the cooling stop temperature of the primary cooling to 600 ° C. or higher.

(Ii) Secondary cooling After cooling to the above temperature range as primary cooling, 3 to 20 at an average cooling rate of 30 ° C./sec or less from the cooling stop temperature of primary cooling as secondary cooling. It is preferable to cool for 2 seconds.

  By performing the secondary cooling for 3 to 20 seconds at an average cooling rate of 30 ° C./second or less from the cooling stop temperature of the primary cooling, ferrite can be generated effectively. If the average cooling rate of the secondary cooling is more than 30 ° C./second, the hardness of the generated ferrite varies and the stretch flangeability is deteriorated. The temperature is preferably 20 ° C./second or less. Although the lower limit of the average cooling rate of the secondary cooling is not particularly limited, it is usually that no auxiliary heating equipment is provided at the place where the secondary cooling is performed. / Second or more, preferably 7 ° C./second or more.

  When the cooling time is less than 3 seconds, ferrite is not sufficiently generated, and ductility may be deteriorated or strength may be reduced. On the other hand, if it exceeds 20 seconds, pearlite or coarse cementite may be generated and stretch flangeability may deteriorate. It is particularly preferable that the time be 7 seconds or more and 15 seconds or less.

(Iii) Third cooling and winding After the second cooling, as the third cooling, from the cooling stop temperature of the second cooling to a temperature range of 550 ° C. or less at an average cooling rate of 30 ° C./second or more. It is preferable to cool and wind up.

  When the average cooling rate in the third cooling is less than 30 ° C./second, stretch flangeability deteriorates due to generation of coarse pearlite and coarsening of bainite lath. It is preferable to set it to 50 ° C./second or more.

  When the cooling stop temperature of the third cooling, that is, the coiling temperature is higher than 550 ° C., the stretch flangeability is deteriorated by the formation of coarse cementite and the segregation of P. More preferably, the cooling stop temperature of the third cooling, that is, the coiling temperature, is set to 450 ° C. or lower. However, if the temperature is lower than 300 ° C., martensite or MA may be generated and stretch flangeability may be deteriorated.

(4) Sheet thickness center temperature control in the initial stage of cooling In the manufacturing process of the steel sheet from the start of hot rolling to winding, the temperature at the 1 / 2t position of the steel sheet, that is, the sheet thickness center temperature satisfies the following formula (4). _The residence time in the temperature range of _pef (° C.) or less and (Ae ₃ points−50 ° C.) or more is preferably 180 seconds or less.

  If this residence time is longer than 180 seconds, carbonitrides containing coarse Ti are formed in the steel, and the balance between strength and stretch flangeability may be reduced. The residence time is more preferably within 150 seconds, and particularly preferably within 135 seconds.

1. Manufacture of hot-rolled steel sheet For each steel having the chemical composition shown in Table 1, an ingot melted by vacuum melting is forged to a width of 200 mm and a thickness of 40 mm, the length is appropriately cut into a steel piece, and then hot It used for rolling. The steel slab was heated by a small heating furnace simulating a heating furnace atmosphere in an actual production line.

  At that time, a small hole having a depth of 15 mm was formed in the vicinity of the thickness center at the rear end of the steel slab in the rolling direction, and a thermocouple was attached to the small hole so that the steel slab center temperature could be measured. The small hole after the thermocouple was attached was sealed with a heat-resistant ceramic filler.

In addition, the underline in each table | surface shows that it is outside prescription | regulation of this invention.
The steel pieces thus prepared were subjected to a homogenization treatment for 1 hour or more at a temperature shown in Table 2 in a heating furnace, and rough rolling was performed with a small lever rolling mill. At that time, the steel slab center temperature was measured with a thermocouple, and the slab surface temperature was measured with a radiation thermometer installed on the upper upstream side of the rolling stand. This is to obtain a correction value when calculating the internal temperature from the slab surface temperature by heat transfer calculation.

  The scale on the steel surface hinders temperature measurement, and also causes surface flaws and cooling unevenness. Therefore, descaling was appropriately performed using a hydraulic descaling device installed up and down on the stand exit side. Moreover, the time of the rough rolling process was controlled by the time between passes.

  After the thickness of the steel slab was reduced to 16 mm by lever rolling, the thermocouple was intentionally cut off and continuous rolling was performed with a small three stand mill to obtain a 2 mm thick hot rolled steel sheet. Radiation thermometers were installed on the entrance and exit sides of each stand, and the steel surface temperature was measured, and the center thickness of the plate thickness was calculated by heat transfer calculation. Note that the internal temperature was also calculated from the surface temperature obtained by the radiation thermometer even in the case where an erroneous measurement that seems to be caused by cutting or short-circuiting of the thermocouple in the steel occurred during the above rough rolling process.

  The rolled steel sheet was transported to a water cooling tank on a feeding table on which small rollers were installed, and was water cooled at a predetermined cooling rate shown in Table 2. A radiation thermometer capable of changing the mounting position was installed on the feed plate table, and the temperature was measured appropriately.

  Furthermore, after satisfying the cooling pattern shown in Table 2, the steel plate was carried out from the water cooling tank onto the feeding table, air-cooled, and then again carried into the water cooling tank. Thus, when the predetermined temperature (winding temperature) was reached, the steel plate was again transported onto the feeding table, and directly led into a slow cooling furnace set at a predetermined initial temperature (winding temperature).

The slow cooling pattern of the slow cooling furnace simulated the actual temperature in the coil. That is, after holding at the initial temperature for 30 seconds, it was gradually cooled to near room temperature at 30 ° C./hour.
2. Evaluation From the hot-rolled steel sheet after slow cooling, a JIS No. 5 tensile test piece, a 90 mm square hole expansion test piece, a microstructural observation and hardness measurement test piece, and a 50 mm × 70 mm chemical conversion treatment evaluation test piece were cut out.

In the structure observation, after performing nitral corrosion, the surface was subjected to C vapor deposition, and the center of the plate thickness and the vicinity of the 1/4 t portion were identified by SEM observation.
Regarding mechanical properties, tensile properties and stretch flangeability were evaluated. As for the tensile properties, a tensile test was performed on the test piece after pickling according to a conventional method, and yield strength (MPa), tensile strength (MPa), and total elongation (%) were obtained. Evaluation of stretch flangeability was performed in accordance with Japan Iron and Steel Federation Standard (JFST) 1001-1996. A punched hole of 10 mmΦ is made with a crank press at a clearance of 12% in the center of the above-extracted 90 mm square hole expansion test piece (pickled), and this hole is formed into a conical punch having a 60 ° apex angle. The hole expansion ratio (%) was obtained from the value obtained by dividing the difference between the hole diameter at the time of breaking the hole and the initial hole diameter by the initial hole diameter. Further, a TS × HER value (MPa ·%), which is a product of the tensile strength, was also obtained.

Hardness measurement was performed in the vicinity of a thickness of 1/4 t of the collected specimen. That is, 30 points each of ferrite and second phase were arbitrarily extracted, and Vickers hardness was measured at 20 mN. At this time, in order to reduce the surface shape of the second phase and the effect of residual stress due to polishing as much as possible, the hardness measurement surface is polished by buffing, further subjected to chemical corrosion polishing with colloidal silica to remove strain, and then subjected to electrolytic corrosion. The tissue was visualized.
For the chemical conversion treatment evaluation, the steel plate surface was pickled and then subjected to chemical conversion treatment using Valbond WL35 chemical conversion solution manufactured by Nihon Parkerizing. The adhesion amount of the chemical crystals on the surface of the sample steel plate immersed in the chemical liquid for 120 seconds at room temperature was investigated and the coating form of the chemical crystals was observed with a scanning electron microscope. As a criterion for judgment, the case where the adhesion amount was 3.0 g / m ² or more and there was no gap in the chemical conversion crystal coating on the surface of the sample steel plate was considered good. On the other hand, when satisfy | filling at least one of the case where the adhesion amount is less than 3.0 g / m < ² > and the case where there exists a part which is not coat | covered with a chemical conversion crystal | crystallization on the sample steel plate surface, it was judged that it was inferior.

3. Evaluation results Table 3 shows the evaluation results.

  Each of the hot-rolled steel sheets according to Test Nos. 1 to 12 having the chemical composition according to the present invention and manufactured by an appropriate manufacturing method has a tensile strength of 950 MPa or more and a TS × HER value of 45000 MPa · % Or more.

On the other hand, in the steel plate of test number 13, pearlite was precipitated because the contents of Mn and Al were outside the range of the chemical composition defined in the present invention. For this reason, hole expansibility deteriorated and TS * HER value was unsatisfactory. In Table 3, “−” in the ferrite area ratio (| Hv _α −Hv _β | / Hv _α ) is practically impossible to measure the area ratio because the amount of ferrite produced is particularly small. Means that.

  The steel plate of test number 14 was insufficient in strength because of low C content, and the steel plate of test number 15 was also inferior in strength because of low C and Al content. Test No. 16 having a small Ti content and Test No. 17 having a low V content also showed insufficient strength.

  Since test number 18 did not satisfy the definition of the relational expression of C, Ti, N, Nb, and V (exceeding the upper limit), the balance between strength and hole expansibility (TS × HER value) was unsatisfactory. On the other hand, the test number 19 was less than the lower limit and did not satisfy the above definition, and the TS × HER value was unsatisfactory. In addition, in this case, since the definition of the relational expression of Si, Al, and V was not satisfied, the chemical conversion treatment property was unsatisfactory.

Test No. 20 was unsatisfactory in strength because the P content was excessive and was outside the range of the above relational expression (exceeding the upper limit).
Test No. 21 is a case where the elapsed time in the temperature range of T _{pef to} Ae ₃ -50 ° C. is not specified with respect to the center temperature of the steel, and strength deterioration was observed. In Test No. 22, since the rolling completion temperature was less than Ae ₃ -50 ° C., a processed ferrite phase was formed, the formability deteriorated, and the TS × HER value particularly decreased. In Test No. 23, since the cooling rate (secondary cooling rate) in the secondary cooling was large, the amount of ferrite produced decreased (the amount of ferrite produced 48%), and the target strength could not be obtained. In Test No. 24, since the cooling rate (secondary cooling rate) in the secondary cooling was too large, sufficient ferrite could not be obtained (ferrite area ratio 35%), the ductility deteriorated, and the TS × HER value decreased. . In Test No. 25, since the cooling rate in the third cooling (third cooling rate) was small, coarse iron carbide was formed, and the TS × HER value also decreased. In test number 26, since the cooling time (secondary cooling time) in the secondary cooling was short, the generation of ferrite became insufficient (ferrite area ratio 10%), and the strength decreased. In Test No. 27, pearlite was generated due to the high winding temperature, the hole expandability deteriorated, and the TS × HER value decreased. In Test No. 28, since the stop temperature (primary cooling stop temperature) in the primary cooling was too low, ferrite was not generated (ferrite area ratio 0%), and the strength decreased.

Claims (5)

In mass%, C: 0.05% or more and less than 0.20%, Si: less than 1.0%, Mn: 0.7% or more and 2.0% or less, Al: more than 0.1% and less than 1.0% Ti: 0.05% or more and 0.3% or less, Nb: 0.1% or less, V: 0.05% or more and 1.0% or less, P: 0.1% or less, S: 0.01% or less And N: 0.01% or less, comprising the balance Fe and impurities, and having a chemical composition satisfying the following formula (1):
Having a steel structure containing ferrite of 50 area% or more and the balance being bainite,
Tensile strength TS is not less than 900 MPa, Ri der TS × HER value 45000 (MPa ·%) or more the product of the tensile strength TS (MPa) and hole expansion ratio HER (%),
Hot-rolled steel sheet according to claim <br/> that the average Hv _beta Vickers hardness of the bainite and the average value Hv _alpha Vickers hardness of the ferrite satisfy the following equation (3).

Here, C, Ti, N, Nb, and V in the formula indicate the content (unit: mass%) of each element in the steel.

The hot rolled steel sheet according to claim 1, wherein the chemical composition satisfies the following formula (2).

Here, Si, Al and V in the formula indicate the content (unit: mass%) of each element in the steel.   The chemical composition contains one or more selected from the group consisting of Ca, Mg, Nd and B in place of a part of Fe in a total amount of 0.1% by mass or less. The hot rolled steel sheet according to 1 or 2.   The said chemical composition replaces a part of Fe and contains 1 type or 2 types chosen from Cr and Mo in total 0.6 mass% or less, The any one of Claims 1-3 characterized by the above-mentioned. Hot rolled steel sheet. The steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 is heated to 1250 ° C or higher, and then hot-rolled, and (Ae ₃ points + 100 ° C) to (Ae ₃ points -50 ° C) ) Complete hot rolling in the temperature range
The obtained hot-rolled steel sheet is subjected to primary cooling which starts cooling within 3 seconds after completion of the hot rolling and cools to a temperature range of 750 to 550 ° C. at an average cooling rate of 30 ° C./second or more,
Secondary cooling is performed by cooling for 3 to 20 seconds at an average cooling rate of 30 ° C./second or less from the cooling stop temperature of the primary cooling,
While performing the third cooling to cool from the cooling stop temperature of the secondary cooling to a temperature range of 550 ° C. or less at an average cooling rate of 30 ° C./second or more, winding up,
In the process from the start of the hot rolling to the winding, the time during which the plate thickness center temperature of the steel sheet is T _pef (° C.) or less (Ae ₃ points−50 ° C.) defined by the following formula (4) is 180. A method for producing a hot-rolled steel sheet, characterized in that it is not more than 2 seconds.

Here, C and Ti in a formula show content (unit: mass%) of each element in steel.