JP4436348B2 - Hot-rolled steel sheet excellent in paint bake-hardening performance and room temperature aging resistance and method for producing the same - Google Patents

Description

  The present invention relates to a steel sheet having paint bake hardening performance (BH), room temperature slow aging, and formability, and a method for producing the same. The steel sheet according to the present invention is used for automobiles, home appliances, buildings, and the like. And it includes a hot-rolled steel sheet in a narrow sense not subjected to surface treatment and a hot-rolled steel sheet in a broad sense subjected to surface treatment such as alloying hot-dip Zn plating and electroplating for rust prevention.

  Since the steel plate according to the present invention is a steel plate having paint bake hardening performance, in use, the plate thickness can be reduced as compared with conventional steel plates, that is, the weight can be reduced. Therefore, it is thought that it can contribute to global environmental conservation. Furthermore, since the steel plate according to the present invention is excellent in the collision energy absorption characteristic, it contributes to the improvement of the safety of the automobile.

  With the recent progress in vacuum degassing of molten steel, it has become easier to produce ultra-low carbon steel, and now the demand for ultra-low carbon steel sheets with good workability is increasing. Among these, for example, the ultra-low carbon steel sheet combined with Ti and Nb disclosed in Patent Document 1 has extremely good workability and has paint bake hardening (BH) properties, and is hot dip galvanized. It is also occupying an important position because of its excellent properties.

  However, the amount of BH does not exceed the level of a normal BH steel sheet, and if a further amount of BH is to be imparted, there is a disadvantage that normal temperature non-aging cannot be ensured. For example, Patent Document 2 discloses a technique related to a steel sheet having both high BH properties and room temperature slow aging properties.

  This is because a large amount of Nb and B, and further Ti are added to ultra low carbon steel, and the structure after annealing becomes a composite structure of a ferrite phase and a low temperature transformation generation phase, and has a high r value, high BH, and high ductility. And a cold-rolled steel sheet having non-aging properties at room temperature.

  However, it has been clarified that this technique has problems in actual operation such as 1) and 2) below.

  1) In steels containing a large amount of Nb, B, and even Ti, the α → γ transformation point does not decrease, and in order to obtain a composite structure, annealing at an extremely high temperature is essential. It causes troubles such as plate breakage.

  2) Since the temperature range of α + γ is extremely narrow, the structure changes in the plate width direction. As a result, the material may vary greatly, or the annealing temperature of several degrees C. may not result in a composite structure. Very unstable.

  In Patent Document 3, in an ultra-low carbon cold-rolled steel sheet to which Nb is added, the carbon concentration in the grain boundary is increased by controlling the cooling rate after annealing, thereby achieving both high BH and room temperature slow aging. Has been shown to be possible. However, the balance between high BH and room temperature slow aging is not sufficient.

  Furthermore, in the conventional BH steel sheet, a predetermined amount of BH can be obtained if the heat treatment condition of BH is 170 ° C.-20 minutes, but if this condition is 160 ° C.-10 minutes or 150 ° C.-10 minutes, BH is There is a problem that it falls.

JP 59-31827 A Japanese Patent Publication No. 3-2224 JP-A-7-300623

  As described above, the conventional BH steel sheet has drawbacks that it is difficult to produce stably, and that the amount of BH is increased and at the same time the room temperature slow aging is lost. Further, when the coating baking temperature is as low as 160 ° C. to 150 ° C. with respect to the current 170 ° C., there is a problem that a sufficient amount of BH cannot be obtained.

  The present invention provides a hot-rolled steel sheet having both high BH properties and room temperature slow aging, and having a sufficient amount of BH even when the temperature of BH is low, and a method for producing the same.

  The inventors of the present invention diligently conducted research to achieve the above-described goal, and gained knowledge that has not existed in the past as described below. That is, by adding Cr, Mo, V, etc. to the steel in which solute N remains, even if it has a high BH and room temperature slow aging, and the coating baking conditions are low temperature and short time, it has a high BH property. It has been found that it is possible to ensure.

  The present invention is a completely new hot-rolled steel sheet and a method for producing the same, which have been constructed based on such a concept and new knowledge, and the gist thereof is as follows.

(1) in mass%, C = 0.0001~0.20%, Si = 2.0% or less, Mn = 3.0% or less, P = 0.15% or less, S = 0.015% hereinafter , Al = 0.20% or less, N = 0.001 to 0.10%, solute N = .0005-0.004%, and, Al and N so as to satisfy the 0.52Al / N <10 And one or more of Cr, Mo, and V, respectively, Cr = 2.5% or less, Mo = 1.0% or less, V = 0.1% or less, and (Cr + 3 .5Mo + 39V) ≧ 0.1, comprising the balance Fe and unavoidable impurities, BH170 evaluated by heat treatment at 170 ° C. for 20 minutes after 2% tensile deformation is 45 MPa or more, and B evaluated by heat treatment at 160 ° C. for 10 minutes after 2% tensile deformation BH150 evaluated by performing heat treatment at 150 ° C. for 10 minutes after H160 and 2% tensile deformation is 35 MPa or more, and further yield point in tensile test after heat treatment at 100 ° C. for 1 hour. A hot-rolled steel sheet excellent in paint bake-hardening performance and room temperature aging resistance, characterized by an elongation of 0.6% or less.

(2) in mass%, the characterized in that it contains Ca 0.0005 to 0.01% (1) Symbol placement bake hardenability performance and anti-aging properties superior hot-rolled steel sheet.

( 3 ) The hot-rolled steel sheet excellent in paint bake hardening performance and room temperature aging resistance as described in (1) or ( 2), wherein B is contained in an amount of 0.0001 to 0.001% by mass.

( 4 ) Mass% and Nb is contained in an amount of 0.001 to 0.03%, which is excellent in paint bake-hardening performance and room temperature aging resistance according to any one of the above (1) to ( 3 ) Hot rolled steel sheet.

( 5 ) The above (1) to ( 4 ), characterized by containing Ti so as to satisfy Ti = 0.0001 to 0.10% and N−0.29Ti> 0.0005 by mass%. A hot-rolled steel sheet having excellent bake hardening performance and room temperature aging resistance.

( 6 ) The above-mentioned, characterized by containing one or more of Sn, Cu, Ni, Co, Zn, W, Zr and Mg in 0.001 to 1.0% in total (% by mass). A hot-rolled steel sheet excellent in paint bake hardening performance and room temperature aging resistance according to any one of 1) to ( 5 ).

( 7 ) Paint bake-hardening performance and room temperature aging resistance, characterized in that hot-rolled steel sheet according to any one of (1) to ( 6 ) is subjected to hot dip galvanizing, alloying hot dip galvanizing, or electrogalvanizing. Galvanized hot rolled steel sheet with excellent properties.

( 8 ) The slab having the chemical component according to any one of (1) to ( 6 ) above is hot-rolled at a temperature of (Ar ₃ points-100) ° C. or higher and 600 ° C. or lower from the hot rolling end temperature. A method for producing a hot-rolled steel sheet excellent in paint bake-hardening performance and room temperature aging resistance, characterized in that it is cooled at an average cooling rate of 10 ° C./s or higher and then wound up at a temperature of 550 ° C. or lower.

  According to the present invention, a hot-rolled steel sheet and a galvanized steel sheet having both a high BH property and a normal temperature slow aging property and having a sufficient amount of BH can be obtained even when the BH temperature is low.

  Here, the reason why the steel composition and production conditions are limited as described above in the present invention will be further described. In addition,% means the mass%.

  Since C is an element that increases the strength at a low cost, the amount of addition varies depending on the target strength level, but it is difficult to make C less than 0.0001% in terms of steelmaking technology, resulting in an increase in cost. In addition, since the fatigue characteristics of the welded portion deteriorate, the lower limit of the C addition amount is 0.0001%.

  On the other hand, if the amount of C exceeds 0.20%, not only deterioration of formability will be caused or weldability will be impaired, but it will also be difficult to achieve both high BH properties and room temperature non-aging properties important in the present invention. The upper limit of the amount of C added is 0.20%. When the present invention is applied to a member that requires deep drawability, the C content is preferably in the range of 0.0001 to 0.0020% or 0.012 to 0.024%.

  The amount of solute C is preferably 0.0020% or less. In the present invention, high BH property and room temperature slow aging are ensured mainly by N. Therefore, if the amount of dissolved C is too large, it is difficult to secure room temperature slow aging. The solute C is more preferably less than 0.0010%. The adjustment of the solid solution C amount may be the total C amount not more than the above upper limit, or may be reduced to a predetermined level depending on the coiling temperature and the overaging treatment conditions.

  In addition to increasing the strength as a solid solution strengthening element, Si is effective for obtaining a structure containing martensite, bainite, and residual γ. The Si addition amount varies depending on the target strength level. However, if it exceeds 2.0%, the press formability is deteriorated or the chemical conversion property is deteriorated. The upper limit is 2.0%.

  When alloying hot dip galvanizing is performed, problems such as a decrease in plating adhesion and a decrease in productivity due to a delay in the alloying reaction occur. A lower limit is not particularly provided, but if it is 0.001% or less, the manufacturing cost increases, so 0.001% is a substantial lower limit. In addition, when it is difficult to perform Al deoxidation from the viewpoint of controlling the amount of Al, it may be deoxidized with Si. In this case, 0.04% or more of Si is contained. Become.

  In addition to being useful as a solid solution strengthening element, Mn forms MnS, suppresses ear cracking due to S during hot rolling, refines the hot rolled sheet structure, martensite, bainite, and residual γ. It is also effective to obtain a tissue that contains it. Further, since Mn has an effect of suppressing normal temperature aging caused by solid solution N, it is preferable to add 0.3% or more.

  However, when deep drawability is required, it is preferably 0.15% or less, and more preferably less than 0.10%. On the other hand, if the addition amount exceeds 3.0%, the strength becomes too high and the ductility is lowered, or the adhesion of galvanization is hindered, so the Mn addition amount has an upper limit of 3.0%. .

  Like Si, P is known as an element that increases the strength at a low cost, and when it is necessary to increase the strength, P is further actively added. P also has the effect of making the hot-rolled structure fine and improving workability.

  However, if the addition amount exceeds 0.15%, the fatigue strength after spot welding becomes poor, or the yield strength increases excessively, causing surface shape defects during pressing. Furthermore, during continuous hot dip galvanization, the alloying reaction becomes extremely slow and productivity is reduced. Also, the secondary workability is deteriorated. Therefore, the upper limit of the P addition amount is set to 0.15%.

  If S exceeds 0.015%, it causes hot cracking or deteriorates workability. Therefore, the upper limit of S addition is 0.015%.

  Al may be used as a deoxidation preparation agent. However, since Al combines with N to form AlN and lowers the BH property, it is desirable to add it to the minimum necessary within a range that is not unreasonable in terms of manufacturing technology.

  In the case of a hot-rolled steel sheet, even if Al has an atomic ratio of 1 or more with respect to N, it is possible to ensure solid solution N if it is rapidly cooled after hot rolling, so the upper limit of Al is 0. 20% is sufficient. If Al is 0.05 or less, further 0.02% or less, the production becomes even easier.

  N is an important element in the present invention. That is, in the present invention, high BH property is achieved mainly by N. Therefore, addition of 0.001% or more is essential. On the other hand, when N is too much, it becomes difficult to ensure the room temperature slow aging property or the workability deteriorates, so the upper limit is made 0.10%. Preferably, it is 0.002 to 0.020%, more preferably 0.002 to 0.008%.

  Further, since N easily bonds to Al to form AlN, it is necessary to set 0.52 Al / N to a certain value or less in order to secure N that contributes to BH. In a hot-rolled steel sheet, it limits as follows.

  When 0.52 Al / N is 10 or more, AlN easily precipitates during the cooling process after hot rolling or during winding, so 0.52 Al / N has an upper limit of less than 10. If 0.52 Al / N is less than 10, it is possible to avoid excessive precipitation of AlN in consideration of the cooling rate and coiling temperature after hot rolling, so that high BH properties can be obtained. A more preferable upper limit of 0.52Al / N is 5.

  Cr, Mo, and V are important elements in the present invention. It is essential to add one or more of these elements. For the first time, the addition of these elements makes it possible to achieve both high BH properties and room temperature aging resistance.

  Since N has a higher diffusion rate than C, it is known that it is difficult to ensure normal temperature aging resistance if a predetermined amount or more of N is present. For this reason, the BH steel plate using N is not applied to a member whose appearance is important, such as an outer panel of an automobile.

  However, it has been newly found that by adding Cr, Mo and V positively, it is possible to obtain room temperature slow aging without impairing BH properties. The mechanism for improving the normal temperature aging resistance by these elements is not necessarily clear, but is presumed as follows.

  In the vicinity of normal temperature, these elements and N form a pair or cluster and suppress the diffusion of N, so that normal temperature aging resistance is ensured. On the other hand, in the coating baking process at 150 to 170 ° C., N escapes from these pairs and clusters and fixes dislocations, so that a high BH property is exhibited.

  The upper limit of the addition amount of Cr, Mo, V is determined from the viewpoint of ensuring workability and cost, and is 2.5%, 1.0%, and 0.1%, respectively.

  V is preferably 0.04% or less because if V is added in an excessive amount, nitrides are formed and it is difficult to ensure solid solution N. In order to ensure normal temperature aging resistance, Cr, Mo, and V must be added so as to satisfy (Cr + 3.5Mo + 39V) ≧ 0.1. (Cr + 3.5Mo + 39V) ≧ 0.4 is a more preferable range.

  Moreover, in order to ensure normal temperature aging resistance, it is much more effective to add two or more types in combination than to add Cr, Mo, and V alone.

  The total amount of solute N is 0.0005 to 0.004%. Here, the solid solution N includes not only N present alone in Fe but also N forming a pair or cluster with a substitutional solid solution element such as Cr, Mo, V, Mn, Si, and P.

  The amount of solute N is preferably obtained by a heat extraction method in a hydrogen stream. In this method, a sample is heated to a temperature range of about 200 to 500 ° C., and solid solution N and hydrogen are reacted to form ammonia. Ammonia is mass analyzed, and the analysis value is converted to obtain the amount of solid solution N. It is.

  Furthermore, the amount of solute N can also be obtained from a value obtained by subtracting the amount of N existing as a compound such as AlN, NbN, VN, TiN, BN, etc. (quantitative analysis from the chemical analysis of the extraction residue) from the total N amount. . Further, it may be determined by an internal friction method or FIM (Field Ion Microscopy).

  If the solute N is less than 0.0005%, sufficient BH property cannot be obtained. On the other hand, if the solid solution N exceeds 0.004%, it becomes difficult to obtain a normal aging property even if the BH property is improved. The amount of solute N is more preferably 0.0012 to 0.003%.

  Ca is not only useful as a deoxidizing element but also an element that has an effect on the form control of sulfides, so Ca may be added in the range of 0.0005 to 0.01%. If it is less than 0.0005%, the effect of addition is not sufficient, and if it exceeds 0.01%, the workability deteriorates, so the amount of Ca added is in the range of 0.0005 to 0.01%.

  Since B is an element effective for preventing secondary work embrittlement, it is added in the range of 0.0001 to 0.001% as necessary. When the amount added is less than 0.0001%, there is almost no effect of addition, and even if added over 0.001%, not only the effect of addition is saturated, but also BN is easily formed and solid solution N is secured. It becomes difficult. 0.0001 to 0.0004% is a more desirable range.

  Nb is an element effective for improving workability, increasing strength, and further miniaturizing and homogenizing the structure. Therefore, Nb is added in a range of 0.001 to 0.03% as necessary. However, when the addition amount is less than 0.001%, the effect of addition does not appear. On the other hand, when the addition amount exceeds 0.03%, NbN is easily formed, and it is difficult to secure solid solution N. 0.001 to 0.012% is a more preferable range.

  Since Ti is an element having the same effect as Nb, it is added in a range of 0.0001 to 0.10% as necessary. However, when the addition amount is less than 0.0001%, the effect of addition does not appear. On the other hand, when the addition amount exceeds 0.10%, a large amount of N precipitates or crystallizes as TiN, and the solid solution N is ensured. It becomes difficult. 0.001 to 0.020% is preferable, and 0.001 to 0.012% is a more preferable range.

  Furthermore, in order to secure solid solution N, Ti must be added within a range satisfying N−0.29Ti> 0.0005. More preferably, N−0.29Ti> 0.0010.

  Even if steel which has these as a main component contains 1 type or 2 types or more of Sn, Cu, Ni, Co, Zn, W, Zr, and Mg in the range of 0.001-1.0% in total. I do not care. However, since Zr forms ZrN, the amount of Zr added is preferably 0.01% or less. Next, the reasons for limiting the manufacturing conditions will be described.

  The slab to be subjected to hot rolling is not particularly limited by the production conditions. That is, what was manufactured with the continuous casting slab, the thin slab caster, etc. should just be used. A slab manufactured by a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting is also suitable for the present invention. When a hot-rolled steel sheet is used as a final product, it is necessary to limit production conditions as follows.

That is, the finishing temperature of hot rolling is (Ar ₃ -100) ° C. or higher. If it is less than (Ar ₃ -100) ° C., it is difficult to ensure workability, and a problem of plate thickness accuracy occurs. Ar ₃ or more is a more preferable range. Although the upper limit of the hot rolling finishing temperature is not particularly defined, it is preferably 1100 ° C. or lower from the viewpoint of preventing the coarsening of crystal grains and protecting the hot rolling roll.

  In addition, although the heating temperature of hot rolling is not particularly limited, when it is necessary to dissolve AlN in order to secure solid solution N, the heating temperature is preferably set to 1200 ° C. or higher.

  After hot rolling, it is necessary to cool from the hot rolling end temperature to at least 600 ° C. so that the average cooling rate is 10 ° C./s. This is for suppressing precipitation of AlN.

  In addition, when N is excessively added to Al, that is, when 0.52 Al / N <1, the cooling rate should be 10 ° C./s or more to achieve high BH resistance and resistance. It has been found that it is important to ensure normal temperature aging. A cooling rate of 30 ° C./s or more is even more preferable for BH properties and normal temperature aging resistance. The upper limit of the cooling rate is not particularly defined, but is preferably 200 ° C./s or less from the viewpoint of productivity.

  The winding temperature is set to 550 ° C. or lower in order to suppress the precipitation of AlN. Preferably, it is 450 degrees C or less. The structure of the hot-rolled steel sheet obtained by the present invention has ferrite or bainite as the main phase, but both phases may be mixed, and martensite, austenite, carbide, and nitride exist in these. Also good. That is, it is only necessary to create an organization according to required characteristics.

  After hot rolling, pickling may be performed as necessary, and then, in-line or off-line, a skin pass with a reduction rate of 10% or less or cold rolling to a reduction rate of about 40% may be performed.

  In the steel sheet obtained by the present invention, BH170 is 45 MPa or more, and BH160 and BH150 are both 35 MPa or more. BH170 is 60 MPa or more, and BH160 and BH150 are more preferably 50 MPa or more. The upper limit of BH is not particularly limited, but when BH170 exceeds 140 MPa and BH160 and BH150 exceed 130 MPa, it becomes difficult to ensure normal temperature aging resistance.

  BH170 is evaluated by heat treatment at 170 ° C. for 20 minutes after 2% tensile deformation, and BH160 is evaluated by heat treatment at 160 ° C. for 10 minutes after 2% tensile deformation. BH and BH150 represent BH evaluated by heat treatment at 150 ° C. for 10 minutes after 2% tensile deformation.

  Room temperature aging resistance is evaluated by the yield point elongation after artificial aging. The steel sheet obtained by the present invention has a yield point elongation of 0.6% or less in a tensile test after heat treatment at 100 ° C. for 1 hour. Preferably, it is 0.4% or less, more preferably 0.3% or less. The yield point elongation after heat treatment at 40 ° C. for 70 days is 0.5% or less, preferably 0.3% or less, and more preferably 0.2% or less.

  Next, an example explains the present invention.

<Example 1>
Steel having the composition shown in Table 1 was melted and hot rolled under the conditions shown in Table 2. At this time, all heating temperatures were 1250 ° C. The temper rolling ratio was 1.0%, JIS No. 5 tensile test specimens were collected, and the elongation of yield point after BH and artificial aging was measured. The obtained structure and mechanical properties are shown in Table 2.

  As is clear from this, when the steel having the chemical component of the present invention was hot-rolled under appropriate conditions, both high BH properties and room temperature aging resistance could be achieved.

<Reference Example 1>
Among the steels in Table 1, A, C, D, E, F, I, N, O, and P steels were hot rolled at a slab heating temperature of 1250 ° C, a finishing temperature of 930 ° C, and a winding temperature of 650 ° C. A steel strip with a thickness of 0 mm was used.

  After pickling, cold rolling at a reduction rate of 80% is performed to obtain a cold-rolled sheet having a thickness of 0.8 mm, followed by annealing at a heating rate of 10 ° C./s and a maximum temperature of 800 ° C. in a continuous annealing facility. After that, it was cooled at various cooling rates shown in Table 3, and the overaging treatment temperature was also changed.

  The overaging treatment time was 300 seconds (constant). Furthermore, temper rolling was performed at a rolling reduction of 1.0%, and a JIS No. 5 tensile test piece was collected, and BH and elongation at yield point after artificial aging were measured.

  The results are shown in Table 3. As is clear from this, when the steel having the chemical component of the present invention was annealed under appropriate conditions, both high BH properties and normal temperature aging resistance could be achieved.

<Reference Example 2>
Of the steels in Table 1, steels A and D were hot-rolled at a slab heating temperature of 1250 ° C, a finishing temperature of 930 ° C, and a winding temperature of 650 ° C to form a steel strip having a thickness of 4.0 mm.

  After pickling, cold rolling with a reduction rate of 80% is performed to obtain a cold-rolled sheet having a thickness of 0.8 mm, followed by annealing at a heating rate of 10 ° C./s and a maximum temperature of 800 ° C. in a continuous hot dip galvanizing facility. After that, it was cooled at various cooling rates shown in Table 4, immersed in a 460 ° C. zinc bath, reheated to 500 ° C. at 15 ° C./s, and held for 15 seconds.

  Furthermore, temper rolling was performed at a reduction rate of 0.8%, JIS No. 5 tensile test specimens were collected, and AI, BH and elongation at yield point after artificial aging were measured.

  The results are shown in Table 4. As apparent from this, when manufactured under appropriate conditions, both high BH properties and normal temperature aging resistance could be achieved.

Claims (8)

% By mass
C = 0.0001 to 0.20%, Si = 2.0% or less,
Mn = 3.0% or less, P = 0.15% or less,
S = 0.015% or less, Al = 0.20% or less,
N = 0.001 to 0.10%, solute N = from .0005 to .004%, and containing Al and N so as to satisfy the 0.52Al / N <10, and, Cr, Mo, One type or two or more types of V satisfy Cr = 2.5% or less, Mo = 1.0% or less, V = 0.1% or less, and (Cr + 3.5Mo + 39V) ≧ 0.1, respectively. BH170 is 45MPa or more and is evaluated by performing heat treatment at 170 ° C for 20 minutes after 2% tensile deformation and at 160 ° C after 2% tensile deformation. BH160 evaluated by performing heat treatment for 10 minutes and BH150 evaluated by performing heat treatment for 10 minutes at 150 ° C. after 2% tensile deformation are both 35 MPa or more, and further, at 100 ° C. for 1 hour. Heat of Bake hardenability performance and anti-aging properties superior hot-rolled steel sheet, characterized in that the yield point elongation at a tensile test after applying a physical is 0.6% or less. By mass%, bake hardenability performance and anti-aging properties superior hot-rolled steel sheet according to claim 1 Symbol mounting, characterized in that it contains Ca 0.0005 to 0.01%. The hot-rolled steel sheet excellent in paint bake hardening performance and room temperature aging resistance according to claim 1 or 2 , characterized by containing 0.0001 to 0.001% B in mass%. The hot-rolled steel sheet excellent in paint bake hardening performance and room temperature aging resistance according to any one of claims 1 to 3 , characterized by containing 0.001 to 0.03% of Nb in mass%. By mass%, Ti = 0.0001~0.10%, and, N-0.29Ti> 0.0005 in any one of claims 1 to 4, characterized in that it contains Ti so as to satisfy the Hot rolled steel sheet with excellent paint bake hardening performance and room temperature aging resistance. By mass%, Sn, Cu, Ni, Co, Zn, W, claim the one or more of Zr and Mg, characterized in that it contains 0.001 to 1.0% in total 1-5 A hot-rolled steel sheet excellent in paint bake-hardening performance and normal temperature aging resistance according to any one of the above. The hot-rolled steel sheet according to any one of claims 1 to 6 is hot-dip galvanized, alloyed hot-dip galvanized or electrogalvanized, and is excellent in paint bake hardening performance and room temperature aging resistance. Galvanized hot rolled steel sheet. The slab having the chemical component according to any one of claims 1 to 6 is hot-rolled at a temperature of (Ar ₃ point-100) ° C or higher, and from the hot rolling end temperature to a temperature of 600 ° C or lower. A method for producing a hot-rolled steel sheet excellent in paint bake hardening performance and room temperature aging resistance, characterized by cooling at an average cooling rate of 10 ° C./s or higher and then winding at a temperature of 550 ° C. or lower.