JP2003155541A - High strength hot rolled steel sheet having excellent corrosion resistance and stretch-flanging property, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which has excellent corrosion resistance and stretch-flanging properties. SOLUTION: The hot rolled steel sheet has a composition containing, by mass, 0.03 to 0.10% C, 0.05 to 1.2% Si, 1.0 to 2.0% Mn, <=0.05% P, <=0.01% S, <=0.005% N and 0.01 to 0.05% Al, and, if required, containing either or both selected from Ti and Nb so as to satisfy -0.05<= Ti+(48/93)×Nb-(48/12)×C-(48/14)×N-(48/32)×S}<=0.2, and, if required, containing Cu, Ni and Ca, and the balance Fe. The thickness of an oxide film containing Fe2 SiO4 on the surface is <5 μm, and the area ratio of a bainitic ferrite phase in the cross-sectional structure is >=80%. In the method of producing the hot rolled steel sheet, the steel is heated to >=1,150 deg.C, is rough-rolled, is thereafter coiled at <=1,150 deg.C, is subjected to finish rolling as-uncoiled so as to be completed at an Ar3 point or higher, is subjected to high speed cooling to <=500 deg.C, and is they coiled at 300 to 500 deg.C.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength hot-rolled steel sheet having excellent corrosion resistance and stretch flangeability, and a method for producing the same.

[0002]

2. Description of the Related Art High-strength hot-rolled steel sheets are widely used for automobile parts, particularly members and arms called suspension parts. The characteristics of the steel sheet required for these parts include strength and ductility as well as stretch-flangeability evaluated by a hole expanding test. This is because these parts are often combined with other parts by methods such as bolting,
This is because the bolt holes at that time are generally formed by the protrusion processing (hole expansion processing) of the punched holes.

More specifically, it is required that cracks that penetrate the steel sheet cross section do not occur during the extrusion processing of the punched hole, and that the hole diameter can be expanded to a larger extent without causing through cracks. It is an indicator of superiority or inferiority.

Many proposals have been made for steel sheets having excellent stretch flangeability. For example, JP-A-6-17292
4, gazette 7-11382, and gazette 7-7.
That is the 0696 publication. These are the ones that have found a method for achieving both strength and stretch-flangeability through the examination of the chemical composition and manufacturing method of the steel sheet, while suppressing the inclusions that can be the base points of cracks during hole expanding,
The gist is the proposal of a steel sheet having a bainite structure or a bainitic ferrite structure as the main phase.

The inventors of the present invention also worked on research and development of a high-strength hot-rolled steel sheet excellent in stretch-flangeability, and in the same manner as the above proposal,
It was concluded that the inclusion control and the bainitic phase of the steel sheet structure, or the bainitic ferrite phase is basically the means to obtain an excellent steel sheet, and we were trying to improve it further.

Above all, the control of cooling conditions after rolling, which is the most important for obtaining a steel sheet having the structure, is devised, and the cooling conditions are likely to become unstable in the ordinary rolling method. However, we were also studying the application of the endless rolling method in order to create the material including the part from the exit of the finish rolling mill to the winding of the steel sheet. The endless rolling method referred to here means that rough rolled hot rolled materials (hereinafter referred to as rough bars) are joined one after another before finish rolling to continuously finish roll a plurality of rough bars and appropriately cut them. It is a method of manufacturing a hot rolled coil.

[0007]

On the other hand, recently, it has become clear that the average usage period of automobiles becomes longer, and in response to this, automobile manufacturers have come to demand higher durability for each member than ever before. . Suspension parts are no exception, and corrosion resistance, which has not been so strictly demanded in the past, is now required.

Since the underbody parts are much thicker than the outer panel, etc., and unlike stainless steel, there is little concern about local severe corrosion. Although it has been attempted to ensure corrosion resistance by designing parts, as the demand for weight reduction of vehicle bodies has increased, these ideas have been forced to change, and the requirement for corrosion resistance has been avoided. It became impossible.

Under these circumstances, corrosion resistance was required in addition to the strength, ductility and stretch flangeability that conventional high strength hot rolled steel sheets for underbody have had, but proposals for steel sheets from this viewpoint Is almost empty.

In general, it is pointed out that it is difficult to form a chemical conversion film for the purpose of ensuring paint adhesion on a steel sheet having a high Si content, which is not desirable from the viewpoint of ensuring corrosion resistance, but Si is a stretch flange. Since it is an element effective for increasing the strength without deteriorating the property, it is not preferable to reduce it unnecessarily. Therefore, it is an object of the present invention to provide a high-strength hot-rolled steel sheet that secures excellent chemical conversion treatability without failing to suppress the amount of Si, by extension, corrosion resistance, and also has excellent stretch flangeability. is there.

[0011]

As described above, the present inventors have been studying to apply endless rolling to a target steel sheet manufacturing method. We have been studying whether or not it is possible to add more characteristics that were not available in the past by using this method. The present invention was discovered in such a process, and completed through a detailed study, and the gist thereof is (1)% by mass, C: 0.03 to 0.10%, Si: 0.05 to 1.2%, Mn: 1.0 to 2.0%, P: 0.05% or less, S: 0.01% or less, N: 0.005% or less, Al: 0.01 .About.0.05%, Ti, Nb
-0.05 ≤ (Ti + (48/93) × Nb- (48/1
2) × C − (48/14) × N− (48/32) × S} ≦ 0.2, the balance is Fe and unavoidable impurities, and the surface of the oxide film containing Fe ₂ SiO ₄ is formed. A high-strength hot-rolled steel sheet having excellent corrosion resistance and stretch-flangeability, which has a thickness of less than 5 μm and an area ratio of bainitic ferrite phase in the cross-sectional structure of 80 to 100%.

(2) Further, in terms of mass%, Cu: 0.5 to 1.5% and Ni: 0.2 to 0.7% are contained, and the corrosion resistance and elongation described in (1) above. High strength hot rolled steel sheet with excellent flangeability.

(3) Further, in mass%, Ca: 0.000
The high-strength hot-rolled steel sheet having excellent corrosion resistance and stretch flangeability according to the above (1) or (2), characterized by containing 5 to 0.0025%.

(4) A method for producing the steel sheet according to any one of (1) to (3) above, which comprises (1) to
The steel material having the chemical composition according to any one of (3) is 1
After heating to 150 to 1250 ° C. and rough rolling, 1150
At a temperature below ℃
Finishing rolling is completed at r3 point to Ar3 point + 100 ° C., further cooled to 300 to 500 ° C. at an average cooling rate of 40 ° C./sec or more, and wound at 300 to 500 ° C., corrosion resistance and stretch flangeability. A method for producing a high-strength hot-rolled steel sheet which is excellent in Is.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION First, the experiments that have led to the completion of the present invention will be described below. The present inventors first conducted research on the Si content and chemical conversion treatability of the steel sheet. Si
Steels having different contents were hot rolled and pickled by a conventional method (hot rolling method other than endless rolling), and then subjected to chemical conversion treatment under the same conditions. When the amount (mass) of forming the chemical conversion film per unit area was examined, it was found that the amount of forming the chemical conversion film decreased as the Si content of the steel sheet increased.

A more detailed examination revealed that the chemical conversion treatment film of the steel sheet having a low Si content was composed of dense phosphate crystals, whereas the steel sheet having a high Si content had phosphate crystals. It was found that not only was it coarse, but there were even places where no film was formed. It was also clarified that a substance containing Fe ₂ SiO ₄ as a main component (hereinafter referred to as Fe—Si spinel) was thickly present in such a place in parallel with the surface of the steel sheet. From this, it was thought that if the thickness of the Fe-Si based spinel layer could be suppressed by some method, excellent chemical conversion treatability could be secured regardless of the Si content, and further studies were conducted.

For the purpose of simply thinning the Fe-Si spinel layer (including completely eliminating it), the pickling time after hot rolling should be lengthened. Increase the concentration, increase the temperature of the pickling solution, or
Although it may be possible to select such as mechanically removing by a method such as grinding, since the method only raises the manufacturing cost and impairs the original cost performance of the hot-rolled steel sheet, the present inventors have For removal, general pickling (for example, soaking in 5% HCl at 80 ° C for 30 seconds)
It was supposed to be done by.

On the other hand, as a part of studying the application of the endless rolling method, a test piece obtained by subjecting a rough bar to a work simulating coil winding after rough rolling was finish-rolled, pickled, and subjected to chemical conversion treatment. However, it has been found that the chemical conversion treatability of the steel sheet subjected to the working before finish rolling is superior to that of the steel sheet not subjected to the working. It was also found that such a difference is caused by the difference in the thickness of the Fe-Si based spinel layer on the surface of the steel sheet.

The present inventors promoted research based on this discovery, and made clear the chemical composition and manufacturing conditions for obtaining a hot-rolled steel sheet having excellent corrosion resistance in addition to strength, ductility, stretch flangeability, and the present invention. Has been completed.

The reasons for limiting the present invention will be described below. First, the chemical components will be described. The content rate of the component is mass%. C: 0.03 to 0.10%. C is an essential element for ensuring the strength of the steel sheet, and at least 0.03% is necessary to obtain a high-strength steel sheet. However, if it is contained in excess, it becomes unavoidable to generate carbides due to Ti or Nb and generation of a cementite phase unfavorable for stretch flangeability even if the cooling conditions are fully utilized, so the content is made 0.1% or less.

Si: 0.05 to 1.2%. Si is an element effective for ensuring the strength without deteriorating the stretch-flangeability, but if it is contained in excess, it tends to generate a polygonal ferrite phase which is unfavorable for the stretch-flangeability. Further, even if the manufacturing method of the present invention is used, deterioration of corrosion resistance cannot be suppressed, so the upper limit is made 1.2%.

Mn: 1.0 to 2.0%. Mn is C, Si
Along with 1.0%, it is an element effective in strengthening steel sheets.
The above should be contained, but if it exceeds 2.0%, it adversely affects the rough bar bondability for endless rolling, so the upper limit is made 2.0%.

P: 0.05% or less. P is effective as a solid solution strengthening element, but since it deteriorates the workability due to segregation and adversely affects the bondability of the rough bar like Mn, it must be made 0.05% or less.

S: 0.01% or less. S forms an inclusion such as MnS to deteriorate the stretch flangeability, and also has a harmful effect such as a decrease in the yield by combining with Ti contained for the purpose of making C a carbide. Therefore, the amount should be suppressed as much as possible, but 0.01% or less is acceptable.

N: 0.005% or less. N forms inclusions such as AlN and deteriorates the spreadability, and also binds with Ti contained for the purpose of converting C into a carbide to reduce the yield. Therefore, it should be suppressed as much as possible, but 0.005% or less is acceptable.

Al: 0.01 to 0.05%. Al needs to be added in an amount of 0.01% or more for deoxidation of molten steel,
If the content is more than 0.05%, the product tends to aggregate and coarsen to cause clogging of the continuous casting nozzle or to become a starting point of cracking during hole expansion processing.
The range is 0.05%.

One or both of Ti and Nb: -0.05≤ {Ti + (48/93) × Nb- (48/12) × C- (48/14) × N- (48/3
2) × S} ≦ 0.2 Ti and Nb improve workability of steel sheet by fixing C, S and N as precipitates (so-called sc
avenging effect) work. On the other hand, if added more than necessary, they are present in the steel as solid solution Ti or solid solution Nb and raise the temperature of recrystallization, tending to leave a hot work structure and impairing ductility. Then, the optimum range of the addition amount can be appropriately expressed by using the middle side of the above formula described using the chemical equivalent of each element as an index, as shown in Examples. That is, if the value is less than -0.05, the ductility and hole expandability are poor, and if it exceeds 0.2, the ductility is deteriorated. For the above reasons, it must be limited to satisfy the above formula.

Cu: 0.5-1.5%. Cu can be used as a solid solution strengthening element or a precipitation strengthening element for increasing the strength of the steel sheet. However, the effect is small unless 0.5% or more is added. On the other hand, if the content exceeds 1.5%, the surface properties of the steel sheet during hot rolling deteriorate, so the upper limit is 1.5%.

Ni: 0.2 to 0.7%. Ni is the above Cu
It has the effect of alleviating the deterioration of the hot rolled surface quality due to, and it is desirable to add 0.2% or more which is about half of Cu.
On the other hand, even if added over 0.7%, the effect is saturated,
Since it only raises the price of steel materials, the upper limit is 0.7%.

Ca: 0.0005 to 0.0025%. C
When a is contained in an amount of 0.0005% or more, it has an action of suppressing the formation of inclusions into a spherical shape by lowering the melting point thereof and becoming a starting point of cracks at the time of hole expansion. The effect is almost saturated at 0.0025%, so addition of less than that is sufficient.

In the present invention, the components other than the above are F
However, inevitable impurities that are mixed in from melted raw materials such as scrap are allowed.

Next, heating, rolling, cooling, and winding conditions will be described. Heating temperature: 1150 to 1250 ° C. In order to form a solid solution of TiC, NbC, etc., it is necessary to set the heating temperature to 1150 ° C or higher. By forming a solid solution of these, the formation of polygonal ferrite is suppressed in the cooling process after rolling, and a structure mainly composed of a bainitic ferrite phase for stretch flangeability can be obtained. On the other hand, when the heating temperature exceeds 1250 ° C, the slab surface is significantly oxidized, and in particular, wedge-shaped surface defects that are considered to be caused by selective oxidation of grain boundaries remain after descaling, which is the surface after rolling. The upper limit is 12 because it impairs quality.
Set to 50 ° C.

Temperature at which the coarse bar is wound into a coil: 115
0 ° C or less. This temperature is one of the most important conditions in the present invention. As will be described later in Examples, when a rough bar is wound at 1150 ° C. or lower, chemical conversion treatability is remarkably improved.
The mechanism is not always clear, but probably
When the rough bar is wound in the appropriate temperature range, Si in the steel plate
Of the hot-rolled scale (immediately below the hot-rolled scale formed during rough rolling) is suppressed, and as a result, the hot-rolled scale lower portion (directly above the hot-rolled steel) formed during finish rolling and after finish rolling. Fe-Si-based spinels are less likely to be generated, and they are completely removed or extremely reduced in the subsequent pickling step, which seems to be related to the improvement of chemical conversion treatability.

Further, the lower limit of the temperature at which the rough bar is wound may be set so that the finish rolling completion temperature is Ar3 to Ar3 + 100 ° C. in consideration of the temperature decrease during rolling and heat generation during processing. The temperature is approximately 950 ° C., preferably 980 ° C.
Note that the present invention is premised on providing a target steel sheet by endless rolling, but although the rough bar is wound in a coil shape, it is not joined with other rough bars, but simply rolled back for finish rolling. Methods are of course included.

Finishing rolling completion temperature: Ar3 point to Ar3 point +
100 ° C. In order to obtain a structure mainly composed of bainitic ferrite phase which is preferable for stretch flangeability,
It is necessary to perform rolling and cooling in the γ range. Therefore, the finish rolling completion temperature is set to Ar3 point or higher. On the other hand, the finishing temperature is A
If the temperature exceeds r3 point + 100 ° C, a polygonal ferrite phase is easily generated, so the upper limit is made Ar3 point + 100 ° C.

Cooling: 30 at an average cooling rate of 40 ° C./sec or more
From 0 to 500 ° C. In order to obtain a steel sheet having excellent stretch flangeability, it is necessary to suppress the formation of polygonal ferrite phase. For that purpose, it is necessary to cool to 300 to 500 ° C. at an average cooling rate of 40 ° C./second or more. on the other hand,
Although it is not necessary to set an upper limit on the cooling rate for structural control, too high a cooling rate may cause uneven cooling of the steel sheet, and it is expensive to manufacture equipment that enables such cooling. Is required, which may lead to an increase in steel plate price. From such a viewpoint, the upper limit of the cooling rate is preferably 100 ° C./second. Further, when the cooling stop temperature is lower than 300 ° C., a martensite phase which is unfavorable for stretch flangeability is generated, so the lower limit is set to 300 ° C.

Winding temperature: 300 to 500 ° C. 300 ° C
When it is rapidly cooled to a temperature below 1, the martensite phase is formed and stretch flangeability is extremely deteriorated. Therefore, the winding temperature needs to be 300 ° C. or higher. On the other hand, it is necessary to set the temperature to 500 ° C. or lower in order to suppress the formation of the polygonal ferrite phase. Further, by winding at 500 ° C. or lower, TiC and NbC are precipitated in the subsequent cooling process, and the amount of solid solution C in the ferrite phase is significantly reduced, and the stretch flangeability is improved.

Next, the surface and structure of the steel sheet will be described. Thickness of oxide film containing Fe ₂ SiO ₄ is less than 5 μm (including 0). The present inventors investigated in detail the surface of the steel sheet from which the hot rolled scale was removed using hydrochloric acid. A test piece prepared by gradually changing the immersion time in a dilute hydrochloric acid aqueous solution was prepared, and the oxide film thickness was measured by ellipsometry and the oxide was identified by replica observation by a transmission electron microscope.

As a result, SiO is formed on the base metal.₂ Mainly includes
Layer, Fe₂ SiO_Four A layer mainly containing Fe₃ O_Four And Fe₂ 
O₃ Is mainly laminated in this order to form an oxide film.
Cage, Fe₃ O_Four And Fe₂ O₃ Layer containing mainly is normal heat
Almost removable by pickling scale
₂ The layer mainly containing is not affected by chemical conversion treatability.
And, and Fe₂ SiO_Four The layer mainly containing is removed by pickling
It is difficult to get rid of, and if the thickness is 5 μm or more, chemical conversion treatment
It was found that the property significantly deteriorates. Fe₂ SiO _Four Including
This is the reason why the thickness of the oxide film is limited to less than 5 μm.
Yes, thinner is preferable, and it is preferable that this oxide film is not present.
Good

The "thickness of the oxide film containing Fe ₂ SiO ₄ " means the layer containing mainly SiO ₂ on the base metal and Fe ₂ SiO 4.
_It is the total thickness of the layers mainly containing _4, and was measured for each of three steel strips taken from the top, middle, and bottom of the hot rolled coil, and the averaged value was taken as the film thickness. . The content of SiO ₂ is a layer containing SiO ₂ mainly 5
A layer containing 0% or more and mainly containing Fe ₂ SiO ₄ is Fe.
The content of ₂ SiO ₄ is 50% or more, and the layer mainly containing Fe ₃ O ₄ and Fe ₂ O ₃ is defined as the total content of these two oxides being 50% or more.

On the other hand, since the thinner chemical conversion treatability if the thickness is thin oxide film improves containing Fe ₂ SiO ₄ Fe ₂ S
A state in which an oxide film containing iO ₄ does not exist (this is referred to as “Fe ₂
The thickness of the oxide film containing SiO ₄ is referred to as "0 (zero)") is also included in the present invention.

Area ratio of bainitic ferrite phase:
80-100%. In order to obtain excellent stretch flangeability, it is necessary to make the structure mainly composed of bainitic ferrite, and the area ratio thereof is 80% as shown in the examples.
Or more, preferably 90% or more. Further, the balance can contain 20% or less of bainite phase and polygonal ferrite phase, and it is desirable to avoid inclusion of martensite phase as much as possible.

[0043]

EXAMPLES Examples of the present invention will be described below together with comparative examples. (Example 1) A plurality of steel slabs having the chemical compositions shown in Table 1 were manufactured. These slabs were made of two sets of the same steel, reheated to 1250 ° C., rough-rolled, and subjected to endless rolling. The rough bar is wound up by a device called a coil box, then rewound, and the rear ends of the leading coarse bar and the leading ends of the trailing rough bars are shear-adjusted, and heat is applied to the abutted sheared surfaces at 5.2 kJ / It was joined with a cm laser and supplied to a finish rolling mill. The steel plate exiting the finishing rolling mill is cooled in the cooling zone,
It was cut by a shearing device immediately before the winder so as to remove the front and rear of the rough bar joint, and wound into two hot rolled coils. Table 2 shows the rough bar winding temperature, finish rolling completion temperature, cooling rate after finish rolling, and winding temperature.

The strength, ductility, hole expandability, chemical conversion treatability, and cross-sectional structure of the steel sheet thus obtained were examined.
The results are shown in Table 3 for each combination of steel and conditions. These are the averages of the test results performed on two coils of the same steel. The strength and ductility were determined by a tensile test of JIS No. 5 test pieces taken parallel to the rolling direction. The hole expandability is a diameter of 1 in the center of a 150 × 150 mm steel plate.
A 0 mm punched hole is spread with a 60 ° conical punch and the hole diameter D (mm) at the time when a through-thickness crack occurs is measured.
= (D-10) / 10 was evaluated by λ. The chemical conversion treatability was evaluated by the phosphate film amount W (g / m ² ) that could be formed on the surface of the steel sheet. The chemical conversion treatment is
The suspension was sprayed on a steel sheet that had been pickled, degreased, and washed with water, and the excess was removed by a squeezing roll to make the adhered amount constant, and then dried by a dryer.

As is clear from Table 3, by using the method of the present invention, it is possible to obtain a steel sheet excellent in strength, ductility, hole expandability and chemical conversion treatability.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

(Example 2) In mass%, C: 0.05%,
Si: 0.51%, Mn: 1.50%, P: 0.021
%, S: 0.0009%, N: 0.0008%, Al:
0.03%, Ti: 0.3%, Nb: 0.03%, C
u: 0.94%, Ni: 0.51%, Ca: 0.002
Multiple slabs containing 1% with the balance Fe and unavoidable impurities were produced. After reheating these slabs to 1250 ° C., they were roughly rolled and supplied to a coil box. At that time, the coarse bar was cooled by using the descaling device until immediately before, and the coarse bar winding temperature was changed in the range of 980 to 1200 ° C. The wound coarse bar was unwound and passed through a finish rolling mill. For comparison, after cooling in the same manner, rolling was also performed by passing through a finish rolling mill without winding in a coil box. Finishing rolling finish temperature is 850 ° C, cooling rate is 5
The coiling temperature was 0 ° C / sec and the coiling temperature was 450 ° C.

The steel sheet thus obtained was examined for strength, ductility, hole expandability, and chemical conversion treatability. The evaluation method is the same as in Example 1. As a result, the area ratio of the bainitic ferrite phase is 80% or more, the strength,
The ductility and hole expandability were hardly affected by the rough bar winding temperature, but the chemical conversion treatability was strongly affected.
The results, taking the rough bar coiling temperature on the horizontal axis Fe ₂ S
It is shown in FIG. 1 together with the thickness of the oxide film containing iO ₄ . As shown in the figure, when the winding temperature of the coarse bar exceeds 1150 ° C, the Fe ₂ S on the surface is not removed even if the coarse bar is wound and unwound.
Since the oxide film containing iO ₄ has a thickness of 5 μm or more, the effect of improving the chemical conversion treatability is not recognized.

[0051]

According to the method of the present invention, it is possible to obtain a high-strength hot-rolled steel sheet which is excellent in chemical conversion treatment property, eventually corrosion resistance, and stretch flangeability.

[Brief description of drawings]

FIG. 1 Winding temperature of coarse bar and chemical conversion treatability, and Fe
₂ is a graph showing the relationship of the thickness of an oxide film containing ₂ SiO ₄ . When finish rolling was performed without winding the rough bar, the temperature was plotted as the temperature at which the rough bar passed the coil box position.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/16 C22C 38/16 F term (reference) 4E002 AA07 AD04 BC05 BC07 BD07 CB01 4K037 EA01 EA05 EA09 EA13 EA15 EA18 EA20 EA23 EA25 EA27 EA28 EB05 EB07 EB08 EB09 FA02 FA03 FB10 FC03 FC04 FC07 FD04 FE01 GA02 HA02

Claims (4)

[Claims] 1. In mass%, C: 0.03 to 0.10%, Si: 0.05 to 1.2%, Mn: 1.0 to 2.0%, P: 0.05% or less , S: 0.01% or less, N: 0.005% or less, Al: 0.01 to 0.05%, and Ti, Nb.
-0.05 ≤ (Ti + (48/93) x Nb- (48/12) x C- (48/14) x N- (48/3
2) × S} ≦ 0.2 is contained, the balance is Fe and unavoidable impurities, and the thickness of the oxide film containing Fe ₂ SiO ₄ on the surface is 5
A high-strength hot-rolled steel sheet excellent in corrosion resistance and stretch-flangeability, characterized in that the area ratio of bainitic ferrite phase in the cross-sectional structure is 80 to 100%. 2. Corrosion resistance and stretch flangeability according to claim 1, further comprising Cu: 0.5 to 1.5% and Ni: 0.2 to 0.7% in mass%. Excellent high strength hot rolled steel sheet. 3. A high-strength hot-rolled steel sheet excellent in corrosion resistance and stretch-flangeability according to claim 1 or 2, further containing Ca: 0.0005 to 0.0025% by mass. . 4. A method for manufacturing the steel sheet according to any one of claims 1 to 3, wherein a steel material having the chemical composition according to any one of claims 1 to 3 is used at 1150 to 1250 ° C. After heating and rough rolling, it is wound into a coil at a temperature of 1150 ° C or lower, and while being rewound, Ar3 point to Ar3 point + 1
Finishing rolling is completed at 00 ° C., and further cooled to 300 to 500 ° C. at an average cooling rate of 40 ° C./sec or more, 300 to 50 ° C.
A method for producing a high-strength hot-rolled steel sheet excellent in corrosion resistance and stretch-flangeability, which comprises winding at 0 ° C.