JP3235416B2 - Manufacturing method of high strength hot rolled steel sheet with excellent workability and fatigue properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for a high-strength member such as an automobile or an industrial machine, and particularly to a steel plate having excellent formability and a durable strength as a structural member typified by a wheel disk of an automobile. The present invention relates to a method for producing a high-strength hot-rolled steel sheet having a tensile strength of 540 N / mm ² or more, which has excellent surface properties as an exterior member.

[0002]

2. Description of the Related Art Hot-rolled steel sheets produced by continuous hot rolling are widely used in automobiles and various other industrial machines as relatively inexpensive structural materials. Many members are formed by press working from the viewpoint of economy, and excellent workability is required.

[0003] In recent years, from the viewpoint of protection of the global environment, regulations for improving fuel efficiency of automobiles have been strengthened, and the development of high fuel efficiency engines typified by lean burn engines has been actively carried out, and the height of components has been increased. Reduction of vehicle weight by strength and thinning is being studied.

For example, as a steel plate used for a wheel disk, a high-tensile steel plate having a tensile strength of 590 N / mm ² or less is generally used. In addition to high static strength (tensile strength), excellent durability strength (fatigue strength) is required, and in addition, press formability such as ductility and stretch flangeability (hole expanding property) must be good. is there.

However, as the tensile strength increases, the fatigue strength of a normal smooth test piece improves, but the fatigue strength of a notched test piece is said to be almost saturated when the tensile strength is 540 N / mm ² or more. I have. Further, since workability usually decreases with an increase in tensile strength, it is not easy to achieve both high static strength or high durability and excellent workability.

As a material having both high static strength and workability, for example, Japanese Patent Publication No. 61-15128 or Japanese Patent Publication No. 61-1
A Dual-Phase microstructure steel as described in Japanese Patent No. 1291 is known. This Dual-Phase steel has a two-phase mixed structure of ferrite phase and martensite phase, low yield ratio (ratio of yield strength to tensile strength), high uniform elongation and high elongation at break, and excellent balance between strength and ductility. It is characterized by showing. However, in order to stably produce polygonal ferrite (equiaxial ferrite phase) which is preferable for improving workability, it is necessary to contain a large amount of Si or Cr.

Further, as a steel material having both high static strength and excellent workability, for example, a Tri-Phase structure steel as disclosed in Japanese Patent Application Laid-Open No. 3-264646 is known.
This Tri-Phase steel has a three-phase structure of ferrite phase, bainite phase and martensite phase. It has the excellent stretch flangeability of ferrite + bainite dual phase steel and the excellent ferrite + martensite dual phase steel. However, in order to stably produce polygonal ferrite necessary for improving workability, it is necessary to contain a large amount of Si.

[0008] However, in the steel containing a large amount of Si, Fe ₂ SiO ₄ is generated in the scale at the time of slab heating, which is deteriorates the descaling property during rolling, leaving scale defects or roughened surface of the steel sheet after pickling Therefore, it is not preferable to apply the present invention to a member that appears outside such as for a wheel. Further, if the surface of the steel sheet is rough, it is not easy to detect harmful scabs and the like, and it is necessary to reduce the sheet passing speed at the time of surface inspection, which causes a problem that the productivity is reduced.

Furthermore, oxides generated in steel sheets containing a large amount of Si and Cr deteriorate the pickling properties, and scales are apt to remain on the surface layer of the steel sheets even after pickling, and further, the chemical conversion property is reduced. However, there is a concern that the corrosion resistance after painting is reduced.

On the other hand, in a Tri-Phase steel, the volume ratio of each phase of ferrite, bainite, and martensite fluctuates due to temperature and heat history, and the characteristics are greatly affected by the temperature. There is also a problem that the variation is easily caused.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a high-strength hot-rolled steel sheet for processing having a high tensile strength of 540 N / mm ² or more, having excellent ductility, stretch flangeability, fatigue strength and surface properties. It is an object of the present invention to establish a method capable of stably producing a product.

[0012]

In order to achieve the above object, the present inventors have studied the effects of the alloying elements and metallographic structure or the conditions of hot rolling, cooling and winding on the mechanical properties of low Si steel. As a result, the following findings were obtained.

When the Si content is reduced, the surface properties of the steel sheet and the chemical conversion property can be improved. However, the decrease in strength due to the reduction of Si can be compensated by adding other solid solution strengthening elements or increasing the amount of hard bainite and martensite by controlling hot rolling conditions and cooling conditions, but the workability is poor. Slightly deteriorates.

By including a trace amount of Nb,
Actively introduces working strain into austenite during hot rolling, promotes the formation of a ferrite phase in the subsequent cooling process, and strengthens the ferrite phase by adding an appropriate amount of Nb and Ti in combination. The strength can be improved along with the improvement of the properties. In addition, coarse TiN
It is possible to significantly improve ductility by suppressing the formation of precipitates.

The basic composition is 0.05% C-0.01% Si-1.5%
Mn-0.002% N steel, without adding Nb,
A steel slab (slab) in which the Ti content was variously changed was melted with a steel containing 0.02% of b. These slabs are heated to 1220 ° C, hot-rolled, finish-rolled at 860 ° C, accelerated to 670 ° C at 40 ° C / s, and cooled for 5 seconds.
After air cooling, accelerate cooling at 70 ° C / s to 100 ° C, and then continue cooling from 100 ° C to room temperature, assuming the cooling state after winding.
A 3.2 mm-thick hot-rolled steel sheet was manufactured by cooling the furnace at a rate of ° C / h. Then, a JIS No. 5 tensile test piece was sampled from the steel sheet, and the tensile properties were examined.

FIG. 1 shows the results of examination of the effect of the Ti content on the tensile strength and elongation. As is apparent from this figure, by adding an appropriate amount of Ti to the low N N-added steel and controlling the cooling conditions after the finish rolling, the decrease in ductility can be reduced and the strength can be improved. / Mm
It is possible to achieve both a tensile strength of ² or more and a high elongation at break of 30% or more. This is presumably because the generation of processed ferrite was suppressed, the generation of polygonal ferrite for improving workability was promoted, and the generation of martensite was appropriately suppressed.

[0017] From rolled steel sheet under the above conditions of steel with varying the Ti content is 0.02% of the above Nb, the fatigue test piece was taken, subjected to Reversed plane bending fatigue test, withstand 10 ⁷ times repeated bending The obtained stress was defined as a fatigue limit or fatigue strength.

FIG. 2 shows the results. Generally, the fatigue limit ratio (the ratio of fatigue strength to tensile strength) hardly changes, and the fatigue strength increases in proportion to the increase in strength. However, as can be seen from the comparison between FIG. 1 and FIG. 2, it can be seen that as the Ti content increases, the fatigue limit ratio increases as the strength increases, and the fatigue strength is greatly improved. This is considered because the soft ferrite phase is strengthened by TiC.

Next, the influence of the carbon content was investigated. First, Fe
-0.01% Si-1.65% Mn-0.10% Ti-0.02% Nb-
Based on the composition of 0.002% N, C content is 0.03-0.13%
The slab changed in the range of was melted. 12 of these slabs
After heating to 50 ° C, hot rolling is performed, finish rolling at 840 ° C, accelerated cooling to 680 ° C, air-cooled for 3 seconds, accelerated cooling to 200 ° C, and slow cooling equivalent to winding. To produce a 3.2 mm thick hot rolled steel sheet. A JIS No. 5 tensile test piece was sampled from those steel sheets to investigate the tensile properties, and a 100 mm x 100 mm square test piece was cut out and punched with a 10 mmφ punch hole with a clearance of 15%, and then a 60 mm conical punch. A hole expansion test was performed to determine the critical hole expansion rate.

FIG. 3 shows the change in the tensile test value and the limit hole expansion ratio with the change in the C content.
It can be seen that when the value exceeds, the hole expandability is greatly reduced.

Although the present invention has been made based on the above findings,
The summary is as follows.

C: 0.03-0.08% by weight, S
i: 0.30% or less (excluding 0.30%) , Mn: 1.
00-2.00%, P: 0.010-0.030%, T
i: 0.05 to 0.15%, Nb: 0.01 to 0.06
%, N: 0.0040% or less, and Cr: 0 to 0.6
%, V: 0 to 0.06 %, Ca: 0 to 0.01 %, Z
r: 0~ 0.10% you and rare earth element: 0 to 0.10%
In, the steel and the balance of iron and inevitable impurities, 780
After completion of hot rolling at 950 ° C. to 950 ° C., accelerated cooling is performed at a cooling rate of 30 ° C./s or more to a temperature in the range of 600 ° C. to 700 ° C., and air-cooled from that temperature for 2 to 10 s.
Accelerated cooling to 250 ° C or less at a cooling rate of 200 ° C / s or more,
A method for producing a high-strength hot-rolled steel sheet having a tensile strength of 540 N / mm ² or more, which is excellent in workability, fatigue characteristics, and surface properties characterized by winding.

[0023]

Embodiments of the present invention will be described below in detail.

A. Chemical composition of steel sheet (1) C: 0.03 to 0.08% C is an element that increases the strength of the steel sheet, and is an especially important element for producing a high-strength steel excellent in workability. That is, if it is less than 0.03%, a sufficient amount of a hard second phase such as martensite cannot be secured, and a high-strength steel sheet having a tensile strength of 540 N / mm ² or more cannot be easily produced. On the other hand, when the content exceeds 0.08%, the amount of the second phase is excessively increased, which causes deterioration of hole expandability. Therefore, the C content is determined to be 0.03 to 0.08%. When the workability is particularly important, the upper limit of the C content is preferably set to 0.06%.

(2) Si: 0.30% or less (excluding 0.30%) Si can not only strengthen the ferrite phase by solid solution strengthening, but also promote the formation of polygonal ferrite and improve ductility. Therefore, it is a preferable element for producing a hot-rolled steel sheet having both high strength and excellent workability. In particular, it is an element to be used to obtain a two-phase mixed structure with a low yield ratio.

However, it is better not to contain Si from the viewpoint of deteriorating the surface quality, such as generation of surface flaws due to deterioration of the descaling property and adversely affecting the chemical conversion property.

Therefore, in the present invention, emphasis is placed on the surface properties, and at most 0.30% or less (excluding 0.30%) as a limit that does not significantly affect the surface properties. In order to stably obtain a surface having more excellent properties, the content is desirably less than 0.05%.

(3) Mn: 1.0 to 2.0% Mn has the effect of increasing the strength of the steel sheet through solid solution strengthening and transformation strengthening. However, if the content is less than 1.0%, the desired effect cannot be obtained. On the other hand, if the content is more than 2.0%, the martensite phase is excessively increased, and the hole expandability is deteriorated. Therefore, the content range was set to 1.0 to 2.0%.

(4) P: 0.010 to 0.030% P has the effect of strengthening the ferrite phase by solid solution strengthening and increasing the strength of the steel sheet. In addition, in the presence of Si, F generated at the interface between FeO and ground iron during slab heating
It has the effect of forming a solid solution in e ₂ SiO ₄ to form Fe ₂ (Si, P) O ₄ and improving the descaling property. If the content is less than 0.01%, these effects cannot be obtained.
If the content exceeds 0.03%, the weldability and workability are deteriorated due to the segregation of the center of the continuous cast slab, so the content is limited to 0.010 to 0.030%.

In particular, when the Si content is 0.05% or more,
When the P content is 0.015% or more, it is possible to more stably obtain a steel sheet having good surface properties.

(5) Ti: 0.05 to 0.15% Ti is one of the most important elements together with Nb in the present invention. Ti has an effect of strengthening the ferrite phase by precipitation of TiC, and is generally used as one of relatively inexpensive strengthening elements. However, since precipitation strengthening involves a significant decrease in ductility, it is necessary to limit the content of high-strength steel when good press formability is required. However, especially in steel with reduced N, if Ti is added in combination with a trace amount of Nb, the strength can be improved without a significant decrease in ductility.

First, by reducing the N content, formation of coarse TiN precipitates harmful to workability is prevented.
Next, at the time of hot rolling finish rolling, the addition of a small amount of Nb delays the progress of recrystallization in the austenite region, and is processed at the next stand before recrystallization is completed between rolling stands.
The cumulative amount of austenite processing strain increases. For this reason, precipitation in the austenite region during hot rolling is promoted, and TiC becomes uniform and fine. In this state, austenite is transformed into ferrite, and TiC is formed.
The C precipitate loses consistency with the generated ferrite ground. In this way, it is presumed that the formation of coherent precipitates of TiC which significantly deteriorates ductility is prevented, and the strength is increased but the deterioration of ductility is reduced.

Further, by increasing the amount of TiC precipitates generated during hot rolling by controlling the Ti content to be relatively high and increasing the cumulative amount of the above-mentioned strain, the accelerated cooling conditions after hot rolling, Variations in the TiC precipitate distribution due to the winding temperature are reduced. For the reasons described above, the soft ferrite phase is strengthened by finely dispersed TiC, and ordinary fatigue strength and notch fatigue strength are stably improved.

If the Ti content is less than 0.05 %, the desired effect due to the above-mentioned effects cannot be obtained because the content is too small.
If the content exceeds 15%, not only the above-mentioned action is saturated, but also the precipitation of coarse TiN is promoted and the workability is lowered, so the content is set to 0.05 to 0.15%. In addition, a desirable content for stably obtaining more excellent workability is 0.08 to 0.13%.

(6) Nb: 0.01 to 0.06% Nb is one of the most important elements together with Ti in the present invention. Nb is generally used as a relatively inexpensive strengthening element because Nb has an effect of strengthening the ferrite phase by precipitation of Nb (C, N). However, the decrease in ductility due to precipitation strengthening is unavoidable, and
Since it is easy to reduce the workability of a steel sheet by suppressing the formation of polygonal ferrite, it is necessary to limit the content of the steel sheet when press formability is required.

However, the presence of a small amount of Nb expands the non-recrystallization rolling temperature range of austenite and increases the amount of strain accumulation, and this becomes a driving force, whereby a ferrite phase can be stably formed. become. Therefore, in the present invention, a small amount of Nb is included.

If the Nb content is less than 0.01%, the above effects cannot be sufficiently obtained. On the other hand, if the Nb content exceeds 0.06%, the formation of polygonal ferrite is suppressed and the workability is reduced. Therefore, the range of the Nb content is determined to be 0.01% to 0.06%. The content range desirable for stably improving workability is 0.015% to 0.03%.

(7) N: 0.0040% or less N is one of the impurities inevitably mixed in the steel making or casting process, and usually forms a nitride by combining with Al, Ti or the like in the steel. However, in the case of steel containing Ti, when the N content is high, coarse TiN is likely to be generated in the process of solidifying the molten steel, and this may be a starting point of crack generation, which may lower the workability of the steel sheet. In addition, the precipitation temperature of carbonitride of Nb rises and precipitates and coarsens in the rough rolling step, so that Nb does not work effectively in the finish rolling step. Therefore, the smaller the content of N, the better, but as a limit that the effect is not remarkable, the content is set to 0.0040% or less. Desirable is 0.0020% or less.

(8) Cr: 0.6% or less Cr has an effect of promoting the formation of martensite, and is effective in improving the strength through its transformation enhancement. When it is contained, a desired effect can be obtained at 0.02% or more. On the other hand, if the content exceeds 0.6%, the pickling property and the chemical conversion property deteriorate remarkably, so the content is set to 0.6% or less.

It should be noted that it is better not to include the metal oxide, particularly when the surface properties and chemical conversion property are emphasized, and it is desirable to control the content to 0.05% or less at most.

(9) V: 0.06% or less V may not be added, but is an element effective for improving strength through precipitation strengthening by bonding with N and C, and may be contained as necessary. When it is contained, a desired effect can be obtained with a content of 0.001% or more. On the other hand, if the content exceeds 0.06%, the above effect is saturated, and the economic efficiency is impaired. Therefore, the content was determined to be 0.06% or less.

(10) Ca: 0.01% or less, Zr: 0.10% or less and rare earth element: 0.10% or less Any addition of Ca, Zr and rare earth elements changes the properties of MnS of inclusions and expands during hot rolling. This has the effect of making it difficult to elongate, and is particularly effective in preventing cracking in processing that undergoes tensile deformation in the width direction of rolling.

If the occurrence of such cracks is not a problem, there is no need to add them. However, if the effect is expected, the effect is not exhibited if the amount is small. Therefore, it is preferable that the content of Ca is 0.0002% or more, the content of Zr is 0.01% or more, and the content of rare earth element is 0.002% or more. However, Ca is 0.01%, Zr is 0.10%, and rare earth element is 0.10%
, The inclusions in the steel become too large and the workability deteriorates.

Therefore, each content is set to Ca: 0.01
%, Zr: 0.10% or less, rare earth element: 0.10% or less. When the S content can be reduced to less than 0.001%, there is no deterioration in workability due to a change in the form of MnS.
It is effective to add when the S content exceeds 0.001%.

Al is added as a deoxidizing agent in order to obtain a sound slab during steel making.
It has little effect on the properties of the steel sheet, in particular its content is not regulated. However, if added in a large amount, the workability is reduced, so that the content is desirably 0.10% or less. Note that 0.03% or less is sufficient to obtain a deoxidizing effect.

S, which is one of the inevitable impurities, reacts with Mn to form MnS-based inclusions and lowers the press workability, so the smaller the better, the better. The desirable upper limit of the content is 0.02%, but is preferably 0.005% or less, and particularly preferably is reduced to 0.001% or less when the form control of MnS by Ca treatment or the like is not performed.

In addition, examples of "unavoidable impurities" unavoidably mixed into steel include O, Cu, Mo, and the like. The smaller the content of these impurity elements, the more desirable.

The steel having the above-described composition can be melted in, for example, an electric furnace. However, in order to keep the N content low, melting in a converter is preferred. Also, the production of hot-rolled slabs may be performed by any of ingot-bulking rolling and continuous casting.

B. Hot Rolling Conditions The hot rolling step of the production method of the present invention involves rolling the heated slab and completing the rolling at a finishing temperature of 780 to 950 ° C., and then 30 ° C. to a temperature in the range of 600 to 700 ° C. / S accelerated cooling at a cooling rate of at least
Winding is performed by accelerated cooling to 400 ° C or less at a cooling rate of at least ° C / s. Hereinafter, the conditions of each step will be described.

(1) Slab Heating Temperature In this hot rolling, a continuous cast slab or a bulk rolled slab may be used, or a directly-rolled slab or a slab once cooled may be reheated and used.

However, when reheating the slab, N
The reheating temperature is preferably 1150 ° C. or higher in order to completely re-dissolve the carbonitride b and TiC in b. When the heating temperature is lower than 1150 ° C., the re-dissolution of the Nb carbonitride once precipitated becomes insufficient, and the N
The work hardening of austenite is not promoted due to the shortage of b, and the precipitation of TiC in the austenite region becomes insufficient. Also, the amount of ferrite generated during accelerated cooling after finish rolling is reduced, leading to a reduction in ductility.

(2) Finishing temperature Next, the austenite grains are refined by completing rolling at a finishing temperature in the range of 780 to 950 ° C., and the austenite is work-hardened. Production can be promoted.
In this case, when the finish rolling temperature is less than 780 ° C, ferrite is generated during hot rolling and becomes processed ferrite,
Workability of the hot-rolled steel sheet is reduced. On the other hand, the finishing temperature
If the temperature exceeds 950 ° C., work hardening of austenite becomes insufficient, and in the subsequent cooling process, polygonal ferrite is not sufficiently generated, resulting in deterioration of workability. Therefore, the finishing temperature of hot rolling is set to 780 to 950 ° C. More desirable
By performing finish rolling at 780 to 860 ° C., polygonal ferrite can be generated more stably.

(3) Cooling process In the cooling process after the completion of hot rolling, accelerated cooling to a temperature falling within the range of 600 to 700 ° C. and air cooling from that temperature for 2 to 10 s give a larger amount of polygonal material. Ferrite can be generated stably. This allows
A high-strength hot-rolled steel sheet having excellent workability, particularly excellent ductility, can be obtained.

If the temperature at which air cooling is started is less than 600 ° C., or if the air cooling time is less than 2 seconds, the formation of polygonal ferrite is insufficient, and the air cooling starting temperature exceeds 700 ° C.
Alternatively, if the air cooling time exceeds 10 s even within the predetermined temperature range, pearlite is generated, and the strength and ductility of the steel sheet decrease. Therefore, the air cooling conditions during the cooling process should be 600-700.
Air cooling was performed for 2 to 10 seconds in the temperature range of ° C. The cooling rate during air cooling varies depending on the thickness of the steel sheet and the passing speed.
In case of 4mm thickness, it is about 6 ~ 12 ℃ / s.

The reason why the accelerated cooling to the air-cooling start temperature after hot rolling is set to a cooling rate of 30 ° C./s or more is, in part, to form ferrite finely.
This is to make it easy to secure an air cooling time of up to 10 seconds.

The reason why the cooling after the completion of the air cooling is set to 50 ° C./s or more is that if the cooling rate is less than 50 ° C./s, bainite is formed, the yield ratio increases, and the ductility decreases. Without the formation of bainite, there is no need to increase the cooling rate any further.

Regarding the winding temperature, as in the first hot rolling condition, at a temperature exceeding 250 ° C. , bainite is formed after winding, the yield ratio increases, and the elongation decreases. Therefore, the winding temperature was set to 250 ° C. or less . Winding temperature is 2
If it is below 50 ° C., there is no need for excessive cooling.

[0058]

EXAMPLES Slabs of steel types A to X having the chemical compositions shown in Table 1 were subjected to hot rolling and rolling after cooling under the conditions shown in Table 2 to obtain hot-rolled steel sheets having a thickness of 3.2 mm.

Regarding the surface properties of the steel sheet after rolling, the surface of the steel sheet after pickling was visually observed, and those having no scale residue were evaluated as ◎ (very good), and those with almost completely removed scale were evaluated as ○ ( Almost good), those with partial scale residue were rated as Δ, and those with scale flaws were rated as x. Table 2 also shows the results.

[0060]

[Table 1]

[0061]

[Table 2]

From the obtained steel sheet, a direction parallel to the rolling direction
A JIS No. 5 tensile test piece was collected and subjected to a tensile test.
Further, a steel slab having a width of 100 mm and a length of 100 mm was sampled, a hole of 10 mmφ was punched out, and a hole expansion test was performed with a conical punch having a vertex angle of 60 °. In the fatigue test, a bilateral plane bending and a pulsating pull with a notched test piece were performed. As shown in Fig. 4, plane bending uses a No. 1 test piece with a width of 20 mm, and a notched test piece uses a test piece punched out with a clearance of 10% with a hole of 12 mm in diameter to withstand 10 ⁷ repetitions. That is, the fatigue limit was determined, and this was defined as the fatigue strength. Table 3 shows the results together. The table also shows the fatigue limit ratio (fatigue strength / tensile strength).

[0063]

[Table 3]

As shown in Table 2, the surface after rolling is good for steels falling within the chemical composition range specified in the present invention (test numbers 1 to 21). Further, as shown in Table 3, hot rolled steel sheets (test numbers 1 to 15) manufactured by rolling and cooling these steels under the conditions specified in the present invention were all 540 N / mm ^2.
At the same time as satisfying the above tensile strength, a larger elongation, a higher hole expansion ratio, and a higher fatigue strength can be obtained with respect to the comparative steel having the same strength.

On the other hand, in Test No. 16 in which the steel had a chemical composition within the range of the present invention but the finishing temperature was below the lower limit of the specified value, the strength, ductility, hole expandability, and fatigue limit ratio were not improved. This is because processed ferrite and pearlite were formed. In Test No. 19, in which the air cooling time exceeded the upper limit of the specified value, and in Test No. 20, in which the cooling rate after air cooling exceeded the lower limit of the specified value, pearlite was formed and strength and hole expandability were not improved, and the fatigue limit ratio was not improved. Not good either. On the other hand, in Test No. 17 in which the air cooling temperature was out of the upper limit of the specified value and in Test No. 21 in which the air cooling temperature was out of the lower limit, the amount of polygonal ferrite generated was insufficient, the ductility and hole expandability were not improved, and the notch fatigue was not improved. The fatigue limit ratio does not improve. Further, in Test No. 18, in which the winding temperature was outside the upper limit of the specified value, martensite was not formed, the strength and ductility were insufficient, and the notch fatigue ratio was not good.

In Test Nos. 22 and 31 in which the C content was outside the upper limit specified in the present invention, the elongation and the hole expandability were not improved.
Test number 23 in which the amount of Si exceeded the upper limit of the specified value, test number 27 in which the amount of Cr exceeded the upper limit of the specified value, test number 28 in which the amount of Si and Cr exceeded the upper limit of the specified value, and the specified amount of P In Test No. 29, which fell outside the lower limit of, the surface properties were inferior. Further, in Test Nos. 24 and 30 in which the Ti content was below the lower limit of the specified value, the tensile strength or the fatigue strength was insufficient, and the fatigue limit ratio was low. This is considered to be due to insufficient precipitation strengthening. In Test No. 26 in which the amount of Mn was below the lower limit of the specified value, the tensile strength and the fatigue strength were low and the fatigue limit ratio of the notch fatigue was low, but this was because martensite was not generated.
In test numbers 25, 30, 31 in which the Nb amount was outside the lower limit of the specified value, and in test numbers 27, 29 in which the N amount was out of the upper limit of the specified value,
Growth is poor.

As described above, it can be seen that the steel sheet manufactured under the conditions in the range defined by the present invention has excellent performance as compared with the steel sheets manufactured under the manufacturing conditions outside the range.

[0068]

As described above, according to the present invention, the tensile strength of 540 N / mm ² or more, the elongation and the hole expanding property,
Hot rolled steel sheets with excellent fatigue strength and good surface properties can be stably mass-produced. The hot-rolled steel sheet is suitable as a high-strength member for automobiles or industrial machines, particularly as a material for wheels, and has an extremely useful effect in industry.

[Brief description of the drawings]

FIG. 1 is a view showing the effect of Ti content on tensile strength and elongation of a hot-rolled steel sheet.

FIG. 2 is a diagram showing the relationship between the fatigue strength of a hot-rolled steel sheet and the Ti content.

FIG. 3 is a view showing the influence of the C content on the tensile strength, elongation, and hole expandability of a hot-rolled steel sheet.

FIG. 4 is a diagram showing the shapes of two types of test pieces used in a fatigue test.

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-6-293910 (JP, A) JP-A-5-179346 (JP, A) JP-A-7-118740 (JP, A) JP-A-8-118 188847 (JP, A) JP-A-8-149542 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C21D 9/46 C22C 38/00- 38/60

Claims (1)

(57) [Claims] C .: 0.03 to 0.08% by weight, S
i: 0.30% or less (excluding 0.30%) , Mn: 1.
00-2.00%, P: 0.010-0.030%, T
i: 0.05 to 0.15%, Nb: 0.01 to 0.06
%, N: 0.0040% or less, and Cr: 0 to 0.6
%, V: 0 to 0.06 %, Ca: 0 to 0.01 %, Z
r: 0~ 0.10% you and rare earth element: 0 to 0.10%
In, the steel and the balance of iron and inevitable impurities, 780
After completing the hot rolling at 950 ° C. to 950 ° C., accelerated cooling is performed at a cooling rate of 30 ° C./s or more to a temperature in the range of 600 ° C. to 700 ° C.
Accelerated cooling to 250 ° C or less at a cooling rate of 200 ° C / s or more,
A method for producing a high-strength hot-rolled steel sheet having a tensile strength of 540 N / mm ² or more, which is excellent in workability, fatigue characteristics, and surface properties characterized by winding.