JP2002180190A - High strength hot rolled steel sheet having excellent hole expandability and ductility and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which has strength of >=690 N/mm2, and combines high hole expandability and ductility, and to provide a method for producing the steel sheet. SOLUTION: Steel having a composition containing, by mass, 0.01 to 0.08% C, 0.30 to 1.50% Si, 0.50 to 2.50% Mn, <=0.03% P, <=0.005% S and one or two kinds of 0.01 to 0.20% Ti and 0.01 to 0.04% Nb, and the balance iron with inevitable impurities is hot-rolled into a steel sheet with a ferrite-bainite dual steel structure in which the ratio of ferrite with a grain size of >=2 μm is >=80%. Further, the steel sheet may contain one or two kinds of Ca and rare earth metals by 0.0005 to 0.01%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to an undercarriage part of an automobile to be pressed and has a diameter of 1.0 to 6.0 mm.
The present invention relates to a high-strength hot-rolled steel sheet having a thickness of about 690 N / mm ² or more and having excellent hole expandability and ductility, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for cost reduction by reducing the weight of a vehicle body and integrally molding parts as a measure to improve fuel economy due to environmental problems of automobiles, and the development of a high-strength hot-rolled steel sheet having excellent press workability has been developed. It has been advanced. Conventionally, as such high-strength hot-rolled steel sheets for processing, those having a mixed structure composed of a ferrite-martensite structure and a ferrite-bainite structure, or those having a substantially single-phase structure mainly composed of bainite and ferrite are widely known.

[0003] However, in the ferrite-martensite structure, microvoids are generated around the martensite from the initial stage of deformation and cracks are generated, so that there is a problem that the hole-expandability is inferior. It was not suitable for applications requiring

Japanese Patent Application Laid-Open Nos. 4-88125 and 3-180426 disclose a steel sheet having a structure mainly composed of bainite. Although excellent in ductility, it is inferior in ductility because there is little soft ferrite phase. Further, JP-A-6-172924 and JP-A-7-11382 disclose steel sheets having a structure mainly composed of ferrite, but also have excellent hole expandability.
Since hard carbide is precipitated to secure strength, it is inferior in ductility.

Japanese Patent Application Laid-Open No. 6-200351 discloses a steel sheet having a ferrite bainite structure and excellent in hole expandability and ductility.
Japanese Patent Application Laid-Open Publication No. H11-209,199 discloses a method for producing a steel sheet in which hole expansion and ductility are compatible by controlling the ferrite occupancy by using two-stage cooling. However, with the background of lighter automobiles and more complex parts, higher hole expandability and ductility are required, and recent high-strength hot-rolled steel sheets require advanced workability that cannot be handled by the above-mentioned technologies. Have been.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and is intended to solve the problems of the 690N.
/ mm ² prevents or more hole expandability and ductility degradation associated with high strength, to provide a high-strength hot-rolled steel sheet and a manufacturing method of the steel sheet having high bore expandability and ductility even at high intensity Aim.

[0007]

Means for Solving the Problems The high-strength hot-rolled steel sheet having excellent hole expandability and ductility of the present invention, which has been made to solve the above-mentioned problems, has a carbon content of 0.01 to 0.08% and 0.30 to 0.3% by mass. 1.50
%, Mn 0.50 to 2.50%, P ≤ 0.03%, S ≤ 0.005%, and
A high-strength hot-rolled steel sheet containing one or two kinds of Ti 0.01 to 0.20% and Nb 0.01 to 0.04%, with the balance of iron and unavoidable impurities having a steel structure in which the proportion of ferrite with a grain size of 2 μm or more is It is characterized by having a ferrite-bainite two-phase structure of 80% or more and a strength of 690 N / mm ² or more. The high-strength hot-rolled steel sheet can contain 0.0005 to 0.01% of one or two of Ca and REM.

The method for producing a high-strength hot-rolled steel sheet excellent in hole expandability and ductility according to the present invention is characterized in that the rolling end temperature is set to the Ar ₃ transformation point to 950 ° C., and then the hot rolling is terminated, and then 20 ° C./sec. At a cooling rate of 650-800 ° C., then 2-15
After air-cooling for 20 seconds, the cooling rate is 20 ° C / sec or more.
It is characterized by cooling to 0 to 600 ° C and winding.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is well known that, in a high-strength hot-rolled steel sheet, hole expandability and ductility tend to contradict each other. The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, it has been found that the ductility can be improved without deteriorating the hole expandability by making the ferrite grain size in the ferrite bainite steel a grain size of a certain value or more as much as possible. And completed the present invention. In other words, in ferrite bainite steel, attention is paid to precipitates composed of ferrite that increases ductility and TiC and NbC that secure strength, and ductility is improved without lowering hole expandability by sufficiently growing ferrite grains. The object has been achieved by forming a precipitate to secure the strength.

In the present invention, C in the high-strength hot-rolled steel sheet is
0.01 to 0.08%. C is an element necessary for preserving the strength by precipitating carbide, and if it is less than 0.01%, it becomes difficult to secure the desired strength. On the other hand, if it exceeds 0.08%, the ductility is greatly reduced.

Si is one of the most important elements in the present invention, and is important for suppressing the formation of harmful carbides and for forming a structure mainly composed of ferrite into a composite structure of the remaining bainite.
In addition, the addition of Si makes it possible to achieve both strength and ductility. To obtain such an effect, it is necessary to add 0.3% or more. However, when the added amount increases, the chemical conversion property decreases and the spot weldability also deteriorates, so the upper limit is 1.5%.
In addition, it is preferable that the range of Si is set to 0.9 to 1.2% because hole expandability and ductility can be effectively compatible.

Mn is one of the important elements in the present invention.
It is an element necessary for securing the strength, and for this purpose, addition of 0.50% or more is required. However, when added in a large amount exceeding 2.5%, micro segregation and macro segregation are likely to occur,
Deterioration of hole spreadability. Note that, in order to effectively achieve both hole expandability and ductility, the range of Mn is preferably set to 1.00 to 1.50%.

Since P forms a solid solution in ferrite and lowers its ductility, its content is made 0.03% or less. Also, S
Since MnS forms MnS and acts as a starting point of fracture and significantly reduces hole expandability and ductility, the content is made 0.005% or less.

[0014] Ti and Nb are also one of the most important elements in the present invention, and are elements effective for precipitating fine carbides such as TiC and NbC to secure the strength. For this purpose it is necessary to add one or two of Ti 0.05-0.20%, Nb 0.01-0.04%, Ti less than 0.05%, Nb
If it is less than 0.01%, it is difficult to secure strength, and Ti
If the Nb content is more than 0.20% and the Nb content is more than 0.04%, a large amount of precipitates will be formed and ductility will deteriorate.

Ca and REM are effective elements for controlling the morphology of sulfide-based inclusions and improving hole-expandability. In order to make this morphological control effect effective, one or two of Ca and REM
It is desirable to add 0.0005% or more. On the other hand, the addition of a large amount leads to coarsening of sulfide-based inclusions, which not only deteriorates cleanliness and lowers ductility, but also increases costs, so the upper limit is made 0.01%.

The size of the ferrite grain size is one of the most important indicators in the present invention. As a result of intensive studies, the present inventors have found that when the area ratio of ferrite having a particle size of 2 μm or more is 80% or more, both the hole expandability and the ductility are excellent. That is, FIG. 1 (tensile strength 780-82)
0N / mm ² , high-strength hot-rolled steel sheet with a λ value of 100 to 115), the percentage of ferrite grains having a grain size of 2 μm or more is 80%.
Above, the steel sheet shows high ductility. When the particle size is less than 2 μm, the crystal grains are not sufficiently recovered and grown, which causes a decrease in ductility. For this reason, in order to achieve both good hole expandability and ductility, the ratio of ferrite grains having a grain size of 2 μm or more needs to be 80% or more.
In order to obtain a more remarkable effect, it is desirable that the ratio of ferrite grains having a grain size of 3 μm or more be 80% or more. still,
The particle size can be determined by converting the area of each particle into a circle equivalent diameter.

The steel structure of the high-strength hot-rolled steel sheet is composed of ferrite and bainite. Here, since the steel structure contains 80% or more of ferrite having a grain size of 2 μm or more, the steel structure becomes a ferrite-bainite two-phase structure of 80% or more of ferrite. For example, as the organization of the present invention,
80% or more of ferrite having a particle size of 2 μm or more, with the balance being 2%
Ferrite having a particle size of less than μm and bainite, or ferrite having a particle size of 2 μm or more having a particle size of 80% or more and the remainder having only bainite can be used. The reason why the content of bainite is set to 20% or less in this way is that when the amount of bainite is larger than this, the decrease in ductility increases.

In producing a high-strength hot-rolled steel sheet by hot rolling, the finish rolling end temperature must be equal to or higher than the Ar ₃ transformation point in order to suppress ferrite formation and improve hole expandability. However, if the temperature is too high, the strength and ductility are reduced due to the coarsening of the structure, so the finish rolling finish temperature must be 950 ° C. or less.

Rapid cooling of the steel sheet immediately after the end of rolling is important for obtaining high hole expandability, and the cooling rate needs to be 20 ° C./sec or more. If the temperature is less than 20 ° C./sec, it is difficult to suppress the formation of carbides that are harmful to hole expandability.

It is important to temporarily stop the rapid cooling of the steel sheet and perform air cooling to precipitate ferrite, increase its occupancy, and improve ductility. However, if the air-cooling start temperature is lower than 650 ° C, pearlite, which is harmful to hole expandability, will be generated from an early stage. On the other hand, when the air-cooling start temperature exceeds 800 ° C., the formation of ferrite is slow and the effect of air-cooling is not easily obtained, and pearlite is likely to be generated during the subsequent cooling. Therefore, the air cooling start temperature is 650 ~
Set to 800 ° C. Further, even if the air cooling time exceeds 15 seconds, the increase in ferrite not only saturates, but also a load is imposed on the control of the cooling rate and the winding temperature thereafter. Therefore, the air cooling time is set to 15 seconds or less. If the air cooling time is less than 2 seconds, ferrite cannot be sufficiently precipitated.

After air cooling, the steel sheet is rapidly cooled again, but the cooling rate also needs to be 20 ° C./sec or more. 20 ° C / s
If it is less than ec, harmful pearlite is likely to be generated. Then, the quenching stop temperature, that is, the winding temperature is 350
~ 600 ° C. If the winding temperature is lower than 350 ° C., hard martensite which is harmful to the hole expandability is generated, while if it exceeds 600 ° C., pearlite and cementite which are harmful to the hole expandability tend to be generated.

By the combination of the above components and hot rolling conditions, the proportion of ferrite having a grain size of 2 μm or more in the steel structure is
A high-strength hot-rolled steel sheet having a ferrite-bainite dual phase structure of 80% or more and a strength of 690 N / mm ² or more and excellent in hole expandability and ductility can be produced. Further, even if a surface treatment (for example, galvanizing) is applied to the surface of the steel sheet of the present invention, the effect of the present invention is obtained, and the present invention does not deviate from the present invention.

[0023]

EXAMPLE A steel having the chemical composition shown in Table 1 was melted in a converter and converted into a slab by continuous casting. The slab was rolled and cooled under the same hot rolling conditions as shown in Table 1, and had a sheet thickness of 2.6 to 3.2 mm. Hot rolled steel sheets were manufactured. The rapid cooling speed was 40 ° C./sec, and the air cooling time was 10 seconds.

[0024]

[Table 1]

[0025]

[Table 2]

The hot rolled steel sheet thus obtained was subjected to a tensile test using a JIS No. 5 test piece, a hole expansion test, and a structure observation. In the observation of the structure, after corrosion with nital, ferrite and bainite were identified by a scanning electron microscope, and the area ratio of the ferrite having a particle size of 2 μm or more was measured by image analysis. In addition, the hole expansion test was performed using the initial hole diameter (d ₀ :
10mm) is punched out with a 60 ° conical punch and the hole expansion value (λ value) = (dd ₀ ) / d ₀ × 100 is calculated from the hole diameter (d) at the time when the crack penetrates the plate thickness. Was evaluated. Table 2 shows the results.

No. 1 to 11 are chemical components, finishing temperature,
Both the air-cooling start temperature and the winding temperature are within the range of the present invention, and are examples of the present invention in which the structure is made of ferrite bainite, and the ratio of ferrite having a particle size of 2 μm or more is 80% or more. It is a high-strength hot-rolled steel sheet with lambda value and elongation that is excellent in hole expandability and ductility. On the other hand, No. 12-21
Comparative Examples out of the conditions of the present invention are inferior in balance of strength, hole expandability and ductility.

Although not shown in the table, no. Finishing temperature 920 ° C, air cooling starting temperature 6
When hot rolling was performed at 25 ° C. and a winding temperature of 460 ° C., pearlite was generated in the structure because the air cooling starting temperature was too low within the range of the present invention, and the area ratio of ferrite having a grain size of 2 μm or more was 75%. Low, so elongation 19%, lambda value
It became 95%, and the hole spreading property and ductility balance were inferior. Similarly, No. Finishing temperature 910 ° C, air cooling start temperature 680 ° C, winding temperature 3
When hot rolling as 20 ° C., martensite is generated in the structure because the winding temperature was too low than the range of the present invention,
Further, the area ratio of ferrite having a grain size of 2 μm or more was as low as 63%, and as a result, the elongation was 20% and the λ value was 63%, so that the hole expandability and the ductility balance were also poor.

[0029]

As described in detail above, according to the present invention, a high-strength hot-rolled steel sheet having high tensile strength of 690 N / mm ² or more and having both hole expandability and ductility can be provided economically. Therefore, the present invention is suitable as a high-strength hot-rolled steel sheet having high workability. In addition, the high-strength hot-rolled steel sheet of the present invention is industrially valuable as it can reduce the weight of a vehicle body, integrate parts, and streamline processing steps, thereby improving fuel efficiency and reducing manufacturing costs. It is a big thing.

[Brief description of the drawings]

FIG. 1 is a scatter diagram showing the correlation between the ratio of ferrite having a grain size of 2 μm or more and the elongation in a high-strength hot-rolled steel sheet.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Riki Okamoto 5-3 Tokai-cho, Tokai-shi, Aichi F-term in Nippon Steel Corporation Nagoya Works (reference) 4K037 EA05 EA15 EA16 EA19 EA23 EA25 EA27 EA28 EA31 EB05 EB08 EB09 EB11 FC04 FC07 FD03 FD08 FE06 JA06 JA07

Claims (3)

[Claims] (1) In mass%, C is 0.01 to 0.08% and Si is 0.30 to
1.50%, Mn 0.50-2.50%, P ≦ 0.03%, S ≦ 0.005%,
And high-strength hot-rolled steel sheets containing one or two of Ti 0.01 to 0.20% and Nb 0.01 to 0.04%, with the balance being iron and unavoidable impurities, and having a steel structure with a grain size of 2 μm or more A high-strength hot-rolled steel sheet excellent in hole expandability and ductility, characterized by having a ferrite-bainite dual phase structure of 80% or more and a strength of 690 N / mm ² or more. 2. The method according to claim 1, wherein one or two of Ca and REM are contained in an amount of 0.0005 to 0.2.
The high-strength hot-rolled steel sheet according to claim 1, which contains 01% by weight. 3. After the hot rolling is completed by setting the rolling end temperature to the Ar ₃ transformation point to 950 ° C., at a cooling rate of 20 ° C./sec or more.
3. The method according to claim 1, wherein the material is cooled to 650 to 800 DEG C., air-cooled for 2 to 15 seconds, and further cooled to 350 to 600 DEG C. at a cooling rate of 20 DEG C./sec or more. For producing high-strength hot-rolled steel sheets with excellent hole expandability and ductility.