JP2002180189A - High strength hot rolled steel sheet excellent in hole expandability and ductility and method for producing the same - Google Patents

Abstract

(57) [Summary] [Problem] 690 N / mm strength with high hole expandability and ductility
Provided are ^two or more high-strength hot-rolled steel sheets and a method for producing the steel sheets. SOLUTION: In mass%, C 0.01-0.08%, Si 0.30-
1.50%, Mn 0.50-2.50%, P ≦ 0.03%, S ≦ 0.005%,
And one or two of Ti 0.01 to 0.20% and Nb 0.01 to 0.04%, and the balance consisting of iron and unavoidable impurities is hot-rolled, and the minor diameter / major diameter ratio (ds / dl) ≧
The steel sheet has a crystal grain of 0.1% or more and has a steel structure of 80% or more of ferrite and a balance of bainite. The high-strength hot-rolled steel sheet can contain 0.0005 to 0.01% of one or two of Ca and REM.

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 in order to secure strength, ductility is poor.

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 C content of 0.01 to 0.08% and a Si of 0.30% 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 types of Ti 0.01 to 0.20% and Nb 0.01 to 0.04%, the balance being iron and unavoidable impurities. 80% or more crystal grains having a ratio (ds / dl) of (dl) of 0.1 or more,
Further, the steel structure is characterized in that the steel structure is 80% or more of ferrite and the balance is bainite, and the strength is 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 produces the high-strength hot-rolled steel sheet excellent in hole spreadability and ductility according to claim 1 or 2. upon the steel of the composition, the rolling end temperature Ar ₃ transformation point to 950
℃ and hot-rolled continuously, at a cooling rate of 20 ℃ / sec or more
It is characterized by cooling to 650 to 800 ° C, air cooling for 2 to 15 seconds, cooling to 350 to 600 ° C at a cooling rate of 20 ° C / sec or more, 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 in order to solve the above-described problems, and as a result, found that the ductility can be improved without deteriorating hole expandability by making the crystal grains as spherical as possible in ferritic bainite steel. Was completed. In other words, ferrite that increases ductility in ferrite bainite steel and TiC and NbC that secure strength
Focusing on precipitates consisting of, improving the ductility without reducing the hole expandability as ferrite grains are sufficiently spherical, and solving the above problem by ensuring the strength by generating precipitates thereafter is there.

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 be deteriorated.

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, it is desirable to add one or two of Ca and REM in an amount of 0.0005% or more. On the other hand, a large amount of addition causes coarsening of the sulfide-based inclusions, which not only deteriorates the cleanliness and lowers the ductility, but also increases the cost, so the upper limit is made 0.01%.

The ratio (d) of the minor axis (ds) to the major axis (dl) of the crystal grain
(s / dl) is an index indicating the degree of grain growth, and is one of the most important indexes in the present invention. In order to achieve both hole expandability and ductility, grains grow and the ratio of minor axis to major axis (ds / dl) is 0.
Must be at least 1. When the ratio of the minor axis / major axis of the crystal grains is less than 0.1, the crystal grains are flattened, not sufficiently recovered crystal grains, and cause a decrease in ductility. It is necessary that the ratio of grains having such a short diameter / major diameter ratio to all crystal grains is 80% or more. If this ratio is less than 80%, the ductility decreases, and if the tensile strength is 690 N / mm ² or more, compatibility with the hole expandability cannot be achieved. Tensile strength 780~820N / mm ² in FIG. 1, lambda value
(Hole expansion value) Minor diameter of high strength hot rolled steel sheet of 100 to 115 /
There is a correlation between the proportion of grains having a major axis ratio of ≧ 0.1 and the elongation. It can be seen that when the proportion is less than 80%, the ductility decreases. Therefore, in order to achieve both hole expandability and ductility, it is necessary that the ratio of the crystal grains having a minor axis / major axis ratio of ≧ 0.1 to all the crystal grains is 80% or more. In order to obtain a more remarkable effect, it is desirable that the ratio of the crystal grains satisfying the ratio of minor axis / major axis ≧ 0.2 is 80% or more.

The high-strength hot-rolled steel sheet of the present invention having excellent hole expandability and ductility may be manufactured by hot rolling a slab or the like containing the above-mentioned components. The steel structure of the steel sheet is a two-phase structure composed of at least 80% ferrite and the balance bainite. The reason is that when the ferrite content is less than 80%, the ductility is greatly reduced.
It needs to be 80% or more.

Thus, in order to produce a high-strength hot-rolled steel sheet excellent in hole expandability and ductility according to the present invention, when producing by hot rolling, the rolling end temperature is set to the Ar ₃ transformation point to 950 ° C. After cooling to 650 to 800 ° C at a cooling rate of 20 ° C / sec or more, air-cooling is performed for 2 to 15 seconds, and then cooled to 350 to 600 ° C at a cooling rate of 20 ° C / sec or more and wound up. The rolling end temperature is set to Ar ₃ in order to suppress the formation of ferrite and improve hole expandability.
It is necessary to keep the transformation point or higher. However, if the temperature is too high, the strength and ductility are reduced due to the coarsening of the structure. Therefore, the finish rolling end temperature must be 950 ° C. or less.

It is important to rapidly cool the steel sheet in order to obtain high hole expandability immediately after the end of rolling. When the cooling rate is less than 20 ° C./sec, the formation of carbides harmful to the hole expandability is suppressed. Therefore, it is necessary to maintain the temperature at 20 ° C./sec or more.

Next, it is important to temporarily stop the rapid cooling of the steel sheet and to perform air cooling in order to precipitate the ferrite, increase its occupancy, and improve the 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, the air cooling start temperature is 80
When the temperature exceeds 0 ° C., the formation of ferrite is slow and it is difficult to obtain the effect of air cooling, and further, the formation of pearlite tends to occur during the subsequent cooling. Therefore, the air-cooling start temperature is 650-800 ° 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. If the cooling rate is less than 20 ° C./sec, harmful pearlite is likely to be generated. . The quenching stop temperature, that is, the winding temperature is set at 350 to 600 ° C. Winding temperature 350 ℃
If the temperature is less than the range, hard martensite harmful to the hole-expanding property is generated. On the other hand, if the temperature exceeds 600 ° C., pearlite and cementite harmful to the hole-expanding property are easily generated.

By the combination of the above components and hot rolling conditions, it is possible to produce a high strength hot rolled steel sheet having a ferrite occupancy of 80% or more and having a residual bainite steel structure with excellent hole expandability and ductility. . Furthermore, even if a surface treatment (for example, galvanizing) is performed on the surface of the steel sheet of the present invention, the effect of the present invention is obtained, and the present invention is not deviated 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.

[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. All crystal grains were traced from an optical microscope photograph in 30 visual fields, and the ratio of the minor axis to the major axis (ds / dl) of each traced crystal grain was determined. In the hole expansion test, a punched hole having an initial hole diameter (d ₀ : 10 mm) was pushed and expanded with a 60 ° conical punch, and the hole expansion value (λ value) was calculated from the hole diameter (d) at the time when the crack penetrated the plate thickness. ) = (Dd ₀ ) / d ₀ × 100.
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 scope of the present invention, and the ratio of crystal grains having a minor axis / major axis ratio (ds / dl) of 0.1 or more is 80% or more. It is a high-strength hot-rolled steel sheet having high lambda value and elongation and excellent in hole expandability and ductility. On the other hand, No. The comparative examples 11 to 21 out of the conditions of the present invention are inferior in balance of strength, hole expandability and ductility.

In addition, No. When hot rolling was performed using steel having the composition shown in Table 1 at a finishing temperature of 920 ° C, an air-cooling start temperature of 625 ° C, and a winding temperature of 460 ° C, pearlite was found in the structure because the air-cooling start temperature was too low within the range of the present invention. The ferrite occupation rate was as low as 76%, and thus the elongation was 20% and the λ value was 93%, resulting in poor hole expandability and ductility balance. Similarly, No. Finishing temperature 910 ° C, air cooling starting temperature 690 using steel of the composition shown in 1.
° C, the rolling temperature was 330 ° C, and hot rolling was performed at a temperature lower than the range of the present invention, so that martensite was generated in the structure and the occupancy of ferrite was as low as 64%. As a result, the elongation was 20% and the λ value was 64%, 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 crystal grains satisfying ds / dl ≧ 0.1 and elongation.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Riki Okamoto 5-3 Tokaicho, Tokai City, Aichi Prefecture F-term in Nagoya Works, Nippon Steel Corporation (Reference) 4K037 EA05 EA09 EA15 EA16 EA19 EA23 EA25 EA27 EA31 EA31 EA36 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 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%, the balance being iron and unavoidable impurities. d
s) and major axis (dl) ratio (ds / dl) is 0.1 or more
% High-strength hot-rolled steel sheet with excellent hole expandability and ductility characterized by having a ferrite content of 80% or more, a balance of bainite, 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. When producing a high-strength hot-rolled steel sheet excellent in hole expandability and ductility according to claim 1 or 2, the steel having the composition is heated at an end rolling temperature of Ar _{3 to} 950 ° C. 650-800 ° C at a cooling rate of 20 ° C / sec or more
After cooling to 2-15 seconds, further cool at 20 ° C / sec.
A method for producing a high-strength hot-rolled steel sheet having excellent hole expandability and ductility, characterized in that it is cooled to 350 to 600 ° C. at the above cooling rate and wound up.