JPH08246097A - High strength hot rolled steel sheet excellent in stretch-flanging workability and its production - Google Patents

Abstract

PURPOSE: To produce a high strength hot rolled steel sheet in which deterioration in stretch-flanging workability in accordance with the increase of its strength to >=750N/mm<2> is prevented and having sufficient stretch-flanging workability even if being high in strength and to provide a method for producing the same. CONSTITUTION: This high strength hot rolled steel sheet with >=750N/mm<2> strength excellent in stretch-flanging workability is the one having a compsn. contg., by weight, 0.02 to 0.10% C, <=0.2% Si, <=3.0% Mn, <=0.1% P, <=0.01% S and 0.1 to 1.0% Cr, and the balance iron with inevitable impurities, having a lath-shaped structure, having no cementitic and pearlitic structures and having >=80% ferritic structure. This high strength hot rolled steel sheet is obtd. by forming a steel having chemical components into a slab, heating the same at >=1100 deg.C, thereafter, finishing hot rolling at a temp. in the range of (Ar3 -50) deg.C to 950 deg.C, executing cooling to a temp. in the range of 500 to 200 deg.C and coiling the same at the same temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet having a strength of 750 N / mm ² or more and having excellent stretch-flange formability, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, high-strength hot-rolled steel sheets have been widely used for various purposes. In particular, when they are used for various members including automobile bumpers, they have strong stretch-flange formability in press forming. Required. As a method for improving the stretch flangeability, for example, JP-A-58-4
As disclosed in Japanese Patent No. 2726, a method of changing the structure of a steel sheet to a ferrite bainite structure, JP-A-57-57
As described in Japanese Patent No. 70257, a method of forming a ferrite-bainite-martensite structure is known.

Further, in addition to these, JP-A-57-23
Japanese Patent Laid-Open No. 4-88125, a method of dispersing fine pearlite in ferrite as described in JP-A-4-88125.
As described in JP-A No. 54-88827, a method for producing fine cementite, and a method for producing fine cementite, as described in JP-A-54-88827.
As described in JP-A-56-55525, a method for controlling bainite lath is known.
Further, a method of reducing the amount of C is also known.

However, the conventionally known hot-rolled steel sheets produced by these methods have limitations in stretch-flange formability, and moreover, these hot-rolled steel sheets have a higher strength of 75.
The stretch-flange formability of the hot-rolled steel sheet of 0 N / mm ² or more is extremely deteriorated. Thus, the conventional high-strength hot-rolled steel sheet cannot satisfy the requirement of workability required for manufacturing members of automobiles, and the reduction of automobile weight due to thinning,
Fuel economy has not been improved.

[0005]

The present invention has been made to solve the above-mentioned problems in the production of conventional high-strength hot-rolled steel sheets, and it is intended to increase the strength to 750 N / mm ² or more. A high-strength hot-rolled steel sheet which prevents deterioration of stretch-flange formability associated therewith and has sufficient stretch-flange formability even with high strength, usually 90% or more of the limit hole expansion rate of punched holes, and such hot-rolling. An object is to provide a method for manufacturing a steel sheet.

[0006]

According to the present invention, the weight percent is
C 0.02 to 0.10%, Si 0.2% or less, Mn
3.0% or less, P 0.1% or less, S 0.01% or less,
And Cr 0.1-1.0%, balance iron and unavoidable impurities, has lath-like structure, has no cementite and pearlite structure, and has 80% or more ferrite structure for stretch flangeability. Excellent strength 750 N / mm ²
The above high strength hot rolled steel sheet is provided.

The hot rolled steel sheet according to the present invention further comprises Ni 1.
It may contain at least one element selected from the group consisting of 0% or less and Cu 1.0% or less.

According to the invention, further C 0.
02-0.10%, Si 0.2% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, and Cr.
A steel containing 0.1 to 1.0% and a balance of iron and unavoidable impurities is used as a slab, and the slab is heated to a temperature of 1100 ° C or higher, and then in a range of (Ar ₃ -50) ° C or higher and 950 ° C or lower. With the strength of 750 N / mm ² or more, the hot rolling is completed at the temperature of, the temperature is cooled to a temperature in the range of 500 to 200 ° C. at a cooling rate of 30 ° C./sec or more, and the coil is wound at that temperature. A method for producing a hot-rolled steel sheet having a lath-like structure, having no cementite and pearlite structure, and having a ferrite structure of 80% or more and having excellent stretch flange formability is provided.

First, the reason for limiting the chemical composition of steel in the present invention will be explained. C is an element necessary to secure the strength of the steel sheet, but when added in an excessive amount, a structure having carbides such as bainite and pearlite is formed beyond the range specified in the present invention. It is impossible to obtain a high-strength hot-rolled steel sheet having excellent stretch flangeability. However, when the addition amount is insufficient, it is not possible to obtain a ferrite structure having a lath structure during cooling after completion of hot rolling, and similarly, high strength hot rolling excellent in desired stretch flange workability is also obtained. I can't get a steel plate. Therefore, according to the present invention, in order to obtain a ferrite structure having a lath structure without forming carbides, C
Is added in the range of 0.02 to 0.10%.

Since Si inhibits the formation of a ferrite structure having a lath-like structure and is a harmful element in the present invention, it is preferable to minimize the content in steel. In particular, when the Si content exceeds 0.2%, it is not possible to obtain a ferrite structure having a lath structure depending on the cooling conditions in ordinary manufacturing equipment. Furthermore, depending on the cooling rate, a dual-phase martensite structure is generated, and the stretch flange formability is extremely deteriorated. In order to obtain a ferrite structure having a lath-like structure in a relatively stable manner according to the present invention, the Si content is preferably 0.1% or less.

Mn is one of the elements that contributes to the formation of the structure aimed at by the present invention. However, if too much is added, a large amount of low-temperature transformation products such as bainite structure are generated, and the stretch-flange formability of the steel sheet is reduced.
It is 3.0% or less. In the present invention, P is important as an element for solid solution strengthening. However, if excessively added, desirable properties other than stretch flangeability such as spot weldability are deteriorated, so the upper limit of the addition amount is made 0.1%.

Since S forms a sulfide that deteriorates stretch-flange formability, it is necessary to reduce S as much as possible. In the present invention, S is 0.005% or less. As described later, by adding Ca, S can be further reduced and the stretch-flangeability of the obtained hot-rolled steel sheet can be further enhanced.

[0013] Cr has the effect of suppressing bainite transformation and pearlite transformation and controlling the formation of carbides that hinder stretch-flange formability. Also, fine Cr compounds are produced during winding at 200 to 500 ° C, Since it can contribute to the improvement of strength, it is an important element in the present invention. Conventionally, it is known that if Ti or Nb is added, carbide precipitates during winding after hot rolling, and this forms a precipitate effective for improving the strength of the steel sheet.
It requires a relatively high winding temperature and no precipitate is formed at the winding temperature in the process of the invention.

According to the present invention, the effect of Cr is effective from the addition amount of 0.1%, but the effect is saturated even if it is added in excess. Therefore, considering the economical efficiency of the manufacturing process, the upper limit of the amount added is set to 1.0%. Particularly, according to the present invention, the highest effect can be obtained when the Cr addition amount is in the range of 0.3 to 0.7%.

In the present invention, in addition to the above-mentioned elements, at least one element selected from the group consisting of Ni 1.0% or less and Cu 1.0% or less can be added to steel.

[0016] Cu is an element effective for obtaining corrosion resistance in the steel sheet, but if it is added excessively, it may cause hot embrittlement of the steel, so the addition amount is made 1.0% or less. However, the addition of Cu does not reduce the stretch flange formability of the steel sheet.

As described above, Ni is usually added in an amount almost equal to that of Cu in order to prevent intergranular cracking and dent defects during hot rolling by adding Cu. However, the addition of Ni does not reduce the stretch flangeability, and the combined addition of Ni and Cu does not reduce the stretch flangeability.

According to the present invention, it is possible to add Ca to steel. Ca has the effect of reducing sulfides that deteriorate stretch flange formability. Therefore, according to the present invention, if necessary, Ca can be added to the steel in the range of 20 ppm or less to remove S as CaS. In this way, the stretch-flange formability of the obtained hot-rolled steel sheet does not deteriorate even if Ca treatment is performed.

Next, the structure of the hot rolled steel sheet according to the present invention will be described. The hot rolled steel sheet according to the present invention is 750 N /
In order to have high strength of mm ² or more and excellent stretch flangeability, it is necessary to have a ferrite structure having a lath-like structure, that is, a bainitic ferrite structure. Here, the lath-like structure is a structure observed when observed with a high-magnification transmission electron microscope or the like, and is usually different from a twin crystal or a lens-like structure in a substructure such as martensite. It refers to a needle-shaped structure and is also called lath martensite. Above bainitic
The ferrite structure is very similar to the bainite structure when observed at a relatively low magnification (about 100 to 5000 times) with an optical microscope or a scanning electron microscope. However, according to observation at a high magnification (about 1000 to 5000 times) with a transmission electron microscope or the like, the bainite structure is a structure containing carbides (such as cementite) in its internal structure, whereas the bainitic ferrite structure is Is a structure that does not include carbide such as cementite and is different from the bainite structure. Also,
The bainitic ferrite structure is different from the polygonal ferrite structure in the lath-like structure, and thus is also a structure different from the polygonal ferrite structure.

The high-strength hot-rolled steel sheet according to the present invention needs to contain 80% or more of such bainitic ferrite structure. However, the remaining 20% or less of the organization
Not only the bainite structure and polygonal ferrite structure, but also any structure including carbides, other low temperature transformation products, second phase and the like may be used.

According to the present invention, such a high-strength hot-rolled steel sheet is made of a steel having the above-mentioned chemical composition as a slab, which is heated to a temperature of 1100 ° C. or higher and then (Ar ₃
Hot rolling is completed at a temperature of −50) ° C. or higher, and 30 ° C. /
It can be obtained by cooling to a temperature in the range of 500 to 200 ° C. at a cooling rate of at least 2 seconds and winding at that temperature.

In the method of the present invention, it is necessary that the heating temperature of the slab is not less than 1100 ° C., which is the solution temperature of steel, and preferably the solution is sufficiently solutionized to ensure heating in the γ region. In addition, the heating temperature is 1200 ° C. or higher. However, the heating temperature is usually in the range of 1350 ° C. or lower because the manufacturing cost is increased even if the temperature is excessively increased.

In hot rolling, it is desirable to finish hot rolling at a temperature just above Ar ₃ . This is because at a temperature just above Ar ₃ , the structure is uniform and becomes fine γ particles, and ferrite particles having a uniform structure can be obtained even after hot rolling.
When rolling is performed in the α + γ region of Ar ₃ points or less, strain is applied to the α grains, and uneven processed α grains are likely to remain. However, even if the rolling temperature is below the Ar ₃ point, (Ar ₃ −50)
Up to ° C, the volume fraction of the α phase is small and the degree of processing is low, so that stretch flangeability and other properties are not deteriorated. Therefore, in the present invention, the hot rolling finishing temperature is set to a temperature of (Ar ₃ -50) ° C or higher. Thus, according to the present invention, it is desirable to finish the hot rolling at a temperature just above Ar ₃ , but if it is 950 ° C. or lower,
There is no hindrance.

The cooling rate after the hot rolling is at least 30 ° C./in order to obtain a ferrite structure having a lath-like structure.
More than a second is needed. The upper limit of the cooling rate is not particularly limited, but is usually 100 ° C./second in consideration of actual factory facility capacity and the like.

In the present invention, the coiling temperature is required to be a low temperature of 200 to 500 ° C. in order to avoid the formation of bainite and pearlite. It is preferably in the range.

[0026]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples. Steel having the chemical composition shown in Table 1 was melted by vacuum melting. In Table 1, steel numbers 1 to 4 are the effects of the amount of C, 3,
5 and 6 are effects of Si amount, 3 and 7 are effects of Mn amount, 3
And 8 show the influence of the amount of P, respectively. Steel No. 9 shows the case where Cr is not added, 10 to 12 show the effect of adding Ni and Cu, and 13 shows the effect of adding Ca.

After holding each slab having the chemical composition shown in Table 1 at a temperature of 1250 ° C. for 30 minutes, the hot rolling end temperature was set in the range of about 950 to 750 ° C. and the plate thickness was 30.
mm to a plate thickness of 2.5 mm. This is further cooled by air, mist (about 40 to 80 ° C./sec) and water to 550.
Cooled to a temperature equivalent to the coiling treatment in the temperature range of up to 150 ° C., held at that temperature for 1 hour, and then cooled to normal temperature by furnace cooling to obtain a hot rolled steel sheet.

From the hot-rolled steel sheet thus obtained, 70
A square mm test piece was prepared, a hole with a diameter of 10 mm was made in the center, and this hole was expanded with a conical punch with a tip angle of 60 °. The limit hole expansion ratio (λ value) was calculated. The results are shown in Tables 2 and 3.

Further, in Tables 2 and 3, the type of tissue was identified by a transmission electron microscope (15,000 times),
The fraction was determined from the results of observation with an optical microscope structure (400 times) and a scanning electron microscope (2000 times).

In Tables 2 and 3, Experiment Nos. 1 to 13
Indicates the influence of the components, and 14 to 27 indicate the influence of the winding temperature. In addition, FIG. 1 shows the effect of the winding temperature on the tensile strength of steel No. 3 (inventive steel) and steel No. 6 (comparative steel) in Table 1, and FIG. 2 shows steel No. 3 in Table 1.
The influence of the winding temperature on the λ value of (invention steel) and steel No. 6 (comparative steel) is shown. Experiment numbers 28 to 36 show the influence of the hot rolling finish temperature. In addition, FIG. 3 shows the effect of the hot rolling finish temperature on the tensile strength of Steel No. 3 (inventive steel) and Steel No. 6 (comparative steel) in Table 1, and FIG. 4 shows steel No. 3 in Table 1. The influence of hot rolling finishing temperature on the stretch flangeability (λ value) of (inventive steel) and steel No. 6 (comparative steel) is shown. Experiment numbers 37-39 show the influence by the cooling rate.

FIG. 5 shows steel number 3 (inventive steel) in Table 1.
And tensile strength of steel No. 6 (comparative steel) -stretch flangeability (λ
It is a graph which shows the influence of the winding temperature which exerts on (value). Here, tensile strength-stretch flangeability balance, tensile strength ×
It is defined by the product of stretch flangeability (λ value), and the larger the value, the better the balance.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

As described above, according to the present invention, the strength is 7
At 50 N / mm ² or more, it has a lath-like structure, has no cementite or pearlite structure, and has 80% ferrite structure.
It is possible to obtain a hot-rolled steel sheet having the above properties and excellent stretch flange formability with a marginal hole expansion ratio of 90% or more.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of winding temperature on the tensile strength of steel No. 3 (inventive steel) and steel No. 6 (comparative steel) in Table 1.

FIG. 2 is a graph showing the influence of the winding temperature on the λ values of steel No. 3 (inventive steel) and steel No. 6 (comparative steel) in Table 1.

FIG. 3 is a graph showing the effect of hot rolling finishing temperature on the tensile strength of steel No. 3 (inventive steel) and steel No. 6 (comparative steel) in Table 1.

FIG. 4 is a graph showing the effect of hot rolling finish temperature on the stretch flangeability (λ value) of steel No. 3 (inventive steel) and steel No. 6 (comparative steel) in Table 1.

FIG. 5 is a graph showing the effect of winding temperature on the tensile strength-stretch flangeability (λ value) of steel No. 3 (inventive steel) and steel No. 6 (comparative steel) in Table 1.

Claims (6)

[Claims] 1. In weight%, C 0.02 to 0.10%, Si 0.2% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, and Cr 0. 1 to 1.0%, balance iron and unavoidable impurities, has lath-like structure, has no cementite and pearlite structure, has 80% or more ferrite structure and has excellent stretch flange formability 750N High-strength hot-rolled steel sheet of / mm ² or more. 2. A weight percentage of (a) C 0.02 to 0.10%, Si 0.2% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, and Cr 0.1-1.0%, and (b) at least 1 selected from the group consisting of Ni 1.0% or less and Cu 1.0% or less.
Including the elements of the species, the balance iron and inevitable impurities,
A high-strength hot-rolled steel sheet having a lath-like structure, having no cementite or pearlite structure, and having a ferrite structure of 80% or more and having an excellent stretch flange formability of 750 N / mm ² or more. 3. The high-strength hot-rolled steel sheet according to claim 1 or 2, further containing Ca in a range of 20 ppm or less. 4. In weight%, C 0.02 to 0.10%, Si 0.2% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, and Cr 0. A steel containing 1 to 1.0% and a balance of iron and unavoidable impurities is used as a slab, and the slab is heated to a temperature of 1100 ° C. or higher, and then a temperature in the range of (Ar ₃ −50) ° C. or higher and 950 ° C. or lower. The hot rolling is terminated at, the temperature is cooled to a temperature in the range of 500 to 200 ° C. at a cooling rate of 30 ° C./second or more, and the coil is wound at that temperature.
/ Mm ² or more, having a lath-like structure, having no cementite or pearlite structure, and having a ferrite structure of 80% or more, a method for producing a hot-rolled steel sheet having excellent stretch flange formability. 5. In weight%, (a) C 0.02 to 0.10%, Si 0.2% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, and Cr 0.1-1.0%, and (b) at least 1 selected from the group consisting of Ni 1.0% or less and Cu 1.0% or less.
A slab made of steel containing the same kind of elements and the balance of iron and inevitable impurities is heated to a temperature of 1100 ° C or higher, and then heated at a temperature in the range of (Ar ₃ -50) ° C to 950 ° C. Finishing the hot rolling, cooling to a temperature in the range of 500 to 200 ° C. at a cooling rate of 30 ° C./sec or more, and winding at that temperature, strength of 750 N / mm ² or more, lath shape A method for producing a hot-rolled steel sheet having a structure, having no cementite or pearlite structure, and having a ferrite structure of 80% or more and having excellent stretch flange formability. 6. The method for producing a hot-rolled steel sheet according to claim 4, wherein Ca is further added to the steel in a range of 20 ppm or less.