JPH08157957A - Production of high strength hot rolled steel plate excellent in stretch flange workability - Google Patents

Abstract

PURPOSE: To produce a hot rolled steel plate, in which a part or the whole of structure is formed into bainitic ferrite structure and which has excellent stretch-flange formability, by hot-rolling a slab of low carbon steel of specific composition under specific conditions and subjecting the resultant plate to rapid cooling and to coiling. CONSTITUTION: A low carbon steel, which has a composition containing, by weight, 0.02-0.10% C, <1.0% Si, <3.0% Mn, <0.1% P, <0.01% S, and 0.1-1.0% Ti in the range satisfying the inequality in the relations among the contents of C, Ti, N, and S and also containing 0.2-2.0% Cu and <2.0% Ni in the range satisfying the relation of 0.2<Ni/Cu<1.5, is used. A slab of this low carbon steel is heated to >=1100 deg.C and hot-rolled into a plate at >=(Ar3 -50) deg.C finishing temp. This hot rolled plate is cooled rapidly down to 400-750 deg.C at (30 to 100) deg.C/sec cooling rate and then coiled at the temp. By this method, the high strength hot rolled steel plate, in which a part or the whole of structure is composed of bainitic ferrite and which has >=600N/mm<2> tensile strength and >=90% critical bore-expand ratio at bore-expand test, can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-rolled steel sheet having a tensile strength of 600 N / mm ² or higher, which is excellent in stretch-flange formability.

[0002]

^2. Description of the Related Art Conventionally, when a high-strength hot-rolled steel sheet of 600 N / mm ² grade or higher is used for various members of an automobile, such as a bumper, it is particularly required to have stretch-flange formability in its press forming. There is. As a method for improving the workability of the stretch flange, JP-A-58-4272 is known.
As described in Japanese Patent Laid-Open No. 6-56, a method of changing the structure of a steel sheet to a ferrite-bainite structure, or JP-A-57 / 57
As described in JP-A-70257, a method of forming a ferrite bainite martensite structure is already known. In addition to this, JP-A-57-23025
There is known a method of dispersing fine pearlite in ferrite as described in Japanese Patent Laid-Open Publication No. Hei 4-88125 or generating fine cementite as described in Japanese Patent Laid-Open No. 4-88125. However, these conventionally known high-strength hot-rolled steel sheets have carbides and a second phase, which deteriorate the stretch-flange formability. It is not possible to meet the requirement of workability required for molding into the above member, and the use of high-strength materials for weight reduction due to thinning and improvement of fuel consumption has not been sufficiently advanced under the present circumstances.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 5-271759 discloses a method for producing a high-strength steel sheet which is substantially composed of ferrite and has Ti and Nb carbides precipitated in its ferrite structure. In such a hot-rolled steel sheet, local concentration is caused around dislocation precipitates, and at a strength of about 600 N / mm ² , conversely, the hole expansion value is greatly reduced.

[0004]

DISCLOSURE OF THE INVENTION The present invention provides a conventional 60
A high-strength hot-rolled steel sheet having a grade of 0 N / mm ² or more was made to solve the above-mentioned problems.
It is an object of the present invention to provide a method for producing a hot-rolled steel sheet having a sufficient stretch-flange formability even when the strength is high, while preventing deterioration of the stretch-flange formability associated with an increase in strength of / mm ² grade or higher.

[0005]

A method for producing a high strength hot rolled steel sheet excellent in stretch flange formability according to the present invention is as follows: C 0.02 to 0.10% by weight, Si 1.0% or less, M
n 3.0% or less, P 0.1% or less, S 0.0
1% or less, Ti 0.1 to 1.0%, Cu 0.2 to
2.0% and Ni 2.0% or less, the addition amount of Ti satisfies C <[(Ti-3.43N-1.5S) / 4], and the addition amounts of Cu and Ni are 0. A steel satisfying 2 <Ni / Cu <1.5 and having a balance of iron and unavoidable impurities is used as a slab, and after heating, hot rolling is finished at a temperature of (Ar ₃ −50) ° C. or higher, and then, After cooling to a temperature in the range of 400 to 750 ° C at a cold rolling speed of 30 to 100 ° C / sec, 4
It is wound at a temperature in the range of 00 to 750 ° C. and has a bainitic ferrite structure partially or entirely, and a tensile strength of 6
Strength of 00N / mm ² or more and limit hole expansion rate of punched holes 9
A hot-rolled steel sheet having a stretch flangeability of 0% or more is manufactured.

According to the present invention, in addition to the above elements, Nb may be added to the steel in the range of 0.1% or less.

First, the structure of the high-strength hot-rolled steel sheet excellent in stretch flange formability according to the present invention will be described. The structure of the hot-rolled steel sheet according to the present invention is bainitic ferrite, and this structure is pseudopolygonal
Distinguished from ferrite and bainite structures. In general,
The bainite structure is composed of a lath-like lower structure having a high dislocation density and carbides that are parallel to or precipitate at a certain angle. However, the bainitic ferrite in the present invention has a lath-like structure as a lower structure, but unlike a normal bainite structure, means a structure in which cementite carbide does not exist. Depending on the heat treatment conditions, a lath-like structure is recovered and a ferrite structure having a high dislocation density without a clear subgrain boundary is also included. In addition, the pseudopolygonal ferrite structure is very similar to the bainitic ferrite structure in the optical microscope structure, but the pseudopolygonal ferrite structure in the lower structure in the transmission electron micrograph shows clear subgrain boundaries. Because I have
The lath structure is different from the bainitic ferrite structure recovered by heat treatment. Such a bainitic ferrite structure has a tensile strength of 600 N / mm ² or more even when mixed with a polygonal ferrite structure.
Further, the limit hole expansion ratio (λ value) in the hole expansion test is 10
It has a property of 0% or more.

Further, the ferrite structure as described above has a T
Unlike the structure formed by precipitating i or Nb carbides, the bainitic ferrite structure has a uniform dislocation density, so that even if the tensile strength is 600 N / mm ² or more, the hole expansion value does not decrease.

FIG. 1 shows the quasi-polygonal ferrite structure, and FIG. 2 shows the polygonal ferrite and bainitic.
A structure composed of ferrite is shown, and a bainitic ferrite structure is shown in FIG.

Next, the chemical composition of the hot rolled steel sheet according to the present invention will be described. C is the most important element for giving the required strength to the steel sheet. C is an important element because it has a driving force for forming a bainitic ferrite structure during cooling after hot rolling by forming a solid solution in γ-ferrite. Further, after the bainitic ferrite structure is formed, precipitates such as Ti and Nb are formed to increase the strength without deteriorating the elongation franchicity.

In the present invention, the amount of C added must satisfy the following formula C <[(Ti-3.43N-1.5S) / 4] (1) in relation to the amount of Ti added. Is. When Nb is added to steel together with Ti, the addition amount of C is expressed by the following formula C <[(Ti-3.43N-1.5S) /4+Nb/7.75] in relation to the addition amounts of Ti and Nb. … It is necessary to satisfy (2). When the amount of C does not satisfy the above formula (1) or (2), the solid solution C remains in the ferrite even after winding, deteriorating the stretch flangeability.

Further in accordance with the present invention, a bainitic
In order to obtain a ferrite structure, C is at least 0.0
2% addition is required, but the amount added is 0.10
%, It is not only necessary to excessively add a precipitate-forming element such as Ti or Nb, but also a coarse precipitate is formed or solid solution C remains in the ferrite. A second phase structure is generated to deteriorate stretch flangeability.

Si is an important element as a solid solution strengthening element. However, when it is added in excess of 1.0%, the formation of painitic ferrite is suppressed,
Alternatively, since the oxide scale is excessively generated on the surface of the steel sheet, which causes a problem in manufacturing, the upper limit of the addition amount is set to 1.0%.

Mn contributes somewhat to the formation of the structure of the steel sheet according to the invention. In particular, the addition of Mn has an effect of affecting the solid solubility limit of C existing in ferrite, changing the form of TiC and NbC precipitates, and lowering the yield ratio. However, when Mn is excessively added, a bainite structure or the like and a second phase structure such as a residual γ structure, which is related to the amount of Si, are generated and the stretch flangeability is deteriorated. There, M
The upper limit of the amount of n added is 3.0%.

P is important as an element for solid solution strengthening. However, when excessively added, other properties such as spot weldability are deteriorated, so the upper limit is made 0.1%. S
Generates sulfide that deteriorates stretch-flange formability, so it is necessary to reduce it as much as possible. However, considering the improvement of the stretch flange formability in the present invention, the upper limit is set to 0.005%.

Ti is added to form TiC during winding after hot rolling and reduce the amount of solid solution C.
In order to effectively obtain the above effects, Ti needs to be added at least 0.1%. However, in the balance between the amount of C and the amount of N, if added in excess of the limit for forming carbonitride precipitates, solid solution Ti remains and the characteristics deteriorate. Therefore, the upper limit of the added amount is set to 1.0%. .

Like Ti, Nb also has the effect of reducing NbC during the winding after hot rolling to reduce the amount of solute C. However, when it is added excessively, the recrystallization temperature is raised and the necessary material cannot be obtained, so the upper limit of the addition amount is made 0.1%.

Cu is an element that suppresses polygonal ferrite transformation. Therefore, the addition of Cu suppresses the formation of polygonal ferrite and forms bainitic ferrite even after cooling at a relatively slow rate after hot rolling. However, when Cu is not added together with Ni, ε-Cu is generated, so the above effect as solid solution Cu does not occur. Therefore, in the present invention, Cu
Requires complex addition with Ni. As described above, in order to effectively obtain the effect as the solid solution Cu by adding Ni together, it is necessary to add at least 0.2% of Cu. However, the above effect is saturated even if it is added in excess of 2.0%, so the upper limit of the amount added is set to 2.0%.

As described above, Ni has the effect of suppressing the formation of ε-Cu, and has a high stretch-flangeability by keeping the steel compounded with Cu together with cooling according to the present invention to a relatively low temperature. A bainitic ferrite structure can be generated. When Cu and Ni are not added together, the temperature at which ε-Cu precipitates is 500.
Since it is 750 degreeC, the effect of solid solution Cu mentioned above cannot be acquired.

As described above, Ni has the greatest effect of suppressing the precipitation of ε-Cu when Cu and Ni are added in the same amount. However, in practice, Cu and Ni have the following formula 0.2 < If Ni / Cu <1.5 (3) is satisfied, a bainitic ferrite structure having sufficiently high strength and high stretch flangeability can be obtained. Therefore, in the present invention, as in the case of Cu,
The upper limit of the amount of Ni added is also 2.0%.

According to the invention, separately from Nb or Nb
At the same time, at least one element selected from the group consisting of Mo, Cr and V can be added. These elements are effective as strengthening elements. However, if these elements are excessively added, ductility is deteriorated, so the upper limit of the addition amount of each element is set to 1.0%.

Ca has the effect of reducing sulfides which deteriorate the stretch flange formability. Therefore, according to the present invention, S can be removed as CaS by adding Ca in the range of 20 ppm or less, if necessary. Thus, even if Ca is added, the Ca does not deteriorate the stretch flange formability.

According to the present invention, a steel having the above-mentioned chemical composition and the balance of iron and unavoidable impurities is made into a slab, which is heated to a temperature of 1100 ° C. or higher and then (A
hot rolling is completed at a temperature of r ₃ −50) ° C. or higher, and
After cooling to a temperature in the range of 400 to 750 ° C. at a cold rolling speed of 30 to 100 ° C./second, it is wound at a temperature in the range of 400 to 750 ° C., that is, cooled to a temperature in the range of 400 to 750 ° C. Then, by winding at that temperature, a part or all of which has a bainitic ferrite structure and has a tensile strength of 600 N / mm ² or more and an elongation of 90% or more of the limit hole expansion rate of punched holes. A hot rolled steel sheet having excellent flange formability can be obtained.

In the method of the present invention, the slab heating temperature is set to 1100 ° C. or higher. By setting the slab heating temperature to 1100 ° C. or higher, TiC or NbC can be sufficiently dissolved in the γ range, while when it is lower than 1100 ° C., TiC or NbC is coarsely precipitated in the r range. During the cooling after the hot rolling, the solid solution C, which is the driving force for generating the bainitic ferrite structure, disappears.

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 a fine r-grain with a uniform structure, and ferrite particles having a uniform structure can be obtained even after hot rolling. When rolling in the α + γ range of ₃ Ar or less,
Strain is added to the α grains, and unevenly processed α grains are likely to remain. However, even if the rolling temperature is below the Ar ₃ point, (Ar
If up to ₃ -50) ° C., the volume fraction of α phase is small and also, the processing degree since also low, does not degrade the stretch flangeability and other properties. Accordingly, in the present invention, the hot rolling finishing temperature and _(Ar 3 -50) ℃ or higher.

The cooling rate after the hot rolling is 30 to 100 ° C.
/ Second range. When the cooling rate is slower than 30 ° C./sec, a polygonal ferrite structure is formed and a bainitic ferrite structure is not formed. On the other hand, when the cooling rate exceeds 100 ° C./second, a martensite structure that deteriorates stretch flange formability is likely to be formed. Thus, after hot rolling, after cooling to a temperature in the range of 400 to 750 ° C., it is wound at a temperature in the range of 400 to 750 ° C.

If a steel having an ordinary composition outside the range specified in the present invention is kept at a temperature in the range of 400 to 750 ° C., a bainite structure is formed or e-
Precipitation of Cu occurs and deteriorates stretch flange formability. However, according to the invention, a certain amount of Ni and C
A steel having a component system in which u is added in combination is hot-rolled under the above conditions, cooled, and then held at a temperature in the range of 400 to 750 ° C., so that high-strength stretch-flange formability without cementite carbide is obtained. Excellent bainitic
A ferrite structure can be generated. When the temperature is higher than 750 ° C, a polygonal ferrite structure is formed with a volume ratio of 100%, and when the temperature is lower than 400 ° C, a martensite structure is generated, which rapidly deteriorates stretch flangeability. It is impossible to obtain a hot-rolled steel sheet having high strength and high stretch-flange formability.

[0028]

As described above, according to the method of the present invention,
A slab to which a predetermined chemical component is added is hot-rolled under predetermined conditions, cooled, and wound to have a bainitic ferrite structure partially or wholly and a tensile strength of 60.
With 0 N / mm ² or more, the limit hole expansion rate of punched holes is 90%
It is possible to obtain a hot-rolled steel sheet having excellent stretch flange formability as described above.

[0029]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples.

Example 1 Table 1 shows the chemical composition of the steel types used. These steels were melted by vacuum melting. In Table 1, Steels A to E show the influence of the C content, and Steels B, F and G show the influence of the C content in relation to Ti and Nb defined by the formula (1) or (2). H to J show the effect of the added amount of Cu, steel K to N
Is a steel type for examining the effect of Ni / Cu, and steels O to S are steel types for examining the effect of each additive element.

In hot rolling, the slab is held at a temperature of 1250 ° C. for 30 minutes and then the hot rolling end temperature is set to about 900 to 7.
Change from 50 ℃ to a plate thickness of 30mm to 2.5mm
Rolled up. Furthermore, air cooling and mist cooling (3-80 ° C /
It was cooled to a temperature corresponding to the winding treatment in the temperature range of 500 ° C. to 800 ° C. for about 1 second and kept at that temperature for 1 hour.
Then, it cooled to normal temperature by furnace cooling.

Tables 2 and 3 show the steel types used, their hot rolling conditions, cooling conditions and winding conditions, as well as the mechanical properties and microstructure of the hot rolled steel sheets obtained. In displaying the organization,
PF is polygonal ferrite, BF is bainitic ferrite, M is martensite, and B is bainite. In Tables 2 and 3, Steels 1 to 5 show the influence of the C content, and Steels 6 to 8 show the influence of the C content in relation to Ti and Nb defined by the formula (1) or (2).
For steels 9 to 17, the effects of Cu addition amount and Ni / Cu
8 to 22 show the influences of various additive elements, respectively. Furthermore, the steels 23 to 28 are affected by the winding temperature,
Steels 29 to 38 show the effect of cooling conditions.

The stretch-flange formability of the obtained hot-rolled steel sheet was determined by sampling a square test piece of 70 mm in length and width from the material having a plate thickness of 2.5 mm after the above hot rolling, and having 0 clearance in the center. Drill a punching hole with a diameter of 10 mm of 15 mm and expand this hole with a conical punch with a tip angle of 60 °.
The evaluation was made by the critical hole expansion rate (λ value) calculated from the critical hole diameter at which cracks were generated at the hole edges.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

In order to more clearly show the relationship between the chemical composition and hot rolling conditions and the mechanical properties of the obtained steel sheet, the relationship between the C content and the mechanical properties is shown in FIG. 4 and (Ti ^* + Nb) in FIG. / C
6 shows the relationship between (Ti ^* + Nb) / C and the limit hole expansion ratio (λ value) in FIG. 6, and FIG. 7 shows the relationship between the Ni / Cu ratio and the limit hole expansion ratio (λ value). FIG. 8 shows the relationship between the Cu amount and the limit hole expansion ratio (λ value), FIG. 9 shows the relationship between the winding temperature and tensile strength, and FIG. 10 shows the winding temperature and the limit hole expansion ratio (λ value).
Value), FIG. 11 shows the relationship between the cooling rate and the tensile strength, and FIG. 12 shows the relationship between the cooling rate and the critical hole expansion ratio (λ value). However, (Ti ^* + Nb) / C
Is [(Ti-3.43N-1.5S) / 4 + Nb /
7.75] / C is meant.

[Brief description of drawings]

FIG. 1 is an electron micrograph showing a pseudopolygonal ferrite structure,

[Figure 2] Polygonal ferrite and bainitic
Electron micrograph of the structure consisting of ferrite,

FIG. 3 is an electron micrograph showing a bainitic ferrite structure.

FIG. 4 is a graph showing the relationship between C content and mechanical properties,

FIG. 5 is a graph showing the relationship between (Ti ^* + Nb) / C and tensile strength,

FIG. 6 is (Ti ^* + Nb) / C and the limit hole expansion ratio (λ
Value), a graph showing the relationship with

FIG. 7 is a graph showing the relationship between the Ni / Cu ratio and the critical hole expansion ratio (λ value),

FIG. 8 is a graph showing the relationship between the Cu amount and the critical hole expansion ratio (λ value),

FIG. 9 is a graph showing the relationship between winding temperature and tensile strength,

FIG. 10 is a graph showing the relationship between the winding temperature and the limiting hole expansion ratio (λ value),

FIG. 11 is a graph showing the relationship between cooling rate and tensile strength,

FIG. 12 is a graph showing the relationship between the cooling rate and the limit hole expansion rate (λ value).

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical indication C22C 38/58

Claims (5)

[Claims] 1. By weight%, C 0.02 to 0.10%, Si 1.0% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, Ti 0.1. To 1.0%, Cu 0.2 to 2.0%, and Ni 2.0% or less, and the addition amount of Ti satisfies C <[(Ti-3.43N-1.5S) / 4]. However, after adding a Cu and Ni content satisfying 0.2 <Ni / Cu <1.5 and making the slab a steel consisting of balance iron and unavoidable impurities and heating it to a temperature of 1100 ° C. or higher,
Hot rolling is terminated at a temperature of (Ar ₃ -50) ° C or higher, and then 400 to 750 ° C at a cold rolling rate of 30 to 100 ° C / sec.
After being cooled to a temperature in the range of 100 to 750 ° C., it is wound at a temperature in the range of 400 to 750 ° C., partially or entirely having a bainitic ferrite structure and having a tensile strength of 600 N.
/ Mm ² or more and a method for producing a hot-rolled steel sheet excellent in stretch flange formability having a punched hole limit hole expansion ratio of 90% or more. 2. By weight%, C 0.02 to 0.10%, Si 1.0% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, Ti 0.1. .About.1.0%, Nb 0.1% or less, Cu 0.2 to 2.0%, and Ni 2.0% or less, and the addition amount of Ti and Nb is C <[(Ti-1.43N- 1.5S) / 4 + Nb /
7.75], the addition amounts of Cu and Ni satisfy 0.2 <Ni / Cu <1.5, and the balance steel and unavoidable impurities are used as slabs, and the slab is used at a temperature of 1100 ° C or higher. After heating to
Hot rolling is terminated at a temperature of (Ar ₃ -50) ° C or higher, and then 400 to 750 ° C at a cold rolling rate of 30 to 100 ° C / sec.
After being cooled to a temperature in the range of 100 to 750 ° C., it is wound at a temperature in the range of 400 to 750 ° C., partially or entirely having a bainitic ferrite structure and having a tensile strength of 600 N.
/ Mm ² or more and a method for producing a hot-rolled steel sheet excellent in stretch flange formability having a punched hole limit hole expansion ratio of 90% or more. 3. In weight%, (a) C 0.02 to 0.10%, Si 1.0% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, Ti 0.1 to 1.0%, Cu 0.2 to 2.0%, and Ni 2.0% or less, and (b) Mo 1.0% or less, Cr 1.0% or less, and V. Addition of Cu and Ni containing at least one element selected from the group consisting of 1.0% or less, and the addition amount of Ti satisfies C <[(Ti-1.43N-1.5S) / 4] The amount of the steel satisfies 0.2 <Ni / Cu <1.5, and the steel consisting of the balance iron and unavoidable impurities is used as a slab.
Hot rolling is terminated at a temperature of (Ar ₃ -50) ° C or higher, and then 400 to 750 ° C at a cold rolling rate of 30 to 100 ° C / sec.
After being cooled to a temperature in the range of 100 to 750 ° C., it is wound at a temperature in the range of 400 to 750 ° C., partially or entirely having a bainitic ferrite structure and having a tensile strength of 600 N.
/ Mm ² or more and a method for producing a hot-rolled steel sheet excellent in stretch flange formability having a punched hole limit hole expansion ratio of 90% or more. 4. In weight%, (a) C 0.02 to 0.10%, Si 1.0% or less, Mn 3.0% or less, P 0.1% or less, S 0.01% or less, Ti 0.1 to 1.0%, Nb 0.1% or less, Cu 0.2 to 2.0%, and Ni 2.0% or less, and (b) Mo 1.0% or less, Cr 1 0.0% or less, and at least one element selected from the group consisting of V 1.0% or less, and the addition amount of Ti is C <[(Ti-3.43N-1.5S) / 4 + Nb /
7.75], the addition amounts of Cu and Ni satisfy 0.2 <Ni / Cu <1.5, and the balance steel and unavoidable impurities are used as slabs, and the slab is used at a temperature of 1100 ° C or higher. After heating to
Hot rolling is terminated at a temperature of (Ar ₃ -50) ° C or higher, and then 400 to 750 ° C at a cold rolling rate of 30 to 100 ° C / sec.
After being cooled to a temperature in the range of 100 to 750 ° C., it is wound at a temperature in the range of 400 to 750 ° C., partially or entirely having a bainitic ferrite structure and having a tensile strength of 600 N.
/ Mm ² or more and a method for producing a hot-rolled steel sheet excellent in stretch flange formability having a punched hole limit hole expansion ratio of 90% or more. 5. A steel containing, in addition to the elements (a) and (b), (c) Ca in an amount of 20 ppm or less.
The method for manufacturing a hot rolled steel sheet according to any one of claims.