JPH0718383A - High strength hot rolled steel sheet excellent in stretch-flanging property and ductility and its production - Google Patents

Abstract

PURPOSE:To produce a hot rolled steel sheet having high strength and excellent in a balance of stretch-flanging properties and ductility and to provide the producing method of the same hot rolled steel sheet. CONSTITUTION:This high strength hot rolled steel sheet excellent in stretch- flanging properties and ductility in the one contg., by weight, 0.06 to 0.12% C, 0.5 to 1.6% Si, 1.5 to 2.2%. Mn, <=0.0050% S and 0.03 to 0.08% Ti, and the balance Fe with inevitable impurities and having a structure of retained austenite of >=3% volume ratio and the balance fine bainite. This steel is produced by subjecting the steel having the same compsn. to finish rolling in the temp. range of the Ar3+30 to the Ar3+110 deg.C and coiling it in the temp. range of 300 to 480 deg.C at >=140 deg.C/sec cooling rate immediately after the finish rolling.

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 excellent in stretch flangeability and ductility and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, as the movement to protect the global environment has increased,
There is a greater demand than ever for measures to reduce exhaust gas from automobiles and to reduce fuel consumption to save energy. One of the effective measures for that is to reduce the weight of the vehicle body. In addition, efforts are being made to reduce the strength and thickness of the steel sheets used, while also improving the safety of the vehicle body. Above all, hot-rolled steel sheets used for automotive undercarriage parts such as lower arms, members, wheels, etc. are subjected to severe forming mainly by stretch flange forming, and as a product, high part strength as an important safety part is required. It is said that Therefore, the strength of the press-formability 70 has a good balance between stretch-flangeability and ductility.
There is an increasing demand for high strength steel sheets of up to 80 kgf / mm ² .

In the past, as a high-strength hot-rolled steel sheet having an excellent balance between stretch flangeability and ductility, a steel sheet composed of three phases of ferrite + bainite + martensite has been proposed (Japanese Patent Publication No. 1-43005). The strength obtained there is at most about 65 kgf / mm ² , which is rather low for recent needs.

Further, as utilizing the residual γ, one based on a high carbon bainite structure (JP-A-1-159317) and one based on a ferrite + bainite structure (JP-A-3-180445).
No.)

The former, however, leaves a large amount of residual γ of 10% or more, and C is 0.35 to 0.55%.
The weldability is significantly hardened and the weldability is poor. In addition, the hole expansion ratio d / do (d: hole diameter when crack penetrates through plate thickness, do: initial hole diameter, punched hole diameter) is 1.32 to 1.4 because of the hard bainite structure showing stretch flangeability. There is not enough.

The latter is a low C system with a C content of 0.05 to 0.15% and has no problem in weldability, but since it is based on two phases, ferrite and bainite, it has a hole expansion ratio showing stretch flangeability. Is as low as d / do ≥1.4 and is not sufficient.

As a 100% bainite material with a high Si content (hereinafter referred to as full bainite), Japanese Patent Laid-Open No. 3-1804
However, it is still not enough performance for the recent needs.

JP-B-63-37166 describes TS: 80.
8 kgf / mm ² , El: 17.5%, hole expansion ratio: 1.
Although 67 was obtained, the El was less than 20% and the ductility was insufficient.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and is 70-80 kgf /
It is an object of the present invention to provide a hot-rolled steel sheet having a strength of mm ² and excellent balance of elongation bulging property and ductility, and a method for manufacturing such a hot-rolled steel sheet at low cost.

[0010]

Means and Actions for Solving the Problems A hot-rolled steel sheet according to the present invention, which is excellent in elongation bulging and ductility balance,
C: 0.06 to 0.12% by weight%, Si: 0.5 to
1.6%, Mn: 1.5 to 2.2%, S: 0.0050
%, Ti: 0.03 to 0.08%, balance F
e and unavoidable impurities, and has a structure consisting of retained austenite with a volume ratio of 3% or more and the balance of fine bainite.

The method for producing a high-strength hot-rolled steel sheet excellent in stretch flangeability and ductility according to the present invention is C:% by weight.
0.06-0.12%, Si: 0.5-1.6%, M
n: 1.5 to 2.2%, S: 0.0050% or less, T
i: contains 0.03 to 0.08 percent, the steel and the balance Fe and unavoidable impurities, Ar ₃ + 30~Ar ₃ +110
Finish rolling in the temperature range between ℃ and immediately after finishing rolling 1
It is characterized in that it is wound in a temperature range of 300 to 480 ° C. at a cooling rate of 40 ° C./second or more.

In this case, in addition to the above composition of steel, C
r: 0.40 to 0.70% may be contained, or N
b: 0.02-0.06%, V: 0.02-0.06%
One or two of them may be contained. Also, C
After containing r: 0.40 to 0.70%, further Nb: 0.02 to 0.06%, V: 0.02 to 0.06
You may contain 1 type or 2 types in%.

The inventors of the present invention should solve the above-mentioned problems from the viewpoints that the problems of the prior art are that stretch flangeability and ductility balance are poor and weldability is not good. , The composition and composition of steel, the structure of steel, and hot rolling conditions were repeatedly studied. Among them, the inventors of the present application investigated in detail the mechanical properties and microstructure of steel plates of various components having a plate thickness of 2.6 nm. As a result, it has been found that the strength-elongation (TS-El) balance is stratified by the amount of retained austenite (hereinafter referred to as residual γ) in a material having a structure of full bainite. That is, it was found that, when the base is full bainite, the TS-El balance becomes better as the residual γ amount increases. This leads to the fact that the stretch flangeability is secured using full bainite as a base, and the elongation (El), which has been a drawback of conventional bainite materials, can be compensated by residual γ. Figure 1
Shows the relationship between the strength-stretch flangeability (TS-λ) balance and the strength-elongation (TS-El) balance and the residual γ amount at that time. From this figure, it is understood that the TS-El balance becomes better as the residual γ amount increases, and the target TS-El balance is obtained when the residual γ amount is 3% or more.

The present invention having the above-mentioned structure is based on such knowledge of the inventors of the present application to refine the structure by adding Si and Ti to a low C steel as a base, and to retain the low C bainite single phase structure. By containing γ, a high-strength hot-rolled steel sheet excellent in stretch flangeability and ductility balance and a method for producing the same are provided.

The present invention will be described in detail below. The hot-rolled steel sheet according to the present invention is characterized by having a specific composition and having a structure composed of retained austenite with a volume ratio of 3% or more and the balance of fine bainite, and has a strength of 70 to 80 k.
gf / mm ² grade, strength-stretch flangeability (TS ×
λ ≧ 6000 kgf / mm ² ·%) balance and strength-elongation (TS × El ≧ 1600 kgf / mm ² ·%) balance excellent properties. Here, TS represents tensile strength, λ represents hole expansion ratio, and El represents elongation. The hole expansion was performed by punching a punched hole 10 mmφ with a clearance of 12% and using a 60 ° conical punch inside the burr. From the ratio of the hole diameter (d _B ) and the initial hole diameter (d _o ) when the plate thickness penetration crack occurred at the hole edge, the hole expansion ratio λ was calculated by the following formula.

Λ = (d _B −d _o ) / d _o × 100 (%) Next, the components of the steel according to the present invention will be described. C is
It is added to secure residual austenite (hereinafter abbreviated as residual γ). However, the amount is 0.06% (% by weight,
If the ratio is less than the same), the effect cannot be obtained. Therefore, the lower limit is set to 0.06%. On the other hand, in order to prevent embrittlement of the welded portion and ensure spot weldability, its upper limit is set to 0.
Specify to 12%.

Si is a strengthening element of steel, and also has an effect of suppressing the precipitation of C as a carbide due to the interaction with C, and increases the C concentration in austenite to indirectly increase austenite. Stabilize. However, if the amount is less than 0.5%, the effect cannot be obtained.
The lower limit is set to 0.5%. On the other hand, the amount is 1.6
%, Not only the effect is saturated, but also the spot weldability deteriorates, so the upper limit is set to 1.6%.

Mn is added in order to secure hardenability, form a bainite structure in the matrix, secure strength, and stabilize γ to secure residual γ. However, if the amount is less than 1.5%, the effect cannot be obtained, so the lower limit is defined as 1.5%. On the other hand, if the amount exceeds 2.2%, not only the effect is saturated but also the spot weldability deteriorates. Therefore, the upper limit is set to 2.2%.

Since S is an impurity element that binds to Mn in steel to form A-type inclusions (MnS-type inclusions) and deteriorates stretch flangeability, it is desirable to reduce S as much as possible, and deterioration of stretch flangeability is caused. From the viewpoint of prevention, the upper limit is set to 0.0050%.

Ti is a relatively low C-based component that is the subject of the present invention, and is an essential element for ensuring a residual γ of 3% or more in a steel sheet whose base is bainite. Such an effect of Ti was first discovered by the present inventors. Although the detailed mechanism of such an action is not clear, it is considered that the Ti-containing material has a refined bainite structure and an increased number of sites where residual γ exists. Since such an effect of Ti can be substantially exhibited when the amount is 0.03% or more, the lower limit is 0.03.
Specify as%. On the other hand, even if the amount exceeds 0.08%, the effect of securing the residual γ due to the refinement of the structure is saturated, so the upper limit is set to 0.08%.

If necessary, Cr: 0.40 to 0.70% is added as another component, or Nb:
0.02-0.06%, V: 0.02-0.06% of 1
Seed or two are added. Or Cr: 0.40 to 0.70
%, And further Nb: 0.02-0.06
%, V: 0.02 to 0.06% of 1 type or 2 types is added.

Cr is added to improve hardenability. And, in the low C-high Si system like the steel of the present invention, it contributes to the prevention of ferrite formation. The effect appears from 0.4% or more, while the effect is saturated even if it exceeds 0.7%, so when Cr is added, the amount is 0.40 to 0.7.
Specify 0%.

Nb and V make the austenite grains during hot rolling fine by forming carbon / nitride or by forming a solid solution to bring about the refinement of the base structure bainite, which is the final structure. Improves flangeability and ductility. The effect is expressed by 0.02% or more, while the effect is saturated even if it exceeds 0.06%. Therefore, when Nb and V are added, the amounts are both set to 0.
It is specified to be 02 to 0.06%.

In the present invention, on the premise of the above-described component system, the structure of the fine bainite matrix contains retained austenite in a volume ratio of 3% or more. With such a structure, it is possible to obtain a high-strength steel sheet excellent in stretch flangeability and ductility balance.

Next, the manufacturing conditions of the method of the present invention will be described. The steel having the above chemical composition is made into a slab by ingot making or continuous casting and then made into a hot coil. The hot rolling and cooling conditions at that time are determined as follows.

Slab heating temperature: In the present invention, since the target steel contains Ti as an essential component and Nb and V are optionally added, the slab heating temperature is set to 1200 ° C. or higher at which these elements form a solid solution. Stipulate.

Finishing temperature: The finishing temperature for hot rolling is Ar
_{It is set to 3} +30 to Ar ₃ + 110 ° C. If the finishing temperature is too high, the austenite grains become coarse and the final structure becomes coarse, so that the workability of the final product deteriorates. Therefore, from the viewpoint of not roughening the final structure, the upper limit of the finishing temperature is set to Ar ₃ + 110 ° C. On the other hand, the lower limit of the finishing temperature is defined as Ar ₃ + 30 ° C. in consideration of the stability of the finishing temperature in order to prevent the deterioration of ductility due to the processed structure.

Cooling start time after finish rolling: Cooling is started immediately after finish rolling. The steel of the present invention has a relatively high Si content,
Moreover, since the finishing temperature is low, ferrite is easily generated. In order to obtain the fine bainite which is the matrix structure of the steel of the present invention, it is necessary to suppress ferrite formation by quenching immediately after finish rolling. The shorter the water cooling start time is, the better, and it is better within 2 seconds after finish rolling.

Cooling rate: The cooling rate after finish rolling is 14
0 ° C / sec or more. The lower limit of cooling rate of 140 ° C / sec is
As in the present invention, the minimum cooling rate for obtaining the fine bainite structure of the matrix is the minimum cooling rate for a steel sheet with a relatively high Si. If it is less than this, ferrite transformation occurs during cooling and workability, especially stretch flangeability, deteriorates.

Winding temperature: The winding temperature is 300 to 480 ° C.
And The lower limit is defined to suppress the formation of martensite, and the upper limit is defined to suppress the formation of pearlite and to form a fine bainite structure.

By manufacturing the steel having the above components and composition under the above conditions, it is possible to obtain a structure containing a retained austenite with a volume ratio of 3% or more in the base of fine bainite.

[0032]

EXAMPLES Examples of the present invention will be described below. Fourteen steel types having the component compositions shown in Table 1 were melted. Steel A ~
G is steel satisfying the components and composition specified in the present invention,
Steels H to N are comparative steels.

[0033]

[Table 1]

Using the steels A to N shown in Table 1, hot rolling and cooling were performed under the conditions specified in the present invention to produce hot rolled steel sheets having a plate thickness of 2.6 mm. The conditions at that time are shown in Table 2. In order to examine the mechanical properties of the obtained steel sheet, a tensile test and a hole expansion test were conducted. The stretch flangeability was evaluated by the hole expansion ratio (λ) described above.

The structure was confirmed by etching the cross section of the steel sheet with nital, observing the structure with a microscope, and measuring the retained austenite (γ) by X-ray diffraction.

The weldability was evaluated by spot-welding the test material and whether or not the nugget portion (the portion which was melted during spot welding and solidified thereafter) was broken. The results are also shown in Table 2. In Table 2, the inventive materials Nos. 1 to 7 correspond to the inventive steels A to G, and the comparative materials 8 to 14 correspond to the comparative steels H to N. In the column of weldability, the case where there is no fracture in the nugget of the spot welded portion is indicated by ◯, and the case where there is fracture is indicated by x.

[0037]

[Table 2]

As is clear from Table 2, the number 1 of the present invention
7 to 7: TS: 70 kgf / mm ² or more, strength-stretch flangeability balance TS × λ is 6000 kgf / mm ²
-% Or more, strength-ductility balance TS x El is 1600 k
gf / mm ² ·% or more, strength-stretch flangeability-
It was confirmed that the ductility balance was excellent. The weldability was also excellent.

On the other hand, comparative steel Nos. 8 to 14
Is TS × El of 1600 kgf / mm ² ·% or more, T
Either S × λ: 6000 kgf / mm ² ·% or more was not satisfied, or the weldability was deteriorated. For example, in Nos. 8 and 12, since C and Mn are low, ferrite is generated and λ is lowered. No. 10 has a low Si amount and a small residual γ amount. The number 13 has a large amount of S and a low λ. Number 1
No. 4 has a small amount of Ti and no residual γ. As described above, in Nos. 8, 10, 12, 13, and 14, since the fine bainization of the base and the utilization of the residual γ are not sufficiently performed,
The balance of strength-stretch flangeability-ductility was insufficient. Further, since the reference numerals 9 and 11 have a high C content and a high Si content,
It was confirmed that the weldability had deteriorated.

Next, using the steels A to C having the composition range of the present invention in Table 1, various conditions of hot rolling and cooling were variously changed to manufacture hot rolled steel sheets having a plate thickness of 2.6 mm. The conditions in that case are shown in Table 3. Here, at least one of the hot rolling / cooling conditions does not satisfy the range specified in the present invention, and the numbers 15 to 20 in Table 3 are all comparative materials. The comparative materials Nos. 15 to 20 manufactured in this manner were evaluated for the same items as in Table 2. The results are also shown in Table 3.

[0041]

[Table 3]

As is clear from this table, the comparative material No. 1 which does not satisfy each of the hot rolling and cooling conditions of the method of the present invention
5 to 20 is the target value of the present invention, TS × λ ≧ 600
0, at least one of TS × El ≧ 1600 kgf / mm ² ·% is not satisfied. This is because the structure of the present invention containing residual γ was not obtained in the fine bainite matrix.

That is, since the comparative materials 15 and 16 do not satisfy the conditions of the finishing temperature of the present invention, the former does not satisfy TS × λ ≧ 6000 kgf / mm ² ·% due to the formation of ferrite, and the latter has coarsening of bainite. Therefore TS × El ≧ 1
Does not satisfy 600 kgf / mm ² ·%.

In Comparative Material 17, since the cooling time after finish rolling is long, ferrite is generated and λ is low. The comparative material 18 has a large amount of ferrite produced due to the slow cooling rate,
After all λ is low. The comparative materials 19 and 20 have a winding temperature outside the scope of the present invention, the former has a low El because a hard structure is generated, and the latter has not been optimized for bainite.
Is low.

[0045]

As described above, according to the present invention, a hot-rolled steel sheet having a strength of 70 to 80 kgf / mm ² and an excellent balance of elongation bulge and ductility, and such a hot-rolled steel sheet A manufacturing method is provided. The steel sheet of the present invention can be produced without making any special changes to the current hot rolling process, and since it does not use a particularly expensive material, it is low cost and industrially very useful. is there.

[Brief description of drawings]

1] Strength-stretch flangeability (TS-λ) balance,
And the figure which showed the relationship between strength-elongation (TS-El) balance and the amount of residual (gamma).

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location C22C 38/14 (72) Inventor Hiroshi Owada 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Within the corporation

Claims (8)

[Claims] 1. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, balance Fe and unavoidable impurities, volume ratio 3%
A high-strength hot-rolled steel sheet having excellent stretch flangeability and ductility, which has a structure composed of the above retained austenite and the balance of fine bainite. 2. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, C
r: 0.40 to 0.70% contained, balance Fe and unavoidable impurities, and a structure having residual austenite with a volume ratio of 3% or more and balance fine bainite, and stretch flangeability and ductility Excellent high strength hot rolled steel sheet. 3. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, containing Ti: 0.03 to 0.08%, Nb: 0.02 to 0.06%, V: 0.0
2 to 0.06% of 1 type or 2 types, and the balance F
A high-strength hot-rolled steel sheet having excellent stretch flange formability and ductility, characterized by having a structure composed of e and unavoidable impurities and having a volume ratio of retained austenite of 3% or more and the balance of fine bainite. 4. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, C
r: 0.40 to 0.70%, and Nb: 0.
02-0.06%, V: 0.02-0.06% of 1
Type or two types, high-strength heat with excellent stretch-flangeability and ductility, characterized by having a structure consisting of residual Fe and unavoidable impurities and having a volume fraction of 3% or more of retained austenite and the balance of fine bainite Rolled steel sheet. 5. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
Steel containing 0.0050% or less, Ti: 0.03 to 0.08%, and the balance Fe and unavoidable impurities, Ar ₃
Stretch-flangeability and ductility characterized by finish rolling in a temperature range of +30 to Ar ₃ + 110 ° C, and immediately after finish rolling, winding in a temperature range of 300 to 480 ° C at a cooling rate of 140 ° C / sec or more. A method for producing a high-strength hot-rolled steel sheet excellent in heat resistance. 6. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, C
r: contains 0.40 to 0.70 percent, the steel and the balance Fe and unavoidable impurities, Ar ₃ + 30~Ar ₃ +110
Finish rolling in the temperature range between ℃ and immediately after finishing rolling 1
A method for producing a high-strength hot-rolled steel sheet excellent in stretch-flangeability and ductility, which comprises winding in a temperature range of 300 to 480 ° C at a cooling rate of 40 ° C / sec or more. 7. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, containing Ti: 0.03 to 0.08%, Nb: 0.02 to 0.06%, V: 0.0
2 to 0.06% of 1 type or 2 types, and the balance F
steel consisting of e and unavoidable impurities, Ar ₃ +30 ~ Ar
Finish rolling in a temperature range of ₃ + 110 ° C., and immediately after finishing rolling, 300 to 48 at a cooling rate of 140 ° C./sec or more.
A method for producing a high-strength hot-rolled steel sheet excellent in stretch-flangeability and ductility, which comprises winding in a temperature range of 0 ° C. 8. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, C
r: 0.40 to 0.70%, and Nb: 0.
02-0.06%, V: 0.02-0.06% Balance Fe
And steel consisting of inevitable impurities, Ar ₃ +30 to Ar ₃
Finish rolling in the temperature range between + 110 ° C, and after finish rolling,
Immediately 300-480 ℃ at a cooling rate of 140 ℃ / sec or more
A method for producing a high-strength hot-rolled steel sheet having excellent stretch-flangeability and ductility, which is characterized in that it is wound in the temperature range of.