JP3290019B2 - Manufacturing method of high strength hot rolled steel sheet with excellent fatigue properties and stretch flangeability - Google Patents

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 excellent fatigue properties and excellent strength-elongation balance and stretch flangeability. It can be suitably used.

[0002]

2. Description of the Related Art In recent years,
Improvements in vehicle safety and fuel efficiency have been studied, and many attempts have been made to reduce the strength and thickness of automotive steel sheets.
Among these, reduction of the weight of the wheel or the undercarriage member serving as the unsprung member is an extremely effective means for improving the fuel efficiency of automobiles. Therefore, application of a high-strength steel plate to the wheel or the underbody member has been studied.

[0003] Such high-strength steel sheets include ferrite and steel.
A martensite composite structure steel sheet, a so-called dual phase steel sheet (DP steel sheet) is already known. This steel sheet has a low yield ratio, a large elongation compared to the strength, is excellent in formability and shape-freezing properties, and has good fatigue properties. However, this high-strength DP steel sheet is inferior in terms of stretch flangeability. Therefore, when applied to a wheel disk or the like, cracks are generated from a hole expanding portion during disk forming, or during a fatigue test or a running test. There is a problem of the occurrence of cracks from the hole expanding portion, and these problems have not been solved yet.

[0004] Recently, in order to improve stretch flangeability, which is a drawback of DP steel sheets, ferrite bainite,
Steel plate with the optimally adjusted martensite three-phase structure,
So-called tri-phase steel sheets (JP-A-57-70257, JP-A-57-145965) have been proposed. Although these are excellent in strength-elongation balance and stretch flangeability, they are not satisfactory in terms of improvement in fatigue strength required for underbody members and wheels that require strict fatigue characteristics.

[0005] Under such circumstances, the present invention is to provide a high-strength hot-rolled steel sheet which has improved fatigue strength by eliminating the drawbacks of the tri-phase steel sheet and without impairing the strength-elongation balance and stretch flangeability. It is intended to provide a manufacturing method.

[0006]

Means for Solving the Problems Accordingly, the present inventors have:
As a result of intensive studies to solve the above-mentioned problems, even in the case of a tri-phase steel sheet, in particular, by controlling the fraction of the bainite area ratio and the martensite area ratio, the strength-
It has been found that the fatigue strength can be improved without significantly impairing the elongation balance and the stretch flangeability, and the present invention has been made here.

That is, according to the present invention, C: 0.05 to 0.2
0%, Si: 0.20 to 2.0%, Mn: 0.60 to 2.5
%, S: 0.005% or less, P: 0.1% or less, Al: 0.00%
1% or less, Ca: 0.005% or less, if necessary, further Cu: 0.1 to 2.0%, V: 0.01 to 0.50
%, Ti: 0.01 to 0.30%, Nb: 0.01 to 0.30
% Or more of the group consisting of
i: 0.1 to 2.0%, Cr: 0.05 to 2.0%, Mo: 0.1%
When hot-rolling a steel containing one or more members of the group consisting of 0.05 to 1.0% and B: 0.0005 to 0.01%, with the balance being Fe and unavoidable impurity elements, The temperature is rolled between Ar ₃ + 50 ° C. and Ar ₃ -20 ° C.
The primary cooling is performed at a cooling rate of 15
C./sec or more, followed by secondary cooling at 600 ° C. or more
A cooling rate of less than 15 ° C / sec within a temperature range of 0 ° C or less
Slow cooling for at least 15 seconds but not more than 15 seconds, followed by tertiary cooling at a cooling rate of 15 ° C./second or more, followed by Ms point or more and 500 ° C.
By winding below, the structure is composed of three phases of polygonal ferrite, bainite and martensite, bainite area ratio (VfB) 1 to 15%, martensite area ratio (VfM) 2 to 30%, and VfM / VfB
The gist of the present invention is to provide a method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability, characterized in that the steel sheet has a tensile strength of 2 to 20.

[0008]

The present invention will be described below in more detail.

In the present invention, the microstructure of the hot-rolled steel sheet is selected from polygonal ferrite, bainite and martensite.
It is essentially important to phase and control VfM / VfB.

That is, as shown in FIG. 1, the bainite steel sheet has the best strength-stretch flange balance as shown in FIG.
It is the order of P steel plate. Since the steel sheet obtained in the present invention is a tri-phase steel sheet composed of three phases of polygonal ferrite, bainite and martensite, it shows almost the same strength-stretch flange balance as a conventional tri-phase steel sheet.

As shown in FIG. 2, the strength-fatigue strength ratio of the DP steel sheet is the best, followed by the tri-phase steel sheet and the bainite steel sheet. The steel sheet obtained in the present invention has an area ratio of bainite and martensite and VfM
By controlling / VfB, a strength-fatigue strength ratio equivalent to that of the DP steel sheet is obtained.

Therefore, the steel sheet obtained by the present invention has excellent workability comparable to that of a tri-phase steel sheet, has the same fatigue strength as a DP steel sheet, and is excellent in all.

For this purpose, as described above, a structure composed of polygonal ferrite, bainite, and martensite is used.
It must be in the range of 15%. When the bainite area ratio exceeds 15%, the strength-fatigue strength ratio decreases even when the above-described martensite is introduced. On the other hand, if it is less than 1%, only the stretch flangeability comparable to that of the DP steel sheet can be obtained, and the workability for severe hole expanding and the like becomes insufficient. Particularly preferred bainite area ratio is 1.5 to 5
% Range.

Further, the martensite area ratio is 2 to 30%.
Must be in the range of When the martensite area ratio exceeds 30%, the stretch flangeability decreases, and the workability for severe hole expanding and the like becomes insufficient.
If it is less than 2%, the strength-fatigue strength ratio becomes low. A particularly preferred martensite area ratio is in the range of 5 to 20%. By setting the average particle size of bainite and martensite to 10 μm or less, a better strength-fatigue strength ratio and stretch flangeability can be obtained.

The martensite specified in the present invention partially contains retained austenite.

The adjustment of the steel sheet structure as described above can be obtained by adjusting the hot rolling of the steel sheet and the cooling conditions after the rolling in consideration of the chemical composition of the steel sheet described below.

Next, the reasons for limiting the chemical components of steel in the present invention will be described.

C: C has an effect on strengthening the steel, and is an element necessary for forming low-temperature transformation products such as bainite and martensite, for which at least 0.05% is added. There is a need. However, if added excessively, the ductility is remarkably deteriorated and the weldability is also reduced, so the upper limit is 0.20%.
And

Si: Si has a role as a deoxidizing element of molten steel and is also an effective element as a substitutional solid solution strengthening element, and has a function of giving high ductility. Further, in the composite structure steel sheet as in the present invention, in the γ-α transformation after hot rolling, it has the effect of promoting the transformation to α and discharging the solute carbon in α into γ. The cleanliness of the phase is enhanced, and the concentration of carbon in γ stabilizes γ, facilitating the formation of a hard phase, thereby improving the mechanical properties. This requires an addition of at least 0.20%. However, excessive addition not only causes embrittlement of the welded portion but also deteriorates the surface properties due to the formation of oxide scale.
Is the upper limit.

Mn: Mn is an element necessary for improving hardenability and obtaining a desired structure. To effectively exhibit this effect, it is necessary to add at least 0.60%. However, if added excessively, not only decreases ductility but also impairs weldability, so the upper limit is made 2.5%.

S: S is usually contained in an amount of about 0.008%.
In particular, in order to improve the stretch flangeability, in the present invention, the content is particularly restricted to the range of 0.005% or less.

P: P is an element effective as a substitutional solid solution strengthening element and has an effect of giving high ductility. However, if added excessively, it segregates at grain boundaries and causes embrittlement, so that 0.1% or less. And

Al: Al is added as a deoxidizing agent, but an excessive addition not only saturates the effect but also increases the cost.
0.1% or less.

Ca: Ca has the effect of improving ductility, especially stretch flangeability, through sulfide morphology control. However, excessive addition not only saturates the effect but also lowers the cleanliness of the steel, so the content is 0.005% or less.

In the present invention, if necessary, Cu, V, T
one or more of the group consisting of i, Nb, and / or
One or more of the group consisting of Ni, Cr, Mo, and B can be contained in an appropriate amount.

Cu: Cu is an element effective for solid solution strengthening and precipitation strengthening, and has a function of strengthening a steel sheet without impairing stretch flangeability. It also has the effect of improving fatigue strength. In order to exhibit these effects, it is necessary to add at least 0.1%. However, an excessive addition not only saturates the effect but also increases the cost, so the upper limit is 2.0%.

V, Ti, and Nb: V, Ti, and Nb are precipitation strengthening elements and are effective not only in increasing the strength but also in coexisting with Mn and the like to have an effect of easily obtaining a low-temperature transformation product. In order to exhibit these effects, V: 0.01 to 0.50%, Ti: 0.01 to 0.3.
0%, Nb: 0.01 to 0.30%.

Ni: Ni has the effect of improving the hardenability and toughness without impairing the weldability, and at least 0.1 is required to exhibit this effect.
% Must be added. However, adding too much increases the cost, so the upper limit is 2.0%.

Cr: Cr is an element that improves hardenability and advantageously gives a desired structure. In order to exert this effect, it is necessary to add at least 0.05%. However, even if it is added excessively, the effect is saturated and the cost increases,
The upper limit is set to 2.0%.

Mo: Mo is an element that improves the hardenability and advantageously gives a desired structure. In order to exert this effect, it is necessary to add at least 0.05%. However, even if it is added excessively, the effect is saturated and the cost increases,
The upper limit is set to 1.0%.

B: B is an element effective for improving hardenability. In order to exhibit this effect, it is necessary to add at least 0.0005%. However, an excessive addition not only saturates the effect but also reduces ductility, so the upper limit is made 0.01%.

In the present invention, the structure of the hot-rolled steel sheet having the above-described chemical components is made into three phases of polygonal ferrite, bainite and martensite, and the bainite area ratio and the martensite area ratio and the ratio of both are controlled. The hot rolling and cooling conditions after rolling are regulated as follows.

That is, the above steel is melted and cast by a conventional method, and hot-rolled.
This is a condition that has an important effect on the total elongation, stretch flangeability, and fatigue properties of a hot-rolled steel sheet. Finishing temperature is Ar ₃ + 50 ° C ~
The reason for defining the range of Ar ₃ −20 ° C. is to increase the number of ferrite nuclei formed in the Ar ₃ transformation, and to form fine ferrite required for ductility in a short time. If the finishing temperature exceeds Ar ₃ + 50 ° C., it is difficult to obtain sufficient ferrite, fine bainite and martensite in the cooling process after rolling. The finishing temperature is Ar ₃ −
If the temperature is lower than 20 ° C., a large amount of strain will enter ferrite during finish rolling, impairing in-plane directionality and ductility and deteriorating workability.

In the present invention, the cooling process after finish rolling is important, and three stages of cooling are performed. First, the primary cooling needs to be rapidly cooled to a secondary cooling temperature range at a cooling rate of 15 ° C./s or more. This is for refining the structure and obtaining bainite and martensite. If the cooling rate is less than 15 ° C./s, pearlite is generated, and it becomes difficult to obtain the intended bainite and martensite. Fatigue properties cannot be obtained.

Next, the secondary cooling is performed at 750 ° C. or less and 600 ° C.
In the above temperature range, it is necessary to perform slow cooling at a cooling rate of less than 15 ° C. and for 3 seconds to 15 seconds. The reason for this is to generate sufficient ferrite and an appropriate amount of fine bainite by slow cooling during this time. If less than 3 seconds, sufficient ferrite cannot be obtained, and workability deteriorates. On the other hand, when the time exceeds 15 seconds, the area ratio of bainite increases, and it becomes difficult to secure the ratio to the desired martensite, and good fatigue characteristics cannot be obtained. Also, the slow cooling start temperature is 75
If the temperature exceeds 0 ° C., ferrite, bainite and martensite become coarse, and it is difficult to sufficiently obtain ferrite required for ductility. On the other hand, when the annealing temperature is lower than 600 ° C., a large amount of pearlite or bainite is generated, and it becomes difficult to maintain a desired ratio to martensite, and good fatigue characteristics cannot be obtained.

In the subsequent tertiary cooling, it is necessary to cool at a cooling rate of 15 ° C./s or more to a winding temperature of 500 ° C. or less. This is to prevent a large amount of pearlite or bainite from being generated during the cooling process to the winding temperature.

If the winding temperature exceeds 500 ° C., the amount of pearlite or bainite increases, and sufficient martensite cannot be obtained, and good fatigue characteristics cannot be obtained. On the other hand, when the winding temperature is lower than the Ms point, it is difficult to obtain bainite produced at a low temperature, which plays an important role in ductility and stretch flangeability. Therefore, the range of the winding temperature is set to be equal to or higher than the Ms point and equal to or lower than 500 ° C.

By controlling the hot rolling and the cooling conditions after the rolling, a predetermined steel sheet structure can be obtained. The cooling after rolling is usually performed on-line, but may be performed by off-line heat treatment in some cases.

Next, examples of the present invention will be described.

[0040]

EXAMPLE A steel having the chemical composition shown in Table 1 was vacuum-melted and rough-rolled, and a slab having a thickness of 30 mm was formed into a hot-rolled sheet having a thickness of 3.5 mm by three to four passes. Specimens were obtained by cooling under the following conditions. Table 2 shows the mechanical properties and the results of observation of the microstructure of the test material. FIG. 1 and FIG. 2 summarize the characteristics and organization.

As for the mechanical properties, the tensile test was performed using a JIS No. 5 tensile test piece, and the stretch flangeability was tested using a conical punch having a tip angle of 60 ° by punching a steel plate with a 10 mmφ hole. The evaluation was based on the hole expansion rate (λ). Fatigue strength was the stress that was not broken in both swing plane bending fatigue test ¹⁰⁷ cycles or more.

The steels No. 1 to No. 9 in Table 1 are steels having the chemical components within the scope of the present invention, and the steels No. 10 to No. 12 are comparative steels. In Table 2, No. 1, No. 5 to No. 7, and No.
12 to No. 18 are examples of the present invention, and others are comparative examples.

In Table 2, the test Nos.
No. 5 to No. 7 and No. 12 to No. 18 are 590 N / mm ²
At the above high strength, σw / TS shows excellent fatigue properties of 0.5 or more and good stretch flangeability.

On the other hand, the tests No. 2 to No.
4, No. 8 to No. 10, even if the steel has a chemical composition within the range of the present invention, since the hot rolling conditions are out of the range of the present invention, a large amount of bainite and the like are generated, and sufficient martensite is formed. Is not obtained, VfM / VfB becomes less than 2, and σW /
When the TS is less than 0.5, excellent fatigue properties are not obtained. Comparative Example No. 11 in which VfM / VfB exceeds 20
Is low in λ, and good stretch flangeability cannot be obtained. In Comparative Examples Nos. 19 to 20, the chemical components were outside the range of the present invention, the structure of the present invention was not obtained, and the fatigue properties were poor. Further, as in Comparative Example No. 21, even if a structure within the range of the present invention is obtained, the amount of S is large, and good stretch flangeability cannot be obtained with a chemical component outside the range of the present invention.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

As described in detail above, according to the present invention,
Tri-phase steel sheet with excellent fatigue properties and strength
A high-strength hot-rolled steel sheet having excellent stretch balance and stretch flangeability can be provided, and is particularly suitable as a material for automobile underbody members and wheel discs.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the strength of various materials and stretch flangeability.

FIG. 2 is a diagram showing a relationship between strength of various materials and fatigue strength.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-126813 (JP, A) JP-A-57-126813 (JP, A) Patent 2640065 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02-8/04 C21D 9/46-9/48 C22C 38/00-38/60

Claims (3)

(57) [Claims] C .: 0.05 to 0.20%, Si: 0.20 to 2.0%, Mn: 0.60 to 2.5%, S: 0. 005% or less, P: 0.1% or less, Al: 0.1% or less, Ca: 0.005% or less , the balance being the finish when hot rolling steel consisting of Fe and unavoidable impurity elements Temperature is Ar ₃ + 50 ° C ~
When rolling at Ar ₃ −20 ° C. and then performing three-stage cooling, primary cooling is performed at a cooling rate of 15 ° C./sec or more, and then secondary cooling is performed within a temperature range of 600 ° C. to 750 ° C. The cooling is performed at a cooling rate of less than 15 ° C./sec and the cooling is performed for 3 seconds to 15 seconds, and then the third cooling is performed at a cooling rate of 15 ° C./sec. Thereby, the structure is composed of three phases of polygonal ferrite, bainite and martensite, and the bainite area ratio (Vf
B) 1 to 15%, martensite area ratio (VfM) 2 to 30
%, And a method for producing a high-strength hot-rolled steel sheet having excellent fatigue properties and stretch flangeability, wherein VfM / VfB is 2 or more and 20 or less. 2. The steel according to claim 1, further comprising : Cu: 0.1 to 2.0%;
V: 0.01 to 0.50%, Ti: 0.01 to 0.30%,
2. The method according to claim 1, comprising one or more of the group consisting of Nb: 0.01 to 0.30% . 3. The steel according to claim 1, further comprising: Ni: 0.1 to 2.0%;
Cr: 0.05 to 2.0%, Mo: 0.05 to 1.0%, B:
One or more of the group consisting of 0.0005 to 0.01%
3. A method according to claim 1 or claim 2 containing: