JPH07252592A - Hot rolled high strength steel sheet with excellent formability, low temperature toughness and fatigue characteristics - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a hot rolled high strength steel sheet excellent in formability, low temperature toughness and fatigue characteristics at low cost and stably. [Structure] C = 0.05 to less than 0.25 wt% as chemical components, Si + Al = 0.5 to 3.5 wt%, Mn =
0.5-3.5% by weight, P ≦ 0.05% by weight, S ≦ 0.
It contains 01 wt% and Fe as main components, and has a microstructure of ferrite, bainite, and retained austenite.
50% or more of ferrite having a Vickers hardness of 150 or more and a grain size of 5 μm or less, and a carbon concentration of 0.1.
9% or more and 5% or more of retained austenite with a grain size of 2 μm or less are contained, and tensile strength (TS) = 490 to 490 as a characteristic.
1180 MPa, strength-ductility balance (tensile strength x total elongation) ≥ 20000 (MPa%), strength-stretch flangeability balance (tensile strength x hole expansion ratio) ≥ 75000 (MP
a%), fracture surface transition temperature ≤ -40 ° C, fatigue limit ratio ≥ 0.
A hot-rolled high-strength steel sheet excellent in formability, low-temperature toughness, and fatigue characteristics, which is characterized by including 45.

Description

Detailed Description of the Invention

[0001]

The present invention relates to excellent moldability (strength-ductility balance, strength-stretch flangeability balance), excellent low temperature toughness and excellent fatigue properties intended for use in automobiles, industrial machines and the like. The present invention relates to a hot rolled high strength steel sheet having both.

[0002]

2. Description of the Related Art Demand for high-strength steel sheets is increasing mainly due to weight reduction of steel sheets for automobiles and ensuring safety in the event of a collision. However, even for high strength steel sheets, the requirements for formability thereof are strict, and high strength steel sheets having excellent formability are desired. Furthermore, deterioration of low temperature toughness due to higher strength and insufficient fatigue strength due to weight reduction (thickness reduction) are becoming apparent, and not only excellent formability but also excellent low temperature toughness and excellent fatigue strength are provided. High strength steel sheets are strongly desired. However, it is the actual situation that the prior art has not sufficiently obtained the above characteristics.

[0003]

In order to solve the above problems, the present invention has excellent moldability (strength-ductility balance, strength-
The purpose of the present invention is to provide a hot-rolled high-strength steel sheet having a balance between stretch flangeability), excellent low temperature toughness, and excellent fatigue properties.

[0004]

In order to achieve the above-mentioned object, the present invention has the above-mentioned constitution as means. (1) As chemical components, C = 0.05 to less than 0.25% by weight, Si + Al = 0.5 to 3.5% by weight, Mn = 0.5
.About.3.5% by weight, P = 0.05% by weight, S.ltoreq.0.01% by weight and Fe as main components, and the microstructure has three phases of ferrite, bainite and retained austenite as main phases, and Vickers hardness is 150 or more and particle size 5 μm
It contains 50% or more of the following ferrites, 5% or more of retained austenite with a carbon concentration of 0.9% or more and a grain size of 2 μm or less, and as a characteristic, tensile strength (TS) = 490 to 11
80 MPa, strength-ductility balance (tensile strength x total elongation)
≧ 20,000 (MPa ·%), strength-stretch flangeability balance (tensile strength × hole expansion ratio) ≧ 75000 (MPa ·
%), Fracture surface transition temperature ≤ -40 ° C, fatigue limit ratio ≥ 0.45
A hot-rolled high-strength steel sheet having excellent formability, low-temperature toughness, and fatigue characteristics. (2) Ca = 0.005-0.01 wt% or REM
= 0.005 to 0.05% by weight, the hot-rolled high-strength steel sheet having excellent formability, low-temperature toughness, and fatigue characteristics according to (1).

[0005]

As a result of various experiments conducted by the present inventors, as a result of dispersing appropriate amounts of residual austenite and bainite with controlled C concentration and grain size in a ferrite matrix with controlled hardness and grain size, Since the problems of the technology have been solved and excellent formability (strength-ductility balance, strength-stretch flangeability balance), excellent low temperature toughness and excellent fatigue properties can be achieved at the same time, the present invention has been accomplished. is there. The summary will be described below. First, the steel sheet microstructure of the present invention will be described in detail. The steel sheet microstructure is mainly composed of three phases: ferrite, bainite, and retained austenite. The inclusion of martensite and pearlite reduces the amount of retained austenite and impairs the characteristics such as moldability, and therefore must be avoided. However, you may mix in the range (less than 5%) which does not impair the characteristic substantially.

Ferrite has a Vickers hardness of 150.
Above, the particle size is 5 μm or less, and the space factor is 50% or more. Fatigue cracks occur first in ferrite, which is a soft phase, so it is necessary to improve the Vickers hardness of ferrite and make its grain size fine in order to secure excellent fatigue properties. If the diameter exceeds 5 μm, excellent fatigue characteristics cannot be obtained.
If the space factor is less than 50%, the second phase, which is a hard phase other than ferrite, is connected to deteriorate the formability, low temperature toughness, and fatigue properties. Therefore, if any one of the conditions is not satisfied, it becomes impossible to combine excellent formability, excellent low temperature toughness, and excellent fatigue properties.

The carbon concentration of retained austenite is 0.
The particle size is 9% or more, the particle size is 2 μm or less, and the space factor is 5% or more. In order to suppress fatigue crack initiation and subsequent growth in ferrite, it is effective that retained austenite undergoes martensitic transformation due to stress load during fatigue. However, its carbon concentration is less than 0.9%,
If the particle size exceeds 2 μm, the retained austenite is unstable and the above effects cannot be sufficiently exhibited. On the other hand, if the space factor is less than 5%, the dispersion is sparse and it is not sufficient to exert the above-mentioned effect. Further, from the viewpoints of formability and low temperature toughness, the residual austenite becomes unstable if the carbon concentration is less than 0.9% and the particle size exceeds 2 μm, and if the space factor is less than 5%, the dispersion is low. Because of the sparseness, if any one of the conditions is not satisfied, excellent characteristics cannot be obtained.

Next, the regulated value of the chemical component and the reason for the limitation will be described. C is added in an amount of 0.05% by weight or more in order to secure retained austenite (hereinafter referred to as residual γ), but the upper limit of addition is 0.2 from the viewpoint of spot weldability.
It is less than 5% by weight. Si and Al play a very important role for remaining austenite and also act as a deoxidizing element and a strengthening element. That is, by promoting the formation of ferrite and suppressing the formation of carbides,
It has a function to secure residual γ, and also has a deoxidizing effect and a strengthening effect. From the above viewpoint, the lower limit of addition of Si + Al needs to be 0.5% by weight or more. However, Si,
Even if Al is excessively added, the above effect is saturated and rather steel is embrittled. Therefore, the upper limit of addition of Si + Al must be 3.5% by weight or less. Further, when particularly excellent surface properties are required, Si <0.1 wt% is used to avoid Si scale, or conversely, Si ≧ 1.0 wt% is used to improve Si scale. It is desirable to generate it on the entire surface to make it inconspicuous.

Mn is a strengthening element, and also has the function of stabilizing γ and securing residual γ. From the above viewpoint, M
The minimum addition amount of n must be 0.5% by weight or more.
However, even if Mn is excessively added, the above effect is saturated, and adverse effects such as ferrite transformation suppression are caused.
The upper limit of the addition of n must be 3.5% by weight or less.
Although P is effective in securing the residual γ, in the present invention, the upper limit is set to 0.05% by weight from the viewpoint of secondary workability, toughness, spot weldability, and recycling. If these requirements are not strict, it may be added in excess of 0.05%. The upper limit of S is 0.01% by weight in order to prevent the stretch flangeability (hole expansion ratio) from deteriorating due to sulfide inclusions.

[0010] Ca is 0.00 to improve the hole expansion ratio by controlling the shape of the sulfide inclusions (spheroidizing).
Although it is added in an amount of not less than 05% by weight, its upper limit is made 0.01% by weight from the viewpoint of saturation of the effect and adverse effect (deterioration of hole expansion ratio) due to increase of inclusions. For the same reason, the addition amount of REM is 0.005 to 0.05% by weight. The above is the reason for adding the main components of the present invention, but Nb, Ti, Cr, Cu, Ni,
You may add 1 type, or 2 or more types of V, B, and Mo. However, if the total amount of addition exceeds 0.2%, it becomes difficult to obtain the microstructure of the present invention and the cost increases, so the upper limit is made 0.2%.

[0011]

EXAMPLES Hot-rolling, cooling, and winding treatments were performed using steel pieces obtained by casting the steel having the chemical composition shown in Table 1 to obtain thin steel sheets of 2 to 4 mmt. Table 2 shows the steel sheet microstructure, and Table 3 shows the formability, low temperature toughness and fatigue characteristics of the steel sheet. Examples of the present invention are A steel to F steel. A comparative example is G steel. Fig. 1 shows a magnification of 1000 which corroded the cross section of A steel in the rolling direction.
It is a double optical microscope photograph. In the examples of the present invention, hot-rolled high-strength steel sheets having excellent formability, excellent low-temperature toughness, and excellent fatigue properties that far exceed those of the comparative examples are obtained. The inventive examples were also good in bendability, secondary workability, and spot weldability.

[0012]

[Table 1]

[0013]

[Table 2]

[0014]

[Table 3]

The microstructure was evaluated by the following method. Ferrite grain size and space factor are measured by Nital reagent and JP-A-59-59.
The cross-section in the rolling direction of the steel sheet was corroded by the reagent disclosed in JP-A-219473, and it was determined from an optical micrograph at a magnification of 1000 times. The ferrite hardness was obtained by the micro Vickers test. The particle size of retained austenite is Japanese Patent Application No. 3-3.
The cross-section in the rolling direction was corroded by the reagent disclosed in No. 51209, and it was determined from an optical microscope photograph at a magnification of 1000 times. The residual austenite space factor ( _Vγ : unit is%) is Mo-K.
It was calculated according to the following formula by X-ray analysis using α rays.

[0016]

[Equation 1]

The C concentration (C _γ ) of retained austenite is C
X-ray analysis by o-Kα ray (20
The lattice constant was calculated from the reflection angles of the (0) plane, the (220) plane, the (113) plane, and the (111) plane, and was calculated according to the following equation. C _γ (%) = (lattice constant −3.572) /0.033 [lattice constant is angstrom]

The characteristic evaluation was carried out by the following methods. The tensile test was carried out according to JIS No. 5, and tensile strength (TS), yield strength (YP), total elongation (T.El), uniform elongation (U.E.)
l), local elongation (L.El), strength-ductility balance (T
S × T. El) was determined. Stretch-flangeability is a hole with a hole diameter (d) at the time when a crack penetrates the plate thickness by expanding a punched hole of 20 mm with a 30-degree conical punch from the surface without burr and an initial hole diameter (d ₀ , 20 mm). Spreading ratio (d / d ₀ ×
100), and strength-stretch flangeability balance (TS
× d / d ₀ ) was calculated. Toughness is 1 / mm of 2mmV notch
The fracture surface transition temperature (vTrs) at which the brittle fracture surface ratio becomes 50% was obtained by performing the test with 4 subsize test pieces. Regarding the fatigue characteristics, the fatigue limit ratio (F) = 2,000,000 times fatigue strength / tensile strength was determined by a double swing plane bending fatigue test.

The bendability was such that a 35 mm × 70 mm test piece was subjected to 90 degree V-bending with the tip 0.5R with the burr on the outside.
The crack was observed. As for the secondary workability, a 90 mmφ punched plate was cup-molded with a drawing ratio of 1.8, crushed at −50 ° C., and cracks were observed. Regarding the spot weldability, the presence or absence of breakage in the nugget (the portion that was melted during spot welding and then solidified) when the spot welding test piece was peeled off with a chisel was observed.

[0020]

EFFECTS OF THE INVENTION According to the present invention, a hot rolled high strength steel sheet having unprecedented composite characteristics, that is, a hot rolled high strength steel sheet having excellent formability, excellent low temperature toughness and excellent fatigue characteristics, can be stably manufactured at low cost. Since it has become possible to provide the hot rolled high strength steel sheet, the applications and conditions of use of the hot rolled high strength steel sheet are remarkably expanded, and the industrial and economic effects are very large.

[Brief description of drawings]

FIG. 1 is an optical micrograph (magnification: 1000) of a metal structure in which the present invention steel A is corroded in a rolling direction cross section by a reagent disclosed in Japanese Patent Application No. 3-351209.

Claims (2)

[Claims] 1. As chemical components, C = 0.05 to less than 0.25% by weight, Si + Al = 0.5 to 3.5% by weight, Mn = 0.5 to 3.5% by weight, P ≦ 0.05. Wt%, S ≤ 0.01 wt% and Fe as main components, with a microstructure having three phases of ferrite, bainite and retained austenite as main phases, and ferrite having a Vickers hardness of 150 or more and a grain size of 5 μm or less. % Or more, the carbon concentration is 0.9% or more, and the residual austenite having a particle size of 2 μm or less is included by 5% or more, and the tensile strength (TS) = 490 to 1180 M as a characteristic
Pa, strength-ductility balance (tensile strength x total elongation) ≧ 20
000 (MPa ·%), strength-stretch flangeability balance (tensile strength × hole expansion ratio) ≧ 75000 (MPa ·%),
A hot-rolled high-strength steel sheet excellent in formability, low-temperature toughness, and fatigue characteristics, characterized by having a fracture surface transition temperature ≤ -40 ° C and a fatigue limit ratio ≥ 0.45. 2. Excellent formability, low temperature toughness and fatigue characteristics according to claim 1, characterized in that it contains Ca = 0.005 to 0.01% by weight or REM = 0.005 to 0.05% by weight. Hot rolled high strength steel sheet.