JPH0617203A - High strength thin steel sheet with composite structure excellent in formability - Google Patents

Abstract

PURPOSE:To manufacture a high strength thin steel sheet high in fatigue limit and repeated yield stress and excellent in formability by subjecting a low carbon slab having a specified compsn. to hot rolling and cooling and making the slab to a thin steel sheet having a specified structure. CONSTITUTION:The continuous cast slab of a low carbon steel having a compsn. contg., by weight, 0.05 to 0.15% C, 1.2 to 2.5% Si, 1.5 to 2.3% Mn and one or both of 0.005 to 0.15% rare earth elements and 0.005 to 0.1% Ca is subjected to hot rolling to form its shape into a thin sheet one, is subjected to air cooling on a run out table and is furthermore cooled to an ordinary temp. The high strength thin steel sheet with a composite structure in which the area ratio of ferrite having 200 to 280 Vickers hardness is regulated to 60 to 85%, that of a hard phase in the case of martensite and bainite is regulated to 15 to 40% (in which the area ratio of bainite is regulated to <=5%) or residual austenite is furthermore incorporated in the range of 20 to 50% and the maximum length of the martensite and residual austenite is regulated to 20 to 150% of the ferritic grain size and the dimension of metallic carbides present on the grain boundaries is regulated to <=0.5mum is manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength thin steel plate having excellent formability for the purpose of reducing the weight of vehicles such as automobiles, and particularly used for underbody parts and the like which are strongly required to have fatigue durability. Regarding

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight of automobiles and the like, the strength of steel sheets and the weight reduction of aluminum alloys have been promoted. However, in the case of an Al alloy, the price is significantly higher than that of steel in terms of weight reduction, and since the fatigue strength is low, the advantage that the specific gravity is small cannot be fully utilized, so it is used for special applications. At present, there is a strong demand for more economical high-strength steel sheets for passenger cars, which are mass-produced products. However, even if the static yield strength and tensile strength are improved, the fatigue strength is not improved accordingly.
The higher the strength, the more problematic. In particular, in the case of products manufactured by press molding or welding such as automobile parts, simply increasing the strength of the material will impair these properties and is not preferable in practice, and therefore the fatigue strength is high even with the same static strength. A high-strength thin steel sheet has been demanded. So far, fine precipitation strengthened steels containing Ti, Nb, V, ferrite / bainite composite structure strengthened steels and ferrite / martensite composite structure strengthened steels (so-called Dual ph
as) steel and a composite microstructure-strengthened steel containing retained austenite with excellent ductility. However, these had the following problems.

[0003]

That is, precipitation-strengthened steel is used in a large amount because it is relatively easy to manufacture, but its fatigue properties are at a low level and its formability is also high. Since it is inferior to a steel plate, there is a limit to its application to foot parts having a complicated shape. Ferrite / bainite composite structure strengthened steel has particularly high local ductility and excellent hole expandability, but its fatigue properties are low due to the presence of coarse bainite. Ferrite / martensite composite structure steel (so-called dual phase steel). Although tension in the study of inventors strength describes the knowledge about the mechanism of fatigue strength excellent in 60 kg / mm ² class high strength thin steel sheet, although higher 70 kg / mm ² or more strength of a high strength thin steel sheet Fatigue strength did not improve. Furthermore, the literature (Yokohama et al .:
1991), the reason why fatigue strength is not improved is
It has been disclosed that the hardness of the hard phase is saturated, so that the improvement of fatigue strength is also saturated.
No technology has been found for producing a high-strength steel sheet with a composite structure having a fatigue property and formability of 0 kg / mm ² or more.

Further, a composite structure steel containing good retained ductility of retained austenite is characterized in that the uniform elongation is remarkably high, but the fatigue characteristics are so large that it cannot be put to practical use. Further, since the roughness of the surface layer is involved in the improvement of the fatigue limit, the fatigue characteristics are remarkably improved by grinding the surface layer to reduce the stress concentration, but at the time of manufacturing by hot rolling, the scale generated by heating is so-called. It is practically impossible to eliminate the surface roughness in the case of a high strength thin steel sheet for forming which is used in an as-pickled state for removing black skin. As another method for increasing fatigue strength, there is a method for improving fatigue durability by introducing a compressive residual stress and a hardened part in the surface layer by shot peening or induction hardening.
This method cannot be applied to high strength thin steel sheets for forming because it lowers formability.

In addition, from the viewpoint of use performance, high strength thin steel sheets for automobile suspension parts have not only a fatigue limit such that they will not be broken by fatigue during use, but also the number of repetitions due to sharp turns during driving and lifting of greenstone. Fatigue strength is required even at high stress amplitudes above the yield strength. Ie 1
2 of the fatigue limit, which is the maximum stress amplitude that does not break at 0 ⁷ times, and the cyclic yield stress, which is determined by the plastic strain amplitude of 0.2%
There is a demand for a high-strength thin steel sheet that satisfies both fatigue characteristics and has excellent formability. The present invention has been made in view of the above situation, and an object thereof is to provide stably a composite microstructure high strength thin steel sheet having excellent press formability, high fatigue limit and high cyclic yield stress and excellent formability. It is what

As mentioned above, FIG. 1 is a diagram showing the relationship between the fatigue limit and the cyclic yield stress of the high strength thin steel sheet using the test results of the invention in comparison with the durability test effect of the automobile wheel as compared with the comparative steel. Is. The fatigue limit here is 3000
It is the maximum stress amplitude that does not rupture at 10 ⁷ times at the fatigue limit by the double-sided plane bending fatigue test performed at a speed of 1 time / minute, and the cyclic yield stress is 0.4% / second at a strain rate of 0.4% / sec. It is expressed as the stress amplitude at a plastic strain amplitude of 0.2% from the obtained stress-strain curve at the time of stability. The wheel used is a passenger car wheel, the rim is a 60 kg / mm ² grade steel plate with a plate thickness of 2.8 mm, and the disk is 80-100 kg / with a plate thickness of 3.5 mm.
A steel plate of mm ² grade was used for forming, and the rim and the disk were joined by carbon dioxide arc welding. The durability test of the wheel was carried out at a bending moment of 180 kgm at a speed of 500 times per minute.
The durability life of a wheel manufactured from a steel sheet of grade / mm ² was compared. The durability life of a wheel is expressed as the time when a fatigue crack is generated in the disk portion and propagates, and the rigidity of the wheel decreases by 8% from the initial value.

[0007]

Means for Solving the Problems The present inventor has investigated in detail the relationship between the fatigue durability of automobile parts, the strengthening mechanism of steel materials and fatigue damage, and has found the location of fatigue slip due to fatigue and the location of cracks and small cracks. The crack growth affects the fatigue properties, and the conventional knowledge is that fatigue properties do not depend on the growth inhibition of fatigue cracks or the diminishing growth force due to bypassing cracks due to the hard phase. Of carbides existing at the grain boundary that inhibits the strength of the ferrite ground and the ferrite grain boundary to suppress the mutual connection and the growth of micro cracks and the growth, or the strength of the interface between the ferrite and martensite is dominant in fatigue damage. It was found that the relative size of the hard phase ferrite such as martensite with the grain size of the ferrite affects the fatigue properties in order to exert such effects.

The steel of the present invention not only excels in the fatigue characteristics of the material, but is also used in a bending moment durability test when an automobile wheel that requires formability as one of the foot parts is manufactured. The product exhibited excellent fatigue life. That is, it was found that when the durability is improved in the bending moment durability test of the wheel as shown in FIG. 1 described above, not only the fatigue limit of the material but also the repeated yield stress are both high at the same time. This is because even if the fatigue limit is high, the fatigue strength cannot be ensured even at high stress amplitudes higher than the yield strength, and the durability as a wheel is not satisfied. Further, it has been found that the durability as a wheel is not satisfied because fatigue cracks are likely to occur even if the cyclic yield stress is high.

Based on such knowledge, the present inventor has made the hardness of ferrite high and the area ratio of ferrite as a condition of a high strength thin steel sheet having a high fatigue limit and a high cyclic yield stress and excellent in formability. It is possible to prevent fatigue damage from easily accumulating at a specific place by uniformly increasing the amount in the structure, and to limit the size and area ratio of martensite, bainite, etc., which are hard phases, and become the starting point of fatigue. And the coarse carbides that easily precipitate at grain boundaries are suppressed, the fatigue limit becomes 3
8 kg / mm ² or more and cyclic yield stress is 50 kg /
We have found a new finding that high strength thin steel sheets with a formability of mm ² or more are possible. This makes it possible to achieve both the formability and the fatigue strength, which were particularly serious problems with high strength thin steel sheets.

The gist of the present invention is as follows: (1) In weight%, 0.05 to 0.15% of C, 1.2 to 2.5% of Si, and 1.5 to 2.3% of Mn are included. A steel containing at least one element selected from the group consisting of 0.005 to 0.15% rare earth elements and 0.005 to 0.1% Ca, and having a Vickers hardness of 200 The area ratio of ferrite having a hardness of 280 to 280 is 60% to 85%, and the sum of hard phases of martensite and bainite is 15% to 40.
% Or less, but the area ratio of bainite is 5% or less in any case, the maximum length of martensite is 20% or more to 150% or less of the average ferrite grain size, and the size of grain boundary carbides. With a microstructure of 0.5 μm or less and excellent composite formability

(2) 0.15 to 0.2 of C in wt%
5%, Si 1.4-2.3%, Mn 1.5-2.3
%, And a steel containing at least one element selected from the group consisting of 0.005 to 0.15% rare earth elements and 0.005 to 0.1% Ca, the structure of which is Vickers. The area ratio of ferrite having a hardness of 210 or more and 300 or less in hardness is 50% to 80%, the sum of hard phases of martensite, bainite and retained austenite is 20% to 50%, and martens Site is 7% or less and residual austenite is 5% or more to 1
8% or less, the maximum length of martensite or retained austenite is 20% or more to 150% or less of the average ferrite grain size, and the grain boundary carbide size is 0.
5 μm or less, excellent composite formability high strength thin steel sheet (3) Fatigue limit of 38 kg / mm ² or more, cyclic yield stress of 50 kg / mm ² or more, and 80 to 100 kg / mm ²
The high-strength thin steel sheet having a composite structure having excellent formability according to the above (1) or (2), which has a tensile strength of a grade.

[0012]

The respective constituent elements of the steel of the present invention will be described below. Not only is the fatigue limit of automobile underbody parts not to be destroyed by fatigue during use, but the fatigue strength at high stress amplitudes above the yield strength, that is, the cyclic yield stress, is important, not only the fatigue limit. Therefore, in the present invention, the fatigue limit is 38 kg / mm ² or more and the cyclic yield stress is 50 kg / mm ²
Each component was defined so that the above two fatigue characteristics could be satisfied at the same time.

First, regarding the chemical composition of the first aspect of the present invention,
If C (% by weight) is less than 0.05%, the tensile strength will be 80 kg.
/ Mm ^{2 or} less than 0.15%, the strength becomes too high and the ductility is hindered.
It was set to 5% or less. When Si is less than 1.2%, the tensile strength is 8
If it does not reach 0 kg / mm ² and exceeds 2.5%, the effect on ductility and fatigue strength saturates.
Below. If Mn is less than 1.6%, the tensile strength is 80k.
If it does not reach g / mm ² and exceeds 2.3, the bainite structure increases and the fatigue properties commensurate with the improvement in tensile strength cannot be obtained, so the content was made 1.5% or more and 2.3% or less.

Next, regarding the chemical composition of the second aspect of the present invention,
If C is less than 0.15, the desired amount of retained austenite cannot be secured, and if it exceeds 0.25%, toughness and weldability are impaired, so 0.15% or more and 0.25% or less. . If Si is less than 1.4%, the tensile strength is 80 kg.
/ Mm ² and the required amount of retained austenite cannot be secured. If it exceeds 2.3%, the effect on ductility and fatigue strength is saturated, so 1.4% or more is 2.3%.
Below. If Mn is less than 1.5%, the tensile strength is 80k.
If it does not reach g / mm ² and exceeds 2.3%, the bainite structure increases and the fatigue properties commensurate with the tensile strength cannot be obtained, so the content was made 1.5% or more and 2.3% or less.

In the present invention, in addition to the above elements, the steel sheet may contain the following elements. That is, N
b, V, and Ti are precipitation strengthening elements that control the ferrite grain size and the size of the hard phase. Regarding the chemical compositions of the first and second aspects of the present invention, Nb is 0.2% or less and V is 0.2. % Or less, and 0.15% or less of Ti may be contained alone or in combination. Rare earth elements (REM) and Ca are effective for controlling the morphology of sulfide inclusions, and are effective for formability and fatigue properties. The upper limit value is determined from the fact that the effect is saturated and, conversely, when it is excessive, the cleanliness of the steel is lowered, and the lower limit value is determined by the minimum value effective for shape control. Therefore, it is necessary to contain at least one element of the group consisting of REM and Ca, and the REM addition amount is 0.005% or more,
0.15% or less, Ca 0.005% or more, 0.1%
Below.

Next, the metal structure will be described. First, in the case of a steel sheet having a composite structure containing ferrite and martensite as the main structure, ferrite hardness is the point where the fatigue slip occurs first, and the micro Vickers hardness is 200.
If it is less than the above range, the hardness is too low and fatigue damage is likely to occur locally, so that neither the fatigue limit nor the cyclic yield stress is improved. If it exceeds 280, the difference between the hard phase and the hard phase will be small and the cyclic yield stress will be improved. However, the origin of fatigue damage will be more likely to concentrate at the grain boundary with the hard phase, and the fatigue limit will be increased despite the improvement in static strength. Does not improve. Further, since the static strength becomes too high, ductility is impaired and it becomes unsuitable for molding automobile parts. Therefore, the fine Vickers hardness is set to 200 or more and 280 or less. Further, in the case of a composite structure steel sheet containing retained austenite, the upper and lower limits of the microhardness of ferrite are determined for the same reason, so the Vickers hardness is set to 2
It was set to 10 or more and 300 or less.

In the case of a steel sheet having a composite structure containing ferrite / martensite as the main structure, the ferrite area ratio is less than 60%, and the ratio of the hard phase increases, so that the structure surrounds the ferrite phase, resulting in too high strength and hard phase. As a result, the deformation of the ferrite is restrained, so that not only the formability is lowered, but also the fatigue limit is saturated and does not increase. Again 80
%, The formability is excellent but the static strength is low and the desired fatigue strength cannot be obtained. Therefore, the ferrite area ratio is 60
% To 80%. In the case of a composite structure steel sheet containing retained austenite, the area ratio of ferrite is set to 50% or more and 80% or less because the upper and lower limits are determined for the same reason.

The area ratio of hard phase such as martensite is the desired static strength and fatigue strength when the sum of martensite and bainite is less than 15% in the case of a steel sheet having a composite structure mainly composed of ferrite and martensite. On the other hand, if it exceeds 40%, the ferrite phase is surrounded by a hard phase to form a knitted structure, which not only lowers the formability but also saturates the fatigue limit and does not increase. Therefore, it is set to 15% to 40%. However, in any case, since bainite is likely to become coarse and fatigue characteristics are significantly deteriorated, the content is made 5% or less. This steel may contain a small amount of retained austenite, but since it is unstable with respect to strain, it disappears during forming and is not an essential constituent element of the first aspect of the present invention.

In the case of a composite structure steel sheet containing retained austenite, the retained austenite which is stable against strain is positively utilized and is a constituent element of the second aspect of the present invention, and the retained austenite is 5%. The amount is not less than 18%. If it is less than 5%, the ductility becomes insufficient and the cyclic yield stress also decreases. Further, if it exceeds 18%, the fatigue strength is saturated. If the sum of the hard phases of martensite, bainite and retained austenite is 20% or less, neither the desired static strength nor fatigue strength can be obtained, and if it exceeds 40%, the ferrite phase is surrounded by a hard phase to form a network structure and formability is improved. Not only does it decrease, but the fatigue limit also saturates and does not increase. Therefore, it is set to 20% to 40%. In this case, if the area ratio of martensite exceeds 7%, the fatigue limit will rather decrease, so it must be 7% or less.

The size of the hard phase such as martensite must be specified in relation to the average grain size of ferrite.
In the first aspect of the present invention, the second aspect of the present invention of martensite
In the item, when the maximum length of martensite or retained austenite is less than 20% of the average grain size of ferrite, a large number of fine hard phases are present as compared with the ferrite grain size, so that the hard phases are easily connected to each other. On the contrary, it inhibits ductility. On the other hand, when the maximum length is 150% or more and is coarse, the fatigue starting point itself occurs in these hard phases, so that the fatigue strength is not improved. Therefore, the average grain size of hard phase such as martensite is 2 of the average grain size of ferrite.
It was set to 0% or more and 150% or less. It is desirable that the size of the grain boundary carbide is as small as possible, and only the maximum value is specified. If it is 0.5 μm or more, a crack is generated from the grain boundary carbide before cracking in the ferrite, and these carbides propagate, so fatigue characteristics are improved. Remarkably damages. Therefore, the maximum value is 0.5 μm
Below.

Hereinafter, the present invention will be further described with reference to examples.

EXAMPLES Steels 1 to 3 of the present invention are composite structure strengthened steels having ferrite and martensite as the main structures, and steels having the chemical composition shown in Table 1 are melted and continuously cast, and then in a temperature range of 830 to 930 ° C. Rolled to 3.5 mm, and run on the runout table for 3
Cool to 740 to 640 ° C at a rate of 0 ° C / sec or more, and air cool in this temperature range for 4 to 10 seconds, then to 50 ° C / 200 ° C or less
It is cooled in more than a second. The steels 4 to 6 of the present invention are composite structure strengthened steels containing retained austenite, and are 750 to 750.
After rolling in the temperature range of 850 ° and air cooling for 4 to 10 seconds,
350 at a speed of 30 ° C / sec or more on the runout table
It was cooled to ~ 450 ° C. Comparative steels 7 to 11 are steels by precipitation strengthening, rolled in the temperature range of 850 to 950 ° C, and cooled to 550 to 650 ° C at a speed of 20 to 30 ° C / sec. Comparative steels 12 and 13 are ferritic / martensitic composite structure strengthened steels, and comparative steels 14 to
No. 16 is a steel containing retained austenite, which was produced within the ranges shown in the production conditions of the invention steels 1 to 3 and 4 to 6, respectively.

[0022]

[Table 1]

The embodiment will be described in detail below. As shown in Table 2, the steels 1 to 3 of the present invention are steels having a main structure of ferrite / martensite, and all have a high fatigue limit and a high cyclic yield stress, and have good wheel molding and wheel durability. It is 2 to 4 times that of the comparative steel. Steels 4 to 6 of the present invention are steels containing retained austenite, all of which have high fatigue limit and high cyclic yield stress, good wheel forming, and wheel life of 2 to 4 times that of comparative steels. Is shown.

Next, the results of the comparative steel will be described. Comparative steels 7 to 11 are precipitation-strengthened steels containing Nb, Ti, V, etc., and have ferrite bainite as a main structure,
Although the structure of the steel of the present invention is different, the comparative steel 7 has Si less than the lower limit, the comparative steel 8 has Si and Mn less than the lower limits, and the sizes of grain boundary carbides are all above the upper limit. . As a result, as shown in Table 3, the fatigue limit is low and the wheel durability life is also low. In Comparative Steel 9, Si is below the lower limit,
With no addition of REM / Ca, the ferrite hardness and the grain boundary carbide dimensions are outside the upper limits. As a result, the fatigue limit was low, and cracking occurred in the wheel forming process, so the wheel durability was not evaluated.

Comparative Steel 10 is out of the lower limit of Si and the size of the grain boundary carbides, and therefore has a low fatigue limit and is poor in forming. In Comparative Steel 11, C is out of the upper limit, Mn is out of the lower limit, Ca is out of the upper limit, ferrite hardness is out of the upper limit, and the grain boundary carbides are out of size, so that the fatigue limit is low. Comparative steels 12 and 13 have a main structure of ferrite / martensite, and in the case of comparative steel 12, Ca is the lower limit,
The ferrite hardness was below the lower limit, and the maximum length of martensite and the area ratio of bainite were outside the upper limits, respectively, and the tensile strength and cyclic yield stress were insufficient, and cracks were formed during wheel molding. Comparative Steel 13 had an insufficient fatigue limit because the maximum size of martensite and the size of the grain boundary carbides were deviated, and small cracks were generated in the wheel forming process.

Comparative Steels 14 to 16 are steels containing retained austenite. In Comparative Steel 14, the area ratio of retained austenite is below the lower limit and both the maximum lengths of martensite and retained austenite are out of the range. The cyclic yield stress is insufficient because the grain boundary carbides are out of size. Comparative Steel 15 has no addition of REM / Ca,
The area ratio of martensite deviates from the upper limit, and the size of grain boundary carbide deviates, so that the fatigue limit becomes low. Comparative steel 16
Has no addition of REM / Ca, the ferrite area ratio is below the lower limit value, that is, the sum of hard phases of martensite, bainite and retained austenite exceeds the upper limit value, so the fatigue limit is low and cracking occurs during wheel molding. Occurred.

[0027]

[Table 2]

[0028]

[Table 3]

1) It is the limit of bidirectional plane bending fatigue of the as-received material. 2) Cyclic stress corresponding to 0.2% cyclic plastic strain in the steady stable stress / strain characteristics in the strain control fatigue test of the as-received material. 3) When the wheel hub hole processing does not cause cracks, the mark is ○, minute cracks are Δ, and when large cracks occur, ×. 4) Wheel durability life (unit: 10,000 times). 5) Comparison wheel (rim and disc tensile strength 60k
g / mm ² steel plate: life ratio to average life 150,000 times).

[0030]

As described above, the steel according to the present invention is excellent in fatigue properties and formability as properties of the raw material, so that it does not require an additional treatment such as a shot peening process in order to increase fatigue strength. It is an invention that can be produced in the same way as the production method and is economically and industrially effective. The fatigue strength can be further improved by performing additional treatment.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between fatigue limit and cyclic yield stress of a high-strength steel sheet using the test results of the invention steel in comparison with the durability test results of automobile wheels compared with the comparative steel.

[Explanation of symbols]

 1 Lower limit line of the area showing the life

Claims (3)

[Claims] 1. The weight percentage of C is 0.05 to 0.15.
%, Si 1.2 to 2.5%, Mn 1.5 to 2.3%
Is a steel containing at least one element selected from the group consisting of 0.005 to 0.15% rare earth elements and 0.005 to 0.1% Ca, the structure of which is Vickers hardness. The area ratio of ferrite having a hardness of 200 or more and 280 or less is 60% or more and 85% or less, and the sum of hard phases of martensite and bainite is 15% or more and 40% or less, provided that the area of bainite is The ratio is 5% or less, the maximum length of martensite is 20% or more to 150% or less of the average ferrite grain size, and the grain boundary carbide size is 0.5 μm or less. Microstructure high strength thin steel plate 2. The C content is 0.15-0.25 in weight%.
%, Si 1.4 to 2.3%, Mn 1.5 to 2.3%
Is a steel containing at least one element selected from the group consisting of 0.005 to 0.15% rare earth elements and 0.005 to 0.1% Ca, the structure of which is Vickers hardness. The area ratio of ferrite having a hardness of 210 or more and 300 or less is 50% or more to 80% or less, and the sum of hard phases of martensite, bainite, and retained austenite is 20% or more to 50% or less, and martensite is Is 7% or less and residual austenite is 5% or more to 18
% Or less, and the maximum length of martensite or retained austenite is 20% or more of the average ferrite grain size.
150% or less and the grain boundary carbide size is 0.5
High-strength steel sheet with a composite structure of less than μm and excellent formability 3. A fatigue limit of 38 kg / mm ² or more, a cyclic yield stress of 50 kg / mm ² or more, and 80 to 100 k.
The composite structure high-strength thin steel sheet with excellent formability according to claim 1 or 2, having a tensile strength of g / mm ² grade.