JPH03187738A - Composite steel sheet with excellent fatigue strength and preparation thereof - Google Patents

Abstract

PURPOSE:To obtain a composite steel sheet with extremely excellent fatigue strength by specifying a mean hardness of the surface layer and an inner mean hardness. CONSTITUTION:The surface layer part of a composite steel sheet contains 0.01-0.15 % C, 0.10-2.0 % Mn, 0.03 % or less P, 0.03 % or less S, 0.01-0.07 % Al, 0.008 % or less N and C+Si/24+Mn/4> 0.3 and the remaining consists of Fe and unavoidable impurities. The inner part contains 0.10 or less C, 0.5 % or less Si, 0.1-1.0 % Mn, 0.03 % or less P, 0.03 % or less S, 0.01-0.07 % Al, 0.008 % or less N and C+Si/24+Mn/4<0.3 and the remaining consists of Fe and unavoidable impurities. The mean hardness of the surface layer part up to 25 % of the sheet thickness is Hv=140-200 and the mean hardness of the inner part is Hv=50-130. Fatigue strength of the composite steel sheet is excellent by surface hardening and the press processability is not spoiled and cracks are hardly generated by providing a soft hardness distribution inside.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is characterized in that the average hardness of the surface layer within 25% of the plate thickness is H.
v -140-200 and its internal hardness is Hv
The present invention relates to a composite steel plate with extremely excellent fatigue strength due to the ratio of 50 to 130, and a method for manufacturing the same.

(Prior Art) A conventional method is disclosed in Japanese Unexamined Patent Publication No. 13332/1983. However, this method produces an expensive composite steel plate, which is a clad steel plate with excellent formability and high fatigue limit, to which special elements such as RIM, Zr, and Ca are added. The present invention does not relate to a method for manufacturing an inexpensive composite steel plate with excellent fatigue strength, in which C is added as necessary.

Many hot rolled steel sheets are subjected to cold working. In particular, press working is widely used.

Fatigue strength after pressing is an important characteristic required of steel sheets for pressing. Generally, when manufacturing a car body for an automobile, several steps of press processing including "drawing,""shearing," and "bending" are performed to form a steel plate into a predetermined part. The obtained molded product is then assembled into the vehicle body through the steps of "joining" and "painting."

Therefore, suspension parts such as wheel discs and wheel houses are safety parts, and their fatigue strength due to repeated tension-compression loads during driving is an important characteristic. The fatigue strength of this part is a major safety issue for automobiles. Most of the conventional research has focused on increasing material strength or increasing plate thickness. This increases the weight of the car body,
The increased strength of the material has led to higher costs.

Particularly, as automobiles are becoming lighter, it is necessary to avoid increasing the weight of the vehicle body.

(Problem to be solved by the invention) Therefore, the inventors have conducted repeated research on composite steel plates with excellent fatigue strength, and have focused on the relationship between fatigue strength and steel plate hardness, and have found that composite steel plates with an optimal hardness distribution are good. I discovered something.

(Means for solving problems) The gist of the present invention is as follows.

1 The surface layer is in weight% G 0.01-0.15% Mn 0.10-2.0% P 0.03% or less S 0.03% or less All 0.01-0.07% N 0.008% The following contains C+ Si/ 24+ Mn/ 4 > 0.3, the remainder consists of Fe and unavoidable impurities, the inside is co, to below 51 0.5% or less Mn 0.1-1.0% by weight P 0.03% or less S 0.03% or less^1 0.01-0.07% N 0.008% or less C+Si/24+Mn/4 <0.3, the balance consists of Fe and unavoidable impurities, and the board The average hardness of the surface layer within 25% of the thickness is Hv -140-
200, and its internal average hardness is Hv-50-1
A composite steel plate with excellent fatigue strength characterized by a strength of 30.

2 Cr 1.0% in one or both of the surface layer and the inside
The composite steel plate with excellent fatigue strength as described in item 1 above, characterized by comprising the following:

3 In continuous casting, the surface layer part is G 0.01-0.15% st less than 0.05% Mn 0.1-2.5% p 0.03% or less S 0.03% or less^1 0. 01-0.07% N 0.008% or less C+SI/24+Mn/4 >0.3, the balance consists of Fe and unavoidable impurities, the interior is C0.10 or less Si 0.5% or less Mn in weight% 0.1-1.0% P 0.03% or less 5 0.03% or less Al 0.01-0.07% N 0.008% or less c+ Sl/ 24+ Mn/ 4 < 0.3, the remainder Producing a steel billet made of Fe and unavoidable impurities,
The steel slab is hot-rolled directly or after reheating at a finishing temperature of 800°C or higher and a coiling temperature of 400°C or lower to reduce the thickness to 25% of the plate thickness.
A method for manufacturing a composite steel plate with excellent fatigue strength, characterized in that the average hardness of the surface layer is Hv -140-200, and the average internal hardness is Hv15G-130.

4 Cr 1.0% in one or both of the surface layer and the inside
The method for manufacturing a composite steel plate with excellent fatigue strength according to item 3 above, which comprises the following steps:

The composite steel sheet of the present invention has excellent fatigue strength due to surface hardening of the steel sheet, and has a soft internal hardness distribution that does not impair press workability. Since the soft material inside is restrained two-dimensionally or three-dimensionally, cracks are less likely to occur. However, if there is no surface hardening, cracks will easily occur from the surface layer.

In the present invention, as shown in FIG. 1, the average hardness of the surface layer up to 25% of the plate thickness is set to Hv -140-200, and the internal average hardness is limited to Hv -50-130.

The reason for this limitation will be explained below. In this invention, the average hardness of the surface layer up to 25% of the plate thickness is Hv = 14
The reason why it is set to 0 or more is because if it is less than this, there will be no difference in hardness between the surface layer and the inside, and there is a risk that surface hardening will be impaired. On the other hand,
The reason why the upper limit of the average hardness of the surface layer part is set to Hv-200 is because if the surface is made harder than this, there is a risk of impairing the moldability.

The reason why the internal average hardness is set to Hvm5G or higher is that a predetermined fatigue strength cannot be obtained with a hardness lower than this.

On the other hand, the reason why the upper limit of the internal average hardness was set to lv 130 is that if the material is made harder than this, the moldability may be impaired.

In manufacturing the steel plate of the present invention, in order to impart fatigue strength to the steel plate, the reasons for limiting the components of the steel in the manufacturing method of the present invention are as follows. All percentages used in the following description are wt%.

The surface layer is high tensile strength, and its constituent elements will be described.

The amount of C is an important element for surface hardening, but if it is less than 0.01%, surface hardening is difficult. However, if it exceeds 0.15%, spot weldability will be impaired, so it should be kept at less than 0.15%.
The amount of 51 is an element that inhibits chemical processability if it is contained in an excessive amount, and even if it is present unavoidably, it should be kept at less than 0.05%.

The amount of 11n is 2.5% because too much will deteriorate weldability.
Do the following. The lower limit is preferably 0.1% or less to prevent S embrittlement.

If too much S and P are included, press forming will be impaired, so the lower the better, the upper limit is set to 0.03% or less.

The amount of A2 is an element necessary for non-aging, but if it is less than 0.01%, it is difficult to make it non-aging. However, if it is contained in a large amount, it will cause inclusions, so it should be kept below 0.07%. The amount of N promotes the precipitation of 8IN and improves the r value, but o, oo
Adding more than a% lowers the r value, so 0.008
Keep it below %1.

The amount of Cr is an important element for dual-phase steel, but 1.0%
If it exceeds 1.0%, dual-phase steel cannot be obtained, so it should be kept at 1.0% or less.

C+Si/24+Mn/4 was limited to >0.3. 0.
If it is less than 3%, Hv -140-200 cannot be obtained.
The interior is mainly made of steel, and its constituent elements are described below.

If the C content exceeds 0.10%, it becomes hard and press forming is impaired, so it is limited to 0.10% or less.

The amount of St may be contained as an impurity, and the upper limit needs to be 0.5% or less because if it is too large, it will harden and deteriorate the workability.

Since the amount of un deteriorates the r value, it is necessary to keep it below 1.0%. The lower limit is 0.0 to prevent hot embrittlement due to S.
0.5% or more.

Hekoryo is a necessary element for non-aging, but 0.01%
If it is less than 0.0%, non-aging is difficult, but if it is contained in a large amount, it becomes hard and press formability is impaired, so the content should be 0.07% or less.

As for the amount of P and the amount of S, the lower the content, the softer the purchase, so the upper limit of each was set to 0.03% or less.

The amount of N combines with the ^ intersection to form AiN and improve press formability, but if it exceeds 0.008%, AIN increases too much and press formability deteriorates, so the N amount is adjusted to o, ooa.
% or less.

The amount of Cr improves stretch flangeability, but too much Cr causes a decrease in ductility, so the upper limit is set to 1.0% or less.

It was limited to C+ Si/ 24+ Mn/ 4 < OJ. If it exceeds 0.3, Hv-50-130 cannot be obtained.

A steel having the above-mentioned composition is produced by a continuous casting method and sent to a hot rolling process. In the present invention, the finishing temperature of hot rolling is aooot: or higher (preferably 850-910°C).
The product is rolled up at a winding temperature of 400° C. or lower (preferably 250° C. or lower) and provided as a product.

(Example) The components shown in Table 1 were melted by continuous casting, sent to a hot rolling process, and rolled at a finishing temperature of aoot or higher and a coiling temperature of 400°C or lower. The results of the fatigue strength of the obtained steel plates are shown in Table 2. The fatigue test was carried out using a Schenck type fatigue testing machine in double-sided plane bending at a repetition rate of 3000 cps. FIG. 2 shows a diagram of repeated stress and number of repetitions. The test piece was a rectangular steel plate with a width of 40 x length of 250 am, and the test was evaluated by the number of repetitions and repeated stress when a constant load was applied and the specimen broke.

Fatigue limit refers to the repeated stress when the number of repetitions exceeds 10,000,000 times.
) is a composite steel plate with a hard surface and a soft interior, and although it has a strength of 40 kg, it has a fatigue limit of 29-31.
5 kgf/■ Otori 2 and comparative example (sample steel No. 4) are homogeneous strength 60 kg class thermal steel plates (S) with no difference in hardness between the surface and inner layers.
APH60) No fatigue limit 30. It is at a high level, equal to or higher than Okgf/am', and greatly improves durability.

Composite steel plates can be obtained, and wide-width products can be manufactured at low cost.

Furthermore, it is also possible to use the steel plate provided with the optimum hardness distribution according to the present invention and then subjected to plating treatment.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the distribution of hardness differences within the plate thickness, and FIG. 2 is a diagram showing repeated stress and number of repetitions. (Effect of the invention) According to the present invention, the average hardness of the surface layer up to 25% of the plate thickness is Hv -140-200, and the average internal hardness is Hv -140-200.
By setting v=50-130, a composite steel plate with extremely excellent fatigue strength can be provided. According to the present invention, the fatigue durability of automobile wheel disks is improved, and the life of the automobile can be significantly extended. Moreover, according to the present invention, the optimum hardness distribution can be easily achieved. 11: ill tank≧

Claims (1)

[Claims] 1. The surface layer portion is C 0.01-0.15% Mn 0.10-2.0% P 0.03% or less S 0.03% or less Al 0.01-0.07% Contains N 0.008% or less C+Si/24+Mn/4>0.3, the remainder consists of Fe and unavoidable impurities, the interior contains C 0.10% or less Si 0.5% or less Mn0.1-1. 0% P 0.03% or less S 0.03% or less Al 0.01-0.07% N 0.008% or less The average hardness of the surface layer within 25% of the thickness is Hv = 140-2
00, and its internal average hardness is Hv=50-13
A composite steel plate with excellent fatigue strength. 2. The composite steel plate with excellent fatigue strength according to claim 1, characterized in that one or both of the surface layer portion and the interior portion contains 1.0% or less of Cr. 3 In continuous casting, the surface layer part is C 0.01-0.15% Si less than 0.05% Mn 0.1-2.5% P 0.03% or less S 0.03% or less Al 0.01-0. 07% N 0.008% or less C+Si/24+Mn/4>0.3, the balance consists of Fe and unavoidable impurities, the inside is C 0.10 or less Si 0.5% or less Mn 0.1-1 .0% P 0.03% or less S 0.03% or less Al 0.01-0.07% N 0.008% or less C + Si/24 + Mn/4 < 0.3 Steel with the balance consisting of Fe and inevitable impurities A steel piece is manufactured at a finishing temperature of 800°C or higher and a coiling temperature of 400°C.
The average hardness of the surface layer is Hv=140-200 to within 25% of the plate thickness by hot rolling directly or after reheating at a temperature below ℃.
A method for manufacturing a composite steel plate with excellent fatigue strength, characterized in that the average internal hardness is Hv=50-130. 4. The method for producing a composite steel plate with excellent fatigue strength according to claim 3, wherein one or both of the surface layer portion and the interior portion contains 1.0% or less of Cr.