JPH03147840A - Composite steel plate excellent in fatigue strength and manufacture thereof - Google Patents

Abstract

PURPOSE:To manufacture composite steel plate having the optimum hardness distribution in view of the relationship between the fatigue strength and the hardness of the steel plate by a constitution wherein the surface layer part and interior of the plate are specified. CONSTITUTION:The surface layer part of the composite steel plate concerned contains 0.01-0.15wt.% C, 0.05-2.0wt.% Si, 0.1-2.5wt.% Mn, 0.03wt.% P maximum, 0.03wt.% N maximum and C+Si/24+Mn/4>=0.3 and remainder Fe and inevitable impurities. Its interior contains 0.10wt.% maximum, 0.05-0.5wt.% Si, 0.1-1.0wt.% Mn, 0.03wt.% P maximum, 0.03wt.% S maximum, 0.01-0.07wt.% Al, 0.008wt.% N maximum and C+Si/24+Mn/4<0.3 and remainder Fe and inevitable impurities. The mean hardness of the surface layer pact, which takes 25% or less of the plate thickness, is set to be 140-200 Vickers diamond pyramid hardness and, at the same time, the interior mean hardness is set to be 50-130 Vickers diamond pyramid hardness.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is characterized in that the average hardness of the surface layer within 250 mm of the plate thickness is H.
v=140-200 and its internal hardness) IV
The present invention relates to a composite steel plate having an extremely excellent fatigue strength of -50 to 130, and a method for manufacturing the same.

(Prior art) As a conventional steel plate with excellent fatigue strength, JP-A-52-13
There is one disclosed in Publication No. 332. However, this method produces an expensive composite steel plate, which is a clad steel plate with excellent formability and high fatigue limit ratio, to which special elements such as REM, Zr, and Ca are added. ,
The present invention does not relate to a method of manufacturing a composite steel plate with low fatigue strength and excellent fatigue strength, in which Cr is added as necessary.

Many hot rolled steel sheets are subjected to cold working. In particular, fatigue strength after pressing is an important characteristic required of steel sheets for pressing, where pressing is widely adopted.

In general, when manufacturing an automobile body, first, several press processes including "drawing", "shearing", and "bending" are performed to form a steel plate into a predetermined part. The resulting molded product is then assembled into the vehicle body through the processes of "joining" and "painting." Therefore, suspension parts such as wheel discs and wheel houses are safety parts, and 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 methods for increasing material strength or increasing plate thickness. This increases the weight of the vehicle body and increases the strength of the material, resulting in higher costs.

In particular, automobiles are becoming lighter and lighter, and increasing the weight of the vehicle body must be avoided.

(Problems to be Solved by the Invention) Therefore, the present inventors have repeatedly researched composite steel plates with excellent fatigue strength, and have focused on the relationship between fatigue strength and steel plate hardness, and found that composite steel plates with an optimal hardness distribution are good. I found that.

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

(1) The surface layer is expressed in weight%, C0.01-0.15 zero St 0.05-2.0 zero Mn 0.1-2.5k P 0.03 or less S 0.03 or less Al 0. 01 to 0.07 N N 0.008 or less C+Si/24+Mn/4≧0.3, the remainder consists of Fe and unavoidable impurities, and the inside is C0.10 in weight%! Below Si 0.05~0.5! Mn 0.1-1. ON P 0.03 contains the following S 0.03 or less Al 0.01~0.0796 N 0.008 contains the following C + Si/24 + Mn/4 < 0.3, the balance consists of Fe and unavoidable impurities, plate thickness The average hardness of the surface layer within 25 zero) Iv-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) Cr1 is applied to one or both of the surface and internal parts of the eight pieces.
．． The composite steel plate with excellent fatigue strength according to item (1), characterized in that it contains 0X or less.

(3) In continuous casting, the surface layer is weight%, co, oi ~ 0.15 to 51 0.05 to 2,0 Mn 0.1 to 2.5 zero P 0.03 or less S 0.03 0.01 to 0.07 zero N 0.008 zero or less C + Si/24 + Mn/4 ≧ 0.3, the remainder consists of Fe and unavoidable impurities, the inside is C0.10 or less in weight% Si 0.05~0.5! k Mn 0.1-1. O Zero P 0.03 or less S 0.03 or less A20.01~0.07 Zero N 0.008% or less C+St/24+Mn/4<0.3 Steel billet consisting of the balance Fe and inevitable impurities The steel billet is finished at a temperature of 800°C or higher and a coiling temperature of 40°C.
Hot rolling is carried out at 0°C or lower, and the average hardness of the surface layer within 250 degrees of the plate thickness is IV-140 to 200, and the internal average hardness is Hv 50 to 130. A method for manufacturing composite steel plates with excellent fatigue strength.

(4) Both the surface layer and the inside of the steel piece have Cr1.0
The method for producing a composite steel plate with excellent fatigue strength according to item (3), characterized in that the composite steel plate has a fatigue strength of less than zero.

(Function) The composite steel plate of the present invention is a steel plate characterized in that it has excellent fatigue strength due to surface hardening of the steel plate, and has a soft internal hardness distribution so that press workability is not impaired.

The effect of surface hardening is that the inner soft material is restrained two-dimensionally or three-dimensionally due to surface hardening, making it difficult for cracks 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 mm of the plate thickness is set to lv-140 to 200, and the average internal hardness is limited to lv-50 to lv-130.

The reason for this limitation will be explained below.

In the present invention, the average hardness of the surface layer within 2596 of the plate thickness is set to Hv = 140 or more.
This is because the internal hardness difference disappears, which may impair the effect of surface hardening. On the other hand, the upper limit of the average hardness of the surface layer is H
The reason for setting v=200 is that if the surface is made harder than this, the moldability may be impaired. internal average hardness)l
The reason why the hardness is v-5O or more is that if the hardness is less than this, the specified fatigue strength cannot be obtained. On the other hand, the upper limit of the internal average hardness) Iv
-130 was chosen because if it becomes 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 explanation below are zero.

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, it is difficult to harden the surface. However, if it exceeds 0.15, spot weldability will be impaired, so it should be kept at less than 0.15.

2. The amount of Si is an element that inhibits chemical processability if added too much. It is necessary to keep it below H. The lower limit is set to 0.05 or more to ensure strength.

If the Mn density is too large, weldability deteriorates, so it is set to 2.5 or less. The lower limit is preferably 0.1 or more to prevent S embrittlement.

When the amount of S and the amount of P are included in a large amount, press formability is impaired, so the smaller the amount, the better, and the upper limit thereof is set to 0.031 or less.

The amount of AM is an element necessary for non-aging, but if it is less than 0.01X, it is difficult to make it non-aging. However, if it is included in a large amount, it will cause inclusions, so it should be set to less than 0.071.

The amount of N promotes the precipitation of AIN and improves the γ value, but 0.
Adding more than 0.08N will lower the γ value, so 0.0
08! Stay below 4 companies.

The amount of Cr is in two-phase RII4! ! Although it is an important element, 1
．． If it exceeds zero, two-phase Ia steel cannot be obtained, so 1.
Keep it below zero.

It was limited to C+Si/24+Mn/4≧0.3. 0.3
Hv=140-200 cannot be obtained below, inside is ^
R- is steel and its constituent elements are described below.

If the C content exceeds 0.1 OK, it becomes hard and press forming is impaired, so Q and IQ were limited to zero or less.

The amount of Si may be included as an impurity, and the lower limit is o
, os96. The upper limit needs to be set to 0.5 or less because if it increases, it will harden and the workability will deteriorate.

Since the Mn content deteriorates the γ value, it needs to be 1.0 or less. The lower limit is o to prevent hot embrittlement due to S.
OSX or higher.

The amount of 0.011 is a necessary element for non-aging.
If it is less than that, it is difficult to non-age, but if it is contained in a large amount, it becomes hard and press formability is impaired, so it should be 0.07X or less.

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

The amount of N combines with the shoulders to form AIN and improves press formability, but if it exceeds 0.008, ^2N will increase too much and press formability will deteriorate, so the N amount should be set to 0.008 or less. do.

The amount of Cr improves stretch flangeability, but if too much Cr is added, surface flaws will occur. Set the upper limit to 1. Must be ON or below.

It was limited to C+Si/24+Mn/4<0.3. 0.
If it exceeds 3, H sea of 50 to 130 cannot be obtained).

Steel having the above composition is manufactured by a continuous casting method and sent to a hot rolling process, but in the present invention, the finishing temperature of hot rolling is 800°C or higher (preferably 850 to 910°C).
The product is then 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 aOO°C or higher and a coiling temperature of 400°C or lower. The results of the fatigue strength of the obtained steel plate are shown in Table 2. The fatigue test was carried out using a Schenck type fatigue tester in bidirectional plane bending at a repetition rate of 3000 cpts.

Figure 342 shows a diagram of repeated stress and number of repetitions.

The test piece was a rectangular steel plate with a width of 40 mm and a length of 250 mm.A constant load was applied to the test piece, and the test was evaluated based on the number of repetitions and the repeated stress at the time of breakage.

The fatigue limit refers to the repeated stress when the number of repetitions exceeds 107 times.

The products of the present invention (sample @ No. 1 to 3) have a hardened surface and a soft interior made of composite PJ steel plates, and have a strength of 40 kg, compared to the fatigue limit of 29 to 30.5 kgf/IIIm. It is at a high level, equivalent to or higher than the fatigue limit of 29 kgf/m- of a homogeneous strength 60 kg class hot rolled steel plate (SAPH80) with no difference in hardness between the surface layer and inner layer of the example (sample 1i1 No. 4), and has a long durable life. significantly improve.

Table 2 (Effects of the invention) According to the present invention, by setting the average hardness of the surface layer up to the plate thickness of 250 to II V -140 to 200, and setting the internal average hardness to Hv・50 to 130, extremely excellent fatigue strength can be achieved. According to the present invention, the fatigue durability of wheel disks of automobiles, for example, can be improved, and the life of automobiles can be greatly extended.

Further, according to the present invention, a composite steel plate having an optimum hardness distribution and extremely excellent fatigue strength after press forming can be easily obtained, and a homogeneous wide material product can be manufactured at low cost.

Furthermore, it is also effective to prepare a sieve plate having an optimum hardness distribution according to the present invention and then perform a plating process on it or scrap it.

[Brief explanation of the drawing]

Figure 1 shows a model delta diagram showing the distribution of hardness difference within the plate thickness, and Figure 2 shows a diagram of repeated stress and number of repetitions. Figure Hv Figure Repetition Number Fatigue Limit Procedure Amendment Heisei χ Hand July 28 Z

Claims (1)

[Scope of Claims] 1 The surface layer portion is in weight percent: C0.01-0.15% Si0.05-2.0% Mn0.1-2.5% P0.03% or less S0.03% or less Al0. 01-0.07% N0.008% or less C+Si/24+Mn/4≧0.3, the balance consists of Fe and unavoidable impurities, the inside is C0.10% or less in weight% Si0.05-0.5 % 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 The average hardness of the surface layer within 25% of the board thickness is H_v = 140 ~
200, and its internal average hardness is H_v=50~
A composite steel plate with excellent fatigue strength characterized by a tensile strength of 130. 2 Cr1.0 on one or both of the surface layer and the inside of the steel piece
% or less, the composite steel plate having excellent fatigue strength according to claim 1. 3 In continuous casting, the surface layer is expressed in weight percent: C0.01-0.15% Si0.05-2.0% Mn0.1-2.5% P0.03% or less S0.03% or less Al0.01-0 .07%N0.008% or lessC+Si/24+Mn/4≧0.3, the balance consists of Fe and unavoidable impurities, and the inside is C0.10% or less in weight%, Si0.05~0.5%Mn0. 1-1.0% P0.03% or less S0.03% or less Al0.01-0.07% N0.008% 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 40°C.
Hot rolling is performed at a temperature of 0°C or lower, and the average hardness of the surface layer within 25% of the plate thickness is H_v = 140 to 200, and the internal average hardness is H_v = 50 to 130. A method for manufacturing composite steel plates with excellent fatigue strength. 4 Both the surface layer and the interior of the steel piece contain 1.0% Cr.
4. The method for manufacturing a composite steel plate with excellent fatigue strength according to claim 3, comprising the following steps: