JPS61264160A - High-strength hot-rolled steel sheet of 60kgf/mm2 level for direct current butt welding - Google Patents

Abstract

PURPOSE:To provide a high-strength hot-rolled steel sheet having improved DC butt weldability and formability by adding proper amounts of Nb and Ti to a steel contg. specified amounts of C, Si, Mn, Al, P and S and by specifying the carbon equiv. CONSTITUTION:The composition of a high-strength hot-rolled steel sheet of 60kgf/mm<2> level is composed of, by weight, 0.04-0.10% C, <=0.3% Si, 1.0-1.6% Mn, 0.01-0.05% Al, <=0.03% P, <=0.01% S, <=0.08% in total of 0.02-0.06% Nb and/or 0.03-0.08% Ti and the balance essentially Fe, and the carbon, equiv. Ceq represented by a formula Ceq=C+Si/24+Mn/6 is regulated to 0.28-0.34. By the composition, the maximum hardness Hv of the weld zone of the steel sheet can be restricted to <=about 300, superior total elongation is ensured, and when the steel sheet is formed into wheel rims, etc., rejects are produced at a lowered rate.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a 60 kgf/mm class 2 high-strength hot rolled steel plate for direct current butt welding. Excellent sex 6
0 kg4/mmz class high strength hot rolled m for DC butt welding
Regarding the board.

(Prior Art) In order to reduce the weight of automobiles and reduce fuel costs, various attempts have been made to reduce the size of automobile bodies and to change materials by using high-strength steel materials. In particular, reducing the weight of wheels is said to be effective in reducing fuel costs, and the application of high-strength hot-rolled steel sheets to wheel rims is being intensively studied.

Wheel rims have conventionally been manufactured by forming a steel plate cut into a predetermined width and length into a ring shape, welding the ends, and then performing several roll forging steps. Conventionally, flash butt welding is used for the above welding, but
In recent years, because the flash does not fly off, there is no deterioration of the working environment due to dust, no noise is generated, and since there are no parts that are consumed by the flash, it is possible to improve the yield by about 1%. , DC butt welding tends to be preferred.

However, new problems have arisen in this new method of manufacturing wheel rims that employs DC butt welding. That is, in the past, when using steel plates with a tensile strength of 45 kgf/mi or less, there was almost no problem, but when using high-strength steel plates with a large amount of C or Mn, welding with a large current, during the upset process, or Due to the welding conditions that conduct electricity even if there is moss after the upset, the austenite grains become coarse at the end of welding, and as a result, the hardenability becomes high during the cooling process, making it easy to transform into martensite or hard bainite. As a result, the hardness of the welded part becomes extremely high, and as a result, cracks are likely to occur from the welded joint surface during processing.The maximum hardness of this welded part is
Although it is not clear in the past, according to the research of the present inventors, it is preferable to set the upper limit to about HV300;
According to conventional high-strength steel, the maximum hardness of the welded part of DC butt welded materials often exceeds Hv300, which is more than 100 Hv higher than that of flash butt welded materials, and the structure is close to martensite. .

(Object of the Invention) According to DC butt welding, the reason why the hardness of the welded part increases is that a larger current is passed through welding than in flash butt welding. As a result of intensive research to solve this problem from the perspective of steel materials, we found that by suppressing the carbon equivalent (Ceq) to 0.34% or less and adding a predetermined amount of Nb and/or Ti,
We have discovered that it is possible to suppress the maximum hardness of the welded part to below Hv 3 Q O, ensure excellent total elongation, and significantly reduce the defective rate in wheel rim molding,
This led to the present invention.

Therefore, the present invention provides, for example, a 660-600 600-6006 having excellent direct current acceptability and formability suitable for manufacturing wheel rims for automobiles.
The purpose of the present invention is to provide a high-strength hot-rolled steel sheet for direct current butt welding of the 0 kgf/mm" class.

(Structure of the Invention) The 60 kgf/mm2 class DC butt welded high-strength hot rolled steel sheet according to the present invention has (a) C 0.04 to 0.10%, Si 0.3% or less, Mn 1.0 to (b) Nb
0.02-0.06% and Ti0.03-0.08
% in a total amount of 0.08% or less, and has a carbon equivalent of 0.28 to 0.34%.

First, the reasons for limiting the chemical components in the steel of the present invention will be explained.

C is an element necessary to give the steel the required strength, and must be added in an amount of at least 0.05%. on the other hand,
As mentioned above, DC butt welding has the property of increasing the hardness of the welded part, so it is not preferable to add too much C. In the steel of the present invention, the upper limit of the amount of C added is set to 0.10. %.

In the case of flash butt welding, St is added to suppress the formation of venetrator, which is an oxide generated during the welding process. However, in the case of DC butt welding, it is difficult to generate penetrators due to the welding mechanism.
If the amount added is regulated to 0.3% or less, it is sufficient to suppress the formation of penetrators. When added in a large amount exceeding 0.3%, it is not preferable because it increases the electrical resistance value of the base metal and tends to cause welding variations.

Mn is necessary to increase the strength of steel, and at 60 k
In order to obtain a high-strength steel plate with a tensile strength of 1.0% or more, it is necessary to add 1.0% or more.

However, when adding too much, the hardness of the weld increases, causing cracks, and the second layer tends to form in layers, which also deteriorates ductility, so the upper limit of the amount added is 1.
6%.

When excessive P is contained in steel, it deteriorates the welded part, so the content is regulated to 0.03% or less.

S increases the amount of elongated MnS and causes cracks along the metal flow, so it is better to keep its content as low as possible, but in practice it is sufficient to limit it to 0.01% or less. It is.

A2 needs to be added in an amount of 0.01% or more to deoxidize the steel. However, when added in excess, it increases alumina-based inclusions and causes hook-like cracks like MnS, so the upper limit of the amount added is set at 0.05%.

In addition to the above-mentioned elements, the steel of the present invention contains Nb0.02~
One or two elements selected from the group consisting of 0.06% and 0.03 to 0.08% Ti can be added in a total amount of 0.08% or less.

Nb and Ti are elements that increase steel strength through precipitation strengthening, and in order to effectively express this effect, at least 0.02% of Nb and at least 0.03% of Ti are added. It is necessary.

This strengthening effect is proportional to the amount of each element added, but adding too much will lead to deterioration of the ductility of the base material and increase manufacturing costs, so the upper limit of the amount added is set for Nb. is 0.06%, and Ti is 0.06%.
08%. These elements may be added in combination, but
However, when the total amount of Nb and Ti added exceeds 0.08%, the
Since it deteriorates the base material, the total amount added should be 0.08% or less.

Furthermore, in the steel of the present invention, at least one element selected from the group consisting of Ca and REM is added together with or separately from at least one element selected from the group consisting of Nb and Ti. be able to. These elements are effective in making the elongated MnS spheroidal and preventing the occurrence of hook-like cracks. In order to effectively obtain such an effect, Ca should be set at 0. OOO5%
, 0 for REM. It is necessary to add 5% or more of OO. However, adding too much C not only saturates the effect, but also creates penetrators in the weld, making cracks more likely to start from these.
It is assumed that a is 0.01% and REM is 0.1%.

Next, the steel of the present invention has the above chemical components, and
As mentioned above, Ceq is required to be in the range of 0.28 to 0.34%. The present inventors performed direct current butt welding to manufacture wheel rims using Ceq steel.
It was found that there is a close relationship between the hardness of the weld and the maximum hardness of the weld. That is, as shown in FIG. 2, which shows the relationship between Ceq and the maximum hardness of the weld, when Ceq exceeds 0.34%, the maximum hardness of the weld exceeds HV300. Next, the defect rate during rim forming will decrease as the maximum hardness of the welded part is lowered, but if the handling network is limited to 60 kgf/mm2 grade steel,
As Ceq is reduced, N, b is added to ensure strength.
It is necessary to add a large amount of Ti, or introduce martensite by cryogenic winding. However, on the other hand, Nb
Addition of a large amount of Ti or Ti will result in insufficient total elongation, and the introduction of martensite will cause a decrease in hardness in the heat-affected zone compared to the base material, resulting in a decrease in plate thickness due to heat influence, and in any case, This will increase the defective rate or significantly increase manufacturing costs. Therefore, in the steel of the present invention, C
eq is 0.28% or more, and Nb and Ti are each 0.
．． 0.06% or less and 0.08% or less, total elongation 23%
The above is to be ensured.

Thus, according to the steel of the present invention, Ceq is 0.28 to 0.
．． Ferrite bainite limited to a range of 34%,
Ferrite, bainite, pearlite or ferrite
Due to the pearlite structure, a total elongation of 23% or more can be ensured.

(Effects of the Invention) As described above, the steel of the present invention uses 60 kgf/n+m2 class high tensile strength steel as the basic steel, and contains 0 each of Nb and/or Ti.
．． 0.06% and 0.08% or less, and the total amount is 0.08
% or less, and add Ceq to 0.
．． By regulating the content within the range of 28 to 0.34%, a 60 kgf/mm" class high-strength hot rolled steel plate for direct current butt welding, in which the hardness of the welded part is HV300 or less and the total elongation of the base material is 23% or more, can be obtained. be able to.

Example 1 Steel having the chemical composition shown in Table 1 was melted and a slab with a thickness of 220 ml was manufactured by continuous casting or ingot forming.

After this, it is made into a steel plate with a thickness of 2.6 mm using the normal rolling method,
It was wound up at a temperature between 400 and 650°C. After skin pass and pickling, it was formed into a slit coil, then DC butt welded under the welding conditions shown in FIG. 1, and subsequently rim-formed.

The mechanical properties and rim forming defect rate of the obtained hot rolled steel sheet are shown in the table. Moreover, the relationship between Ceq of steel and the maximum hardness of the welded part is shown in FIG. Note that the appended numbers indicate the steel numbers in the table (the same applies to FIG. 3).

As Ceq increases, the maximum hardness of the weld increases and reaches 0°3.
When it exceeds 4%, the hardness exceeds Hv300.

FIG. 3 shows the relationship between Ceq and rim molding defect rate.

When Ceq is in the range of 0.28 to 0.34%, the rim molding defect rate is suppressed to 3% or less, but 0°34
%, the defective rate increases rapidly.

This defect is mainly caused by cracking at the welded joint that occurs during the flaring step, which is the first step of rim forming, and is caused by the hardness of the welded portion being excessively high.

On the other hand, when Ceq is smaller than 0.28%, the defect rate also increases. This defect is mainly due to thinning in the weld heat-affected zone or heavily processed zone, and is due to lack of total elongation.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the conditions of DC butt welding in the example, Fig. 2 is a graph showing the relationship between Ceq and the maximum hardness of the welded part by DC butt welding, and Fig. 3 is a graph showing the relationship between Ceq and rim forming defect rate. It is a graph showing the relationship. Figure 3 Ce ((〆)

Claims (2)

[Claims] (1) In weight% (a) C0.04-0.10%, Si0.3% or less, Mn1.0-1.6%, Al0.01-0.05%, P0.03% or less, and S0. (b) 0.08% or less in total of one or two elements selected from the group consisting of Nb 0.02-0.06% and Ti 0.03-0.08%. and the carbon equivalent Ceq=C+(Si/24)+(Mn/6) is 0.28
~0.34% 60kgf/mm
^2nd class DC butt welded high strength hot rolled steel plate. (2) In weight% (a) C0.04-0.10%, Si0.3% or less, Mn1.0-1.6%, Al0.01-0.05%, P0.03% or less, and S0. (b) one or two elements selected from the group consisting of Nb 0.02-0.06% and Ti 0.03-0.08% in a total amount of 0.08% or less; (c) contains at least one element selected from the group consisting of Ca0.0005-0.01% and REM0.005-0.1%, and carbon equivalent Ceq=C+(Si/24 )+(Mn/6) is 0.28
~0.34% 60kgf/mm
^2nd class DC butt welded high strength hot rolled steel plate.