JPH0456725A - Production of hot rolled steel plate excellent in workability - Google Patents

Abstract

PURPOSE:To produce a hot rolled steel plate excellent in workability by subjecting a steel having a specific composition in which respective contents of C, Mn, and Cu are specified to hot rolling and to coiling under respectively specified conditions. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.08-0.13% C, 0.9-1.5% Mn, <=0.10% Si, <=0.010% S, 0.010-0.08% Al, 1.2-2.0% Cu, 0.3-0.7% Ni, and the balance Fe with inevitable impurities and satisfying 2.2<=10XC(%)+Mn(%)<=2.5 is hot-rolled at a finishing temp. not lower than the Ar3 point and coiled at 350-500 deg.C. By this method, the hot rolled steel plate excellent in workability in spite of its high strength before working and capable of increasing its strength to a greater extent by means of heat treatment after working can be produced.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a hot-rolled steel sheet with excellent workability, specifically a hot-rolled steel sheet that has high strength and excellent workability, and whose strength is greatly increased by heat treatment after processing. This invention relates to a method for manufacturing steel plates.

(Conventional technology) Hot-rolled steel sheets are widely used in fields such as automobiles, construction, and home appliances, but in recent years, hot-rolled steel sheets with high strength have been adopted to reduce the weight of products. It is progressing. However, if high-strength hot-rolled steel sheets are used, the sheet thickness can be made thinner, making the product lighter, but there is a problem that high-strength hot-rolled steel sheets are difficult to process. Most of them are used after being pressed into a predetermined shape, so excellent workability is required, but workability and strength are in a contradictory relationship, and the higher the strength, the lower the workability.

For this reason, research is being conducted on high-strength hot-rolled steel sheets with excellent workability.One example of this is hot-rolled steel sheets that are soft and have low strength before processing, but can be made to have high strength by heat treatment after processing. Various developments have been made (for example, the
No. 49, JP-A No. 64-4429, JP-A No. 64-79347
issue).

(Problems to be Solved by the Invention) All of the hot-rolled steel sheets proposed so far have a strength of about 45 kgf/ms as hot-rolled, and are easy to press, and can be easily press-formed by heat treatment after forming. Strength is 65kg
f/ms". However, in recent years, advances in press technology have made it possible to process materials of the 60 kgf/+u+' class, and the strength can be increased to about 65 kgf/am" through post-processing heat treatment. The benefits of heat treatment are waning. Furthermore, in the automobile industry, fuel efficiency regulations stemming from environmental issues require further weight reduction, so it is thought that the materials used will become even stronger in the future. In fact, there is a demand for hot-rolled steel sheets whose strength before heat treatment is 60 kgf/-12 class and whose strength after heat treatment is higher than this.

The purpose of the present invention is to meet such demands. Specifically, the strength before heat treatment is in the 60 kgf/am" class, but it has excellent workability, and the strength is increased to 15 kgf/am" class after heat treatment.
The object of the present invention is to provide a method capable of producing a hot rolled steel sheet with an increase in the temperature of 100 kgf/am or more.

(Means for Solving the Problems) The present inventor uses steel with adjusted contents of C and Mn and adds an appropriate amount of Cu, which is a precipitation hardening element, as a material, and under certain conditions. If hot-rolled and coiled, it is possible to obtain a hot-rolled steel sheet that has a strength of 60 kgf/■■8 class as it is hot-rolled, has excellent workability, and whose strength increases greatly by heat treatment after processing. I found it.

The gist of the present invention based on the above knowledge is "In weight %, C:
0.08-0.13%, Mn: 0.9-1.5%,
Si: 0.10% or less, S: 0.010% or less, ^I: 0.010 to 0.08% or less, Cu:
1.2 to 2.0%, Ni: 0.3 to 0.7%, the balance consists of Fe and inevitable impurities, and 2
．． The steel satisfying 2≦IQXc (%) + Mn (%)≦2.5 is Ar.

After hot rolling at intended finishing temperature, 350-500
A method for producing a hot-rolled steel sheet with excellent workability, which is characterized by winding at a temperature of °C.

(Function) Hereinafter, in the method of the present invention, the chemical composition ratio of the material steel,
The reason why the hot finishing temperature and the winding temperature are numerically limited as described above will be explained.

A) Chemical composition ratio of the steel material■ C A certain amount of C is required to achieve a strength of 60 kgf/ms" class before heat treatment. If C is less than 0.08%, the strength before heat treatment will be 60 kgf/ms" class. If the C content is below /--t and exceeds 0.13%, the stretch flangeability will decrease and the workability will be poor, so the C content should be 0.08 to 0.
．． It was set at 13%.

■ Mn Mn increases the strength of steel through solid solution strengthening, but if it is less than 0.9%, this effect is small (the strength before heat treatment is 60%).
kgf/as". On the other hand, if the content exceeds 1.5%, good elongation cannot be ensured. For this reason, the Mn content was determined to be 0.9 to 1.5%.

The content of C and Mn is within the above range and satisfies the following formula.

2.2≦10×C (%) + Mn (%)≦2.5 In the present invention, the strength of the hot rolled steel sheet before heat treatment is adjusted by C and Mn, and the respective contents of C and Mn are Even if it is within the range, if the value calculated from the above formula is smaller than 2.2, the strength before heat treatment will be less than 60 kgf/ms,
If it is larger than 2.5, the strength before heat treatment becomes too high, impairing workability.

■ 5i Si has a deoxidizing effect, but if it exceeds 0.10%, island-like scales will be formed on the surface of the hot rolled steel sheet, deteriorating the surface quality, so the St content should be 0.1%. It was limited to 10% or less.

■ SS Since S causes the formation of sulfide-based inclusions and deteriorates the workability of hot rolled steel sheets, the content was limited to 0.010% or less.

■ Al Al is added as a deoxidizer to steel, but if it is less than 0.010%, sufficient deoxidizing effect will not be exhibited;
If it exceeds 0.010 to 0.0%, the steel will harden and alumina-based inclusions will increase, deteriorating workability.
．． The content was set to 0.08% or less.

(2) Cu Cu is an important element in the present invention, and is present as a solid solution in the hot-rolled steel sheet, and precipitates during post-working heat treatment to increase strength. However, if it is less than 1.2%, the increase in strength due to heat treatment will be low, and if it exceeds 2.0%, CuS will be formed and the steel will become brittle, and processing cracks will occur on the steel plate surface during hot working. was set at 1.2% to 2.0%.

■　Nl N1 has the effect of preventing cracking caused by the addition of Cu.

If the content is less than 0.3%, the above effect is small, and if the content exceeds 0.7%, the above effect will be saturated and the cost of the steel will only increase. Therefore, Ni is 0.3 to 0.7%. The content of

B) Finishing temperature During hot rolling, if finishing at a temperature lower than A r 3 point, the structure will become processed ferrite and the workability of the resulting hot rolled steel sheet will be significantly deteriorated, so the finishing temperature should be set at A r
Scored 3 points or more.

C) Coiling temperature If the coiling temperature is too high, solid solution Cu will precipitate during the cooling process, and if it is too low, the steel used in the present invention will contain C, Mn,
Because it contains elements that have a hardening effect, such as Cu and Ni, the hot-rolled steel sheet hardens, and in both cases, the strength becomes higher than necessary, impairing workability, and furthermore, tempering softens during heat treatment. The increase in strength due to Cu precipitation is impaired.

Figure 1 shows c: o, ii%, Si: 0.08%, M
n: 1.1%, P: 0.008%, S: 0
．． 004%, At: 0.032%, N: 0.0
026, Cu: 1.52%, Ni: 0.6%,
After heating the steel, the remainder of which is Fe, at 1200°C,
The tensile properties of hot-rolled steel sheets that were hot-rolled to a thickness of 3.2-cm at a finishing temperature of 30°C and wound at different coiling temperatures were investigated in relation to the coiling temperature. The tensile properties were examined by taking JIS No. 5 test pieces from each hot-rolled steel sheet after winding.

As is clear from FIG. 1, the strength is high and the elongation is low when the winding temperature is below 350°C and above 500°C.

In the present invention, there are no particular restrictions on the heating conditions prior to hot rolling, and the hot charging method, which has been developed in recent years for the purpose of energy saving, in which the slab after continuous casting is inserted into a heating furnace at high temperature, or the hot charging method, in which the slab is rolled as it is, Even if the direct roll method is adopted, the effects of the present invention will not be impaired in any way.
In the hot-rolled steel sheet manufactured by the method of the present invention, solid solution Cu is precipitated by the heat treatment after processing, and the strength increases greatly, but in order to ensure this effect, the heat treatment must be performed at a temperature of 500 to 700
It is best to carry out at a temperature range of 0°C.

Next, the present invention will be further explained with reference to Examples.

(Example) By continuous casting, 250 mm thick slabs with the chemical composition shown in Table 1 were cast, heated, hot rolled to a thickness of 2.91 mm, and wound into one roll. JIS No. 5 test pieces were taken from each hot-rolled steel sheet, and the mechanical properties and stretch flangeability of the hot-rolled steel sheets were investigated. In addition, in order to investigate the increase in strength due to post-processing heat treatment, a similar test piece was treated at 600° C. for 10 minutes and then subjected to a tensile test.

Table 2 shows hot rolling conditions, mechanical properties as hot rolled, stretch flangeability (HLE), and strength increase after heat treatment.

(Hereinafter, blank space) From Table 2, it can be seen that the hot-rolled steel sheets manufactured by the method of the present invention all have a TS of 60 kgf/++n" or more as hot-rolled, but an EL of 28% or more and a HEL of 65%. It has excellent workability as above, and the TS is 15kgf/
■It can be seen that the price has increased by more than WLT.

(Effects of the Invention) As explained above, according to the method of the present invention, it is possible to produce a hot-rolled steel sheet that has high strength and excellent workability before processing, and whose strength further increases by heat treatment after processing. .

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between winding temperature, tensile strength, and elongation.

Claims (1)

[Claims] In weight%, C: 0.08~0.13%, Mn: 0.9~
1.5%, Si: 0.10% or less, S: 0.010% or less, Al: 0.010 to 0.08% or less, Cu: 1.2
~2.0%, Ni: 0.3~0.7%, the balance consists of Fe and unavoidable impurities, and 2.2≦10
×C (%) + Mn (%)≦2.5
_350~5 after hot rolling at finishing temperature of 3 points or more
A method for producing a hot-rolled steel sheet with excellent workability, which is characterized by winding at a temperature of 00°C.