JPS63247314A - Manufacture of hot-rolled sheet metal having delayed ageing at ordinary temperature and baking hardenability - Google Patents

Abstract

PURPOSE:To easily manufacture a high-tensile hot-rolled sheet metal excellent in delayed ageing characteristic at ordinary temp. and baking hardenability and having superior workability, by subjecting a steel having a specific composition containing C, Mn, P, Al, and N to specific hot rolling, cooling, and winding. CONSTITUTION:A steel containing, by weight, 0.02-0.10% C, 0.5-1.2% Mn, 0.04-0.10% P, 0.02-0.05% Al, and 0.005-0.020% N is hot-rolled at a temp. of Ar3-20 deg.C or above. Subsequently, the hot rolled steel plate is cooled at >=30 deg.C/s average cooling rate so as to inhibit AlN precipitation and also to provide a fine ferritic structure. Then, winding is carried out at 150-450 deg.C so as to accelerate the proper precipitation of C. By this method, the high-tensile hot-rolled sheet metal showing baking hardening amount as high as >=about 5kgf/mm<2>, practically free from age deterioration at room temp., and having superior workability of about 40-50kgf/mm<2> by T.S. can be obtained.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing a high-strength hot-rolled thin steel sheet suitable for use in automobile bodies, etc., which require high strength after forming. Paint baking after molding is a process in which the yield point increases significantly when the coating is subjected to baking treatment, so-called baking is intended to improve hardenability and slow aging properties at room temperature.

(Prior Art) High tensile strength thin steel plates have been widely used for automobile body exterior panels and the like in order to reduce the weight of the vehicle body. High-strength thin steel sheets for automobiles are subjected to forming processes such as press working, so they not only need to be relatively soft and have good formability, but also have sufficient strength. Therefore, recently, steel sheets are being used that are relatively soft during forming, but whose strength increases through age hardening during the painting process after forming, that is, steel plates that are highly hardenable during baking. It looks like this.

Seizure is an index of hardenability. The amount of seize and hardening is generally measured as follows. That is, first, a prestrain of about 2%, which corresponds to press molding, is applied, and then heat treatment is performed at 170° C. for 220 minutes, which corresponds to baking treatment.

Then, the difference between the deformation stress at 2% pre-strain and the yield stress after heat treatment is calculated, and this value is taken as the amount of hardening by baking.

By the way, there are various types of conventional steel plates that have hardening properties, but recently, particularly high baking properties have been developed as high-strength, high-ductility copper plates that have hardening properties, such as Japanese Patent Publication No. 55-485.
As described in Publication No. 75, quenched and tempered steel sheets made of high Nl steel with increased nitrogen (N)I content, "Japan Institute of Metals Aji J 19 (1980) P2S5" or "Japan Institute of Metals Aji J 19 (1980)" PIO, r iron and steel J V
ol 6B (1982) No. 9. P134B
Dual phase steel sheet (Dualhas) as described in
e steel plate) etc. are known.

(Problems to be Solved by the Invention) However, the steel plate described in the above-mentioned Japanese Patent Publication No. 55-48575 has a problem of large variations in material quality because it is difficult to adjust the amount of N.

On the other hand, the dual-phase steel sheet listed above has an average hardening amount of 5k during baking.
gf/me”, and if we tried to further increase the hardenability of the baking, the problem of material deterioration due to aging at room temperature was unavoidable.Also, with dual-phase steel sheets, the strength level would be too high, and the T , S, it has been difficult to manufacture a steel plate with good workability of about 40 to 60 kgf/mm''.

This invention was developed in view of the above-mentioned circumstances.High baking indicates hardening, specifically, high baking of 5 kgf 7 mm" or more indicates hardening, and there is almost no aging deterioration at room temperature. T, S,: 40-60 kgf/
The object of the present invention is to provide a high-tensile hot-rolled thin steel sheet having workability as good as that of a flat plate.

(Means for Solving the Problems) As a result of various studies to achieve the above object, the inventors have found that by appropriately controlling the chemical composition, rolling conditions, and cooling conditions, the desired results can be achieved. They discovered that the object could be advantageously achieved and completed this invention.

That is, in this invention, C: 0.02 to 0.10 wt% (hereinafter simply expressed as %)
, Mn: 0.5-1.2%, P: 0.04-0.10%, Affi: 0.02-0.05% and N
: A steel having a composition containing 0.005 to 0.020% is hot-rolled in Ar at a temperature of 20°C or higher, and then cooled at an average cooling rate of 30°C/s or higher. consisting of winding in a temperature range of 450°C,
Constant slow aging and baking is a method for producing hardenable hot rolled thin steel sheets.

This invention will be explained in detail below.

First, the reason why the composition of the starting materials is limited to the above range in this invention will be explained.

C: 0.02 to 0.10% If C is less than 0.02%, the transformation temperature is too high and the lower limit of the finish rolling temperature becomes high, which is disadvantageous in terms of operation.
If C exceeds 0.10%, welding becomes difficult during the spot welding process performed during forming and assembly in thin plate applications, and baking also tends to reduce hardenability, so the C content should be 0.10%. It was limited to a range of 0.02 to 0.10%.

Mn: 0.5 to 1.2% Mn is important as a reinforcing element, but in this invention it is not only used as a reinforcing element (it also has the effect of suppressing aging deterioration at room temperature through interaction with C and N). We are using.

This effect is small below 0.5%, so set the lower limit to 0.5%.
It was set to 0%.

On the other hand, even if it is added in a large amount exceeding 1.2%, an increase in strength can be expected, but the ductility deteriorates, and problems such as increased cost and deterioration of weldability occur, so the upper limit is set at 1.2%. And so.

P: 0.04-0.10% P, like Mn mentioned above, is a useful element that strengthens steel while suppressing aging deterioration at room temperature through interaction with C and N. However, if the content is less than 0°04%, the effect of the addition will be poor, while if the content exceeds 0°10%, not only will the effect of the addition reach almost saturation, but it will also cause problems such as secondary processing brittleness, which is undesirable. , P is 0.04~
It was supposed to be added in a range of 0.10%.

The term "secondary work embrittlement" here refers to a phenomenon in which a material that has been subjected to strong cold working becomes significantly brittle.

Al2: 0.02-0.05% Al is useful as a deoxidizing element and as a room temperature aging suppressing element by fixing solid solution N, but if the content is less than 0.02%, its addition effect will be poor. , while 0.050%
When reheating the slab at low temperatures, the AI
The dissolved N will remain as N, and the solid solute N will decrease in the hot-rolled sheet state, and large seizures will not only result in poor hardenability.
Deterioration of surface properties due to AfzO3 clusters and the like also becomes significant. Therefore, it was decided to add Al in a range of 0.02 to 0.050%.

N: 0.005 to 0.020% If N is less than 0.0050%, hardening cannot be expected due to large seizures due to solid solution N, so the lower limit was set to o, oos%.

On the other hand, even if the amount of N exceeds 0.0200%, the amount of baking and hardening will be
The upper limit was set at 0.0200% because not only does it hardly increase, but it also makes melting difficult and causes disadvantages such as deterioration of hot rolling properties.

The basic components have been explained above, but in the present invention, St can be further added as a strength improving element.

5i11% or less Si is an advantageous element for improving strength without significantly reducing ductility, but if it exceeds 1%, the steel hardens and rollability deteriorates, so Si is contained at 1% or less. It is preferable.

Next, the reasons for limiting the hot rolling conditions will be described.

First, regarding the finish rolling temperature, it is desirable that the baking temperature be relatively high in order to increase hardenability.

This temperature corresponds to the so-called Ar transformation point, but taking this point into consideration, Ar3 20° C. is the lower limit temperature, and finishing at a temperature lower than this lowers the hardenability of baking.

However, simply applying high-temperature finishing does not necessarily improve the quality of all materials, and it is not preferable for workability because it leads to coarsening of crystal grains. 0.08C-0,10S
i-1,00Mn-0,007ON -0,07P-
0,040A/! Steel is used as the material, and after heating to 1200"C, the finish rolling temperature is varied to harden (baking)
BH) changes in the amount were investigated.

The results obtained are shown in FIG. Note that the cooling rate and coiling temperature after rolling at this time were 40"Q/s, respectively.
The temperature was kept constant at 200°C. Further, the Ar1 point is about 870°C.

As is clear from the figure, by finishing the hot rolling at a temperature of Arz 20°C or higher, 5 kgf/mm
”Excellent BH properties as described above have been obtained.

After cooling, it is then wound up. In order to suppress the precipitation of AfN during cooling and to obtain good baking and hardenability by creating a fine-grained ferrite structure, it is necessary to control the cooling rate. This is important, and specifically, it is necessary to set the cooling rate after finish rolling until winding at an average rate of 30°C/s or more.

Regarding the winding temperature, there is an appropriate temperature range in order to suppress the precipitation of AfN, promote appropriate precipitation of C, and obtain an appropriate fine-grained ferrite structure.

If the winding temperature is less than 150°C, the amount of solid solute C will be too large, and aging deterioration at room temperature will become noticeable;
If the temperature exceeds 150℃, the solid solution C1N becomes thick (reduced), and baking causes deterioration of hardenability, which is undesirable.
-450'C.

(Function) In this invention, in particular, N addition and P addition have component characteristics, and by combining these with controlled rolling and controlled cooling, N and C remain in a solid solution state, and baking is hardened. Moreover, the interaction between P and these solid solution elements suppresses aging at room temperature.

(Example) A steel slab having the composition shown in Table 1 was hot rolled under the conditions shown in Table 2, and the tensile properties, BH properties, and weldability of the obtained hot rolled steel sheet were investigated. Also listed in

Here, ΔY, Ef are Y, l! when aged for 6 months at 40°C after 1% skin bath. It shows the amount of increase in %, and BH is 170'C,2 after 2% pre-straining.
0 minute baking is the yield stress after aging and 2 before aging.
% deformation stress (flow 5trss), and is expressed as stress.

In this invention, ΔY and Ef are required to be approximately 0.5% or less, and BH is required to be 5 kgf/mm'' or more.

Table 1 As is clear from the table, all the hot rolled sheets obtained according to the present invention have good relationships between T, S, and Ef, and furthermore, ΔY, l! is 0, and BH is 5kgf/mm2
The above-mentioned good baking was indicative of hardenability.

(Effects of the Invention) Thus, according to the present invention, it is possible to easily obtain a hot-rolled thin steel sheet with excellent room temperature slow aging properties and hardening properties, and it is possible to easily obtain a hot-rolled thin steel sheet with excellent slow aging properties and hardenability at room temperature. It can also be advantageously used for applications such as car body exterior panels.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between finish rolling temperature and BH amount. Figure 1 Finishing ° Rolling 1 degree (7)

Claims (1)

[Claims] 1. C: 0.02 to 0.10 wt%, Mn: 0.5 to 1.2 wt%, P: 0.04 to 0.10 wt%, Al: 0.02 to 0.05 wt% % and N: 0.005 to 0.020 wt% is hot rolled at a temperature of Ar_3-20°C or higher, and then cooled at an average cooling rate of 30°C/s or higher. , a method for producing a hot-rolled thin steel sheet having constant slow aging and bake hardenability, the method comprising coiling at a temperature range of 150 to 450°C.