JPS595652B2 - Manufacturing method of high tensile strength cold rolled steel sheet - Google Patents

Description

Detailed Description of the Invention This invention has a yield strength of 70 to 90 kg/7IL77L.
The present invention relates to a method for manufacturing a cold-rolled steel sheet with a high yield point and high tensile strength.

It is well known that in recent years, there has been a strong demand for cold-rolled steel sheets that have a high yield point, high tensile strength, and ductility.

In particular, high-strength cold-rolled steel sheets are required from the viewpoint of safety for strong members such as automobile door guards and bumpers.

As a result of various studies on the characteristics required for these high-strength members, we have found that the fracture strength of high-strength structural members is determined not only by the tensile strength (TS) of the material, but also by the yield point (YP). It was concluded that it would be appropriate to use a high yield type high tensile strength steel plate.

This invention has a yield strength of 70 to 90 kg suitable for this invention.
The present invention proposes a method for producing a high yield point, high tensile strength, cold rolled steel sheet having a high tensile strength of /- at low cost.

This invention includes 0.20% or less of carbon, 2.0% or less of silicon, 0.15 to 2.5% of manganese, and 0.01% of aluminum.
5 to 0.150%, nitrogen 0.0040 to 0.020%, and one or two of vanadium 0.005 to 0.10% and niobium 0.005 to 0.08%, Furthermore, for the purpose of controlling the morphology of nonmetallic inclusions, titanium 0.
Contains one or more of the following: 30 to 0.05%, calcium 0.0050% or less, magnesium 0.0050% or less, and rare earth elements such as lanthanum and cellulum 0.0050% or less, with the remainder being iron and unavoidable impurities. Steel made of
Hot rolled at Ar3 transformation point or above, 450°C below 700°C
It is characterized in that it is coiled at the above temperature, further cold rolled at a reduction rate of 30% or more, and then annealed at 350° C. or more below the recrystallization temperature.

In this invention, the chemical composition of the target steel is limited for the following reason.

Carbon coexists with manganese and is effective in improving the strength of steel, but if it exceeds 0.20%, weldability deteriorates, which is not desirable.

Silicon is necessary for deoxidation, and is also effective as a solid solution strengthening component to improve ductility. However, if silicon is contained in large amounts, silicon oxides produced during hot rolling will cause problems in cold rolled steel sheets. Since the surface quality deteriorates, it is limited to 2.01 or less.

Manganese coexists with carbon and is effective in strengthening steel, but 0
．． If it is less than 15%, surface flaws or cracks will occur due to red heat brittleness, and if it is more than 2.5%, melting will be difficult and the cost will be high.

Aluminum is effective in stabilizing silicon and manganese as aluminium-killed steel, but if it is less than 0.015%, it has no effect (and if it is more than 0.150%, the ingot formation becomes unstable, which is not desirable.

Nitrogen is effective as a strengthening element for steel, but 0.004
If the content is less than 0%, the effect will be lost (and if the content is more than 0,020%, it will be difficult to form an agglomerate, which is not desirable).

Both niobium and vanadium form carbides and are effective as steel strengthening elements due to their precipitation strengthening, but both niobium and vanadium have no effect when less than 0.005%, and 0.08% niobium and 0.08% vanadium. If it exceeds 10%, it is not desirable because the content ratio has no reinforcing effect and the cost is high.

In this invention, titanium, which is used as an element for controlling inclusion morphology, is effective in strengthening steel and controlling inclusion morphology.
If it is less than 0.05%, the effect will be small, and if it exceeds 0.30%, the effect will not change and the cost will increase, which is not desirable.

Calcium, magnesium, and rare earth elements such as lantum and cerium are all effective in making sulfide-based inclusions (type A) spheroidized long in the rolling direction and preventing deterioration in ductility in the direction perpendicular to the rolling direction. Since the respective contents can sufficiently exhibit their effects at 0.0050% or less, it is not necessary to contain more than that.

In addition, this invention provides that after hot rolling at Ar3 transformation point or higher,
It is characterized by being rolled at a temperature of 00°C or lower and 450°C or higher, which is common for hot rolling at a temperature higher than the Ar3 transformation point, but if the winding temperature is set to a high temperature of 700°C or higher, The crystal grains of the rolled sheet become thick, which reduces the strength by reducing the effect of precipitation strengthening with niobium, vanadium, etc. At temperatures below 450℃, the strength is high for a cold rolled material, but the ductility is poor. Cold rolling properties (thickness accuracy, shape,
The winding temperature should not be lower than 700°C (450°C) as the efficiency will decrease.
℃ or higher.

Further, the reason why the rolling reduction ratio is set to 30% or more is because a cold rolled steel sheet with good dimensional accuracy and surface texture cannot be obtained with a rolling reduction ratio of 30% or less.

Note that the upper limit of the rolling reduction is not particularly specified, but in this invention, the subsequent annealing is performed at a temperature of 350°C or higher below the recrystallization temperature, so in order to increase the ductility of the product, the rolling reduction should be smaller. Although this is preferable, there are cases where rolling is performed at a high reduction rate due to the structure of the finished product.

Furthermore, the reason why the annealing temperature is lower than the recrystallization temperature or higher than 350°C is to obtain a high yield point, which is the original purpose.
In particular, annealing at a temperature just below the recrystallization temperature is preferred for the purpose of increasing ductility.

This is because annealing at a temperature higher than the recrystallization temperature lowers the yield point, resulting in a normal high-strength steel sheet.

In addition, the ductility of the product decreases when the annealing temperature is low;
Since practical workability is lost below 50°C, the lower limit of the annealing temperature is 350°C.

Next, embodiments of the invention will be described.

Steel having the chemical composition shown in Table 1 was melted in a converter and made into a 210 mm thick slab by continuous casting.

After heating these slabs to 1250-1150℃,
Finish rolling at 860-800℃ to a thickness of 2.6-2.3
mmO steel strips were each wound into coils at a winding temperature of 700° C. or less.

Next, it was cold rolled at a reduction rate of 54 to 48% to produce a cold rolled steel sheet with a thickness of 1.2 mm.

Then, annealing was performed in a batch-type annealing furnace and a continuous annealing furnace.

Thereafter, 1.0% temper rolling was performed and the properties were tested.

The results are shown in Table 2.

Table 2 shows the annealing temperature and soaking time of each annealing furnace.

From the results in Table 2, all of the products according to the present invention had a yield point of 70 kg/mft or more, and also had a significantly high tensile strength.

Claims (1)

[Claims] 1 Carbon 0.20% or less, silicon 2.0% or less, manganese 0.15-2.5%, aluminum 0.015-0.
150%, containing 0.0040 to 0.020% nitrogen,
To this, 0.005 to 0.10% vanadium and 0.0% niobium.
0.005-0.08% of titanium or titanium 0.30-0.08% for the purpose of controlling the form of nonmetallic inclusions.
．． Steel containing one or more of the following: 0.05% or less of calcium, 0.0050% or less of magnesium, 0.0050% or less of rare earth elements such as lanthanum and cerium, and the remainder consisting of iron and unavoidable impurities. After hot rolling at Ar3 transformation point or higher, coiling at a temperature of 700°C or lower and 450°C or higher, and further cold rolling at a rolling reduction of 30% or higher, annealing is performed at a temperature of 350°C or higher below the recrystallization temperature. A method for producing high-strength cold-rolled steel sheets characterized by the following.