JPH01119617A - Production of steel sheet - Google Patents

Abstract

PURPOSE:To produce a steel sheet having excellent strength and toughness by subjecting steel contg. specific ratios of C, Si and Mn to light-draft rolling down and cooling under specific conditions. CONSTITUTION:The steel which contains, by which %, 0.03-0.25% C, 0.01-0.5% Si, 0.3-1.8% Mn, further, contains 1 or >=2 kinds among <=0.05% Al, <=1.5% Cu, <=5% Ni, <=1% Cr, <=1% Mo, <=0.2% Nb, <=0.05% Ti, <=0.5% V, and <=0.0025% Bi, and consists of the balance iron and inevitable impurities is subjected to casting or the light-draft rolling down to the extent of unifying the surface conditions after the casting. The rolled steel is then cooled at >=2 deg.C/sec and <=50 deg.C/sec cooling rate from 800 deg.C down to <=600 deg.C in the course of cooling. The fine transformation structure is thereby formed from the inside of the coarse solidified austenite grains, by which the steel sheet having the excellent strength and toughness is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing a steel plate having excellent properties by light reduction rolling to improve the surface properties of the steel plate as cast or after casting.

[Prior Art] In the manufacturing technology of steel plates, there has been a strong trend toward omitting, simplifying, and directing processes in order to reduce manufacturing costs. One example of this is the idea of directly linking casting and hot rolling by omitting the reheating process, and even omitting hot rolling by casting into slabs of the thickness of the product.

However, in steel plates that are subjected to light rolling to adjust the surface properties as cast or after casting, it is impossible to improve the material quality by rolling, that is, to refine the austenite by rolling recrystallization. Prior austenite grain boundaries are usually the preferred nucleation sites for ferrite transformation.

For this reason, the transformed structure from coarse solidified austenite grains as cast is such that the prior austenite grain boundaries become coarse acicular ferrite, and the interior of the prior austenite grains also becomes considerably coarse ferrite.

The properties of steel sheets having such a structure are generally not very good, and in particular they tend to be poor in low-temperature toughness.

[Problems to be Solved by the Invention] The present invention provides a method for manufacturing a steel plate with excellent strength and toughness.

[Means for Solving the Problems] The present invention eliminates the drawbacks of the conventional methods as described above, and provides a method for manufacturing steel sheets with excellent properties either as cast or by light rolling to improve the surface properties after casting. This is what we are trying to provide.

The metallurgical difference between the prior art and the present invention is the behavior of ferrite transformation. Conventionally, it has been said that prior austenite grain boundaries become preferential nucleation sites for ferrite transformation. For this reason, many technologies to date have aimed to refine the austenite grains by rolling in the recrystallization zone, and then flatten the austenite grains by rolling in the non-recrystallization zone to increase the austenite grain boundary area per unit volume. .

However, the austenite grains in steel sheets as cast or lightly rolled to improve the surface texture after casting are coarse.
The grain boundary area is small (transformed ferrite naturally becomes coarse.

The present invention suppresses the above-described ferrite transformation from the austenite grain boundaries and effectively utilizes the transformation from within the austenite grains to form a fine final structure.

The present invention will be explained in detail below.

First, the reasons for limiting the components of the steel of the present invention will be described.

C is an essential element for strengthening steel, and 0.0
If it is less than 3%, the required strength cannot be obtained, and if it is less than 0.25%,
0.0 because the toughness of the base metal and weld zone will be impaired if
It was limited to 3% or more and 0.25% or less.

Sl is effective as a deoxidizing element and a steel strengthening element.
．． If it is added in an amount of 0.01% or more, but in excess of 0.5%, workability deteriorates and the surface quality of the steel sheet is impaired.

Mn is effective in strengthening steel, but if it is less than 0.3%, it has no effect, and if it exceeds 1.8%, workability deteriorates.

Ag is added as a deoxidizing element, but its effect is saturated if it exceeds 0.05%, so the upper limit is limited to 0.05%.

Cu is effective in improving the corrosion resistance and strength of steel, but excessive addition causes hot cracking of the weld metal, so the content is limited to 1.5% or less.

Although Ni increases the strength and low-temperature toughness of steel, excessive addition makes it difficult to obtain an economical effect, so it is limited to 5% or less.

Cr, Mo, and B improve the hardenability of steel and are effective in stabilizing the structure within prior austenite grains, which is characteristic of the present invention. However, excessive addition causes hot cracking during transformation, so Cr and Mo are each kept at 1% or less, and B at 0.0025% or less.

In the present invention, Nb and V precipitate as fine carbonitrides in the cooling process after rolling and increase the strength of the steel.
Excessive addition impairs low-temperature toughness, so add 0.2% each.
Hereinafter, it shall be 0.5% or less.

In addition to the same effects as Nb and V, TI oxides and nitrides generated during the solidification and cooling process effectively act as nuclei for ferrite transformation from within the austenite grains.

However, if it exceeds 0.05%, the toughness deteriorates significantly, so the upper limit is set at 0.05%.

Next, we will talk about the manufacturing method.

In the present invention, the molten steel that satisfies the above-mentioned composition is subjected to light rolling to adjust the surface properties as it is cast or after casting, and then rolled from 800°C to 80°C during cooling.
Cooling is performed at a cooling rate of 2° C./second or more and 50° C./second or less until the temperature reaches 00° C. or lower.

During this cooling process, fine ferrite is transformed from within the austenite grains. Here, the transformation nuclei are oxides of St and Mn, Mn51, and for steels to which TI is added, oxides or nitrides of TI, either alone or in combination. .

If the cooling rate of 800°C or less is too high, the structure will become coarse bainite or martensite, and if it is too low, it will become a coarse ferrite/pearlite structure, producing fine ferrite within the austenite grains that are the object of the present invention. I can't.

For this reason, the cooling rate from 800°C to 600°C or less is limited to 2°C/second or more and 50°C/second or less.

[Example] Table 2 shows the results of experiments conducted on the invention steel and comparative steel having the components shown in Table 1.

Note that the underlined items in the table do not meet the conditions of the present invention.

In Table 1, Steel A to Steel F are steels of the present invention.

Steel G and Steel H are comparative steels with excessive C and St, respectively.

Steels 1 to 8 in Table 2 are the steels of the present invention and exhibit excellent strength and toughness.

In Nos. 9 and 11, the cooling rate at 800 to 600° C. or less was too fast, resulting in coarse bainite or martensite structure, resulting in significantly poor toughness. On the other hand, in the case of 10°12, the water cooling stop temperature is high and the cooling rate is too slow, so the structure is coarse ferrite/pearlite mainly caused by ferrite transformation from prior austenite grain boundaries, and the toughness is poor.

13.14 is the result of comparative steel with excess C or Sl, but the toughness is considerably inferior.

As mentioned above, samples 9 to 14 did not have satisfactory toughness. As described above, if even one of the conditions deviates from the scope of the present invention, the object of the present invention will not be achieved.

[Effects of the Invention] As described above, according to the present invention, by generating a fine transformed structure from within the grains of coarse solidified austenite grains, it is possible to improve It is extremely useful industrially because it allows the production of steel sheets with excellent properties.

Agent Patent Attorney Tatsuo Chanoki Procedural Amendment (
Voluntary) February 19, 1988

Claims (2)

[Claims] (1) Steel consisting of C: 0.03 to 0.25%, Si: 0.01 to 0.5%, Mn: 0.3 to 1.8%, the balance being iron and unavoidable impurities in weight percent, After performing light reduction rolling to adjust the surface properties as cast or after casting, during the cooling process, the product is cooled from 800°C to 600°C at a cooling rate of 2°C/second or more and 50°C/second or less. A method of manufacturing a steel plate, characterized by the following. (2) C: 0.03-0.25%, Si: 0.01-0.5%, Mn: 0.3-1.8%, further Ag: ≦0.5%, Cu: ≦1.5%, Ni: ≦5%, Cr: ≦1%, Mo: ≦1%, Nb: ≦0.2%, Ti: ≦0.05%, V: ≦0.5%, B: ≦0.0025% of steel containing one or more of the following, with the balance consisting of iron and unavoidable impurities, as cast or after being lightly rolled to the extent that the surface texture is adjusted, and during cooling. A method for manufacturing a steel plate, characterized by cooling from 800°C to 600°C at a cooling rate of 2°C/second to 50°C/second.