JPS589812B2 - Manufacturing method of high-strength steel plate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for economically producing a steel sheet with superior material quality compared to hot-rolled steel strips containing carbonitride-forming elements produced by conventional processes. It is.

Currently, hot rolled steel sheets are manufactured using continuous hot rolling mills.
It is supplied by slabs made by blooming steel ingots using the ordinary ingot making method and slabs made by the continuous casting method.

Currently, these slabs are cooled to room temperature, then heated in a heating furnace at high temperatures for a long period of time, and then rolled into a continuous hot rolling mill to produce hot rolled steel strips. be.

That is, in the conventional method, 12
The slab is heated to a high temperature of 00 to 1,300°C for a long time of one hour or more, and the amount of heating is enormous.

In order to save this energy, the inventors of the present invention (1) directly hot-roll the highly mixed slab produced by continuous casting.

■Also, by directly hot rolling a slab containing high-temperature carbonitride-forming elements that has been continuously cast at the same time, superior material quality can be obtained compared to the conventional process of reheating the slab and then continuously hot rolling it. thing.

■Furthermore, what are the manufacturing constraints in the continuous casting-direct hot rolling process?

■The latest continuous hot rolling mill has a total length of 800m to 1000m.
Considering that the equipment is extremely long, measuring up to 5000 m long, and requires a vast site, and its construction costs are also enormous, what is the solution to continuous casting and direct hot rolling equipment, which is compact and has low construction costs? We have carefully considered whether or not this is the case.

As a result, the method of the present invention directly hot-rolls continuously cast slabs containing carbonitride-forming elements such as Nb, Ti, and V under specific conditions, thereby omitting the high-temperature and long-time heating of conventional methods. At the same time, the precipitation of precipitates in the continuous cooling process after continuous casting is slower than that in thermal equilibrium, and the precipitation in the austenite region is slower than that in the ferrite region. ,
Furthermore, when carbonitrides are precipitated, it is attempted to actively utilize the fact that they are precipitated in a uniformly dispersed manner.

That is, in the conventional method, once the slab is turned into a cold mass, the precipitation of carbonitrides is completed during the cooling process, and the carbonitrides grow large.

Therefore, in order to completely bring this precipitate into a solid solution state before continuous hot rolling, reheating at high temperature and for a long period of time is required as described above.

Since the elements are in a solid solution state in a continuously cast high-temperature slab, the present invention effectively utilizes this state as a starting point for manufacturing steel sheets.

1 C in weight percent: 0.06-0.20%, M
n: 0.50-2.00%, Si: 0.03-0.50
Nb in steel with basic components of 0.01-0.10%
, Ti: 0.01-0.10%, V: 0.01-0.1
When producing high-strength steel sheets containing 5% of one or two kinds of additives, the slab produced by a continuous casting machine is directly cast at 90% by weight.
Rough rolling is started at a temperature of 0°C or higher, and the finishing temperature is set to Ar.
1. A method for producing a high-strength steel plate, characterized by finishing at a temperature of 3 or higher.

C: 0.06-0.20%, Mn at 2% by weight
:0.50-2.00%, Si:0.03-0.50%
Nb: 0.01-0.10%,
Ti: 0.01-0.10%, V: 0.01-0.15
% or two, and Cu: 0.2 to 1.0%, C
When producing high-strength steel sheets containing one or more of r: 0.2 to 1.0% and Ni: 0.2 to 1.0%, slabs produced by a continuous casting machine are directly cast into
Rough rolling is started at a temperature of ℃ or higher, and the finishing temperature is set to Ar3.
A method for producing a high-strength steel plate characterized by finishing at a temperature higher than or equal to the temperature above.

The contents of the present invention will be described in detail below.

Nb (CN) (
The study focused on the behavior of TiC (titanium carbide) and V (CN) (vanadium carbonitride) in continuous casting and continuous hot rolling processes, and steel sheets containing Nb, Ti, and V can be manufactured. The following conditions were considered.

Various precipitates can be used for various purposes.

One is the control of crystal grains by the precipitates, and the other is the securing of material quality (strength) by the precipitates, and the other is the control of the texture.

When using a precipitate, the time of precipitation of the precipitate is of course extremely important in order to effectively exhibit the effect of the precipitate.

In steel sheets containing Nb, Ti, and V, Nb(CN), T
Increasing the material strength by precipitation of iC, V (CN) is important in manufacturing high strength steel sheets containing Nb, Ti, and V.

For this purpose, in the slab before hot rolling, Nb, T
i, V, C, and N are kept in a solid solution state, and TNb (CN), TiC are added in the process of winding the steel strip after hot rolling.
, V(CN) must be precipitated.

The present invention focuses on Nb (CN) in Nb, Ti, and V-containing steel sheets.
, TiC, and V (CN) precipitation, it was concluded that the strength of the steel plate can be ensured if the high-temperature slab after continuous casting is put into a rough rolling mill at a temperature of 900'C or higher. This is what I got.

In the conventional method, the cold slab is reheated and then subjected to rough rolling, but once the temperature of the slab has dropped to room temperature, precipitates precipitate out during cooling. In order to make it a solid solution again, for example, 1200℃
Heating must be carried out at a higher temperature for more than one hour.

Even if the slab, which has once cooled to room temperature, is reheated to a low temperature of 1000℃, the precipitates cannot be completely re-dissolved, and therefore the precipitates have no effect on material quality control. be.

Even if the rough rolling temperature is as low as 900°C as in the present invention, the precipitates will complete their precipitation if the slab has just been continuously cast and the depth does not drop to room temperature. The effect of precipitates can be effectively utilized.

Next, we investigated whether or not it would be possible to subject a continuously cast high-temperature slab to a rough rolling mill without dropping the temperature below 900°C on an actual production line.

FIG. 1 specifically represents an example of the present invention.

The upper half of the figure shows the arrangement of the continuous casting machine, high-frequency heating furnace, rough rolling mill, finish rolling mill, and winding machine, and the lower half of the figure shows the arrangement of the continuous casting machine, high-frequency heating furnace, rough rolling mill, finishing rolling mill, and winding machine. The results are shown below, using calculations to determine what kind of temperature history will follow.

When no heating furnace is used, the slab thickness is 170mm.
If the slab is thick, the gas cutting of the continuously cast slab and its transportation to the roughing mill will be done quickly so that the slab enters the roughing mill within 24 minutes after casting. It is possible to heat the slab to a temperature of 1000°C or higher.

Since the residence time allowed from continuous casting to entering the rough rolling mill is quite short, the continuous casting machine and continuous hot rolling mill should be placed in series on almost the same line, or both should be placed in series. It is also important for the present invention that the slabs be transported extremely quickly even if they are on separate lines.

In the example shown in Figure 1, the slab may be directly subjected to a rough rolling mill within 24 minutes, but if it is necessary to equalize the temperature in the thickness and width direction of the slab, it is necessary to compensate for the surface temperature. There is no problem even if it is heated in the middle of the process.

Even if the heating furnace (soaking furnace) in this case is in-line,
Although it may be off-line, it is natural that the heating furnace used in the line of the present invention should be capable of high-speed heating in order to match the subsequent high rolling speed.

I think an example that would meet that purpose would be a high-frequency heating furnace, but depending on the combination of the number of continuous casting machines and the number of heating furnaces,
There is no need to limit yourself to high-frequency heating furnaces.

Another method is to uniformize the cooling in the thickness and width direction of the slab by recuperating heat from the slab itself.

In other words, a few tens of minutes to one hour should be placed in a simple cover to prevent heat radiation.
By simply holding the slab for a period of about 1 hour, it is possible to sufficiently equalize the temperature without lowering the average temperature of the slab too much.

This method is extremely simple and highly effective.

From the above, the term "direct" in the claims includes cases where hot rolling is performed directly after continuous casting, cases where reheating is performed, and cases where heating is performed to an extent that compensates for the surface temperature. It does not increase the extreme temperature of the slab.

Furthermore, the example in Figure 1 shows the results for a slab with a thin thickness of 170 mm, but as the slab gets thicker, the heat dissipates less. You will have more time to maintain this level.

If an appropriate heat radiation prevention cover is installed on the line between the continuous casting machine and the rough rolling mill to prevent heat radiation, there will be time for the temperature to decrease.

In this way, in the present invention, in order to produce a high-strength hot-rolled steel sheet containing Nb, Ti, and V from a high-temperature slab immediately after continuous casting,
Continuous hot rolling is started in such a way that the temperature of the hot slab immediately after continuous casting never drops below 900°C.

Next, let's talk about finishing temperature.

This is already well known, but there is a close relationship between the crystal grains of a hot-rolled steel strip and the finishing temperature, and the crystal grains in portions where finish rolling is not completed at a temperature of Ar3 or higher become coarse.

Since the crystal grains of a steel sheet in which coarse grains are partially generated are too large, when processed, the workability of the steel sheet itself deteriorates, such as roughening of the surface of the steel sheet and deterioration of ductility.

As described above, the finishing rolling temperature is closely related to the grain size and workability of the hot rolled steel sheet, so it is an unavoidable condition that finishing rolling is finished at a temperature of Ar3 or higher even in the method of the present invention. think.

Naturally, the Ar3 temperature varies depending on the components of the steel sheet, and in the steel sheet to which the present invention is applied, it is 700°C to 850°C.

In the above integrated process of continuous casting and continuous hot rolling, Nb,
The conditions for manufacturing Ti, V-containing high-strength hot-rolled steel strip while ensuring material quality superior to conventional manufacturing methods are as follows: Nb (CN), Ti
In the Nb, Ti, and V-containing high-strength hot-rolled steel sheet of the present invention whose tensile strength is approximately in the range of 50 to 70 kg/mm from the viewpoint of C, V (CN) precipitation, the composition range is patented. It is intended that the limitations be within the scope of the claims.

Carbon (C), anganese (Mn), and silicon (Si) are used at 0.06% or more, 0.50% or more, and 0.03%, respectively, as basic components to increase the strength level while ensuring workability.
The above is necessary.

Furthermore, if these component elements are present in an amount greater than necessary, workability as a hot rolled steel strip will be lost.

Therefore, C, Mn, and Si are each 0.20% or less,
It becomes necessary to keep the content below 2.00% and below 0.50%.

Nb: 0.01% or less, Ti: 0.01% or less, v: o
．． At o1% or less, Nb, Ti, and V have almost no effect on increasing the strength of each steel sheet, and when Nb: 0.1
0% or more, Ti: 0.10% or more, (2) 0.15% or more, the effects of increasing the strength of the steel plate of Nb, Ti, and V reach a saturated state.

Therefore, the amounts of TNb and Ti are limited to 0.01-0.10%, and the amount of V is limited to 0.01-0.15%.

Note that these may be contained in combination.

If it is necessary to further increase the strength or weather resistance, one or more of Cu, Ni, and Cr may be added in an amount of 1.0% or less according to the second aspect of the invention.

These elements may be added singly or in combination of two or more, without losing the effects of Nb, Ti, and V.

Note that the reason why the upper limit of Cu, Ni, and Cr is limited to 1.0% is that even if more than this is added, the effect of economically increasing the strength of the steel sheet and improving the weather resistance becomes saturated.

On the other hand, the lower limit value of the addition amount is 0.2% for each of Cu, Ni, and Cr, and if it is less than this value, the desired effect cannot be obtained.

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

Example A high-temperature slab was made by solidifying in a continuous casting machine and cutting it to prevent heat radiation by covering the upper surface of the high-temperature slab corresponding to one slab length with a part of the high-temperature slab that came out of the continuous casting machine to prevent heat radiation. .

This hot slab was transported from the continuous casting plant to the continuous hot rolling plant.

The temperature at the ends of the Takafuchi slab that comes out of the continuous casting machine is lower than that at the center.

In order to reduce the temperature difference between the end portion and the central protruding portion by recuperation of the slab itself, the slab was kept inserted in a heat insulating cover for 0 minutes to 3 hours.

Thereafter, the heat insulating cover was removed and the high temperature slab was directly subjected to continuous hot rolling.

Continuous hot rolling work can be carried out without any problems, and 3
．． A hot-rolled plate with a thickness of 2 mm to 13.0 mm was produced.

The hot rolling finishing temperature was 765°C to 865°C.

On the other hand, in one and the same charge, slabs other than the one slab that was directly hot-rolled by being transported from the continuous casting machine to the continuous hot rolling machine at high temperature are cooled to room temperature as per the normal process after continuous casting. did.

The cooled slab is transported from Jobuchi to 12
After reheating to 80° C. for 1 hour, continuous hot rolling was performed to obtain a comparison material of continuous casting and direct hot rolling.

The material test results for hot rolled steel sheets are shown below.

A-1, B-1, C-1, D-1, E- of the process materials of the present invention
1, F-1 and conventional process materials A-2, B-2, C-2, D
-2, E-2, and F-2 made from the same slab, there is not much difference in total elongation between the two, but the yield point and tensile strength of the inventive material are lower than that of the comparative material. It is about 1 to 3 kg/1 higher than the yield point and tensile strength.

This seems to be because in the Nb, Ti, and V-containing steel produced by the process of the present invention, Nb, Ti, and V are more effective in increasing the strength than in the case of conventional process materials.

As mentioned earlier, in order to increase the strength of a steel sheet through precipitates, the elements are completely in a solid solution state before hot rolling, and the elements precipitate between the end of hot rolling and the winding of the hot rolled sheet. You must do so.

Conventionally, processed materials are heated to 1280°C for 1 hour to keep the elements in a solid solution state, and most of the precipitates are precipitated after hot rolling is completed to increase the strength of the steel sheet. First, the effect of precipitates does not work effectively in conventional process materials.

This is because in conventional process materials, precipitates are temporarily dissolved when heated at 1280°C for 1 hour, but since the dissolved elements are not uniformly dispersed in the steel, they may precipitate again as precipitates. This is thought to be because the precipitates inevitably precipitate in a limited area, making it difficult to form fine and uniformly dispersed precipitates.

In comparison, in the case of the process of the present invention, a high-temperature slab immediately after continuous casting is used, so no precipitates have ever precipitated, and all elements are considered to be uniformly distributed.

Therefore, when it first precipitates in the latter half of hot rolling, it precipitates very uniformly and as fine precipitates, which seems to effectively increase the strength of the steel sheet.

Next, we will touch on the heat retention time of high-temperature slabs before rolling.

In A-1, the heat retention time is 0, which means that a continuously cast high-temperature slab was hot-rolled as it was.

In this case, the time from when the slab came out of the continuous casting machine until the start of hot rolling was 32 minutes.

As shown in Examples B-1, C-1, D-1, E-1, and F-1, the high-temperature slab immediately after continuous casting was allowed to wait for up to 3 hours in a heat-insulating cover before hot rolling. The effect of the heat retention time was investigated.

C-1 is the case where the heat retention time is 3 hours, but the comparative material C-
As can be seen from the comparison with 2, C-1 has higher strength than C-2, indicating that the precipitates are working more effectively.

Even in the case of other heat retention times, the strength of the processed material of the present invention is high.

From these facts, it can be said that the features of the present invention are fully utilized when the heat retention time is up to 3 hours.

As described above in detail, Nb,
If a hot rolled steel strip is produced by directly hot rolling a high temperature slab of steel containing Ti and V after continuous casting, it is possible to simply save the enormous amount of reheating energy required for reheating in the conventional process. Rather, it is possible to make the material properties of the hot rolled steel strip even better than those of conventionally processed materials.

[Brief explanation of drawings]

FIG. 1 is an example of the present invention, and shows continuous casting, continuous hot rolling, and the temperature change of the plate during the process. 1 is a tundish, 2 is a mold, 3 is a continuously cast high-temperature slab, 3' is a cut slab, 4 is a high-frequency heating furnace, 5 is a rough rolling mill, 6 is a finishing mill, and 7 is a coiler.

Claims (1)

[Claims] 1 C: 0.06 to 0.20% by weight, M
n: 0.50-2.00%, Si: 0.03-0.50
Nb in steel with basic components of 0.01-0.10%
, Ti: 0.01-0.10%, V: 0.01-0.1
When producing high-strength steel sheets containing 5% of one or two kinds of additives, the slab produced by a continuous casting machine is directly cast at 90% by weight.
Rough rolling is started at a temperature of 0°C or higher, and the finishing temperature is set to Ar.
1. A method for producing a high-strength steel plate, characterized by finishing at a temperature of 3 or higher. C: 0.06-0.20%, Mn at 2% by weight
:0.50~2.00%, Si:0.03~0.50%
Nb: 0.01-0.10%,
Ti: 0.01-0.10%, V: 0.01-0.15
% or two, and Cu: 0.2 to 1.0%, C
When producing high-strength steel sheets containing one or more of r: 0.2-1.0% and Ni: 0.2-1.0%, slabs produced by a continuous casting machine are directly heated to 900°C.
A method for producing a high-strength steel plate, characterized by starting rough rolling at a temperature above and finishing at a finishing temperature with a deviation of Ar3 or above.