JPS6070123A - Method for hot rolling continuously cast aluminum killed steel - Google Patents

Abstract

PURPOSE:To obtain an Al killed steel with superior workability at a low cost by adding prescribed percentages of C, Mn, Al, N, Nb and Ti and carrying out rolling at a prescribed temp. CONSTITUTION:An Al killed steel slab or strip consisting of, by weight, <=0.1% C, <=0.5% Mn, <=0.015% N, 0.015-0.1% acid-sol. Al and the balance Fe or further contg. <=0.1% in total of 0.002-0.1% each of Nb and/or Ti is formed by continuous casting. The slab or strip is cooled once to the Ar3 transformation point-the Ar3 transformation point -100 deg.C immediately after solidification. It is immediately reheated to <=1,100 deg.C and rolled at 900-1,100 deg.C starting temp. The hot rolling is finished at the Ar3 transformation point or above.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for hot rolling continuously cast aluminum killed steel, and particularly to a method for hot rolling aluminum killed steel that is low cost and has excellent workability.

The conventional manufacturing process for thin steel sheets is to first cool the slab obtained by continuous casting or ingot blooming to room temperature, perform surface treatments, etc., and then reheat it to a high temperature in a heating furnace and hot-roll it. This process produces hot-rolled steel sheets, which are then further cold-rolled and annealed to produce cold-rolled steel sheets.

However, in recent years, for the purpose of energy saving and continuity, in the hot rolling process (hot rolling process), continuous casting slabs are pressure charged into a heating furnace and reheated without being cooled to room temperature (hot charging). Direct rolling, which involves hot rolling without reheating, has come to be practiced. - Continuous annealing is now performed instead of box annealing in the multi-thickness annealing process, and in this case, if the continuously cast slab can be rolled without being reheated at all, the energy saving effect will be very large. It is known that good results can be obtained. For example, special public service 56-2
1330, the disclosed formula t'1. According to the FC technology, when a cold rolled sheet is box annealed, the quality of the annealed material is improved by the direct rolling process.

However, in the case of continuous annealing, it is known that application of the direct rolling process causes significant material deterioration of the cold rolled sheet.

Therefore, it has been desired to develop a method for hot rolling steel sheets for drawing υ that is suitable for continuous annealing and achieves energy savings.

SUMMARY OF THE INVENTION The object of the present invention is to provide a method for hot rolling continuously cast aluminum killed steel, which has excellent material quality and can achieve energy savings, in response to the above-mentioned demands for the prior art.

The above objects of the present invention are achieved by the following two inventions.

The gist of the first invention is as follows. That is, C: 0.1% or less, Mn: 0.50% or less, acid-soluble At
Nb: 0.015 to 0.100%, N: 0.0150% or less, and if necessary, one or both of Nb and Ti, each in the range of 0.002 to 0.100% and in total. Immediately after solidification, a continuously cast aluminum guild steel slab or a continuously cast copper strip consisting of 0.100% or less and the remainder Fe and unavoidable impurities is heated to Ar, transformation point ~ (Ar,
A step of cooling to a temperature range of transformation point -100'C), and immediately reheating the cooled slab or steel strip to a temperature of 1100 °C or less and starting rolling in a temperature range of 900 to 1100 °C, Ar, transformation point A method for hot rolling continuously cast aluminum killed steel, comprising the step of terminating hot rolling at a temperature above.

The gist of the second invention is as follows. That is, a continuous casting slab or continuous casting -FC copper strip of aluminum killed steel having the same composition as the first invention is subjected to a step of rolling down 5% or more in each of the width direction and thickness direction immediately after solidification, and thereafter the same as in the first invention. This is a method for hot rolling continuously cast aluminum guild steel, which is characterized by comprising a cooling step and a hot rolling step.

Generally, when producing cold-rolled steel sheets for drawing using a continuous annealing method using low carbon aluminum killed steel, the hot-rolled steel sheets after hot rolling are
It is known that winding at a high temperature of around 00° C. is necessary to improve the material quality, and other manufacturing conditions are also significantly different from box annealing. That is, as mentioned above, when a cold rolled sheet is annealed by the box annealing method, the material quality of the annealed sheet is improved by the direct rolling process, and in the case of a continuous annealing method, this direct rolling causes deterioration of the material quality of the annealed sheet. Yes, but
The cause of this is not clear, but MnS, At
It is speculated that this is because the state of precipitation of N, etc. has an adverse effect on grain growth and texture development during continuous annealing. As a result of various studies, the inventors of the present invention have discovered a method for manufacturing hot-rolled sheets suitable for manufacturing steel sheets for drawing by continuous annealing by controlling the temperature history of continuous casting → hot rolling slabs. That is, after cooling the continuous casting slab to below the transformation point, it is reheated at low temperature and hot rolled to form MnS, AtN.
etc. are precipitated in an appropriate amount, making it possible to produce hot-rolled sheets with excellent material quality. The present invention was developed based on this knowledge.

Explaining the reasons for limiting the composition of aluminum killed steel in the present invention: The present invention is intended for hot rolling of steel sheets for drawing, and it is desirable that the amount of C be as low as possible in order to soften the steel. The content was limited to 0.1% or less because the properties deteriorated.

Mn: It is desirable that Mn is also small, and if it exceeds 0.50%, workability will be significantly impaired, so the upper limit was set at 50%.

Acid-soluble At: At is added to deoxidize and fix N as 7'CAtN, but if acid-soluble At is less than 0.015%, N fixation will be incomplete, and if it exceeds 0 or 100%. Since surface scratches are likely to occur, the acid-soluble At was reduced to 0.0.
It was limited to a range of 15 to 0.100%.

N: The lower the N content, the better, but if it exceeds 0.0150%, the quality of the material deteriorates, so the upper limit was limited to 0.0150%.

Above, C, Mn, oxidation bath A7. Each limited amount of H is considered to be the basic component of the aluminized steel according to the present invention, but the object of the present invention can be achieved more effectively even when limited amounts of Nb and Ti are also contained at the same time. . The reasons for this limitation are as follows. That is, Nb, Ti
Both are effective in fixing C and N and making the steel sheet even softer, but if it is less than 0.002%, it has no effect, and even if it exceeds 0.100% alone or in total, the effect is 0.002 to 0.1 alone due to saturation and high cost
00% and the total amount was limited to 0.100% or less.

Next, the manufacturing conditions for aluminum killed steel having the above-mentioned limiting components will be explained. Controlling the temperature history from solidification to hot rolling of a continuously cast slab or continuously cast steel strip is most important in the present invention, and by changing the temperature history during this period, the γ grain structure of the slab and the state of precipitates can be thickened. Can change.

The present inventors conducted the following basic experiment for the purpose of investigating the influence of these temperature histories on the material properties of cold-rolled annealed sheets. That is, in a small vacuum melting furnace, C・0.03%, Mn:
0.30%, 5otAt: 0.04% steel was melted and cast into a slab with a thickness of 30+o+, width of 100m, and length of 8200m. Immediately after solidification, the mold is removed from the mold, cooled to room temperature, then reheated (A) and directly rolled (B).
and cooled to 800℃ and reheated to 1000℃ (C)
After giving three types of temperature history, each sample was hot rolled from 30 m to 3.5 m using a small rolling mill. This hot rolled sheet was cold rolled to 0.8 dew and annealed. Annealing: 850℃, 1
After soaking for 1 minute, it was rapidly cooled in a trench at 400°C and subjected to a continuous annealing heat cycle in which it was overaged at 400°C for 2 minutes. The mechanical properties of these cold-rolled annealed sheets were investigated, and the results are shown in FIG. From Fig. 2, case (C) was cooled to 800°C and then reheated to 1000°C and rolled.
It can be seen that the yield stress (ys) and tensile strength (TS) are the lowest, and the elongation (Et) and Lankford@(r匝) are the highest.

Although the mechanism by which the material quality improves due to such temperature history is not clear, the following phenomenon may be considered.

At, N, and Mn are present in high-temperature slabs that are punched immediately after casting.
, S, etc. are all considered to be in a solid solution state.

By cooling this in a trench around 800℃, MnS,
At first, AtN etc. precipitate, but even if the material is reheated after that, if the heating temperature is as low as 1000°C, these precipitates remain, and the density of the precipitates is coarser than when the material is cooled to room temperature. It is thought that the state of the precipitates in the hot rolled sheet became favorable for the growth of grains in the SK grains and the development of the (111) recrystallization texture after cold rolling. Also, A
r, cooling below the transformation point and reheating γ→α→γ
It is thought that the fact that the influence of the cast structure, which is seen in slabs without transformation, is reduced by the transformation also contributes greatly to the improvement of the material quality.

In addition, we investigated the Lankford @ of the cold-rolled annealed sheet by varying the initial cooling temperature Tc Y of the temperature history shown in Figure 3 (3) in case (C) of the basic experiment described above, and the results were summarized as follows. 3
It is shown in Figure (B). In Figure 3 (B), △T =
When Ar, point -Tc is in the range of 0 to 100°C, that is, in the range of Tc z ArH point to (Ar, point -100°C)? Since the saliva shows a good value of 1.4 or more,
In the present invention, the temperature range for cooling after solidification is limited to the range from Ar H point to (Ara point - 100°C).

Next, the reheating temperature after cooling and the hot rolling finishing temperature will be explained. In the present invention, since the precipitation phenomenon that occurs by cooling once below the Ar point is utilized, it is impossible to completely dissolve these precipitates by reheating, so the upper limit of the reheating temperature is limited to 1100°C. Hot rolling start temperature is 900~
However, the upper limit of 1100°C is limited by the upper limit of the reheating temperature, and the lower limit of 900°C must be secured in order to finish the hot rolling above the Ar point. In addition, if the hot rolling finishing temperature is below the Ars point, the structure of the hot rolled sheet will have mixed grains and the workability of the cold rolled sheet after potash annealing will deteriorate, so A r 1
Limited to points or more.

The effect of improving the material quality by providing a thermal history as described above is further enhanced by adding carbonitride elements such as Ti and Nb. Preliminary processing as shown in the second aspect of the invention also promotes the above-mentioned thermal history effect. In other words, immediately after solidification of a continuously cast slab, the solidified structure of the slab is destroyed by applying a reduction of 5% or more in the width direction and the thickness direction, and by introducing dislocations near the surface of the slab, precipitation near the surface is reduced. This reduces the difference in the amount of precipitation depending on the position within the slab and improves workability. If the amount of reduction in the width direction and the thickness direction is less than 5%, the above effect is insufficient, so the amount of reduction in the width direction and thickness direction is limited to 5% or more, respectively.

In the process from continuous casting to hot rolling as mentioned above,
The energy saving effect is extremely large compared to conventional processes.
It is also effective in preventing hot embrittlement, which is a problem in so-called direct rolling, in which hot rolling is completed without any reheating after casting. This is because the precipitates lined up at the γ grain boundaries, which are the cause of hot embrittlement, become intragranular precipitates due to the change in the position of the γ grain boundaries due to the γ→α-gamma transformation.

Furthermore, the hot rolled sheet rolling method of the present invention improves the tensile properties of the hot rolled sheet as described above because the influence of the cast structure is reduced, and the precipitation of AAN is promoted, so material deterioration and aging deterioration of the hot rolled sheet are also reduced. .

Although the present invention has mainly been described with respect to hot rolling of slabs produced by a continuous slab caster, the present invention can also be applied to a continuous Hiko hot rolling process for thin steel strips produced by rotary casters or the like.

Example 1 C: 0.031%, Mn: 0.21%, 5otA
After melting low carbon aluminum guild steel with T: 0.041% and N: 0.0050% in a converter, it was made into a slab using a continuous casting machine. These slabs were hot rolled to a thickness of 3.0 m under various heat history and hot rolling conditions shown in Table 1.

Next, this hot-rolled plate was cold rolled, soaked at 850°C for 30 seconds, rapidly cooled, and continuously annealed at 400°C for 2 minutes for overaging. The materials of these hot-rolled sheets and cold-rolled materials after annealing were investigated, and the results are shown in Tables 1 and 2.

As for the hot-rolled sheets, as shown in Table 1, the material quality of the examples of the present invention is almost the same as that of conventional materials that are cooled to room temperature and then reheated.
-Compared with the directly rolled material that was not subjected to any heating, the yield stress (YS) and tensile strength (TS) of the example were lower, and the elongation (Et) and 7 value were almost the same.

As for the cold rolled sheet, as shown in Table 2, compared to the material cooled to room temperature and then reheated, when the material is coiled at a high temperature after hot rolling, the results of the examples of the present invention are almost the same as those of the conventional example, or even lower. In the case of low-temperature winding, the example clearly shows better properties. On the other hand, when comparing the directly rolled material without any reheating and the example, the example has superior material quality.

Example 2 Steel with the same composition as Example 1 was made into a slab using a continuous casting machine,
After applying a reduction of 15% in the width direction and 5% in the thickness direction using a blooming mill, it was cooled to 800°C, then reheated to 1000°C and hot rolled, and then cold rolled in the same manner as in Example 1. , continuous annealing was performed. In addition, a cold-rolled plate was produced by the same method except for rolling in the width direction and thickness direction. The distribution of seven values in the width direction of these cold-rolled annealed sheets was investigated, and the results are shown in FIG. 4 for comparison. It is clear that by applying rolling in the width direction and thickness direction as shown in FIG. 4, the variation in material quality in the width direction is reduced and the material quality is improved.

Example 3 ASB, C3 type ultra-low carbon aluminum guild steel as shown in Table 3 was hot continuous casted, and the thermal history under various conditions shown in Table 4,
A hot-rolled sheet was obtained by hot rolling, and then the hot-rolled sheet shown in Table 3 was cold-rolled and continuously annealed at 830° C. for 40 seconds. The material properties of this cold-rolled annealed plate were investigated, and the results are also shown in Table 4. As is clear from Table 4, even in ultra-low carbon steel like this example, this example shows the same or better material quality than the conventional example which was cooled to room temperature and then reheated,
It has better properties than conventional direct rolling without reheating.

In addition, the comparative example in which the heating temperature after cooling is 1250°C, which is higher than the limiting conditions of the present invention, has a yield stress (ys) and a tensile strength of 1 (TS).
is high and is not suitable for processing.

As is clear from the various embodiments described above, the present invention provides continuous casting slabs or continuously cast steel strips of aluminum-killed steel with a limited composition, immediately after hot solidification, by applying Ar to the transformation point to (, Ar, transformation point -100°C). ), followed by cooling to a temperature range of 1100
By reheating the material to a temperature below ℃, starting rolling in the temperature range of 900 to 1100℃, and hot rolling at a temperature above the Ar3 transformation point, the workability of the continuously annealed material after cold rolling is improved and the effect of energy saving is achieved. I can give it to you! Ta.

[Brief explanation of drawings]

Figure 1 shows the various temperature histories given to the slab. Line diagram shown, 2nd
The figures are diagrams showing the relationship between temperature history and mechanical properties of cold rolled annealed sheets, Figure 3 (A) is a diagram showing temperature history Z, Figure 3 (B
) is a diagram showing the relationship between the temperature difference between the Ar point and the cooling temperature and the 7 values of the cold-rolled annealed sheet, and Figure 4 shows the presence or absence of rolling in the width/atomic bomb direction and the 7 values in the width direction of the cold-rolled annealed material. FIG. 3 is a diagram illustrating the relationship between Agent: Takeo Nakaji (19), patent attorney

Claims (2)

[Claims] (1) Weight ratio C: 0.1% or less, Mn: 0.50% or less, acid soluble kt
:o, 0.015% to 0.100%, N: 0.0150% or less, and if necessary, one or both of Nb and Ti in the range of 0.002 to 0.100% each and in total. Immediately after solidification, a continuously cast slab of aluminum killed steel containing 0.100% or less and the balance consisting of Fe and unavoidable impurities or continuously cast metal is heated to Ar, transformation point ~ (Ar3
A step of cooling the slab or copper strip to a temperature range of (transformation point -100°C), immediately reheating the cooled slab or copper strip to a temperature of 1100°C or lower, and starting rolling in a temperature range of 900 to 1100°C to reach a temperature higher than the Ars transformation point. 1. A method for hot rolling continuously cast aluminum killed steel, comprising the step of terminating hot rolling at a temperature. (2) Weight ratio C: 0.1% or less, Mn + 0.50% or less, acid-soluble At: 0.015 to 0.100%, N: 0.0150
% or less, and if necessary, contains one or both of Nb and Ti in the range of 0.002 to 0.100% each, and a total of 0.100% or less, with the balance being Fe and unavoidable impurities. Continuously cast aluminum killed steel slab
'fC is a step of rolling down a continuously cast copper strip by 7n in the width direction and thickness direction by 5% or more immediately after solidification, and immediately rolling the rolled slab or steel strip once to the Arm transformation point (krH transformation point). cooling the slab or copper strip to a temperature range of 1100° C.);
Continuously cast aluminum killed, comprising the steps of reheating to a temperature below °C, starting rolling at a temperature range of 900 to 1100 °C, and finishing hot rolling at a temperature above the transformation point. Method of hot rolling steel.