KR20020052857A - Method for manufacturing steel of transformation induced plasticity - Google Patents

Abstract

PURPOSE: A method for manufacturing steel of transformation induced plasticity is provided to obtain sufficient amount of residual austenite by appropriately controlling reduction ratio and cooling rate during hot rolling. CONSTITUTION: The method for hot rolling steel of transformation induced plasticity comprises the steps of obtaining a reduction ratio according to finish rolling outlet target temperature of a slab, a target thickness of a strip and coiling temperature and cooling rate from a target residual austenite amount before finish rolling the slab that is heated in a heating furnace and rough rolled in a rough rolling mill using the following mathematical expression 1: £Mathematical Expression 1| γR(%)=(-2.25x10-4xTc-7.21x10-4xRr+1.55x10-3xCr+0.278)x100 where γR is a volume fraction(%) of residual austenite, Tc is a coiling temperature(deg.C), Rr is a reduction ratio(%), and Cr is a cooling rate(deg.C/sec)(70,71); determining an optimum reduction ratio(Rr) within the range that the residual austenite amount(γR) is 12 to 14%, the coiling temperature(Tc) is 450 to 600 deg.C and the cooling rate(Cr) is 5 to 35 deg.C/sec in the calculation values calculated in the reduction ratio calculation step(72); determining a reduction ration of final stand and load distribution ratios of each finish rolling mills from the determined optimum reduction ratio(Rr)(73); and finishing rolling the slab according to the optimum reduction ratio, coiling temperature and cooling rate determined in the above steps, and cooling the finish rolled steel in a continuous cooler(74).

Description

Method for Manufacturing Steel of Transformation Induced Plasticity

The present invention relates to a method for producing a modified organic plastic steel, and more particularly, to a method for manufacturing a modified organic plastic steel that can obtain a sufficient amount of retained austenite by optimally adjusting the reduction ratio and cooling rate during hot rolling.

Transformed organic plastic (TRIP) steel is an ultra-high strength steel sheet with high tensile strength and elongation because it contains 5-20% of retained austenite after hot rolling.It is an ultra-high strength steel door impact beam or bumper. It is also used for parts requiring high strength, such as wheels.

The metamorphic organic plastic steel has such high strength and is excellent in formability and lightweight, and thus has recently been expanding its application range to various materials of automobiles.

A hot rolling process for producing a general modified organic plastic steel will be described with reference to FIG.

First, the slab 10 having a thickness of about 200-250 mm is continuously heated in the heating furnace 20 in the range of 1120-1300 ° C. The heated slab 10 is rolled into a bar material having a thickness of about 20 to 60 mm while passing through the roughing mill 30. The bar material rolled in the rough mill 30 is manufactured in the form of a steel plate having a thickness of about 1.0-20 mm while passing through the finishing mill 40.

The roughing mill 30 is hot rolled by the reversible rolling method in which the slab 10 is rolled while reciprocating 2-4 sets of roughing mills because it cannot apply a large reduction force to the slab 10 at one time. However, the finishing mill 40 rolls the slab 10 into the bar material while passing the slab 10 through 6-7 sets of finishing mills 41, 42, 43, 44, 45, 46, 47 without reversible rolling. The steel sheet rolled in the finishing mill 40 is cooled with cooling water while passing through the continuous cooling apparatus 50, and then wound by coils in the winding machines 60 and 60.

Here, the continuous cooling device 50 is arranged in a row of about 12-15 banks (51) for spraying water, each bank 51 is provided with a pipe-shaped header is usually installed in four sets of cylindrical Each header is provided with a plurality of nozzles. The number of nozzles provided in each header is usually about 15-40. Each bank 51 can inject about 15-25 liters / min of cooling water into the material.

In general, when manufacturing steel sheet by hot rolling process, process metallurgy technology or structure / property prediction and control technology is applied to control the manufacturing conditions to predict the mechanical properties of the steel sheet. To prepare.

For example, when the metamorphic organic plastic steel is manufactured by hot rolling, when the thickness, width and temperature FDT at the end of finishing rolling are determined in the finishing mill 40, the continuous cooling apparatus 50 is cooled accordingly. Speed and winding temperature (CT) are controlled. The program used for the purpose of temperature control in the continuous cooling device 40 is called CTC. In the CTC, the amount of material temperature drop by air cooling and the opening of each bank 51 are injected to predict the amount of material temperature drop. The number of banks 51 is determined.

When cooling is controlled by CTC, the unit is controlled in units of banks 51. Normal finishing end temperature (FDT) is set at 850-950 ° C. and winding temperature is set at 400-750 ° C. Control to cool at a cooling rate of -25 ° C / sec.

Since the high strength properties of metamorphic organic plastic steel are determined by the austenite remaining in the material after hot rolling, it is usually required to cool very rapidly from 900-950 ° C to 400-450 ° C during continuous cooling. At the same time, the metamorphic organic plastic steel should be light in weight, so it is usually made of thin steel plate of about 1.5-3.4mm to meet these characteristics.

Rolling conditions and cooling conditions of the tip of the finishing mill are particularly important when producing a modified organic plastic sheet by hot rolling. Here, the leading end of the finishing mill means the part which starts to roll out from the finishing mill.

In the case of the tip portion is not yet entered into the winding machine 60 at the time of the initial rolling operation, and is pushed out of the finishing mill, thus it is in a state of not receiving any force toward the winding machine 60. Therefore, as shown in FIG. 2, defective products such as the cold winding 15 are generated when overcooled. However, when the tip of the rolled steel sheet enters the winding machine 60, the winding machine 60 pulls the steel sheet, so that a tensile force is generated between the winding machine 60 and the final finishing mill 47. Even in this state, hot rolling can be performed in a stable state.

As such, when the cold winding occurs, the material cannot be used as a whole product, and it takes a lot of time to remove the cold winding, and thus it is emerging as an important problem of steel sheet manufacturing.

For this reason, in the state where the tip portion is not drawn into the winding machine 60, the rolling speed of the finishing mill is slowed to prevent the cold winding 15.

However, if the rolling speed is reduced to prevent cold winding, the running performance of the steel sheet may be improved in the continuous cooling apparatus, but the cooling speed may be slowed, and thus sufficient residual austenite may not remain. In other words, the transformation organic plastic steel is not manufactured under these conditions.

In order to solve this problem, it is possible to consider a method of rolling by setting the cooling temperature itself as high as possible, but if the cooling temperature is set high, the amount of retained austenite is increased because austenite phase-transforms into either ferrite or ferrite. The steel sheet produced by reduction is unable to secure the formability and strength that can be used as the transformation organic plastic steel.

Until now, there have been suggested methods for adjusting the composition of the material or changing the cooling method in an effort to solve such problems, but there is no known method for controlling the hot rolling and continuous cooling processes themselves.

The present invention has been made to solve the above problems, and an object of the present invention is to produce a modified organic plastic steel by hot rolling, from the final thickness of the steel sheet and the target residual austenite amount to the rolling reduction and cooling rate at the time of hot rolling In addition, the present invention relates to a method for producing a modified organic plastic steel which can generate an appropriate amount of retained austenite without hot quenching by determining the coiling temperature and then applying an optimal rolling ratio and cooling conditions.

1 is a layout view of a hot rolling mill.

2 is a schematic diagram showing a state in which a poor product occurs during quenching in a hot rolling mill.

3 is a process control diagram of hot rolling according to the present invention.

4 is a graph showing the experimental results for the correlation between the cooling rate and the amount of retained austenite.

In the present invention for achieving the above object in the method for producing a metamorphic organic-plastic steel by hot rolling low carbon steel having a carbon content of 0.2-05%, Mn and Si are added as a special element and other inevitable impurities, Before rolling the slab heated in the furnace and roughly rolled in the roughing mill, the reduction ratio according to the winding temperature and the cooling rate is calculated from the target rolling exit target temperature of the slab, the target thickness of the steel sheet and the target residual austenite. Obtaining according to 1;

γ _R (%) = (-2.25 × 10 ^-4 × Tc-7.21 × 10 ^-4 × Rr + 1.55 × 10 ^-3 × Cr + 0.278) × 100 (where _R is the volume fraction of residual austenite (% ), Tc means winding temperature (℃), Rr means reduction ratio (%) and Cr means cooling rate (℃ / sec) respectively.)

Residual austenite amount (γ _R ) is 12-14 Vol.%, Winding temperature (Tc) is 450-600 ° C and cooling rate (Cr) is 5-35 ° C / sec. Determining an optimum reduction ratio Rr within a range; Determining a reduction ratio of the final stand and a load distribution ratio of each finishing mill from the determined optimal reduction ratio Rr; Rolling the slab according to the optimum reduction ratio, the winding temperature, and the cooling rate determined in each step and cooling in a continuous cooler; It provides a hot rolling method of metamorphic organic plastic steel comprising a.

In the present invention, it is preferable to control the reduction ratio of the final stand to 15-45%.

In the present invention, in the case of the load distribution ratio of each finishing mill, it is preferable to gradually increase the reduction ratio from the initial stand and to apply the maximum reduction ratio at the stand immediately before the final stand.

In addition, the present invention is preferably controlled to 25 ℃ / sec or less in the case of the optimum cooling rate.

Hereinafter, a method of manufacturing a metamorphic organic plastic steel by the hot rolling process according to the present invention will be described.

The metamorphic organic plastic steel of the present invention is composed of carbon steel containing inevitable impurities in low carbon steel having a carbon content of 0.2-0.5 wt%. The present invention is based on the carbon steel is added to a special element that causes transformation organic plasticity, such as Si and Mn, the base material, when the addition of Si as a special element is added 1.5 wt%, Mn is added as a special element In this case, the amount of addition is preferably 2.0wt%.

A manufacturing method for producing a metamorphic organic plastic steel according to the present invention will be described with reference to FIG. 3.

First, the exit target temperature Ti in the last rolling mill of the finishing mill, the target thickness hi of the steel sheet, and the target amount of retained austenite γ _R are determined (70). The outgoing target temperature (Ti) is set at around 900 ° C, the target thickness (hi) of the steel sheet is set at 1.0-3.5mm and the target amount of retained austenite (γ _R ) is set at 12-14%. It is a preferable condition for producing an organoplastic steel.

After the continuous cooling is finished, the winding temperature (Tc) and the cooling rate (Cr) of the steel sheet are set from the Ti, hi, and γ _R values set as described above. Calculate the reduction ratio (Rr) of.

γ _R (%) = (-2.25 × 10 ^-4 × Tc-7.21 × 10 ^-4 × Rr + 1.55 × 10 ^-3 × Cr + 0.278) × 100 where γ _R is the volume fraction of residual austenite (%), Tc means the winding temperature (° C.), Rr means the reduction ratio (%) and Cr means the cooling rate (° C./sec).

Equation (1) is obtained by regression analysis using experimental results for various conditions when manufacturing transformed organic plastic steel for hot rolling using carbon containing 0.2wt% carbon and Mn and Si added as special elements. The rolling reduction rate at the final stand of finishing rolling is 15-45%, and the coiling temperature of the coil is applied in the range of 450-600 ° C.

The rolling reduction rate Rr represents the ratio of the steel sheet thickness in the finishing mill and the steel sheet thickness in the total rolling mill, and can be expressed by Equation 2 below.

Rr = [(h _i-1 -h _i ) × 100] / h _i-1

Using the above equation, the reduction ratio (Rr) for obtaining the target residual austenite amount (γ _R ) in the winding temperature (Tc) in the range of 450-600 ° C and the cooling rate (Cr) in the range of 5-25 ° C / sec is obtained. Calculate (71).

From the calculated result, it is determined by selecting the optimum reduction ratio (Rr), the optimum winding temperature (Tc) and the cooling rate (Cr) according to the steel sheet thickness (72).

The reduction ratio of the final stand calculated above is applied and the reduction ratio load distribution of each stand of the finishing mill to be described below is corrected (73).

In this way, after determining the optimum reduction ratio load distribution, the rolling is applied by applying the optimum cooling rate described below, followed by cooling the steel sheet in the continuous cooling apparatus to produce the modified organic plastic steel having the target residual austenite (74). ).

Table 1 to Table 3 show the results of comparing the specific experimental measured values hot rolled according to the conditions of the present invention and the values calculated according to the above equation.

The experimental values shown in Tables 1 to 3 are the results of analyzing the hot rolling conditions and the crystal structure of the steel produced according to the conditions when the metamorphic organic plastic steel is manufactured using the 140m continuous cooling system.

division Cooling rate 25 ℃ / sec Winding temperature (℃) 450 500 550 600 Rolling reduction (%) 15 30 45 15 30 45 15 30 45 15 30 45 γ residual amount (%, calculated value) 20 19 18 19 18 17 18 17 16 17 16 15 γ residual amount (%, experimental value) 19 17 17 18 18 16 17 17 15 18 15 13 Rolling speed (m / min) 447 447 447 500 500 500 568 568 568 656 656 656

division Cooling rate 20 ℃ / sec Winding temperature (℃) 450 500 550 600 Rolling reduction (%) 15 30 45 15 30 45 15 30 45 15 30 45 γ residual amount (%, calculated value) 20 19 17 19 17 16 17 16 15 16 15 14 γ residual amount (%, experimental value) 19 19 17 18 16 17 17 16 15 14 16 13 Rolling speed (m / min) 357 357 357 400 400 400 454 454 454 525 525 525

division Cooling rate 15 ℃ / sec Winding temperature (℃) 450 500 550 600 Rolling reduction (%) 15 30 45 15 30 45 15 30 45 15 30 45 γ residual amount (%, calculated value) 19 18 17 18 17 16 17 16 14 15 14 13 γ residual amount (%, experimental value) 18 18 18 17 17 16 15 17 15 14 13 14 Rolling speed (m / min) 268 268 268 300 300 300 341 341 341 394 394 394

As shown in Tables 1 to 3, the austenite residual value calculated according to Equation 1 is 15-45% reduction ratio, the cooling rate 15-25 ℃ / sec, the austenite residual experimental value in the winding temperature 450-600 ℃ range Shows good agreement with. In addition, the shaded parts in Tables 1 to 3 generally produce target organic-plastic steels in this range in consideration of the fact that hot rolling is performed at a tip rolling speed of 400-750 m / min in the hot rolling process. It shows the conditions that can be done.

As described above, the final reduction rate range is selected in consideration of the rolling speed, the cooling rate, and the winding temperature, and then the rolling amount distribution pattern for each stand of the finishing mill is determined (73).

The rolling reduction distribution pattern of each stand is determined in consideration of the rolling reduction of the final stand set above, and the transformation of steel sheet produced by the conventional hot rolling method and the hot rolling method of the present invention and inspection of the structure of the steel sheet manufactured The results are shown in Table 4.

Experimental results shown in Table 4, to prepare a 2.5mm thick steel plate rolled 30mm thick bar material with a finishing mill of carbon steel added Mn 1.5wt%, Si 2.0wt% to 0.2wt% carbon steel and the extrusion temperature was 920 It is made into metamorphic organic-plastic steel by making it into ° C. The comparative example in Table 4 is based on the conventional reduction ratio pattern and is for a steel sheet manufactured by gradually rolling down the 50-60% reduction ratio in the final stand while gradually increasing the reduction amount, and the embodiment is the reduction reduction determined in the present invention. It is about the steel plate manufactured according to the rate pattern.

division Stand number of finishing mill Residual amount of γ (Vol.%) Runability of steel plate Rolling Condition Winding Temperature (℃) Cooling Speed (℃ / sec) One 2 3 4 5 6 7 Comparative Example 1 Coil thickness (mm) 26.2 22.2 17.5 12.8 8.4 5.0 2.5 10 Bad Winding temperature: 480 Cooling speed: 20 Rolling reduction (%) 14.7 15.3 21.1 26.8 34.4 41.0 50.0 Comparative Example 2 Coil thickness (mm) 26.2 22.2 17.5 12.8 8.4 5.0 2.5 11 Cold spot Winding temperature: 480 Cooling speed: 35 Rolling reduction (%) 14.7 15.3 21.1 26.8 34.4 41.0 50.0 Example 1 Coil thickness (mm) 26.2 22.2 17.5 12.8 8.4 2.9 2.5 18 Good Winding temperature: 480 Cooling speed: 20 Rolling reduction (%) 14.7 15.3 21.1 26.8 34.4 65.5 15.0 Example 2 Coil thickness (mm) 26.2 22.2 17.5 12.8 8.4 3.5 2.5 17 Very good Winding temperature: 480 Cooling speed: 20 Rolling reduction (%) 14.7 15.3 21.1 26.8 34.4 58.3 30.0 Example 3 Coil thickness (mm) 26.2 22.2 17.5 12.8 8.4 4.5 2.5 16 Very good Winding temperature: 480 Cooling speed: 20 Rolling reduction (%) 14.7 15.3 21.1 26.8 34.4 46.4 45.0

As shown in Table 4, Comparative Examples and Examples have the same rolling pattern from stand 1 to 5, and the difference is that at the 6 and 7 stands, the comparative example gradually increases the rolling reduction rate to obtain the best rolling reduction at the final stand. While the ratio (50-60%) was added, the steel sheet was manufactured by adding the highest reduction ratio (45-65%) in the sixth stand and only 15-45% in the final stand.

When the high reduction ratio is applied to the final stand as in the comparative example, even if the winding temperature is lowered and the cooling rate is increased, it is difficult to generate a sufficient amount of austenite. In addition, when working as in the comparative example, a cold winding occurs during hot rolling, and thus the hot rolled coil cannot be used.

However, as in the examples, a reduction rate of 15-45% at the final stand produces more than 16% of sufficient retained austenite even at low cooling rates (below 20 ° C / sec) and relatively high winding temperatures (above 450 ° C). You can. In addition, when hot rolling is carried out under the same conditions as in the embodiment, the steel sheet running in the continuous cooling zone is good, so that a poor product does not occur.

Hereinafter, the influence on the change in the amount of retained austenite according to the cooling rate changes. The experimental results for the amount of retained austenite according to the cooling rate change are shown in FIG. 4. 4 is a result of the experiment under the rolling reduction conditions of Table 4.

As can be seen in Figure 4, it can be seen that the amount of retained austenite does not increase significantly even though the cooling rate is increased when only the cooling rate is changed by the rolling conditions of the comparative example.

However, it can be seen that when the cooling rate is changed by the rolling conditions of the embodiment, the amount of retained austenite is increased by 15-21%.

As shown in Figure 4 it can be seen that the cooling rate that can produce the maximum amount of retained austenite under the rolling conditions according to the present invention is 30 ℃ / sec. However, considering that the cooling time is generally 10-15 seconds in the continuous cooling zone, if the cooling rate is more than 25 ℃ / sec, there is a high possibility that the cold winding is caused by excessive injection of the cooling water.

Therefore, when producing the transformation organic plastic steel under the rolling conditions according to the present invention, the cooling rate is preferably adjusted to 25 ℃ / sec or less.

As described above, according to the present invention, when the modified organic plastic steel is manufactured by hot rolling, an optimum rolling reduction pattern, particularly an optimum rolling reduction pattern in a finishing mill, is obtained and hot rolled under such conditions to produce a target amount of retained austenite. Can be.

In addition, when the metamorphic organic plastic steel is manufactured under the hot rolling condition according to the present invention, rapid cooling can be avoided in the continuous cooling zone, and the running property of the steel sheet can be expanded, thereby minimizing the incidence of defective products such as cold rolling.

Claims (5)

In the method for producing metamorphic organic-plastic steel by hot rolling low carbon steel containing 0.2-05% of carbon, Mn and Si as special elements, and other unavoidable impurities, Before rolling the slab heated in the furnace and roughly rolled in the roughing mill, the reduction ratio according to the winding temperature and the cooling rate is calculated from the target rolling exit target temperature of the slab, the target thickness of the steel sheet and the target residual austenite. Obtaining according to 1; γ _R (%) = (-2.25 × 10 ^-4 × Tc-7.21 × 10 ^-4 × Rr + 1.55 × 10 ^-3 × Cr + 0.278) × 100 (where _R is the volume fraction of residual austenite (% ), Tc means winding temperature (℃), Rr means reduction ratio (%) and Cr means cooling rate (℃ / sec) respectively.) Residual austenite amount (γ _R ) is 12-14 Vol.%, Winding temperature (Tc) is 450-600 ° C and cooling rate (Cr) is 5-35 ° C / sec. Determining an optimum reduction ratio Rr within a range; Determining a reduction ratio of the final stand and a load distribution ratio of each finishing mill from the determined optimal reduction ratio Rr; Rolling the slab according to the optimum reduction ratio, the winding temperature, and the cooling rate determined in each step and cooling in a continuous cooler; Hot rolling method of metamorphic organic plastic steel comprising a. The hot rolling method of transformation organic plastic steel according to claim 1, wherein the reduction ratio of the final stand is 15-45%. The hot rolling method of transformation organic plastic steel according to claim 2, wherein the load distribution ratio of each finishing mill gradually increases the reduction ratio from the initial stand and applies the maximum reduction ratio at the stand immediately before the final stand. The hot rolling method of transformation organic plastic steel according to claim 3, wherein the optimum cooling rate is 25 ° C / sec or less. The method of claim 4, wherein the amount of Mn added to the carbon steel is 1.5 wt% and the amount of Si is 2.0 wt%.