JPS6119322B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a hot-rolled steel sheet, and more particularly to a method for producing a hot-rolled high-strength steel sheet having a composite structure with excellent homogeneity by controlling the amount of transformation on a run-out table. Conventionally, temperature control of a hot-rolled steel sheet between the finishing process and the winding process is important in determining the material quality, and control has been performed mainly based on the rolling finishing temperature and the winding temperature. Later, it was discovered that even if the finishing rolling temperature and winding temperature were the same, changing the cooling pattern would change the material properties. Method of selecting cooling pattern by
As proposed in No. 42779, there are well-known methods such as installing a thermometer in the middle of the runout table and dividing the cooling zone into two zones, coarse cooling and fine cooling, to improve the accuracy of the winding temperature. . Furthermore, for the purpose of strengthening by a two-phase mixed structure, a method has been proposed in which the material is rapidly cooled from the α+γ region (Ar ₃ to Ar ₁ ) and then rolled up at a low temperature (Japanese Patent Publication No. 56-52090).
However, each of the above-mentioned conventional means has the disadvantage that material quality control with excellent homogeneity that satisfies recent strict quality requirements cannot be carried out satisfactorily. The present invention is a new method for producing hot rolled steel sheets that was devised as a result of investigating the relationship between the transformation rate during cooling of steel sheets, the subsequent cooling rate, and the material quality. The transformation rate at the middle of the runout table is controlled within a certain range based on the board thickness, line speed, FT, CT, target CT, etc. obtained online and offline information separately from the actual measurement values, and then the cooling rate or winding temperature is controlled. This is done by changing the amount of cooling water. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram of an example of a manufacturing process in which the method of the present invention is carried out. A thickness gauge 8 and a finishing thermometer 7 are provided, and a transformation rate gauge 3 and an intermediate thermometer 9 consisting of a magnetic field generator 3a and a sensor 3b are provided approximately in the middle of the runout table 10, and a winding gauge 9 is provided immediately before the winding machine 2. A thermometer 6 is provided. A pre-cooling step 4 is installed between the finishing thermometer 7 and the transformation rate meter 3, and a pair is provided above and below the run-out table 10. Similarly, a post-cooling step 5 is installed between the intermediate thermometer 9 and the transformation rate meter 3. It is provided between the winding thermometer 6 and the winding thermometer 6. The transformation rate meter 3 can employ any measuring means such as well-known X-ray diffraction or magnetic permeability measurement.
In this example, a magnetic permeability method was used, and as shown in the figure, a magnetic field generator 3a was used in combination with a magnetic field generator 3a at the bottom of the steel plate, and a sensor 3b at the top. Next, a cooling control method in this manufacturing process will be described in detail. First, when passing a hot rolled steel sheet in the above manufacturing process, a target transformation rate is determined in advance for each steel type and target mechanical properties of the rolled steel sheet based on experience. This target transformation rate is the γ→α transformation rate (α/γ+α), which is set within a certain range in consideration of hunting during cooling control, which will be described later. Then, the transformation rate actually measured by the transformation rate meter 3 is compared with the target transformation rate, and the amount of cooling water in the pre-cooling step 4 is increased or decreased according to the difference in the transformation rate. In other words, when the measured transformation rate a exceeds the target transformation rate b, the amount of water is decreased in proportion to a-b to slow down the progress of transformation, and when the measured transformation rate a is smaller than the target transformation rate b, b-a
The amount of cooling in the pre-cooling step 4 is corrected so that the amount of water is increased in proportion to the amount of water to accelerate the progress of transformation and approximate the target transformation rate. Next, based on the temperatures measured by the intermediate thermometer 9 and the winding thermometer 6, the actual cooling rate between the intermediate thermometer and the winding thermometer is calculated using the following equation. CR=TM-CT/(L/S)...(1) However, CR: Cooling rate (℃/sec) TM: Intermediate temperature (℃) CT: Winding temperature (℃) L: Intermediate thermometer ~ Winding thermometer Distance between (m) S: Threading speed (m/sec) The present inventors have researched the relationship between the cooling rate at the latter stage of the runout table, the transformation rate, and the component system, and as a result, the cooling rate in this cooling process has been determined. It was discovered that this has a large effect on the material quality of the steel sheet, and that by controlling the cooling rate in the post-cooling process within an appropriate range, a composite steel with excellent homogeneity can be obtained. The following formula (2) shows the appropriate cooling rate range. After finish rolling steel with C0.03 to 0.15%, Si≦2.0%, and Mn 0.1 to 2.0% at Ar ₃ to _{Ar 3} +60℃, It was obtained as an experimental formula when the amount of cooling water in the pre-cooling step was corrected to set the γ→α transformation rate to 0 to 90% (target transformation rate) in terms of α amount. (730−5.3γ) (1/1+0.7Si) (1/1+4.5Mn)≦CR≦(950−3.6γ) (1/1+0.7Si) (1/1+4.5Mn) …(2) However, Si, Mn: wt% γ: Actual austenite rate%
(γ/γ+α) In other words, the actual cooling rate CR determined by equation (1)
The cooling rate in the post-cooling process, that is, the amount of cooling water, is adjusted so that it falls within the CR range of equation (2). The actual target cooling rate is the upper limit cooling rate {(950−3.6γ)
1/(1+0.7Si)・1/(1+4.5Mn)} and lower limit cooling rate {730−5.3γ)
1/(1+0.7Si)(1+4.5Mn)}, that is, the average value of {(840-4.5γ)1/(1+0.7Si)(1+4.5
The amount of water is controlled using Mn)} as the target cooling rate. Figure 2 shows the austenite percentage in the middle of the runout table for 0.1%Si-1.4%Mn steel and 1.2%Si-1.4%Mn steel and the appropriate range of cooling rate thereafter up to the winding thermometer. When the cooling rate is lower than the lower limit, pearlite transformation occurs and no low-temperature transformed phase is generated, and two-phase formation is no longer possible.When the cooling rate is higher than the upper limit, the low-temperature transformed phase becomes too large and the strength increases significantly.
Ductility and workability are significantly reduced. Also, α% is 90%
If the temperature exceeds that level, pearlite transformation begins to occur before the middle of the runout table, making it difficult to generate a low-temperature transformed phase during subsequent cooling. Next, Table 1 shows the effect of improving material quality by the method of the present invention.

[Table] Comparative steels A, D, and G have too high a cooling rate, so their strength is too high. Steels C, E, and F have too low a cooling rate, so their strength is too low, and they all have a poor strength-ductility balance. or the yield ratio is high.
B and H are steels to which the method of the present invention has been applied, with appropriate strength;
High ductility and low yield ratio have been obtained.

[Table] Table 2 above is a comparison table of the variation in the material inside the coil cooled by the conventional method and the method of the present invention. It can be seen that the uniformity of the material inside the coil has improved. As described in detail above, the present invention places a transformation rate meter and an intermediate thermometer together in the middle of the runout table,
This method controls the cooling rate at the same time as controlling the amount of transformation. Compared to the conventional method that only controls the cooling rate by simply measuring temperature, material quality control is much more accurate. This is an industrially excellent invention that allows steel plates to be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of one example of a manufacturing process for implementing the method of the present invention, and FIG. 2 is a graph showing an appropriate range of cooling rate in the post-cooling step of the present invention. 1: Final rolling stand, 2: Winding machine, 3: Transformation rate meter, 4: Pre-cooling process, 5: Post-cooling process, 6: Winding thermometer, 7: Finishing thermometer, 8: Plate thickness gauge, 9 : Intermediate thermometer.

Claims (1)

[Scope of Claims] 1. A method for producing a hot-rolled steel sheet, in which a cooling step is provided between the hot-rolling finishing step and the winding-up step, and the required cooling amount is determined from the finishing outlet temperature and the winding-up temperature to cool the hot-rolled steel sheet, The cooling process is divided into a pre-cooling process and a post-cooling process, and the transformation rate and temperature of the hot-rolled steel plate are actually measured between the pre-cooling process and the post-cooling process, and the measured transformation rate approximates a target transformation rate that is set in advance for the hot-rolled steel plate. The cooling amount in the pre-cooling step is corrected so that the cooling amount in the pre-cooling step is corrected, and then the actual cooling rate is determined from the measured temperature and the measured winding temperature, and the cooling amount in the post-cooling step is corrected based on the actual cooling rate. Method of manufacturing rolled steel plate.