JP2003320403A - Manufacturing method for hot rolled steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method which enables stable manufacturing of a hot rolled steel strip with uniform super-fine-grained ferrite microstructure from the head end to the tail end through a rolling equipment of a common specification, causing deterioration of neither thickness profile nor strip shape. <P>SOLUTION: In hot finish rolling of a hot rolled steel strip through a finish rolling mill with plural rolling stands, the manufacturing method for the hot rolled steel strip is characterized in that thickness of a hot bar and a reduction distribution and rolling speed at each stand pass is set so as to have an average temperature in the cross section of the material to be rolled at each stand stay almost uniform at a temperature immediately above a transition temperature Ar<SB>3</SB>through the whole length of the material and then the finish rolling of the material is executed keeping the average cross sectional temperature of the material immediately above the transition temperature Ar<SB>3</SB>at the entry side of the hot finish rolling mill. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention has an average particle size of 3 μm.
The present invention relates to a method for manufacturing a hot rolled steel strip having the following fine ferrite structure and excellent in strength and toughness.

[0002]

2. Description of the Related Art As a method for producing a fine grain structure steel material having an extremely fine grain ferrite crystal structure having a grain size of 3 to 4 .mu.m as hot rolled and having excellent ductility, Japanese Patent Publication Nos. 62-7247 and 62-39228 are disclosed. No. 1 was subjected to hot working in the process of cooling from the temperature range of Ac ₃ transformation point or higher, and at the final stage, the temperature range of (Ar ₁ + 50 ° C) to (Ar ₃ + 100 ° C) was practically within 1 second. During one or two or more times, hot working by which the total surface reduction rate is 50% or more and 95% or less is added, and after completion of the hot working, 600 at a cooling rate of 20 ° C./second or more and 2000 ° C./second or less. A method of cooling to a temperature range of ℃ or less is shown.

Further, in Japanese Patent Laid-Open No. 10-8142, hot finish rolling is reversely transformed by heat generated by rolling when the temperature of a material to be rolled passes through one of the rolling stands of the finishing rolling mill train, After finishing rolling the temperature so as to be Ar ₃ −50 ° C. or higher and winding at a temperature of 550 to 750 ° C.,
A method for producing a steel sheet for can making which has good workability and is free from surface roughening by performing scale removal, cold rolling, continuous annealing, and temper rolling is disclosed.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the methods of No. 47 and Japanese Patent Publication No. 62-39228, one-pass large reduction is performed in either stand in the finishing rolling mill, but the final strip thickness of the hot-rolled steel strip is about several mm. Therefore, when one pass large reduction is applied at any one of the rolling stands in the finishing rolling line, bending deflection occurs in the rolling roll due to the large rolling load, and the strip thickness profile of the strip is centered in the strip width direction. The convex cross-sectional shape in which the plate thickness is thicker at the part and the plate thickness decreases toward the plate width end, that is, the so-called plate crown becomes very large, and plate shape defects such as selvages or middle elongation are likely to occur. In addition, in order to perform such large reduction rolling, a strong rolling mill that can withstand a large rolling load and a large torque, including the drive system, is required. In order to prevent deterioration of the properties, high-speed rolling with a large-capacity motor is required, and it is extremely difficult to realize it with a rolling facility having general specifications.

Further, in the method of the above-mentioned Japanese Patent Laid-Open No. 10-8142, in order to cause the reverse transformation utilizing the heat generated by the working in the finish rolling mill, a large working work in the stand of the rolling mill, that is, a large reduction, a large rolling Processing speed conditions are essential,
Japanese Patent Publication No. 62-7247 and Japanese Patent Publication No. 62-39228
As in the case of No. 1, the deterioration of the strip thickness profile and strip shape becomes a major problem along with the ability of the rolling mill.

Further, in the conventional finish rolling, accelerated rolling is performed in order to compensate for the temperature drop of the rough bar which occurs between the start of rolling of the rough bar and the rolling of the tail end. Since it is performed, it is inevitable that the heat history in each part of the coil is different, which causes unevenness of the material from the coil tip to the tail end.

The present invention solves the problems of the prior art as described above, and in a rolling facility of general specifications, without deteriorating the strip thickness profile and strip shape, and from the tip to the tail of the coil. An object of the present invention is to provide a method for manufacturing a hot-rolled steel strip capable of stably manufacturing a hot-rolled steel strip having a uniform ultrafine ferrite structure.

[0008]

Means for Solving the Problems The present inventors have stably manufactured a hot-rolled steel strip having a fine ferrite structure without deteriorating the strip thickness profile and strip shape in a rolling facility with general specifications. The method for manufacturing hot-rolled steel strips that can be used was investigated. In the hot working of steel materials, it is known that grain refinement can be achieved by applying a large strain in one pass. The effect of this refinement is especially large rolling at temperatures near the Ar ₃ transformation point. It is known to be large in processing. On the other hand, as the temperature of the rolled material decreases, the recovery speed of the strain added by rolling slows down, so the strain does not completely recover between rolling passes, and there is an effect that it accumulates in the next pass rolling as residual strain. It has been known. That is, it is possible to apply the strain equivalent to the one-pass large reduction by utilizing the accumulated effect of the strain, without applying the large reduction processing in the one-pass. As shown in FIG. 1, normally in hot finish rolling, A
A finish target temperature is set in the vicinity of the r ₃ transformation point, and the temperature is lowered so that the finish target temperature is reached from the former rolling mill to the latter rolling mill. At this time, it is considered that the accumulated strain occurs in the second to third stands where the rolled material temperature is close to the Ar ₃ transformation point.

As described above, 1 in the hot finish rolling mill is used.
Under a large pass pressure, it is a fatal problem that the plate thickness profile and the plate shape are deteriorated, so the present inventors effectively utilize the cumulative effect of strain as a means for refining the crystal grains of the rolled material. As a result of examining the method, it is possible to maintain the rolled material temperature in the vicinity of the Ar ₃ transformation point over the entire stand by adjusting the rough bar thickness, the reduction ratio distribution in each pass, and the rolling speed. , It was found that a large strain accumulation effect can be obtained.

The temperature change of the rolled material in continuous rolling such as hot finish rolling is caused by processing heat generation in each rolling pass, friction heat generation, heat removal to the rolling roll, air cooling between each pass and spray between stands. -Is affected by the cooling effect of. The work heat is generated by converting the work applied by rolling into heat, and depends on the amount of strain applied and the deformation resistance of the material. Friction heat is generated by converting frictional work between a rolled material and a rolling roll into heat.
It is affected by the contact pressure between the two parts, the coefficient of friction, and the relative sliding speed. The heat removal to the rolling roll is a phenomenon of heat conduction between substances and is determined by the temperature difference between the rolling roll and the surface of the rolled material and the contact time between the two. The amount of temperature drop of the rolled material between rolling passes is due to heat loss due to radiation and convection.It depends on the temperature difference between the rolled material temperature and the atmosphere, and the time between stands determined by the distance between rolling mill stands and the passing speed. It is determined. As described above, the temperature change of the rolled material in the hot finish rolling largely depends on the rough bar thickness, the distribution of the rolling reduction at each stand, and the rolling speed. Therefore, the mass flow, which is the product of the plate thickness and the speed at the entrance and exit of each rolling mill, must be constant, and each cannot be controlled independently. However, by optimizing the rough bar thickness, the reduction ratio distribution at each stand, and the rolling speed for an arbitrary finished plate thickness, the rolled material temperature can be kept almost constant from the first stand to the final stand. It will be possible. By carrying out such rolling at a temperature near the Ar ₃ transformation point, a large strain accumulation effect can be obtained.

Further, as described above, in the normal hot finish rolling, the rolling speed is increased at a substantially constant acceleration rate from the start of the rolling of the rolled material to the start of the rolling of the tail end. Speed up the finishing temperature on the exit side of the finishing mill to A
Accelerated rolling is performed by controlling the temperature to around the r ₃ transformation point.
This is due to the above heat generation by increasing the rolling speed.
The direction of heat removal and cooling reduces the temperature drop of the rolled material,
In other words, this is because it has the effect of increasing the amount of heat generation and decreasing the amount of heat removal and cooling, and compensates for the decrease in temperature between the start of rolling at the tip and the start of rolling at the tail. This is because. However, the present invention aims to obtain a uniform microstructure by utilizing the cumulative effect of strain over the entire length of the hot-rolled steel strip, and the first stand to the final stand over the entire length of the hot-rolled steel strip. It is important to keep the rolled material temperature almost constant. In order to achieve such an object, for example, a temperature compensation equipment such as a tunnel furnace or an induction heating device, or a coil box or the like is installed on the entrance side of the finishing mill so that a constant temperature and a constant speed are maintained over the entire length of the steel strip. Rolling to obtain a constant heat history.

Further, it is generally known that a finer structure can be obtained by performing rapid cooling immediately after the hot working is finished. This is because the rapid cooling accelerates the rate of transformation nucleation into ferrite and slows down the grain growth rate. As a result of diligent studies on the influence of the cooling rate, the present inventors have found that the average grain size is 3 μm as hot rolled by cooling at a cooling rate of 50 ° C./sec or more after the final rolling pass of finish rolling according to the present invention. It has been found that a hot-rolled steel strip having the following fine ferrite structure can be manufactured. At this time, the smaller the initial austenite grain size on the finish rolling entry side, the greater the effect of the present invention. Therefore, it is desirable to keep the austenite grain size on the finish rolling entry side as small as possible.

The present invention has been made on the basis of such findings, and its features are as follows.

(1) In hot finish rolling of a hot-rolled steel strip using a finish rolling mill composed of a plurality of rolling stands, the average sectional temperature of the rolled material at each rolling stand is the Ar ₃ transformation point over the entire length of the rolled material. The coarse bar thickness, the distribution of the rolling reduction in each pass, and the rolling speed are set so that the temperature directly above is almost constant, and the average cross-sectional temperature of the rolled material on the entry side of the finishing rolling mill is set to A.
A method for producing a hot-rolled steel strip, comprising performing finish rolling of a rolled material at a temperature just above the r ₃ transformation point.

(2) The method for producing a hot-rolled steel strip according to the above (1), characterized in that the rapid cooling immediately after the final rolling pass of the finish rolling is performed at a cooling rate of 50 ° C./sec or more.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is an explanatory view showing an embodiment of a hot-rolled steel strip manufacturing facility row used for carrying out the present invention.

The hot-rolled steel strip production line shown in FIG. 2 includes a rough rolling machine 3 for reducing the thickness of the slab 2 reheated in the heating furnace 1 to a rough bar having a predetermined thickness, and a rough rolling. An intermediate cooling device 4 installed immediately near the machine exit side, a coil box 5 for winding and keeping the rough bar 3 warm, and finish rolling for reducing the thickness of the rough bar to a hot-rolled steel strip having a predetermined thickness. The machine 6, a quick cooling device 7 located immediately near the exit side of the finish rolling mill, for rapidly cooling the hot-rolled steel strip, a cooling device 8 for adjusting the winding temperature, and the hot-rolled steel strip are wound. And a winding machine 9 for winding.

The intermediate cooling device 4 is a cooling device for preventing grain growth after rough rolling and for performing temperature controlled cooling to near a target temperature in finish rolling.

The intermediate cooling device 4 is preferably installed near the exit side of the rough rolling final rolling stand. The recrystallized austenite grains generated under the rolling in the rough rolling usually show a large grain growth behavior in several tens of seconds between the rough rolling and the finish rolling.

In recent years, it has been reported that the austenite grain size on the entry side of finish rolling affects the final grain refinement of the hot-rolled steel strip, and the grain size of austenite on the entry side of finishing rolling is as much as possible. It is preferable to make it small. For this reason, the intermediate cooling device is installed immediately near the exit side of the rough rolling final rolling stand to prevent grain growth after rough rolling and to perform temperature controlled cooling to a temperature close to the target temperature in finish rolling.

The coil box 5 is used for winding a coarse bar and keeping it warm so that the temperature of the rough bar on the side of finish rolling is constant. Since the finish rolling of the present invention does not normally carry out accelerated rolling, the coil box 5 is provided in order to compensate for the temperature drop at the tail end of the rough bar.
In order to compensate for this temperature drop, a tunnel furnace or an induction heating device may be provided instead of the coil box.

As another embodiment, a continuous process is performed between the coil box 5 and the finish rolling mill 6 by welding or pressure welding the tail end of the front rough bar and the tip of the next rough bar. The present invention can be applied to the form of hot rolling as well, and particularly when the finished plate thickness is thin, high speed constant speed rolling by continuous hot rolling is effective.

In the rapid cooling device 7, the rate of nucleation of transformation nuclei into ferrite is increased by the rapid cooling, and the effect of slowing down the grain growth rate is obtained. Therefore, by performing the rapid cooling immediately after the hot working, a finer grain size is obtained. A cooling device for obtaining tissue. It is preferable to perform rapid cooling immediately after the end of the final rolling pass of the finish rolling at a cooling rate of 50 ° C./sec or more.

In order to enable rapid cooling immediately after the rolling mill, it is desirable to arrange it as close as possible to the exit side of the rolling mill. In the embodiment of FIG. 2, the rapid cooling device is arranged near the exit side of the finishing mill. .

From the viewpoint of material adjustment, the winder 9
The temperature after being taken up by the winding machine is also important, and in the embodiment of FIG. 2, the cooling device 8 for adjusting the winding temperature is provided immediately before the winding machine 9.
Are arranged.

Hereinafter, an embodiment of the method of the present invention using the above-mentioned device configuration will be described.

In the hot-rolled steel strip production line shown in FIG. 2, the slab 2 is reheated in the heating furnace 1, reduced by the rough rolling mill 3 into a rough bar having a predetermined roughness, and rolled. Next, the rolled rough bar is subjected to temperature control cooling to a temperature close to a target temperature for finish rolling by the intermediate cooling device 4 installed immediately near the exit side of the rough rolling mill. The coarse bar which has been subjected to the temperature adjustment cooling is wound around the coil box 5 to keep it warm. Next, the rough bar is taken out from the coil box 5, reduced by a finish rolling machine 6 into a hot-rolled steel strip having a predetermined thickness, and rolled.

Then, immediately after the end of the final rolling pass of finish rolling, a rapid cooling device 7 for rapidly cooling the hot-rolled steel strip is used for rapid cooling at a cooling rate of 50 ° C./sec or more to adjust the coiling temperature. After the temperature is adjusted by the cooling device 8, the steel strip is wound by the winder 9.

[0029]

EXAMPLE Using the hot-rolled steel strip manufacturing equipment row shown in FIG.
A hot rolled steel strip was produced by the method of the present invention. That is, a low carbon steel slab with an initial plate thickness of 250 mm is heated to about 1100 in a heating furnace.
After heating to ℃, this slab was reduced to 50 mm by 5-pass rolling with a rough rolling machine to obtain a rough bar. This coarse bar is cooled to about 870 ° C. by an intermediate cooling device and wound around a coil box. The coarse bar wound around the coil box is unrolled in sequence and at a constant speed from the tip to the tail end.
Finish rolling of the pass was performed.

Table 1 shows the rolling results at this time (thickness on the inlet side, thickness on the outlet side, reduction ratio, speed, temperature, rolling load, rolling power at each rolling stand). 4m
m, finishing rolling speed is 800 mpm, target finishing temperature is 85
It is 0 ° C. The temperature change of the rolled material from the 1st pass to the 7th pass was at most several degrees Celsius over the entire length of the coil.
Fig. 1 shows the transition of the temperature of the rolled material at each finishing rolling stand.

On the other hand, in Tables 2 and 3, a rough bar having an entrance side thickness of 34 mm is finished by a conventional method and a finishing plate thickness of 4 mm is used.
It is a finish rolling result of the hot-rolled steel strip having a target finishing temperature of 850 ° C. Here, Table 2 shows the finish rolling result at the rolled material tip portion, and Table 3 shows the finish rolling result at the rolled material tail end portion.

In the conventional methods shown in Tables 2 and 3, the pattern is such that the temperature decreases as the steel strip advances from the former rolling stand to the latter rolling stand, and the action of accelerated rolling causes the tip portion and the tail end portion to be reduced. Although the finish rolling temperature of No. 1 is almost the target, the rolling temperature history is different in the coil, and the microstructure of the hot-rolled steel strip is nonuniform from the coil tip to the tail end. FIG. 1 shows the transition of the temperature at each finish rolling stand between the rolled material front end portion and the tail end portion together with the transition of the temperature of the rolled material of the present invention.

Further, comparing the speed of the rolled material tip portion in Table 2 with the speed of the rolled material tail end portion in the same finish rolling stand of Table 3, Table 3 is higher, and accelerated rolling is performed. According to the method of the present invention shown in Table 1, rolling is performed at a constant speed over the entire length of the rolled material in each finishing rolling stand, and accelerated rolling is not performed.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

In both the present invention and the conventional method, immediately after the end of the final rolling pass of finish rolling, a cooling rate of 50 ° C./sec was applied for 6
When the structure of the hot rolled steel strip was examined after cooling to 00 ° C. and winding and cooling to room temperature by air cooling, it was found that the conventional method had a ferrite crystal structure with an average grain size of about 5 to 8 μm. The method according to the present invention was found to have an ultrafine ferrite crystal structure with an average grain size of about 2 μm over the entire length of the coil, and the method according to the present invention could be confirmed.

[0038]

As described above, according to the present invention,
A hot-rolled steel strip having an ultrafine ferrite structure with a grain size of 3 μm or less can be produced without adding an alloying element, and a steel strip having high strength and high toughness can be produced.

[Brief description of drawings]

FIG. 1 is a graph showing a comparison of temperature transitions between finish rolling stands according to the present invention and a conventional method.

FIG. 2 is an explanatory view showing an embodiment of a hot-rolled steel strip manufacturing equipment row used for carrying out the present invention.

[Explanation of symbols]

1 heating furnace 2 slabs 3 rough rolling mill 4 Intercooler 5 coil box 6 Finishing mill 7 Rapid cooling device 8 cooling device 9 Winder

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Claims (2)

[Claims] 1. In hot finish rolling of a hot-rolled steel strip using a finish rolling mill comprising a plurality of rolling stands, the average cross-sectional temperature of the rolled material at each rolling stand is directly above the Ar ₃ transformation point over the entire length of the rolled material. The rough bar thickness, the distribution of the rolling reduction in each pass and the rolling speed are set so as to be substantially constant at the temperature of, and the average cross-sectional temperature of the rolled material on the inlet side of the finishing rolling mill is set to Ar ₃
A method for producing a hot-rolled steel strip, which comprises performing finish rolling of a rolled material at a temperature just above a transformation point. 2. The method for producing a hot-rolled steel strip according to claim 1, wherein the rapid cooling immediately after the final rolling pass of the finish rolling is performed at a cooling rate of 50 ° C./sec or more.