JPS58120745A - Continuous heat treatment for high tensile cold-rolled steel strip - Google Patents

Abstract

PURPOSE:To make the economization of blank material alloys possible without reheating and to cool a steel strip with good shapes by incorporating a cooling method by immersion in static hot water and a cooling method by hot water jets in hot water of prescribed temp. in the post stage to a primary cooling method after soaking. CONSTITUTION:In a primary cooling stage of titled treatment consisting of heating, soaking primary cooling and post-treatment stages, the steel strip S after holding at temp. Ac1-Ac3 is immersed in static hot water W kept at >=90 deg.C, and is cooled down to 710-550 deg.C at 20-90 deg.C/sec cooling rate. As the 2nd stage, the steel strip is cooled by the hot water jets from underwater headers 23 immediately down to the Ms point lower than 200 deg.C at 100-500 deg.C/sec cooling rate in the hot water W' kept at 60-75 deg.C. Since the steel strip can be cooled without cutting the nose of CCT curves by the 2nd stage of the quick cooling, the blank material alloys can be economized. The shape of the steel strip is assured by the above-mentioned selection of the inflection point of the two-stage cooling. If the steel strip after soaking is slowly cooled with gaseous jets down to about 800-600 deg.C in the fore stage of the primary cooling, the shapes of the steel strip is assured and the easy tendency to cutting of the nose of the CCT curves is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous heat treatment method for high tensile strength cold rolled steel strip.

In recent years, as a method for producing cold-rolled steel strips, continuous annealing has been replaced by the inefficient patch annealing method. The continuous annealing method is classified according to the method of primary cooling after heating and soaking.

The primary cooling methods currently in practical use are gas jet cooling (hereinafter abbreviated as GJC) and water quench cooling (hereinafter referred to as W
(abbreviated as Q).

Typical annealing cycle patterns in the GJC method are shown in $11 FIAOb for cold-rolled steel strips for processing, and in Zaob for high-tensile cold-rolled steel strips, and typical annealing cycles in the WQ method. 12 of $1111 for cold rolled steel strip for pattern t1 processing, Jl for high tensile strength cold rolled steel strip! Figure 2b2 Diagram 2゜These embodiments are described in Japanese Patent Publication No. 51-5335, the GJe method and 4111, respectively.
The WQ method is disclosed in Publication No. 18415/1983.

However, each of these three methods includes problems.

That is, (1J In the GJC method, the cooling rate [is 1Qp
It is possible to control the so-called end point temperature by stopping the cooling at an arbitrary temperature (for example, the overaging temperature), so there is no need to place a reheating zone before the overaging zone, which reduces equipment costs. While it has the advantage of low energy costs, due to its low cooling rate, it is necessary to increase the alloy content of the material when producing high-strength cold-rolled steel strip (41 and 11M dual-phase structure), which has become increasingly important in recent years. (2) With the WQ method, the cooling rate is on the order of 1,000 liters, and it is difficult to use low alloy materials when manufacturing high-strength steel. Although it has the advantage of being able to use the During manufacturing, it is necessary to temper due to metallurgical requirements.
In the end, reheating CaF (Fig. 2 and section R in Fig. 2) is indispensable in the production of any type of product, resulting in increased equipment costs and energy costs. Not only that but WQ
This reheating in the process causes fine noble compounds to be distributed within the grains, which tends to deteriorate the workability (especially ductility) of both cold-rolled steel strips and high-strength cold-rolled steel strips.

A static hot water immersion cooling method has recently been considered as another cooling method that solves the problem of existing methods as described above. This cooling method is as shown in Japanese Patent Publication No. 52-93619, in which the steel strip is cooled by immersion in hot water with a boiling point close to 9.
It is something. This results in uniform 1 on the surface of the copper strip in the high temperature range.
Raise the film boiling #, 40 c/41! In the low-temperature region, rapid cooling of about 150 cm is performed, and rapid cooling of about 150 cm is performed.
"Two-stage cooling" occurs naturally.

The purpose of this static hot water immersion cooling method is to control the cooling end point temperature at the overaging temperature (approximately 400°C) when producing cold rolled steel strip for processing, and to attempts to save the raw material alloy without reheating by gentle rapid cooling, but there are actually some problems with the latter aim. That is, the inflection point of the two-stage cooling is about 3 in the natural state.
Since the temperature is 00°C, cooling of about 40 degrees will continue to reach that temperature, which is not rapid. Therefore, depending on the material, cooling during production of high-strength cold-rolled steel strip (especially dual-phase structure 'fJ) may be difficult. The curve cuts off the nose of the continuous cooling transformation curve (hereinafter abbreviated as OCT curve) (see curve d in FIG. 3), and the desired effect of alloy saving cannot be obtained.

In addition, in static hot water immersion cooling, the bath temperature is limited to a temperature extremely close to the boiling point due to the need to maintain the shape, so the cooling** is uniquely determined by the thickness of the copper strip. The cooling rate and the inflection point of two-stage cooling are also non-variable. this is,
When considering future characteristics requirements for fluid high-tensile cold-rolled steel strips, it lacks flexibility and cannot be said to be desirable.

Furthermore, the inflection point of two-stage cooling, which occurs during natural immersion cooling in still hot water, may destroy the shape of the copper strip due to the large difference in cooling rate before and after the inflection point (Tetsu to Hagane, 62 (1976)).
6. ``Development of water quenching technology for cold rolled strips'' (see JP636).

Based on the above 11, the simple static hot water immersion cooling method is suitable for the conventional GJC method,
This means that not only is there less advantage than the WQ method, but there is also a possibility that a product with a satisfactory shape cannot be obtained.

An object of the present invention is to provide a continuous heat treatment method for high-strength cold-rolled steel strip, which can solve the problems of the above-mentioned static hot water immersion cooling method.

In view of the above object, the present inventors focused on the following points. That is, in the primary cooling method after soaking in continuous heat treatment of high-strength cold-rolled steel strip, a two-stage cooling type is used, and at least in the latter stage, a cooling method other than immersion cooling flow in still hot water is used. This method attempts to avoid the drawbacks of cooling methods.

The inventors of the present invention adopted the "immersion cooling method" as a result of various studies, but the above-mentioned stationary hot water immersion cooling method was adopted. Since the film had to be removed uniformly, a method was used in which a jet was blown onto the copper strip in hot water during the subsequent cooling step. At that time, if the water temperature is too low, the copper strip will be cooled too rapidly and will require the ninth reheating of the tempering process as described above. Therefore, in order to keep the cooling rate below t 500 C/-, water is 60
It is preferable to use hot water with a temperature of ℃ or higher.On the other hand, if the water temperature is too high, the cooling effect will decrease and the equipment cost will not only increase, but also it will not be possible to obtain a cooling rate of 100 Roux or higher to obtain a hardened structure, so the temperature should not exceed 75℃. Preferably hot water at a temperature of .

To summarize the above, the post-cooling method is a, 60℃ or higher, 7
The underwater jet cooling method (hereinafter abbreviated as the hot water jet cooling method) using hot water of 5°C or less is i! This means that it is uniform.

If this cooling method is used in the latter stage, the cooling rate will be 100
~500! -, starting point of post-cooling t-40
If the temperature is set to 0°C or above, the CCT Curzo can be cooled without cutting the nose (see the curve in Figure 3), and the material alloy can be saved.Also, since the starting temperature of the post-cooling is variable, the cooling curve can also be changed. This hot water jet cooling method and the static hot water immersion cooling method described above are collectively referred to as the hot water cooling method.

On the other hand, as the pre-cooling method for the second cooling, the static hot water immersion cooling method is used as described above. This cooling method is 20~90'
In the production of high-strength cold-rolled steel strips, gradual cooling at CS- level is preferred because it promotes diffusion and concentration of carbon and alloying elements into the austenite phase and facilitates martensite formation by rapid cooling in the subsequent stage. Cooling rate is 20! - less than CC
It is easy to cut the nose of the T, and the effect of alloy saving is diminished, so
If it exceeds 'c+i, the effect of concentrating alloying elements such as C into one phase will be diminished.

In this two-stage cooling, by setting the inflection point to 550° C. or higher, the alloy saving effect becomes greater.

However, at high temperatures, due to the solid solubility limit, the above diffusion 'aI
Since the effect of k11 is lost, a temperature of 710°C or lower is required.

Based on the method of the present invention, the rough annealing cycle pattern is shown in 1 in FIG. This annealing cycle allows the material alloy to be saved by gentle rapid cooling to temperatures below the M5 point without reheating.

In the hot water jet cooling at the final stage of primary cooling when producing high tensile strength cold rolled steel strip (especially of the two-phase structure type), the third
As shown by curve C in the figure, the cooling field must be set so that the copper strip temperature completely cuts through the point M.

Although it depends on the material composition, the M point is usually around 300°C, and in order to completely lower this temperature, the end point temperature of hot water jet cooling should be 200°C or less.

In particular, in jet cooling using hot water, unlike when using cold water, the cooling rate is not excessively high and gentle rapid cooling is performed, so there is no need for post-cooling tempering to ensure the workability of the product. There are advantages in that the reheating energy required for this is saved, and difficult end point control in this temperature range (~200°C) is not required.

As a primary cooling facility, if only static hot water immersion cooling is used, the inflection point from film boiling to nucleate boiling is approximately 300°C.
If the nose of the T curve is cut, it will not become martensite, so from a temperature of 550℃ or higher and 71O℃ or lower,
In order to obtain a cooling rate of ~500Q-, and to uniformly remove the vapor film from both sides of the copper strip to ensure its shape, hot water at 60-75℃ is applied to the surface of the steel strip, which is conveyed directly to the surface of the copper strip. A vertical tank is used so that a nozzle can be installed to spray the jet stream. ・A horizontal tank not only increases the line length and increases equipment costs, but also increases the cooling condition of the upper and lower surfaces of the copper strip (causing differences in the generation and removal of air bubbles, The shape of the belt is easily broken.

The reason for performing slow cooling by gas jet cooling in the preceding stage of primary cooling according to the second feature of the present invention will be explained.

In order to lower the O steel strip temperature at the start of hot water immersion to a temperature necessary for metallurgical reasons and to ensure the shape of the copper strip, the second method of the present invention
The copper strip after soaking is cooled using a gas jet cooling device before the hot water cooling tank according to the characteristics of Or, since there is no slow cooling effect to secure the shape, use a gas jet cooling device.5Q4a: 30"A@eJJ,
After cooling to a temperature of 600°C or more and 800°C or less at the cooling rate shown below, it is immersed in a hot water cooling tank.

As described above, according to the second feature of the present invention, the cooling from the soaking temperature by gas jet cooling is reduced to 800°C or less.
The reason why the temperature is higher than 800°C is that if the temperature at which static hot water immersion cooling starts exceeds 800°C, the shape of the steel strip will be broken.
This is because if the GJC is used to cool the CCT to a temperature lower than 0° C., the alloy saving effect that has been achieved by cutting the nose of the CCT will be lost.

Embodiments of the present invention will be described in detail below with reference to the drawings.

Figure 4 shows the original F! This shows the entire continuous annealing equipment for performing Aft, where 1 is an unwinding machine, 2 is a welding machine, 3 is a cleaning device, 4 is an entrance looper, 5 is a heating zone, 6 is a soaking zone,
7 is a primary cooling device which constitutes the main body of the present invention, and 1
1 is an exit looper, 12 is a temper rolling mill, 13 is an inspection and finishing section, 14 is a shearing machine, and 15 is a winding machine.

In FIG. 4, the fact that the entire furnace is a vertical furnace constructed entirely of vertical paths indicates that this line is connected to a high-capacity, high-speed line from the point of view of space saving.

Next, an embodiment of the primary cooling device 7 shown in FIG. 4 is shown in FIG. This secondary cooling system basically consists of two
In figure 0, 21 and 211 are the first and second hot water immersion tanks, respectively, and 22 and 221 are the steel strips SYI.
: Equipment that rotates while dipping (e.g. zinc roll),
23 is an underwater jet header installed on the downstream path of the second immersion tank 21&. The aqueous bath W in the first immersion tank 21 is maintained at approximately the boiling point, and the immersion length of the steel strip S is adjusted by raising and lowering the bath surface or by raising and lowering the turning device 22, making it possible to control the end point. The aqueous bath V of the second immersion tank 21m is usually, for example, 70 m.
℃ more specifically 60℃ or higher, preferably 75℃ or higher
The bath temperature is maintained at the bath temperature, and in some cases, the bath surface L'
Lower it to the bottom of the turning device 22a and pass the copper strip St1 through the air.
can do. The method of using this device is as follows. That is, after the first-stage cooling is performed using the stationary boiling bath wt- in the first tank 21, the second-stage cooling is performed in the second tank 21m using an aqueous bath W' (the bath temperature is around 70°C using ice and heated water) using the submersible header 23. This is done by jet cooling.The inflection point of this two-stage cooling is
Selected for metallurgical reasons or the need to secure shape,
The temperature is usually in the range of 450 to 710°C, but in order to save alloy, the higher the temperature, the better, and preferably 550°C or higher.

When the soaking temperature is high (for example, 800° C. or higher), a primary cooling zone 6& is installed between the soaking zone 6 and the secondary cooling device 70 in accordance with the second feature of the present invention to ensure the shape. .

Another embodiment of the apparatus for carrying out the invention is shown in FIG.

Basically, it consists of multiple hot water immersion tanks connected to each other.
121m and 21b are hot water immersion cooling tanks, 211! is a hot water jet cooling tank with an underwater jet header 23. (In this example, there is one hot water jet cooling tank, but basically there may be a plurality of hot water jet cooling tanks.) By having a plurality of hot water immersion cooling tanks and hot water jet cooling tanks, respectively, it can be applied to a large capacity line.

Another feature of the embodiment shown in FIG. 6 is that there is no "pneumatic section", i.e. the species are in close contact with each other, separated only by partition walls 26, and The upper turning roll (deflector roll) 27 has only one roller between the seeds, and has another μm roll 28 on the upper part of itself to form a pinch roll-like shape, and the steel strip S is shaped like a bathtub. It is designed to be continuously cooled underwater without being exposed to the air. This eliminates the need to shut off the outside air during cooling p1, and also eliminates the "shelf" on the cooling curve that would substantially reduce the cooling speed v.

In addition, to explain the goo to the underwater jet, hot water is supplied to the underwater jet header 23 by drawing out, for example, hot water in the hot water immersion tank 21 by a jet water supply pump f25. In other words, since it is an internal circulation jet system,
Just by operating the jet system, the bath surface water level and bath temperature do not change, allowing stable operation to continue.

In addition, the schematic system diagram for adjusting the bath surface water level in Figure 6 is shown in Figure 7.
As shown in the figure. 21 is a hot water immersion tank, 31 is a reserve tank located below the hot water immersion tank 21, and 32 is a hot water immersion tank 2.
A head tank is located above 1, 33 is a water pump, and 34 and 35 are valves.

When raising the bath surface water level Lt of the hot water immersion tank 21, the valve 34' is opened and the water in the bath 21 is quickly dropped into the reserve tank 31 by gravity. The water dropped into the reserve tank 31 is raised to the head tank 32 through the bonder 33 while the hot water immersion tank 21 is operating at a low water level.
When raising the water level Lt of 1, the valve 35t is opened and the water in the head tank 32 is quickly dropped into the hot water immersion tank 24 by gravity again.

By using this method of gravity, the water level at the bath surface can be controlled.

In a hot water jet cooling tank, when cooling a high-tensile cold-rolled steel strip, it is necessary to maintain the bath temperature tm at a constant temperature of, for example, 60° C. or higher and 75° C. or lower, as described above.

A specific method for adjusting the bath temperature is, for example, an indirect cooling method as shown in FIG.

21m is a hot water immersion tank (second tank shown in Figure 5), 41 is a circulation bonder, 42 is a heat converter, 43 is an external cooling water system, 4
4 is a three-way valve, and 45 is a bath temperature detector. Hot water immersion tank 21
The water in a is sucked out by the bonder 41 and cooled by external cooling water 43 in the heat exchanger 42, and then returned to the hot water immersion tank 21.
Return to m. At this time, the bath temperature is detected by the bath temperature detector 45 and the three-way valve 44 is adjusted to control the amount of external cooling water going to the heat exchanger 42. Thereby, the bath V discharges the heat brought in by the steel strip S to the outside, and can maintain a constant bath temperature.

FIG. 9 shows an equipment arrangement when initial slow cooling is required for primary cooling according to the second feature of the invention.

21 is a hot water immersion tank, and 51 is a gas jet cooler. After being slowly cooled by the gas jet cooler 51, the steel strip S is cooled at a required cooling rate in the hot water immersion tank 21.

At that time, it is necessary to cut off the outside air until the steel strip S is cooled in the bath W in the hot water immersion tank 21, so the steel strip S is installed in a hood 521 whose tip is submerged in the bath W, and then the hot water Steam generated in the immersion tank 21 is sent to the gas cooler 5.
A throat 53 is installed to prevent the flow from flowing backwards toward the flow direction.

As is clear from the above description, by skillfully combining static boiling bath immersion cooling and hot water jet immersion cooling according to the present invention, extremely rapid cooling can be achieved, and in the cooling process, inflection This enables two-stage cooling with variable point temperature. This makes it possible to produce high-strength cold-rolled steel strip with excellent properties at a low manufacturing cost and with a low investment cost. Furthermore, its flexible annealing cycle provides high potential for new product development in the future.

Below, the effects of the present invention will be explained in detail in comparison with conventional methods. As conventional methods, the GJC method and the WQ method are adopted.

First, a comparative evaluation of product quality will be made based on specific examples.

EXAMPLE In this example, the objective was to manufacture a high-strength cold-rolled steel strip having a tensile strength of 60-2 class and having a two-phase structure and good workability. The basic components of the material are c: 0.079, Si: 0.
．． 58 regrets, p:o, ots regrets.

S: 0.009, At: 0.0611# N:
0.0052 Sorry, specified tensile strength 60#/l1l
In order to increase the t'', the Mu content was changed as shown in Table ka11 depending on the continuous annealing method.

These steels were hot-rolled to a thickness of 2.31111 at a finishing temperature of 890°C and a winding temperature of 610°C, and after pickling, cold-rolled to a thickness of 0.7mm, and then subjected to the continuous annealing method of the present invention and the conventional method. The product was made by continuous annealing using the continuous annealing method.

The annealing cycle pattern at that time is as shown in g2mEl, where a is for the present invention, b is for the GJC method, and b2
corresponds to the WQ method.

The results are shown in Table 1 together with the continuous annealing conditions.The bath temperature of the immersion tank in the second cooling was 98C1 in the first tank.
Similarly, the second tank was heated to 68°C, and the conventional method-2 (WQ method) was heated to about 40°C.

As is clear from Table 1, the continuous annealing method of the present invention achieves extremely rapid cooling compared to conventional method-2 (WQ method), so the required amount of alloy Mn is slightly higher. However, there is no reheating and tempering process, and
It can be seen that a high-strength cold-rolled steel strip with excellent workability and excellent workability can be produced in terms of material.

Furthermore, when compared with Conventional Method-1 (GJC method), it can be seen that the material alloy components are significantly reduced, especially since the cooling rate in the latter stage is rapid.

Next, based on the above examples, a comprehensive comparative evaluation will be attempted between the method according to the present invention and the conventional methods (GJC method and WQ method).

Table 2 shows a comparative evaluation of the above methods regarding product materials, manufacturing costs, and equipment costs. The details of Table 2 are as follows. That is, 1) G-product material: In the method of the present invention, there is no conventional method-21'Q method) and i4 p-reheating after air quenching-tempering (see Examples), so conventional method-1C
(GJC method) A product with similar excellent workability can be obtained.

2) 14 products, manufacturing cost: As seen in the examples, in the present invention, conventional method-1 (GJC method)
Since the amount of alloy components in the material is reduced compared to the previous method, the material cost and, ultimately, the product manufacturing cost can be reduced. Conventional method-2 (WQ
Compared to the method (method), the present invention does not require reheating, so the cost of fertilizers is reduced; however, depending on the product grade, as shown in Table 1, a slightly longer material alloy content is required, which increases the material cost. There is.

3) Capital investment amount: Main F! The required investment amount for continuous annealing equipment for carrying out step 4 is considerably cheaper than conventional method-2 (WQ method), which does not require a relatively expensive reheating zone.

As can be seen from the above explanation, the present invention provides a useful gross process that is superior to conventional methods (GJC method, WQ method) in terms of product characteristics, product manufacturing cost, and required capital investment for high-strength cold-rolled steel strip. [This is what we provide.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of an annealing and cooling cycle for cold-rolled steel strip for processing.
Figure 2 is a schematic diagram of a heat treatment cycle for high-strength cold-rolled steel strips, Figure 3 is a continuous cooling curve diagram for continuous heat treatment of high-strength cold-rolled steel strips, and Figure 4 includes continuous heat treatment equipment of the present invention. An overall view of the cold rolled steel strip continuous processing equipment, FIG. 5 is a detailed view of the primary cooling device 7 in FIG. 4, fs6 is a diagram of another embodiment of the primary cooling device 7 in FIG. 4, and FIG. 7 is a cooling FIG. 8 is a schematic diagram of a water level control device for the bath surface, FIG. 8 is a schematic diagram of a temperature control device for a primary cooling tank, and FIG. 9 is a schematic diagram of a primary cooling device equipped with a gas jet cooler. Figure 1. Mathematics 2 Figure 5 Kuni g Figure 7

Claims (2)

[Claims] (1) In a continuous heat treatment method for high-strength cold-rolled steel strip consisting of heating, soaking, secondary cooling, and post-treatment steps, the copper strip after soaking is heated to a temperature of %A, 3 or higher than the transformation point of M e1. , immersed in still hot water of 90°C or higher as the first stage of cooling,
20'C/4H up to temperature f below 710℃ and above 550℃
! - or above, 9 Q 'Q7hag, cool at a cooling rate of below, and as a second step, perform jet cooling in hot water of 60°C or above and 75°C or below, 100Q- or above, 500! - Continuous heat treatment method for high-strength cold-rolled steel strip characterized by cooling to a temperature of 200°C or less at the following cooling rate: (2) In a continuous heat treatment method for high tensile strength cold rolled steel strip consisting of heating, soaking, secondary cooling, and post-treatment steps, the temperature is higher than the Ae1 transformation point. 5 After soaking to a temperature below the transformation point, the OS zone is heated to a temperature of 800°C or lower and 600°C or higher before starting the primary cooling.5! -After slow cooling by gas jet cooling at a cooling rate of -30φ or less, the first step of the next cooling is immersion in still hot water of 90C or more, and the Cool at a cooling rate of 1 or more and 9QcS or less, and as a second step, perform jet cooling in hot water of 60°C or more and 75°C or less to achieve a cooling rate of 100'Cy4mc or more5 Q Q
A method for continuous heat treatment of high-strength cold-rolled steel strip, which aims at cooling at a temperature tLt of 200° C. or less at a cooling rate of `Cym or less.