EP3395964A1

EP3395964A1 - Method and device for manufacturing martensite-containing steel sheet

Info

Publication number: EP3395964A1
Application number: EP16879266.1A
Authority: EP
Inventors: In-Suk Han
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2015-12-21
Filing date: 2016-12-16
Publication date: 2018-10-31
Also published as: JP2019505667A; KR20170089045A; EP3395964A4; CN108431249A; WO2017111399A1

Abstract

The present invention relates to a method and a device for manufacturing a martensite-containing steel sheet, the method comprising the steps of: preparing, in a coil shape, a steel sheet in which at least one part of austenite is transformed into martensite during water cooling; heating the steel sheet coil such that at least one part of the microstructure of the steel sheet becomes austenite; and unwinding the heated steel sheet coil, and water-cooling the steel sheet at a cooling speed of 500°C/sec or more while transferring the steel sheet at a transferring speed of 300 mpm or more, so as to transform the at least one part of austenite into the martensite.

Description

[Technical Field]

The present disclosure relates to a method and a device for manufacturing a martensite-containing steel sheet used for automobile structural steel, and more particularly, to a method and a device for manufacturing a martensite-containing steel sheet via batchwise heating and continuous cooling.

[Background Art]

Demand for high strength automotive structural steel is increasing, in order to satisfy requirements for high fuel efficiency and strict safety regulations through lightweight components.
Generally, alloying elements such as nonferrous metals, rare earth elements or the like may be added to steel to produce high strength steel.
However, when alloying elements are added, a problem in which manufacturing costs increase and the manufacturing efficiency and product processability, for example, weldability, are deteriorated, may occur.
Thus, attempts to improve the strength of steel have been made via rapid cooling in heat treatment processes, while significantly reducing alloying components.
As an example of such a rapid cooling method, there may be a method using quenching.
The quenching cooling method may generally be a method in which a steel sheet is cooled at a cooling rate of, for example, 1000°C/second in the case of a 1 mm steel sheet.
In the quenching and cooling process, a quenching cooling bath storing cooling water may be used.
The cooling bath may include a cooler for maintaining cooling water at a constant temperature and a jet injector for increasing a cooling rate. As such, a steel strip passes through a cooling bath, a quenching device, to obtain a desired degree of cooling.
However, there may be a problem in which, when a high-temperature steel sheet is quenched in the existing quenching-cooling device, the shape of a steel sheet may be deteriorated.
For example, in a heating zone, a steel sheet may be heated to a temperature of 800°C or higher, and when the steel sheet is quenched, the steel sheet may be rapidly cooled to room temperature in one second. In this case, in the steel sheet, a relatively high temperature difference may occur in longitudinal and width directions, and high thermal stress may occur, thereby causing deformation of the steel sheet.
To solve this problem, there have been various methods to increase tensile strength of a steel sheet, to adjust a water temperature of cooling water, to lower the temperature of a steel sheet at the start of quenching and to add an alloying component to prevent the deformation of ferrite, and the like, to obtain a flat steel sheet in a quenching process.
However, there is a limit to obtaining good flatness by quenching a high-temperature steel sheet through the related art techniques.
On the other hand, there are a batch type and a continuous type processes in a process of heat treating a steel sheet. In the batch type process, in which a steel sheet is heated and cooled in a coil state, a cooling rate is relatively slow. In the continuous type process, in which the steel sheet is heated and cooled while moving, there is a feature that heat treatment equipment may be relatively large.
Generally, the continuous type process may be advantageous for mass production, and the batch type process may be advantageous for small-scale production, as compared with that of the continuous type.

[Disclosure]

[Technical Problem]

An aspect of the present disclosure is to provide a method of manufacturing a martensite-containing steel sheet in which shape transformation is reduced via heating in a batch manner and via continuous cooling, economically.
Another aspect of the present disclosure is to provide a device for manufacturing a martensite-containing steel sheet in which shape transformation is reduced via heating in a batch manner and via continuous cooling, economically.

[Technical Solution]

According to an aspect of the present disclosure, a method of manufacturing a martensite-containing steel sheet includes preparing, in coil form, a steel sheet in which at least a portion of austenite is transformed into martensite during water cooling; heating the steel sheet having the coil form such that at least a portion of a microstructure of the steel sheet becomes austenite; and unwinding the heated steel sheet having the coil form, and water-cooling the steel sheet at a cooling rate of 500°C/sec or more while transferring the steel sheet at a transferring speed of 300 mpm or more, to transform the at least a portion of the austenite into the martensite.
A water-cooling termination temperature during the water cooling may be 150 °C or higher, a temperature equal to or lower than a martensitic transformation start temperature (Ms).
According to an aspect of the present disclosure, a device for manufacturing a martensite-containing steel sheet includes a heating furnace heating a steel sheet in the form of a coil; a pay-off reel unwinding a heated coil; a cooling device cooling a heated steel sheet transferred by being unwound from the coil; and a coiler winding a cooled steel sheet.
The cooling device may be configured to provide a cooling rate of 500°C/sec or more when cooling the steel sheet.
The cooling device may include a cooling bath in which a cooling liquid cooling the heated steel sheet is stored, and a cooling member configured to jet the cooling liquid toward a steel strip.
Between the pay-off reel and the cooling device, and between the cooling device and the coiler, a guide roller is provided to guide the steel sheet.

[Advantageous Effects]

According to an aspect of the present disclosure, facility costs may be significantly reduced by satisfying a cooling rate required for manufacturing martensite steel while reducing the size of equipment. Further, by separating heating and cooling, a transfer speed of a steel strip may increase in a cooling process, such that a change in a longitudinal temperature gradient of the steel strip may be significantly decreased even at a high cooling rate of 500 °C/sec or more, thereby decreasing thermal stress to significantly reduce shape deformation of the steel strip.

[Description of Drawings]

FIG. 1 is a schematic view illustrating an example of a device for manufacturing a martensite-containing steel sheet according to an exemplary embodiment in the present disclosure.

[Best Mode for Invention]

A method and a device for manufacturing a martensite-containing steel sheet are provided, which are effective in terms of facility investment and operation, and in which shape deformation of a steel strip may also be significantly reduced by dispersing and significantly decreasing concentration of thermal stress in a longitudinal direction of a steel sheet during rapid cooling of the steel sheet.
According to an exemplary embodiment in the present disclosure, a steel sheet, in which at least a portion of austenite is transformed into martensite during water cooling, is prepared in the form of a coil, to produce a martensite-containing steel sheet.
As the steel sheet, a steel sheet obtained as at least a portion of a microstructure, becomes austenite during heating and at least a portion of the austenite is transformed into martensite during water cooling may be used, and is not limited thereto.
As the steel sheet which may be appropriately used in the present disclosure, a martensitic steel sheet, a dual-phase steel sheet, a complex-phase steel sheet or the like may be used.
As an example of the steel sheet, a steel sheet including 0.05 to 0.4 wt% of carbon, 2 wt% or less of silicon, 3 wt% or less (including 0%) of manganese, 0.05 wt% or less of phosphorus, 0.02 wt% or less of sulfur, and Fe and impurities as a remainder thereof, may be used.
The steel sheet is not particularly limited, and may be, for example, a hot-rolled steel sheet or a cold-rolled steel sheet.
Next, the steel sheet coil prepared as described above may be heated such that at least a portion of the microstructure of the steel sheet becomes austenite.
Heating conditions of the steel sheet may be appropriately controlled depending on a steel sheet to be produced.
For example, heating may be performed in such a manner that the entirety of the microstructure of the steel sheet may become austenite, to transform the entirety of the microstructure into martensite.
When the steel sheet is heated, a heating temperature may be 700 to 900°C, and a heating atmosphere may be a reducing atmosphere. The reducing atmosphere may be an atmosphere of 90 to 98% by volume of nitrogen and 2 to 10% by volume of hydrogen. A heating method is not particularly limited. For example, examples of the heating method may include methods using a radiant tube provided as a radiant heating furnace or a muffle-type heating furnace.
The heating time may be 20 to 80 hours, for example, when a coil width is within a range of 600 to 1400 mm.
The steel sheet coil heated as described above may be unwound and may be water-cooled at a cooling rate of 500°C/sec or more while being transferred at a transfer speed of 300 mpm or more, to transform at least a portion of the austenite into martensite, thereby manufacturing a martensite-containing steel sheet.
For example, when the steel sheet has a transfer speed of less than 300 mpm, there may be a problem in which a temperature gradient and a decrease in thermal stress are insufficient as compared with those in a continuous annealing method of the related art, and the effect of improving the shape is relatively low.
The transfer speed of the steel sheet may further be increased. On the other hand, when the transfer speed of the steel sheet is too fast, there may be difficulty in controlling warpage and facilities. Thus, an upper limit thereof may be limited to 800 mpm.
In further detail, the transfer speed of the steel sheet may be within 400 to 600 mpm.
For example, if the cooling rate of the steel sheet is less than 500°C/sec, a cooling deviation may increase in a film boiling region and a shape of the steel sheet may thus deteriorate.
On the other hand, if the cooling rate is too high, the temperature gradient and the thermal stress may increase and the steel sheet shape may deteriorate. Thus, an upper limit thereof may be limited to 2000°C/sec. In detail, the cooling rate of the steel sheet may be 500 to 1500°C/sec. In further detail, the cooling rate of the steel sheet may be 800 to 1200°C/sec.
In water-cooling, a water-cooling termination temperature is not particularly limited as long as it is equal to or lower than a martensitic transformation start temperature (Ms), and may be, for example, room temperature.
In the case of self-tempering, the water-cooling termination temperature may be 150°C or more, a Ms (martensitic transformation start temperature) or lower.
As described above, for example, when the water-cooling termination temperature in water-cooling is Ms or less, 150°C or more, self-tempering may be performed to improve an elongation rate without further tempering, and to evaporate remaining water on a surface of a steel strip, thereby preventing formation of scale.
Hereinafter, an example of a device for manufacturing a martensite-containing steel sheet according to an exemplary embodiment in the present disclosure will be described with reference to FIG. 1.
As illustrated in FIG. 1, a device 100 for manufacturing a martensite-containing steel sheet according to an exemplary embodiment may include a heating furnace 1 heating a steel sheet in the form of a coil 10; a pay-off reel 2 unwinding the heated coil 10; a cooling device 3 cooling a heated steel sheet unwound from the coil to be transferred therefrom; and a coiler 4 winding the cooled steel sheet.
The cooling device 3 may include a cooling bath 31 in which a cooling liquid is stored to cool the heated steel sheet, and a cooling member 32 provided to jet the cooling liquid toward the steel strip.
The cooling device 3 may be configured to provide a cooling rate of 500°C/sec or more when cooling the steel sheet.
The device for manufacturing a martensite-containing steel sheet may include guide rollers 5a and 5b guiding the steel sheet and disposed between the pay-off reel 2 and the cooling device 3 and between the cooling device 3 and the coiler 4, respectively.
In FIG. 1, reference numeral 6 denotes a transfer truck, reference numeral 11 denotes a heating furnace door, and reference numeral 33 denotes a cooling tub roller.
A method of manufacturing a martensite-containing steel sheet using the device 100 for manufacturing a martensite-containing steel sheet of FIG. 1 will be described.
First, the coil 10 for a steel sheet, to be heated, may be located on a transfer truck 6 using a crane or the like.
The transfer truck 6 carrying the coil may move to the heating furnace 1. When the coil 10 is inserted into the pay-off reel 2, the transfer truck 6 may emerge from the heating furnace 1.
Thereafter, the steel sheet may be connected to the guide roller 5a, the cooling tub roller 33, the guide roller 5b and the coiler 4. Alternatively, a steel sheet of the coil 10 newly-inserted into the pay-off reel 2 may be welded to be connected to the already connected steel sheet.
Then, the heating furnace door 11 may be closed, to heat the coil 10.
For example, a heating temperature may be 700 to 900°C, and the interior of the heating furnace may have 90 to 98% by volume of nitrogen and 2 to 10% by volume of hydrogen in a reducing atmosphere.
The heating furnace 1 is not particularly limited, and for example, a radiant tube provided as a radiant heating furnace, a muffle-type heating furnace, or the like may be used.
A heating period of time may be 20 to 80 hours when a coil width is 600 to 1400 mm.
When a required temperature reaches the interior of the coil, the pay-off reel 2 and the coiler 4 may be operated, such that the steel sheet passes through the cooling bath 31 of the cooling device 3. The cooling bath 31 may be filled with water as a refrigerant for cooling. Further, the cooling member 32 may be provided in the cooling bath, to spray a coolant on front and rear surfaces of the steel strip, to cool the steel sheet at a rate of 500°C/sec or more.
The transfer speed of the steel sheet during cooling may be 300mpm or more, 1.5 times faster than 200 mpm, a transfer speed in a normal continuous annealing process, and in detail, may be 400 to 600 mpm, 2 to 3 times faster than 200 mpm.
As a result, in the case of a general continuous heat treatment process, for example, a temperature gradient of a steel strip is 300°C/meter in quenching, rapid cooling, but according to an exemplary embodiment in the present disclosure, a temperature gradient of a steel strip may decrease to 150 to 100°C/meter, and thus, thermal stress may also decrease. Numerical results indicate that the thermal stress decreases to one-third (1/3) when the speed is doubled. Thus, even in the case in which the cooling rate is the same as a cooling rate of 1000°C/sec in a general continuous annealing quenching process, the shape of the steel sheet may be good. On the other hand, since the transfer speed of the steel strip is relatively fast during cooling, time taken to cool the entirety of a coil may be as short as about 4 minutes.

Claims

A method of manufacturing a martensite-containing steel sheet, the method comprising:
preparing, in coil form, a steel sheet in which at least a portion of austenite is transformed into martensite during water cooling;

heating the steel sheet having the coil form such that at least a portion of a microstructure of the steel sheet becomes austenite; and

unwinding the heated steel sheet having the coil form, and water-cooling the steel sheet at a cooling rate of 500°C/sec or more while transferring the steel sheet at a transfer speed of 300 mpm or more, to transform the at least a portion of the austenite into the martensite.
The method of claim 1, wherein the transfer speed of the steel sheet is 300 mpm to 800 mpm.
The method of claim 1 or 2, wherein a water-cooling termination temperature during the water cooling is equal to or lower than a martensitic transformation start temperature (Ms) .
The method of claim 1 or 2, wherein the water-cooling termination temperature during the water cooling is 150°C or higher, a temperature equal to or lower than a martensitic transformation start temperature (Ms).
The method of claim 1, wherein the cooling rate of the steel sheet is 800°C/sec to 1200°C/sec.
The method of claim 1, wherein the steel sheet is one of a martensitic steel sheet, a dual-phase steel sheet and a complex-phase steel sheet.
The method of claim 1, wherein the steel sheet comprises 0.05 to 0.4% by weight of carbon, 2% by weight or less of silicon, 3% by weight or less (including 0%) of manganese, 0.05 % by weight or less of phosphorus, 0.02 % by weight or less of sulfur, the balance Fe and other impurities.
A device for manufacturing a martensite-containing steel sheet, the device comprising:
a heating furnace heating a steel sheet in the form of a coil;

a pay-off reel unwinding a heated coil;

a cooling device cooling a heated steel sheet transferred by being unwound from the coil; and

a coiler winding a cooled steel sheet.
The device of claim 8, wherein the cooling device is configured to provide a cooling rate of 500°C/sec or more when cooling the steel sheet.
The device of claim 8, wherein the cooling device comprises a cooling bath in which a cooling liquid cooling the heated steel sheet is stored, and a cooling member configured to jet the cooling liquid toward a steel strip.
The device of claim 8, wherein between the pay-off reel and the cooling device, and between the cooling device and the coiler, a guide roller is provided to guide the steel sheet.