KR20140127570A - Manufacturing method of transformation induced plasticity steel with excellent coatability and coating adhesion - Google Patents

Abstract

Disclosed are a TRIP steel capable of ensuring excellent coatability and coating adhesion by changing a temperature rising pattern at the time of annealing heat treatment process, and a method for manufacturing the same. According to the present invention, the method for manufacturing TRIP steel includes: a hot-rolling step for forming hot rolled steel by reheating, hot-rolling, and coiling a steel slab containing silicon (Si) and manganese (Mn); a cold-rolling step for cold-rolling the hot-rolled steel after pickling; an annealing heat treatment step for annealing heat treating the cold-rolled steel up to a temperature of 750-850°C after heating the cold-rolled steel in a radiant tub heating furnace up to a temperature of 450-650°C and maintaining the heated steel for 50-300 seconds in a section of maintaining at a temperature of 450-650°C; and a galvanizing step for galvanizing the annealing heat-treated steel at a temperature of 440-480°C.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a TRIP steel having excellent plating and plating adhesion,

The present invention relates to a TRIP steel manufacturing technology, and more particularly, to a TRIP steel manufacturing method capable of securing an excellent plating property and a plating adhesion property by changing a temperature rising pattern in an annealing heat treatment process.

Transformation Induced Plasticity (TRIP) steel is manufactured through a series of processes including a hot rolling process, a cold rolling process and an annealing process.

In particular, TRIP steels are produced by forming austenite in an annealing process, partially retaining austenite at room temperature through cooling control, and transforming the retained austenite to martensite during plastic deformation.

As described above, since the process of transforming the residual austenite into martensite during the plastic deformation is included in the TRIP steel manufacturing process, the stress concentration is relaxed and the ductility can be improved. Therefore, the produced TRIP steel is advantageous in strength and ductility at the same time.

The TRIP steel generally contains silicon (Si) in an amount of 0.5 wt% or more. Therefore, oxidizing elements such as Si and Mn are selectively oxidized and oxidized on the surface to form an oxide film on the surface, which has a problem of deteriorating the plating quality due to the oxide film.

In order to solve this problem, a conventional method has been used in which a preliminary oxide is formed on a surface of a furnace by controlling the air-fuel ratio and a surface oxide of Si and Mn is suppressed by a pre-oxide. However, since it is very difficult to precisely control the air-fuel ratio in the direct-burning furnace, if the air-fuel ratio is high, the preliminary oxide film becomes too thick so that there is residual oxide that has not been reduced in the indirect heating furnace in the reducing atmosphere. Soot is deposited on the surface of the substrate.

A related prior art document is Korean Patent Laid-Open Publication No. 10-2013-0026131 (published on Mar. 13, 2013), which discloses a hot-dip galvanized steel sheet and a manufacturing method thereof.

An object of the present invention is to provide a method of manufacturing a TRIP steel capable of securing an excellent plating property and a plating adhesion property through a change in a temperature rising pattern in an annealing heat treatment step.

In order to accomplish the above object, a TRIP steel manufacturing method according to an embodiment of the present invention includes a hot rolling step of reheating, hot rolling and winding a steel slab containing silicon (Si) and manganese (Mn) to form hot rolled steel; A cold rolling step of pickling the hot-rolled steel and cold-rolling the hot-rolled steel; The cold-rolled steel is heated in an indirect heating furnace (Radiant Tube Heating Furnace) to 450 to 650 ° C, maintained in the holding section maintained at 450 to 650 ° C for 50 to 300 seconds, and then heated to 750 to 850 ° C An annealing heat treatment step of heating and annealing; And a hot-dip galvanizing step of hot-dipping the annealed annealed steel at 440 to 480 ° C.

The TRIP steel manufacturing method according to the present invention strictly controls the annealing heat treatment atmosphere and is heated from 450 to 650 占 폚 in the initial heating zone and then heated to a temperature in the range of 750 to 850 占By performing the annealing heat treatment, it is possible to control the surface oxide form to be deformed into the granular Mn-based oxide film in the dense thin film-like Si oxide film, thereby securing the excellent plating property and the plating adhesion property.

1 is a process flow diagram schematically showing a method of manufacturing a TRIP steel according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and how to accomplish them, will become apparent with reference to the embodiments and drawings described in detail below. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims.

Hereinafter, a method for manufacturing a TRIP steel excellent in plating and plating adhesion according to the present invention will be described in detail.

1 is a process flow diagram schematically showing a method of manufacturing a TRIP steel according to an embodiment of the present invention.

Referring to FIG. 1, a TRIP steel manufacturing method according to an exemplary embodiment of the present invention includes a hot rolling step S110, a cold rolling step S120, a annealing heat treatment step S130, an oust tempering step S140, (S150) and an alloying heat treatment step (S160). At this time, in the method of manufacturing a TRIP steel according to an embodiment of the present invention, the tempering step (S140) and the annealing heat treatment step (S160) are not necessarily performed and may be omitted if necessary.

Hot rolling

In the hot rolling step (S110), the steel slab containing silicon (Si) and manganese (Mn) is reheated, hot rolled and wound to form hot rolled steel.

The steel slab in the semi-finished product state to be subjected to the hot rolling process in the method of manufacturing TRIP steel according to the present invention is not particularly limited, but is preferably 0.05 to 0.30% by weight, Si: 0.5 to 1.5% Mn, 1.0 to 3.0% P, 0.03% or less S, 0.005% or less of S, 0.1 to 1.0% of Al, 0.02 to 0.06% of Nb and 0.006% or less of N, .

Hereinafter, the content of each component contained in the TRIP steel according to the embodiment of the present invention and the reason for the addition will be described as follows.

Carbon (C) is added to ensure strength of the steel. In addition, the carbon serves to stabilize the austenite phase according to the amount that is concentrated in the austenite phase. The carbon is preferably added in a content ratio of 0.05 to 0.30% by weight of the total weight of the steel. When the content of carbon is less than 0.05 wt%, the second phase fraction in the TRIP steel according to the present invention is lowered and it is difficult to secure sufficient strength. On the other hand, if the content of carbon exceeds 0.30% by weight, the strength is increased but the weldability may be greatly reduced.

Silicon (Si) acts as a deoxidizer in the steel. Silicon also stabilizes the ferrite and contributes to strength. Silicon also serves to increase the ferrite fraction by promoting austenite-ferrite transformation. The silicon is preferably added in a content ratio of 0.5 to 1.5% by weight of the total weight of the steel. If the content of silicon is less than 0.5% by weight, the effect of adding silicon can not be obtained properly. On the contrary, when the addition amount of silicon exceeds 1.5% by weight, an oxide such as Mn ₂ SiO ₄ is formed on the surface in the annealing step despite the rapid heating of the steel sheet, thereby deteriorating the plating property.

Manganese (Mn) contributes to the improvement of the strength of the steel through solid solution strengthening and penetration. In addition, manganese stabilizes austenite to facilitate the formation of retained austenite and martensite. The manganese is preferably added in a content ratio of 1.0 to 3.0 wt% of the total weight of the steel. When the content of manganese is less than 1.0% by weight, the effect of the addition is insufficient and it is difficult to secure the strength. On the other hand, when the addition amount of manganese exceeds 3.0 wt%, a manganese band structure is formed and the segregation increases sharply, which deteriorates the workability and weldability of the steel.

Phosphorus (P) contributes to improving the strength of steel by solid solution strengthening, but if it is contained excessively, it causes hot brittleness and deteriorates weldability. Therefore, in the present invention, the content of phosphorus is limited to 0.03% by weight or less of the steel total weight in consideration of the above points.

Sulfur (S) inhibits the toughness and weldability of steel and increases MnS nonmetallic inclusions in steel. Accordingly, in the present invention, the content of sulfur is limited to 0.005% by weight or less of the total weight of the steel in consideration of the above points.

Aluminum (Al) acts as deoxidizer with silicon to remove oxygen in steel to prevent cracking during slab manufacturing. In particular, aluminum forms an FeAl ₂ O ₄ oxide on the surface favorable for plating wettability. In addition, aluminum bonds with nitrogen (N) in the steel to form AlN to make the structure finer. In the present invention, in order to compensate for the decrease in the amount of silicon (Si) that reduces the plating ability by forming an oxide such as Mn ₂ SiO ₄ on the surface of the steel in the overdosage, the addition amount of aluminum (Al) By weight or more. However, if aluminum is added in an amount exceeding 1.0 wt% of the total weight of the steel, the heating and holding temperature in the annealing step after the cold rolling step becomes higher than the normal working temperature, which may hinder productivity. Therefore, in the present invention, the addition amount of aluminum is limited to 0.1 to 1.0 wt% of the total weight of the steel.

Niobium (Nb) forms a niobium-based carbonitride precipitate. The niobium-based carbonitride precipitates interfere with grain boundary growth during hot rolling and anomalous reverse annealing, and is refined when grain refinement is performed, thereby improving strength and ductility. In addition, niobium improves steel strength through solid solution strengthening in iron (Fe). The niobium is preferably added in a content ratio of 0.02 to 0.06% by weight of the total weight of the steel. When the content of niobium is less than 0.02% by weight, the effect of the addition is insufficient and it is difficult to expect improvement in strength and the like. On the contrary, when the content of niobium exceeds 0.06% by weight, there is a problem that workability and moldability are lowered.

Nitrogen (N) contributes to the formation of niobium carbonitride, but when contained in large amounts, the molten zinc has a problem in that it is supersaturated in the cooling process or the cooling process of the alloying process to lower the uniform elongation. Therefore, in the present invention, the content of nitrogen is limited to 0.006% by weight or less of the total weight of the steel.

At this time, the slab reheating is performed in order to reuse the segregated components in casting. In the slab reheating step, the steel slab having the above composition is preferably reheated to a slab reheating temperature (SRT) of 1150 to 1250 ° C. When the slab reheating temperature (SRT) is less than 1150 ° C, the segregated components can not be reused. On the other hand, when the slab reheating temperature (SRT) exceeds 1250 DEG C, the austenite grain size increases and the crystal grains of the ferrite are coarsened, so that it becomes difficult to secure the strength.

In the hot rolling step, the slab reheated steel is finely hot-rolled under FDT (Final Delivery Temperature): 850 to 950 ° C. At this time, when the final hot rolling temperature (FDT) is less than 850 DEG C, excessive electric potential is introduced into the ferrite during hot rolling, and coarse grains may be formed on the surface of the steel during winding. On the other hand, when the finish hot rolling temperature (FDT) exceeds 950 占 폚, the size of the ferrite grains increases and the strength can be reduced.

In the winding step, finishing hot rolled steel is cooled to a CT (Coiling Temperature): 550 to 650 ° C and wound. At this time, when the coiling temperature (CT) exceeds 650 DEG C, manganese, silicon, etc. may be segregated. On the other hand, if the coiling temperature (CT) is lower than 550 占 폚, there is a problem that ductility is lowered and workability is hardly secured.

Cold rolling

In the cold rolling step (S120), the hot-rolled steel is pickled and cold-rolled.

At this time, it is preferable that the cold reduction ratio is 60 to 80%. If the cold rolling reduction is less than 60%, there is a problem that the amount of annealed recrystallized nuclei is small, so that the crystal grains are excessively grown during the annealing heat treatment and the strength is rapidly lowered. On the other hand, when the cold rolling reduction is more than 80%, the amount of nucleation becomes too large, so that the annealing grains are rather too fine to reduce the ductility and deteriorate the formability.

Annealing heat treatment

In the annealing heat treatment step (S130), the cold-rolled steel is heated in an indirect heating furnace (Direct Fired Furnace) to 450 to 650 ° C, maintained in a holding section maintained at 450 to 650 ° C for 50 to 300 seconds, To 850 캜 and annealing is performed.

In particular, in the present invention, the annealing heat treatment is carried out in a holding section maintained at a temperature of 450 to 650 ° C corresponding to a low temperature region for 50 to 300 seconds after heating to 450 to 650 ° C, And heating is performed.

In the conventional TRIP steel manufacturing process, the annealing heat treatment is performed in the furnace at a temperature of about 700 DEG C or higher for annealing heat treatment. In this case, the Si-based oxide, Si-Mn oxide, That is, among the oxidizing elements such as Si and Mn, there is a problem that Si mainly diffuses to the surface predominantly at a high temperature of 700 ° C or higher to generate a Si-based oxide, thereby deteriorating the plating characteristics. Particularly, in a high tensile steel to which Si is added in an amount of 0.5 wt% or more, a Si-rich oxide film having a dense film shape is generated, which causes a drastic decrease in the plating ability.

On the other hand, in the present invention, the particles are heated to 450 to 650 ° C. in an indirect heating furnace (Radiant Tube Heating Furnace) and then maintained in a holding section maintained at 450 to 650 ° C. for 50 to 300 seconds, So that the plating adhesion can be further improved. That is, as in the present invention, when a temperature rising period is maintained for a period of from 50 to 300 seconds, more preferably from 100 to 200 seconds, in a temperature range of 450 to 650 ° C. corresponding to a low temperature range, Mn is predominantly diffused The formation of a Si-rich oxide film can be avoided.

At this time, when the holding temperature in the holding section is less than 450 DEG C or the holding time in the holding section is less than 50 seconds, it may be difficult to exhibit the above effect properly. On the other hand, when the holding temperature in the holding section exceeds 650 ° C or the holding time in the holding section exceeds 300 seconds, not only does the manufacturing cost increase without further increase of the effect, but also causes generation of the Si- The generation of the Mn-rich oxide film in the form of particles is hindered, and there is a great possibility that the plating ability is hindered.

At this time, the dew point is preferably controlled to 0 to -40 占 폚, and the hydrogen concentration is preferably controlled to 5 to 15%. This is because if the dew point and water concentration are not controlled within the above range, plating adherence may be deteriorated even by an excessive amount of Mn oxide due to an oxidizing atmosphere.

On the other hand, the steel which has passed through the holding section is rapidly heated from 750 to 850 ° C for 5 to 200 seconds, and then cooled to the bainite transformation temperature at a rate of 10 to 70 ° C / s.

At this time, when the annealing temperature is less than 750 캜, the property values, particularly, the tensile strength are lowered, and when the annealing temperature exceeds 850 캜, productivity may be a problem. When the heating holding time is less than 5 seconds, the austenite phase is not sufficiently formed during the heating and holding, and it is difficult to control the fraction of the ferrite and the second phase. If the heating holding time exceeds 200 seconds, productivity is lowered.

In the annealing step, the ferrite transformation and the pearlite transformation can be avoided at an average cooling rate of 10 to 70 DEG C / sec from the annealing heat treatment temperature, and quenched to the bainite transformation temperature. At this time, when the cooling rate is less than 10 ° C / sec, pearlite is generated at the time of cooling, and residual austenite (γ) finally obtained is decreased. On the other hand, if the cooling rate exceeds 70 DEG C / sec, it is difficult to apply to a cooling method using roll quenching or gas jet.

The cooling rate is controlled to the bainite transformation temperature. In the present invention, the bainite transformation temperature range can be approximately 400 to 500 占 폚. When the control is terminated prematurely at a higher temperature than the bainite transformation temperature zone and thereafter cooled at a significantly slower rate, for example, residual? Is hardly generated and an excellent elongation can not be ensured. On the other hand, when cooling is carried out at the cooling rate to the lower temperature region than the bainite transformation temperature region, the amount of transformation of the residual? Into martensite is increased, so that it is difficult to ensure excellent elongation.

Austempering

In the oust tempering step (S140), the annealing heat treated steel is subjected to constant temperature transformation for 50 to 300 seconds at the bainite transformation temperature region. At this time, by maintaining the temperature at the bainite temperature for more than 50 seconds, the concentration of C in the residual? Can be efficiently advanced in a short time to obtain a stable large amount of residual?, And as a result, the TRIP effect due to the residual? have. On the other hand, if the temperature holding time exceeds 300 seconds, the TRIP effect due to the residual? Is not sufficiently exerted.

Hot-dip galvanizing

In the hot-dip galvanizing step (S150), hot-tempered steel is hot-dip galvanized. In this step, if the oust tempering step (S150) is omitted, the annealed annealed steel is hot-dip galvanized.

Hot dip galvanizing can be carried out by continuously immersing the annealed steel in a plating bath maintained at a temperature of 440 to 480 캜. At this time, when the plating temperature is lower than 440 DEG C, it is difficult to sufficiently coat the surface of the steel. Conversely, when the plating temperature exceeds 480 DEG C, the plating adhesion may be lowered.

Alloying heat treatment

In the alloying heat treatment step (S160), the steel in which the hot dip galvanized steel is reheated to a temperature of 490 to 540 占 폚, followed by alloying heat treatment, and then cooled to 200 to 300 占 폚 at a rate of 20 to 50 占 폚 / sec.

In this step, when the alloying heat treatment temperature is lower than 490 DEG C, stable growth of the hot-dip galvanized layer is difficult. On the other hand, if the alloying heat treatment temperature exceeds 540 占 폚, the plating adhesion may be lowered.

Further, the cooling performed after the alloying heat treatment can be carried out for securing the martensite fraction. When the cooling rate is less than 20 DEG C / sec or when the cooling termination temperature exceeds 300 DEG C, the martensite fraction securing effect is insignificant. On the other hand, when the cooling rate exceeds 50 DEG C / sec or the cooling end temperature is less than 200 DEG C, it is difficult to control the martensite fraction due to excessive quenching.

The TRIP steel produced in the above-described processes (S110 to S160) has a composite structure in which the final microstructure includes ferrite, retained austenite and a second phase, and the second phase contains at least one of bainite and martensite do.

In particular, the TRIP steel according to the present invention produced by the above-mentioned method has an unplated area of less than 0.07% and has a plating adhesion of 1 to 2, which indicates that the plating property and the plating adhesion are excellent.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense. The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Specimen Manufacturing

Specimens according to Examples 1 to 6 and Comparative Examples 1 to 3 were prepared with the compositions shown in Table 1 and the process conditions shown in Table 2. The specimens according to Examples 1 to 6 and Comparative Examples 1 to 3 were reheated in a steel slab having the composition shown in Table 1 at 1200 ° C for 2 hours and then subjected to finish hot rolling at 900 ° C, . Thereafter, pickling treatment was carried out and then cold rolling was carried out at a reduction ratio of 50%, and then each specimen was produced under the process conditions shown in Table 2. [

[Table 1] (unit:% by weight)

[Table 2]

2. Property evaluation

Table 3 shows the results of measuring the plating properties and the plating adhesion of the specimens according to Examples 1 to 6 and Comparative Examples 1 to 3.

(1) Plating property evaluation: The plating property evaluation was performed based on the measurement of the unplated area.

(2) Evaluation of plating adhesion: The plating adhesion was evaluated by the following criteria based on the degree of peeling of the plating layer during the taping test of the 0T-bending test and the bending test.

1 Class: No peeling

Class 2: Less than 5% exfoliation

Class 3: Less than 5 ~ 10% exfoliation

4 Rating: Less than 10 ~ 30% exfoliation

5 Rating: More than 30% exfoliation

[Table 3]

Referring to Tables 1 to 3, in the case of the specimens according to Examples 1 to 6, the uncoated area was measured at 0.03 to 0.06%, and the coating adhesion was measured at 1 to 2 grades.

On the other hand, in the case of the samples according to Comparative Examples 1 to 3, the uncoated area was measured at 0.7 to 0.9% and the coating adhesion was measured at 3 to 5 grades.

Compared with the comparative examples 1 to 3, the specimens according to Examples 1 to 6 were excellent in the plating ability and the plating adhesion property by changing the heat treatment temperature raising pattern so as to have sufficient holding time in the temperature rising section maintained at about 450 to 650 ° C, -rich oxide film is formed and the plating ability is improved.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Hot rolling step
S120: Cold rolling step
S130: annealing heat treatment step
S140: Osterming step
S150: Hot dip galvanizing step
S160: Alloying heat treatment step

Claims (4)

A hot rolling step of reheating, hot rolling and winding steel slabs containing silicon (Si) and manganese (Mn) to form hot rolled steel;
A cold rolling step of pickling the hot-rolled steel and cold-rolling the hot-rolled steel;
The cold-rolled steel is heated in an indirect heating furnace (Radiant Tube Heating Furnace) to 450 to 650 ° C, maintained in the holding section maintained at 450 to 650 ° C for 50 to 300 seconds, and then heated to 750 to 850 ° C An annealing heat treatment step of heating and annealing; And
And a hot dip galvanizing step of hot dip galvanizing the annealed annealed steel at 440 to 480 캜.
The method according to claim 1,
The steel
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.30% of C, 0.5 to 1.5% of Si, 1.0 to 3.0% of Mn, 0.03% or less of P, 0.005% or less of S, , N: 0.006 wt% or less, and the balance of Fe and unavoidable impurities.
The method according to claim 1,
In the annealing heat treatment step,
And waiting for 100 to 200 seconds in the holding period.
The method according to claim 1,
Between the annealing heat treatment step and the hot-dip galvanizing step,
Further comprising a tempering step of subjecting the annealed heat treated steel to a constant temperature transformation at a bainite transformation temperature range for 50 to 300 seconds.

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