KR101467051B1 - Method of manufacturing dual phase steel - Google Patents

Abstract

The present invention relates to a method for manufacturing a two-phase fine grain steel having a fine grain and excellent abrasion resistance.
According to the present invention, there is provided a method for manufacturing a two-phase high-strength steel, comprising the steps of: firstly annealing a steel subjected to hot rolling or cold rolling at an A3 transformation temperature or higher; Subjecting the annealed steel to Equal Channel Angular Pressing (ECAP) at a temperature lower than the A1 transformation temperature; And secondarily annealing the ECAP-treated steel at an Al transformation temperature or higher.

Description

METHOD OF MANUFACTURING DUAL PHASE STEEL [0002]

TECHNICAL FIELD The present invention relates to a method for manufacturing a two-phase straight steel (DP steel), and more particularly, to a method for manufacturing a DP steel having an ultra fine grain and excellent wear resistance by using ECAP (Equal Channel Angular Pressing) treatment.

In recent years, many researches have been conducted to improve the mechanical properties of structural materials such as grain refinement and alloying element addition.

Among them, the method of improving the mechanical properties by the addition of the alloying element has a disadvantage that it is difficult to reduce ductility and toughness, expensive manufacturing cost and recycling.

On the other hand, ultrafine fine grained (UFG) materials having a size of 1 탆 or less have great potential as next-generation structural materials. The reason for this is that as the grain size becomes very small, the strength and ductility at room temperature and the improved mechanical properties such as deformation and superplasticity at low temperatures can be obtained.

On the other hand, the two-phase straight steel is a steel in which martensite, which is the second phase, is dispersed in a soft ferrite base and has high ductility of ferrite and high strength of martensite. In addition, the two-phase steels have a low yield ratio, exhibit excellent formability, and have excellent mechanical properties such as excellent fatigue and impact resistance.

Therefore, if the two-phase straight steel is made of ultra fine grain, it can exhibit more excellent physical properties, particularly abrasion resistance.

As a background art related to the present invention, there is a high strength steel sheet having excellent resistance to brittleness characteristics disclosed in Korean Patent Laid-Open Publication No. 10-2011-0046689 (published on May 25, 2011) and a method for manufacturing the same.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a duplex stainless steel having excellent mechanical properties, particularly abrasion resistance, by using ECAP (Equal Channel Angular Pressing) treatment.

According to another aspect of the present invention, there is provided a method of manufacturing a two-phase high-strength steel according to the present invention, comprising the steps of: firstly annealing a steel subjected to hot rolling or cold rolling at an A3 transformation temperature or higher; Subjecting the annealed steel to Equal Channel Angular Pressing (ECAP) at a temperature lower than the A1 transformation temperature; And secondarily annealing the ECAP-treated steel at an Al transformation temperature or higher.

Wherein the steel contains 0.13 to 0.18 wt% of carbon (C), 0.3 to 0.6 wt% of manganese (Mn), 0 to 0.04 wt% of phosphorus (P) By weight or less and the balance of iron (Fe) and unavoidable impurities.

It is preferable that the secondary annealing is performed at the A1 transformation temperature to the A3 transformation temperature.

After the secondary annealing, the steel can be water-cooled to the martensite temperature range.

According to the method for producing a two-phase fine grain steel according to the present invention, after the hot rolling or the cold rolling, a two-phase structure having a fine grain of not more than 1 mu m in ferrite grain size through primary annealing, ECAP (Equal Channel Angular Pressing) Steel can be manufactured.

In the case of the two-phase structure steel produced, there is an advantage in that mechanical properties, particularly abrasion resistance, are excellent through the ultra fine grain.

FIG. 1 schematically shows a method of manufacturing a two-phase high-strength steel according to an embodiment of the present invention.
2 shows the microstructure of a cold-rolled specimen.
Fig. 3 shows the microstructure of the specimen heat-treated in accordance with Example 1. Fig.
Fig. 4 shows the microstructure of the specimen heat-treated in accordance with Comparative Example 1. Fig.
Fig. 5 shows the microstructure of the specimen heat-treated in accordance with Comparative Example 2. Fig.
6 shows the strain-stress curves of the heat-treated specimens according to Example 1, Comparative Example 2, and Comparative Example 3. Fig.
7 shows the abrasion rate of the specimens heat-treated in accordance with Example 1, Comparative Example 1, and Comparative Example 2 with regard to degradation.

The features of the present invention and the method for achieving the same will be apparent from the accompanying drawings and the embodiments described below. However, the present invention is not limited to the embodiments described below, but may be embodied in various forms. The present embodiments are provided so that the disclosure of the present invention is complete and that those skilled in the art will fully understand the scope of the present invention. The invention is only defined by the description of the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, with reference to the accompanying drawings, a method for manufacturing a two-quadrangular solid steel according to a preferred embodiment of the present invention will be described in detail.

FIG. 1 schematically shows a method of manufacturing a two-phase high-strength steel according to an embodiment of the present invention.

Referring to FIG. 1, the method for manufacturing a two-phase rectilinear steel according to the present invention includes a first annealing step (S110), an ECAP processing step (S120), and a second annealing step (S130).

First, in the first annealing step (S110), the hot-rolled or cold-rolled steel is annealed at a temperature not lower than the A3 transformation temperature for about 2 to 4 hours to homogenize the steel structure. After the annealing, air cooling or water cooling may be performed.

The steel contains 0.13 to 0.18 weight% of carbon (C), 0.3 to 0.6 weight% of manganese (Mn), 0.04 weight% or more of phosphorus (P) 0.05% by weight or less and the balance of iron (Fe) and unavoidable impurities.

Carbon (C) is an element contributing to strength. The carbon is preferably added in an amount of 0.13 to 0.18 wt% of the total weight of the steel. When the addition amount of carbon is less than 0.13% by weight, it is difficult to secure a sufficient martensite fraction. On the other hand, if the addition amount of carbon exceeds 0.18% by weight, toughness and weldability are deteriorated.

Manganese (Mn) is an element useful for improving strength without deteriorating toughness.

The manganese is preferably added in an amount of 0.3 to 0.6% by weight based on the total weight of the steel. When the addition amount of manganese is less than 0.3% by weight, the effect of addition thereof is insufficient. On the contrary, when the addition amount of manganese exceeds 0.6% by weight, the weldability is deteriorated.

When phosphorus (P) and sulfur (S) are contained in excess, it causes red embrittlement or significantly reduces toughness. The content of phosphorus (P) is limited to more than 0 wt% to 0.04 wt%, and the content of sulfur (S) is limited to 0 wt% to 0.05 wt% or less.

Next, in the Equal Channel Angular Pressing (ECAP) processing step (S120), the first annealed steel is put into the ECAP apparatus and subjected to the rigid processing. In the case of ECAP treatment, as is well known, shear deformation is applied to the steel, which can impose severe plastic deformation inside the steel without reducing the cross-sectional area.

At this time, in order to enhance the ECAP effect, it is preferable that the ECAP process is performed at a temperature lower than the A1 transformation temperature. After the ECAP treatment, water cooling or air cooling may be performed.

On the other hand, if the steel has a rectangular shape, it is preferable that the steel is subjected to ECAP treatment once, then rotated 90 °, and ECAP treatment is performed once again. In this case, the outer shape of the steel can be maintained for every even-numbered machining.

The steel can be ultra-fine-grained by ECAP treatment.

Next, in the second annealing step (S130), the ECAP-treated steel is secondarily annealed at a temperature higher than the A1 transformation temperature for about 10 minutes to 2 hours. After the second annealing, cooling is performed to the martensite temperature region. The cooling is preferably a water cooling method so that martensite can be formed while preventing undesired transformation to pearlite and the like.

If the secondary annealing is not performed, the final microstructure becomes a ferrite + pearlite structure, and the target ferrite + martensite bimodal steel can not be produced, so that secondary annealing is essential.

The secondary annealing can be performed in a single phase of the austenite above the A3 transformation temperature, but in this case the grains can be coarsened. Therefore, it is preferable that the secondary annealing is carried out at the A 1 transformation temperature to the A 3 transformation temperature, that is, at the austenite-ferrite two-phase region.

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. Heat treatment of steel

(1) Example 1

A cold rolled steel sheet consisting of 0.15% of carbon (C), 0.5% of manganese (Mn), 0.03% of phosphorus (P), 0.01% of sulfur (S) and the balance of iron (Fe) Lt; 0 > C for 1 hour and then cooled to room temperature at an average cooling rate of 10 [deg.] C / sec. Thereafter, ECAP treatment was performed at 500 ° C. At this time, the quadrangular specimen was subjected to ECAP treatment once, followed by 90 ° rotation, and then subjected to ECAP treatment four times, followed by air cooling. Thereafter, the secondary annealing was performed at 730 캜 for 10 minutes and cooled to a martensite temperature region at an average cooling rate of 20 캜 / sec in a water-cooling manner.

(2) Comparative Example 1

The composition of the steel was the same as that of Example 1, and was maintained at 1200 ° C for 1 hour and then cooled to room temperature.

(3) Comparative Example 2

Heat treatment was performed in the same manner as in Example 1, except that ECAP treatment was not performed.

(4) Comparative Example 3

The heat treatment was performed in the same manner as in Example 1, except that the secondary annealing was not performed.

2. Characterization

2 shows the microstructure of a cold-rolled specimen. Fig. 3 shows the microstructure of the specimen heat-treated in accordance with Example 1. Fig. Fig. 4 shows the microstructure of the specimen heat-treated in accordance with Comparative Example 1. Fig. Fig. 5 shows the microstructure of the specimen heat-treated in accordance with Comparative Example 2. Fig.

2 to 5, in the case of the specimen heat-treated according to Example 1 (FIG. 3), the crystal grains are very fine as compared with the remaining specimens (FIGS. 2, 4 and 5).

6 shows the strain-stress curves of the heat-treated specimens according to Example 1, Comparative Example 2, and Comparative Example 3. Fig.

Referring to FIG. 6, in the case of the specimen heat-treated in accordance with Example 1, the strength was the highest and the tensile strength was the highest.

7 shows the abrasion rate of the specimens heat-treated in accordance with Example 1, Comparative Example 1, and Comparative Example 2 with regard to degradation.

Referring to FIG. 7, it can be seen that the wear rate with respect to the load is the slowest in the case of the first embodiment.

Therefore, it can be seen that the DP steel produced according to the present invention exhibits excellent wear resistance while having ultra fine grain.

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.

Claims (4)

0.1 to 0.18 wt% of carbon (C), 0.3 to 0.6 wt% of manganese (Mn), 0.04 wt% or more of phosphorus (P) First annealing the steel which is made of the remaining iron (Fe) and unavoidable impurities, and subjected to the hot rolling or the cold rolling at an A3 transformation temperature or more;
Subjecting the annealed steel to Equal Channel Angular Pressing (ECAP) at a temperature lower than the A1 transformation temperature; And
And secondarily annealing the ECAP-treated steel at an Al transformation temperature or higher.
delete The method according to claim 1,
Wherein the secondary annealing is performed at an Al transformation temperature to an A3 transformation temperature.
The method according to claim 1,
After the secondary annealing, the steel is water-cooled to the martensite temperature.

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