KR20130052215A

KR20130052215A - Method for minimizing crystal gain of low-carbon steel by asymmetric rolling

Info

Publication number: KR20130052215A
Application number: KR1020110117538A
Authority: KR
Inventors: 이경문; 이종국; 한도석; 강혁
Original assignee: 현대자동차주식회사
Priority date: 2011-11-11
Filing date: 2011-11-11
Publication date: 2013-05-22

Abstract

The present invention relates to a method for refining grains of low carbon steel through an asymmetric rolling method to improve mechanical properties by asymmetrically rolling low carbon steel to refine grains, and a low carbon steel manufactured by the method. It consists of asymmetrically rolling the upper and lower rolls to refine the ferrite grains of the low carbon steel rolled through to improve the mechanical properties.

Description

Grain refinement of low carbon steel through asymmetrical rolling {METHOD FOR MINIMIZING CRYSTAL GAIN OF LOW-CARBON STEEL BY ASYMMETRIC ROLLING}

The present invention relates to a method for refining grains of low carbon steel through an asymmetric rolling method to improve mechanical properties by asymmetrically rolling low carbon steel to refine grains, and to produce low carbon steel produced by the method.

As a method of improving the mechanical properties of the steel, there are various reinforcing methods such as precipitate strengthening, solid solution strengthening, martensite strengthening, and fine pearlite strengthening. However, these steel reinforcement methods are accompanied by deterioration of yield strength, tensile strength and elongation.

In the case of improving the mechanical properties of steel by miniaturizing the grains, it is possible to solve the deterioration problem of yield strength, tensile strength, and elongation accompanying high strength, and to reduce the impact transition temperature. Many technical advances have been made.

In particular, low carbon structural steels, which are used mainly when welding is required in the fabrication of structures or when the impact toughness of steel materials are required, are mostly made of ferrite except for quenching (quenching). In the following, the technology for refining grains of ferrite steel has been remarkably developed.

Among them, a so-called TMCP (Thermo-Mechanical Controlled Process) method is developed in recent years, in which a steel sheet is rolled in a non-recrystallized zone to generate austenite strain bands, and then accelerated cooling to increase the rate of nucleation of ferrite and to refine grains. It has provided breakthrough electricity for grain refinement technology.

The TMCP method was groundbreaking at the time of development, but recently, it has been evaluated as a generalized fine grain steel manufacturing technology, and when applied to low carbon ferritic steel, it is known that the ferrite grains can be refined to about 5 μm.

However, when the steel is strengthened through grain refinement, the strength increases depending on the reciprocal of the grain size. Thus, when the grain size of the ferrite is 5 µm or less, the rate of increase in strength due to grain refinement becomes rapid.

Therefore, in recent years, ultrafine grain refinement technology has been developed in which the size of the ferrite grains is 5 µm or less. The most influential factor among the requirements for ultrafine ferrite is the grain size of austenite.

In addition, when finish-rolling low carbon steel near the Ar3 temperature, the total reduction ratio is 75% or more through a single pass or a multi-stage pass, and accelerated cooling with a holding time between passes of 1 second or less. There is also a technique for miniaturizing so that there is also a manufacturing method of ultra-fine grain steel in which the ferrite grain size becomes 5 µm or less by heating the ultra low carbon steel after heating and rolling the finish rolling in the range of Ar1 or less, which is a ferrite stable temperature. .

The techniques presented in the prior art presuppose the concept that ultrafine ferrite can be obtained only by applying a large pressure in a hot or warm processing process, which is a major process for manufacturing steel materials, and accordingly, there is a slight difference in each technique. As a prerequisite for refining, the minimum reduction rate per pass or the maximum holding time between passes is specified.

However, in order to impart a high pressure during hot processing as in the prior art, it is almost impossible to achieve it with a conventional facility because it requires a hot processing equipment such as a rolling mill with a huge capacity. There was a limit to the formation of ultra-fine ferrite tissue, such as the easy growth of ferrite tissue formed by heat.

The present invention has been invented to solve the above problems, the size of the upper roll to improve the mechanical properties (yield strength, tensile strength, loss rate, etc.) by miniaturizing the ferrite grains of low carbon steel rolled through the upper / lower rolls It is an object of the present invention to provide a method for refining grains of low carbon steel through asymmetrical rolling with asymmetrical rolling to a size lower than that of a lower roll, and a low carbon steel manufactured by the method.

The present invention for achieving the above object consists of asymmetrically rolling the upper / lower rolls to refine the ferrite grains of low carbon steel rolled through the upper / lower rolls to improve mechanical properties.

And the asymmetrical rolling rate of the upper and lower rolls is 30-50%, the asymmetrical rolling, the upper roll is rolled larger than the lower roll, the size ratio of the upper roll and the lower roll is the upper roll 1.3-1.5: lower Roll 1.

The carbon content of the low carbon steel is 0.12-0.2 wt%, and the rolling of the low carbon steel is hot rolled.

In addition, the present invention is a low carbon steel obtained by miniaturizing ferrite grains by hot asymmetrically rolling a steel material having a carbon content of 0.12-0.2 wt% with an upper / lower roll.

The asymmetrical rolling ratio of the upper / lower roll is 30-50%, the size ratio of the upper roll and the lower roll is the upper roll 1.3-1.5: the lower roll 1, and the size of the ferrite grains is 1-2 μm. .

The present invention as described above, by applying an effective stress applied simultaneously to the compressive stress and shear stress to the low carbon steel rolled by the method of asymmetrical rolling of low carbon steel larger than symmetrical rolling, to refine the ferrite grains to improve the mechanical properties Of course, there are cost savings.

Figure 1a and 1b is a schematic diagram showing the configuration and asymmetrical rolling process of the asymmetrical rolling roll of the present invention,
Figure 2 is a tissue photograph showing the shear strain of the 50% asymmetrical reduction rate,
3 (a) to (d) is a micrograph showing the change in microstructure according to the 20%, 30%, 40%, 50% asymmetric reduction rate,
Figure 4 is a micrograph of the analysis of the low carbon steel by electron microscope (TEM) after 50% asymmetrical rolling,
(a) is a photographic image of the electron microscope,
(b) is an electron micrograph.
(c) is a photograph of the grain boundary carbide analysis by electron microscopy analysis,
(d) is a Diffraction Pattern.
5 is a graph showing tensile strength change according to cooling rate of 50% asymmetrically rolled low carbon steel,
6 is a photograph of a low carbon steel 30% asymmetrically rolled at 15 ℃ / s cooling rate,
7 is a photograph of the analysis of high-angle ferrite grains and flat grains by backscattered electron microscopy analysis of the 50% asymmetrically rolled material of the present invention;
8 is a graph analyzing the average grain of the low carbon steel produced by the present invention,
9 is a graph showing the mechanical properties according to the reduction ratio of the low carbon steel produced by the present invention,
10 is a graph showing the mechanical properties according to the cooling rate of the low carbon steel produced by the present invention.

Hereinafter, with reference to the accompanying drawings, a method for refining grains of low carbon steel through asymmetrical rolling and a low carbon steel manufactured by the method according to the preferred embodiment of the present invention will be described in detail.

The present invention is to improve the mechanical properties by miniaturizing the ferrite grains through the machining process of low carbon steel, that is, asymmetrical rolling process, unlike the commonly carried out (TMr: Thermo-Mechanical Controlled Process).

When the steel is cooled at a high temperature, the transformation occurs in the ferrite phase. When the stress is applied during the ferrite transformation, the ferrite transformation is further promoted and the grains are refined.

Therefore, the present invention is to improve the mechanical properties by miniaturizing the grain size to 1 ~ 2㎛ level by applying asymmetrical rolling with different sizes of upper and lower rolls as shown in Figure 1 of the ferrite transformation.

Unlike the symmetrical rolling, the asymmetrical rolling is applied simultaneously to the compressive stress and the shear stress on the material. Therefore, the effective stress is applied to the low carbon steel material is larger than symmetrical rolling.

The principle that the effective stress is applied more than symmetrical rolling as described above is to roll by varying the size ratio of the lower / upper roll, as shown in Figure 1, the asymmetrical rolling rate of the upper / lower roll is 30-50% It is preferable to.

More specifically, in the asymmetrical rolling, the upper roll is rolled larger than the lower roll, and the size ratio of the upper roll and the lower roll is most preferably in the ratio of the upper roll 1.3-1.5: lower roll 1.

When the asymmetrical rolling of low carbon steel in the same manner as above, the total shear stress is about 0.91, and thus the total effective stress is about 1.6, so that the size of ferrite grains can be reduced to 1-2 ㎛ due to such effective stress. will be.

On the other hand, the fine-grained ferrite generation fraction also reaches about 80%, the theoretical limit of 0.15C.

As described above, when the size of the ferrite grains becomes 1-2㎛ and becomes fine grains, the yield strength, which is a mechanical property, shows a value of about 650 MPa while satisfying the general Hall-Petch relationship, but a large number of dislocations occur in the ferrite by steel processing. Therefore, hardening of work does not occur during plastic processing of materials. As a result, the maximum tensile strength reaches a level of about 720 MPa as shown in FIG. 9.

Therefore, the yield ratio representing the ratio of the yield strength and the tensile strength of the material is higher (0.9) than the general low carbon steel (0.6). This is because it is preferable to maintain the yield ratio at 0.7 or less in structural steel because the strength band from the elastic behavior to the fracture during the plastic deformation becomes short.

In the present invention, after the low rolling reduction (less than 30%) asymmetrical rolling, a mixed structure of ferrite and Lath martensite produced by 2-3 μm level and water cooling was produced through fast cooling at about 15 ° C./s. .

The mechanical properties of the mixed structure thus produced are as shown in Figure 5 and 10 yield strength is about 500MPa, the maximum tensile strength is about 720MPa value. This lowered the yield ratio to about 0.69.

It has a higher yield (250 MPa) and a higher tensile strength (460 MPa) and similar yield ratio than general low carbon steel (0,15).

Table 1 shows the mechanical properties of the low carbon steel according to the cooling rate after asymmetrical rolling according to the present invention.

Cooling rate Rolling (%) Y.S. (MPa) T.S. (MPa) YR
Yield Ration EL (%)
3 ℃ / s
20
30
40
50 469 ± 19
499 ± 6
535
591 ± 2 546 ± 11
563 ± 3
587
614 ± 11 0.86 ± 0.2
0.89 ± 0.05
0.91
0.97 ± 0.01 18 ± 3
27 ± 1
23
24
5 ℃ / s
20
30
50 500
563 ± 31
620 ± 18 571
619 ± 32
629 ± 20 0.87
0.91
0.98 28 ± 2
25
24 ± 3
10 ℃ / s 20
30
50 491 ± 6
622
679 ± 13 671 ± 33
698
729 ± 7 0.73 ± 0.05
0.89
0.93 ± 0.03 25
15
20 ± 2
13 ℃ / s 20
30
40
50 473 ± 40
481 ± 43
611
600 ± 31 691 ± 30
724 ± 10
713
722 ± 3 0.69 ± 0.09
0.67 ± 0.07
0.85
0.83 ± 0.04 21 ± 1
14
13
16 ± 3
15 ℃ / s 20
30
40
50 444 ± 16
479
547 ± 5
600 727 ± 12
744
749 ± 11
738 0.62 ± 0.04
0.64
0.73 ± 0.02
0.81 18 ± 2
16
16 ± 1
17

As described above, the present invention uses a dynamic transformation phenomenon in which the number of nucleation sites of ferrite increases during rolling through 50% asymmetrical rolling. It can be refined, and when rolled at 30% asymmetrical rolling and 15 ° C./s cooling rate, as shown in FIG. 6, thereby improving yield and tensile strength compared to general low carbon steel, and yield ratio is general low carbon As well as maintaining a level similar to steel, there is an effect of improving the mechanical properties by 50% or more by asymmetrical rolling according to the present invention.

In addition, low-pressure reduction rate (below 30%) asymmetrical rolling to form a ferrite of 5㎛ or less and then accelerated cooling of untransformed austenite to produce martensite, the fraction of the martensite produced is less than 5% Sufficient work hardening can be achieved after elastic deformation.

Meanwhile, the present invention utilizes precipitation hardening through the addition of alloying elements as shown in Table 2 to increase the mechanical properties, but it is possible to improve mechanical properties without adding additional alloying elements as shown in Table 2, thereby greatly reducing the cost. .

C (wt%) Si (wt%) Mn (wt%) Ti (wt%) B (wt%) Al (wt%) Fe (wt%) 0.154 0.26 1.34 0.015 0.0093 0.004 Honey.

The present invention as described above, the size of the upper roll to lower the size of the upper roll to improve the mechanical properties (yield strength, tensile strength, firing rate, etc.) by miniaturizing the ferrite grains of the low carbon steel rolled through the difference between the upper and lower rolls By asymmetrical rolling to larger than the size of the low-carbon steel grain size of 1 ~ 2㎛ fine to improve the mechanical properties, there is an advantage of cost reduction.

Claims

Asymmetrical rolling is characterized in that the upper and lower rolls are asymmetrically hot rolled to improve the mechanical properties by miniaturizing the ferrite grains of low carbon steel which are 0.12 to 0.2 wt% of carbon and hot rolled through the upper and lower rolls. Grain refinement method of low carbon steel through

The method according to claim 1,
The asymmetrical rolling rate of the upper and lower rolls is 30 to 50%, characterized in that the grain refining method of low carbon steel through asymmetrical rolling.

The method according to claim 1,
The asymmetrical rolling is a low-carbon steel grain refining method through asymmetrical rolling, characterized in that the upper roll is rolled larger than the lower roll.

The method according to claim 3,
The ratio of the size of the upper roll and the lower roll is the upper roll 1.3-1.5: lower roll 1, characterized in that the grain refining method of low carbon steel through asymmetrical rolling.

A low carbon steel, characterized in that the ferrite grains are refined to have a grain size of 1 to 2 μm by hot asymmetrical rolling of a steel material having a carbon content of 0.12 to 0.2 wt% with an upper / lower roll.