KR20140084771A

KR20140084771A - Non-quenched and tempered steel wire rod having excellent toughness and manufacturing method thereof

Info

Publication number: KR20140084771A
Application number: KR1020120154612A
Authority: KR
Inventors: 이상윤; 박수동
Original assignee: 주식회사 포스코
Priority date: 2012-12-27
Filing date: 2012-12-27
Publication date: 2014-07-07

Abstract

0.001-0.40%, Mn: 0.6-1.90%, Cr: 0.05-1.50%, P: 0.03% or less, S: 0.02% or less, N: 0.01% or less, B : 0.0005-0.003%, Ti: 0.005-0.04%, V: 0-0.3%, Ni: 0-1.0%, and the balance Fe and other unavoidable impurities, 1.297 <Mn + 0.65 xCr + 0.52 xV + 0.1ⅹNi Wherein the microstructure is bainite or bainite and tempered martensite, and a method for producing the same.
According to the present invention, the toughness of the steel can be improved by producing low / medium carbon bainite or bainite + tempered martensitic steel during the wire rod manufacturing process.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength non-cored steel wire rod,

The present invention relates to a tough non-tempered steel wire and a method of manufacturing the same.

Medium carbon steel wire rod is generally used for cold working. In order to improve the processing characteristics, a low temperature annealing heat treatment, a spheroidizing heat treatment, and the like are performed, and a quenching and tempering heat treatment is performed to impart physical properties after forging. However, for parts requiring reduced heat treatment costs or a high level of numerical accuracy, non-tempered steels are used to omit such heat treatment.

In the carbon steel wire rod, most of the structure is a structure in which pearlite and a little ferrite are mixed. Especially, in non-tempered steel which is directly cold-forged through a drawing process without cold annealing heat treatment or spheroidizing heat treatment, The fraction is known to affect impact toughness of steel.

A well-known method for improving the toughness of the non-tempered steel wire is to add a carbon / nitride forming element capable of fixing the ferrite grain boundaries such as Ti, Nb and V and prevent the austenite grain refinement and coarsening through controlled rolling, So that the toughness is improved by refining the ferrite. However, this method requires control of precipitates such as Ti, Nb and V, and the temperature of the heating furnace must be raised to 1200 ° C. or higher.

One aspect of the present invention is to propose a manufacturing method and a high-tenacity noncondensated steel wire which can secure the toughness of a low-carbon steel wire rod which does not undergo a low-temperature annealing heat treatment and a spheroidizing heat treatment in a post-manufacturing process of the wire rod.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to accomplish the above object, one aspect of the present invention provides a steel sheet comprising: 0.05-0.20% of C, 0.001-0.40% of Si, 0.6-1.90% of Mn, 0.05-1.50% of Cr, %, S: not more than 0.02%, N: not more than 0.01%, B: 0.0005-0.003%, Ti: 0.005-0.04%, V: 0-0.3%, Ni: 0-1.0%, the balance being Fe and other unavoidable impurities And provides a tough non-tempered steel wire with a microstructure of bainite or bainite and tempered martensite satisfying 1.297 < Mn + 0.65 xCr + 0.52 xV + 0.1 x Ni.

Another aspect of the present invention is to provide a method for producing a steel sheet, which comprises 0.05-0.20% of C, 0.001-0.40% of Si, 0.6-1.90% of Mn, 0.05-1.50% of Cr, 0.03% or less of P, Comprising the steps of: preparing a steel comprising N: 0.01% or less, B: 0.0005-0.003%, Ti: 0.005-0.04%, V: 0-0.3%, Ni: 0-1.0%, balance Fe and other unavoidable impurities, And a step of hot rolling at 1250 캜 for a period of from 80 to 700 minutes for homogenizing and cooling at a cooling rate of 1 to 20 캜 / sec.

According to the present invention, toughness of the steel can be improved by producing low / heavy carbon bainite or bainite + tempered martensitic steel during the wire rod manufacturing process.

1 is a photograph showing a microstructure of a wire according to an embodiment of the present invention.
2 is a photograph showing a microstructure of a wire according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown.

An aspect of the present invention relates to a steel sheet comprising, by weight, 0.05-0.20% of C, 0.001-0.40% of Si, 0.6-1.90% of Mn, 0.05-1.50% of Cr, 0.03% or less of P, And the balance of Fe and other unavoidable impurities, and 1.297 < Mn + 0.65 x C < + 0.52 ⅹ V + 0.1 ⅹ Ni, and the microstructure is bainite or bainite and tempered martensite.

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

C: 0.05-0.20%

If the content of carbon is more than 0.2%, the hardness of the low-temperature structure is too high, which makes processing after wire rod manufacture difficult. Since non-durability products generally do not have a heat treatment process after the production of wire rods, the physical properties of the wire rods affect the final product, so that wire rod hardness, strength and ductility play a very important role. At a carbon content of 0.05% or less, low temperature formation such as bainite can be produced by a minimum of 5 to 10 ° C / sec even if B is added. In a general wire rod process, it is difficult to achieve the same cooling rate as water cooling rate, but the target strength can not be obtained, so the carbon content is limited to 0.05-0.20%.

Si : 0.001-0.40%

Si is a representative substitutional element and has a great influence on the amount of work hardening of steel. Especially, in non - tempered steel, there is no softening heat treatment process, and the increase of Si content in steel subjected to cold pressing immediately after drawing leads to increase of work hardening and deterioration of die life. In the case of this component system, in case of exceeding 0.4%, the amount of work hardening is increased and the ductility of steel is lowered, and the impact toughness is considered to be lowered. Si of less than 0.001% is considered to be impossible to manufacture when considering the steelmaking situation.

Mn : 0.6-1.90%

Manganese (Mn) plays a role in delaying ferrite formation and improving the ingotability of steel, and its content is limited to 0.6-1.90%. When manganese is added in excess of 1.9%, it has been reported that the mechanical properties such as impact toughness are increased but the texture heterogeneity due to manganese grains has a more detrimental effect on the non-tempered steel. Macroscopic segregation and micro segregation are easy to occur. Due to the relatively low diffusion coefficient of the mesostructured seaweed, the segregation zone is promoted and the resulting hardening ability is the main cause of the core martensite. When manganese is added in an amount of less than 0.6%, it is difficult to obtain the bainite phase pursued by the present invention because the effect of the segregation due to the manganese gypsum is relatively small but it is difficult to obtain sufficient incombustibility.

Cr : 0.05-1.50%

Chromium is added to inhibit ferrite formation and to enhance the ability of the steel to ingest. The role is expected to play the same role as Mn. If the Cr content exceeds 1.50%, Cr carbide is excessively formed on the surface of the steel, which may adversely affect the physical properties of the steel. If the Cr content is less than 0.05%, it is unlikely to increase the ingotability.

P: not more than 0.03%

Phosphorus (P) is segregated at the grain boundaries to reduce toughness and reduce delayed fracture resistance, so its upper limit is limited to 0.03%.

S: not more than 0.02%

Sulfur is segregated by low-melting point elements to deteriorate toughness and form emulsions, which adversely affects the delayed fracture resistance and stress relaxation characteristics, so that the upper limit is preferably limited to 0.02%.

N: not more than 0.01%

Nitrogen is generally combined with precipitate-forming elements such as Ti, Al and V to form carbonitrides. If the nitrogen content exceeds 0.01%, the precipitate grows to a great extent and the ferrite can act as a nucleation site. The nitrogen content is limited as above for homogeneous microstructure generation.

B: 0.0005-0.003%

In the content of boron is less than 0.0005%, so that boron is insufficient to suppress the grain boundary segregation of ferrite formation, the amount mimihayeo is not possible to sufficiently form the low-temperature tissue 0.003% more than the crude precipitate of such as Fe ₂₃ (C, B) ₆ over Formation of cracks can be a major cause of cracking of inclusions.

Ti : 0.005-0.04%, V: 0-0.3%

Titanium (TI) and vanadium (V) form Ti ₂ O ₃ , TiO ₂ , VC, VN and V (C, N). In the present invention, these fine precipitates act as bainite forming nuclei. When accompanied by adequate rolling, it serves to improve the toughness of non-tempered steel through refining of ferrite. If the content of Ti is less than 0.005%, the role of bainite-forming nuclei is insignificant as the distribution of Ti oxide and vanadium precipitates in the base material is decreased. If the content of Ti is more than 0.04% or V is more than 0.3% , A coarse vanadium carbonitride is formed to adversely affect toughness.

Ni : 0-1.0%

Ni is also inferior to Mn and Cr, but the incombustibility is increased. When the Ni content exceeds 1.0%, the effect of increasing the incombustibility is insignificant.

The remainder consists of Fe and unavoidable impurities.

Further, the contents of Mn, Cr, V and Ni should satisfy the relational expression (1.297 <Mn + 0.65 xCr + 0.52 xV + 0.1ⅹNi).

In the present invention, Mn, Cr, V, Ni, and the like are added to improve the incombustibility, and the above relation must be satisfied as a basis for forming bainite. If the above-mentioned relational expression is not satisfied, the microstructure may contain a ferrite + pearlite structure or a mixed structure of ferrite + pearlite + bainite or the like.

The microstructure of the high-strength non-tempered steel wire of the present invention is composed of bainite or bainite and tempered martensite.

Bainite and tempered martensite show high values at room temperature and low temperature impact toughness compared to ferrite and pearlite structures at the same carbon content, which is a very favorable microstructure for non - tempered steels which require shock characteristics guarantee.

Some of the bainites are produced as nuclei in at least one of precipitates of Ti oxide, V nitride and B nitride.

The present invention is based on a bainite or bainite + tempered martensite structure in a wire rod state, not a non-welded steel based on a conventional ferrite + pearlite structure, and a Ti oxide, a V nitride, Nucleation is provided to promote the formation of bainite, thereby improving the toughness of the steel.

It is preferable that 95% or more of the microstructure in the area fraction is bainite or bainite and tempered martensite.

If the area fraction is less than 95%, the fraction of the two phases is increased and the interface of the two phases becomes relatively weak. It is already known in various studies that the interface between microstructures having two-phase structure is weak. The upper limit of the area fraction is not set separately, because it is advantageous for impact toughness to have a single-phase structure.

The impact resistance of the wire is preferably 80 J / mm 2 or more.

The toughness value as described above has a significantly higher value than the impact toughness value of 20 to 60 J / mm < 2 > of the ferrite + pearlite structure, which is a conventional wire rod. The reason for limiting the range as described above is that when the impact toughness value is less than 80 J / mm 2, there is an increase in the designing of the steel species and the cost of the process in order to have a bainite or tempered martensite structure. The higher the impact toughness value is, the better the higher the impact toughness value is, so do not set the upper limit separately.

In order to produce a high-strength non-tempered steel wire having the above-described composition and microstructure, the following procedure is performed.

First, in terms of weight%, 0.05-0.20% of C, 0.001-0.40% of Si, 0.6-1.90% of Mn, 0.05-1.50% of Cr, 0.03% or less of P, 0.02% or less of S, , Fe: 0.001-0.03%, V: 0.03-0.3%, Ni: 0-1.0%, and Fe and other unavoidable impurities are prepared, and the steel is heated at 1000 to 1250 ° C for 80 to 700 minutes Homogenization heat treatment and hot rolling.

Mn, Cr, B and the like are added in order to secure the physical properties in the present invention. In particular, Mn and Cr are likely to be segregated during the performance process. To solve this problem, it is necessary to perform a diffusion treatment at a sufficient temperature for a sufficient time before hot rolling. If the temperature exceeds 1250 ° C, the surface of the steel will undergo oxidation due to oxidation and partial dissolution depending on the component system. Therefore, heat treatment should be performed at 1250 ° C or below. When the temperature is less than 1000 ° C, the homogenization effect and physical properties for hot rolling It is difficult to secure. When the homogenization heat treatment time exceeds 700 minutes, the glass is homogeneous in terms of homogenization but the surface deterioration in the steel becomes severe. In the case of less than 80 minutes, the whole billet or bloom is not uniformly heated.

After the hot rolling, the steel sheet is cooled at a cooling rate of 1 to 20 ° C / sec.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

[ Example ]

The steel having the composition shown in the following Table 1 was dissolved in a 50 kg ingot, and then subjected to homogenization heat treatment at a temperature of 1200 캜 for 10 hours and hot-rolled in a 15 mm steel bar. At this time, the finishing temperature was set at 1200 ° C. And then cooled at a cooling rate of 1 to 20 ° C / sec after hot rolling.

The composition of each of the inventive materials and comparative materials shown in Table 1 is not described. The content of Si is 0.2%, Cr is 0.15 to 0.2%, P is 0.015 or less, S is 0.01 or less, N is 0.005% 0.1% or less. (Note that the unit of the content is% by weight).

Microstructure and mechanical properties were measured. The relational expression means the value of Mn + 0.65 xCr + 0.52 xV + 0.1 xNi.

C Mn B Ti V Relation Cooling rate
(° C / sec) Seal
burglar
(Mpa) Shock
tenacity
(J / mm ² ) minuteness
group Inventory 1 0.08 1.22 0.0015 0.015 0 1.29 10 686 142 B Inventory 2 0.15 1.21 0.0013 0.015 0 1.29 10 779 130 B Inventory 3 0.1 0.8 0.002 0.015 0 1.37 20 690 127 B Invention 4 0.1 1.4 0.0024 0.015 0 1.69 One 752 119 B Invention Article 5 0.1 1.13 0.0022 0.015 0.1 1.55 5 724 131 B Inventions 6 0.1 1.2 0.0018 0.015 0.2 1.52 5 727 127 B Comparison 1 0.02 1.2 0.0015 0.015 0 0.80 10 324 - F Comparative material 2 0.25 1.2 0.0015 0.015 0 1.15 10 820 42 M + B Comparative material 3 0.1 1.7 0.0015 0.015 0 2.12 0.1 620 68 F + B Comparison 4 0.1 0.4 0.0015 0.015 0 0.73 10 540 78 F + B

In Table 1, B represents bainite, F represents ferrite, and M represents martensite. As can be seen from Table 1, the impact strength (U notch) of the impact toughness of 100 J / mm ² or more was obtained in the inventive material, but it was not as good as that of the comparative material. The inventive material was composed of bainite and some precipitates as a single structure, but in the case of the comparative material, the ferrite and bainite composite phases were composed of less than 50% of bainite and had a bad influence on tensile strength and impact value. The comparative material 2 has a considerable martensite structure, but the amount of carbon and Mn is higher than that of the other compositions.

The bainite of the inventive material starts from the grain boundaries, but when Ti precipitates are contained, nucleation is also observed in Ti oxides. This will play an important role in the formation of the bainite phase in the boundary conditions in which the bainite phase is generated. Figs. 1 and 2 are photographs of microstructures after the transformation of Invention Material 1 and Invention Material 3, respectively. Microstructures are seen in the form of acicular ferrite or acicular bainite, and these structures are known to be very positive for impact toughness.

In the present invention, Mn, Cr, V, Ni and the like were added to improve the entrapment property and the relationship (1.297 <Mn + 0.65ⅹCr + 0.52 × V + 0.1ⅹNi) was satisfied as a criterion for forming bainite.

As shown in Table 1, the impact toughness value is good at a value of 1.29 or more in the relational expression.

Claims

0.001-0.40%, Mn: 0.6-1.90%, Cr: 0.05-1.50%, P: 0.03% or less, S: 0.02% or less, N: 0.01% or less, B : 0.0005-0.003%, Ti: 0.005-0.04%, V: 0-0.3%, Ni: 0-1.0%, and the balance Fe and other unavoidable impurities, 1.297 <Mn + 0.65 xCr + 0.52 xV + 0.1ⅹNi And the microstructure is bainite or bainite and tempered martensite.

The method according to claim 1,
Wherein a part of the bainite is produced by nucleation in at least one of precipitates of Ti oxide, V nitride and B nitride.

The method according to claim 1,
Wherein at least 95% of the microstructure in the area fraction is bainite or bainite and tempered martensite.

The method according to claim 1,
Wherein said wire rod has an impact toughness value of 80 J / mm2 or more.

0.001-0.40%, Mn: 0.6-1.90%, Cr: 0.05-1.50%, P: 0.03% or less, S: 0.02% or less, N: 0.01% or less, B : 0.0005-0.003%, Ti: 0.005-0.04%, V: 0-0.3%, Ni: 0-1.0%, the balance Fe and other unavoidable impurities;
Subjecting to a homogenizing heat treatment at 1000 to 1250 캜 for 80 to 700 minutes and hot rolling;
And cooling at a cooling rate of 1 to 20 占 폚 / sec.