KR101736590B1 - Non heat treated wire rod having excellent high strength and method for manafacturing thereof - Google Patents

Abstract

More particularly, the present invention relates to a non-cored wire having excellent strength, which can be suitably used for machine parts such as industrial machines and automobiles exposed to various external load environments, and a method for manufacturing the same .

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength nonconducting wire and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

More particularly, the present invention relates to a non-cored wire having excellent strength, which can be suitably used for machine parts such as industrial machines and automobiles exposed to various external load environments, and a method for manufacturing the same .

Recently, efforts to reduce the emission of carbon dioxide (CO ₂ ), which is considered to be the main cause of environmental pollution, have become a global issue.

As a part of this, there has been an active regulation to regulate the exhaust gas of automobiles, and measures for improving the fuel efficiency of automobiles have been continuously studied as countermeasures thereto.

As described above, in order to improve the fuel efficiency of automobiles, it is required to reduce the weight and high performance of automobiles, and accordingly, the necessity of high strength of automobile materials or parts is increasing.

On the other hand, there is a limit in ensuring high strength of ferrite or pearlite structure in the wire rods. The materials having these microstructures are usually characterized by relatively low strength. In order to increase the strength, additional cold drawing must be performed.

Therefore, in order to secure an excellent strength, a bainite structure or a tempered martensite structure is generally utilized.

The bainite structure can be obtained by subjecting the hot rolled steel to a constant temperature transformation heat treatment, and the tempered martensite structure can be obtained by quenching and tempering heat treatment. However, since such a low-temperature structure can not be stably obtained only by the ordinary hot rolling and continuous cooling processes, there is a disadvantage that an additional heat treatment step must be performed on the hot-rolled steel material.

If the high strength can be secured without performing the additional heat treatment as described above, a number of processes from the material to the part production can be omitted or simplified, thereby improving the productivity and lowering the manufacturing cost.

However, to date, there has been no development of non-brittle wire materials stably having bainite or martensite structure by using hot rolling and continuous cooling processes without additional heat treatment, and a demand for technology development thereof is increasing.

An aspect of the present invention is to provide a non-tempered wire having excellent strength characteristics only by a hot rolling and a continuous cooling process without additional heat treatment such as constant temperature transformation, quenching and tempering, and a manufacturing method thereof.

An aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.05 to 0.15% carbon, 1.0 to 2.0% silicon, 1.5 to 2.5% manganese, 0.05 to 0.25% chromium, 0.05 to 0.25% of molybdenum (Mo), 0.020% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.001 to 0.003% of boron (B) (N): 0.0050% or less, the balance Fe and other unavoidable impurities,

(M / A) of 90% or more in bainite and residual martensite (M / A) in an area fraction of the microstructure.

According to another aspect of the present invention, there is provided a method of manufacturing a steel plate, comprising: preparing a steel material satisfying the above-mentioned composition; Subjecting the reheated steel material to a finish hot rolling in a temperature range of 850 to 950 占 폚; Cooling the hot-rolled steel to a temperature range of Bf to Bf-50 占 폚 at a cooling rate of 3 to 8 占 폚 / s; And cooling and air-cooling the high-strength non-tempered wire.

According to the present invention, it is possible to provide a non-tempered wire having excellent strength required in industrial machines, automobile materials, parts, and the like, without an additional heat treatment process.

Further, since no additional heat treatment process is required, there is an advantageous effect to reduce manufacturing cost.

The inventors of the present invention have found that even when a heat treatment (constant temperature transformation, quenching and tempering process, etc.) is carried out for securing low-temperature structure such as bainite, martensite and the like, it is possible to secure a low-temperature structure, I have studied in depth how to do this. As a result, it has been found that a non-tempered wire having excellent strength characteristics can be provided not only in securing a low-temperature structure from the optimization of the alloy component and the manufacturing conditions but also in making the crystal grain finer, and completed the present invention.

Hereinafter, the present invention will be described in detail.

The high strength non-tempered wire according to one aspect of the present invention comprises 0.05 to 0.15% carbon, 1.0 to 2.0% silicon, 1.5 to 2.5% manganese (Mn) 0.05 to 0.25% of chromium (Cr), 0.05 to 0.25% of molybdenum (Mo), 0.020% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.001 to 0.003% of boron (B) (Ti): 0.01 to 0.03%, and nitrogen (N): 0.0050% or less.

Hereinafter, the reason why the alloy composition of the steel sheet provided in the present invention is limited as described above will be described in detail. Here, the content of each component means weight% unless otherwise specified.

C: 0.05 to 0.15%

Carbon (C) is an indispensable element for ensuring strength and is either dissolved in steel or in the form of carbide or cementite. The easiest way to improve the strength is to increase the content of C in the steel to form a carbide or cementite, but in such a case, there is a problem that the ductility and the impact toughness are lowered. Therefore, the addition amount of C needs to be limited within a certain range .

In the case of the present invention, it is preferable to control the content of C to 0.05 to 0.15%. If the content of C is less than 0.05%, it is difficult to obtain the desired strength. On the other hand, when the content of C is more than 0.15% It is not desirable.

Si: 1.0 to 2.0%

Silicon (Si) is dissolved in ferrite and is a very effective element for enhancing the strength through strengthening of solid solution of steel. If the content of Si is less than 1.0%, the effect of strengthening solid solution due to Si can not be sufficiently obtained, so that the increase in strength is small. On the other hand, if the content exceeds 2.0%, the increase in strength becomes too large, There is a problem and it is not desirable.

Therefore, in the present invention, the Si content is preferably limited to 1.0 to 2.0%, more preferably 1.1% or more.

Mn: 1.5 to 2.5%

Manganese (Mn) is an element that increases the strength of the steel and improves the hardenability, thereby facilitating the formation of low-temperature structure such as bainite or martensite even at a wide range of cooling rates.

If the content of Mn is less than 1.5%, the hardenability is not sufficient and it is difficult to stably obtain a low-temperature structure in the continuous cooling step after the hot rolling. On the other hand, if the content exceeds 2.5%, the curing ability becomes excessively high, and if the cooling rate is high, the martensite structure may excessively be formed, which is not preferable.

Therefore, in the present invention, the content of Mn is preferably limited to 1.5 to 2.5%, more advantageously 1.6% or more, and more preferably 1.7% or more.

Cr: 0.05 to 0.25%

Cr (Cr) is an element effective for increasing the strength and hardenability of a steel similar to Mn.

However, if the content of Cr is less than 0.05%, the effect of improving the strength and hardenability is not significant. On the other hand, if the content exceeds 0.25%, the strength and hardenability can be secured, while the ductility is lowered.

Taking this into consideration, in the present invention, it is preferable to limit the content of Cr to 0.05 to 0.25%.

Mo: 0.05 to 0.25%

Molybdenum (Mo) is an element which improves the hardenability of steel and facilitates generation of low-temperature structure such as bainite or martensite. In other words, formation of ferrite is suppressed and the transformation temperature of bainite is lowered, which makes it possible to secure strength and low-temperature toughness. In addition, when boron is added together with boron (B), the effect tends to be further increased.

In order to advantageously obtain the above-mentioned effect, it is preferable to add Mo at 0.05% or more, but if the content exceeds 0.25%, the impact toughness may be deteriorated due to the formation of carbide.

In view of this, in the present invention, the content of Mo is preferably limited to 0.05 to 0.25%.

P: not more than 0.020%

Phosphorus (P) is segregated at the grain boundaries to lower the toughness and reduce the delayed fracture resistance. Therefore, in the present invention, the P content is preferably limited to 0.020% or less.

S: not more than 0.020%

Sulfur (S) is segregated in grain boundaries similar to P above to decrease toughness and form a low melting point emulsion, which is a main cause of inhibiting hot rolling. In the present invention, it is preferable to limit the content of S to 0.020% or less Do.

B: 0.001 to 0.003%

Boron (B) diffuses into the austenite grain boundary system to improve the hardenability and suppresses the formation of ferrite during cooling, and facilitates the formation of low-temperature structure such as bainite or martensite.

In order to obtain the above-mentioned effect, B must be added in an amount of 0.001% or more. However, if the content exceeds 0.003%, if the content is excessively high, the effect obtained by B is saturated and boron nitride is precipitated in the grain boundaries, There is a fear that the strength is lowered and the hot workability is lowered.

In view of this, in the present invention, the content of B is preferably limited to 0.001 to 0.003%.

Ti: 0.01 to 0.03%

Titanium (Ti) has the highest reactivity with nitrogen and is the first element to form nitride. When Ti is formed by the addition of Ti to exhaust most of the nitrogen in the steel, boron is inhibited from being precipitated and the boron exists in a soluble state, that is, in a free B state, which is advantageous for improving the hardenability .

If the content of Ti is less than 0.01%, the above-mentioned effect can not be sufficiently ensured. On the other hand, if the content exceeds 0.03%, there is a problem that physical properties of the steel are degraded by forming a coarse nitride.

Taking this into consideration, in the present invention, the content of Ti is preferably limited to 0.01 to 0.03%.

N: 0.0050% or less

It is preferable that the content of nitrogen (N) is limited to 0.0050% or less so that the boron added to the present invention is maintained in a soluble state to sufficiently exhibit the effect of improving the hardenability.

Among the above-mentioned composition, Mn, Cr, and Mo serve to enhance the hardenability of the steel and to facilitate the production of bainite even when the cooling rate is relatively slow. Further, Ti bonds with N to form a nitride, so that B is sufficiently dissolved in the steel, thereby suppressing ferrite formation and playing a favorable role in facilitating bainite formation.

The remainder of the present invention is iron (Fe). However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making.

The non-dwelling wire of the present invention satisfying the above-mentioned composition is preferably a microstructure containing bainite having an area fraction of 90% or more and residual martensite (M / A).

In the present invention, the residual martensite (M / A), which is the residual structure, is formed along the grain boundaries of the main phase bainite. When the fraction is high, the strength of the steel becomes high while impact toughness may be lowered. M / A) is preferably controlled to be low. In the present invention, it is preferable to control the fraction of martensite (M / A) on the surface by an area fraction of 10% or less (excluding 0%), and the average grain size of the martensite (M / A) desirable.

In the present invention, the fraction of the amorphous martensite (M / A) can be achieved by controlling the manufacturing conditions of the steel, particularly the cooling rate upon cooling after hot rolling.

The non-woven wire of the present invention having such a microstructure has an effect of securing a tensile strength of 800 to 1000 MPa and a ductility of 10% or more.

Hereinafter, a method of manufacturing a high strength non-tempered wire according to an aspect of the present invention will be described in detail.

The non-cored wire according to the present invention can be manufactured by preparing a steel material satisfying the composition of the composition proposed in the present invention and then subjecting it to a reheating-hot rolling-cooling-air cooling process. Hereinafter, Will be described in detail.

Reheating process

In the present invention, it is preferable to carry out a step of reheating the prepared steel material before performing the hot rolling.

At this time, reheating is preferably performed in a temperature range of 1000 to 1100 占 폚. If the reheating temperature is less than 1000 ° C, the temperature of the steel during the subsequent hot rolling process may become too low to cause surface defects. On the other hand, when the temperature exceeds 1100 ° C, the austenite grains grow to a great extent, there is a problem.

Hot rolling process

The reheated steel material is preferably hot rolled to form a wire rod, and the hot rolled steel sheet is preferably subjected to finish hot rolling at a temperature ranging from 850 to 950 ° C.

If the final hot rolling temperature is lower than 850 ° C, there is a problem that the possibility of causing surface defects increases. On the other hand, when the final hot rolling temperature is higher than 950 ° C, the crystal grains do not become finer and the desired physical properties can not be secured.

Cooling process

Preferably, the hot-rolled steel is cooled, and at this time, it is preferable to carry out the cooling at a cooling rate of 3 to 8 ° C / s to a temperature range of Bf to Bf-50 ° C.

When the cooling end temperature during cooling exceeds Bf (bainite transformation end temperature), it is difficult to form the desired bainite in a sufficient fraction. On the other hand, when the cooling end temperature is lower than Bf-50 ° C, the treatment is easy, I can not.

When the cooling rate is lower than 3 ° C / s, the formation of pro-eutectoid ferrite and pearlite becomes excessive. On the other hand, when the cooling rate is more than 8 ° C / s, martensite is excessively formed So that impact toughness is inferior.

It is preferable that the cooling step is completed by performing the cooling step according to the above-mentioned method and then air cooling to room temperature.

The non-cored wire of the present invention, which has been manufactured through the above-described processes, has an effect of ensuring excellent strength characteristics by securing bainite having an area fraction of 90% or more with a microstructure.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

Molten steel having the composition shown in Table 1 was cast into an ingot, and homogenized at 1250 占 폚 for 12 hours. Thereafter, the homogenized steel material was reheated in a temperature range of 1000 to 1100 DEG C, and then subjected to hot rolling at a finishing hot rolling temperature of 850 to 950 DEG C to a final thickness of 25 mm, followed by air cooling. Each of the prepared steels was subjected to solution treatment at 900 ° C and then cooled at the cooling rate shown in Table 2 below to prepare respective non-tempered wire rods.

The microstructure fraction and average grain size of the non-cored wire thus prepared were measured, and the mechanical properties (tensile strength and ductility) were measured and shown in Table 2 below.

The fraction and the grain size of the microstructure were measured by using an image analyzer. The room temperature tensile test was carried out at a rate of 0.9 mm / min until the yield point and then at a rate of 6 mm / min. < / RTI >

division Component composition (% by weight) C Si Mn P S Cr Mo Ti B N Inventive Steel 1 0.13 1.1 1.5 0.012 0.012 0.10 0.24 0.016 0.0020 0.0047 Invention river 2 0.07 1.5 2.3 0.010 0.017 0.15 0.10 0.019 0.0015 0.0043 Invention steel 3 0.09 1.6 2.1 0.015 0.013 0.12 0.08 0.017 0.0026 0.0045 Inventive Steel 4 0.06 1.8 1.9 0.016 0.011 0.23 0.12 0.023 0.0028 0.0046 Invention steel 5 0.12 1.2 2.0 0.018 0.015 0.10 0.11 0.028 0.0022 0.0038 Invention steel 6 0.05 1.7 2.5 0.012 0.014 0.05 0.07 0.013 0.0019 0.0039 Invention steel 7 0.08 1.3 1.7 0.009 0.012 0.16 0.16 0.020 0.0017 0.0037 Comparative River 1 0.23 1.5 1.8 0.012 0.013 0.17 0.19 0.019 0.0025 0.0046 Comparative River 2 0.13 0.4 2.0 0.017 0.015 0.09 0.10 0.023 0.0020 0.0039 Comparative Steel 3 0.12 1.7 0.8 0.011 0.012 0.15 0.21 0.017 0.0005 0.0042 Inventive Steel 8 0.10 1.9 2.1 0.011 0.008 0.11 0.08 0.016 0.0025 0.0049 Invention river 9 0.08 1.5 1.6 0.014 0.007 0.23 0.16 0.025 0.0021 0.0033 Comparative Steel 4 0.07 1.4 1.9 0.014 0.016 0.22 0.15 0.005 0.0027 0.0036 Comparative Steel 5 0.06 1.6 3.2 0.013 0.011 0.19 0.11 0.015 0.0018 0.0035 Comparative Steel 6 0.09 1.8 1.9 0.010 0.014 0.50 0.23 0.020 0.0016 0.0047 Comparative Steel 7 0.11 1.2 2.3 0.017 0.010 0.08 0.62 0.022 0.0019 0.0045 Comparative Steel 8 0.08 3.5 2.2 0.015 0.007 0.16 0.13 0.018 0.0016 0.0048

Steel grade
Manufacturing conditions Microstructure Mechanical properties division
Cooling rate
(° C / s) M / A fraction
(%) M / A
Grain size (탆) The tensile strength
(MPa) ductility
(%) Inventive Steel 1 4 4 2.8 845 26 Inventory 1 Invention river 2 5.5 5 2.5 891 23 Inventory 2 Invention steel 3 3 4 3.1 918 19 Inventory 3 Inventive Steel 4 8 8 1.8 954 16 Honorable 4 Invention steel 5 6 5 2.3 886 22 Inventory 5 Invention steel 6 7.5 7 1.9 927 18 Inventory 6 Invention steel 7 7 6 2.1 905 20 Honorable 7 Comparative River 1 8 14 2.0 1090 9 Comparative Example 1 Comparative River 2 5.5 6 2.7 784 29 Comparative Example 2 Comparative Steel 3 4 5 2.9 712 32 Comparative Example 3 Inventive Steel 8 11 14 1.3 1156 7 Comparative Example 4 Invention river 9 0.5 3 5.6 756 30 Comparative Example 5 Comparative Steel 4 5 One 2.6 740 30 Comparative Example 6 Comparative Steel 5 7.5 12 2.0 1135 7 Comparative Example 7 Comparative Steel 6 7 11 2.2 1110 8 Comparative Example 8 Comparative Steel 7 6 13 2.3 1089 9 Comparative Example 9 Comparative Steel 8 5 6 2.9 1153 6 Comparative Example 10

(In Table 2, Examples 1 to 7 include bainite phases on the remainder other than the M / A fraction.)

As shown in Tables 1 and 2, all of the non-tempered wire materials produced by satisfying all of the composition and manufacturing conditions proposed in the present invention have a bainite structure with an area fraction of 90% or more, and thus have tensile strengths of 800 to 1000 MPa And it is possible to secure a maximum of 26% in ductility.

On the other hand, in the case of Comparative Example 1 in which the content of C was excessively added without satisfying the range of the present invention, it was confirmed that the strength was excessively increased and ductility was improved, To increase the phase fraction of the stable M / A.

In the case of Comparative Example 2 in which the content of Si was insufficient, the effect of solid solution strengthening by Si could not be sufficiently obtained, and strength of less than 800 MPa was obtained.

On the other hand, in the case of Comparative Example 3 in which the content of Mn and B was insufficient, securing of hardenability of steel was insufficient, and ferrite, pearlite and bainite structure were mixed together even though the production conditions satisfied the present invention.

The steel composition composition satisfied the present invention, but in the case of Comparative Example 4 in which the cooling rate exceeded 8 캜 / s and was excessively applied, martensite was formed and the strength was excessively increased and ductility was weakened.

Further, in the case of Comparative Example 5 in which a too slow cooling rate was applied, ferrite and pearlite were produced, and as the coarse M / A phase was formed, strength of less than 800 MPa was obtained.

In the case of Comparative Example 6 in which the content of Ti was insufficient, the amount of soluble B was so small that the curing ability could not be sufficiently secured. As the ferrite and pearlite transformation amount increased, strength of less than 800 MPa was obtained.

In Comparative Examples 7, 8 and 9 in which the contents of Mn, Cr and Mo were excessively added, the curing ability was excessively increased, so that martensite was generated even under the conditions of the present invention, And the ductility was inferior.

In the case of Comparative Example 10 in which Si was added excessively, the effect of solid solution strengthening by Si was too large, the strength was excessively increased, and the ductility was inferior.

Claims (5)

(C): 0.05 to 0.15%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.5 to 2.5%, chromium (Cr): 0.05 to 0.25%, molybdenum (Mo): 0.05 (B): 0.001 to 0.003%, titanium (Ti): 0.01 to 0.03%, nitrogen (N): 0.0050% or less , The balance Fe and other unavoidable impurities,
Wherein the microstructure is composed of 90% or more of bainite and residual martensite (M / A) in an area fraction, and the mean grain size of the on-road martensite (M / A) is 5 占 퐉 or less.
delete The method according to claim 1,
Wherein the wire has a tensile strength of 800 to 1000 MPa and a ductility of 10% or more.
(C): 0.05 to 0.15%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.5 to 2.5%, chromium (Cr): 0.05 to 0.25%, molybdenum (Mo): 0.05 (B): 0.001 to 0.003%, titanium (Ti): 0.01 to 0.03%, nitrogen (N): 0.0050% or less , The remainder Fe and other unavoidable impurities, and then reheating the steel material;
Subjecting the reheated steel material to a finish hot rolling in a temperature range of 850 to 950 占 폚;
Cooling the hot-rolled steel to a temperature range of Bf to Bf-50 占 폚 at a cooling rate of 3 to 8 占 폚 / s; And
Air cooling after cooling,
Wherein the air-cooled microstructure is composed of 90% or more of bainite and residual martensite (M / A) in an areal proportion, and the average grain size of the on-road martensite (M / A) Gt;
5. The method of claim 4,
Wherein the reheating temperature is in the range of 1000 to 1100 ° C.