JP6806905B2 - High-strength wire with excellent impact toughness and its manufacturing method - Google Patents

Description

The present invention relates to a high-strength wire rod having excellent impact toughness and a method for manufacturing the same. More specifically, the present invention has excellent impact toughness that can be suitably used as a material for industrial machines and automobiles exposed to various external load environments. Regarding high-strength wire rod and its manufacturing method.

Recently, reducing carbon dioxide emissions, which has been pointed out as one of the major causes of environmental pollution, has become a global issue, and great efforts have been made to deal with it. As part of this, there is an active movement to regulate automobile exhaust gas, and as a countermeasure, automobile manufacturers are trying to solve this problem by improving fuel efficiency. However, in order to improve fuel efficiency, it is required to reduce the weight and performance of automobiles. Along with this, there is an increasing need for high strength for automobile materials and parts. In addition, as the need for stability against external impact is increasing, impact toughness is also recognized as an important physical property of materials and parts.

Wire rods with a ferrite or pearlite structure have limitations in achieving excellent strength and impact toughness. Materials with these structures usually have high impact toughness but relatively low strength, and high strength can be obtained by cold drawing to increase the strength, but the impact toughness is high. There is a drawback that it decreases sharply in proportion to the increase in strength.

Therefore, in general, a bainite structure or a tempered martensite structure is used in order to achieve high strength and excellent impact toughness. The bainite structure can be obtained through constant temperature transformation heat treatment using hot-rolled steel, and the tempered martensitic structure can be obtained through quenching and tempering heat treatment. However, since such a structure cannot be stably obtained only by a normal hot rolling or continuous cooling step, it is necessary to perform an additional heat treatment step as described above using the hot rolled steel material.

If high strength and excellent impact toughness are achieved without this additional heat treatment, some steps from the material to the production of parts can be omitted or simplified to improve productivity. , The advantage that the manufacturing cost can be reduced comes out.

However, a wire rod capable of stably obtaining a bainite or martensite structure by using hot rolling and continuous cooling steps without performing an additional heat treatment step has not yet been developed, and as a result, further development of the wire rod. The need for is emerging.

One of some objects of the present invention is to provide a high-strength wire rod having excellent impact toughness and a method for producing the same without performing additional heat treatment.

One aspect of the present invention is C: less than 0.05% (excluding 0%), Si: 0.05% or less (excluding 0%), Mn: 3.0 to 4.0%, by weight%. P: 0.020% or less, S: 0.020% or less, Ni: 1.0 to 3.0%, B: 0.0010 to 0.0030%, Ti: 0.010 to 0.030%, N : Less than 0.0030%, Al: 0.010 to 0.050%, the balance is composed of Fe and other unavoidable impurities, and the fine structure is 3 area% or less (including 0 area%). Provided is a high-strength wire rod composed of site (MA), an eutectoid ferrite having 2 area% or less (including 0 area%), and a vanitic ferrite having 95 area% or more (including 100 area%).

Another aspect of the present invention is the method for producing a wire rod according to claim 1, wherein in terms of weight%, C: less than 0.05% (excluding 0%), Si: 0.05% or less (0%). ), Mn: 3.0 to 4.0%, P: 0.020% or less, S: 0.020% or less, Ni: 1.0 to 3.0%, B: 0.0010 to 0. Reheat a steel material having a composition of 0030%, Ti: 0.010 to 0.030%, N: less than 0.0030%, Al: 0.010 to 0.050%, and the balance consisting of Fe and other unavoidable impurities. The step, the step of hot rolling the reheated steel material to obtain a wire rod, the step of primary cooling the wire rod from Bs ° C. to (Bs + 50) ° C. at a rate of 10 to 20 ° C./sec, and the above 1 It is characterized by performing a step of secondary cooling the second cooled wire from (Bf-50) ° C. to Bf ° C at a rate of 2 to 5 ° C./sec and a step of air-cooling the second cooled wire. And. (However, Bs is 600 to 650 ° C. and Bf is 350 to 400 ° C.).

As one of the effects of the present invention, the wire rod according to the present invention is excellent in strength and impact toughness. As a result, it can be suitably used as a material for industrial machines and automobiles exposed to various external load environments.

Further, the wire rod according to the present invention has an advantage that excellent strength and impact toughness can be ensured without performing additional heat treatment, which is advantageous from an economical point of view.

The advantages and effects of the present invention are not limited to those described above, and can be more easily understood in the process of explaining the specific embodiments of the present invention.

Hereinafter, a high-strength wire rod having excellent impact toughness according to one aspect of the present invention will be described in detail.

First, the alloy component and the preferable content range of the high-strength wire rod of the present invention will be described in detail. Unless otherwise specified, the contents of each component described later are all based on weight.

C: Less than 0.05% (excluding 0%)
Carbon contributes to increasing the strength of the wire rod by being dissolved in the steel or existing in the form of carbide or cementite, but in the present invention, it is not intentionally added and contains carbon. At least, it does not pose a big obstacle from the viewpoint of ensuring physical properties. However, 0% is excluded in consideration of the amount of unavoidable contamination in manufacturing.
On the other hand, as the carbon content increases, ductility and impact toughness decrease, so it is necessary to adjust the carbon content within a certain range. In addition, as the carbon content increases, the formation of island-like martensite (MA) during bainite transformation increases, which may inhibit impact toughness. In the present invention, in consideration of this, the carbon content is controlled to less than 0.05%.

Si: 0.05% or less (excluding 0%)
Silicon is a deoxidizing element together with aluminum, and is known as an element that is dissolved in ferrite and has a great effect on increasing strength through solid solution strengthening of steel materials. However, in the present invention, silicon is intentionally added. However, even if it does not contain silicon, it does not pose a major problem from the viewpoint of ensuring predetermined physical properties. However, 0% is excluded in consideration of the amount of unavoidable contamination in manufacturing.
On the other hand, when silicon is contained, the strength is significantly increased, but the ductility and impact toughness are sharply decreased, so that the silicon content is very limited in the case of cold forged parts that require sufficient ductility. In addition, silicon interferes with the precipitation of cementite during bainite transformation so that carbon is concentrated in the austenite phase, so that island-like martensite (M / A) can be easily formed. In the present invention, the silicon content is controlled to 0.05% or less in order to ensure excellent impact toughness.

Mn: 3.0 to 4.0%
Manganese facilitates the formation of cold structures such as bainite or martensite at a wide range of cooling rates by increasing the strength of the steel and improving its curability.
If the manganese content is less than 3.0%, the curing ability is not sufficient, and it becomes difficult to stably secure a low temperature structure in the continuous cooling step after hot rolling. On the other hand, if it exceeds 4.0%, the curing ability becomes too high and a martensite structure may be formed even during air cooling, which is not preferable.

P: 0.020% or less Phosphorus is an impurity that is inevitably contained in steel and is segregated at grain boundaries to reduce the toughness of steel and reduce delayed fracture resistance, so it is not contained as much as possible. Is preferable. In the present invention, the upper limit of phosphorus is controlled to 0.020%.

S: 0.020% or less Sulfur is an impurity that is inevitably contained in steel, and like phosphorus, it segregates at grain boundaries to reduce toughness, form low melting point sulfide, and heat. It is preferable not to include it as much as possible because it hinders inter-rolling. In the present invention, the upper limit of sulfur is controlled to 0.020%.

Ni: 1.0-3.0%
Nickel acts together with manganese as an element that enhances curability. This can reduce the formation of island-shaped martensite (M / A). If the nickel content is less than 1.0%, the curing ability is not sufficient and the effect of suppressing the formation of island-like martensite (M / A) is slight. On the other hand, if it exceeds 3.0%, the curing ability becomes too high and a martensite structure may be formed, which is not preferable. More preferably, it is 1.2 to 2.8%.

B: 0.0010 to 0.0030%
Boron is an element that improves curability and diffuses into austenite grain boundaries to suppress the formation of ferrite during cooling, facilitating the formation of bainite or martensite. However, if the boron content is less than 0.0010%, the effect of adding the boron cannot be expected, and if it exceeds 0.0030%, no further increase in the effect can be expected and the grain boundaries. Boron-based nitrides may be deposited on the surface, lowering the grain boundary strength and impairing hot workability.

Ti: 0.010 to 0.030%
Titanium has the highest reactivity with nitrogen, and as a result, forms a nitride first. When TiN is formed by the addition of titanium and nitrogen in the steel is almost eliminated, the effect of improving the curing ability is obtained by preventing the precipitation of BN and helping the boron to exist in a dissolved state. Can be done. However, if the titanium content is less than 0.010%, the addition effect is insufficient, and if it exceeds 0.030%, coarse nitrides may be formed and the mechanical properties may be impaired.

N: Less than 0.0030% Nitrogen is maintained in a solution state with boron, and is preferably not contained as much as possible in order to improve curability. In addition, it is necessary to limit the formation of island-shaped martensite (M / A) during bainite transformation. In the present invention, the nitrogen content is controlled to less than 0.0030%.

Al: 0.010 to 0.050%
Aluminum is a strong deoxidizing element that not only removes oxygen in steel to improve cleanliness, but also combines with nitrogen dissolved in steel to form AlN, thereby increasing impact toughness. Can be improved. Therefore, in the present invention, aluminum is positively added. However, if the content of aluminum is less than 0.010%, it is difficult to expect the effect of addition thereof, and if it exceeds 0.050%, a large amount of alumina inclusions are generated, which greatly impairs the mechanical characteristics. Sometimes.

The remaining component other than the above composition is iron (Fe). However, in a normal manufacturing process, unintended impurities are inevitably mixed in from the raw material or the surrounding environment, so this cannot be eliminated. Since these impurities are easily understood by an engineer having ordinary knowledge in the technical field, all the contents thereof are not specifically described in this specification.

On the other hand, when designing an alloy of a steel material having the above component range, it is preferable to control the contents of C, Si, and Ni so as to satisfy the following relational expression 1.
[Relationship formula 1]

In the present invention, carbon may form cementite and island martensite (M / A) to reduce impact toughness, and silicon may be dissolved in steel or island martensite (M / A). It may form M / A) and reduce impact toughness. On the other hand, nickel can suppress the formation of island-like martensite (M / A) by increasing the curing ability. As a result of repeated studies and experiments focusing on the above points, the present inventors have excellent strength and impact toughness when the contents of carbon, silicon, and nickel satisfy the above relational expression 1. It was confirmed that a wire rod having a bainitic ferrite structure could be obtained.

Further, when designing an alloy of a steel material having the above component range, it is preferable to control the contents of Mn, Ti, N, and B so as to satisfy the following relational expression 2. The more preferable range of the following relational expression 2 is 10.0 or more, and the more preferable range is 12.0 or more.
[Relational expression 2]

In the present invention, manganese enhances the curability to facilitate the formation of bainitic ferrite even when the cooling rate is relatively low. Then, titanium combines with nitrogen to form a nitride so that boron is sufficiently dissolved in the steel, thereby suppressing the formation of ferrite and facilitating the formation of bainitic ferrite. As a result of repeated studies and experiments focusing on the above points, the inventors of the present invention have excellent strength and impact toughness when the contents of manganese, titanium, boron, and nitrogen satisfy the above relational expression 2. It was confirmed that a wire rod having a bainitic ferrite structure was obtained.

Hereinafter, the fine structure of the high-strength wire rod having excellent impact toughness of the present invention will be described in detail.

The wire rod of the present invention has island-shaped martensite (MA) of 3 area% or less (including 0 area%), proeutectoid ferrite of 2 area% or less (including 0 area%), and 95 area as its fine structure. % Or more (including 100 area%) of bainitic ferrite can be included. That is, the wire rod of the present invention has bainitic ferrite as the main structure and can contain island-like martensite (MA) and proeutectoid ferrite as the second phase, but the area ratios of these are 3% and 2%, respectively. Limited to: Bainite is referred to by various terms depending on the carbon content and morphology, but usually above the medium carbon range (about 0.2 to 0.45% by weight), upper / lower bainite (upper / lower bainite). ), And in the low carbon range of 0.2% or less, it is called bainitic ferrite, acicular ferrite, granular ferrite, or the like, depending on the temperature range. The wire rod of the present invention has a bainitic ferrite structure among these.

In the wire rod of the present invention, excellent strength and impact toughness can be achieved by using bainitic ferrite as the main structure as described above. When ordinary ferrite is the main structure instead of bainitic ferrite, it may be advantageous from the viewpoint of impact toughness, but it is not preferable because it cannot prevent a decrease in strength.

The higher the surface integral of the island-shaped martensite, the more advantageous it is in terms of the strength of the wire, but it may impair the impact toughness. Considering this, it is preferable to control the area ratio of island-shaped martensite as low as possible, and as described above, it is controlled to 3% or less in the present invention.

The proeutectoid ferrite is formed mainly along the grain boundaries of the former austenite and greatly reduces the impact toughness. Therefore, it is preferable to control the area ratio of proeutectoid ferrite as low as possible, and as described above, it is controlled to 2% or less in the present invention.

According to one example, the crystal grain size of island-shaped martensite is 5 μm or less (excluding 0 μm). If the crystal grain size exceeds 5 μm, the contact point with the bainitic ferrite region may become large and the impact toughness may deteriorate. Here, the crystal grain size means the equivalent circular diameter of the particles detected by observing one cross section of the wire rod.

The high-strength wire rod of the present invention described above can be produced by various methods, and the production method is not particularly limited. However, as a preferable example, it can be produced by the following method.

Hereinafter, a method for producing a high-strength wire rod having excellent impact toughness according to another aspect of the present invention will be described in detail.

First, a steel material having the above-mentioned component system is provided, and then this is reheated. Here, the form of the steel material is not particularly limited, but is generally in the form of bloom or billet.

At this time, the reheating temperature is preferably in the range of 950 to 1050 ° C. This is to prevent coarsening of crystal grains by reheating the steel material at a relatively low temperature.

Next, the reheated steel material is finished and hot-rolled to obtain a wire rod.
At this time, the finishing hot rolling temperature is preferably in the range of 750 to 850 ° C. This is because the austenite crystal grains are refined through sufficient low-temperature rolling, and after the phase transformation, a fine bainite structure is finally obtained to improve the impact toughness.

Next, the wire rod is first cooled from Bs ° C. to (Bs + 50) ° C. at a rate of 10 to 20 ° C./sec. Here, Bs is the temperature at which the bainite phase transformation starts on the continuous cooling curve, and in the present invention, by cooling the wire rod at a relatively high speed until just before the bainite phase transformation, it is the first time along the austenite grain boundaries. It positively suppresses the formation of bainite.
In the present invention, Bs is preferably in the temperature range of 600 to 650 ° C.

Next, the primary cooled wire rod is secondarily cooled from (Bf-50) ° C. to Bf ° C. at a rate of 2 to 5 ° C./sec and then air-cooled. Here, Bf is the temperature at which the bainite phase transformation ends on the continuous cooling curve. If the secondary cooling end temperature exceeds Bf ° C, it becomes difficult to secure a sufficient amount of bainitic ferrite structure, and if it is less than Bf-50 ° C, the steel material is sufficiently cooled and easy to handle. , May lead to a decrease in productivity.

Further, if the secondary cooling rate is less than 2 ° C./sec, a large amount of proeutectoid ferrite may be formed, and if it exceeds 5 ° C./sec, martensite is formed in the steel to provide strength and impact toughness. May be damaged.

Hereinafter, the present invention will be described in more detail through examples. However, the description of such examples is for exemplifying the practice of the present invention, and the description of such examples does not limit the present invention. This is because the scope of rights of the present invention is determined by the matters stated in the claims and the matters reasonably inferred from them.

A molten steel having the alloy composition shown in Table 1 below was cast, reheated at 1000 ° C., and then rolled with a wire rod having a diameter of 15 mm (finishing hot rolling temperature: 750 ° C.). Then, the wire rod was manufactured by primary and secondary cooling under the conditions shown in Table 2 below and air-cooled from a temperature of 350 ° C. or lower, which is lower than the Bf temperature. On the other hand, Bf, which is the end temperature of the bainite phase transformation, was measured using a dilatometer and found to be in the range of about 350 to 400 ° C., although it was slightly different depending on the chemical composition.

Table 2 shows the analysis results of the microstructure and the measurement results of tensile strength and impact toughness of the wire rod produced in this manner. The area fraction and crystal grain size of island-shaped martensite (MA) among the fine structures of the wire rod were measured using an image analyzer.

In the normal temperature tensile test, the crosshead speed was measured at a speed of 0.9 mm / min up to the yield point and 6 mm / min thereafter. Further, the impact test was carried out at room temperature by using an impact tester having a curvature of the edge portion of the striker that gives an impact to the test piece of 2 mm and a test capacity of 500 J.

As shown in Tables 1 and 2 above, the test pieces 1 to 5 satisfying all of the alloy compositions and process conditions proposed in the present invention not only have a tensile strength of 600 MPa or more, but also have an extremely excellent impact toughness of 200 J or more. The results are shown.

On the other hand, in the test piece 6, since the nickel content did not reach the range proposed by the present invention, many MA phases were formed and the impact toughness was inferior.
In the test piece 7, since the carbon content exceeded the range proposed by the present invention, the tensile strength was excellent, but the impact toughness was inferior. This is because carbon was dissolved in the MA phase to form a stable MA phase.
In the test piece 8, when the content of silicon exceeds the range proposed in the present invention, the amount of silicon dissolved in the matrix increases as the amount of silicon added increases, as a result. Since the effect of solid solution strengthening was exhibited and the MA phase was also increased, the tensile strength was excellent, but the impact toughness was inferior.
In the test piece 9, since the contents of manganese and boron do not reach the range proposed by the present invention, the hardening ability of the steel material is low, and as a result, ferrite and vanitic are obtained even if the cooling conditions proposed by the present invention are satisfied. Ferrite structure was mixed and the tensile strength was inferior.
In the test piece 10, the alloy composition satisfies the proposed range of the present invention, but the component relational expression (relational formula 1) and the secondary cooling rate in the manufacturing process exceed the proposed range of the present invention. Martensite and martensite were formed and the tensile strength was excellent, but the impact toughness was inferior.
In the test piece 11, the alloy composition satisfied the proposed range of the present invention, but the secondary cooling rate did not reach the proposed range of the present invention, so that ferrite was formed and the tensile strength was inferior.
In the test piece 12, when the titanium content does not reach the proposed range of the present invention and the amount of boron in the solute decreases, the curing ability decreases and the cooling rate is low. The amount of precipitated ferrite increased and the tensile strength decreased.
The test pieces 13 and 14, respectively, were in the case where the contents of manganese and nickel exceeded the proposed range of the present invention, and the curability was relatively too large. Martensite was formed and the strength was increased, but the impact toughness was inferior.

Although the embodiments of the present invention have been described in detail above, the scope of rights of the present invention is not limited to this, and various modifications and modifications and modifications are made within the scope of the technical idea of the present invention described in the claims. It is clear to those with ordinary knowledge in the art that the transformation is possible.

Claims (8)

By weight%, C: less than 0.05% (excluding 0%), Si: 0.05% or less (excluding 0%), Mn: 3.0 to 4.0%, P: 0.020% or less , S: 0.020% or less, Ni: 1.0 to 3.0%, B: 0.0010 to 0.0030%, Ti: 0.010 to 0.030%, N: less than 0.0030%, Al: 0.010 to 0.050%, with the balance consisting of Fe and other unavoidable impurities.
Its microstructure is 3 area% or less (including 0 area%) island martensite (MA), 2 area% or less (including 0 area%) proeutectoid ferrite, and 95 area% or more (100 area%). A high-strength wire rod characterized by being a bainitic ferrite (including). The high-strength wire rod according to claim 1, wherein the high-strength wire rod satisfies the following relational expression 1.
[Relationship formula 1]

The high-strength wire rod according to claim 1, wherein the high-strength wire rod satisfies the following relational expression 2.
[Relational expression 2]

The high-strength wire rod according to claim 1, wherein the island-shaped martensite has a crystal grain size of 5 μm or less (excluding 0 μm). The high-strength wire rod according to claim 1, wherein the Ni content is 1.2 to 2.8% by weight. The method for manufacturing a wire rod according to claim 1.
By weight%, C: less than 0.05% (excluding 0%), Si: 0.05% or less (excluding 0%), Mn: 3.0 to 4.0%, P: 0.020% or less , S: 0.020% or less, Ni: 1.0 to 3.0%, B: 0.0010 to 0.0030%, Ti: 0.010 to 0.030%, N: less than 0.0030%, Al: 0.010 to 0.050%, the stage of reheating the steel material with the composition of Fe and other unavoidable impurities as the balance,
At the stage of hot rolling the reheated steel material to obtain a wire rod,
A step of primary cooling the wire from Bs ° C. to (Bs + 50) ° C. at a rate of 10 to 20 ° C./sec,
A step of secondary cooling of the primary cooled wire rod from (Bf-50) ° C. to Bf ° C. at a rate of 2 to 5 ° C./sec.
The stage of air-cooling the secondary cooled wire and
A method for manufacturing a high-strength wire rod, which is characterized by the above.
(However, Bs is 600 to 650 ° C. and Bf is 350 to 400 ° C.).
The method for producing a high-strength wire rod according to claim 6, wherein the reheating temperature at the time of reheating the steel material is 950 to 1050 ° C. The method for producing a high-strength wire rod according to claim 6, wherein the finishing hot rolling temperature at the time of hot rolling is 750 to 850 ° C.