JP6488008B2 - Wire material excellent in strength and impact toughness and method for producing the same - Google Patents

Description

  The present invention relates to a wire rod excellent in strength and impact toughness that can be used for parts such as industrial machines and automobiles that are exposed to various external load environments, and a method for manufacturing the same.

In recent years, attempts to reduce carbon dioxide emissions, which are the main cause of environmental pollution, have become a global concern, and as part of this, movements to regulate automobile exhaust gas are active. Automobile manufacturers are trying to solve this problem by improving fuel efficiency.
In order to improve the fuel efficiency, it is necessary to reduce the weight and performance of automobiles, and further increase the strength of automobile materials and parts. At the same time, automotive materials and parts are also strongly required to improve stability against external impacts, and impact toughness of materials and parts has been recognized as an important physical property item.

  Ferrite and pearlite-structured wire rods have limitations in securing high strength and excellent impact toughness. A material having such a structure is usually excellent in impact toughness but tends to have a relatively low strength. In addition, a method of performing cold wire drawing to increase the strength is known, but the method using cold wire drawing can provide high strength, but the impact toughness rapidly decreases in proportion to the strength improvement. Has the disadvantage of

  For this reason, in order to realize both high strength and excellent impact toughness, it is common to use a bainite structure or a tempered martensite structure. The bainite structure can be obtained by performing isothermal transformation heat treatment using a hot-rolled steel material, and the tempered martensite structure can be obtained by performing quenching and tempering heat treatment. However, since these structures cannot be obtained stably only by a general hot rolling and continuous cooling process, it is necessary to go through an additional heat treatment process as described above using a hot rolled steel material.

  If the required strength and impact toughness can be secured without such additional heat treatment, part of the process from material production to parts production can be omitted or simplified. The manufacturing cost can be reduced by improving the performance.

  However, a wire rod that can stably obtain a bainite or martensite structure using a hot rolling and continuous cooling step without an additional heat treatment step has not yet been developed, and development of such a wire rod is strongly desired. ing.

  The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to have high strength and excellent impact toughness only by hot rolling and continuous cooling processes without requiring an additional heat treatment process. An object of the present invention is to provide a wire rod and a method of manufacturing the same.

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

  In order to achieve the above-mentioned object, the wire material having excellent strength and impact toughness according to an embodiment of the present invention is carbon (C): 0.05 to 0.15%, silicon (Si): 0 by weight%. 0.2% or less, manganese (Mn): 3.0 to 4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010 0.0030%, Titanium (Ti): 0.010 to 0.030%, Nitrogen (N): 0.0050% or less, Aluminum (Al): 0.010 to 0.050%, the remainder is Fe and inevitable impurities The fine structure contains 90% or more of bainitic ferrite and the remainder includes island martensite (M / A) in area fraction.

  In order to achieve the above object, a method for manufacturing a wire rod having excellent strength and impact toughness according to an embodiment of the present invention is represented by weight%, carbon (C): 0.05 to 0.15%, silicon (Si ): 0.2% or less, manganese (Mn): 3.0-4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0 0.0010% to 0.0030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010 to 0.050%, the rest is Fe And a step of reheating the steel material containing inevitable impurities, a step of hot rolling the reheated steel material, and after the hot rolling, a phase transformation finish temperature Bf to a temperature range of Bf-50 ° C. is 0.1. A step of cooling at a rate of ˜2 ° C./s, and a step of air-cooling the cooled steel material , Characterized in that it comprises a.

  ADVANTAGE OF THE INVENTION According to this invention, the wire excellent in the intensity | strength and impact toughness which are requested | required of an industrial machine and the raw material or components for motor vehicles only using a hot rolling and a continuous cooling process can be provided.

  In addition, since the conventional additional heat treatment step can be omitted, it is very advantageous to reduce the entire manufacturing cost.

  Hereinafter, embodiments of the present invention will be described in detail.

  First, the wire according to the embodiment of the present invention will be described in detail. The wire of the present embodiment is, by weight, carbon (C): 0.05 to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0 to 4.0%, Phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.010 to 0.030%, Nitrogen (N): 0.0050% or less, Aluminum (Al): 0.010-0.050%, the remainder contains Fe and inevitable impurities.

  Hereinafter, the reason for limiting the steel component and the composition range of the wire rod of the present embodiment will be described in detail (hereinafter referred to as “% by weight”).

Carbon (C): 0.05 to 0.15%.
Carbon is an essential element for ensuring strength, and is dissolved in steel, or exists in the form of carbide or cementite. The simplest way to increase strength is to increase the carbon content to form carbides or cementite, but increasing the carbon content conversely reduces ductility and impact toughness, so It is necessary to adjust the content within a certain range. In the present embodiment, it is preferable to add so that the carbon content is in the range of 0.05 to 0.15%. This is because when the carbon content is less than 0.05%, it is difficult to obtain the target strength, and when it exceeds 0.15%, the impact toughness may rapidly decrease.

Silicon (Si): 0.2% or less.
Silicon, together with aluminum, is known as a deoxidizing element and is an element that improves strength. Silicon is known as an element that is dissolved in ferrite at the time of addition and has a very large effect on the increase in strength due to solid solution strengthening of the steel material. However, the strength is greatly increased by the addition of silicon, but the ductility and impact toughness are drastically reduced. Therefore, in the case of cold forged parts that require sufficient ductility, the addition of silicon is very limited. In the present embodiment, the silicon content is set to 0.2% or less in order to ensure excellent impact toughness while minimizing the decrease in strength. If the silicon content exceeds 0.2%, it may be difficult to ensure the target impact toughness, and it is more preferably 0.1% or less.

Manganese (Mn): 3.0-4.0%.
Manganese increases the strength of the steel and improves the hardenability, thereby facilitating the formation of low temperature structures such as bainite or martensite at a wide range of cooling rates. However, if the manganese content is less than 3.0%, the hardenability is not sufficient, and thus it is 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 suitable. Considering this, in the present embodiment, the manganese content is preferably set to 3.0 to 4.0%.

Phosphorus (P): 0.020% or less.
Phosphorus segregates at the grain boundaries to lower the toughness and cause a major cause of reduced delayed fracture resistance, so it is preferable not to include phosphorus as much as possible.For this reason, the upper limit is set in this embodiment. It is limited to 0.020%.

Sulfur (S): 0.020% or less.
Sulfur segregates at the grain boundaries to reduce toughness and forms a low melting point sulfide to inhibit hot rolling, so it is preferable that sulfur is not contained as much as possible. For this reason, the upper limit is limited to 0.020% in this embodiment.

Boron (B): 0.0010 to 0.0030%.
Boron is an element that improves the hardenability and is an element that diffuses into the austenite grain boundaries to suppress the formation of ferrite during cooling and facilitates the formation of bainite or martensite. However, if the added amount of boron is less than 0.0010%, the effect due to addition cannot be expected, and if it exceeds 0.0030%, further increase in the effect cannot be expected, The boron-based nitride is precipitated at the grain boundary, so that the grain boundary strength is lowered and hot workability may be lowered. Therefore, in consideration of such points, in this embodiment, the boron addition range is set to 0.0010 to 0.0030%.

Titanium (Ti): 0.010 to 0.030%.
Since titanium has the highest reactivity with nitrogen, nitride is formed first. The effect of improving hardenability by allowing boron to exist in a dissolved state by preventing the precipitation of BN when TiN is formed by the addition of titanium and the nitrogen in the steel is almost exhausted. Can be obtained. However, if the amount of titanium added is less than 0.010%, the effect of the addition becomes insufficient, and if it exceeds 0.030%, coarse nitrides may be formed and mechanical properties may be lowered. Considering such points, in this embodiment, the content of titanium is set to 0.010 to 0.030%.

Nitrogen (N): 0.0050% or less.
Nitrogen is maintained in a dissolved state with boron. In order to sufficiently exhibit the effect of improving the curability, it is preferable not to include as much as possible. Therefore, in this embodiment, the nitrogen content is preferably 0.0050% or less.

Aluminum (Al): 0.010 to 0.050%.
Aluminum is a powerful deoxidizing element that not only improves the cleanliness by removing oxygen in the steel, but also improves the impact toughness by forming AlN by combining with the solid solution nitrogen in the steel. Can be made. In the present embodiment, aluminum is positively added. However, if the content is less than 0.010%, it is difficult to expect the effect of adding aluminum. If the content exceeds 0.050%, alumina inclusions are present. It may be produced in large quantities and the mechanical properties may be greatly reduced. In consideration of such points, in the present embodiment, the aluminum content is preferably in the range of 0.010 to 0.050%.

  In addition to the above composition, chromium (Cr) may further be contained in an amount of less than 0.3%. Chromium, like manganese, increases the strength and hardenability of the steel. When the chromium content is 0.3% or more, the strength can be increased by the effect of improving the hardening ability and strengthening the solid solution, but the impact toughness may be lowered. In consideration of this, in this embodiment, it is preferable that the chromium content is in a range of less than 0.3%.

  The wire of this embodiment contains Fe and unavoidable impurities in addition to the above composition. In this embodiment, in addition to the alloy composition mentioned above, addition of other alloys is not excluded.

  On the other hand, in the present embodiment, it is preferable that the aforementioned manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) are contained so as to satisfy the following relational expression 1.

[Relational expression 1]
Mn + 5 (Ti-3.5N) /B≧5.0
(However, in the above relational expression 1, each of manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) means the weight-based content of the corresponding element.)

  In the present embodiment, manganese enhances the hardenability so that bainitic ferrite is easily generated even when the cooling rate is relatively low. In addition, titanium combines with nitrogen to form nitrides, and boron is sufficiently dissolved in steel to suppress the formation of ferrite so that bainitic ferrite is easily generated. To.

  As a result of repeated research and experiment focusing on the above points, the inventors of the present invention have found that the relationship between manganese, titanium, boron, and nitrogen is Mn + 5 (Ti-3.5N) / When satisfying B ≧ 5.0, it was recognized that a wire material having a bainitic ferrite structure having better strength and impact toughness can be provided, and the above relational expression 1 was derived.

  Moreover, it is preferable that manganese (Mn) and silicon (Si) of this embodiment are contained so as to satisfy the following relational expression 2.

[Relational expression 2]
Mn / Si ≧ 18
(However, in the above relational expression 2, each of manganese (Mn) and silicon (Si) means the weight-based content of the corresponding element.)

  In the present embodiment, manganese increases the hardenability so that bainite can be easily generated even when the cooling rate is relatively low. In addition, silicon has a disadvantage that it dissolves in steel and increases strength while decreasing impact toughness.

  As a result of repeated research and experiments focusing on the above points, the present inventors have obtained superior strength and impact toughness when the relationship between manganese and silicon satisfies Mn / Si ≧ 18 based on weight%. It has been confirmed that a wire material having a bainitic ferrite structure having the above can be provided, and the relational expressions of the present composition components have been presented.

On the other hand, in the wire according to the present embodiment, it is preferable that the ratio of the maximum manganese concentration [Mn _max ] and the minimum concentration [Mn _min ] in an arbitrary cross-sectional region satisfies the following relational expression 3.

[Relational expression 3]
[Mn _max ] / [Mn _min ] ≦ 3

  In the present embodiment, manganese enhances the hardenability so that bainitic ferrite is easily generated even when the cooling rate is relatively low. However, when manganese is segregated locally, martensite may be easily formed, and ferrite may be formed in the area depleted of manganese, resulting in uneven microstructure and impact toughness. May decrease.

  As a result of repeating research and experiment focusing on the above points, the present inventors have obtained excellent strength when the ratio of the maximum concentration and the minimum concentration of manganese in any cross-sectional area of the wire is 3 or less. In addition, the present inventors have confirmed that a wire rod having a bainitic ferrite structure having impact toughness can be provided, and have come to present this relational expression.

  Hereinafter, the microstructure of this embodiment will be described in detail.

  It is preferable that the microstructure of the wire according to this embodiment includes 90 area% or more bainitic ferrite and the remaining island martensite (M / A). Bainite, on the other hand, can be referred to in various terms depending on the carbon content or morphology. Generally, above medium carbon (about 0.2-0.45 wt%), it is called upper / lower bainite. However, in the low carbon range of 0.2% or less, it is called bainitic ferrite, acicular ferrite, granular ferrite, etc. depending on the temperature range. In the present embodiment, since it is a low carbon region, a bainitic ferrite structure is included.

  In the present embodiment, since the microstructure of the wire includes 90% by area or more of bainitic ferrite, it is possible to ensure excellent strength and impact toughness. If the phase fraction of a general ferrite that is not bainitic ferrite is too large, it may be advantageous in terms of impact toughness, but it is not preferable because a decrease in strength cannot be prevented.

  On the other hand, the island-shaped martensite is formed along the columnar bainitic ferrite crystal grain boundaries, and when the fraction is high, the strength of the steel material can be increased, but there is a risk of reducing the impact toughness. It is preferable to manage the fraction as low as possible. In consideration of this, in this embodiment, it is preferable to manage the fraction of the island-shaped martensite to 10% or less (in other words, the columnar bainitic ferrite structure is 90% or more) in area%. . In order to obtain such a microstructure of the wire, in this embodiment, after the steel material is hot-rolled, it is effective to obtain the microstructure of the wire by adjusting the cooling end temperature and the cooling rate during cooling. Can be achieved.

  The crystal grain size of the island martensite (M / A) is preferably 5 μm or less. If the crystal grain size of the island martensite (M / A) exceeds 5 μm, the area of the interface in contact with the bainitic ferrite matrix increases, so that impact toughness may be reduced.

  Next, a method for manufacturing the wire rod according to the present embodiment will be described in detail.

  The method of manufacturing the wire according to the present embodiment includes a step of reheating the steel having the above-described composition, a step of hot rolling the reheated steel material, a hot rolling, and then completing the phase transformation. A step of cooling from a temperature Bf to a temperature range of Bf-50 ° C. at a rate of 0.1 to 2 ° C./s, and a step of air-cooling the cooled steel material.

  First, in this embodiment, after providing the steel material which has the composition component mentioned above, this is reheated. The reheating temperature range that can be employed in the present embodiment may use a range of 1000 to 1100 ° C.

  The form of the steel material is not particularly limited, but in general, it is preferably in the form of a bloom or billet.

  Subsequently, the reheated steel material is hot-rolled to produce a wire. Although the finish hot rolling temperature in hot rolling is not specifically limited, It is preferable to manage in the range of 850-950 degreeC.

  The hot-rolled steel material is cooled, and this cooling is preferably performed at a cooling rate of 0.1 to 2 ° C / s from the phase transformation end temperature Bf to a temperature range of Bf-50 ° C. When the cooling end temperature is higher than Bf, it is difficult to secure a sufficient amount of bainitic ferrite structure. When the cooling end temperature is lower than Bf-50 ° C., the steel is sufficiently cooled and easy to handle. However, in order to reduce productivity, it is preferable that the cooling end temperature is in a temperature range from the phase transformation end temperature Bf to Bf−50 ° C. The Bf is a temperature at which the phase transformation from austenite to bainite or bainitic ferrite is completed.

  In this embodiment, the continuous cooling after hot rolling ensures the strength and impact toughness of the steel material by securing the bainitic ferrite structure. As a result, heat treatment such as quenching and tempering that has been conventionally performed can be omitted. Therefore, no additional steps are required, and the method of the present embodiment is very advantageous in terms of manufacturing cost.

  In the present embodiment, it is preferable to cool the section from the cooling start temperature to the cooling end temperature at a cooling rate of 0.1 to 2 ° C./s. If this cooling rate is less than 0.1 ° C./s, the formation of proeutectoid ferrite increases, and if it exceeds 2 ° C./s, the formation of martensite increases and the strength and impact toughness decrease. It is preferable to manage the cooling rate of embodiment at 0.1-2 degreeC / s.

  As described above, by securing the cooling rate in the cooling section, it is possible to obtain a wire having excellent strength and impact toughness having bainitic ferrite having an area fraction of 90% or more.

  Examples of the present invention will be described in detail below. The following examples are only for helping understanding of the present invention, and are not intended to limit the present invention.

  After casting molten steel having the composition components shown in Table 1 below, reheating at 1100 ° C. and rolling the wire to a diameter of 15 mm, cooling to 300 ° C. below the phase transformation end temperature Bf at the cooling rate shown in Table 2. Then, the wire was manufactured by air cooling. On the other hand, Bf, which is the bainite phase transformation end temperature, was measured using a dilatometer, and was slightly different depending on the chemical composition, but was in the range of 300 to 350 ° C.

  The microstructure of the wire thus produced was analyzed and shown in Table 2, and the tensile strength and impact toughness were measured and shown in Table 2. The area fraction and crystal grain size of island martensite (M / A) in the microstructure of the wire are measured using an image analyzer, and the manganese concentration is EPMA (Electron Probe Micro-Analysis). And measured.

  In the room temperature tensile test, the cross head speed was measured at a speed of 0.9 mm / min until the yield point and then 6 mm / min. The impact test was carried out at room temperature using an impact tester in which the curvature of the edge portion of the striker that applies impact to the specimen is 2 mm and the test capacity is 500 J.

（上記表１において、関係式１はＭｎ＋５（Ｔｉ−３．５Ｎ）／Ｂ、関係式２はＭｎ／Ｓｉであり、残りはＦｅ及び不可避不純物である。）

(In Table 1 above, relational expression 1 is Mn + 5 (Ti−3.5N) / B, relational expression 2 is Mn / Si, and the rest are Fe and inevitable impurities.)

（上記表２において、関係式３は［Ｍｎ_ｍａｘ］／［Ｍｎ_ｍｉｎ］である。）

(In Table 2 above, relational expression 3 is [Mn _max ] / [Mn _min ].)

  As shown in Tables 1 and 2 above, Examples 1 to 11 satisfying the composition and manufacturing method of the steel of the present invention all obtained bainitic ferrite of 90 area% or more, and mechanical properties were also It can be seen that a tensile strength of 600 to 700 MPa and an excellent impact toughness of 150 to 200 J are exhibited.

  In Example 8, it can be confirmed that when the silicon content is 0.1% by weight or less, the impact toughness is further improved. Among the above invention examples, the relational expression 1 of manganese, titanium, boron and nitrogen (Mn + 5 (Ti−3.5N) /B≧5.0) and the relational expression 2 of manganese and silicon (Mn / Si ≧ 18) It can be seen that Invention Examples 2, 3, 5, 7, 6, 9, and 11 satisfying all of the above are further superior in impact toughness when compared with the cases other than those.

  That is, among the above Examples, Examples 1 and 4 that do not satisfy Relational Expression 1 (Mn + 5 (Ti−3.5N) /B≧5.0) and / or Relational Expression 2 (Mn / Si ≧ 18), 6 and 10 show that the impact toughness is slightly inferior.

  On the other hand, although the comparative example 12 has high carbon content and is excellent in tensile strength, it can confirm that impact toughness is inferior. This is because carbon was dissolved in the M / A phase to increase the stable M / A phase. Comparative Example 13 is a case where the silicon content is outside the scope of the present invention, and silicon, like carbon, the silicon content added to the base increases as the amount of silicon added increases. The effect of solid solution strengthening comes to be produced. That is, even when the silicon addition amount is at a level of 0.25%, the tensile strength becomes very large, but the impact toughness decreases rapidly with that. In Comparative Example 14, since the addition amount of manganese and boron is small and the hardenability of the steel material is lowered, it is confirmed that even when the cooling condition is satisfied, the structure of ferrite and bainitic ferrite is mixed and the tensile strength is lowered. it can.

  On the other hand, in Comparative Example 15, although the steel composition components satisfy the scope of the present invention, martensite is formed as the cooling rate increases in the manufacturing process, so that the strength increases, but the impact toughness deteriorates. It is shown that. Comparative Example 16 shows that the steel composition components satisfy the scope of the present invention, but the strength decreases because ferrite is formed when the cooling rate is low in the manufacturing process.

  In Comparative Example 17, when the amount of titanium added is small, the amount of dissolved boron decreases, so that the hardenability decreases, and when the cooling rate is slow, the precipitation amount of pro-eutectoid ferrite increases and the tensile strength decreases. It shows that

  Further, in Comparative Example 18, when a large amount of manganese is added, the hardening ability is relatively too large, so that martensite is generated even when cooled at the cooling rate presented in the embodiment of the present invention. While the strength increased, the impact toughness decreased. Moreover, since manganese segregates in steel, it shows that impact toughness becomes inferior due to the formation of a locally non-uniform structure.

Claims (11)

In mass%, carbon (C): 0.05 to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0 to 4.0%, phosphorus (P): 0 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N): 0 .0050 percent, aluminum (Al): 0.010 to 0.050%, the remainder consisting of Fe and unavoidable impurities,
Microstructure, in area fraction, bainitic ferrite and the remaining 90% is seen contains the island martensite (M / A),
A wire rod excellent in strength and impact toughness, wherein the island-shaped martensite (M / A) has a crystal grain size of 5 μm or less .   The wire according to claim 1, wherein the wire further includes chromium (Cr): less than 0.3%. The wire material excellent in strength and impact toughness according to claim 1, wherein the contents of manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) satisfy the following relational expression 1. .
[Relational formula 1] Mn + 5 (Ti-3.5N) /B≧5.0 The wire material excellent in strength and impact toughness according to claim 1, wherein the contents of manganese (Mn) and silicon (Si) satisfy the following relational expression 2.
[Relational expression 2] Mn / Si ≧ 18 2. The wire has excellent strength and impact toughness according to claim 1, wherein the ratio of the maximum manganese concentration [Mn _max ] and the minimum concentration [Mn _min ] in an arbitrary cross section satisfies the following relational expression 3. wire.
[Relational expression 3] [Mn _max ] / [Mn _min ] ≦ 3 A method for manufacturing the wire according to claim 1, comprising:
In mass%, carbon (C): 0.05 to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0 to 4.0%, phosphorus (P): 0.0. 020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N): 0.0. 0050% or less, aluminum (Al): 0.010 to 0.050%, the rest is a stage of reheating a steel material composed of Fe and inevitable impurities;
Hot rolling the reheated steel material;
After the hot rolling, cooling from the cooling start temperature to the cooling end temperature set in the temperature range of the phase transformation end temperature (Bf) to (Bf-50) ° C. at a rate of 0.1 to 2 ° C./s; ,
Air cooling the cooled steel material;
The manufacturing method of the wire excellent in the intensity | strength and impact toughness characterized by including this. The method for producing a wire rod having excellent strength and impact toughness according to claim 6 , wherein the steel material further contains chromium (Cr): less than 0.3%. The wire material excellent in strength and impact toughness according to claim 6 , wherein the contents of manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) satisfy the following relational expression 1. Manufacturing method.
[Relational expression 1] Mn + 5 (Ti-3.5N) /B≧5.0 The method for producing a wire rod excellent in strength and impact toughness according to claim 6 , wherein the contents of manganese (Mn) and silicon (Si) satisfy the following relational expression 2.
[Relational expression 2] Mn / Si ≧ 18 The method of manufacturing a wire excellent in strength and impact toughness according to claim 6 , wherein the reheating step is performed in a temperature range of 1000 to 1100 ° C. The method for producing a wire rod having excellent strength and impact toughness according to claim 6 , wherein the finish hot rolling in the hot rolling step is performed in a temperature range of 850 to 950 ° C.