WO2016072681A1 - Wire rod having enhanced strength and impact toughness and preparation method for same - Google Patents

Abstract

The present invention relates to a wire rod having enhanced strength and impact toughness and a preparation method for same, the wire rod which can be used for components of an industrial machine, vehicle and the like that are exposed to various environments of external loads.

Description

Wire rod with excellent impact toughness and manufacturing method

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

Recently, efforts to reduce the emission of carbon dioxide, which is the main cause of environmental pollution, have become a global issue. As a part of this, the movement to regulate the exhaust gas of automobiles is active, and as a countermeasure, automakers are trying to solve this problem by improving fuel economy. However, in order to improve fuel efficiency, it is required to reduce the weight and performance of automobiles, and thus, the necessity of high strength of automobile materials or components is increasing. In addition, since the demand for stability against external impact is increasing, impact toughness is also recognized as an important physical property of a material or a component.

The wire of ferrite or pearlite structure has a limit in securing excellent strength and impact toughness. Materials with these structures generally have high impact toughness, but relatively low strength, and when cold drawn to increase strength, high strength can be obtained, but impact toughness decreases sharply in proportion to strength increase. There is this.

Therefore, in general, bainite or tempered martensite is used to realize excellent strength and impact toughness simultaneously. The bainite structure can be obtained by constant temperature heat treatment using hot rolled steel, and the temper martensite structure can be obtained by quenching and tempering heat treatment. However, these tissues cannot be stably obtained by the usual hot rolling and continuous cooling processes alone, and thus must be subjected to such additional heat treatment using hot rolled steel.

If it is possible to secure high strength and excellent impact toughness without additional heat treatment, a part of the process from material to part production can be omitted or simplified, thereby improving productivity and lowering manufacturing cost.

However, wire rods that can stably obtain bainite or martensite structure using hot rolling and continuous cooling processes without additional heat treatment have not yet been developed, and thus there is a demand for wire rod development.

The present invention is to provide a wire rod and a method for manufacturing the same that can have a high strength and excellent impact toughness only by hot rolling and continuous cooling process without an additional heat treatment process.

The problem to be solved by the present invention is not limited to the above-mentioned problem, another task that is not mentioned will be clearly understood by those skilled in the art from the following description.

One embodiment of the present invention by weight, carbon (C): 0.05 ~ 0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% 5.0% or less, chromium (Cr): 0.5-2.0% , Phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.010% to 0.050%, the rest includes Fe and unavoidable impurities,

The microstructure provides a wire rod having excellent impact toughness, including an area fraction of 95% or more martensite and the remainder of retained austenite (γ).

Another embodiment of the present invention by weight, carbon (C): 0.05 ~ 0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% 5.0% or less, chromium (Cr): 0.5 ~ 2.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.010 to 0.050%, the rest of which reheats the steel containing Fe and unavoidable impurities;

Hot rolling the reheated steel;

After the hot rolling, cooling to a temperature range of Mf ~ Mf-50 ℃ at a rate of 0.2 ℃ / s or more; And

It provides a method of producing a wire with excellent impact toughness comprising the step of air cooling the cooled steel.

The present invention according to the above-described configuration, by using only the hot rolling and continuous cooling process can provide a wire rod excellent in strength and impact toughness required in the material or parts for industrial machinery and automobiles.

In addition, the conventional additional heat treatment process can be omitted, which is very advantageous to reduce the overall manufacturing cost.

Hereinafter, the present invention will be described in detail. The present invention relates to a wire rod having excellent impact toughness only by hot rolling and continuous cooling process without the additional heat treatment process such as constant temperature transformation, quenching and tempering, in order to secure high strength and excellent impact toughness, and a method of manufacturing the same.

First, the wire rod of the present invention will be described in detail. Wire rod of the present invention in weight%, carbon (C): 0.05 ~ 0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% and 5.0% or less, chromium (Cr): 0.5-2.0%, Phosphorus (P): 0.020% or less, Sulfur (S): 0.020% or less, aluminum (Al): 0.010% to 0.050%, the rest includes Fe and unavoidable impurities.

Hereinafter, the reason for limitation of the steel component and the composition range of the wire rod of the present invention will be described in detail (hereinafter,% by weight).

Carbon (C): 0.05-0.15%

Carbon is an essential element for securing strength and is dissolved in steel or exists in carbide or cementite form. The easiest way to increase the strength is to increase the carbon content to form carbides or cementite, but on the contrary, ductility and impact toughness decrease, so it is necessary to control the amount of carbon added within a certain range. In the present invention, it is preferable to add the C content in the range of 0.05 to 0.15%, because if the carbon content is less than 0.05%, it is difficult to obtain the target strength, and if it exceeds 0.15%, the impact toughness may be drastically reduced.

Silicon (Si): 0.2% or less

Silicon, together with aluminum, is known as a deoxidation element and is an element that improves strength. Silicon is known to be an element that is very effective in increasing the strength through solid solution strengthening of steel as it is dissolved in ferrite when added. However, since the strength is greatly increased by the addition of silicon, but the ductility and impact toughness decrease rapidly, the addition of silicon is very limited in the case of cold forged parts that require sufficient ductility. In the present invention to minimize the drop in strength, in order to ensure excellent impact toughness, the content of the silicon is included in less than 0.2%. If the silicon content exceeds 0.2%, it may be difficult to secure the target impact toughness. More preferably, it contains 0.1% or less.

Manganese (Mn): greater than 3.5%, less than 5.0%

Manganese increases the strength of the steel and improves the hardenability to facilitate the formation of low temperature structures such as bainite or martensite at a wide range of cooling rates. However, if the manganese content is 3.5% or less, the hardenability is not sufficient, so it is difficult to stably secure the low temperature structure by the continuous cooling process after hot rolling. If it exceeds 5.0%, segregation of Mn during coagulation is likely to be facilitated. In consideration of this, in the present invention, it is preferable that the content of manganese more than 3.5%, 5.0% or less.

Chromium (Cr): 0.5-2.0%

Chromium, like manganese, increases the strength and hardenability of steels and improves impact toughness, especially when added with manganese. However, if the chromium content is less than 0.5%, the effect of improving strength, hardenability and impact property is not great. If the chromium content is more than 2.0%, it is effective for improving strength and hardenability, but the impact property may be lowered. In consideration of this, in the present invention, it is preferable to include the content of chromium in 0.5 ~ 2.0%.

Phosphorus (P): 0.020% or less

Since phosphorus is segregated at grain boundaries to lower toughness and reduce delayed fracture resistance, it is preferable not to be included as much as possible, and for this reason, the upper limit thereof is limited to 0.020%.

Sulfur (S): 0.020% or less

The sulfur segregates at grain boundaries, lowers toughness, forms low melting emulsions, and inhibits hot rolling, so it is preferably not included. For this reason, the upper limit of the present invention is limited to 0.020%.

Aluminum (Al): 0.010 ~ 0.050%

Aluminum is a powerful deoxidation element that removes oxygen in steel to improve cleanliness, and also combines with nitrogen dissolved in steel to form AlN, thereby improving impact toughness. In the present invention, although aluminum is actively added, if the content is less than 0.010%, the effect of addition is difficult to be expected. If the content exceeds 0.050%, a large amount of alumina inclusions are generated, and mechanical properties can be greatly reduced. In consideration of this point, in the present invention, it is preferable to make the aluminum content in the range of 0.010% to 0.050%.

In addition to the above composition, the rest includes Fe and unavoidable impurities. The present invention does not exclude the addition of alloys other than the alloy compositions mentioned above.

On the other hand, in the present invention, the content of the manganese (Mn), chromium (Cr) and carbon (C) is preferably contained so as to satisfy the following relational formula (1).

[Relationship 1]

4.0 ≤ C (Mn + Cr) 5/50 ≤ 9.0

However, in the relation 1, manganese (Mn), chromium (Cr) and carbon (C) refer to the weight-based content of the corresponding element, respectively.

In the present invention, by controlling the content of manganese, chromium and carbon as shown in the relation 1, it is possible to manufacture a wire rod having more excellent impact toughness. That is, manganese and chromium increase the hardenability, so that martensite is easily generated even when the cooling rate is relatively low, and carbon and chromium having a low content can greatly contribute to improving the impact toughness of martensite.

In addition, the content of the manganese (Mn) and silicon (Si) in the present invention is preferably contained so as to satisfy the following relation 2.

[Relationship 2]

Mn / Si ≥ 22

However, in the relation 2, manganese (Mn) and silicon (Si) refer to the content by weight of the corresponding element, respectively.

Manganese in the present invention increases the hardenability to help the martensite is easily produced even when the cooling rate is relatively small. In addition, silicon is dissolved in steel to increase strength, but impact toughness is lowered.

The present inventors focused on the above point, and as a result of repeated studies and experiments, when the relationship between the manganese and silicon satisfies Mn / Si ≥ 22 on a weight percent basis, the present invention provides a wire of martensite structure having excellent strength and impact toughness. It is to confirm that it can be done and to present this compositional relation.

On the other hand, in the wire rod of the present invention, it is preferable that the ratio of the maximum concentration [Mn _max ] and the minimum concentration [Mn _min ] of manganese in an arbitrary cross-sectional area satisfies the following expression (3).

[Relationship 3]

[Mn _max ] / [Mn _min ] ≤ 4

In the present invention, the manganese is easy to produce martensite even when the cooling rate is relatively small by increasing the hardenability, but martensite can be easily produced when the manganese is locally segregated, while in the region where manganese is depleted Can be formed, the microstructure becomes non-uniform, impact toughness may be inferior.

The present inventors have focused on the above points, and have repeatedly conducted research and experiments to provide a wire rod of martensite structure having excellent strength and impact toughness when the ratio between the maximum concentration and the minimum concentration of manganese in any cross-sectional area of the wire rod is 4 or less. We can confirm that we can and present this relationship.

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

The microstructure of the wire rod of the present invention contains not less than 95 area% of martensite and residual residual austenite (γ). Martensite of the present invention is characterized by low carbon content, high strength, high ductility, and excellent impact toughness. However, when the amount of bainite or residual austenite other than the martensite is increased, the impact toughness may be somewhat advantageous, but it is not preferable because the reduction in strength cannot be prevented. Therefore, the wire rod of the present invention contains at least 95 area% of martensite.

The wire rod of the present invention preferably has a circular cross section, a tensile strength of 1000 to 1200 MPa, and an impact value of 80 J or more.

Next, the method of manufacturing the wire rod of the present invention will be described in detail.

Method for producing a wire rod of the present invention, after providing a steel having the above-described composition, the step of reheating it; Hot rolling the reheated steel; After the hot rolling, the step of cooling to a temperature range of Mf ~ Mf-50 ℃ at a rate of 0.2 ℃ / s or more; And air-cooling the cooled steel material.

First, in the present invention, after preparing the steel having the above-described composition components, it is reheated. The reheating temperature range employable in the present invention may be in the range of 1000 to 1100 ° C.

Although the form of the said steel is not specifically limited, Usually, it is preferable that it is a bloom or billet form.

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

The hot rolled steel is cooled, the cooling is preferably cooled to a cooling rate of 0.2 ℃ / s or more to the temperature range of Mf ~ Mf-50 ℃. If the cooling end temperature exceeds Mf, it is difficult to secure a sufficient amount of martensite structure. If the cooling end temperature is less than Mf-50 ° C, the steel is sufficiently cooled and easy to handle, but the cooling end temperature is lower than that of Mf ~ Mf-50 ° C. It is preferable to set it as a temperature range. The Mf means the temperature at which the phase transformation from austenite to martensite is terminated.

In the present invention, by performing continuous cooling after hot rolling to secure the martensite structure to ensure excellent strength and impact toughness. Thus, heat treatments such as quenching and tempering, which have been previously performed, may be omitted, and thus, an additional process is not required, and thus, there is a very advantageous advantage in terms of manufacturing cost.

In the present invention, it is preferable to cool the section from the cooling start temperature to the cooling end temperature at a cooling rate of 0.2 ° C / s or more. Cooling at a cooling rate of at least 0.2 ℃ / s, and then air-cooled to obtain a structure with martensite of 95% or more of the area fraction.

Hereinafter, embodiments of the present invention will be described in detail. The following examples are only for the understanding of the present invention, and the present invention is not limited by the examples.

(Example)

After casting the molten steel having a composition component of Table 1, after reheating it to 1100 ℃ and wire rod rolled to a diameter of 15mm, after cooling to 150 ℃ below the Mf temperature at the cooling rate of Table 2 and air-cooled to prepare a wire rod . On the other hand, Mf, the phase transformation end temperature of martensite, was measured using a dilatometer, and was slightly different depending on chemical composition, and showed a range of 150 to 200 ° C.

The wire rod thus prepared was shown in Table 2 by analyzing the microstructure, and the tensile strength and impact toughness were measured and shown in Table 2. Meanwhile, the concentration of manganese was measured by using Electron Probe Micro-Analysis (EPMA).

In addition, the room temperature tensile test was measured by performing a crosshead speed of 0.9mm / min to the yield point, 6mm / min after that. In addition, the impact test was measured at room temperature using an impact tester having a curvature of the edge portion of the striker impacting the specimen of 2mm and a test capacity of 500J.

No.	Ingredients (% by weight)							Relationship 1	Relation 2
	C	Si	Mn	Cr	P	S	Al
One	0.07	0.17	4.1	1.0	0.017	0.020	0.024	4.83	24.1
2	0.09	0.19	3.9	1.4	0.014	0.017	0.029	7.53	20.5
3	0.08	0.15	3.8	0.9	0.011	0.015	0.035	3.67	25.3
4	0.06	0.16	4.7	0.7	0.016	0.013	0.018	5.51	29.4
5	0.12	0.14	3.6	1.5	0.015	0.014	0.034	8.28	25.7
6	0.14	0.20	4.3	1.2	0.011	0.012	0.026	14.09	21.5
7	0.07	0.08	3.7	1.8	0.019	0.013	0.043	7.05	46.2
8	0.11	0.18	4.5	0.8	0.015	0.016	0.015	9.20	25.0
9	0.07	0.16	3.7	2.5	0.014	0.013	0.038	12.83	23.1
10	0.18	0.16	4.2	0.5	0.011	0.015	0.033	8.26	26.3
11	0.11	0.17	5.3	0.8	0.018	0.014	0.027	18.58	31.2
12	0.06	0.15	2.6	1.5	0.016	0.017	0.021	1.39	17.3
13	0.10	0.24	3.8	1.8	0.012	0.011	0.025	11.01	15.8
14	0.08	0.14	3.6	1.4	0.015	0.012	0.032	5.00	25.7
15	0.09	0.18	4.3	0.2	0.017	0.016	0.036	3.32	23.9

(1 equations in Table 1 is ^{C (Mn + Cr) 5/} 50, equation 2 is a Mn / Si, the remainder being Fe and inevitable impurities)

(Equation 3 in Table 2 is [Mn _max ] / [Mn _min ])

As shown in Table 1 and 2, Inventive Examples 1 to 8 satisfying the emphasis and the manufacturing method of the present invention are all obtained in the martensite structure of 95 area% or more, excellent tensile strength of 1000 ~ 1200MPa or more and 80J or more It can be seen that impact toughness is shown.

On the other hand, Example 7 is a case where the content of silicon is 0.1% by weight or less, it can be seen that very excellent impact toughness and elongation can be secured compared to other invention examples. In addition, among the above inventions, satisfies relation 1 (4.0 ≦ C (Mn + Cr) ⁵ /50≦9.0) of manganese, chromium and carbon content and relation 2 (Mn / Si ≧ 22.0) of manganese and silicon. 1, 4, 5 and 7 can be seen that the impact toughness is more excellent than when not.

That is, in Examples 2, 3, 6, and 8 that do not satisfy relation 1 (4.0 ≦ C (Mn + Cr) ⁵ /50≦9.0) and / or relation 2 (Mn / Si ≧ 22.0), It can be seen that the impact toughness is relatively inferior.

Comparative Example 9 is a case where the chromium component is outside the scope of the present invention, but the strength is increased, but the ductility decreases and eventually the impact toughness is inferior. Comparative Example 10 is a case where the content of carbon exceeds the range of the present invention, the strength is greatly increased due to the increase in the martensite matrix solid solution strengthening effect of carbon, there is a problem that the impact toughness is very low.

Comparative Example 11 is a case in which the manganese component is out of the scope of the present invention, but the strength is increased, but the ductility decreases, and thus the impact toughness is worsened. The segregation of manganese in the steel also shows that the impact toughness is inferior due to the formation of locally uneven tissue.

Comparative Example 12 is a case where the manganese is added less than the component range of the present invention, because the relatively low curing ability, when the cooling rate is small, forming a bainite structure instead of martensite, the impact toughness increases, but the strength is reduced Is showing. In addition, Comparative Example 13 is a case where the silicon is contained beyond the component range of the present invention, it can be seen that even when the addition amount of 0.52% level, the tensile strength is greatly increased and the impact toughness is drastically reduced.

Comparative Example 14 shows that when the steel composition of the present invention is satisfied but the cooling rate is too slow, bainite structure is formed instead of martensite, thereby increasing impact toughness but decreasing strength. In addition, Comparative Example 15 containing less chromium can be seen that the impact toughness is not good.

Claims (9)

1. By weight%, carbon (C): 0.05-0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% and 5.0% or less, chromium (Cr): 0.5-2.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.010% to 0.050%, the rest includes Fe and inevitable impurities,

   Wire structure with excellent impact toughness including microstructure as an area fraction and 95% or more of martensite and remaining austenite (γ).
2. The method according to claim 1,

   The content of the manganese (Mn), chromium (Cr) and carbon (C) is excellent wire toughness satisfying the following relational formula (1).

   [Relationship 1]

   4.0 ≤ C (Mn + Cr) 5/50 ≤ 9.0
3. The method according to claim 1,

   The wire of the manganese (Mn) and silicon (Si) is excellent impact toughness that satisfies the following relation 2.

   [Relationship 2]

   Mn / Si ≥ 22.0
4. The method according to claim 1,

   The wire is an excellent impact toughness of the ratio of the maximum concentration [Mn _max ] and the minimum concentration [Mn _min ] of manganese in any cross section satisfies the following equation 3.

   [Relationship 3]

   [Mn _max ] / [Mn _min ] ≤ 4
5. By weight%, carbon (C): 0.05-0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% and 5.0% or less, chromium (Cr): 0.5-2.0%, phosphorus (P): Reheating the steel including 0.020% or less, sulfur (S): 0.020% or less, aluminum (Al): 0.010 to 0.050%, and the remainder Fe and unavoidable impurities;

   Hot rolling the reheated steel;

   After the hot rolling, cooling to a temperature range of Mf ~ Mf-50 ℃ at a rate of 0.2 ℃ / s or more; And

   Air cooling the cooled steel

   Method for producing a wire rod excellent impact toughness comprising a.
6. The method according to claim 5,

   The content of the manganese (Mn), chromium (Cr) and carbon (C) is a method of producing a wire rod excellent impact toughness that satisfies the following relation 1.

   [Relationship 1]

   4.0 ≤ C (Mn + Cr) 5/50 ≤ 9.0
7. The method according to claim 5,

   The content of the manganese (Mn) and silicon (Si) is a method for producing a wire rod excellent impact toughness that satisfies the following relation 2.

   [Relationship 2]

   Mn / Si ≥ 22.0
8. The method according to claim 5,

   The reheating temperature is a method for producing a wire rod excellent in impact toughness performed at 1000 ~ 1100 ℃.
9. The method according to claim 5,

   Finishing of the hot rolling The hot rolling is a method for producing a wire rod having excellent impact toughness at a temperature range of 850 to 950 ° C.