KR102065266B1 - Wire rod for chq capable of reducing softening treatment time, and method for manufaturing the same - Google Patents

Abstract

The present invention provides a wire rod for cold heading quality capable of reducing softening heat treatment time and a manufacturing method thereof. The wire rod for cold heading quality comprises 0.15-0.5 wt% of C, 0.02-0.4 wt% of Si, 0.3-1.2 wt% of Mn, 0.02-0.05 wt% of Al, 0.03 wt% or lower of P, lower than 0.01 wt% of S, lower than 0.01 wt% of N, and the remainder consisting of Fe and inevitable impurities, and has an internal structure comprising 20-90 area% of a pro-eutectoid ferrite structure, 5 area% or lower of a bainite and a martensite structure, and the remainder consisting of a perlite structure. The pro-eutectoid ferrite constituting the internal structure corresponds to 80% or higher in the equilibrium pro-eutectoid ferrite fraction calculated by a lever rule. The pro-eutectoid ferrite has a crystal grain size of 5 μm or lower on average and satisfies the following relation formula 1. The perlite has a perlite colony size of 4 μm or lower on average, and cementite constituting the perlite has an average major axis length of 4 μm or lower.

Description

Soft heat treatment time reduction cold-rolling wire and its manufacturing method {WIRE ROD FOR CHQ CAPABLE OF REDUCING SOFTENING TREATMENT TIME, AND METHOD FOR MANUFATURING THE SAME}

The present invention relates to the production of cold-rolled cold-rolled wire rod for shortening the soft heat treatment time, and more particularly, the cold-rolled wire rod and the method for manufacturing the same can be reduced by controlling the microstructure of the wire rod after rolling. It is about.

In order to soften the wire, spheroidization is generally performed. Spheroidal heat treatment spheroidizes cementite and induces homogeneous particle distribution in order to improve cold workability during cold forming. In addition, the hardness of the material to be processed can be lowered as much as possible in order to improve the life of the processing dies. In order to achieve the above two objects, it has been used as a concept of soft-nitriding materials.

Such spheroidization heat treatment is largely classified into two types. One is a method of heating for a long time below the vacancy temperature, which is mainly used for the spheroidizing treatment of hot rolled products. The other is a method of obtaining spheroidized tissue by ultra-cooling after heating between vacancy and austenitization temperatures.

When the initial structure is composed of pearlite, the process of spheroidization at the spheroidizing heat treatment temperature is performed by the diffusion of carbon at high temperature due to the defect of lamellar cementite or the difference in curvature with the flat interface at the end. Is generated and the lamellar cementite is known to be segmented and then spheronized to reduce interfacial energy.

Since the spheroidization soft nitriding treatment requires a lot of separate processes and costs and time, it is desirable to shorten the process time as much as possible. Therefore, research has been focused on the development of a technique for shortening the above-mentioned nodular soft nitriding treatment process.

Republic of Korea patent application 2018-0072965 (released July 2, 2018)

Accordingly, the present invention provides a cold-rolled wire rod and a method for manufacturing the same, which can shorten the heat treatment time of soft nitriding by controlling the wire structure of the wire rod manufactured by controlling the wire alloy composition and the manufacturing process conditions with the microstructure of the cornerstone ferrite and pearlite composite. For the purpose of providing it.

In addition, the technical problems to be achieved in the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned above are clearly understood by those skilled in the art from the following description. Could be.

The present invention for achieving the above object,

Weight% includes C: 0.15 to 0.5%, Si: 0.02 to 0.4%, Mn: 0.3 to 1.2%, Al: 0.02 to 0.05%, P: 0.03% or less, S: less than 0.01%, N: less than 0.01% And the remaining Fe and other unavoidable impurities,

The internal structure includes 20-90 area% of the cornerstone ferrite structure, 5 area% or less of bainite and martensite structure, and residual pearlite structure, and the cornerstone ferrite constituting the internal structure is applied to the lever rule. More than 80% of the equilibrium cornerstone ferrite fraction calculated by

The cornerstone ferrite has an average grain size of 5㎛ or less and satisfies the following relation 1,

The pearlite has a pearlite colony size of less than 4㎛ on average, and relates to a cold-rolled wire rod that can shorten the soft heat treatment time, characterized in that the average length of cementite major axis of the pearlite is 4㎛ or less.

[Relationship 1]

Cornerstone Ferrite Fraction (%) ≥ -105 * [C] +79

In the present invention, the wire rod may have a shape ratio of cementite (cementite) of 2.5 or less after soft nitridation heat treatment.

In another aspect, the present invention, after heating the steel having the composition component in the 900 ~ 1050 ℃ range, the step of maintaining within 180 minutes;

Controlling the austenite grain size (AGS) of the steel in the range of 5 to 20 μm;

Finishing hot rolling of the steel in which the AGS is controlled to a wire shape at a deformation amount of 0.3 to 2.0 at a temperature of less than or equal to Ae ₃ to 730 ° C .; And

And cooling the finished hot rolled wire at a cooling rate of 3 to 20 ° C./s.

The cooled wire rod has an internal structure of 20-90 area% of the cornerstone ferrite structure, 5 area% or less of bainite and martensite structure, and residual pearlite structure, and the cornerstone ferrite constituting the internal structure is a lever. 80% or more of the equilibrium saltpeter ferrite fraction calculated by a rule rule, wherein the saltpeter ferrite has a grain size of 5 µm or less on average and satisfies the above Equation 1, and the pearlite has a pearlite of 4 µm or less on average. It has a colony size, and the average length of cementite long axis (cementite) forming the pearlite is 4㎛ or less relates to a method for producing a wire rod for cold-rolled cold forging.

In addition, the cooled wire is cooled to 15 ~ 30 ℃ / hr up to 660 ℃ after holding in the temperature range of Ae1 ~ Ae1 + 40 ℃, at this time, the process of spheroidizing heat treatment wherein the temperature maintenance and cooling time is a total of 10 to 15 hours It may further include.

After the spheroidization heat treatment process, the shape ratio of spherical carbides may satisfy 2.5 or less.

As described above, the configuration of the present invention can obtain a wire rod having desired characteristics through optimization of the manufactured wire structure microstructure through a relatively short soft-nitriding heat treatment time, thereby reducing manufacturing cost and time. It has a useful effect.

1 is a texture photograph showing AGS of steel before finishing hot rolling, (a) shows Inventive Example 1, and (b) shows Comparative Example 1. FIG.
2 is a structure showing the microstructure of the wire rod obtained by cooling after wire rod rolling, (a) shows invention example 4, and (b) shows comparative example 4. FIG.
3 is a structure showing the microstructure of the wire rod after spheroidization heat treatment, (a) shows Inventive Example 4, and (b) shows Comparative Example 4. FIG.

Hereinafter, the present invention will be described.

In the present invention, C: 0.15 to 0.5%, Si: 0.02 to 0.4%, Mn: 0.3 to 1.2%, Al: 0.02 to 0.05%, P: 0.03% or less, S: less than 0.01%, N: 0.01% Of heat-treated short-wired wires, which contain less than, steel and other remaining Fe and other unavoidable impurities, to form a cornerstone ferrite by rolling to induce grain refinement to obtain a soft wire by accelerating the diffusion of carbon during soft heat treatment of the material It relates to a manufacturing method.

The wire composition and the reason for the content limitation of the present invention will be described. Here,% means% by weight unless otherwise defined.

C: 0.15-0.5%

The reason for limiting the content of carbon to 0.15 to 0.5% is that if the content is more than 0.5%, almost all tissues are composed of pearlite, and it is difficult to secure the target cornerstone ferrite grains, and at less than 0.15%, the fraction of the cornerstone ferrite increases. This is because the grains are not fine and it is difficult to transform into martensitic microstructure during QT heat treatment, and even in the martensite structure, it is difficult to secure sufficient strength due to the low carbon content.

Si: 0.02-0.4%

The content of the silicon (Si) is limited to 0.02 to 0.4%, for the following reason. Si is a representative substitution type element and has a great influence on securing the strength of steel. If the content is less than 0.02%, it is difficult to secure the strength of the steel, and if the content is more than 0.4%, it is necessary to further remove the cost by encouraging the production of decarburized tissue during wire rolling, and it is difficult to forge because the strength increases during forging.

Mn: 0.3 ~ 1.2%

The manganese (Mn) forms a substituted solid solution in the matrix and lowers the temperature of A1 to refine the interlayer gap between pearlite, and increases the crystal grains in the cornerstone ferrite tissue, so its content is limited to 0.3-1.2%. When the manganese is added in excess of 1.2%, it has a harmful effect due to tissue heterogeneity caused by manganese segregation. When the steel solidifies, macro segregation and micro segregation tend to occur depending on the segregation mechanism. Manganese segregation promotes segregation due to its relatively low diffusion coefficient compared to other elements. This is the main reason for generating. In addition, when the manganese is added in less than 0.3%, it may be difficult to secure sufficient hardenability for securing the martensite structure after QT.

Al: 0.02% to 0.05%

In the present invention, the aluminum content is preferably limited to 0.02 ~ 0.05%. If the content is less than 0.02%, it is difficult to secure sufficient deoxidation power, and if it is 0.05%, hard inclusions such as Al2O3 may increase, and nozzle clogging may occur due to inclusions during playing.

N: less than 0.01%

In the present invention, the content of nitrogen should be controlled to less than 0.01%. This is because, above 0.01%, a decrease in material phosphorus / ductility may occur due to solid nitrogen which is not bound as a precipitate.

P: 0.03% or less, S: less than 0.01%

Since P and S are impurities, P segregates at grain boundaries and degrades recognition, so the content thereof is preferably limited to 0.03% or less. In addition, since S is a low melting point element, the grain boundary segregation lowers toughness and forms an emulsion, which has a detrimental effect on the product.

In addition, the cold-rolled wire rod of the present invention has an internal structure of 20-90 area% of the cornerstone ferrite structure, 5 area% or less of bainite and martensite structure, and the residual pearlite structure, 80 of the equilibrium cornerstone ferrite fraction More than% is the cornerstone ferrite tissue. The cornerstone ferrite has an average grain size of 5 μm or less and satisfies the following Equation 1, and the pearlite has a pearlite colony size of 4 μm or less on average, and an average length of cementite major axes forming the pearlite may be 4 μm or less. .

[Relationship 1]

Cornerstone Ferrite Fraction (%) ≥ -105 * [C] +79

First, the cold-rolled wire rod of the present invention has an internal structure of 20-90 area% of the cornerstone ferrite structure, 5 area% or less of bainite and martensite structure, and residual pearlite structure, the equilibrium of the cornerstone ferrite fraction More than 80% are cornerstone ferrite tissue

In the present invention, the equilibrium saltpeter ferrite fraction means a saltpeter ferrite fraction at a temperature directly above A1 in the state diagram of each composition. In the present invention, Thermo calc. The state diagram calculated in the software was used.

The present invention is characterized by having a cornerstone ferrite structure having such an equilibrium cornerstone ferrite fraction of 80% or more. Compared with the cornerstone ferrite in the wire rod generated and grown during normal cooling, the cornerstone ferrite fraction of the steel of the present invention is produced by the conventional method because the cornerstone ferrite is produced and grown during finishing rolling and grown during cooling at temperatures below Ae ₃ to 730. It is higher than the cornerstone ferrite fraction in the wire rod of the same composition produced.

In the present invention, the reason why the average grain size of the cornerstone ferrite is limited to 5 μm or less is because the cornerstone ferrite is rapidly formed during finishing rolling to refine the grains, which accelerates the diffusion of carbon through the fine grains during the post-soft nitriding heat treatment. This is because spheroidized tissue can be obtained in a shorter time than usual. The reason why the area ratio of the bainite and martensite structure is controlled to 5% or less is that the material may be disconnected during the drawing process or uncoil before the soft nitriding heat treatment if the tissue is present.

 In the present invention, it is preferable to control the cornerstone ferrite fraction so as to satisfy the above relational expression 1. This is because the amount of cornerstone ferrite that satisfies the relation 1 is rapidly generated during finish rolling to obtain a cornerstone size of 5 µm or less, which may shorten the nitriding heat treatment time.

In addition, in the present invention, the pearlite has a pearlite colony size of 4 µm or less on average, and the average length of the cementite major axis forming the pearlite is required to be controlled to 4 µm or less, which facilitates segmentation and spheroidization of cementite by fine grains. Because.

Next, the method of manufacturing an ultrafine wire rod which can accelerate the soft nitridation heat treatment of the present invention will be described in detail.

Cold-rolled wire rod manufacturing method that can shorten the soft nitridation heat treatment time of the present invention, after heating the steel material having the above-mentioned composition in the range 900 ~ 1050 ℃, maintaining within 180 minutes; Controlling the austenite grain size (AGS) of the steel in the range of 5 to 20 μm; Finishing hot rolling of the steel in which the AGS is controlled to a wire shape at a deformation amount of 0.3 to 2.0 at a temperature of less than or equal to Ae ₃ to 730 ° C .; And cooling the finished hot rolled wire at a cooling rate of 3 to 20 ° C / s.

First, the present invention is heated to 900 ~ 1050 ℃ range of the steel having the above-mentioned composition components, and maintained within 180 minutes. If the heating temperature exceeds 1050 ℃ AGS grows significantly to induce the cornerstone ferrite with a higher amount of deformation during the finish rolling, there is a problem to refine the grains, and if it is less than 900 ℃ to overload the equipment by increasing the amount of reduction during rough rolling Because. If the holding time exceeds 180 minutes, AGS grows largely for the same reason as above, and thus, there is a problem in miniaturizing grains by inducing the cornerstone ferrite with more deformation amount during finish rolling.

Subsequently, in the present invention, the austenitic grain size (AGS) of the steel is controlled in the range of 5 to 20 μm immediately before finishing hot rolling. The reason for controlling the austenite grain size (AGS) in this way is 0.3 or more deformation during finish rolling. This is to induce the cornerstone ferrite to refine the grains. If the size is larger than 20㎛, more finish rolling amount is required, so it is difficult to refine the grain, and to make AGS material of 5㎛ or less during rough rolling, more deformation amount is required than the conventional manufacturing method, so that the billet size is increased, There is a problem of process constraints because the material feed rate must be increased to reduce the interpass time.

In the present invention, the AGS-controlled steel material is hot-rolled to a wire shape having a deformation amount of 0.3 to 2.0 at a temperature of Ae ₃ or less to 730 ° C. or more.

At this time, it is preferable to control the hot finishing temperature range of Ae ₃ or less ~ 730 ℃ or more, if the Ae ₃ temperature is exceeded, cornerstone ferrite is not produced, it is disadvantageous to grain refinement, and below 730 ℃ pearlite during rolling This is because it is disadvantageous to grain refinement, and the rolling temperature is low because the rolling temperature is overloaded.

And it is preferable to make the deformation amount to 0.3 ~ 2.0, which is less than 0.3, the deformation amount is small and can not induce the cornerstone ferrite can not refine the crystal grains, if it is 2.0 or more, the rolling amount overload due to the increase in deformation amount and the diameter of the desired material It is difficult to manufacture.

Subsequently, in the present invention, the finished hot rolled wire is cooled at a cooling rate of 3 to 20 ° C./s to obtain a wire in which the internal microstructure as described above is controlled to fine grains. At this time, the reason for controlling the cooling rate in the range of 3 ~ 20 ℃ / s is to suppress the grain growth of ferrite grain size (FGS) 5㎛ or less after the end of hot rolling.

In the present invention, the wire rod manufactured may be spheroidized. Conventional agglomerated steel wire spheroidization heat treatment is prepared by a slow cooling after maintaining the temperature at the temperature of Ae1 ~ Ae1 + 40 ℃, in the present invention 15 ~ 30 ℃ to 660 ℃ after maintaining the material in the temperature range of Ae1 ~ Ae1 + 40 ℃ / hr was cooled and heat treatment was performed for 10-15 hours.

Hereinafter, the present invention will be described in detail through examples.

(Example)

The billet having the component composition shown in Table 1 was rolled 9mm wire rod. The invention examples satisfy the component range and production conditions of the present invention, and the comparative examples are outside the production conditions of the present invention.

division Alloy component (% by weight) Heating condition Finish rolling Cooling condition
C Si Mn Al N Temperature (℃) Holding time (min) AGS (μm) before rolling Rolling temperature (℃) Strain Comparative Example 1 0.17 0.19 0.62 0.026 0.004 1157 92 25 753 One 5 Comparative Example 2 0.36 0.29 0.45 0.025 0.005 1032 115 14 840 1.2 8 Comparative Example 3 0.42 0.26 0.9 0.027 0.005 989 87 10 749 0.1 7 Comparative Example 4 0.48 0.32 0.69 0.025 0.004 979 94 9 762 0.8 One Inventive Example 1 0.18 0.25 0.36 0.023 0.004 962 95 9 761 1.4 5 Inventive Example 2 0.24 0.11 0.49 0.025 0.004 1022 107 10 754 1.6 8 Inventive Example 3 0.36 0.32 0.77 0.026 0.005 1046 121 10 749 0.5 4 Inventive Example 4 0.48 0.19 0.62 0.024 0.005 988 88 11 759 0.8 9 Inventive Example 5 0.44 0.36 0.94 0.027 0.004 994 99 9 768 1.1 11

* Cooling conditions in Table 1 are the cooling rates (℃ / s) at which the wire surface temperature reaches up to 500 ℃.

1 is a texture photograph showing AGS of steel before finishing hot rolling, (a) shows Inventive Example 2, and (b) shows Comparative Example 2. FIG. AGS was measured using the ASTM E112 method.

In the case of Comparative Example 1, since the heating temperature was heated at a high temperature condition of 1157 ℃ compared to other conditions, AGS before the finish rolling was larger than other conditions. The small AGS before finishing rolling can produce a large amount of cornerstone ferrite at grain boundaries by the amount of deformation during finishing rolling, and the grain size of the final wire can be made small by the formation and growth of the cornerstone ferrite during rolling.

Table 2 shows the microstructure and mechanical properties of the wire microstructure and the soft nitriding material of the soft nitriding heat treatment made by the above manufacturing conditions.

division
Wire Rod Microstructure Soft Nitriding Heat Treatment Condition cementi
te average
Aspect ratio Average cornerstone ferrite grain size (㎛) Cornerstone ferrite equilibrium fraction (%) b * Cornerstone ferrite fraction (%) Pearlite Colony Average Size (㎛) cementite long axis average size (㎛) Heat treatment temperature
(℃) Heat treatment time (h) Cooling rate up to 660 ℃ (℃ / h) Comparative Example 1 11 81 61 57 9 8 740 10 17 5.2 Comparative Example 2 13 58 41 37 14 13 742 13 22 6.3 Comparative Example 3 12 48 35 27 19 17 746 11 17 6.8 Comparative Example 4 10 37 29 21 22 19 737 12 25 7.2 Inventive Example 1 4 79 60 78 2 2 742 10 28 1.8 Inventive Example 2 5 72 54 70 3 3 746 11 17 1.7 Inventive Example 3 5 59 41 58 3 2 738 13 22 2.2 Inventive Example 4 4 38 29 36 3 2 744 12 25 1.4 Inventive Example 5 4 44 33 43 4 4 741 11 19 1.9

In Table 2, b * is the value of -105 * [C] +79 in relation 1 for the cornerstone ferrite fraction.

First, in the case of Comparative Example 1, as described above, the AGS before the finish rolling was larger than other conditions, so that the cornerstone ferrite was not sufficiently induced during the rolling.

In the case of Comparative Example 2, because the rolling temperature is finish-rolled at a temperature higher than Ae3 at 840 ° C, the cornerstone ferrite was not induced.

In Comparative Example 3, since the amount of deformation during finish rolling was too low as 0.1, the cornerstone ferrite due to deformation could not be induced.

In Comparative Example 4, the cornerstone ferrite grew too much at a cooling rate of 1 ° C./s at which the wire surface temperature reached 500 ° C., but the average grain size grew too much.

Therefore, in Comparative Example 1-4, the average cornerstone ferrite grain size of the final wire rod was more than 10 μm, and at the same time, the average size of the pearlite colonies and the average length of the cementite major axis were larger than those of the wire rods of the inventive examples. 2 is a structure showing the microstructure of the wire rod obtained by cooling after wire rod rolling, (a) shows invention example 4, and (b) shows comparative example 4. FIG.

Inventive Example 1-5 having a fine wire microstructure in comparison with the comparative examples promoted the spheroidization rate through the accelerated diffusion of carbon during the subsequent soft nitriding heat treatment, it was possible to have a cementite aspect ratio of 2.5 or less after the soft nitriding heat treatment. That is, in the case of the present invention, it can be confirmed that since the formation and growth of the cornerstone ferrite during the finish rolling compared to the comparative example, having a balanced cementite ferrite fraction of more than 80% as a structure, spherical fire material having an excellent cementite shape ratio during the same nodular heat treatment It can be seen that there is an effect to obtain a microstructure. 3 is a structure showing the microstructure of the wire rod after spheroidization heat treatment, (a) shows Inventive Example 4, and (b) shows Comparative Example 4. FIG.

On the other hand, the relational expression 1 is a parameter that expresses the case of 80% or more of the equilibrium saltpeter ferrite fraction 100% of the composition. When the conventional cooling rate for preparing the cornerstone ferrite and pearlite structure in the alloy composition of the present invention steel and comparative steel is applied (not the quench cooling rate at which the low temperature tissue is produced), the cornerstone ferrite and pearlite by the C element content Since the fraction of the tissue is dominant, the linear linear parameters such as the above were obtained by using the C content by regression analysis.

As described above, although the present invention has been described by way of limited embodiments and experimental examples, the present invention is not limited thereto and is intended by those skilled in the art to which the present invention pertains. Of course, various modifications and variations are possible within the scope of equivalents of the claims to be described below.

Claims (5)

Weight% includes C: 0.15 to 0.5%, Si: 0.02 to 0.4%, Mn: 0.3 to 1.2%, Al: 0.02 to 0.05%, P: 0.03% or less, S: less than 0.01%, N: less than 0.01% And the remaining Fe and other unavoidable impurities,
The internal structure includes 20-90 area% of the cornerstone ferrite structure, 5 area% or less of bainite and martensite structure, and residual pearlite structure, and the cornerstone ferrite constituting the internal structure is applied to the lever rule. Equivalent to at least 80% of the equilibrium saltpeter ferrite fraction calculated by
The cornerstone ferrite has an average grain size of 5㎛ or less and satisfies the following relation 1,
The pearlite has a pearlite colony size of less than 4 ㎛ average, and the average length of cementite long axis (cementite) forming the pearlite is characterized in that the soft heat treatment time for shortening the soft heat treatment time.
[Relationship 1]
Cornerstone Ferrite Fraction (%) ≥ -105 * [C] +79
The wire rod for cold rolling as claimed in claim 1, wherein, after soft nitriding heat treatment, a shape ratio of cementite is 2.5 or less.
Weight% includes C: 0.15 to 0.5%, Si: 0.02 to 0.4%, Mn: 0.3 to 1.2%, Al: 0.02 to 0.05%, P: 0.03% or less, S: less than 0.01%, N: less than 0.01% And, after heating the steel material containing the remaining Fe and other unavoidable impurities in the 900 ~ 1050 ℃ range, and maintaining within 180 minutes;
Controlling the austenite grain size (AGS) of the steel in the range of 5 to 20 μm;
Finishing hot rolling of the steel in which the AGS is controlled to a wire shape at a deformation amount of 0.3 to 2.0 at a temperature of less than or equal to Ae ₃ to 730 ° C .; And
And cooling the finished hot rolled wire at a cooling rate of 3 to 20 ° C./s.
The cooled wire rod has an internal structure of 20-90 area% of the cornerstone ferrite structure, 5 area% or less of bainite and martensite structure, and residual pearlite structure, and the cornerstone ferrite constituting the internal structure is a lever. It corresponds to more than 80% of the equilibrium saltpeter ferrite fraction calculated by a rule rule, the saltpeter ferrite has a grain size of 5㎛ or less on average and satisfies the following relation 1, the pearlite is a pearlite of 4㎛ or less on average A method for manufacturing a soft heat treatment short-circuit cold rolling rod having a colony size, and an average length of cementite major axes forming the pearlite is 4 µm or less.
[Relationship 1]
Cornerstone Ferrite Fraction (%) ≥ -105 * [C] +79
The method of claim 3, wherein the cooled wire is cooled to 15 to 30 ℃ / hr up to 660 ℃ after holding in the temperature range of Ae1 ~ Ae1 + 40 ℃, wherein the temperature maintenance and cooling time is a total of 10 to 15 hours A method for producing a soft heat treatment short-circuit cold pressurizing wire rod, further comprising a process of phosphorous nodular heat treatment.
5. The method of claim 4, wherein the spherical carbide has a shape ratio of 2.5 or less after the spheroidizing heat treatment process.

Images