KR102065265B1 - 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 has an average particle diameter of 5 μm or lower and corresponds to 80% or higher in the equilibrium pro-eutectoid ferrite fraction calculated by a lever rule. And the tensile strength of the wire rod satisfies the following relation formula 1. [Relation formula 1] TS(MPa) >=>= 279+864*([C]+[Si]/8+[Mn]/18)

Description

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 rolling wire for shortening the soft heat treatment time, and more particularly, the cold rolling wire for reducing the subsequent soft nitriding heat treatment time by controlling the microstructure of the wire after rolling and It relates to a manufacturing method.

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)

Therefore, the present invention includes the microstructured ferrite having a grain size of 5 μm or less having a maximum grain size of not more than 80% of the equilibrium ferrite fraction of equilibrium, and the bainite / martensite area fraction of 5% or less, and residual pearlite structure. It is an object of the present invention to provide a cold-rolled wire rod and a method of manufacturing the same, by controlling the composite structure to shorten the soft-nitriding heat treatment time.

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 has an average particle diameter of 5 µm or less. It corresponds to more than 80% of the equilibrium saltpeter ferrite fraction calculated by the lever rule, and relates to a cold-rolled wire rod that can shorten the soft heat treatment time that the tensile strength satisfies the following equation (1).

[Relationship 1]

T S (MPa) ≥ 279 + 864 * ([C] + [Si] / 8 + [Mn] / 18)

In another aspect, the present invention after heating the steel having the composition in the 900 ~ 1050 ℃ range, 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 tensile strength of the cooled wire rod is a cold-rolled wire rod manufacturing method that can shorten the soft heat treatment time satisfying the following relational formula 1

[Relationship 1]

T S (MPa) ≥ 279 + 864 * ([C] + [Si] / 8 + [Mn] / 18)

     In addition, the cooled wire rod,

     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 has an average particle diameter of 5 µm or less. It may correspond to 80% or more of the equilibrium saltpeter ferrite fraction calculated by the lever rule.

In addition, the cooled wire is not drawn, the material is maintained in the temperature range of Ae1 ~ Ae1 + 40 ℃, and then cooled to 15 ~ 30 ℃ / hr up to 660 ℃, the temperature maintenance and cooling time total 10 ~ 15 It may further comprise a spheroidizing heat treatment process that is time.

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 2, and (b) shows Comparative Example 2. 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.

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 At least% is a cornerstone ferrite structure having an average particle diameter of 5 µm or less.

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 that the wire rod manufactured by the cooling satisfies the TS parameter of the following relational formula 1. In the case of the present invention, since the tensile strength due to the grain refinement of the manufactured wire rod is usually higher than that of the wire rod, it has a higher tensile strength that satisfies the TS parameter of the following Equation 1, and accordingly softening heat treatment time according to grain refinement It can help you shorten effectively.

The tensile strength of the wire increases with increasing alloying elements (C, Si, Mn), but the tensile strength of the wire is high due to the grain refinement despite the same alloy composition and microstructure (F + P). It is a characteristic of invention steel. Compared with the conventional material it is divided through the following relation 1, and when the following relation 1 is satisfied, there is an effect that can obtain a low tensile strength in the same spheroidized heat treatment material.

[Relationship 1]

T S (MPa) ≥ 279 + 864 * ([C] + [Si] / 8 + [Mn] / 18)

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 material is preferably cooled to 15 to 30 ° C./hr up to 660 ° C. after holding at Ae 1 to Ae 1 + 40 ° C. 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 Ae1 ~ Ae1 + 40 ℃ Cooled to / hr, it is preferable to proceed the heat treatment to maintain the temperature and cooling time for a total of 10 to 15 hours. The wire rod of the present invention subjected to the heat treatment process may exhibit low tensile strength due to diffusion acceleration of C due to grain refinement as compared with the wire rod prepared by a conventional method.

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 0.47 0.25 0.75 0.025 0.004 974 99 11 756 0.1 8 Comparative Example 2 0.26 0.16 0.58 0.026 0.004 1035 207 22 737 1.1 9 Comparative Example 3 0.46 0.29 0.77 0.028 0.005 985 97 12 842 1.2 8 Comparative Example 4 0.31 0.24 0.72 0.023 0.004 994 104 11 748 0.8 One Inventive Example 1 0.23 0.31 0.49 0.026 0.005 1023 92 12 747 1.3 6 Inventive Example 2 0.18 0.18 0.88 0.023 0.005 978 109 9 759 1.6 4 Inventive Example 3 0.44 0.24 0.92 0.025 0.004 1013 98 9 773 0.7 9 Inventive Example 4 0.36 0.22 0.78 0.025 0.005 992 101 11 764 0.9 11 Inventive Example 5 0.41 0.28 0.82 0.026 0.004 989 107 10 752 1.1 7

* Cooling conditions are the cooling rate (℃ / s) at which the wire surface temperature reaches 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 2, since it was heated for a long time compared to other conditions, it can be seen that AGS before finishing rolling is larger than other conditions. On the other hand, the small AGS before finishing rolling can produce a large amount of cornerstone ferrite at grain boundaries due to the deformation amount during finishing rolling, and thus, 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 The tensile strength Soft Nitriding Heat Treatment Condition Tensile Strength after Spherical Heat Treatment
(MPa) Cornerstone Ferrite Grain Size (㎛) Ferrite Equilibrium Ratio (%) Wire Ferrite Fraction (%) Tensile Strength (TS) a * Temperature (℃) Hours (h) Cooling rate up to 660 ℃ (℃ / h) Comparative Example 1 11 37 27 718 748 742 10 18 572 Comparative Example 2 14 72 52 524 549 741 12 25 458 Comparative Example 3 13 47 27 710 745 739 10 22 565 Comparative Example 4 15 61 41 571 607 742 11 27 485 Inventive Example 1 4 72 70 558 535 745 12 17 423 Inventive Example 2 5 77 76 523 496 737 11 22 392 Inventive Example 3 4 48 46 754 729 742 13 19 495 Inventive Example 4 4 52 50 682 651 742 11 25 467 Inventive Example 5 4 49 48 727 703 470 12 22 486

In Table 2, a * is the tensile strength (MPa) calculated by the TS parameter relation 1 {279 + 864 * ([C] + [Si] / 8 + [Mn] / 18)}.

In Comparative Example 1, since the amount of deformation during finish rolling was too small, 0.1, the cornerstone ferrite could not be induced by the deformation.

In the case of Comparative Example 2, as described above, since the furnace charging time was 207 minutes, which was longer than other conditions, AGS was larger than other conditions before finishing rolling, and thus, the cornerstone ferrite was not sufficiently induced during rolling.

In Comparative Example 3, the cornerstone ferrite was not induced because the rolling temperature was finish-rolled at a temperature higher than Ae3 at 842 ° C.

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

Accordingly, in Comparative Example 1-4, the average cornerstone ferrite grain size of the final wire rod is 10 μm or more, and thus the grain size is larger than that of the invention examples, which is a main cause of the wire strength being lower than that of the invention steel.

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.

On the other hand, in the present embodiment, looking at the TS parameter (279 + 864 * ([C] + [Si] / 8 + [Mn] / 18)), the tensile strength of the wire rod of the present invention examples is greater than the TS parameter, the tensile strength of the comparative examples It can be seen that the strength of the wires of the invention examples shows a significantly lower tensile strength of the heat treatment material after spheroidization compared to the wire tensile strength through rapid carbon diffusion during soft nitriding heat treatment due to the fine microstructure. Can be.

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 (4)

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 has an average particle diameter of 5 µm or less. More than 80% of the equilibrium cornerstone ferrite fraction calculated by the lever rule, and
A cold-rolled wire rod that can shorten a soft heat treatment time in which tensile strength satisfies the following expression 1.
[Relationship 1]
T S (MPa) ≥ 279 + 864 * ([C] + [Si] / 8 + [Mn] / 18)
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 tensile strength of the cooled wire rod is a cold-rolled wire rod manufacturing method that can shorten the soft heat treatment time satisfying the following relation 1.
[Relationship 1]
T S (MPa) ≥ 279 + 864 * ([C] + [Si] / 8 + [Mn] / 18)
The method of claim 2, wherein the cooled wire,
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 has an average particle diameter of 5 µm or less. And 80% or more of the equilibrium saltpeter ferrite fraction calculated by the lever rule, wherein the soft heat treatment time can be shortened.
The method of claim 2, wherein the cooled wire is not drawn, the material is maintained in the temperature range of Ae1 ~ Ae1 + 40 ℃ after cooling to 15 to 30 ℃ / hr to 660 ℃, the temperature holding and cooling time A method for manufacturing a cold-rolled wire rod which can shorten the soft heat treatment time, further comprising the spheroidizing heat treatment step of 10 to 15 hours in total.

Images