KR20220059372A - Steel wire rod with excellent impact toughness and its manufacturing method - Google Patents

Abstract

The present invention relates to a wire rod which can be applied as materials for a structure requiring an excellent impact toughness, and a manufacturing method thereof. Disclosed are a wire rod, which can be applied as materials for a welding rod as needed, and a manufacturing method thereof. According to an embodiment of the present invention, the wire rod with excellent impact toughness comprises: 0.05-0.8 wt% of C; 0.05-0.7 wt% of Si; 16-31 wt% of Mn; 0.020 wt% or lower of P; 0.010 wt% or lower of S; 0.15-0.40 wt% of N; 8.0 wt% or lower of Ni; 1.0-3.5 wt% of Cr; 1.0-3.5 wt% of Mo; 0.1-0.3 wt% of V; and remaining Fe and unavoidable impurities. A fine tissue includes 95 area% or more of austenitic phase. The fraction occupied by Σ3 twin crystal boundary among the high inclinometer of the austenite crystal particle may be 20% or more.

Description

Wire rod with excellent impact toughness and manufacturing method thereof

The present invention relates to a wire rod applicable as a structural material requiring excellent impact toughness and a method for manufacturing the same, and to a wire rod applicable as a material for welding rods if necessary and a method for manufacturing the same.

Materials for mechanical structures used in various industrial fields such as automobiles and bridges need to have not only excellent strength but also the ability to absorb external shocks. In particular, bridge steel wires used in cold regions such as northern Europe, North America, and Siberia and ropes used in deep seas must have excellent impact toughness because brittle fracture does not occur at low temperatures and must be maintained stably for a long time.

As for steel whose microstructure is made of ferrite-pearlite, the higher the strength, the poorer the impact toughness tends to be. Therefore, when a material satisfying high strength and high impact toughness at the same time is required, tempered martensite or bainitic steel is usually used. However, since tempered martensite or bainite structural steel does not have enough impact toughness to be used in cryogenic slides such as cold areas or deep seas, a steel material having superior impact toughness is required.

Korean Patent Laid-Open Publication No. 10-2016-0078621 (published on July 05, 2016)

In order to solve the above problems, the present invention is to provide a wire rod excellent in strength and impact toughness and a manufacturing method thereof by controlling the microstructure through the alloy composition and manufacturing method.

As a means for achieving the above object, the wire rod having excellent impact toughness according to an embodiment of the present invention is, by weight, C: 0.05 to 0.8%, Si: 0.05 to 0.7%, Mn: 16 to 31%, P: 0.020 % or less, S: 0.010% or less, N: 0.15 to 0.40%, Ni: 8.0% or less, Cr: 1.0 to 3.5%, Mo: 1.0 to 3.5%, V: 0.1 to 0.3%, the remainder being Fe and unavoidable impurities The microstructure may include 95% or more of the austenite phase as an area fraction, and the fraction occupied by the Σ3 twin boundary among the high hardness boundaries of the austenite grains may be 20% or more.

In addition, in the wire rod having excellent impact toughness of the present invention, the grain size of the austenite may be 5㎛ or less.

In addition, in the wire rod having excellent impact toughness of the present invention, the tensile strength may be 900 to 1100 MPa.

In addition, in the wire rod having excellent impact toughness of the present invention, the impact toughness at -196 ℃ may be 100J or more.

In addition, as another means for achieving the above object, the method for manufacturing a wire rod having excellent impact toughness according to an embodiment of the present invention is, by weight, C: 0.05 to 0.8%, Si: 0.05 to 0.7%, Mn: 16 to 31%, P: 0.020% or less, S: 0.010% or less, N: 0.15 to 0.40%, Ni: 8.0% or less, Cr: 1.0 to 3.5% or less, Mo: 1.0 to 3.5%, V: 0.1 to 0.3%, The remainder is a step of reheating the steel containing Fe and unavoidable impurities to 1050 to 1150 ° C., hot rolling to a finish hot rolling temperature of 850 to 950 ° C., and winding, cooling to 5 ° C. / s or more, reduction in area ratio of 70% Including the step of cold drawing as above and annealing at 600 to 800 ° C., including 95% or more of the austenite phase as an area fraction as the microstructure of the wire rod, and the Σ3 twin boundary among the high hardness boundaries of the austenite grains occupies The fraction may be greater than or equal to 20%.

According to the present invention, by controlling the matrix structure of the wire rod with fine austenite, it is possible to provide a wire rod having excellent impact toughness required for materials or parts for automobiles, construction and marine applications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes preferred embodiments of the present invention. However, the embodiment of the present invention may be modified in various other forms, and the technical idea of the present invention is not limited to the embodiment described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression unless the context clearly requires it to be singular. In addition, as used in this application, terms such as "comprises" or "includes" are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, and other features It should be noted that the use is not intended to preliminarily exclude the existence of elements, steps, functions, components, or combinations thereof.

On the other hand, unless otherwise defined, all terms used herein should be regarded as having the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Accordingly, unless explicitly defined herein, specific terms should not be construed in an unduly idealistic or formal sense. For example, a singular expression herein includes a plural expression unless the context clearly dictates otherwise.

In addition, in this specification, "about", "substantially", etc. are used in or close to the numerical value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used in a precise sense to help the understanding of the present invention. or absolute figures are used to prevent unreasonable use by unconscionable infringers of the mentioned disclosure.

The wire rod having excellent impact toughness according to an embodiment of the present invention is, by weight, C: 0.05 to 0.8%, Si: 0.05 to 0.7%, Mn: 16 to 31%, P: 0.020% or less, S: 0.010% or less, N: 0.15 to 0.40%, Ni: 8.0% or less, Cr: 1.0 to 3.5%, Mo: 1.0 to 3.5%, V: 0.1 to 0.3%, the remainder may include Fe and unavoidable impurities.

The reason for limiting the component range of each alloy element will be described below.

The content of C may be 0.05 to 0.8% by weight.

C is an austenite-forming element, which increases strength and improves impact toughness in low-temperature environments. When the C content is less than 0.05 wt %, it is difficult to secure a target strength, and when the C content exceeds 0.8 wt %, the impact toughness may be inferior due to carbide precipitation.

The content of Si may be 0.05 to 0.7 wt%.

Si is not only useful for deoxidation of steel, but also effective for securing strength through solid solution strengthening, but is an element that has inferior impact properties. In order to obtain sufficient strength, Si may be included in an amount of 0.05 wt% or more. However, when the content exceeds 0.7% by weight, the impact toughness may be reduced.

The content of Mn may be 16 to 31% by weight.

Mn is an advantageous element for stabilizing the austenite phase and greatly improving the impact toughness at room temperature as well as low temperature. If the Mn content is less than 16% by weight, the impact toughness may be inferior due to the processing-induced martensitic transformation at low temperature, and if the content exceeds 31% by weight, the strength may be reduced.

The content of P may be 0.020 wt% or less.

P is segregated at grain boundaries and may reduce toughness, so it is preferable not to be included as much as possible in steel materials. In the present invention, P is limited to 0.020% by weight or less.

The content of S may be 0.010 wt% or less.

Since S is an element that not only reduces toughness by segregating P and grain boundaries, but also inhibits hot rolling by forming a low-melting emulsion, it is preferable not to be included in steel materials as much as possible. In the present invention, S is limited to 0.010% by weight or less.

The content of N may be 0.15 to 0.40 wt%.

N is an element advantageous for stabilizing the austenite phase and greatly improving the impact toughness at room temperature as well as low temperature. When the N content is less than 0.15% by weight, the effect of improving the strength at room temperature is not large, and when it exceeds 0.40% by weight, the impact toughness may be inferior.

The content of Ni may be 8.0 wt% or less.

Ni is an austenite phase stabilizing element and improves impact toughness. However, as Ni is an expensive element, when more than a certain amount is added, the manufacturing cost is greatly increased, and when added in excess, it may cause a decrease in strength at room temperature. Therefore, in the present invention, the Ni content is managed to 8.0% by weight or less.

The content of Cr may be 1.0 to 3.5% by weight.

Cr is a ferrite phase stabilizing element, and promotes the formation of ferrite to reduce the austenite phase region. Increasing the amount of Cr is advantageous in improving the strength at room temperature. If the Cr content is less than 1.0% by weight, the effect of improving the strength at room temperature is not great, and if the Cr content exceeds 3.5% by weight, carbides may be formed to deteriorate the impact toughness, and there is a fear that the austenite phase region may be reduced. .

The content of Mo may be 1.0 to 3.5% by weight.

Mo has a better effect of improving strength at room temperature than Cr, and it is not easy to form carbides, so there is little decrease in impact toughness. For this effect, in the present invention, Mo may be added in an amount of 1.0% by weight or more. However, as Mo is an expensive element, when a certain amount or more is added, the manufacturing cost greatly increases, and when added in excess, it also causes a decrease in cryogenic impact toughness. Accordingly, in the present invention, the Mo content is managed to 3.5% by weight or less.

The content of V may be 0.1 to 0.3 wt%.

V has better room temperature strength improvement effect than Cr and Mo. For this effect, in the present invention, V may be added in an amount of 0.1% by weight or more. However, V is an expensive element and is limited to 0.3% by weight or less because manufacturing cost increases significantly when added in a certain amount or more.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The wire rod having excellent impact toughness of the present invention contains 95% or more of an austenite phase as an area fraction as a microstructure. If a large amount of carbide is formed in the steel, the austenite phase stability is lowered, and if a large amount of processed martensite is formed, the structure becomes non-uniform and there is a problem that the impact toughness is poor. It is desirable to exclude them as much as possible.

The smaller the austenite grain size, the better the impact toughness is improved by the fine grain size. In the wire rod having excellent impact toughness according to an embodiment of the present invention, the grain size of austenite may be 5 μm or less.

The austenite structure of the wire rod according to an embodiment of the present invention is formed through a static recrystallization process, and a misorientation angle between crystals of austenite is 15° or more. High angle boundary The fraction occupied by the Σ3 twin boundary may be 20% or more. As the fraction of twin boundaries increases, the austenite grain size is further refined to improve impact toughness. Therefore, in order to secure excellent impact toughness improvement, it is desirable to control conditions such as alloy composition and manufacturing process so that the fraction of Σ3 twin boundary is 20% or more. Here, the fraction of the Σ3 twin boundary means a value expressed as a percentage of the length occupied by the twin boundary among the total length of the high inclination system.

Next, a method for manufacturing a wire rod having excellent impact toughness according to the present invention will be described in detail. The method of manufacturing a wire rod according to an embodiment of the present invention comprises the steps of reheating the steel material including the alloy composition to 1050 to 1150 ° C., hot rolling to a finish hot rolling temperature of 850 to 950 ° C., and winding, 5 ° C./s It may include a step of cooling to above, cold drawing to a reduction ratio of 70% or more, and annealing at 600 to 800°C.

First, a steel material having the above-described alloy composition is prepared and then reheated. The reheating temperature range may be 1050 to 1150 °C. If the reheating temperature is less than 1050℃, it may be difficult to secure the target microstructure and mechanical strength because the precipitates contained in the steel are completely dissolved and not re-dissolved in austenite. On the other hand, when the temperature exceeds 1150° C., coarse austenite grains grow to deteriorate mechanical properties, and surface oxidation may be severe to cause surface defects.

Then, the reheated steel is hot rolled. At this time, the finish hot rolling temperature may be 850 to 950 ℃. If the finish hot rolling temperature is less than 850°C, the effect of refining the grains may be small because dynamic recrystallization is not sufficiently activated. Therefore, in the present invention, the finish hot rolling temperature is preferably controlled in the temperature range of 850 to 950 ℃.

Then, after winding the hot-rolled steel material may be cooled to 5 ℃ / s or more. If the cooling rate is less than 5°C/s, precipitates are formed during cooling, and the strength is excessively increased, and there is a risk that the impact toughness is inferior.

Then, the cooled steel may be cold drawn with a reduction ratio of 70% or more. If the reduction ratio is less than 70%, it is not preferable because the size of the crystal grains formed through the static recrystallization process during annealing cannot be sufficiently fine because the amount of accumulated deformation is small.

Subsequently, the cold drawn steel may be annealed at 600 to 800°C. If the annealing temperature is less than 600 ℃, the static recrystallization cannot be completed, so the strength is greatly increased, and there is a possibility that the impact toughness may be inferior. On the other hand, when the annealing temperature exceeds 800°C, the strength may be reduced due to excessive growth of crystal grains.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

{Example}

A steel material having an alloy composition according to Table 1 was prepared, and then, a wire rod was manufactured in the order of reheating-hot rolling-winding-cooling-cold drawing-annealing under the conditions of Table 2 below.

Table 2 shows the austenite grain size and Σ3 twin boundary fraction among grain boundaries for the manufactured wire rods. The austenite grain size was measured using an image analyzer, and the twin boundary fraction was obtained using backscattered electron diffraction (EBSD).

In addition, the tensile strength and low-temperature impact toughness of the manufactured wire rods were evaluated through a room temperature tensile test and a low temperature (-196° C.) Charpy impact test (V-notch), and the results are shown in Table 2. Tensile strength was measured according to JIS Z 2241 using a JIS No. 4 test piece, and Charpy impact toughness was measured according to JIS Z 2242 using a JIS Z 2202 test piece.

steel grade Alloy composition (wt%) C Si Mn P S N Ni Cr Mo V One 0.25 0.20 23 0.020 0.007 0.30 3 2.1 2.3 0.1 2 0.32 0.35 28 0.019 0.005 0.25 2 1.8 2.0 0.3 3 0.65 0.40 19 0.017 0.010 0.20 3 3.3 1.4 0.2 4 0.13 0.37 25 0.010 0.007 0.23 6 1.2 3.1 0.2 5 0.41 0.62 21 0.016 0.008 0.35 5 2.5 1.3 0.3 6 0.33 0.33 18 0.011 0.006 0.22 6 1.5 2.7 0.1 7 0.24 0.25 26 0.015 0.009 0.18 3 2.1 1.9 0.2 8 0.52 0.21 25 0.013 0.005 0.48 4 2.7 2.0 0.3 9 0.34 0.35 29 0.014 0.003 0.09 3 1.0 1.1 0.1

steel grade reheat
temperature
(℃) finish
hot
rolled
temperature
(℃) Cooling
speed
(℃/s) cold
drawing
reduction rate
(%) Annealing
temperature
(℃) Auste
Night
decision
granularity
(μm) Σ3
twin
boundary
fraction
(%) Seal
robbery
(MPa) Shock
tenacity
(J) Invention Example 1 One 1100 910 5 70 700 3 25 980 175 Invention example 2 2 1080 890 6 80 650 One 30 1010 160 Invention example 3 3 1130 930 8 75 680 2 27 1090 115 Invention Example 4 4 1090 940 7 85 750 4 34 950 170 Comparative Example 1 5 950 920 5 75 690 2 29 1150 70 Comparative Example 2 6 1250 1050 6 70 700 33 23 790 190 Comparative Example 3 7 1120 1030 5 80 730 16 26 880 175 Comparative Example 4 One 1110 930 One 75 720 3 23 1110 80 Comparative Example 5 2 1100 950 6 40 780 8 20 890 160 Comparative Example 6 3 1070 910 5 78 550 - 0 1580 5 Comparative Example 7 4 1090 930 7 73 900 50 21 680 210 Comparative Example 8 8 1080 940 5 77 710 3 27 1370 55 Comparative Example 9 9 1060 880 6 80 790 5 31 885 180

As shown in the results of Tables 1 and 2 above, the wire rods of Inventive Examples 1-4 satisfying the alloy composition and manufacturing conditions defined in the present invention had a high tensile strength of 900 to 1100 MPa, and at the same time an impact at -196°C. It can be seen that the toughness is 100J or more, and the impact toughness is excellent at low temperature.

On the other hand, in Comparative Example 1, the strength was greatly increased and the impact toughness was inferior because the precipitates contained in the steel were completely dissolved and not re-dissolved in austenite due to the low reheating temperature.

In Comparative Example 2, the reheating temperature and the finish hot rolling temperature were high, so that the austenite grains were very large, and the tensile strength was greatly reduced.

In Comparative Example 3, the high finish hot rolling temperature resulted in very large austenite grains, resulting in a significant decrease in tensile strength.

In Comparative Example 4, the cooling rate after winding was low, and precipitates were formed in the matrix tissue, so that the tensile strength was greatly increased, and the impact toughness was inferior.

In Comparative Example 5, the amount of critical strain capable of static recrystallization during subsequent annealing was insufficient because the cold drawing reduction ratio was too low. Due to this, the final austenite grain size was not sufficiently small, resulting in inferior strength.

In Comparative Example 6, since the annealing temperature was low, recrystallization did not occur properly, and only the recovery process occurred, so it was difficult to form a high hardness boundary, so the grain boundary was not clear, it was difficult to observe twins, and the strength was greatly increased because a large amount of unresolved dislocations remained , the impact toughness became very poor.

In Comparative Example 7, the annealing temperature was too high, and grain growth occurred after recrystallization, which resulted in a significant decrease in strength.

In Comparative Example 8, the solid solution strengthening effect was greatly increased as the amount of nitrogen dissolved in the austenite matrix was greatly increased due to the excessive amount of nitrogen added. As a result, the tensile strength became very large, while the impact toughness was inferior.

In Comparative Example 9, the amount of nitrogen added was too small, and the amount of nitrogen dissolved in the austenite matrix was greatly reduced, so that the solid solution strengthening effect was greatly reduced. As a result, the tensile strength was somewhat inferior.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art may not depart from the concept and scope of the claims described below. It will be appreciated that various modifications and variations are possible.

Claims (5)

By weight%, C: 0.05 to 0.8%, Si: 0.05 to 0.7%, Mn: 16 to 31%, P: 0.020% or less, S: 0.010% or less, N: 0.15 to 0.40%, Ni: 8.0% or less, Cr: 1.0 to 3.5%, Mo: 1.0 to 3.5%, V: 0.1 to 0.3%, the balance contains Fe and unavoidable impurities,
A wire rod having excellent impact toughness, including an austenite phase as an area fraction of 95% or more as a microstructure, and having a Σ3 twin boundary in a high hardness boundary of the austenite grains of 20% or more. The method of claim 1,
The austenite grain size is 5㎛ or less, excellent impact toughness wire rod. The method of claim 1,
A wire rod having excellent impact toughness with a tensile strength of 900 to 1100 MPa. The method of claim 1,
A wire rod with excellent impact toughness of 100J or more at -196°C. By weight%, C: 0.05 to 0.8%, Si: 0.05 to 0.7%, Mn: 16 to 31%, P: 0.020% or less, S: 0.010% or less, N: 0.15 to 0.40%, Ni: 8.0% or less, Cr: 3.5% or less, Mo: 1.0 to 3.5%, V: 0.1 to 0.3%, the rest of the step of reheating the steel containing Fe and unavoidable impurities to 1050 to 1150 ℃;
Hot rolling to a finish hot rolling temperature of 850 to 950 ℃, and winding;
cooling to 5° C./s or more;
cold drawing with a reduction rate of 70% or more; and
Including; annealing at 600 to 800 ° C.
A method of manufacturing a wire rod having excellent impact toughness, comprising 95% or more of an austenite phase as an area fraction as a microstructure, and 20% or more of a Σ3 twin boundary among the high hardness boundaries of the austenite grains.