EP4324945A1

EP4324945A1 - Drawn wire material, and method for producing drawn wire material

Info

Publication number: EP4324945A1
Application number: EP22788204.0A
Authority: EP
Inventors: Kazuhiro Ishimoto
Original assignee: Tokyo Rope Manufacturing Co Ltd
Current assignee: Tokyo Rope Manufacturing Co Ltd
Priority date: 2021-04-15
Filing date: 2022-04-14
Publication date: 2024-02-21
Also published as: CN117222770A; KR20230169306A; JPWO2022220281A1; WO2022220281A1

Abstract

Provided is a wiredrawn product drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, wherein a GOS value/average crystal grain size is greater than or equal to -0.6 × GAM value + 1.5 at a grain boundary setting angle of 2° and a step number of 0.07 µm.

Description

TECHNICAL FIELD

The present invention relates to a wiredrawn product and a method for manufacturing a wiredrawn product.

BACKGROUND ART

Wiredrawn products, typically wires and wire ropes made by twisting multiple wires, are made of steels which are referred to as wire rods produced by hot rolling by steel manufacturers, such as specifically hard carbon steel wire rods (JIS G 3506) and piano wire rods (JIS G 3502). The wire rods such as hard carbon steel wire rods and piano wire rods produced by the steel manufacturers generally have large variations in tensile strength in a longitudinal direction, and in order to manufacture high-quality wires, wire ropes, and the like with stable quality in the longitudinal direction, heat treatment is performed on the wire rods. The wire rods produced by the steel manufacturers generally have a minimum diameter of about 5.5 mm. In order to manufacture finer wires, heat-treated wire rods are drawn. When the diameter of the wire rod is rapidly reduced by one wire drawing, the toughness may be deteriorated. To avoid this, heat treatment and wire drawing may be alternately performed multiple times.
The heat treatment performed on wire rods in order to provide stable quality is generally referred to as "patenting". In patenting, the wire rods are heated to a predetermined temperature, and then cooled by being passed through a medium (for example, molten lead) heated to a predetermined temperature lower than the heating temperature. The patenting can produce the heat-treated steel (wire) having little variation in tensile strength in the longitudinal direction and having moderate toughness. The heat-treated steel has an iron oxide generated on the surface and may be thus drawn after the iron oxide is removed, or it may be drawn after being subjected to coating or plating for preventing seizure with dies. The drawn heat-treated steel may be shipped as it is, or may be shipped after plating or coating. The multiple wires of the drawn heat-treated steel may be twisted to manufacture wire ropes, or may be further plated with brass to manufacture steel cords. In any case, patenting is a very important process in a process of manufacturing high-quality wires, wire ropes, steel cords, or the like.
In order to prevent troubles such as wire breakage during wire drawing, it is essential to achieve both the tensile strength and the toughness. Therefore, the heat-treated steel (steel before wire drawing, which is generally targeted for wire drawing) preferably has a structure referred to as pearlite in which ferrite and plate-shaped cementite (an intermetallic compound of Fe (iron) and C (carbon)) are alternately arranged in layers. Pearlite appears when the steel is heated as described above to have the crystal structure transformed from body-centered cubic to face-centered cubic (austenitized) and the heated steel is rapidly cooled (see Patent Document 1, for example).
If the heating for obtaining the austenitized steel is insufficient, the cementite is not dissolved during heating, resulting in a decrease in the tensile strength of the heat-treated steel and a deterioration in the toughness of the steel after wire drawing. For example, when the steel to be heat-treated has the large thickness (diameter), a surface (surface layer) portion of the steel may be sufficiently heated, but a center (center layer) portion thereof may be insufficiently heated. In general, in order to avoid insufficient heating (to ensure complete austenitization) (to ensure that no undissolved carbides remain and that the carbon in the cementite is uniformly diffused within the austenite), the steel is heated for a long period of time with a margin. Unfortunately, this may grow crystal grains (austenite grains) especially at the surface portion. The large crystal grain size may make a metallographic structure rough, reducing the toughness.

CITATION LIST

PATENT LITERATURE

PATENT LITERATURE 1: JP3599551 B2

SUMMARY OF INVENTION

TECHNICAL PROBLEM

An object of the present invention is to provide a wiredrawn product having high tensile strength and toughness.
Another object of the present invention is to reduce heat radiated when a cooling medium tank is kept warm, thereby reducing the cost of fuel.
A further object of the present invention is to produce a wiredrawn product having a wider range of tensile strength on the higher strength side than conventional steel from the steel with the same composition (same steel grade).
A still further object of the present invention is to provide tensile strength equivalent to that of a heat-treated steel to which alloying elements are added, without adding expensive alloying elements to a heat-treated steel to provide higher strength.
A still further object of the present invention is to provide a wide range of relationships between tensile strength and hardness, and to reduce sheave wear and provide wear resistance when a rope or the like is produced.

SOLUTION TO PROBLEM

As described above, the traditional wiredrawn product having both tensile strength and toughness preferably is drawn from the heat-treated steel having pearlite in which ferrite and cementite are alternately arranged in layers. Meanwhile, according to the inventor's tests and considerations, it was found that a wiredrawn product having both tensile strength and toughness can be provided even if the heat-treated steel has no pearlite in which ferrite and cementite are alternately arranged in layers (even if the metallographic structure has little pearlite).
It was also confirmed that a wiredrawn product according to the present invention has several properties different from a conventional wiredrawn product. As described below, the wiredrawn product according to the present invention can be defined in terms of (1) a GAM (Grain Average Misorientation) value, (2) a GOS (Grain Orientation Spread) value, (3) a relationship between tensile strength and hardness, and (4) a cross section.
Focusing on the GOS value and GAM value, a wiredrawn product according to the present invention is drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, and is characterized in that: when a GAM value is a variable at a grain boundary setting angle of 2° and a step number of 0.07µm, a GOS value/average crystal grain size at a grain boundary setting angle of 2° is greater than or equal to -0.6 × GAM value + 1.5.
Focusing on the GOS value, a wiredrawn product according to the present invention is drawn from a heat-treated steel containing: 0.38 to 1.05 % by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50 % by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, and is characterized in that: when an average crystal grain size at a grain boundary setting angle of 2° is a variable, a GOS value/average crystal grain size at a grain boundary setting angle of 2° is greater than or equal to -0.18 × average crystal grain size + 2.25.
Further, focusing on the GOS value, a wiredrawn product according to the present invention is drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, and is characterized in that: when a degree of integration in a longitudinal direction [101] is a variable, a GOS value/average crystal grain size at a grain boundary setting angle of 2° is greater than or equal to 0.06 × degree of integration + 1.45.
Focusing on the relationship between tensile strength and hardness, a wiredrawn product according to the present invention is drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, and is characterized in that: within a range where a torsional fracture surface is normal in a torsion test, a relationship between tensile strength (TS) and hardness is as follows, and the relationship between tensile strength and hardness is adjustable according to a heating condition and an isothermal transformation temperature during patenting. $0.16 TS + 90 \leq hardness \leq 0.16 TS + 290$
Focusing on the cross section, a wiredrawn product according to the present invention is drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, and is characterized in that: when a structure of the heat-treated steel before wire drawing described above is observed with a backscattered electron (BSE) image, in a two-phase structure of ferrite and iron carbide, an area fraction of the branched, bent, or curved iron carbide is 9% or more in the field of view. The branched, bent, or curved iron carbide looks like a mottled pattern.
According to the present invention, a wiredrawn product having high tensile strength and toughness is provided.
A method for manufacturing a wiredrawn product according to the present invention is characterized by comprising the steps of: preparing a steel containing 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities; causing the steel itself to generate heat to directly heat the steel; passing the heated steel through a bath in which a cooling medium capable of isothermal transformation is stored to cool the steel; and drawing the cooled steel, wherein a temperature gradient in a final stage of heating of the heating step is the largest, and the heated steel is allowed to enter the cooling medium immediately after the steel reaches a predetermined maximum heating temperature in the final stage of heating of the heating step to start the cooling without maintaining the predetermined maximum heating temperature. The heating step may involve heating using electric current or high frequency. Molten lead, or the like can be used as the cooling medium.
A method for manufacturing a wiredrawn product according to the present invention can be also defined as follows. That is, a method for manufacturing a wiredrawn product, includes: heating a steel from room temperature to 800°C or more within a few seconds, the steel containing 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities; cooling the heated steel to 620°C or less within a few seconds without maintaining a maximum heating temperature; and drawing the cooled steel.
According to the manufacturing method, a wiredrawn product having high tensile strength and toughness can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a block diagram schematically showing a patenting device.
[FIG. 2] FIG. 2 is a block diagram schematically showing a dry wire drawing machine.
[FIG. 3] FIG. 3 is a block diagram schematically showing a wet wire drawing machine.
[FIG. 4] FIG. 4 is a graph showing a temperature change of a steel patented with a gas furnace.
[FIG. 5] FIG. 5 is a graph showing a temperature change of a steel patented with the patenting device in FIG. 1.
[FIG. 6] FIG. 6 shows a table providing a steel grade and its composition.
[FIG. 7] FIG. 7 shows a BSE image of a conventional product.
[FIG. 8] FIG. 8 shows a BSE image of a developed product.
[FIG. 9] FIG. 9 is a partially enlarged schematic view of the BSE image of the conventional product.
[FIG. 10] FIG. 10 is a partially enlarged schematic view of the BSE image of the developed product.
[FIG. 11] FIG. 11 is a BSE image of a developed product.
[FIG. 12] FIG. 12 is a BSE image of the developed product.
[FIG. 13] FIGS. 13A, 13B, and 13C, respectively, show an ABF image, an IPF map, and a LOS map of a developed heat-treated steel.
[FIG. 14] FIGS. 14A, 14B, and 14C, respectively, show an ABF image, an IPF map, and a LOS map of a wire drawn from the developed heat-treated steel to ϕ0.76.
[FIG. 15] FIGS. 15A, 15B, and 15C, respectively, show an ABF image, an IPF map, and a LOS map of a wire drawn from the developed heat-treated steel to ϕ0.375.
[FIG. 16] FIGS. 16A, 16B, and 16C, respectively, show an ABF image, an IPF map, and a LOS map of a conventional heat-treated steel.
[FIG. 17] FIGS. 17A, 17B, and 17C, respectively, show an ABF image, an IPF map, and a LOS map of a wire drawn from the conventional heat-treated steel to ϕ0.76.
[FIG. 18] FIGS. 18A, 18B, and 18C, respectively, show an ABF image, an IPF map, and a LOS map of a wire drawn from the conventional heat-treated steel to ϕ0.375.
[FIG. 19] FIG. 19 is a graph showing a relationship between true strain and a ratio of change in cross-sectional area in a longitudinal direction of a wire.
[FIG. 20] FIG. 20 shows a relationship between true strain and an average crystal grain size at a grain boundary setting angle of 15° for each of the developed products and the conventional product.
[FIG. 21] FIG. 21 shows a relationship between true strain and an average crystal grain size at a grain boundary setting angle of 5° for each of the developed products and the conventional product.
[FIG. 22] FIG. 22 shows a relationship between true strain and an average crystal grain size at a grain boundary setting angle of 2° for each of the developed products and the conventional product.
[FIG. 23] FIG. 23 shows a relationship between true strain and an average crystal grain size at a grain boundary setting angle of 15° for each of the developed products and the conventional products.
[FIG. 24] FIG. 24 shows a relationship between true strain and an average crystal grain size at a grain boundary setting angle of 2° for each of the developed products and the conventional products.
[FIG. 25] FIG. 25 shows a relationship between true strain and a degree of integration for each of the developed products and the conventional products.
[FIG. 26] FIG. 26 shows a relationship between an average crystal grain size at a grain boundary setting angle of 15° and a GOS value/average crystal grain size at a grain boundary setting angle of 15° for each of the developed products and the conventional products.
[FIG. 27] FIG. 27 shows a relationship between an average crystal grain size at a grain boundary setting angle of 2° and a GOS value/average crystal grain size at a grain boundary setting angle of 2° for each of the developed products and the conventional products.
[FIG. 28] FIG. 28 shows a relationship between a degree of integration and a GOS value/average crystal grain size at a grain boundary setting angle of 2° for each of the developed products and the conventional products.
[FIG. 29] FIG. 29 shows a relationship between a GAM value at a grain boundary setting angle of 2° and a GOS value/average crystal grain size at a grain boundary setting angle of 2° for each of the developed products and the conventional products.
[FIG. 30] FIG. 30 shows a relationship between tensile strength and hardness for the conventional products.
[FIG. 31] FIG. 31 shows a relationship between tensile strength and hardness for each of the developed products and the conventional products.
[FIG. 32] FIG. 32 shows a relationship between true strain and tensile strength for each of the developed products and the conventional products.
[FIG. 33] FIG. 33 shows a relationship between true strain and tensile strength for each of the developed products and the conventional products.
[FIG. 34] FIG. 34 shows a relationship between true strain and tensile strength for each of the developed products and the conventional products.
[FIG. 35] FIG. 35 shows a relationship between true strain and tensile strength for each of the developed products and the conventional products.
[FIG. 36] FIG. 36 shows a relationship between true strain and tensile strength for each of the developed products and the conventional product.

DESCRIPTION OF EMBODIMENTS

FIGS. 1 to 3 schematically show an apparatus for manufacturing a wiredrawn product, typically a wire. FIG. 1 shows a patenting device constituting the wire manufacturing apparatus, and FIGS. 2 and 3 each show a wire drawing machine constituting the wire manufacturing apparatus. In the following description, a steel before patenting is simply referred to as a "steel 11", a steel after patenting is simply referred to as a "heat-treated steel 12", and the heat-treated steel 12 that has been wiredrawn is referred to as a "wire 13" to distinguish among them.
The wire manufacturing apparatus includes the patenting device and the wire drawing machine.
Referring to FIG. 1, the patenting device includes a power source 14, a feed roll 15, a bath 16, and a molten lead 17 stored in the bath 16.
The steel 11 is supplied in the form of linear body (wire rods). The steel 11 unreeled from a payoff reel (not shown) runs at a constant speed from left to right in FIG. 1, passes through the feed roll 15, and is immersed in the molten lead 17 stored in the bath 16 for a predetermined period of time.
First, heat treatment is performed on the steel 11. The power source 14 provided in the patenting device is connected to the feed roll 15 and the bath 16 to form a closed circuit including the power source 14, the feed roll 15, the molten lead 17, and the bath 16. On the left side (upstream side) of the feed roll 15, an insulating device (not shown) is provided so that that current is not applied to the steel 11. In a section from the feed roll 15 to the liquid surface of the molten lead 17 stored in the bath 16, the steel 11 is energized and heated by the current supplied from the power source 14.
The steel 11 is most heated at the point immediately before entering the liquid surface of the molten lead 17 stored in the bath 16. The heating temperature of the steel 11 (the maximum temperature of the steel 11) is set to 975°C or less in order to exhibit the properties described later. This is because if the heating temperature is too high, crystal grains (austenite grains) will grow and the metallographic structure will become coarse, resulting in a decrease in toughness, especially a reduction of area. However, insufficient heating leads to the non-solution of an iron carbide (cementite as an example), which is an intermetallic compound of Fe and C. Accordingly, the heating temperature of the steel 11 is preferably set to 800°C or higher. The heating temperature of the steel 11 can be controlled by adjusting the voltage or current of the power source 14. The heating time is adjusted by the length of the path from the feed roll 15 to the liquid surface of the molten lead 17 and the running speed of the steel 11.
The molten lead 17 stored in the bath 16 is heated to a constant temperature with a gas furnace (an electric heater may be used). The molten lead 17 has a lower temperature than the heating temperature of the steel 11 described above, and the steel 11 is heated to the maximum temperature immediately before entering the liquid surface of the molten lead 17, and starts cooling as soon as it enters the molten lead 17.
The temperature of the molten lead 17 (lead furnace temperature), that is, an isothermal transformation temperature is set to 620°C or less. This is because the steel 11 is rapidly cooled to precipitate pearlite and carbide from austenite. However, if the steel 11 is cooled too rapidly, martensite or the like will appear to make the product brittle, and thus the lower limit temperature of the molten lead 17 is set to about 350°C.
The patented steel, which has been immersed in the molten lead 17 and then drawn out from the bath 16, that is, the heat-treated steel 12 is then subjected to water washing, coating, and wire drawing.
FIG. 2 and FIG. 3 schematically show a dry wire drawing machine and a wet wire drawing machine, respectively. In general, the dry wire drawing machine is used for manufacturing the wire 13 with a relatively large diameter, and the wet wire drawing machine is used for manufacturing the wire 13 with a small diameter.
Referring to FIG. 2, the dry wire drawing machine includes a lubricant box for storing a dry lubricant 21, a die 22, a die holder 23, and a drawing block 24.
The dry lubricant 21 stored in the lubricant box adheres to the surface of the heat-treated steel 12. The dry lubricant 21 is used to prevent the seizure of the heat-treated steel 12 and the die 22 described below, and to make the heat-treated steel 12 slippery such that the heat-treated steel 12 can be easily drawn from the die 22 to maintain a stable machining shape. As the dry lubricant 21, metal soaps such as sodium-based soaps and calcium-based soaps can be used.
The heat-treated steel 12 with the dry lubricant 21 adhering to its surface is passed through a hole made in the die 22. The hole of the die 22 is formed so that the diameter of the die 22 is reduced from the inlet side to the outlet side. The diameter of the heat-treated steel 12 is reduced as the heat-treated steel 12 is passed through the hole of the die 22.
Cooling water is stored around the die 22 and the die holder 23 for fixing the die 22. The cooling water removes the heat generated by wire drawing, which prevents thermal damage to the heat-treated steel 12 and the die 22.
The wire 13, which has passed through the die 22 and been reduced in diameter, is wound around the drawing block 24. The drawing block 24 provides for drawing and cooling of the wire 13.
Referring to FIG. 3, the wet wire drawing machine includes two drawing capstans 32, 33 provided at intervals and a plurality (three in FIG. 3) of dies 31 provided between the two drawing capstans 32, 33. The drawing capstans 32, 33 each have one or more capstans provided coaxially, and the drawing capstans 32, 33 shown in FIG. 3 each have three capstans with small, medium and large diameters. The heat-treated steel 12 is hung on the small diameter capstan of the one drawing capstan 32, the small diameter capstan of the other drawing capstan 33, the medium diameter capstan of the one drawing capstan 32, and the medium diameter capstan of the other drawing capstan 33, the large diameter capstan of the one drawing capstan 32, and the large diameter capstan of the other drawing capstan 33 in sequence. The heat-treated steel 12 is passed through the holes of the dies 31 provided between the two drawing capstans 32, 33. The diameter of the hole of the dies 31 is also formed to be reduced from the inlet side to the outlet side, and the diameter of the heat-treated steel 12 is reduced each time the heat-treated steel 12 is passed through the holes of the dies 31.
All of the drawing capstans 32, 33 and the dies 31 are immersed in a lubricating liquid, and the lubricating liquid prevents the seizure between the heat-treated steel 12 and the dies 31. In the wet wire drawing machine, the lubricating liquid also serves to cool the heat-treated steel 12 and the dies 31.
The wire 13 thinned by the dry wire drawing machine, the wet wire drawing machine, or both described above is then wound on a winding drum (not shown).
FIG. 4 and FIG. 5 show a temperature change (temperature rise curve) of the steel 11 patented with a gas furnace (heat-treated steel 12) and a temperature change (temperature rise curve) of the steel 11 patented with the patenting device shown in FIG. 1 (heat-treated steel 12), respectively. In both graphs of FIGS. 4 and 5, the temperature drops sharply at the timing when the steel 11 enters the molten lead 17. It should be noted that the scale of the time axis (horizontal axis) differs between FIG. 4 and FIG. 5.
Referring to FIG. 4, when the gas furnace is used, the steel 11 is gradually heated. In an atmosphere heating furnace as a typical gas furnace, the time required for heating is proportional to the wire diameter of the steel 11. The thinner the wire diameter, the shorter the heating time, and the thicker the wire diameter, the longer the heating time. FIG. 4 and FIG. 5 show the graphs of the steel 11 with a wire diameter of ϕ2.11, and when the gas furnace is used, it takes about 40 seconds to reach the maximum temperature (target heating temperature). On the other hand, referring to FIG. 5, when the patenting device shown in FIG. 1 is used, the steel 11 reaches the maximum temperature (target heating temperature) in several seconds. The patenting device shown in FIG. 1 can provide the constant rate of temperature rise regardless of the wire diameter.
Comparing the graph in FIG. 4 and the graph in FIG. 5, the shapes of the temperature rise curves are significantly different. In the graph of FIG. 4, the rate of temperature rise slows down from around 723°C at which austenitization starts, and the ratio of the time required for austenitization increases, while in the graph of FIG. 5, the rate of temperature rise increases at 723°C or more, and the ratio of time required for austenitization decreases. Further, in FIG. 4, after the maximum temperature is reached, it is maintained for about 20 seconds, while in FIG. 5, the cooling starts immediately after the maximum temperature is reached.
The steel 11 as a starting wire rod and the heat-treated steel 12 as the patented steel are carbon steels including iron (Fe) and carbon (C). The carbon content (carbon concentration) of 0.38% (mass%; the same applies hereinafter) or more makes it easier to provide sufficiently high strength, and the carbon content of 1.05% or less prevents the deterioration of workability and the reduction of fatigue limit.
In addition to Fe and C, manganese (Mn), chromium (Cr), and silicon (Si) may be included in the heat-treated steel 12.
Manganese (Mn) is contained as a deoxidizer. The content is limited to 1.0% or less in order to prevent the deterioration of workability.
Chromium (Cr) is generally effective in refining pearlite and improving the toughness. The content is limited to 0.50% or less as adding a large amount of Cr adversely causes a decrease in toughness.
Silicon (Si) is used as a deoxidizing agent. The content is limited to about 1.5% in order to avoid ductility deterioration.
In addition, other elements such as vanadium (V) (0.50% or less), molybdenum (Mo) (0.25% or less), boron (B) (0.005% or less), titanium (Ti) (0.050%), nickel (Ni) (0.50% or less), aluminum (0.10% or less), zirconium (Zr) (0.050% or less) and the like may be added to the steel 11 (heat-treated steel 12) depending on applications.
In the following description, the heat-treated steel 12 that is heated as shown in FIG. 4 and ensures the time of maintaining the maximum temperature for about 20 seconds, and the wire 13 produced by drawing the heat-treated steel 12 are referred to as a "conventional product", and the heat-treated steel 12 that is heated as shown in FIG. 5 and starts cooling immediately after the maximum temperature is reached, and the wire 13 produced by drawing the heat-treated steel 12 are referred to as a "developed product" to distinguish from each other. FIG. 6 summarizes the names of the steel grades of the multiple steels 11 (heat-treated steels 12 and wires 13) described below and their compositions.

(Backscattered electron image)

FIGS. 7 and 8 are Backscattered Electron (BSE) images of the heat-treated steels 12 provided by respective different patenting methods as shown in FIGS. 4 and 5 although all the steels 11 before heat treatment are the same (all SWRH62A having a diameter of 2.11 mm), and FIG. 7 is the BSE image of a conventional product, and FIG. 8 is the BSE image of a developed product. The backscattered electron image is provided by polishing the heat-treated steel 12, milling it with argon gas, and photographing the longitudinal direction of the heat-treated steel 12. The length of the white oblong rectangle shown at the bottom of the BSE images in FIGS. 7 and 8 corresponds to 1 µm (a magnification of 10,000). FIG. 9 and FIG. 10 show a partially enlarged schematic diagram of the BSE image of the conventional product shown in FIG. 7 and a partially enlarged schematic diagram of the BSE image of the developed product shown in FIG. 8, respectively.
FIG. 7 is the BSE image of the conventional heat-treated steel 12 when the temperature of the molten lead 17 is set to 565°C. FIG. 8 is the BSE image of the developed heat-treated steel 12 when the temperature of the molten lead 17 is set to 450°.
As shown in FIGS. 7 and 9, in the BSE images of the conventional product, the layered structure with ferrite and cementite alternately arranged in layers is identified within the prior austenite grain boundary. In the BSE images of the conventional product, the cementite appears as a plurality of parallel and elongated streaks.
The white portions in FIG. 8 include cementite (Fe₃C), but may include an iron carbide (for example, Fe_2-2.5C, Fe_2-3C) different from cementite. The comparison between FIG. 7 and FIG. 8 or FIG. 9 and FIG. 10 shows that the plate thickness (layer thickness) of the multiple iron carbides in the developed product is not uniform and is thick (approximately 60 nm) than that in the conventional product.
In the following description, an iron carbide (Fe₃C, Fe_2-2.5C, Fe_2-3C, or the like) constituting the layered structure identified in the developed product is referred to as "special cementite" to distinguish it from "cementite" (Fe₃C), which is an iron carbide constituting the layered structure identified in the conventional product.
As shown in FIGS. 8 and 10, the layered structure of ferrite and special cementite is identified in the BSE images of the developed product. However, it can be seen that there is very little layered special cementite (streaks that are parallel to each other and extend elongated in the BSE image), the layer thickness (the thickness of the streaks in the BSE image) is not uniform, and there are many branched, bent, or curved portions (an area fraction in the field of view is 9% or more). In the BSE image of the developed product, the special cementite looks like, so to speak, a mottled pattern.
In FIGS. 11 and 12, the steel grade SWRS92A is used. FIG. 11 is a BSE image of the developed heat-treated steel 12 when the temperature of the molten lead 17 is set to 565°C, and FIG. 12 is a BSE image of the developed heat-treated steel 12 when the temperature of the molten lead 17 is set to 450°C. As in the BSE images of the developed product shown in FIGS. 11 and 12, there is little layered special cementite and the special cementite looks like a mottled pattern.
Various measurements are performed to ascertain the properties of the wire 13 produced by drawing the developed heat-treated steel 12 having a structure different from the conventional product. The measurements are also performed on the wire produced by drawing the conventional heat-treated product. The measurement results are described below.
As described in detail below, an EBSD (Electron Back Scattered Diffraction) analysis is used to measure the properties of the developed product and the conventional product. In the EBSD analysis, a measurement area of a cross section of the polished sample (the cross section in the longitudinal direction (longitudinal cross section) of the wire 13 in this example) is divided into measurement points (generally referred to as "pixels"), an electron beam is incident on each of the divided pixels, and the incident electron beam is reflected by the pixels. Based on the thus obtained reflected electrons, a crystal orientation for each of the pixels is measured. The obtained crystal orientation data is analyzed using the EBSD analysis software to calculate various parameters. In this example, the EBSD detector manufactured by TSL Solutions KK is used, and regular hexagonal pixels are employed as pixels.
In the EBSD analysis software, a grain boundary setting angle (grain boundary setting value) is set. In the EBSD analysis, using the crystal orientation obtained for each pixel, the boundary at which the difference in crystal orientation between adjacent pixels is greater than or equal to the grain boundary setting angle described above is regarded as a "grain boundary" and the area enclosed by the grain boundary is regarded as a "crystal grain". When the grain boundary setting angle (grain boundary setting value) is decreased, the crystal grain size decreases and the number of crystals in the observation area increases. Conversely, when the grain boundary setting angle is increased, the crystal grain size increases and the number of crystals in the observation area decreases. The EBSD analysis evaluates the crystal orientation of ferrite because the carbides are too small to be measured.
When the EBSD analysis is performed on the wire 13, an object to be measured, which has been subjected to the wire drawing (plastic working) described above, there may be a portion with an inaccurate measurement result of the crystal orientation due to the distortion of the crystal lattice of the object to be measured by the plastic working. In particular, at the grain boundary, the crystal lattice is distorted, which may cause inaccurate crystal orientation measurement, resulting in a high possibility of an incorrect analysis. A method for processing inaccurate measurement portion differs depending on the manufacturers of EBSD analysis devices. The EBSD analysis device used herein and manufactured by TSL Solutions KK employes the CI (Confidence Index) value indicating a probability that the crystal orientation analyzed for each pixel is correct, and employs only a portion with a correct measurement of the crystal orientation with a probability of 95% or more, that is, a portion with a CI value of 0.1 or more.
It is verified that there is no problem even if the EBSB analysis is performed except for the portion with the inaccurate measurement of the crystal orientation. For the verification, the developed heat-treated steel 12 of SWRH62A with ϕ2.11, and two types of wires 13 drawn from the heat-treated steel 12 to ϕ0.76 and ϕ0.375 are used. For the verification, structure observation by s-TEM (Scanning Transmission Electron Microscopy) and structure observation by t-EBSD (Transmission Electron Backscattered Diffraction) (transmission EBSD) with higher resolution than normal EBSD are used.
FIG. 13A, FIG. 13B, and FIG. 13C show an Annular Bright-Field (ABF) image by s-TEM, an IPF map by t-EBSD, and a LOS map by t-EBSD, respectively. All are observation results of the heat-treated steel 12 (that is, heat-treated steel 12 before wire drawing).
The ABF image by s-TEM shown in FIG. 13A is a combination of an image of the entire field of view and a partially enlarged and clearly captured image. In the ABF image of the developed heat-treated steel 12, grain boundaries and special cementite are observed.
The t-EBSD shown in FIGS. 13B and 13C are analyzed at a grain boundary setting angle of 15° (same applied hereinafter). In a IPF (Inverse Pole Figure) map by t-EBSD shown in FIG. 13B, the measurement points are color-coded according to the crystal orientation. For convenience of illustration, the IPF map in FIG. 13B is not color-coded, and the crystal orientation is indicated only by the density (brightness) of the image (the same applies hereinafter). The LOS (Local Orientation Spread) map shown in FIG. 13C indicates the difference in crystal orientation between adjacent pixels by color coding. As in FIG. 13C, for convenience of illustration, the LOS map is not color-coded and the difference in crystal orientation is indicated only by the density (brightness) of the image (the same applies hereinafter).
Portions with a CI value of less than 0.1 are represented by black dots in the IPF map of FIG. 13B and the LOS map of FIG. 13C. The observation results in FIGS. 13A to 13C show that the portions with a CI value of less than 0.1 in the developed heat-treated steel 12 are concentrated at the grain boundaries, and the portions of the grain boundaries, which are the portions with a CI value of less than 0.1, should be excluded from the analysis. In addition, the LOS map in FIG. 13C also shows that the developed heat-treated steel 12 has almost no sub-grain boundaries within the crystal grains.
FIG. 14A, 14B and 14C, respectively, show an ABF image, an IPF map, and a LOS map of the cross section of the wire 13 drawn from the developed heat-treated steel 12 from ϕ2.11 to ϕ0.76.
The special cementite is unclear in the ABF image shown in FIG. 14A. The IPF map and LOS map shown in FIG. 14B and FIG. 14C show that the portions with a CI value of less than 0.1 are concentrated at the grain boundaries, and the portions at the grain boundaries should be excluded from the analysis. In addition, the LOS map shown in FIG. 14C shows that lines with a difference in crystal orientation of less than 15° between adjacent pixels, which are not seen in the heat-treated steel 12 before wire drawing (FIG. 13C), are observed in the crystal grains, indicating that sub-grain boundaries are generated by wire drawing.
FIG. 15A, 15B and 15C, respectively, show an ABF image, an IPF map, and a LOS map of the cross section of the wire 13 further drawn from the developed heat-treated steel 12 to ϕ0.375.
As with the wire 13 with ϕ0.76 described with reference to FIGS. 14A to 14C, the IPF map shown in FIG. 15B and the LOS map shown in FIG. 15C show that the portions with a CI value of less than 0.1 are concentrated at the grain boundaries, and the portions at the grain boundaries should be excluded from the analysis. In addition, comparing the LOS map of the wire 13 drawn to ϕ0.76 (FIG. 14C) and the LOS map of the wire 13 drawn to ϕ0.375 (FIG. 15C), in the LOS map of the wire 13 drawn to ϕ0.375, there are places where the line with a difference in crystal orientation of less than 15° and the region with a CI value of less than 0.1 are connected, indicating that the sub-grain boundaries generated by the wire drawing become grain boundaries by further wire drawing.
FIG. 16A, 16B and 16C, respectively, show an ABF image, an IPF map, and a LOS map of the conventional heat-treated steel 12.
There are many regions with a CI value of less than 0.1 at the bottom of the IPF map (FIG. 16B) and the LOS map (FIG. 16C). This is because in t-EBSD, the sample is made into a thin film and is irradiated with electron beams, and the transmitted analysis image is used, and when the thin film is made, the sample becomes thicker as it moves away from the edge, resulting in unclear analysis image. Unlike the LOS map of the developed heat-treated steel 12 shown in FIG. 13C, according to the LOS map of FIG. 16C, the conventional heat-treated steel 12 has sub-grain boundaries with a difference in crystal orientation of less than 15° within the crystal grains.
FIGS. 17A, 17B and 17C, respectively, show an ABF image, an IPF map, and a LOS map of the cross section of the wire 13 drawn from the conventional heat-treated steel 12 to ϕ0.76.
In the ABF image shown in FIG. 17A, cementite can be seen more clearly than in the wire 13 (FIG. 14A) drawn from the developed heat-treated steel 12 to cp0.76. The LOS map of FIG. 17C shows that many sub-grain boundaries are generated as in the developed product (FIG. 14C).
FIGS.18A, 18B, and 18C, respectively, show an ABF image, an IPF map, and a LOS map of the cross section of the wire 13 drawn from the conventional heat-treated steel 12 to ϕ0.375.
Similar to the developed product shown in FIG. 15C, the LOS map of the conventional product in FIG. 18C shows more sub-grain boundaries.
In both the developed product and the conventional product, the crystal grain size does not decrease by reduction of area in wire drawing. When the heat-treated steel 12 is drawn, the sub-grain boundaries are generated, and when it is further drawn, the sub-grain boundaries become grain boundaries, indicating that as the reduction of area increases (as the wire is further made thinner), the crystal grain size becomes smaller and smaller.
In comparing the developed product and the conventional product, it is difficult to secure an observation area for observing a statistically sufficient number of grain boundaries in t-EBSD. This is because the sample is made into a thin film in t-EBSD, resulting a very small sample and a narrow range of observation. In addition, the cross section has a high ratio of grain boundaries to the observation range, and a small ratio of accurate crystal orientation measurement. For this reason, the observation is preferably performed by a normal EBSD with a wide range of measurement and in the vertical section (longitudinal section) having a smaller ratio of grain boundaries and a higher ratio of accurate measurement than the cross section. Further, it is known that when the heat-treated steel 12 is subjected to wet wire drawing, friction with the die causes additional shear strain on the surface of the wire, resulting in an increase in [111] crystal orientation. Due to the surface of the wire being greatly affected by the drawing conditions, the EBSD analysis is performed at the center of the wire, which is less affected by friction.
As a condition for measurement by EBSD, acceleration voltage or the like is set under the measurement condition that the ratio of a CI value of 0.1 or more at all measurement points is 70% or more. The interval between the measurement points is referred to as a step number, and the step number is basically 0.07 µm. However, due to the performance of EBSD, when the step number is set to 0.07 µm, the number of measurement points may be too large to be processed by the analysis software. In that case, the step number may be changed up to an upper limit of 0.20 µm as long as the number of crystal grains at a grain boundary setting angle of 2° is 1.5 times or more the number of crystal grains at a grain boundary setting angle of 15°. Here, when the ratio of the number of crystal grains at a grain boundary setting angle of 2° to that at a grain boundary setting angle of 15° is less than 1.5 times, the measurement condition should be changed because a portion with large strain cannot be measured, or the step number is too large and a portion that is not a grain boundary is determined as a grain boundary. For a range of measurement, a length of measurement in the longitudinal direction is set to be at least twice the maximum length in the longitudinal direction of the crystal grains measured at a grain boundary setting angle of 15° and a CI value of 0.1 or higher, because the object to be measured is elongated in the longitudinal direction by the wire drawing. The range where the number of crystal grains whose average crystal grain size (converted to the diameter of a circle with an area equal to the crystal grain area) is greater than or equal to the average value is 30 or more is observed.
FIG. 19 shows a graph with the horizontal axis representing true strain, and the vertical axis representing the calculated ratio of change in cross-sectional area in the longitudinal direction including the central axis (longitudinal cross-sectional area) of a wire drawn from the wire 13 before wire drawing with a diameter of A₀ and a length of L₀ to a diameter of A (A>A₀) and a length of L (L>L₀), with the cross-sectional area in the longitudinal direction of the wire 13 having a diameter of A₀ set to 1 (longitudinal cross-sectional area after wire drawing/longitudinal cross-sectional area before wire drawing). The true strain is given by 2ln (A₀/A) ("ln" is the natural logarithm), wherein A₀ is the longitudinal cross-sectional area of the wire 13 (heat-treated steel 12) before wire drawing, and A is the longitudinal cross-sectional area of the wire 13 after wire drawing. The value of true strain increases as the degree of processing (diameter reduction rate) on the heat-treated steel 12 by a wire drawing machine (die) increases.
The volume of the heat-treated steel 12 does not change before and after wire drawing, and thus the length L of the heat-treated steel 12 drawn from diameter A₀ to diameter A is expressed by (A₀/A) × L₀. In addition, when the heat-treated steel 12 is drawn from diameter A₀ to diameter A, the ratio of the longitudinal cross-sectional area including the central axis in the longitudinal direction is A₀/A regardless of the length of the heat-treated steel 12. Therefore, the relationship between the true strain and the ratio of the longitudinal cross-sectional area is expressed by exp[0.5 × {2 × ln(A₀/A)}]. This formula is shown by the solid line in FIG. 19. Computationally, the larger the true strain value (the smaller the wire diameter), the larger the longitudinal cross-sectional area in the longitudinal direction of the heat-treated steel 12. The same applies to the crystal grain size observed in the cross section in the longitudinal direction.
FIG. 20 is a graph showing a relationship between true strain and an average crystal grain size for each of the developed products and the conventional product with the horizontal axis representing true strain and the vertical axis representing the measurement results of the average crystal grain size (value converted to the diameter of a circle with the same area as the crystal grain area) (µm) when the grain boundary setting angle is set to 15° in the EBSD analysis software. In FIG. 20, the broken lines and the solid line indicate the developed products and the conventional product, respectively. For both the developed products and the conventional product, the measurement results are for the wires 13 drawn from the steels 11 of the same steel grade (SWRH62A) that have been subjected to heat treatment (as mentioned above, the heat treatment differs between the developed product and the conventional product). As for the developed products, the measurement results of three types of wires 13 produced when the temperature of the molten lead 17 is set to 565°C, 450°C, and 425°C are shown. As for the conventional product, the measurement results of one type of wire 13 produced when the temperature of the molten lead 17 is set to 565° C are shown. Below the graph in FIG. 20, the line type shown in the graph, and the steel grade, the distinction between the developed product and the conventional product, and the isothermal transformation temperature for each line type are shown (the same applies hereinafter).
Referring to FIG. 20, the conventional product (solid line) has a relatively large average crystal grain size at a grain boundary setting angle of 15°, and the average crystal grain size increases as the true strain increases up to around 1.0. However, when the true strain exceeds 1.5, the average crystal grain size tends to decrease as the true strain increases. On the other hand, it can be seen that the developed products (broken lines) have a small average crystal grain size at a grain boundary setting angle of 15° (approximately 4 µm) in the undrawn state (true strain is 0.0), and even if the true strain increases, that is, even if the heat-treated steel 12 is subjected to wire drawing, the average crystal grain size at a grain boundary setting angle of 15° does not change as significantly as the conventional product.
FIG. 21 is a graph showing a relationship between true strain and an average crystal grain size for each of the developed products and the conventional product with the horizontal axis representing true strain and the vertical axis representing the measurement results of the average crystal grain size (µm) when the grain boundary setting angle is set to 5° in the EBSD analysis software. As in the graph shown in FIG. 20, for both the developed products (broken lines) and the conventional product (solid line), the measurement results are for the wires 13 drawn from the steels 11 of the same steel grade (SWRH62A) that have been subjected to heat treatment (as mentioned above, the heat treatment differs between the developed product and the conventional product).
When the grain boundary setting angle is set to 5°, the average crystal grain size decreases as the true strain increases for both the conventional product (solid line) and the developed products (broken lines). In addition, the average crystal grain size of the developed products is smaller than that of the conventional product.
FIG. 22 is a graph showing a relationship between true strain and an average crystal grain size for each of the developed products and the conventional product with the horizontal axis representing true strain and the vertical axis representing the measurement results of the average crystal grain size (µm) when the grain boundary setting angle is set to 2° in the EBSD analysis software. As in the graphs shown in FIGS. 20 and 21, for both the developed products (broken lines) and the conventional product (solid line), the measurement results are for the wires 13 drawn from the steels 11 of the same steel grade (SWRH62A) that have been subjected to heat treatment (as mentioned above, the heat treatment differs between the developed product and the conventional product).
Even when the grain boundary setting angle is set to 2°, the average crystal grain size decreases as the true strain increases for both the conventional product (solid line) and the developed products (broken lines). In addition, the average crystal grain size of the developed products is smaller than that of the conventional product.
Comparing the graph when the grain boundary setting angle is set to 15° (FIG. 20), the graph when the grain boundary setting angle is set to 5° (FIG. 21), and the graph when the grain boundary setting angle is set to 2° (FIG. 22), for the conventional product (solid line), the smaller the grain boundary setting angle, the smaller the change in the average crystal grain size according to the degree of wire drawing (degree of true strain). On the other hand, for the developed products (broken lines), it can be seen that the change in the average crystal grain size is almost the same regardless of the size of the grain boundary setting angle and the degree of wire drawing (degree of true strain).
FIG. 23 is a graph showing a relationship between true strain and an average crystal grain size for each of the developed products and the conventional products with the horizontal axis representing true strain and the vertical axis representing the measurement results of the average crystal grain size (µm) when the grain boundary setting angle is set to 15° in the EBSD analysis software. FIG. 23 shows a relationship between true strain and an average crystal grain size at a grain boundary angle of 15° for the wires 13 of steel grades SWRH42A, SWRH62A, SWRH82A, SWRH82B, SWRS92A, 92A-Cr, 92B-Si, and 102A-Cr, for each of the conventional products (solid lines) and the developed products (broken lines).
FIG. 23 shows that for all of the conventional products (solid lines) of various steel grades, the average crystal grain size at a grain boundary setting angle of 15° changes or fluctuates greatly when the true strain is changed. On the other hand, it can be seen that for the developed products (broken lines), the change in the average crystal grain size is small even when the true strain is changed. In addition, the graph in FIG. 23 also shows that for both the conventional products and the developed products, the larger the average crystal grain size before wire drawing (true strain 0), the greater the decrease in the average crystal grain size due to wire drawing. For some conventional products such as conventional SWRH62A and conventional SWRH42A, the average crystal grain size increases as the true strain increases up to a true strain of 2.0.
FIG. 24 is a graph with the horizontal axis representing true strain and the vertical axis representing an average crystal grain size (µm) at a grain boundary setting angle of 2°, in which the broken and solid lines indicate the developed and conventional products, respectively.
Referring to FIG. 24, the change in the average grain size when the grain boundary angle is set to 2° is small regardless of the degree of true strain, and the difference in the average grain size between the conventional products and the developed products is also small. The true strain and the average grain size have a roughly linear relationship, and the larger the true strain, the smaller the average grain size. The average grain size when the grain boundary angle is set to 2° has a correlation with the true strain.
FIG. 25 is a graph with the horizontal axis representing true strain and the vertical axis representing the degree of integration, in which the broken and solid lines indicate the developed and conventional products, respectively.
The degree of integration on the vertical axis indicates the degree of integration in the [101] direction of the longitudinal direction. The degree of integration is a value calculated in EBSD and given by calculating, when the probability of the crystal orientation existing in a completely random state is set to 1, the probability of the crystal orientation of the measured one existing. It is known that the [101] direction is oriented in the longitudinal direction by wire drawing. The greater the true strain, the greater the degree of integration. If the diameter of the heat-treated steel 12 (the wire before wire drawing) and the diameter of the wire 13 after wire drawing are given, the "true strain" can be determined. On the other hand, if the diameter of the heat-treated steel 12 is not given, the "degree of integration" calculated in EBSD can be used as an indicator to determine how much wire drawing has been performed, although it is only a rough estimate. In the following, the degree of integration in the longitudinal direction [101] is used.
FIG. 26 shows measurement results of a plurality of developed products and a plurality of conventional products and is a graph with the horizontal axis representing an average crystal grain size at a grain boundary setting angle of 15°, and the vertical axis representing a GOS value/average crystal grain size at a grain boundary setting angle of 15°. The conventional and developed products are indicated by solid lines and broken lines, respectively.
A GOS (Grain Orientation Spread) value (also referred to as an average GOS value) is determined by calculating and averaging misorientation between two pixels within the same crystal grain, and is used as an indicator of strain. As described above, as the crystal grain boundary varies depending on the grain boundary setting angle, the GOS value varies when the grain boundary setting angle is changed. The GOS value is also a value calculated by the EBSD analysis software. The GOS value represents a wide range of misorientations within the crystal grains. The GOS value is a parameter that reflects the change in the overall crystal orientation of the crystal grains, and corresponds to the integral of the local misorientation (KAM) described above. The GOS value does not depend on the step number, but increases as the crystal grain size increases when the twist of the crystal orientation per unit length is the same. In the following, as the GOS value, the average value determined from an area fraction within the measurement range is used.
Referring to FIG. 26, the developed products (broken lines) tend to have a larger value of the GOS value/average crystal grain size than the conventional products (solid lines). Calculating the GOS value/average grain size at a grain boundary setting angle of 15° makes it possible to roughly distinguish between the developed products and the conventional products.
FIG. 27 shows measurement results of a plurality of developed products and a plurality of conventional products and a graph with the horizontal axis representing an average crystal grain size at a grain boundary setting angle of 2° and the vertical axis representing a GOS value/average crystal grain size at a grain boundary setting angle of 2°. In FIG. 27, a straight line representing -0.18 × average crystal grain size + 2.25 is indicated by a broken line.
When the grain boundary setting angle is set to 2°, the value of GOS value/average crystal grain size is larger in the developed products than in the conventional products at the same average crystal grain size. The conventional products (solid lines) have a value of GOS value/average crystal grain size of less than or equal to "-0.18 × average crystal grain size + 2.25", while the developed products (broken lines) have a value of GOS value/average crystal grain size of greater than or equal to "-0.18 × average crystal grain size + 2.25". The GOS value/average crystal grain size measured when the grain boundary setting angle is set to 2° is used to determine whether the value is greater than or equal to -0.18 × average crystal grain size + 2.25 or less than or equal to -0.18 × average crystal grain size + 2.25 to distinguish between the conventional products and the developed products.
FIG. 28 shows measurement results of a plurality of developed products and a plurality of conventional products, and is a graph with the horizontal axis representing the degree of integration and the vertical axis representing a GOS value/average crystal grain size at a grain boundary setting angle of 2°. In FIG. 28, a straight line representing 0.06 × degree of integration + 1.45 is indicated by a broken line.
When the grain boundary setting angle is set to 2°, the developed products have the larger value of GOS value/average crystal grain size than the conventional products at the same degree of integration. In addition, the conventional products (solid lines) have a GOS value/average crystal grain size of less than or equal to "-0.06 × degree of integration + 1.45", while the developed products (solid lines) have a GOS value/average crystal grain size of greater than or equal to "-0.06 × degree of integration + 1.45". The value calculated by "-0.06 × degree of integration + 1.45" is used as a reference value to determine whether the GOS value/average crystal grain size at a grain boundary setting angle of 2° is greater than or equal to the reference value or less than or equal to the reference value to distinguish between the conventional products and the developed products.
FIG. 29 shows measurement results of a plurality of developed products and a plurality of conventional products with the horizontal axis representing a GAM value at a grain boundary setting angle of 2° and a step number of 0.07 µm, and the vertical axis representing a GOS value/average crystal grain size at a grain boundary setting angle of 2°. The conventional products are indicated by solid lines and the developed products are indicated by broken lines. In FIG. 29, a straight line representing -0.6 × average GAM value +1.5 is shown by a broken line.
A GAM (Grain Average Misorientation) value (also referred to as an average GAM value) is the average value of the misorientation between adjacent pixels within a single crystal grain, and is one of the indicators of the twist of the crystal orientation within the crystal grains. The larger the GAM value, the more distorted the crystal lattice is. The GAM value differs depending on the distance between measurement points (pixels) at the time of measurement (represented by the "step number"). The GAM value is a value calculated by the EBSD analysis software. The GAM value is the average of m misorientations between measurement points within the crystal grain. The GAM value defined from the average of local misorientations corresponds to a value given by averaging local misorientation KAM (Kernel Average Misorientation) values for each crystal grain, and its absolute value depends on the step number of EBSD measurement. The wire 13 with uneven strain has a GAM value changing when the step number is changed. In this example, the step number is fixed at 0.07 µm. In the following, as the GAM value, the average value determined from an area fraction within the measurement range is used.
Referring to FIG. 29, when the grain boundary setting angle is set to 2°, the developed products (broken lines) have a larger value of GOS value/average crystal grain size than the conventional products (solid lines). The value of "-0.6 × GAM value + 1.5" is used to as a reference (threshold) to determine whether the value of GOS value/average crystal grain size at a grain boundary setting angle of 2° is greater than or equal to the reference value or less than or equal to the reference value to distinguish between the conventional products and the developed products.
FIG. 30 is a graph showing measurement results of a plurality of conventional products with tensile strength (MPa) on the horizontal axis and hardness (Hv) on the vertical axis. FIG. 30 shows a relationship between tensile strength and hardness for the conventional wire 13 made from each of the steel grades SWRH42A, SWRH62A, SWRH82A, SWRH82B, SWRS92A, 92A-Cr, 92B-Si and 102A-Cr. In FIG. 30, the products with the normal torsional fracture surface in the torsion test are plotted.
According to the relationship between tensile strength and hardness with reference to FIG. 30, the conventional products have a hardness falling within the range of 0.2TS + 88 ≤ hardness ≤ 0.2TS + 123 (wherein TS is tensile strength).
FIG. 31 shows measurement results of a plurality of developed products, and, similar to FIG. 30, is a graph with tensile strength (MPa) on the horizontal axis and hardness (Hv) on the vertical axis. As in FIG. 31, the products with the normal torsional fracture surface in the torsion test are plotted.
The graphs especially shown by (a) and (b) in FIG. 31 will be described. These graphs show measurement results of two types of developed wires 13 made from the steel 11 of the same steel grade SWRH62A, the two types of developed wires 13 produced under different heating conditions (including wire speed) and isothermal transformation temperatures (temperatures of the molten lead 17) during patenting in the wire manufacturing apparatus. That is, the graphs in FIG. 31 show that the heating conditions or the isothermal transformation temperatures during patenting in the wire manufacturing apparatus can be adjusted to produce the wires 13 with variously adjusted relationships between tensile strength and hardness from the same starting wire rod (steel 11).
Referring to FIG. 31, the developed products have a relationship between tensile strength and hardness included in the range indicated by 0.16TS + 90 ≤ hardness ≤ 0.16TS + 290 (wherein TS is tensile strength), which is particularly remarkable in the developed wire 13 made from the steel grade SWRH62A. It is also found that for the developed product, as described above, the heating condition and isothermal transformation temperature during patenting in the wire manufacturing apparatus can be changed to control the relationship between tensile strength and hardness. Comparing the graph (a) and the graph (b) in the graphs of FIG. 31, when the tensile strength is 2,100MPa, for example, the wire 13 indicated by the graph (a) has a hardness of 570Hv, and the wire 13 indicated by the graph (b) has a hardness of about 480Hv. The wire 13 indicated by the graph (b) has the same tensile strength as the wire 13 indicated by the graph (a) and also has high toughness.
FIG. 32 shows work hardening curves of the wire 13 of steel grade SWRH62A with true strain on the horizontal axis and tensile strength (MPa) on the vertical axis.
FIG. 32 shows graphs (all broken lines) of three developed wires 13, all of which are made from the steel grade SWRH62A and at the different temperatures of the molten lead 17 and graphs (both solid lines) of two conventional wires 13, all of which are made from the steel grade SWRH62A and at the different temperatures of the molten lead 17. It can be seen that among the five wires 13, the conventional wire 13 produced with the temperature of the molten lead 17 (lead furnace temperature) set to 450°C does not have the improved tensile strength as much as the remaining four wires 13 even if the true strain is increased. For the developed products, the tensile strength of the wire 13 increases as the true strain increases, regardless of whether the temperature of the molten lead 17 is set to 450°C or even lower to 425°C. That is, the tensile strength of the conventional wire 13 decreases when the temperature of the molten lead 17 is lowered, but the tensile strength of the developed wire 13 does not decrease even if the molten lead 17 at a low temperature is used. That is, in the developed product, even if the temperature of the molten lead 17 is lowered to 425°C, the wire 13 having high tensile strength can be provided. Setting the temperature of the molten lead 17 to 425°C can reduce heat loss from the bath 16 and reduce fuel costs by about 20% compared to the case when the temperature of the molten lead 17 is set to 565°C. In other words, the developed products have higher energy efficiency than the conventional products because the tensile strength of the developed products does not decrease even if the molten lead 17 at a low temperature is used.
FIG. 35 shows work hardening curves of the wires 13 of steel grades SWRH42A and SWRH62A.
FIG. 33 shows graphs (all broken lines) of four developed products produced at different temperatures of the molten lead 17 and graphs (both broken lines) of two conventional products produced at different temperatures of the molten lead 17.
Comparing the developed wires 13 of the same steel grade SWRH42A or SWRH62A, the tensile strength of the wire 13 produced with the temperature of the molten lead 17 set to 450°C is higher than that of the wire 13 with the temperature of the molten lead 17 set to 565°C. That is, the temperature of the molten lead 17 can be controlled to control the tensile strength of the developed product, and the molten lead 17 at a low temperature can be used to increase the tensile strength. In addition, the graphs in FIG. 33 also show that when comparing the developed wire 13 and the conventional wire 13 of the same steel grade at the same temperature of molten lead 17, the developed product is higher in tensile strength to the conventional product.
FIG. 34 shows work hardening curves of the wires 13 of steel grades SWRH82A and SWRH82B.
For example, the work hardening curve of the developed product of steel grade SWRH82A with a lead furnace temperature set to 450°C and the work hardening curve of the developed product of steel grade SWRH82B (which has a higher manganese content) also with a lead furnace temperature set to 450°C are almost the same. Similarly, the work hardening curve of the developed product of steel grade SWRH82A with a lead furnace temperature set to 565°C and the work hardening curve of the developed product of steel grade SWRH82B also with a lead furnace temperature set to 565°C are almost the same. On the other hand, for the conventional products, the work hardening curve of the steel grade SWRH82B has a slightly larger slope than the work hardening curve of the steel grade SWRH82A, and has high tensile strength. This means that the developed products do not require the addition of expensive alloying elements (manganese as mentioned above) to increase the tensile strength. The developed products can achieve high strength and reduce costs without adopting steel grades including expensive alloying elements (manganese, chromium, or the like) for high strength.
FIG. 35 shows work hardening curves of the wires 13 of steel grades SWRH92A, 92A-Cr (chromium added), and 92B-Si (high manganese content and silicon added). FIG. 35 also shows that the developed products do not require the addition of expensive alloying elements (chromium, silicon, or the like) to increase the tensile strength.
FIG. 36 shows work hardening curves of the steel grade 102A-Cr. It can be seen that the developed products have higher tensile strength than the conventional product.
FIGS. 32 to 36 are compared in terms of carbon content. Focusing on the tensile strength of the developed products when the molten lead 17 is set to 450°C, when comparing the developed product and the conventional product of the same steel grade, the lower the carbon content (see, for example, FIG. 32), the greater the slope of the work hardening curve for the developed product than for the conventional product, and the higher the tensile strength of the developed product is likely to be. Conversely, as the carbon content is higher (see, for example, FIG. 36), the slope of the work hardening curve of the developed product approaches that of the conventional product. However, focusing on the tensile strength immediately after heat treatment (when the true strain is 0), it can be seen that the developed products have higher tensile strength than the conventional products for all the steel grades, indicating that the developed products are higher in tensile strength than the conventional products.
When the temperature of the molten lead 17 is set to 565°C, the difference in slope of the work hardening curves between the developed product and the conventional product does not occur as much as when the temperature of the molten lead 17 is set to 450°C. However, even when the temperature of the molten lead 17 is set to 565°C, the developed products have higher tensile strength than the conventional products for all the steel grades when comparing the developed and conventional products of the same steel grade, indicating that the developed products are higher in tensile strength than the conventional products.
The developed wires 13 having different tensile strength and hardness can be produced with fewer steel grades (fewer types of steel 11) as starting materials than the conventional product, facilitating managing wire manufacturing plants. In addition, the isothermal transformation temperature (the temperature of the molten lead 17) can be changed to achieve higher strength than conventional products at the same true strain.

REFERENCE SIGNS LIST

11: steel
12: heat-treated steel
13: wire
14: power source
15: feed roll
16: bath
17: molten lead
22, 31: die

Claims

A wiredrawn product drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, characterized in that:
a GOS value/average crystal grain size at a grain boundary setting angle of 2° and a step number of 0.07 µm is greater than or equal to -0.6 × GAM value + 1.5.
Awiredrawn product drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, characterized in that:
a GOS value/average crystal grain size at a grain boundary setting angle of 2° is greater than or equal to -0.18 × average crystal grain size + 2.25.
A wiredrawn product drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, characterized in that:
a GOS value/average crystal grain size at a grain boundary setting angle of 2° is greater than or equal to 0.06 × degree of integration in a longitudinal direction [101] + 1.45.
Awiredrawn product drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, characterized in that:
within a range where a torsional fracture surface is normal in a torsion test, a relationship between tensile strength (TS) and hardness is as follows, and the relationship between tensile strength and hardness is adjustable according to a heating condition and an isothermal transformation temperature during patenting. $0.16 TS + 90 \leq hardness \leq 0.16 TS + 290$
Awiredrawn product drawn from a heat-treated steel containing: 0.38 to 1.05% by mass of C; 0.0 to 1.0% by mass of Mn; 0.0 to 0.50% by mass of Cr; and 0.0 to 1.5% by mass of Si, with the remainder being Fe and unavoidable impurities, characterized in that:
when a structure of the heat-treated steel before wire drawing is observed with a backscattered electron (BSE) image, in a two-phase structure of ferrite and iron carbide, an area fraction of bent, curved, or branched iron carbide is 9% or more in the field of view.
A method for producing a wiredrawn product, characterized by comprising the steps of:
preparing a steel containing 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities;

causing the steel itself to generate heat to directly heat the steel;

passing the heated steel through a bath in which a cooling medium capable of isothermal transformation is stored to cool the steel; and

drawing the cooled steel, wherein

a temperature gradient is the largest in a final stage of heating of the heating step, and the heated steel is allowed to enter the cooling medium immediately after the steel reaches a predetermined maximum heating temperature in the final stage of heating of the heating step to start cooling without maintaining the predetermined maximum heating temperature.
A method for producing a wiredrawn product, comprising:
heating a steel from room temperature to 800°C or more within a few seconds, the steel containing 0.38 to 1.05% by mass of C, 0.0 to 1.0% by mass of Mn, 0.0 to 0.50% by mass of Cr, and 0.0 to 1.5% by mass of Si with the remainder being Fe and unavoidable impurities;

cooling the heated steel to 620°C or less within a few seconds without maintaining a maximum heating temperature; and

drawing the cooled steel.