Method for heat treatment of steel materials, method for manufacturing steel materials, and heat treatment apparatus for steel materials
The use of core-shell particles with latent heat storage materials in fluidized bed furnaces addresses the challenge of maintaining cooling medium stability and temperature uniformity, ensuring consistent steel material transformation.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- NIPPON STEEL CORPORATION
- Filing Date
- 2024-12-20
- Publication Date
- 2026-07-02
AI Technical Summary
Existing heat treatment methods for steel materials face challenges in maintaining the stability of the cooling medium and controlling temperature variations, leading to inconsistencies in the transformation structure of steel materials.
Employing core-shell particles with a latent heat storage material core and ceramic shell as the cooling medium in a fluidized bed heat treatment furnace to maintain temperature uniformity and control the metal structure of steel materials.
Facilitates easy maintenance and management of the cooling medium, suppressing temperature variations, and achieving uniform microstructure in steel materials by utilizing the latent heat storage capacity of the core-shell particles.
Smart Images
Figure 2026110343000001_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates to a heat treatment method for steel materials, a manufacturing method for steel materials, and a heat treatment apparatus for steel materials.
Background Art
[0002] There are various methods for heat treatment of steel materials, and the stability of the cooling medium during the transformation of steel materials often affects the quality of the steel materials after heat treatment. Generally, in the case of heat treatment of steel materials, first the steel materials are heated to the austenite state, and then heat treatment is performed using various cooling media. For example, when performing pearlite transformation of steel materials, ideally, a dense pearlite structure can be obtained by passing through the nose portion of the TTT diagram.
[0003] As cooling media used for heat treatment of steel materials, in addition to lead baths and nitrates (salt baths), there are fluidized bed heat treatment furnaces that flow sand such as alumina and zircon and pass the steel materials through them (see, for example, Patent Document 1).
Prior Art Documents
Patent Documents
[0004]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0005] Lead baths and nitrates (salt baths) require a great deal of maintenance management to prevent leakage. In a fluidized bed heat treatment furnace, it is difficult to keep the atmosphere temperature of the heat treatment furnace and the temperature of the flowing sand constant, and the temperature of the cooling medium changes depending on the steel type, the diameter of the steel material, the conveyance speed of the steel material, etc., which is one of the factors causing variations in the transformation structure of the steel material.
[0006] The object of this disclosure is to provide a heat treatment method for steel materials, a method for manufacturing steel materials, and a heat treatment apparatus for steel materials that facilitates the maintenance and management of the cooling medium and suppresses temperature variations. [Means for solving the problem]
[0007] The above problems will be solved by the following means. <1> A heat treatment method for steel, comprising a heat treatment step of heat treating steel using a cooling medium which is a core-shell particle comprising a core made of a latent heat storage material that is a metal or alloy, and a shell made of ceramics that covers the surface of the core. <2> The ceramic of the shell is an oxide of a constituent element of the metal or alloy of the core. <1> The heat treatment method for steel materials described in [the relevant document]. <3> The heat treatment step is a step of heat-treating the steel material by transporting the heated steel material in a fluidized bed in which the core-shell particles are flowing. <1> or <2> The heat treatment method for steel materials described in [the relevant document]. <4> The aforementioned steel material is a wire rod. <1> ~ <3> A heat treatment method for steel materials described in any one of the following. <5> The heat treatment step is a step to control the metal structure of the steel material. <1> ~ <4> A heat treatment method for steel materials described in any one of the following. <6> The step of controlling the metal structure of the steel material is a step of controlling at least one of the ferrite fraction and the pearlite fraction of the steel material. <5> The heat treatment method for steel materials described in [the relevant document]. <7> The process of controlling the metal structure of the steel material is a process of causing the steel material to undergo ferrite and pearlite transformation. <5> The heat treatment method for steel materials described in [the relevant document]. <8> The step of controlling the metal structure of the steel material is a step of controlling the pearlite structure properties of the steel material. <5> The heat treatment method for steel materials described in [the relevant document]. <9> The process of controlling the metal structure of the steel material is a process of controlling the upper bainite transformation behavior and the lower bainite transformation behavior of the steel material. <5> The heat treatment method for steel materials described in [the relevant document]. <10> The process of controlling the metal structure of the steel material is a process of controlling the martensitic transformation of the steel material. <5> The heat treatment method for steel materials described in [the relevant document]. <11> <1> ~ <10> A method for manufacturing steel, comprising the step of heat-treating the steel material by any one of the heat treatment methods for steel materials described in [the relevant document]. <12> A heat treatment furnace containing core-shell particles, which include a core made of a latent heat storage material that is a metal or alloy, and a shell made of ceramics that covers the surface of the core, and which flows the core-shell particles to form a fluidized bed, A conveying device for transporting heated steel materials into the fluidized bed, A heat treatment apparatus for steel materials, including steel. [Effects of the Invention]
[0008] According to this disclosure, a method for heat-treating steel materials, a method for manufacturing steel materials, and a heat-treatment apparatus for steel materials are provided that make it easy to maintain the cooling medium and suppress temperature variations. [Brief explanation of the drawing]
[0009] [Figure 1] A schematic cross-sectional view showing an example of a latent heat storage core-shell particle. [Figure 2] This figure shows an example of a manufacturing process for latent heat storage core-shell particles. [Figure 3] This is a schematic diagram showing an example (first embodiment) of a heat treatment apparatus for heat-treating wire using a latent heat storage material. [Figure 4] Figure 3 is a schematic diagram showing a cross-sectional view along line AA of the apparatus shown. [Figure 5] This is a schematic diagram showing the single-wire and overlapping sections of a coiled wire. [Figure 6] This is a schematic diagram showing an example of a TTT diagram. [Figure 7] This is a schematic diagram showing another example (second embodiment) of a heat treatment apparatus for heat-treating wire using a latent heat storage material. [Figure 8] Figure 7 is a schematic diagram showing a cross-section of the device along the bar line. [Figure 9]This is a graph created by simulation of the temperature changes in the single-wire part and the overlapping part when using a latent heat storage material as the cooling medium for the wire coil and when using a holding heating furnace equipped with a combustion burner.
Embodiments for Carrying Out the Invention
[0010] A heat treatment method for steel materials, a manufacturing method for steel materials, and a heat treatment apparatus for steel materials according to the present disclosure will be described. In the present disclosure, the numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. However, when "more than" or "less than" is attached to the numerical values described before and after "~", the numerical range means a range that does not include these numerical values as the lower limit value or the upper limit value. The term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps as long as the intended purpose of the step is achieved.
[0011] The temperature of the cooling medium changes due to the steel grade, the size (diameter, etc.) of the steel material, the conveying speed of the steel material, the air invading from the outside air, the heat dissipated from the furnace wall, etc., which is one of the factors causing variations in the transformation structure of the steel material. In order to solve these problems, the inventor of the present disclosure studied a method that has no special restrictions in terms of maintenance and management and can equalize the temperature of the cooling medium. As a result, it was found that by using a latent heat storage material as the cooling medium in the heat treatment of steel materials, the above problems can be solved. That is, the heat treatment method for steel materials according to the present disclosure includes a heat treatment step of heat-treating a steel material using a cooling medium that is core-shell particles including a core made of a latent heat storage material that is a metal or an alloy and a shell made of ceramics covering the surface of the core.
[0012] <Steel Material> The steel material to be heat-treated is not particularly limited, and examples include wire rods (steel wire rods), steel bars, steel plates, building materials, steel pipes, etc.
[0013] <Cooling Medium> The heat treatment method for steel materials according to this disclosure uses core-shell particles, which include a core made of a latent heat storage material that is a metal or alloy, and a shell made of ceramics that covers the surface of the core, as a cooling medium. Latent heat storage materials utilize the latent heat of solid-liquid phase transformation required when a substance changes state (phase change) from solid to liquid or from liquid to solid, without causing a change in the substance's temperature. Latent heat has a larger heat storage capacity than sensible heat, and the heat of fusion at the melting point and the heat of solidification at the freezing point of the latent heat storage material are accumulated, allowing the temperature to remain constant during the phase change. Using a latent heat storage material as a cooling medium in the heat treatment of steel materials reduces maintenance constraints and improves the temperature uniformity of the cooling medium.
[0014] Figure 1 shows an example of a cooling medium that can be used in the heat treatment method for steel materials according to this disclosure. The cooling medium 10 shown in Figure 1 is a core-shell particle 10 (sometimes referred to as "latent heat storage core-shell particle" in this disclosure) that includes a core 12 made of a latent heat storage material which is a metal or alloy, and a shell 14 made of ceramics that covers the surface of the core 12. When heat treating steel materials, using latent heat storage core-shell particle 10 in which the core 12 is made of a metal or alloy which is a latent heat storage material reduces maintenance compared to lead baths, salt baths, etc., allows for heat treatment of steel materials with high uniformity, and suppresses contamination of the steel materials by the cooling medium adhering to them.
[0015] (core) The core 12 is composed of a metal or alloy. Examples of latent heat storage materials (core materials) that constitute the core include, but are not limited to, the metals or alloys shown in Table 1 below.
[0016] [Table 1]
[0017] (shell) The shell 14 is made of ceramics. The ceramic material constituting the shell 14 is not particularly limited as long as it can maintain the core-shell structure even when the core 12 is melted due to heat treatment of the steel material. For example, the ceramic material of the shell 14 can preferably be an oxide of the constituent elements (metal or alloy) of the core 12. For example, a latent heat storage core-shell particle can be used in which the core is Al or an Al alloy and the shell is made of Al2O3.
[0018] The method for producing latent heat storage core-shell particles is not particularly limited, but one example is a two-stage process in which metal particles or alloy particles made of a metal or alloy core material are chemically converted, and then heat-oxidized at a temperature above the melting point of the core material. Figure 2 shows a schematic of the production process for latent heat storage core-shell particles in which the core is an Al-based alloy (Al-Si) and the shell is Al2O3. For Al-Si particles 12A, which are latent heat storage material, particles are obtained by coating them with an AlOOH shell 13 through a chemical conversion treatment by reacting Al with water. Next, the Al-Si particles 12A are subjected to a heat-oxidation treatment below the melting point. This makes it possible to obtain latent heat storage core-shell particles 10A in which the surface of the Al-Si particles (core) 12A is coated with an Al2O3 film (shell) 14A.
[0019] The particle size of the latent heat storage core shell particles 10 is not particularly limited, but from the viewpoint of fluidity, ease of manufacturing, and handling, an average particle size of 30 to 100 μm is preferred.
[0020] Since the latent heat storage core shell particles 10 have a surface composed of a Ceramax shell 14, they can be treated as ceramic particles regardless of the phase change of the core 12 and can be repeatedly used as a cooling medium during the heat treatment of steel materials.
[0021] As latent heat storage core-shell particles in this disclosure, for example, latent heat storage bodies disclosed in International Publication No. 2015 / 162929 and latent heat storage body microcapsules disclosed in International Publication No. 2017 / 200021 can be used.
[0022] Next, an example of an embodiment of the heat treatment method for steel materials according to this disclosure will be described. As a representative example, an example of constant temperature heat treatment of wire rod will be described, but the heat treatment method for steel materials according to this disclosure is not limited to the embodiments described below.
[0023] <First Embodiment> Figure 3 is a schematic diagram showing an example (first embodiment) of a heat treatment apparatus for constant-temperature heat treatment of wire using latent heat storage core-shell particles. Figure 4 is a schematic diagram showing a cross-section of the heat treatment apparatus 100 shown in Figure 3 along line AA. In the first embodiment shown in Figures 3 and 4, after hot rolling, a wire (wire coil) 20 that has been continuously wound into a coil is transported in a fluidized bed of latent heat storage core-shell particles 10, cooled, and then subjected to constant-temperature heat treatment.
[0024] After the hot rolling process, the wire, which has been formed into a circular cross-section, is wound into a spiral (coil) shape via a laying head 40. The heated wire (wire coil) 20, wound into a coil shape, is transported at a predetermined speed in the transport direction X by a transport means such as a belt conveyor (not shown). As shown in Figure 5, the wire coil 20 is transported to the heat treatment furnace 32 of the fluidized bed heat treatment line 30 with spacing between adjacent wires so that they do not overlap in the central part 22 of the coil width. The heat treatment furnace 32 contains latent heat storage core shell particles 10. A heating burner 34 is positioned at the top of the heat treatment furnace 32. Below the heat treatment furnace 32, multiple gas supply pipes 36 are arranged in the transport direction X to supply fluidizing gas (air). Each gas supply pipe 36 is equipped with a nozzle 38 that ejects gas from the bottom of the heat treatment furnace 32 to fluidize the latent heat storage core shell particles 10.
[0025] In the fluidized bed heat treatment line 30, the wire coil 20 is transported at a constant speed within the fluidized bed, where latent heat-storing core-shell particles 10 are flowing due to air supplied from the nozzle 38. The wire coil 20 is heated by the burner 34 in the heat treatment furnace 32, and after being cooled to a predetermined temperature by heat exchange with the latent heat-storing core-shell particles 10 through contact, it is transported in a nearly constant temperature state.
[0026] When conveying the wire coil 20, the height of the fluidized bed of the latent heat storage core shell particles 10 is preferably 1.2 times or more the initial height (before flow) from the viewpoint of fluidizing the latent heat storage core shell particles 10. On the other hand, if the height of the fluidized bed is too high, the packing density will decrease, making it difficult for heat exchange between the wire 20 and the latent heat storage core shell particles 10 to function, so it is preferable that the height is 3 times or less the initial height (before flow).
[0027] The wire coils 20 that have passed through the fluidized bed heat treatment line 30 are stored in the storage section 42 in an overlapping state.
[0028] By using a fluidized bed of latent heat-storing core-shell particles 10 to perform constant-temperature thermal treatment on the wire coil 20, the metallic structure of the wire 20 can be controlled. Figure 6 schematically shows an example of a TTT diagram. Generally, the microstructure of steel changes depending on the steel composition, as well as the heat treatment temperature, heat treatment time, and cooling rate. Therefore, the microstructure of the wire 20 can be controlled by selecting latent heat storage core-shell particles 10 suitable for the transformation temperature, taking into account the melting point, heat storage density, and heat storage capacity of the latent heat storage material constituting the core 12, according to the desired microstructure. Examples of microstructure control include the following: (A) Control of either or both of the ferrite and pearlite fractions. (B) Ferrite and pearlite transformation (C) Control of perlite structural properties (lamellar spacing, perlite block particle size, etc.) (D) Control of upper and lower bainite transformation characteristics (such as the precipitation form of carbides like lath bainite) (E) Martensitic transformation
[0029] In the heat treatment process for steel materials described herein, when the steel material undergoes a ferrite / pearlite transformation, pearlite transformation, bainite transformation, or martensitic transformation from an austenite state, the cooling medium (latent heat storage core-shell particles) can utilize the latent heat of the latent heat storage material itself and has a large heat capacity, so it is less affected by the heat received from the steel material and its temperature changes are small. As a result, the heat treatment of the steel material approaches isothermal transformation, and the microstructure becomes uniform. Here, by using a latent heat storage material that utilizes the latent heat of solid-liquid phase transformation of the substance as the cooling medium, the heat received from the steel material is stored as the latent heat of the cooling medium, so the temperature of the cooling medium can be kept constant.
[0030] By changing the operating temperature of the latent heat storage material using the method described herein, it is possible to stabilize the ferrite and pearlite fractions in ferrite / pearlite transformation, densify the microstructure through short-time transformation in the nose portion of the TTT diagram in pearlite transformation, achieve uniform control of the upper and lower bainite transformations, which are unstable in bainite transformation, and prevent quench cracking in martensitic transformation by keeping the temperature of the cooling medium constant. In other words, according to the heat treatment method described herein, when steel undergoing martensitic transformation, uneven quenching of the steel is suppressed, and heat treatment that prevents quench cracking can be achieved.
[0031] (Second Embodiment) Figure 7 is a schematic diagram showing another example (second embodiment) of a heat treatment apparatus for heat-treating wire using a latent heat storage material. Figure 8 is a schematic diagram showing a cross-section of the heat treatment apparatus 200 shown in Figure 7 along line BB. In the second embodiment shown in Figures 7 and 8, a wire coil 20A is wound up and a straight wire is subjected to constant temperature heat treatment. The heat treatment apparatus 200 shown in Figure 7 comprises a heating furnace 60 and a heat treatment furnace 32. Burners 62 and 64 are provided at the top and bottom of the heating furnace 60. In the heat treatment furnace 32, as in the first embodiment, latent heat storage core shell particles 10 are contained as a cooling medium. A burner 34 is provided at the top of the heat treatment furnace 32. A gas supply pipe 36 equipped with a nozzle 38 is arranged at the bottom of the heat treatment furnace 32.
[0032] The wire rods (wire coils) 20A, 20B, and 20C, wound into coils after the hot rolling process, are unwound from carriers 52A, 52B, and 52C to form straight wires and transported in a heating furnace 60. Inside the heating furnace 60, each wire rod 20A, 20B, and 20C is heated to, for example, 850-950°C by upper and lower burners 62 and 64, and transformed into austenite. The austenitic wire rods 20A, 20B, and 20C are then transported in a heat treatment furnace 32. In the second embodiment, since the wire rods 20A, 20B, and 20C are transported in a straight line, multiple wire rods 20A, 20B, and 20C can be transported and heat treated simultaneously.
[0033] In the heat treatment furnace 32, as in the first embodiment, the latent heat storage core shell particles 10 are heated by the burner 34, while air supplied from a nozzle 38 in the gas supply pipe 36 causes them to flow, forming a fluidized bed. The height of the fluidized bed of latent heat storage core shell particles 10 during flow is preferably 1.2 times or more and 3 times or less the initial height (before flow), as in the first embodiment. The austenitic wire is transported through the fluidized bed of the heat treatment furnace 32 to cool it down, and then subjected to constant temperature heat treatment.
[0034] The wires 20A, 20B, and 20C, which have passed through the heat treatment furnace 32 and been transformed into the desired metallic structure, are coiled by the winding machine 70 and wound onto carriers 54A, 54B, and 54C.
[0035] In the second embodiment as well, the metal structure can be controlled by selecting latent heat storage core-shell particles 10 suitable for the transformation temperature, taking into account the melting point, heat storage density, heat storage capacity, etc., of the latent heat storage material (core) according to the desired metal structure.
[0036] Although embodiments of the heat treatment method for steel materials according to this disclosure have been described above, the heat treatment method for steel materials according to this disclosure is not limited to the above embodiments. For example, in each embodiment, multiple heat treatment furnaces may be arranged, and each heat treatment furnace may contain different latent heat storage core shell particles as core material (latent heat storage material) and perform heat treatment in stages. Furthermore, the steel material to be heat-treated is not limited to wire rods. For example, in the second embodiment, drawn wire (steel wire) obtained by intermediate drawing of hot-rolled wire rod may be subjected to constant-temperature heat treatment using latent heat storage core-shell particles. According to the heat treatment method for steel materials described herein, it is possible to easily maintain and manage the cooling medium in the heat treatment process and to produce steel materials having a desired metallic structure. [Examples]
[0037] The following describes examples of the heat treatment method for steel materials according to this disclosure. The following examples are not intended to limit the heat treatment method for steel materials according to this disclosure.
[0038] A 10 mm diameter wire coil was heated to 850°C, and constant-temperature heat treatment was performed in a fluidized bed using latent heat storage core-shell particles (core: Al-25wt%Si alloy, shell: Al2O3, average particle size: 85 μm) and zircon sand (ZrHfO2: 67%, SiO2: 33%, average particle size: 90 μm) as cooling media. The time-dependent changes in the steel temperature of the single-wire section 22 and overlapping section 24 of the wire coil 20 shown in Figure 5 were estimated by simulation. The results are shown in Figure 9.
[0039] As shown in Figure 9, when cooling was performed using zircon sand (labeled "fluidized bed" in Figure 9), the temperature difference between the overlapping and single-wire sections of the wire coil was estimated to be approximately 100°C after 100 seconds. On the other hand, when cooling was performed using latent heat storage core-shell particles (labeled "latent heat storage material" in Figure 9), the temperature difference between the overlapping and single-wire sections of the wire coil was estimated to be approximately 35°C. Based on these simulation results, it can be inferred that using a latent heat storage material as a cooling medium to heat-treat wire coils effectively suppresses temperature variations in the steel and improves the uniformity of the metal structure. [Explanation of symbols]
[0040] 10,10A Latent heat storage core shell particles (cooling medium) 12 cores 14 Shells 20 Wire coil (wire) 20A,20B,20C wire rod 22 Single-track section (central section) 24 Overlapping parts 30 Fluidized Bed Heat Treatment Line 32 Heat treatment furnace 34 Heating burner 36 Gas supply pipe 38 nozzles 40 Reinforcement Head 42 Storage Unit 52A, 52B, 52C carrier 54A, 54B, 54C carrier 60 Furnace 62, 64 burners 70 Winding machine 100 Heat treatment equipment 200 Heat treatment equipment
Claims
1. A heat treatment method for steel, comprising a heat treatment step of heat treating steel using a cooling medium which is a core-shell particle comprising a core made of a latent heat storage material that is a metal or alloy, and a shell made of ceramics that covers the surface of the core.
2. The heat treatment method for steel according to claim 1, wherein the ceramic of the shell is an oxide of a constituent element of the metal or alloy of the core.
3. The heat treatment step is a step of heat treating the steel material by transporting the heated steel material in a fluidized bed in which the core-shell particles are fluidized, according to claim 1 or claim 2.
4. The heat treatment method for steel material according to claim 1 or claim 2, wherein the steel material is a wire rod.
5. The heat treatment step is a step of controlling the metal structure of the steel material, according to claim 1 or claim 2.
6. The heat treatment method for steel according to claim 5, wherein the step of controlling the metal structure of the steel is a step of controlling at least one of the ferrite fraction and the pearlite fraction of the steel.
7. The heat treatment method for steel according to claim 5, wherein the step of controlling the metal structure of the steel is a step of performing ferrite and pearlite transformation of the steel.
8. The heat treatment method for steel according to claim 5, wherein the step of controlling the metal structure of the steel is a step of controlling the pearlite structure properties of the steel.
9. The heat treatment method for steel according to claim 5, wherein the step of controlling the metal structure of the steel is a step of controlling the upper bainite transformation properties and the lower bainite transformation properties of the steel.
10. The heat treatment method for steel according to claim 5, wherein the step of controlling the metallic structure of the steel is a step of controlling the martensitic transformation of the steel.
11. A method for manufacturing steel, comprising the step of heat-treating the steel material by the heat treatment method for steel material described in claim 1 or claim 2.
12. A heat treatment furnace containing core-shell particles, which include a core made of a latent heat storage material that is a metal or alloy, and a shell made of ceramics that covers the surface of the core, and which flows the core-shell particles to form a fluidized bed, A conveying device for transporting heated steel materials into the fluidized bed, A heat treatment apparatus for steel materials, including steel.