WO2016158408A1

WO2016158408A1 - Method for manufacturing spring for suspension device, and spring for suspension device

Info

Publication number: WO2016158408A1
Application number: PCT/JP2016/058284
Authority: WO
Inventors: 彰丹下; 清栗本
Original assignee: 日本発條株式会社
Priority date: 2015-03-31
Filing date: 2016-03-16
Publication date: 2016-10-06
Also published as: JP2016191121A

Abstract

This method for manufacturing a spring for a suspension device is characterized by including a molding step S10 for molding a steel member that contains, in mass%, C: 0.10-0.39%, Mn: 1.00-1.50%, and B: 0.0005-0.003% in hot or cold molding to a spring shape, and a quenching step S20 for carrying out quenching of the molded steel member using a medium having a heat transfer coefficient about equal to that of water or close to that of water, without the steel member subjected to quenching being tempered at 400°C or above.

Description

Suspension device spring manufacturing method and suspension device spring

The present invention relates to a method for manufacturing a suspension device spring and a suspension device spring.

In a vehicle such as an automobile, a suspension device spring such as a coil spring or a leaf spring (lap leaf spring) is provided in a suspension device for suspending an axle. The suspension device spring connects the vehicle body side and the wheel side to support the vehicle weight, and plays a role of buffering an impact applied to the vehicle body through the wheel. In the field of suspension systems, there has been a demand for a lightweight suspension system spring that meets the demand for lighter vehicle weight. In addition, in recent years, there has also been a tendency for vehicle weight to increase with the mounting of electric motors and secondary batteries. Therefore, a suspension device spring that can cope with high stress and can realize weight reduction is desired.

Conventionally, spring steel such as SUP9, SUP9A, and SUP11A (JIS standard) has been used as a material for the suspension spring. The suspension device spring is manufactured by heat-treating a steel wire or a steel plate processed into a predetermined shape and shape into a spring shape in a hot or cold state, and performing a heat treatment. As the heat treatment, oil quenching and tempering are often performed. Suspension device springs are commercialized after such heat treatment, through shot peening, setting, painting, and the like.

As a technique for manufacturing a suspension device spring through tempering as described above, for example, Patent Document 1 describes weight percent (hereinafter the same), C: 0.30 to 0.75%, Si: 1.0. About 4.0%, Mn: 0.5-1.5%, Cr: 0.1-2.0% and Ni ≦ 2.0%, with the balance being Fe and inevitable impurities, Disclosed is a method for producing a high strength spring steel with excellent fatigue strength and sag resistance by quenching and reducing the amount of retained austenite after oil quenching to less than 10%, followed by tempering. .

Japanese Patent Laid-Open No. 06-172847

As disclosed in Patent Document 1, when manufacturing a suspension spring using high-strength steel, oil quenching is generally performed to avoid burning cracks. However, a manufacturing method that undergoes oil quenching is not necessarily an optimal method from the viewpoint of safety and environmental compatibility. This is because a coolant such as mineral oil used in oil quenching has a risk of ignition, and there are restrictions on handling, storage, production equipment design, and the like. Moreover, since the environmental load of the waste oil after use is not low, disposal costs and the like may increase. That is, the manufacturing method that undergoes oil quenching has a number of factors that reduce the productivity of the suspension spring.

Also, it is desired that the suspension spring has excellent toughness with mechanical strength and good toughness. Conventionally, high temperature tempering has often been performed after quenching in order to provide sufficient fatigue durability while supporting high stress design. On the other hand, in recent years, there has been a high demand for manufacturing springs for suspension systems in the vicinity of vehicle manufacturers that are strategically establishing or relocating production bases, and a compact manufacturing line with reduced manufacturing equipment scale, man-hours, operating costs, etc. There is also the present situation that is desired. High-temperature tempering usually involves heat treatment at a high temperature exceeding about 400 ° C., and such heat treatment may be repeated several times. Therefore, a large amount of heat is additionally added to steel that has already been quenched. It takes time to spend. Therefore, the manufacturing method of the suspension device spring that undergoes high temperature tempering does not meet the recent demand for downsizing of the production line.

Therefore, the present invention provides a method for manufacturing a suspension device spring that can manufacture a suspension device spring having good mechanical strength and toughness with high productivity in a compact production line, and a suspension device manufactured by the method. The object is to provide a spring.

In order to solve the above-mentioned problem, the suspension device spring manufacturing method according to the present invention is, in mass%, C: 0.10% or more and 0.39% or less, Mn: 1.00% or more and 1.50% or less. , B: a forming step of forming a steel material containing 0.0005% or more and 0.003% or less into a spring shape in hot or cold, and a heat transfer coefficient equivalent to or close to water in the formed steel material And a quenching step of quenching with a medium having a medium, and the suspension spring is manufactured without tempering the quenched steel material to 400 ° C. or higher.

Further, the suspension device spring according to the present invention is manufactured by the above-described suspension device manufacturing method.

According to the present invention, it is possible to provide a method for manufacturing a suspension spring that can manufacture a suspension spring having good mechanical strength and toughness with high productivity on a compact production line. Since the suspension spring manufactured by the manufacturing method is provided with a compressive residual stress having a deep distribution in the surface layer, it is difficult to cause delayed fracture and has excellent fatigue durability.

It is a perspective view which shows the suspension apparatus provided with the coil spring for suspension apparatuses manufactured by the manufacturing method of the spring for suspension apparatuses which concerns on one Embodiment of this invention. It is a longitudinal cross-sectional view of the coil spring for suspension apparatuses manufactured by the manufacturing method of the spring for suspension apparatuses which concerns on one Embodiment of this invention. It is a side view which shows the suspension apparatus provided with the leaf | plate spring for suspension apparatuses manufactured by the manufacturing method of the spring for suspension apparatuses which concerns on one Embodiment of this invention. It is a front view of the leaf | plate spring for suspension apparatuses manufactured by the manufacturing method of the spring for suspension apparatuses which concerns on one Embodiment of this invention. It is a flowchart which shows the manufacturing process of the manufacturing method of the spring for suspension apparatuses which concerns on one Embodiment of this invention. It is a figure which shows distribution of the residual stress of the surface layer in the spring for suspension apparatuses manufactured by water quenching. It is a figure which shows distribution of the residual stress of the surface layer in the spring for suspension apparatuses manufactured by performing oil quenching.

Hereinafter, a method for manufacturing a suspension device spring according to an embodiment of the present invention and a suspension device spring manufactured by the method will be described. In addition, about the component which is common in each figure, the same code | symbol is attached | subjected and the overlapping description is abbreviate | omitted.

The suspension device spring manufacturing method according to the present embodiment includes a coil spring (suspension device coil spring) provided in a suspension device provided in a vehicle, a plate spring (suspension device leaf spring) constituting a laminated leaf spring, and the like. The present invention relates to a method of manufacturing a suspension device spring. In this manufacturing method, the material is formed into a spring shape in the hot or cold state, subjected to quenching with a medium having a heat transfer coefficient equivalent to or close to that of water, and the quenched steel material is 400 ° C or higher after quenching. It is a method for manufacturing a suspension spring without tempering. As the material for the spring for the suspension device, manganese boron steel having a particularly low carbon content is employed.

[Coil spring for suspension system]
First, the suspension device coil spring manufactured by the suspension device manufacturing method according to the present embodiment will be described.

FIG. 1 is a diagram showing a coil spring for a suspension device manufactured by a method for manufacturing a spring for a suspension device according to an embodiment of the present invention. FIG. 1A is a perspective view showing a suspension device provided with a coil spring for a suspension device, and FIG. 1B is a longitudinal sectional view of the coil spring for the suspension device.

As shown in FIG. 1A, the vehicle is provided with a suspension device 8A for suspending an axle to which wheels W are attached. In FIG. 1A, left and right independent

strut suspension devices

8A and 8A are shown as suspension devices. As shown in FIG. 1A, a pair of left and

right suspension devices

8A and 8A includes a suspension device coil spring (coil spring) 1 manufactured by the suspension device manufacturing method according to the present embodiment, and a spring seat 2a. The spring seat 2b and the shock absorber 3 are provided.

The shock absorber 3 has a damping force generating mechanism composed of a cylindrical cylinder, a rod inserted into the cylinder, an orifice, and the like. A piston is provided at one end of the rod, and the working fluid inside the cylinder flows by the expansion and contraction of the rod, and a damping force is generated by the resistance accompanying the flow. One end of the shock absorber 3 is supported by the knuckle 6 via the bracket 5 and the other end is connected to the vehicle body side (not shown). As shown in FIG. 1A, a spring seat 2a is fixed to the cylinder of the shock absorber 3, and a spring seat 2b is fixed to the other end. A coil spring 1 is interposed between the spring seat 2a and the spring seat 2b.

As shown in FIG. 1B, the coil spring 1 is formed by forming a strand having a circular cross section into a spiral shape. The coil spring 1 elastically supports the vehicle weight in the suspension device 8A and generates a resistance force against a compressive load and a tensile load received in the axial direction. As a result, it plays a role of mitigating the impact applied to the vehicle body through the wheels and vibration during traveling. In the method for manufacturing the suspension device spring according to the present embodiment, the coil average diameter, pitch, spring constant, winding direction, and the like of the coil spring 1 to be manufactured are not particularly limited. 1B shows a cylindrical coil, the shape of the coil spring 1 may be an appropriate shape such as a barrel shape or a drum shape. The diameter of the wire of the coil spring 1 is appropriately set according to the product specifications, but as an example, it is in the range of about 8 mm to 21 mm. As will be described later, N shown in FIG. 1B indicates a torsional moment, and τ1 and τ2 indicate shear stress.

[Leaf spring for suspension system]
Next, the suspension device leaf spring manufactured by the suspension device manufacturing method according to the present embodiment will be described.

FIG. 2 is a view showing a leaf spring for a suspension device manufactured by the method for manufacturing a spring for a suspension device according to an embodiment of the present invention. FIG. 2A is a side view showing a suspension device provided with a leaf spring for a suspension device, and FIG. 2B is a front view of the leaf spring for the suspension device.

As shown in FIG. 2A, a vehicle such as a truck is provided with a suspension device 8B that suspends an axle to which wheels W are attached. In FIG. 2A, only one of the pair of leaf suspensions 8B provided on the left and right sides of the vehicle is shown. As shown in FIG. 2A, the suspension device 8B includes a suspension leaf spring (lap leaf spring) 10 manufactured by the suspension device manufacturing method according to the present embodiment, an axle 17, and a U bolt 18. And a nut 19 and a shock absorber 20 are provided.

As shown in FIG. 2B, the laminated leaf spring 10 is configured by a plurality of suspension device leaf springs (leaf springs) 11 being overlapped. The plurality of leaf springs 11 are fastened and fixed to each other by a center bolt 12 near the center of the leaf portion 11a. Further, near the end of the leaf portion 11a, the clip 13 is pinned and bundled. The uppermost plate spring 11 is formed with

eyeballs

11b and 11b at both ends. A bush (not shown) having a metal outer cylinder and an inner cylinder and an elastic body that joins the outer cylinder and the inner cylinder is press-fitted into the

eyeball portions

11b and 11b.

As shown in FIG. 2A, the laminated leaf spring 10 is supported by the vehicle body frame 14 via a bush (not shown). That is, the eyeball portion 11b on one end side is pivotally attached to the bracket 15a fixed to the vehicle body frame 14, and the eyeball portion 11b on the other end side is supported by the bracket 15b fixed to the vehicle body frame 14 via the shackle 16. The On the other hand, the laminated leaf spring 10 is fixed to an axle (axle beam) 17 by a U bolt 18 and a nut 19 in the leaf portion 11a. A shock absorber 20 is interposed between the axle 17 and the vehicle body frame 14.

As shown in FIGS. 2A and 2B, the leaf spring 11 is formed in a curved shape having a concave warp on the upper surface side. The leaf spring 11 elastically supports the vehicle weight in the suspension device 8B and generates a resistance force against a compressive load or a tensile load received by displacement in the direction of warpage. As a result, it plays a role of mitigating the impact applied to the vehicle body through the wheels and vibration during traveling, and also functions as a coupling mechanism. 2A and 2B, the overlap leaf spring 10 is a multi-leaf spring in which three leaf springs 11 having different lengths are overlapped. However, the suspension spring according to this embodiment is manufactured. In the method, the length, shape, number, and connection form of both ends of the leaf spring 11 to be manufactured are not particularly limited. Further, the leaf spring 11 is not limited to the main spring, and may constitute an auxiliary spring. The width and thickness of the leaf spring 11 are appropriately set according to product specifications, but as an example, the width is about 60 mm to 100 mm and the thickness is 9 mm to 40 mm.

[Manufacturing method of suspension device spring]
Next, a method for manufacturing the suspension device spring (coil spring for suspension device, leaf spring for suspension device) according to the present embodiment will be described. In the following description, processes common to the suspension device coil spring and the suspension device leaf spring will be described collectively, and individual descriptions will be omitted.

The suspension device spring according to the present embodiment is manufactured using manganese boron steel having a low carbon content as a material. By adopting manganese boron steel with low carbon content and excellent hardenability, quenching with a medium having a heat transfer coefficient equivalent to or close to water can be adopted instead of oil quenching. Yes. That is, even if the steel material, which is a material, is quenched at a high cooling rate corresponding to water quenching, the occurrence of quenching cracks and distortion is prevented, so that the suspension spring can be manufactured with high productivity. In addition, a low carbon content manganese boron steel with excellent hardenability has formed a martensitic structure that has good strength (mechanical strength) and good toughness as it is quenched. .

Specifically, the manganese boron steel having a low carbon content is, in mass%, carbon (C): 0.10% to 0.39%, manganese (Mn): 1.00% to 1.50%, Boron (B): 0.0005% or more and 0.003% or less are included. The material for the suspension spring is obtained by subjecting the steel material obtained by hot rolling steel containing such a predetermined chemical component to wire drawing, oil tempering, patenting, etc. as necessary. Can do. In addition, in this specification, a steel material shall be used by the meaning containing the steel materials (steel wire, steel plate, etc.) and semi-finished products in the state which processed and heat-processed.

Low carbon content manganese boron steel has the following formula:
Ceq (IIW) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
The carbon equivalent (Ceq) represented by: can be a chemical component composition satisfying 0.27% or more and 0.64% or less. However, in the formula, C, Mn, Cr, Mo, V, Cu, and Ni represent respective contents (mass%) when each element is included. If the carbon equivalent (Ceq) is 0.27% or more, the hardness generally required for a suspension spring can be realized. On the other hand, when the carbon equivalent (Ceq) is 0.64% or less, it is possible to reliably prevent the occurrence of quenching cracks even when quenching is performed at a high cooling rate corresponding to water quenching while ensuring quenching hardness. be able to. Moreover, the occurrence of delayed fracture can be significantly reduced even after quenching. The carbon equivalent (Ceq) is more preferably 0.36% or more and 0.41% or less.

Manganese boron steel with a low carbon content is, by mass, C: 0.10% to 0.39%, Si: 0.05% to 0.40%, Mn: 1.00% to 1.50. % Or less, B: 0.0005% or more and 0.003% or less may be contained as an essential element, and the balance may be a chemical component composition composed of Fe, an optional additive element, or an unavoidable impurity. As the optional additive element, at least one element selected from the group consisting of Cr, Mo, V, Cu, Ni, Ti, Nb, Al, N, Ca, and Pb can be contained. As an unavoidable impurity, it is allowed to contain elements such as P and S in the range of 0.005% by mass or less.

More specifically, the manganese boron steel having a low carbon content may contain an essential element group composed of C, Si, Mn, and B, and the balance may have a chemical composition composed of Fe and inevitable impurities. In addition, the essential element group composed of C, Si, Mn, and B may be contained, and the balance may have a chemical composition composed of Fe, an optional additive element, and inevitable impurities. If the manganese boron steel having a low carbon content has a chemical composition that does not contain an optional additive element, the material cost of the suspension spring can be reduced. On the other hand, when the chemical composition contains an arbitrarily added element, various characteristics can be modified according to the element type.

マンガン Manganese boron steel with a low carbon content shall contain at least 1.20% of each optional additive element in the chemical composition including the optional additive element. Manganese boron steel having a low carbon content is a standard American Engineering standard 15B23 equivalent steel or 15B26 equivalent steel in a preferred form. In addition, although it is preferable that each arbitrary addition element is contained by content with which a carbon equivalent (Ceq) satisfy | fills the said numerical range, it is not prevented that it is contained with content which does not satisfy | fill the said numerical range. Hereinafter, each component element of the manganese boron steel having a low carbon content will be described.

Carbon (C) is a component that contributes to improvement in strength and hardness. By setting C to 0.10% by mass or more, it is possible to ensure good strength and hardness required for the suspension device spring. On the other hand, when C is 0.39% by mass or less, it is possible to prevent the occurrence of burning cracks and distortions due to transformation stress and the like, and delayed fracture. Moreover, the fall of the corrosion resistance by precipitation of a carbide | carbonized_material can be suppressed. The content of C is more preferably 0.20% by mass or more and 0.26% by mass or less.

Silicon (Si) is a component that contributes to improvement in strength and hardness. Moreover, it is also a component added for the purpose of deoxidation at the time of steelmaking of steel materials. By setting Si to 0.05 mass% or more, good strength, hardness, corrosion resistance, and sag resistance can be ensured. On the other hand, when Si is 0.40 mass% or less, it is possible to avoid the deterioration of toughness and workability. The Si content is preferably 0.15% by mass or more and 0.30% by mass or less.

Manganese (Mn) is a component that contributes to improving hardenability and strength. Moreover, it is also a component added for the purpose of deoxidation at the time of steelmaking of steel materials. By setting Mn to 1.00% by mass or more, good strength and hardenability can be ensured. On the other hand, when Mn is 1.50% by mass or less, it is possible to avoid deterioration of toughness and workability due to segregation and deterioration of corrosion resistance.

Boron (B) is a component that contributes to improving hardenability and strength. By making B 0.0005 mass% or more and 0.003 mass% or less, good hardenability can be ensured. Moreover, toughness and corrosion resistance can be improved by grain boundary strengthening. On the other hand, even if the content of B exceeds 0.003% by mass, the effect of improving the hardenability is saturated and the mechanical properties are deteriorated, so the upper limit of the content is limited.

Phosphorus (P) is an unavoidable impurity that remains from the time of steelmaking. By making P into 0.040 mass% or less, it can avoid that toughness and corrosion resistance are impaired by segregation. The content of P is more preferably 0.030% by mass or less.

Sulfur (S) is an unavoidable impurity that remains from the time of steelmaking. By making S into 0.040 mass% or less, it can avoid that toughness and corrosion resistance are impaired by segregation or precipitation of MnS inclusions. The content of S is more preferably 0.030% by mass or less.

Chromium (Cr) is a component that contributes to improving strength, corrosion resistance, and hardenability. By adding Cr, strength, corrosion resistance and hardenability can be improved. On the other hand, when Cr is excessively contained, toughness and corrosion resistance are impaired due to segregation of carbides, workability is reduced, and material costs are excessive, so the upper limit of the content is limited. The Cr content is preferably 1.20% by mass or less, may be 0.60% by mass or less, or a composition not intentionally added is also preferable.

Molybdenum (Mo) is a component that contributes to improving hardenability, toughness, and corrosion resistance. By adding Mo, hardenability, toughness, and corrosion resistance can be improved. On the other hand, when Mo is excessively contained, the material cost becomes excessive, and thus the upper limit of the content is limited. The Mo content is preferably 0.08% by mass or less, more preferably 0.02% by mass or less, or a composition not intentionally added.

Vanadium (V) is a component that contributes to improvements in toughness and hardness. Moreover, it has the effect | action which couple | bonds with nitrogen (N) and prevents fixation of boron (B) by N. By adding V, toughness and hardness can be improved, and the effect of boron (B) can be effectively expressed. On the other hand, if V is contained excessively, the toughness and corrosion resistance are impaired by the precipitation of carbonitride, and the material cost becomes excessive, so the upper limit of the content is limited. The V content is preferably 0.30% by mass or less, or a composition not intentionally added is preferable.

Copper (Cu) is a component that contributes to improving hardenability and corrosion resistance. By adding Cu, hardenability and corrosion resistance can be improved. However, when Cu is excessively contained, hot surface embrittlement may occur, so the upper limit of the content is limited. The Cu content is preferably 0.30% by mass or less, or a composition not intentionally added is preferable.

Nickel (Ni) is a component that contributes to improving corrosion resistance and hardenability. By adding Ni, it is possible to ensure good corrosion resistance and hardenability, and it is possible to reduce corrosion deterioration and quench cracking. On the other hand, even if Ni is contained excessively, the effect of improving hardenability is saturated and the material cost also increases, so the upper limit of the content is limited. The Ni content is preferably 0.30% by mass or less, or a composition not intentionally added is also preferable.

Titanium (Ti) is a component that contributes to improvement in strength and corrosion resistance. Moreover, it has the effect | action which couple | bonds with nitrogen (N) and prevents fixation of boron (B) by N. By adding Ti, the strength and corrosion resistance can be improved, and the effect of boron (B) can be effectively expressed. On the other hand, when Ti is excessively contained, toughness and corrosion resistance are impaired by precipitation of carbonitrides, so the upper limit of the content is limited. The Ti content is preferably 0.05% by mass or less, or a composition not intentionally added is preferable.

Niobium (Nb) is a component that contributes to improvement of strength and toughness. Moreover, it has the effect | action which couple | bonds with nitrogen (N) and prevents fixation of boron (B) by N. By adding Nb, the strength and toughness can be improved by making the crystal grains finer, and the effect of boron (B) can be effectively expressed. On the other hand, when Nb is contained excessively, toughness and corrosion resistance are impaired by precipitation of carbonitrides, so the upper limit of the content is limited. The Nb content is preferably 0.06% by mass or less, or a composition not intentionally added is also preferable.

Aluminum (Al) is a component that contributes to improvement of toughness. Moreover, it has the effect | action which couple | bonds with nitrogen (N) and prevents fixation of boron (B) by N. Moreover, it is also a component added for the purpose of deoxidation at the time of steelmaking of steel materials. By adding Al, strength and toughness can be improved by making crystal grains finer, and the effect of boron (B) can be effectively expressed. On the other hand, when Al is excessively contained, toughness and corrosion resistance are impaired by precipitation of nitrides and oxides, so the upper limit of the content is limited. The Al content is preferably 0.30% by mass or less, or a composition not intentionally added is preferable. Note that Al as an optional additive element corresponds to Soluble Al.

Nitrogen (N) is an unavoidable impurity that remains from the time of steelmaking. However, since it is a component that contributes to improving the strength, it can also be added intentionally. By containing N in a predetermined content range, it is possible to improve strength while avoiding damage to toughness and corrosion resistance due to precipitation of nitride. The N content is preferably 0.02% by mass or less.

Calcium (Ca) is a component that contributes to improvement of machinability. By adding Ca, the machinability of the steel material can be further improved. The Al content is preferably 0.40% by mass or less, or a composition not intentionally added is preferable.

Lead (Pb) is a component that contributes to improvement of machinability. By adding Pb, the machinability of the steel material can be further improved. The Pb content is preferably 0.40% by mass or less, or a composition not intentionally added is preferable.

FIG. 3 is a flowchart showing a manufacturing process of a method for manufacturing a spring for a suspension device according to an embodiment of the present invention.

As shown in FIG. 3, the suspension device spring manufacturing method includes a molding step S10, a quenching step S20, a low temperature tempering step S30, a shot peening step S40, a setting step S50, and a coating step S60 in this order. It can be a method comprising. In this manufacturing method, the low-temperature tempering step S30 and the shot peening step S40 are not essential steps and can be omitted as will be described later.

The forming step S10 is a step of forming a steel material into a spring shape in a hot or cold state. As the steel material used for forming, the above-described low carbon content manganese boron steel material (steel wire, steel plate, etc.) cut into a predetermined shape according to the specifications of the suspension device spring is used. Specifically, in the coil spring 1 for a suspension device, a steel material (steel wire) is formed in a spiral shape, and end winding portions supported by the spring seats 2a and 2b are formed at the ends. Moreover, in the leaf spring 11 for a suspension device, a steel material (steel plate) is formed into a curved shape, and the

eyeball portions

11b and 11b are formed at the ends as necessary. The heating temperature in the hot forming is not particularly limited as long as it is an austenite region, but is preferably in the range of 850 ° C. or higher and 1100 ° C. or lower.

The quenching step S20 is a heat treatment step in which the steel material is heat treated to austenite and then cooled at a lower critical cooling rate or higher to be quenched. Specifically, in the method for manufacturing a suspension device spring according to the present embodiment, the steel material formed into a spring shape is quenched with a medium having a heat transfer coefficient equal to or higher than that of water. It is preferable that the heat transfer coefficient of the quenching medium (coolant) is within a range of ± 10% with respect to the heat transfer coefficient value of still water or water having flow with respect to the steel material.

The heat treatment in the quenching step S20 is preferably performed at a heating temperature of 850 ° C. or higher and 1100 ° C. or lower. By suppressing the heating temperature to 1100 ° C. or less, coarsening of austenite crystal grains can be reduced. As a result, good toughness can be imparted, and the occurrence of fire cracking and distortion can also be prevented. As a heating method, for example, high frequency induction heating can be used. When hot forming is performed in the forming step S10, the austenitic steel material may be subjected to a quenching treatment immediately after forming and subjected to quenching.

The cooling process in the quenching step S20 is performed with a medium having a heat transfer coefficient equivalent to or close to that of water. Specifically, it is preferable to perform water quenching, aqueous solution quenching or salt water quenching. Water quenching is a process that uses water as a coolant. The water temperature may be in the temperature range of 0 ° C. to 100 ° C., preferably 5 ° C. to 40 ° C. Aqueous solution quenching (polymer quenching) is a treatment using an aqueous solution to which a polymer is added as a coolant. As the polymer, for example, various polymers such as polyalkylene glycol and polyvinyl pyrrolidone can be used. Salt water quenching is a treatment using an aqueous solution to which salts such as sodium chloride are added as a coolant. The cooling process in the quenching step S20 may be performed by stirring the medium at an appropriate speed.

The cooling process in the quenching step S20 may be performed in the form of restraint quenching, spray quenching, injection quenching, or the like. Specifically, the suspension device coil spring 1 is constrained to a helical shape having a predetermined free height, and is rapidly cooled with a coolant to be martensite. Moreover, in the leaf spring 11 for a suspension device, the

eyeball portions

11b and 11b at both ends are fixed and rapidly cooled with a coolant to be martensite. The metal structure in which the low carbon content manganese boron steel is martensitic has few precipitates and has good toughness as it is quenched. Therefore, in the manufacturing method of the suspension device spring according to the present embodiment, as shown in FIG. 3, the quenched steel material is tempered without tempering to 400 ° C. or higher, that is, the low temperature tempering step S30, shot. It shall be used for the peening process S40 or the setting process S50.

The low temperature tempering step S30 is a heat treatment step in which the quenched steel material is soaked and tempered to 200 ° C. or higher and 350 ° C. or lower. When the heating temperature in tempering is 200 ° C. or higher, the yield point is significantly improved by strain aging, and better strength can be imparted. Further, since dehydrogenation can be achieved, hydrogen embrittlement is reduced, and the occurrence of delayed fracture is more reliably prevented. On the other hand, if the heating temperature in tempering is 350 ° C. or less, the hardness is not easily lost, and it is difficult to be affected by low temperature temper embrittlement in combination with the grain boundary strengthening by boron. The tempering temperature is preferably 200 ° C. or higher and 300 ° C. or lower. The tempering time is preferably 60 minutes or less at 300 ° C. However, the low-temperature tempering step S30 can be omitted.

The shot peening step S40 is a step of performing shot peening on the surface of the heat-treated steel material. Shot peening may be performed in either warm or cold form, or may be performed in the form of stress peening. Further, it may be repeated a plurality of times while changing conditions such as particle diameter and projection speed. By applying shot peening, compressive residual stress can be applied to the surface of the steel material, and it is possible to prevent cracking and stress corrosion cracking, and to improve fatigue strength and wear resistance. However, the execution of the shot peening process S40 can be omitted.

The setting step S50 is a step of applying an overload exceeding the elastic limit to the heat-treated steel material. The setting may be performed in either warm or cold form. As temperature conditions in warm, 200 degreeC or more and 300 degrees C or less are preferable. This is because if the temperature is 300 ° C. or lower, the compressive residual stress formed on the surface layer of the steel material is difficult to remove. By setting the working direction of the spring of the steel material formed into a spring shape, the yield point is increased and the permanent deformation is suppressed, so that the elastic limit and the sag resistance are improved. In addition to the setting step S50, warm setting can be performed after the low temperature tempering step S30 and before the shot peening step S40.

The coating step S60 is a step of coating the surface of the steel material that has been heat-treated. In painting, the surface of the steel material can be preliminarily subjected to a cleaning treatment or a base treatment. In the cleaning process, oil and fat on the surface and foreign matters are removed. In the base treatment, for example, a film of zinc phosphate, iron phosphate or the like is formed. For coating, it is preferable to use a powder coating material such as an epoxy resin. The steel material to be used for coating may be preheated as necessary and then coated with a paint on the surface. The applied paint is then baked by furnace heating or infrared heating. The heating temperature in the preheating or baking is preferably in the range of 180 ° C. or more and 200 ° C. or less where normal coating can be applied.

Through the above manufacturing process and product inspection, the suspension device spring is manufactured. Since the suspension spring is made of manganese boron steel with a low carbon content, it has both good strength and good toughness. Specifically, the metal structure of the suspension spring is a martensite structure with a main phase or a tempered martensite structure in which ε carbides are precipitated, and has substantially no troostite or sorbite structure. It is also possible to manufacture a suspension spring in which 90% or more of the central portion of the cross section has a martensite structure. The as-quenched martensite structure can form a single phase extending to the center of the suspension coil spring 1 and the suspension plate spring 11 due to the hardenability of manganese boron steel having a low carbon content. Therefore, it is difficult to form a local battery between the precipitated carbide and the like, which is advantageous in that the suspension spring has excellent corrosion resistance.

The suspension spring to be manufactured preferably has a grain size number G of more than 8 and more preferably 9 or more with respect to the grain size of the prior austenite grain boundaries. By refining the grain size of the prior austenite grain boundaries in this way, the strength can be further improved without impairing toughness. The refinement of the crystal grain size can be realized, for example, by lowering the quenching temperature or increasing the content of Mn or an optional additive element. The crystal grain size of the prior austenite grain boundaries can be measured in accordance with JIS G 0551. The particle size number G can be determined based on a microscope observation image of the as-quenched metal structure, and is preferably obtained as an average value of the particle size numbers of 5 to 10 fields of view.

According to the method for manufacturing a suspension device spring according to the above-described embodiment, the quenched steel material is tempered to 400 ° C. or higher by adopting manganese boron steel having a low carbon content and excellent hardenability. It is possible to manufacture a spring for a suspension device that has both good strength and good toughness without being quenched. Therefore, there is an advantage that it is not necessary to perform high-temperature tempering in which a high amount of heat is consumed and heated to a high temperature exceeding 400 ° C. Therefore, it is possible to reduce the number of man-hours and operating costs related to the heat treatment during the manufacturing process, and the manufacturing method is suitable for downsizing the scale of the manufacturing equipment for the suspension device spring.

Moreover, in the manufacturing method of the spring for suspension devices according to the present embodiment, the suspension device spring having both good strength and good toughness can be manufactured without being quenched, and thus the low temperature tempering step S30 is omitted. It is also possible. This is advantageous in that it is not necessary to install a long tempering furnace on the production line for the suspension device spring. Further, since tempering is not required, the number of man-hours and operating costs related to the heat treatment during the manufacturing process are reduced, and the scale of the manufacturing facility for the suspension device spring is more suitable for downsizing.

Moreover, in the manufacturing method of the suspension device spring according to the present embodiment, quenching with a medium having a heat transfer coefficient equal to or higher than that of water is used instead of conventional oil quenching. This eliminates the need for management security and disposal costs associated with the use of oil-based coolants such as mineral oil, and enables efficient production of suspension springs. Then, the suspension spring to be manufactured is subjected to quenching with a higher cooling rate compared to oil quenching, so that a deep distribution of compressive residual stress is applied to the surface layer as follows.

FIG. 4 is a diagram showing the distribution of residual stress on the surface layer of the suspension spring. FIG. 4A is a diagram showing a distribution of residual stress in a suspension spring manufactured by water quenching, and FIG. 4B is a diagram showing a distribution of residual stress in a suspension spring manufactured by oil quenching. is there.

In FIG. 4, the vertical axis represents the residual stress (MPa) applied to the suspension spring. The (−) side is compressive stress and the (+) side is tensile stress. The solid line shows the residual stress value (with SP) in the suspension spring manufactured through the shot peening process S40, and the broken line shows the residual stress value (without SP) in the suspension spring manufactured without going through the shot peening process S40. ). As schematically shown in FIGS. 4A and 4B, the residual stress caused by the thermal stress and the residual stress caused by the transformation stress are superimposed on the steel that has been subjected to the cooling treatment in the quenching, and a residual stress distribution appears.

In quenching, thermal contraction due to cooling of the steel material proceeds from the surface side. That is, the surface side is rapidly cooled, heat-shrinks more than the inner side, and plastically deforms while dragging the inner side. Thereafter, the deformation on the surface side ends, whereas the heat shrinkage still proceeds on the inner side where the heat conduction is delayed, and the plastic deformation ends while dragging the already solidified surface side. Therefore, the thermal stress generated in the process of such thermal shrinkage tends to have a compressive residual stress dominant on the surface side of the steel material and a tensile residual stress dominant on the inner side.

On the other hand, in quenching, transformation expansion due to martensitic transformation of steel also proceeds from the surface side. That is, the surface side is rapidly cooled, and is lower than the martensite transformation end temperature (Mf) before the inner side, and the volume change of the metal structure is terminated. Thereafter, the transformation on the surface side ends, whereas the transformation expansion still proceeds on the inner side where the heat conduction is delayed, and the plastic deformation ends while dragging the surface side that has already been transformed. For this reason, the transformation stress generated in the process of transformation causes a tendency that the tensile residual stress is dominant on the surface side of the steel material and the compressive residual stress is dominant on the inner side, contrary to the thermal stress.

Conventionally, in oil quenching performed as a heat treatment for a spring for a suspension device, the cooling rate of a steel material is relatively slow, and thus the contribution of thermal stress to such transformation stress is low. Therefore, in the conventional suspension device spring manufactured by oil quenching, as shown by the broken line in FIG. 4B, a tensile residual stress is formed on the surface layer. 4B, even if shot peening is performed, for example, the residual stress (S) at a depth of 0.80 mm usually becomes a tensile stress.

On the other hand, in the manufacturing method of the suspension device spring according to the present embodiment, a high cooling rate corresponding to water quenching is adopted, so that the contribution of thermal stress to transformation stress is high. Therefore, as shown by a broken line in FIG. 4A, a deep distribution of compressive residual stress is formed on the surface layer. That is, the crossing point at which the compressive residual stress changes to the tensile residual stress is at least 0.80 mm or more from the surface, and exceeds the depth of the corrosion pit that normally grows to a depth of about 0.40 mm. A high compressive residual stress (S) of 150 MPa or more is applied from the surface to a depth of 0.80 mm. Furthermore, when performing shot peening, as shown by a solid line in FIG. 4A, it is possible to apply a higher compressive residual stress to the surface side.

As shown in FIG. 1B, the suspension device coil spring 1 manufactured by the suspension device manufacturing method according to the present embodiment is designed to take a desired pitch angle (θ). The pitch angle (θ) is an angle formed by a plane perpendicular to the center line (X) of the coil spring 1 and the center line (Y) of the strand. In general, when the pitch angle (θ) is sufficiently small, the shear stress generated in the axial direction (X-ray direction) of the coil during use is larger than the outside of the coil (τ1) due to the superposition of the torsional moment (N). It becomes larger on the inner side (τ2). In addition, a small-diameter coil spring manufactured by cold forming tends to leave a tensile residual stress in the axial direction of the wire inside the coil due to plastic deformation in coiling. From such a characteristic, it is desired that a sufficient compressive residual stress be applied to the inside of the coil from the viewpoint of appropriately preventing fatigue failure occurring in the coil spring 1.

In this respect, according to the method for manufacturing a suspension spring according to the present embodiment, a compressive residual stress having a deep distribution is easily formed over the entire surface layer of the coil spring 1 through quenching at a high cooling rate corresponding to water quenching. It is possible to make it. Therefore, the fatigue durability of the coil spring 1 can be improved, and the occurrence of delayed fracture such as cracking can be prevented. In particular, it is advantageous in that fatigue fracture starting from the inside of the coil can be satisfactorily prevented. Further, the omission of the shot peening step S40 can be permitted by applying a deep distribution of high compressive residual stress.

In addition, the inside of the coil of the coil spring 1 is a region where it is difficult for the shot peening projection material to reach compared to the outside of the coil. On the other hand, in the coil spring 1 for a suspension apparatus according to the present embodiment, the inner side of the coil of the coil spring 1 is not affected by shot peening by quenching at a high cooling rate corresponding to water quenching. It is possible to have the same hardness as the outside. The inner side and the outer side of the coil mean an inner region and an outer region of a plane that is parallel to the axial direction of the coil and passes through the center of the wire.

The coil spring 1 for a suspension device according to an embodiment of the present invention has the same hardness (± 5% HV) on the inner side and the outer side of the coil when shot peening is not performed by such a manufacturing method. In addition, it is possible to have a compressive residual stress of 150 MPa or more from the surface to a depth of 0.80 mm both inside and outside the coil. Further, the Charpy impact value at room temperature can be 30 J / cm ² or more in the generally required hardness. Since the crossing point is at least 0.80 mm deep from the surface, cracks caused by corrosion pits can be satisfactorily prevented even in the case winding portion where the coating or the like easily peels off due to sliding or collision with the spring seat. This is advantageous in that it can.

On the other hand, as shown in FIG. 2A and FIG. 2B, the suspension device leaf spring 11 manufactured by the suspension device manufacturing method according to the present embodiment has a vehicle body via

eyeballs

11b and 11b formed at both ends. Supported by the frame 14. It is known that the inner peripheral surface 110 of the eyeball portion 11b where loads from the vehicle body side tend to concentrate tends to be a starting point for delayed fracture and fatigue fracture. From such a characteristic, it is desired that sufficient compressive residual stress is applied to the inner peripheral surface 110 of the eyeball portion 11b from the viewpoint of appropriately preventing fatigue failure occurring in the leaf spring 11.

In this respect, according to the manufacturing method of the suspension device spring according to the present embodiment, the compression residual having a deep distribution on the inner peripheral surface 110 of the eyeball portion 11b of the leaf spring 11 through quenching at a high cooling rate corresponding to water quenching. Stress can be easily formed. Therefore, the fatigue durability of the leaf spring 1 can be improved, and the occurrence of delayed fracture such as cracking can be prevented. In particular, it is advantageous in that it is possible to satisfactorily prevent fatigue failure starting from the inner peripheral surface 110 of the eyeball portion 11b. Further, the omission of the shot peening step S40 can be permitted by applying a deep distribution of high compressive residual stress.

In addition, the inner peripheral surface 110 of the center part 11b of the leaf spring 11 is an area that requires man-hours to reach the shot peening projection material, as compared with the leaf part 11a. Moreover, the leaf spring 11 has a substantially rectangular cross-sectional shape, and has a shape in which uniform shot peening is relatively difficult including corners. On the other hand, in the leaf spring 11 for the suspension device according to the present embodiment, the inner peripheral surface 110 of the eyeball portion 11b of the leaf spring 11 is equivalent to the leaf portion 11a by quenching at a fast cooling rate corresponding to water quenching. It is advantageous in that it can be made hard, and a compressive residual stress can be reliably applied to the corners in a cross-sectional view.

The suspension device leaf spring 11 according to an embodiment of the present invention is obtained by such a manufacturing method, in a state where shot peening has not been performed, and the warped side surface of the leaf portion 11a and the inner peripheral surface 110 of the eyeball portion 11b. Is equivalent hardness (± 5% HV), and compressive residual stress of 150 MPa or more from the surface to a depth of 0.80 mm on both the warped side surface of the leaf portion 11a and the inner peripheral surface 110 of the eyeball portion 11b. It can also have. Further, the Charpy impact value at room temperature can be 30 J / cm ² or more in the generally required hardness. Since the crossing point has a depth of at least 0.80 mm from the surface, it is advantageous in that the crossing point is resistant to defects caused by sliding and collision of the stacked leaf springs 11 and the center bolt 12.

<< Other Embodiments >>
1. In the above embodiment, the case where an aqueous coolant having a heat transfer coefficient equivalent to or close to that of water is used has been described as an example, but the quenching target can be rapidly cooled, and the predetermined performance such as the described strength, toughness, etc. Is obtained, the type of medium is not particularly limited. For example, it may be water or oil containing ice, an organic solvent, a liquid or solid having a high heat transfer coefficient. Note that the phase of the medium is not particularly limited, such as a liquid or a liquid containing a solid.

2. Although various configurations have been described in the embodiment, each configuration may be selected, or each configuration may be appropriately selected and combined.

3. The above-described embodiments are examples of the present invention, and the present invention can be modified in various ways within the scope of the claims or the scope described in the embodiments.

DESCRIPTION OF SYMBOLS 1 Coil spring 2a, 2b Spring suspension 3 Shock absorber 5 Bracket 6

Knuckle

8A, 8B Suspension device 10 Lap plate spring 11 for suspension device Leaf spring

11a Leaf part

11b Eye part 12 Center bolt 13 Clip 14

Body frame

15a, 15b Bracket 16 Shackle 17 Axle 18 U Bolt 18
19 Nut 20 Shock absorber S10 Molding process S20 Quenching process S30 Low temperature tempering process S40 Shot peening process S50 Setting process S60 Coating process

Claims

A steel material containing, by mass%, C: 0.10% or more and 0.39% or less, Mn: 1.00% or more and 1.50% or less, and B: 0.0005% or more and 0.003% or less is hot or cold. A molding process for forming a spring shape in between,
A quenching step of quenching the formed steel material with a medium having a heat transfer coefficient equal to or higher than water or close to water,
A method for producing a suspension spring, comprising producing the suspension spring without tempering the quenched steel material to 400 ° C or higher.
The following formula,
Ceq (IIW) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
A forming step of forming a steel material of manganese boron steel having a carbon equivalent (Ceq) represented by: 0.27% or more and 0.64% or less into a spring shape hot or cold;
A quenching step of quenching the formed steel material with a medium having a heat transfer coefficient equal to or higher than water or close to water,
A method for producing a suspension spring, comprising producing the suspension spring without tempering the quenched steel material to 400 ° C or higher.
The steel materials are in mass%, C: 0.10% to 0.39%, Si: 0.05% to 0.40%, Mn: 1.00% to 1.50%, B: 0 .0005% to 0.003% as essential elements, P: 0.040% or less, S: 0.040% or less, and optional addition elements include Cr, Mo, V, Cu, Ni, Ti , Nb, Al, N, Ca and Pb, each containing at least one element selected from the group consisting of Nb, Al, N, Ca and Pb in a range of 1.20% or less, the balance being Fe and inevitable impurities The manufacturing method of the spring for suspension apparatuses of Claim 1 or Claim 2.
The method for manufacturing a spring for a suspension device according to claim 1 or 2, wherein the quenching is water quenching, aqueous quenching, or salt quenching.
3. The method for manufacturing a suspension spring according to claim 1, wherein the quenching is water quenching.
The method for manufacturing a suspension spring according to claim 1 or 2, wherein the suspension spring is manufactured without tempering the quenched steel material to 200 ° C or higher.
The method for manufacturing a suspension spring according to claim 1 or 2, further comprising a tempering step of soaking the tempered steel material at 200 ° C to 350 ° C and tempering.
A shot peening step of performing shot peening on the heat-treated steel,
A setting step for setting the steel material subjected to shot peening;
The method for manufacturing a spring for a suspension device according to claim 1, further comprising a coating step of performing a coating process on the steel material subjected to the setting.
A setting step for setting the steel material that has been heat-treated;
And further including a coating step of performing a coating process on the steel material subjected to the setting,
The method of manufacturing a suspension spring according to claim 1 or 2, wherein the suspension spring is manufactured without performing shot peening on the heat-treated steel material.
A suspension spring is manufactured by the method for manufacturing a suspension spring according to claim 1 or 2.
The suspension spring according to claim 10, wherein a compressive residual stress of 150 MPa or more is distributed from the surface of the steel material to a depth of 0.8 mm.
The suspension spring according to claim 10, wherein the suspension spring is a coil spring for the suspension device.
The suspension spring according to claim 10, wherein the suspension spring is a suspension spring.