JPH04268013A - Heating furnace and heating method for quenching steel material - Google Patents

Abstract

PURPOSE:To heat a steel material to austenizing temp. without decarbonization. CONSTITUTION:This heat furnace is divided into preheating zone 11, first heating zone 12 and second heating zone 13. The first heating zone 12 is set to the furnace length and the duct thereof so that a work having the presumed max. plate thickness or max. wire diameter can sufficiently be heated to the temp. at lower than the transformation point of the steel. The temp. in the second heating zone 13 is set to the austenizing temp. at higher than the temp. in the first heating zone 12 and the work is rapidly heated in alpha+gamma two phase range. The setting temps. in the first heating zone 12 and the second heating zone 13 are clearly distinguished as the above the execute the temp. control.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heating furnace used to heat a workpiece of spring steel to a predetermined temperature when hot forming a suspension spring, for example, and a method for quenching and heating steel materials.

0002

[Prior Art] When manufacturing hot-formed suspension springs (plate springs or coil springs), the steel material is kept in the austenite region for a certain period of time in order to form the steel material into a desired shape or to harden it at a desired temperature. May heat up. At this time, it is necessary to prevent scale formation due to oxidation reactions occurring on the surface of the steel material, reduction in surface hardness due to decarburization, or coarsening of crystal grains due to overheating. In particular, ferrite decarburization causes a significant decrease in surface hardness, so even if shot peening is performed after tempering, sufficient compressive residual stress on the surface cannot be obtained, resulting in a decrease in wear resistance.
This causes an increase in stiffness and a decrease in fatigue life, which significantly reduces the performance of the spring. Therefore, it is important to avoid decarburization when heating steel materials.

0003

[Problems to be Solved by the Invention] Conventional heating furnaces have had the following problems. (a) In walking beam furnaces and walking hearth furnaces, the work path line and the furnace inlet opening are at approximately the same height, so outside air easily enters the furnace, making it difficult to control the furnace internal pressure and having poor thermal efficiency.

(b) Workpieces (steel materials) that can be processed in the same furnace in order to reduce capital investment or improve productivity
The furnace is designed to widen the dimensional specification range as much as possible, but if the furnace size is matched to a workpiece with a large plate thickness or wire diameter, is heated at a higher temperature and for a longer time than necessary, leading to the risk of decarburization or overheating. However, if the size of the furnace is adjusted to workpieces with small plate thicknesses and wire diameters, the takt time will have to be reduced and the furnace time will have to be extended when processing workpieces with large plate thicknesses or wire diameters. This results in a decrease in productivity.

(c) When changing the temperature inside the furnace by zone control, it takes a relatively long time to adjust the temperature. To avoid this, there is a method of shortening the cooling time by introducing outside air into the furnace, but in that case, free oxygen in the furnace increases, oxidation of the steel surface becomes active, and scaling and decarburization occur. becomes intense.

(d) As an indirect heating method, if the furnace atmosphere is actively adjusted by using a controlled furnace atmosphere such as an inert atmosphere, reducing atmosphere, or carburizing atmosphere, scaling or decarburization due to oxidation of the work surface can be avoided. These problems will be eliminated, but the equipment investment and running costs will be high due to the need to adjust the atmosphere of the entire furnace.

(e) In order to prevent or reduce decarburization, the higher the heating rate in the α+γ two-phase region, the better; however, in conventional furnaces, rapid heating in the α+γ two-phase region is difficult;
It is difficult to set a good heat pattern to prevent decarburization. Therefore, an object of the present invention is to provide a heating furnace and a method for quenching and heating steel materials, which can solve the above-mentioned conventional problems.

[0008]

[Means for Solving the Problems] The heating furnace of the present invention developed to achieve the above object includes a work transfer mechanism for transferring the steel material, a preheating zone for preheating the steel material, and a heating furnace for heating the steel material to a temperature below the transformation point. a first heating zone that heats the steel material to a constant temperature; a second heating zone that is provided downstream of the workpiece conveyance side of the first heating zone and rapidly heats the steel material to an austenitizing temperature; and a temperature seal section provided between the heating zone and the heating zone.

[0009]

[Operation] When heating steel materials, it is known that the heating temperature and heating time have a large effect on decarburization. According to experiments conducted by the present inventors, it was found that the relationship between heating conditions and decarburization has the following characteristics.

(1) Even if steel is heated, as long as the temperature is kept from room temperature to below the transformation point, decarburization will hardly proceed. For spring steel, the upper limit is 750°. (2) The higher the temperature increase rate in the α+γ2 phase region, the less decarburization occurs. Utilizing such characteristics, in the present invention, the heating zone is divided into two, the first heating zone and the second heating zone, and the set temperature is set to below the transformation point (for example, up to 750°C) in the first heating zone.
, in the second heating zone the austenitizing temperature (e.g. 900
By clearly distinguishing the temperatures of both heating zones and controlling the temperature, it is possible to process workpieces with a wide range of plate thicknesses or wire diameters even though the same heating furnace is used. Decarburization heating can be performed.

[0011] The workpiece introduced into the preheating zone from the lower side of the hearth through the workpiece inlet by the workpiece carry-in lifter passes through the preheating zone and the first heating zone until it reaches a certain temperature below the transformation point of the steel. heated. In this first heating zone, the furnace length or takt is set so that a workpiece having the expected maximum plate thickness or maximum wire diameter can be sufficiently heated to the above temperature.

Therefore, a workpiece with a relatively small plate thickness or wire diameter reaches the above temperature in a shorter time than a workpiece with a maximum plate thickness or maximum wire diameter, but the temperature reaches a higher temperature in the first heating zone. The temperature is maintained at the same level until the second heating zone is reached. Therefore, as stated in (1) above, there is no significant influence to the extent that decarburization occurs in the first heating zone. In other words, regardless of the thickness or wire diameter of the work, the temperature of the work remains constant until it passes through the first heating zone and just before reaching the second heating zone.

[0013] The workpiece carried into the second heating zone at the above-mentioned temperature at which decarburization does not occur is rapidly heated to a predetermined austenitizing temperature, and then taken out of the furnace by a carry-out lifter or the like. The set temperature of the second heating zone requires some adjustment depending on the thickness or wire diameter of the workpiece, but unlike conventional furnaces, there is no need to adjust the temperature of the entire furnace, but only the second heating zone. This makes it easy to handle workpieces with various dimensional specification ranges.

[0014]

[Embodiment] An embodiment of the present invention will be described below with reference to FIG. The illustrated direct-fired continuous heating furnace 10 includes, in order from the inlet side, a preheating zone 11, a first heating zone 12 with a relatively long furnace length, and a second heating zone 13 with a relatively short furnace length. ing. This heating furnace 10 is used, for example, when hot forming a workpiece (steel material) A such as a suspension spring for a vehicle into a desired shape or performing heat treatment such as quenching. The heating furnace 10 is
It includes a hearth 15 and a hearth wall 16 covering the upper part of the hearth. A workpiece introduction port 20 is opened at the left end of the hearth 15 in the figure, that is, at the entrance of the preheating zone 11 . In this work introduction port 20,
A workpiece carry-in lifter 21 used to take the workpiece A into the furnace from below the hearth 15 is provided so as to be movable up and down.

The right end of the hearth 15 in the figure, that is, the second heating zone 1
A workpiece take-out port 25 is provided at the exit portion of No. 3. A workpiece take-out lifter 26 is provided at the workpiece take-out port 25 for taking out the workpiece A from below the hearth 15 to the outside of the furnace.

Further, a work transfer mechanism 30 is provided inside the heating furnace 10. The work transfer mechanism 30 is connected to the hearth 15
The work path line extends horizontally. By this transfer mechanism 30, the workpiece A is continuously moved in the horizontal direction from the workpiece introduction port 20 to the workpiece removal port 25.

[0017] Inside the preheating zone 11, a workpiece introduction port 20 is provided.
An in-furnace exhaust gas exhaust port 35 is provided near the furnace. A fan 37 is provided in a flue 36 that communicates with the exhaust port 35, and the fan 37 sucks in the furnace exhaust gas. A variable damper 39 is provided in an exhaust passage 38 provided on the exhaust side of the fan 37. The opening degree of this damper 39 is controlled by a controller 45,
The amount of discharge inside the furnace can be adjusted. The controller 45 is an arithmetic device 4 using a microcomputer or the like.
Controlled by 6.

The set temperature of the first heating zone 12 is a temperature slightly below the transformation point of steel (for example, 750° C. or less in the case of spring steel). On the other hand, the set temperature of the second heating zone 13 is the austenitizing temperature of steel (900°C to 100°C).
(around 0°C).

The first heating zone 12 is divided into a front zone 12a located on the upstream side of transport of the workpiece A, and a rear zone 12b located on the downstream side of transport. Front zone 12a
The burner 50 provided in the rear zone 12b and the burner 51 provided in the rear zone 12b are connected to air supply pipes 52, 53 and fuel gas supply pipes 54, 55, respectively. Air supply pipes 52 and 53 are connected to an air supply source 60. The fuel gas supply pipes 54 and 55 are connected to a fuel gas supply source 61. Air supply pipes 52, 53 and fuel gas supply pipes 54, 5
5, the ratio of air to fuel is adjusted from 1.05 to 1.1 by flow rate regulating valves 65, 66, 67, and 68, respectively.
It is controlled to 0 and sent to burners 50 and 51.

An air outlet 71 is provided between the front zone 12a and the rear zone 12b of the first heating zone 12. The air outlet 71 is configured to send temperature-controlled air blown out by a fan 75 to the first heating zone 12 .

A temperature sealing portion 80 is provided between the first heating zone 12 and the second heating zone 13 in order to rapidly heat the workpiece A to a predetermined final austenitizing temperature. There is. This temperature sealing part 80 is connected to an in-furnace exhaust gas inlet 81 that sucks exhaust gas from the second heating zone 13.
In addition, a recuperator 85 is provided between the in-furnace exhaust gas inlet 81 and the fan 75.

The recuperator 85 has a radiant tube type double tube structure installed in the flue, and converts the air sent from the air supply source 86 into the high-temperature exhaust gas taken in from the furnace exhaust gas inlet 81. It can be fed into the burner 91 while being preheated by the heat. By doing this, the high-temperature waste heat of the second heating zone 13 preheats the air sent to the burner 91 of the second heating zone 13, and the air is sent to the first heating zone 12 through the fan 75 and the air outlet 71. Cool the reactor air so that the temperature of the air sent to the reactor is not too high. At the same time,
The furnace pressure in the second heating zone 13 can be maintained and waste heat can be used effectively. The amount of air supplied to the burner 91 is determined by the flow rate adjustment valve 90.
adjusted by. The amount of fuel gas supplied to the burner 91 is determined by a flow rate regulating valve 9 controlled by a calculation device 46.
Adjusted by 2.

Differential pressure detectors 95 and 96 are provided in the first heating zone 12 and the second heating zone 13, respectively. Controllers 97 and 9 are used to maintain the furnace atmosphere in the first heating zone 12.
For the furnace pressure set to 8, the furnace pressure detection ends 95a, 96a
In the first heating zone 12, a fan 37 and a second
In the heating zone 13, the rotation speed of the fan 75 is controlled by an inverter. Further, by controlling the variable damper 39 in accordance with the combustion amount of the second heating zone 13, the exhaust amount of the entire furnace is controlled.

[0024] The heating furnace 10 having the above configuration has a first heating zone 12
is set at a temperature below 750°C, while the second heating zone 13 is set at a predetermined austenitizing temperature (e.g. 900°C).
(around C). The temperature of the preheating zone 11 is set to be equal to or lower than that of the first heating zone 12.

The workpiece introduction port 20 is opened by the carry-in lifter 21.
The workpiece A carried into the preheating zone 11 is transferred to the workpiece transfer mechanism 30, and after being preheated in the preheating zone 11, the workpiece A is heated to a constant temperature below 750°C while passing through the first heating zone 12 by the workpiece transfer mechanism 30. heated to temperature. In this case, the smaller the plate thickness and wire diameter of the workpiece A, the faster the temperature reaches the above temperature, but since the first heating zone 12 is maintained at a non-decarburization temperature, decarburization is not significantly affected. The workpiece A is heated to the second heating zone 13 while being maintained at the above temperature.
sent to. In the second heating zone 13, rapid heating is performed to a predetermined austenitizing temperature. Work A is α+γ
Since it is rapidly heated in a two-phase region in a short period of time, decarburization can be made difficult to occur. After being heated to a desired austenitizing temperature in this manner, the workpiece A is taken out of the furnace from the workpiece take-out port 25 by the take-out lifter 26, and is subjected to a predetermined hot forming or quenching process.

[0026]

[Effects of the Invention] According to the present invention, the first heating zone and the second heating zone which are separated from each other via the temperature seal part,
Rapid heating of the α+γ two phase region of the workpiece is now possible. Further, since the workpieces are inserted and extracted from the bottom of the hearth, it is possible to prevent outside air from entering the furnace when inserting and extracting the workpieces, and it is also easy to control the furnace pressure. Workpieces of various plate thicknesses or wire diameters can be heated uniformly at a temperature that does not affect decarburization, and then rapidly heated to the austenitizing temperature, allowing high-quality heating regardless of the size of the workpiece. The treatments can be carried out in the same furnace.

Furthermore, by combining atmospheric heating using controlled furnace air (for example, reducing gas atmosphere or carburizing gas atmosphere) only in the second heating zone, the effect of preventing decarburization can be further enhanced. In that case, there is no need to atmosphere-heat the entire furnace as in the past, and only the second heating zone needs to be atmospheric-heated, so equipment investment and running costs can be lower than in the past.

[Brief explanation of the drawing]

FIG. 1 is a sectional view schematically showing the structure of a heating furnace according to an embodiment of the present invention.

[Explanation of symbols]

A...Work (steel material), 10...Heating furnace, 11...Pre-preparation zone, 1
2... First heating zone, 13... Second heating zone, 15... Hearth, 20
...Work introduction port, 21...Carry-in lifter, 25...Work removal port, 26...Carry-out lifter, 30...Work transfer mechanism, 50
, 51...Burner, 80...Temperature seal portion, 81...Furnace exhaust gas inlet, 91...Burner, 95, 96...Differential pressure detector.

Claims (4)

[Claims] Claims: 1. A work transfer mechanism for transferring a steel material as a work; a preheating zone for preheating the steel material; a first heating zone for heating the steel material to a constant temperature below a transformation point;
a second heating zone that is provided downstream of the workpiece conveyance side of the heating zone and rapidly heats the steel material to an austenitizing temperature;
A heating furnace comprising: a temperature seal section provided between the first heating zone and the second heating zone. 2. The temperature sealing section includes a furnace exhaust gas inlet located between the first heating zone and the second heating zone, and the furnace exhaust gas inlet allows the temperature of the second heating zone to be increased. 2. The heating furnace according to claim 1, further comprising a recuperator for discharging the furnace exhaust gas and preheating the air guided to the burner of the second heating zone by the heat of the exhaust gas sucked in from the furnace exhaust gas inlet. 3. A workpiece introduction port is opened in the hearth of the preheating zone, a workpiece removal port is opened in the hearth of the second heating zone, and a carry-in lifter is provided in the workpiece introduction port. 2. The heating furnace according to claim 1, further comprising means for controlling the furnace pressures of the first heating zone and the second heating zone. 4. After heating the steel material in the first heating zone to a predetermined temperature below the transformation point of the steel, in the second heating zone α
A method for quenching and heating steel material, characterized by rapidly heating the +γ2 phase region to a predetermined austenitizing temperature.