JP2003268453A - METHOD FOR MANUFACTURING HIGH-Si SPRING STEEL WIRE WITH REDUCED FERRITE DECARBONIZATION - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a high-Si spring steel with reduced ferrite decarbonization, from which a coiled spring for suspension superior in fatigue strength can be manufactured without needing a peeling process. <P>SOLUTION: The method for manufacturing the high-Si spring steel wire with reduced ferrite decarbonization comprises, when producing a wire rod by heating and hot-rolling a slab of the high-Si spring steel containing 1.0% Si or more by weight ratio, finishing the final rolling in a surface temperature range of 800-1,000°C, winding it into a coil shape without water cooling before winding, and cooling it at an air-cooling rate or less until the metamorphosis completes, without suddenly changing a cooling condition in a cooling step after winding. <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitable for use as a material for a suspension coil spring for an automobile or the like, and has a high and stable fatigue strength after being manufactured into a coil spring. The present invention relates to a method for producing a high Si spring steel wire rod capable of ensuring a low ferrite decarburization. 2. Description of the Related Art Suspension spring steels for automobiles are SUP6, SUP7 or SA having a high Si content among conventional spring steels.
E9254 or the like was used. This is because when used as a suspension spring, the phenomenon called "set", in which the spring height gradually decreases during use, cannot be ignored, and when this phenomenon occurs, the bumper height decreases and there is a safety problem. This is because it was revealed in the 1970s that it was better to increase the Si content in order to prevent the above. Thereafter, elements forming carbonitrides such as Nb and V are added, and the dislocation movement during use is suppressed by precipitation hardening,
Numerous new steel types have been developed, including steel that further improves sag resistance.
Although a proposal has been made and a patent has been filed, high Si spring steel is still used as a coil spring steel for suspension even after more than 20 years have passed. This is because irrespective of the addition amount of elements other than Si, it is essential to add a large amount of Si in order to maintain excellent sag resistance. [0003] However, high Si spring steels have significant disadvantages. That is, the thickness of the decarburized layer tends to be thicker than other spring steels during heating and cooling during rolling. What is particularly problematic is that ferrite decarburization having a carbon content close to 0 is more likely to be generated on the surface than spring steels such as SUP9 and SUP10 having a low Si content. If quenching and tempering the wire in the state in which this ferrite decarburization is generated as it is, the surface hardness will be greatly reduced, and naturally the fatigue characteristics will be significantly reduced. The establishment of a method was strongly required. As a countermeasure for this, a method of peeling the surface decarburized layer and removing the decarburized layer and then quenching and tempering is not inevitable. However, in addition to the reduction of the material yield,
Since an additional processing cost is required due to the addition of the peeling step, it is becoming difficult to select this method in order to meet recent severe cost requirements. In particular, in a cold-formed coil spring in which a wire is oil-tempered (quenched and tempered) and then coiled cold.
In many cases, the peeling step is omitted due to strong demand for cost reduction. In this case, since the surface state of the rolled material remains on the product as it is, it has been necessary to find a method for preventing ferrite decarburization in the as-rolled state. Conventionally proposed methods for reducing the decarburization of high-Si spring steel include reducing the decarburization by changing the chemical components to components that are difficult to decarburize and optimizing the rolling conditions such as the heating temperature. The measures can be broadly divided into two, and patent applications have been filed for each. As the invention corresponding to the former, for example, JP-A-62-274058, JP-A-7-278747 (Japanese Patent No. 3031816), and JP-A-8-176737.
No. etc. Among them, Japanese Patent Application Laid-Open No. Sho 62-274058,
The invention described in JP-A-7-278747 is intended to reduce decarburization by adding Sb or Se, which is not usually positively added to spring steel. Further, the invention described in Japanese Patent Application Laid-Open No. 8-176737 has found that the addition of S, Ni, Cu, etc., which are contained as impurities in a small amount even in ordinary spring steel, is effective in reducing decarburization. It is characterized in that an appropriate amount is positively added to reduce decarburization. On the other hand, as the invention corresponding to the latter, there are, for example, JP-A-55-100931 and JP-A-57-4783.
No. 5, JP-A-57-82428 and the like. Although these three inventions do not completely match in their contents, they have a common feature in the temperature range where two phases of α (ferrite) and γ (austenite) are most likely to decarburize. In a time as short as possible to prevent ferrite decarburization. [0008] However, the conventionally proposed invention has the following problems. First, regarding the former invention, when a new steel type is actually changed from a steel type conventionally used and applied, when quenching and tempering is performed at a temperature at which a determined use hardness is obtained,
It is necessary to confirm that many properties such as set resistance, impact value, fatigue properties and the like are not problematic, and to judge that there is no problem in all properties. In particular, when used as a suspension coil spring, those skilled in the art often cannot clearly understand how to decarburize the components (the effects of elements other than Si are not clear). For this reason, it is almost always the case that a component is designed with priority given to securing characteristics required for a coil spring, such as superior set resistance and fatigue strength, rather than whether or not a decarburized layer is easily formed. Therefore, it is general that the degree of decarburization of the steel adopted as a result is determined only by repeating the trial production test after determining the composition. In addition, measures to prevent decarburization by adding alloys have the problem of increasing the cost of the material, and it is essential that the other properties such as set resistance are not degraded at the time of component design, making component design difficult. There is. Therefore, if decarburization can be reduced by taking measures against the manufacturing conditions, there is a great merit that the decarburization can be reduced and at the same time, the component design of spring steel having excellent sag resistance can be easily designed. high. On the other hand, as for the invention corresponding to the latter, the present inventors have conducted detailed experiments and investigated the decarburization phenomenon. Although it was confirmed that it is a phase temperature region, when the cooling rate is increased as described in the publication, the temperature difference between the central portion and the surface portion of the wire becomes large as a natural result,
In the course of the cooling, a state in which the temperature is in the two-phase temperature region only in the vicinity of the surface even though the internal temperature is still in the γ region is more likely to occur than when the cooling rate is low. In such a state, the growth of proeutectoid ferrite occurs intensively on the surface, and instead the formation of a ferrite decarburized layer is promoted. I found out I couldn't do it. Further, the wire rolling mill is generally provided with a water cooling zone for adjusting the winding temperature immediately before winding into a coil shape. It can be used. Furthermore, after being wound up, forced air cooling is carried out when it is carried on the conveyor,
The cooling rate can also be increased by reducing the rate of overlap of the wire rods during conveyance after winding. However, the actual rolling speed of the wire rod having a diameter of about φ10 to 14 which is the material of the suspension coil spring is extremely high, about 20 to 80 m / sec, and the length of the water cooling zone is limited. The actual time for water cooling is limited to a very short time within several seconds. On the other hand, if the surface temperature is lowered too much, there is a possibility that winding after passing through the water-cooled zone may not be performed normally, so that there is a restriction that extremely rapid cooling cannot be performed. When water-cooled under such constraints, the wire temperature immediately after passing through the water-cooled zone decreases to the two-phase temperature of α and γ at the surface or at a position close to the surface, but the inside still has the γ phase. Temperature region is likely to remain in the temperature range, and then the surface temperature rises due to heat conduction from the higher temperature inside, resulting in a longer stagnation time in the two-phase temperature region on the surface, It has been found that ferrite decarburization increases. In addition to the forced cooling in the water-cooled zone, the cooling after winding is accelerated to shorten the stagnation time at the two-phase region temperature of the surface portion. Is likely to increase abnormally high hardness,
There is a problem that production in a later process such as drawing before oil tempering is hindered. The present invention has been made to solve the above-described problems, and has as its object the following.
It is an object of the present invention to provide a novel method of manufacturing a wire for a suspension coil spring with less ferrite decarburization on the surface. SUMMARY OF THE INVENTION The present invention relates to a method for producing a wire rod by heating and hot rolling a slab of high Si spring steel containing 1.0% or more by weight of Si. After finishing the finish rolling in the temperature range of surface temperature of 800 to 1000 ° C., winding in a coil shape without water cooling before winding, and in the cooling process after winding, air cooling or less without abruptly changing cooling conditions. A method for producing a high Si spring steel wire with less ferrite decarburization, wherein the wire is cooled at the speed of completion of the transformation. The most remarkable point of the present invention is that S
Rolling of high Si spring steel containing 1% or more of i is completed at a temperature in the γ region slightly higher than the two-phase region temperature, and is not water-cooled in the water-cooling zone before winding, but is equal to or lower than air-cooling even after winding. In addition, the cooling is performed without suddenly changing the cooling conditions in the middle, so that the surface temperature does not temporarily rise due to heat conduction from the inside. The formation of the decarburized ferrite layer during cooling in the two-phase temperature range after rolling is disclosed in Japanese Patent Application Laid-Open No. 57-47.
It is already known as described in JP-A-835. However, in the conventional invention, only cooling in this temperature region is considered at a high speed, and it is sufficient that the temperature distribution in the steel material generated in that case and the cooling method until the end of the transformation are insufficiently studied. It is a cause that the effect of reducing decarburization cannot be obtained. That is, when the temperature region in the two-phase region is forcibly cooled rapidly, a temperature distribution always occurs in the steel material. Therefore, when trying to cool down to a low temperature region completely passing through the temperature region of the two-phase region to the center of the steel material, the surface must be cooled rapidly and continuously until the surface temperature becomes considerably low. This results in a hard rolled structure, which may hinder the subsequent manufacturing process. On the other hand, when the forced cooling is stopped halfway, that is, when the forced cooling is stopped in a state where only the surface portion passes the two-phase region temperature and the center portion is still in the temperature of the γ region, the surface temperature is thereafter reduced to the central portion. The temperature rises due to heat conduction from the surface, and as a result, the residence time in the two-phase region temperature region on the surface becomes longer and the formation of a ferrite decarburized layer is promoted, so that the effect of reducing the decarburization cannot be obtained. It becomes. In actuality, as described above, since the surface temperature cannot be lowered so much that the winding after passing through the water-cooled zone is normally performed, and the passing time of the water-cooled zone, the winding can be performed normally. When the water cooling in the water cooling zone is performed under the condition that the surface temperature can be achieved, the state always corresponds to the latter. Also, even after winding, it is difficult to perform rapid quenching because it is necessary to avoid an increase in the rolling hardness. In this case, a temporary temperature rise occurs at the surface, resulting in decarburization of ferrite on the surface. Becomes thicker. Therefore, the present inventors have confirmed that the inventions proposed in the past are not sufficiently completed in this respect. Therefore, the present inventors have considered that the conventional rolling method did not sufficiently consider the temperature distribution that always occurs inside the rolled material, and that the reason why the sufficient decarburization reduction effect could not be obtained. Noting that, contrary to the conventional proposal contents, the work of quickly passing through the temperature region of the two-phase region by water cooling using a water cooling zone or the like was not performed, and at the time of passing through the water cooling zone after rolling, During coil winding and during the entire period after the completion of winding transformation, by continuously cooling at a speed equal to or less than air cooling without suddenly changing the cooling conditions,
A test was conducted to prevent as much as possible a temporary rise in temperature of the surface due to heat conduction from the center during cooling, and to cool under conditions where the temperature difference between the surface and the inside does not increase. The results were organized. As a result, it has been confirmed that ferrite decarburization can be significantly reduced, and the present invention has been completed. The reason why the effect of reducing the decarburization of ferrite on the rolled material surface according to the present invention is not clear, but is presumed to be due to the following reason. That is, including cooling in the water-cooled zone, when performing rapid cooling such as water cooling temporarily in the middle stage after rolling to the completion of transformation,
Inside the steel material, the temperature difference between the surface and the center increases, and when the surface is in the two-phase temperature region, the center still has the γ-phase temperature region as a matter of course. Therefore, A
The precipitation reaction of pro-eutectoid ferrite by the r3 transformation occurs intensively on the surface portion, and as a result, a thick ferrite layer is expected to be generated on the surface. On the other hand, when rapid cooling such as water cooling is not performed and cooling is performed at a speed equal to or less than air cooling, a slight temperature difference occurs between the surface portion and the center portion, but the difference is extremely small compared to the former case. . Therefore, the precipitation of pro-eutectoid ferrite does not occur concentrated only on the surface, and the precipitation reaction of pro-eutectoid ferrite proceeds simultaneously in the interior slightly closer to the center than the surface. Therefore, it is considered that a rolled structure in which ferrite is more uniformly precipitated in the steel material is obtained, and as a result, intensive precipitation of ferrite on the surface is reduced. Further, the present invention can be effectively applied to a spring steel containing 1% or more of Si. Specifically, SUP6,
It can be applied to standard steels such as SUP7, SUP12, and SAE9254. In addition, the present invention can be applied to steels in which a carbonitride forming element such as Nb, V, Mo, or the like is added to these steels, or an appropriate amount of Ni is added to improve the set resistance. Next, the reasons for limiting the rolling conditions of the present invention will be described. The surface temperature at the end of finish rolling is 800 ° C to 10 ° C.
The reason for setting the temperature to 00 ° C. is to obtain a fine rolled structure by performing rolling in a temperature range as low as possible within the temperature range of the γ phase and to reduce the thickness of the entire decarburized layer as much as possible. . Until now, only the reduction of ferrite decarburization has been described, but the higher the temperature, the more the carbon on the surface reacts with oxygen in the atmosphere during heating to accelerate the decarburization reaction, and the carbon concentration in the central part increases. At the position corresponding to (ie, the total decarburized layer depth).
Even if ferrite decarburization can be suppressed to a low level, excellent fatigue characteristics cannot be obtained after heat treatment in a state where the thickness of the entire decarburized layer is large. Therefore, the upper limit temperature is limited to 1000 ° C.
It is needless to say that a lower temperature is preferable.
By setting the temperature to 0 ° C. or lower, more desirably 900 ° C. or lower, the amount of decarburization can be further reduced. The reason why the lower limit temperature is set to 800 ° C. is that if the temperature is lower than 800 ° C., it becomes impossible to prevent the steel material surface from temporarily becoming lower than the Ar3 transformation temperature during rolling, and the effect of reducing the decarburization of ferrite is not sufficiently achieved. This is because they cannot be obtained. In other words, during rolling, the temperature does not decrease monotonically and sometimes rises temporarily due to the influence of processing heat. If the rolling end temperature is too low, the two-phase temperature region stays on the surface portion. This is because the time may consequently increase and ferrite decarburization may increase. Next, the reason why water cooling is not performed between the end of rolling and the winding into a coil shape is that, as described above, the time required to pass through the water cooling zone before winding is extremely short in view of the rolling speed. Inevitably, if forced cooling by water cooling here, the temperature of the surface and the part close to the surface will drop significantly compared to the center, and after passing through the water cooling zone, heat conduction from the center will cause This is because the temperature rises temporarily, and as a result, the residence time in the two-phase temperature region on the surface becomes longer, and ferrite decarburization increases. Therefore, in the present invention, the air is passed through normal air cooling or air cooling such as applying cooling air with a blower from the end of rolling to the start of winding. Next, during the period from the start of winding into the coil until the completion of the transformation, the temperature of the surface is temporarily increased by heat conduction from the hotter center as in the case before the start of winding. Cool at a rate less than air cooling to prevent as much as possible. In order to prevent the temporary temperature rise of the surface as much as possible, it can be easily achieved unless the cooling conditions are changed suddenly on the way. For example, if the conveyor speed is slowed down after winding and the coil overlap is large, and the conveyor cover is closed and the blower is not used, the cooling conditions will change before entering the conveyor and the wire surface will change. Care must be taken because the amount of heat due to heat conduction from the inside, which is higher in temperature than the amount of heat transmitted to the atmosphere, increases the possibility of temporary recuperation. Such a method is a method that is often performed to reduce the rolling hardness, but since there is a possibility that ferrite decarburization increases, it is better not to perform the method unless there is a problem with the hardness. By continuing the cooling conditions at a rate equal to or lower than air cooling until the completion of the transformation, the temperature of the steel material maintains a state where the temperature difference between the surface and the central part is small, and there is a temporary rise in temperature until the completion of the transformation. Without cooling. Note that even when water cooling is not performed in the water cooling zone, the surface temperature may rise temporarily immediately after finish rolling due to generation of processing heat during rolling. However, this is a recuperation in a temperature region completely higher than the two-phase region temperature, and there is no problem. Even if recuperation occurs, the temperature starts to decrease after a while, and if it is cooled so that recuperation does not occur,
The effect of the present invention can be sufficiently obtained. The term “cooling process after winding” in the claims is intended to exclude this temporary reheating. The condition of air cooling or lower referred to here may be any condition as long as it does not hinder the subsequent process due to burning or the like. Therefore, not only a case where cooling is performed in the atmosphere without any operation but also a method of applying a cooling air with a blower or the like to such an extent that the reheating does not occur is included. This is because the purpose of preventing decarburization of ferrite can be achieved by cooling so that reheating does not occur. By cooling under the condition that temporary reheating does not occur until the completion of transformation, precipitation of proeutectoid ferrite is prevented from concentrating on the surface part and minimizing the thickness of the ferrite decarburized layer. It can be suppressed. However, if the wire diameter becomes smaller, the cooling speed of the wire itself increases even under the same conditions, so that the wire is naturally easily burned. Therefore, when the wire diameter is small, it is necessary to adjust the cooling speed to be slow by a method such as slowing down the conveyer so that the coil overlaps tightly within a range where the reheating does not occur as described above. Next, the effects of the present invention will be clarified by examples. Table 1 shows the chemical components of the steel used in the examples. [Table 1] One of the steels shown in Table 1 was SAE92.
54 is a steel obtained by adding 0.2% of V to the steel.
Steel with improved sag resistance by adding Nb and V to
The three steels are SUP7. These three types of steel were melted in an electric furnace to produce a 2.6 ton steel ingot, and then subjected to slab rolling and rough rolling to produce a 160 mm square steel slab. This was further coarsely rolled, and a wire rod of φ13 mm was manufactured by a wire rod rolling mill. At the time of finish rolling by the wire rolling mill, as shown in Table 2, the rolling finish temperature, whether or not to perform water cooling in a water cooling zone, the speed of the conveyor, the opening and closing of the conveyor cover, etc. are changed, and the winding start temperature, Temperature change after winding,
The thickness (DM-F, DM-T) of the decarburized layer was measured. The rolling conditions during the rough rolling were exactly the same, and the finishing temperature of the rolling was adjusted by adjusting the extraction temperature from the heating furnace. The decarburized layer was measured by a measuring method using a microscope in accordance with JIS G0558. Table 2 shows the results. [Table 2] As is clear from the results in Table 2, the test No.
When cooling was performed in a water-cooled zone as in 1, 2, 5, 6, 9, and 11, ferrite decarburization exceeding 0.02 mm all occurred. In addition, even when cooling in the water-cooled zone, especially the cooling condition of the conveyor is gradually cooled (see Table 2 for details).
, Ferrite decarburization further increased as compared with the case of air cooling (for details, see Table 2). This is because the difference between the cooling conditions with the immediately preceding water cooling becomes larger when the cooling is performed gradually, and the recuperation of the surface portion after passing through the water cooling zone becomes remarkable, resulting in a longer stagnation time in the two-phase temperature region. It is presumed to be the cause. Further, the test No. When the rolling finishing temperature was high as in Nos. 1 to 4, Dm-T increased. In contrast to the comparative examples, when the cooling in the water-cooled zone was not performed as in the present invention, the thickness of the ferrite decarburized layer was reduced to 0.005 mm or less without being largely influenced by the cooling conditions of the conveyor. It was confirmed that it could be suppressed. We also experimented with three different steel types,
It was confirmed that the same effect was obtained in all cases. As described above, in the method of manufacturing a high Si spring steel wire of the present invention, a method of temporarily cooling rapidly, that is, water cooling after rolling, is not used, but at a speed lower than air cooling. In addition, by cooling until the transformation is completed without suddenly changing the cooling conditions and cooling so that the surface temperature does not recover by heat conduction from the inside, it has become possible to suppress ferrite decarburization to an extremely small thickness. Further, since ferrite decarburization can be suppressed to a small degree regardless of the cooling condition of the conveyor, the optimum cooling condition of the conveyor can be selected according to the hardenability and the wire diameter of the material to be rolled, and no hardening occurs. It is possible to obtain a great effect on the manufacturing side that the condition can be easily adjusted. Accordingly, by applying the method of the present invention to high Si spring steel, it is possible to stably produce a wire rod with less ferrite decarburization, and to provide a coil spring having excellent fatigue strength even if the peeling step is omitted. Can be manufactured.

Claims (1)

Claims 1. When a steel piece of high Si spring steel containing 1.0% or more by weight of Si is heated and hot-rolled to produce a wire, finish rolling is performed at a surface temperature. Is 800-1000 ° C
After finishing in the temperature range of above, winding into a coil without water cooling before winding, and in the cooling process after winding, cool down to the transformation completion at a speed less than air cooling without suddenly changing the cooling conditions. A method for producing a high-Si spring steel wire rod with less ferrite decarburization.