JP2011080105A - Method for manufacturing steel for spring - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel for a spring, which prevents super cooling and improves workability in wire drawing, while securing fatigue characteristics by suppressing ferrite decarburization in a steel material. <P>SOLUTION: The method for manufacturing the steel for the spring includes: extracting the steel material which includes 0.35-0.65% C (mass%, hereinafter the same), 1.4-3.0% Si, 0.1-1.0% Mn, 0.1-2.0% Cr, 0.025% or less P (excluding 0), 0.025% or less S (excluding 0) and the balance Fe with unavoidable impurities, from a heating furnace; then hot-rolling the steel material while setting a temperature before finish rolling at lower than 1,000°C; finish-rolling the plate; keeping the plate in the range of 1,000-1,150°C for 5 sec or shorter and winding up the plate; cooling the coil to 750°C or lower at a cooling rate of 2-8°C/s; and then slowly cooling the coil to 600°C while spending 150 sec or longer after the winding-up. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a spring steel manufacturing method suitable for industrial springs such as automobile suspension springs.

  As is well known, spring steel (rolled material), which is a material for industrial springs represented by high-strength suspension springs for automobiles, ensures the fatigue characteristics of springs, omits the peeling process after rolling and heat treatment, etc. From this point of view, it is required to suppress the decarburization (decarburization layer) of the ferrite structure of steel, and recently, the decarburization of the structure including not only the ferrite structure but also the region where the ferrite occupies a large proportion (called the total decarburization layer) ) Has also become a problem, and the fatigue properties under more severe conditions have been demanded.

  Against this background, various methods have been proposed for the suppression of this decarburized layer, but the main ones are those that control the heating of steel, the temperature of rolling and the cooling rate after rolling. It is.

  For example, in Patent Document 1, a process of heating at 1170 ° C. or higher for at least 2 minutes at the time of hot rolling and cooling a temperature range of 750 to 600 ° C. after rolling at a cooling rate of 5 to 300 ° C./min and a descaling process are performed. In addition, Patent Document 2 discloses a method in which a temperature range of A3 transformation point or more is set in all steps from the start to the end of hot rolling, and the rolling is performed at a cooling rate of 0.5 to 3.0 ° C./min after rolling. Each is shown.

  However, all of these conventional methods require heating to a high temperature and rolling at a high temperature, so that overcooling tends to occur during the cooling process after rolling, and total decarburization tends to increase.

  When overcooling occurs, the steel contains hard and brittle structures (supercooled structure) such as martensite and bainite, and this supercooled structure is the workability (drawing process) of the steel material that is performed after rolling. ). Moreover, when total decarburization becomes large, the bad influence which reduces the fatigue characteristics (atmospheric fatigue characteristics) as a steel material for springs will arise.

JP 2003-268383 A JP 2002-194432 A

  The present invention eliminates the above-mentioned conventional problems, suppresses ferrite decarburization of the steel material to ensure fatigue characteristics, and further prevents overcooling and improves workability at the time of wire drawing. Is the main solution to the problem. In addition to this, the secondary solution is to provide a spring steel manufacturing method capable of suppressing total decarburization.

  The present invention proposes the following spring steel manufacturing method as a specific means for solving this problem.

  (1) C: 0.35-0.65% (mass%, the same applies hereinafter), Si: 1.4-3.0%, Mn: 0.1-1.0%, Cr: 0.1-2.0%, P: 0.025% or less (excluding 0), S: 0.025% or less (excluding 0), steel material with the balance consisting of Fe and inevitable impurities, hot-rolled after extraction in the furnace, with a pre-finishing temperature of less than 1000 ° C, and after finish rolling, 1000-1150 ° C For springs, which is held in the range of 5 seconds or less, cooled to 750 ° C. or less at a cooling rate of 2 to 8 ° C./s, and then gradually cooled to 600 ° C. over 150 seconds after winding. Steel manufacturing method.

  (2) C: 0.35-0.65% (mass%, the same applies hereinafter), Si: 1.4-3.0%, Mn: 0.1-1.0%, Cr: 0.1-2.0%, P: 0.025% or less (excluding 0), S: 0.025% or less (excluding 0), steel material with the balance consisting of Fe and inevitable impurities, hot-rolled after extraction in the furnace, with a pre-finishing temperature of less than 1000 ° C, and after finish rolling, 1000-1150 ° C The steel for springs is characterized in that it is held in the range of 5 sec or less and wound up, then cooled to 750 ° C. or less at a cooling rate of 2 to 8 ° C./s, and then gradually cooled to 600 ° C. over 150 sec after winding. Manufacturing method.

  (3) C: 0.35-0.65% (mass%, the same applies hereinafter), Si: 1.4-3.0%, Mn: 0.1-1.0%, Cr: 0.1-2.0%, P: 0.025% or less (excluding 0), S: 0.025% or less (excluding 0), steel material with the balance consisting of Fe and inevitable impurities is heated at a furnace extraction temperature of 1000-1100 ° C and then hot rolled at a pre-finishing temperature of less than 1000 ° C. After finishing rolling, hold in the range of 1000 to 1150 ° C for 5 seconds or less, wind up, cool to 750 ° C or less at a cooling rate of 2 to 8 ° C / s, and then gradually take up to 600 ° C over 150 seconds from winding. A method for producing spring steel, characterized by cooling.

  (4) The steel material further includes one or more of Ni: 1.0% or less (excluding 0) and Cu: 1.0% (excluding 0). A method for producing spring steel.

  (5) The steel material is further Mo: 1.0% or less (excluding 0), V: 0.3% or less (not including 0), Ti: 0.1% or less (not including 0), Nb: 0.1% or less (0 5), Zr: 0.1% or less (not including 0), or one or more of the following: The method for producing spring steel according to any one of claims 1 to 4.

  According to the present invention (all claims), ferritic decarburization of a steel material can be suppressed to ensure excellent fatigue characteristics, and overcooling can be prevented to improve workability during wire drawing. Further, according to the present invention (Claims 2 and 3), not only ferrite decarburization of steel material but also total decarburization is suppressed, and more excellent fatigue characteristics are ensured. The workability of the can be improved.

  As described above, conventionally, heating and rolling were performed at a relatively high temperature in order to suppress ferrite decarburization. However, when rolled at a high temperature, the hardenability increases due to the coarsening of the γ grain size, and overcooling tends to occur in the subsequent cooling step. In addition, various alloy components are added as the strength increases. In the case of alloy elements that produce carbonitrides, the carbonitrides do not contribute to the hardenability, but they are solidified by high-temperature heating. When melted, the hardenability increases. Therefore, it is preferable that the heating temperature is not too high from the viewpoint of suppressing the coarsening of the γ grain size and suppressing the solid solution of the alloy element.

  Therefore, in the present invention, in order to suppress this overcooling, a method of controlling the heating temperature of the steel material before hot rolling to 1100 ° C. or less, and / or the temperature of the steel material before finish rolling to less than 1000 ° C. adopt.

  First, from the viewpoint of suppressing the coarsening of the γ grain size, in the present invention, the temperature before finish rolling at the time of hot rolling is controlled to less than 1000 ° C. to suppress overcooling. The temperature before the finish rolling should be low for suppressing overcooling, preferably 990 ° C. or less, more preferably 980 ° C. or less, and even more preferably 970 ° C. or less.

  Further, from the viewpoint of suppressing solid solution of carbonitride, in the present invention, the heating temperature of the steel material is controlled to 1100 ° C. or lower. This heating temperature is preferably 1080 ° C. or lower, more preferably 1060 ° C. or lower, and even more preferably 1040 ° C. or lower.

  Either one of these two means is sufficiently effective in suppressing supercooling, but the reduction in heating temperature also has the effect of suppressing total decarburization and is effective in improving the atmospheric fatigue characteristics.

  On the other hand, as in the prior art, a decrease in heating temperature and rolling temperature tends to cause ferrite decarburization. When the heating temperature is low, ferrite decarburization proceeds during heating. For this reason, in this invention, heating temperature shall be 1000 degreeC or more. Preferably it is 1020 degreeC or more, More preferably, it is 1040 degreeC or more.

  Ferrite decarburization also occurs during rolling. In the present invention, the temperature after finish rolling is increased to 1000 to 1150 ° C. and maintained for 5 seconds or less to eliminate ferrite decarburization generated during rolling. If it is less than 1000 ° C, there is no effect of eliminating the ferrite decarburization. If it exceeds 1150 ° C, the ferrite decarburization can be eliminated, but the γ grain size becomes coarse and overcooling occurs. The lower limit of the temperature is preferably 1010 ° C, more preferably 1020 ° C, and even more preferably 1030 ° C or higher. The upper limit of the temperature is preferably 1100 ° C. or lower, more preferably 1080 ° C. or lower, and even more preferably 1050 ° C. or lower. Further, in order to eliminate the ferrite decarburization, it is necessary to hold for a certain period of time in the above temperature range, but as the time becomes longer, the γ grain size becomes coarse and overcooling occurs. . It is preferably 4 seconds or less, more preferably 3 seconds or less, and even more preferably 2 seconds or less.

  Ferrite decarburization also occurs during cooling. In order to suppress ferrite decarburization during cooling, it is necessary to relatively rapidly cool the temperature range from winding to 750 ° C. In the present invention, it is 2 ° C./sec or more. If it is less than 2 ° C / sec, ferrite decarburization occurs. It is preferably 3 ° C./sec or more, more preferably 4 ° C./sec or more, and even more preferably 5 ° C./sec or more. If the cooling rate exceeds 8 ° C./sec, there is a risk of overcooling, so in the present invention, it is set to 8 ° C./sec or less. Preferably it is 7 degrees C / sec or less, More preferably, it is 6 degrees C / sec or less, More preferably, it is 5 degrees C / sec or less.

  While ferrite decarburization during cooling can be suppressed by cooling control up to 750 ° C, control of a temperature range of 750 ° C or lower is also necessary to suppress overcooling. In the present invention, the cooling time from 750 ° C. to 600 ° C. is 150 seconds or more. If it is less than 150 seconds, there is a high risk of overcooling. It is preferably 160 seconds or longer, more preferably 170 seconds or longer, and even more preferably 180 seconds or longer.

  When applied to a high-strength suspension spring, it is necessary to satisfy the final required characteristics in addition to decarburization, supercooling suppression and wire drawing processability. From this viewpoint, the steel components are limited as follows.

  That is, C: 0.35-0.65% (mass%, the same applies hereinafter), Si: 1.4-3.0%, Mn: 0.1-1.0%, Cr: 0.1-2.0%, P: 0.025% or less, S: 0.025% or less are essential In addition to this component, Ni: 1.0% or less (excluding 0), Cu: 1.0% (including 0) or more, and / or Mo: 1.0% or less (0 V: 0.3% or less (not including 0), Ti: 0.1% or less (not including 0), Nb: 0.1% or less (not including 0), Zr: 0.1% or less (not including 0) ), And the balance consists of Fe and inevitable impurities.

  Hereinafter, the reasons for limiting the steel components will be described.

C: 0.35-0.65%
C has a great influence on the strength of steel. Addition of 0.35% or more is necessary to apply to high strength suspension springs. On the other hand, the suspension spring is required to have corrosion resistance, but the corrosion resistance deteriorates as the C content is increased. In order to ensure the corrosion resistance required for high-strength suspension springs, it is necessary to make it 0.65% or less.

Si: 1.4-3.0%
Si is effective in securing the sag resistance necessary for the spring. In order to ensure the sag resistance required for high-strength suspension springs, it is necessary to add 1.4% or more. On the other hand, increasing the amount of Si suppresses cementite precipitation during tempering and increases the residual γ, but the spring characteristics deteriorate due to an increase in the residual γ. Therefore, in the present invention, the upper limit of Si content is set to 3.0%.

Mn: 0.1-1.0%
Mn is effective for fixing S, which is a toughness degrading element, and needs to be added in an amount of 0.1% or more. On the other hand, when the amount of Mn is increased, solidification segregation during casting becomes prominent, and breakage tends to occur at the segregated portion. Therefore, the upper limit is 1.0%.

Cr: 0.1-2.0%
Cr is effective in improving corrosion resistance, and an effect can be obtained by adding 0.1% or more. On the other hand, when the amount of Cr is increased, coarse Cr-based carbides are generated and the toughness is deteriorated. Therefore, the upper limit is set to 2.0%.

P: 0.025% or less (excluding 0)
The lower the P, the better because it segregates at the grain boundaries and degrades the toughness. In order to ensure the characteristics as a high-strength suspension spring, it is necessary to control to 0.025% or less.

S: 0.025% or less (excluding 0)
The lower the S, the better, since it deteriorates toughness due to grain boundary embrittlement and coarse sulfide formation. In order to ensure the characteristics as a high-strength suspension spring, it is necessary to control to 0.025% or less.

  Further, the following elements may be added as appropriate in order to further improve the characteristics of the high-strength suspension spring.

Ni: 1.0% or less (excluding 0)
Ni is an element that improves corrosion resistance. On the other hand, excessive addition increases residual γ and degrades the spring characteristics. Therefore, it is good to control to 1.0% or less.

Cu: 1.0% or less (excluding 0)
Cu is an element that improves corrosion resistance. On the other hand, excessive addition increases residual γ and degrades the spring characteristics. Therefore, it is good to control to 1.0% or less.

Mo: 1.0% or less (excluding 0)
Mo is an element effective for toughening such as securing strength and reducing the adverse effects of P segregation. However, since it is easy to solidify and segregate, if it is added excessively, breakage is likely to occur at the segregated part. Therefore, it is better to control to 1.0% or less.

V: 0.3% or less (excluding 0)
Since V forms carbonitride, a fine structure is obtained and is effective in improving toughness. However, excessive addition makes the carbonitride coarse and deteriorates toughness. Therefore, it is good to control to 0.3% or less.

Ti: 0.1% or less (excluding 0)
Since Ti forms carbonitrides, a fine structure is obtained and effective in improving toughness. However, excessive addition makes the carbonitride coarse and deteriorates toughness. Therefore, it is better to control to 0.1% or less.

Nb: 0.1% or less (excluding 0)
Since Nb forms carbonitride, a fine structure is obtained and effective in improving toughness. However, excessive addition makes the carbonitride coarse and deteriorates toughness. Therefore, it is better to control to 0.1% or less.

Zr: 0.1% (excluding 0)
Since Zr forms carbonitride, a fine structure is obtained and it is effective for improving toughness. However, excessive addition makes the carbonitride coarse and deteriorates toughness. Therefore, it is better to control to 0.1% or less.

  In order to demonstrate the excellent effect of the production method of the present invention, the three types of steel materials for high-strength suspension springs shown in Table 1 are targeted, and these are hot-rolled under the production conditions specified in the present invention and other conditions. The state of ferrite decarburization, total decarburization, and supercooling was investigated for each sample of the obtained hot-rolled wire (wire diameter: 15 mm), and their evaluation was performed. Specific methods for investigating and evaluating ferrite decarburization, total decarburization, and supercooling are as follows.

(1) Investigation and evaluation of ferrite decarburization and total decarburization After cutting a 10 mm long sample from a wire ring taken from hot-rolled wire rod by wet cutting, wet polishing, buffing, and chemical polishing are performed as sample preparation. The sample which reduced the distortion and unevenness | corrugation of grinding | polishing processing as much as possible was created. At this time, it grind | polished so that the observation surface might become the cross section of a steel wire. After corrosion with nital to reveal a metal structure, the presence or absence of ferrite decarburization was observed with an optical microscope. From this observation, the case where no ferrite decarburization was observed was evaluated as ◯, and the case where it was observed was evaluated as ×. Further, the total decarburization amount was measured, and a small amount, that is, a total decarburization depth of 0.2 mm or less was evaluated as ◯, and an amount exceeding this amount was evaluated as Δ.

(2) Investigation and evaluation of supercooled structure After cutting a 10mm long sample from a wire ring taken from a hot-rolled wire rod by wet cutting, we perform wet polishing, buff polishing, chemical polishing as sample preparation, and polishing Samples with as much distortion and irregularities as possible were prepared. At this time, it grind | polished so that the observation surface might become the cross section of a steel wire. After corroding with nital to reveal a metal structure, a supercooled structure (bainite, martensite) was observed with an optical microscope. As a result, the case where no supercooled structure was observed was evaluated as ◯, and the case where it was observed was evaluated as ×.

  These evaluation results in each example (invention example, comparative example) are shown in Table 2 together with the production conditions.

From Table 2, all of the invention examples satisfying the production conditions defined by the present invention are suppressed in ferrite decarburization and supercooling, and are effective in improving fatigue characteristics (atmospheric fatigue characteristics) and wire drawing workability. As a result, some invention examples (steel No. A1-1, A2)
Except for -1 and A3-1), almost all of them were also suppressed at the same time, and it can be seen that the fatigue characteristics were improved even more effectively.

  On the other hand, regarding the comparative example outside the conditions of the present invention, steel material No. A1-2 has high pre-finishing temperature (T2) and post-finishing holding temperature (T3), and thus overcooling occurred. Since No. A1-7 has a low holding temperature (T3) after finishing, ferrite decarburization has occurred, and No. A1-7 has a low heating temperature (T1), so ferrite decarburization has occurred. No.A2-2 has overcooling due to the short cooling time from 750 ° C to 650 ° C, and No.A2-3 has high pre-finishing temperature (T2), so overcooling occurs and winding Ferrite decarburization occurs because the cooling rate (CR1) from 750 to 750 ° C is slow, and overcooling occurs because No.A2-6 has a long holding time (Ht) at 1000 to 1050 ° C after finish rolling. No.A2-8 has a high cooling rate (CR1) from coiling to 750 ° C, so overcooling occurs, and No.A2-12 has a slow cooling rate (CR1), and ferrite decarburization occurs. I am doing. Furthermore, No.A3-1 has overcooling due to its high holding temperature after finishing.

Claims (5)

  C: 0.35-0.65% (mass%, the same applies hereinafter), Si: 1.4-3.0%, Mn: 0.1-1.0%, Cr: 0.1-2.0%, P: 0.025% or less (excluding 0), S: 0.025 % Of steel (not including 0), the balance being Fe and inevitable impurities, hot-rolled after extraction in the furnace, with a pre-finishing temperature of less than 1000 ° C, and after finish rolling, in the range of 1000-1150 ° C Production of spring steel characterized by holding for 5 seconds or less, winding to 750 ° C or less at a cooling rate of 2-8 ° C / s, and then gradually cooling to 600 ° C over 150 seconds after winding Method.   C: 0.35-0.65% (mass%, the same applies hereinafter), Si: 1.4-3.0%, Mn: 0.1-1.0%, Cr: 0.1-2.0%, P: 0.025% or less (excluding 0), S: 0.025 % Or less (excluding 0), with the balance consisting of Fe and inevitable impurities, heated at a furnace extraction temperature of 1000-1100 ° C, hot-rolled, and after finish rolling, in the range of 1000-1150 ° C The steel for springs is characterized in that it is held at 5 sec or less for winding and then cooled to 750 ° C. or less at a cooling rate of 2 to 8 ° C./s, and then gradually cooled to 600 ° C. over 150 sec after winding. Production method.   C: 0.35-0.65% (mass%, the same applies hereinafter), Si: 1.4-3.0%, Mn: 0.1-1.0%, Cr: 0.1-2.0%, P: 0.025% or less (excluding 0), S: 0.025 % Of steel (not including 0), the balance being Fe and unavoidable impurities, heated at a furnace extraction temperature of 1000-1100 ° C, hot rolled at a pre-finishing temperature of less than 1000 ° C, and finished rolling After that, hold it in the range of 1000 to 1150 ° C for 5 seconds or less, wind it down, cool it to 750 ° C or less at a cooling rate of 2 to 8 ° C / s, and then gradually cool it down to 600 ° C over 150 seconds from winding. A method for producing spring steel.   4. The spring according to claim 1, wherein the steel material further includes at least one of Ni: 1.0% or less (excluding 0) and Cu: 1.0% or less (excluding 0). Steel manufacturing method.   The steel is further Mo: 1.0% or less (excluding 0), V: 0.3% or less (not including 0), Ti: 0.1% or less (not including 0), Nb: 0.1% or less (not including 0) ), Zr: 0.1% or less (not including 0), or one or more of the following. 5. The method for producing spring steel according to claim 1.