JPH0314898B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has high strength, high toughness, and high fatigue strength, and is particularly suitable for use as a suspension spring material for vehicles such as automobiles. The present invention relates to a method for producing high-strength, high-toughness spring steel. In recent years, in order to improve the driving performance and fuel efficiency of automobiles, efforts have been made to reduce the weight of automobiles, and suspension springs, which are component parts of automobiles, are also required to be reduced in weight. It is a common idea to meet such demands by increasing the stress of spring steels, and one of the ways to do this is to develop spring steels with improved fatigue resistance. Traditionally, spring steels with excellent resistance to fatigue include:
JIS SUP6 and SUP7, which has a higher Si content, have been used because Si contained in steel is an effective element for resistance to fatigue. Recently, the development of spring steels that are even more resistant to fatigue than SUP7 has progressed, and in some cases SUP6, SUP7
Steel to which Nb, V, etc. are added is used. In order to meet these demands, the present inventors have carried out extensive research and have detailed the manufacturing method with the aim of obtaining a high-strength, high-toughness spring steel with significantly improved fatigue strength as well as fatigue resistance. As a result of our research, we found that by containing a specific amount of C, Si, Mn, and Cr in combination, adding one or more of Al, V, and Nb, and further adding B depending on the purpose of use. In order to further improve the fatigue strength, [S] and [O] contents are regulated, and specific rolling conditions are selected for steel with such specific components to improve the crystallization of the steel. It has been found that the above objective can be achieved by setting the particle size to No. 9. That is, the method for manufacturing a high-strength, high-toughness spring steel according to claim 1 of the present invention includes C: 0.40 to 0.75% and Si: 1.0 to 2.5% by weight.
%, Mn: 0.5-1.0%, Cr: 0.1-1.0%, and
Al: 0.01~0.1%, V: 0.03~0.3%, Nb: 0.01~
Steel containing 0.3% of one or more of the above, with the balance consisting of Fe and impurities is heated to 930-980°C and hot-rolled, and the finish rolling temperature is reduced to 900°C or less and the rolling is performed in the finish rolling. By setting the ratio to 5% or more and the cooling rate to the Ar ₁ transformation point after rolling to 30°C/min or more, a high-strength and high-toughness spring steel with a grain size of No. 9 or more can be obtained. The method for producing high-strength and high-toughness spring steel according to the present invention, which is characterized by the following claim 2, is characterized in that, in weight percent, C: 0.40 to 0.75%, Si: 1.0 to
2.5%, Mn: 0.5~1.0%, Cr: 0.1~1.0%, and
Al: 0.01~0.1%, V: 0.03~0.3%, Nb: 0.01~
Contains one or more of 0.3% of
Steel consisting of Fe and impurities is heated to 930-980°C and hot rolled, the finish rolling temperature is set to 900°C or less, the rolling reduction in finish rolling is set to 5% or more, and the temperature is reduced to Ar ₁ transformation point after rolling. cooling rate of 30℃/
min or more to obtain a high-strength, high-toughness spring steel having a crystal grain size of No. 9 or more, and furthermore, the high The manufacturing method for high-strength/high-toughness spring steel is C: 0.40 to 0.75 in weight%.
%, Si: 1.0~2.5%, Mn: 0.5~1.0%, Cr: 0.1~
1.0%, and Al: 0.01-0.1%, V: 0.03-0.3%,
Nb: Contains 0.01 to 0.3% of one or more types,
Further, the steel containing 0.0005 to 0.01% of B, with the remainder Fe and impurities, is heated to 930 to 980°C and hot rolled, and the finish rolling temperature is set to 900°C or less, and the rolling reduction in the finish rolling is reduced to 5. % or more and after rolling
By setting the cooling rate to the Ar ₁ transformation point at 30°C/min or more, the crystal grain size is 9 or more.
It is characterized by having a configuration for obtaining a high-toughness steel for springs, and furthermore, the method for producing high-strength and high-toughness steel for springs according to claim 4 of the present invention has the following features: C: 0.40-0.75%, Si:
1.0~2.5%, Mn: 0.5~1.0%, Cr: 0.1~1.0%,
and Al: 0.01-0.1%, V: 0.03-0.3%, Nb:
Contains one or more of 0.01 to 0.3%, further contains B: 0.0005 to 0.01%, [S]≦0.010%,
[O] is regulated to 0.0015%, and the steel consisting of the balance Fe and impurities is heated to 930 to 980°C and hot rolled, the finish rolling temperature is set to 900°C or less, and the rolling reduction in the finish rolling is set to 5%. % or more and the cooling rate to the Ar ₁ transformation point after rolling is 30°C/min or more to obtain a high-strength, high-toughness spring steel with a grain size of No. 9 or more. . Next, the reason for limiting the range of steel components (weight %) and rolling conditions for the steel applied in the method for manufacturing high-strength, high-toughness spring steel according to the present invention will be explained. C (carbon): C is an effective element for increasing the strength of steel, but if it is less than 0.40%, it will not be possible to obtain the strength necessary for a spring, and if it exceeds 0.75%, steel-like cementite will be produced. Since the fatigue strength of the spring is impaired, it is set in the range of 0.40 to 0.75%. Si (silicon): Si is an effective element for improving the strength of steel and the fatigue resistance of springs, but if it is less than 1.0%, it may not be possible to obtain the fatigue resistance required for springs. If it exceeds 2.5%, the toughness will deteriorate, so 1.0
The range was set at ~2.5%. Mn (manganese): Mn is an element that is effective in deoxidizing steel as well as preventing damage caused by S. For this purpose, it is necessary to contain 0.5% or more, but 1.0
If it exceeds 0.5%, the hardenability becomes excessive, degrading the toughness and easily causing deformation during quenching. Cr (chromium): Cr is an element effective in preventing decarburization and graphitization of high carbon steel, but if it is less than 0.1%, these effects cannot be fully expected;
Since toughness deteriorates when the content exceeds 0.1% to 1.0%. Al (aluminum), V (vanadium), Nb (niobium); Al, V, and Nb have a large grain refinement effect during low-temperature rolling, and can improve spring characteristics and reliability. , V, and Nb also contribute to precipitation hardening during quenching and tempering. Regarding Al, if it is less than 0.01%, the effect of grain refinement will be small, and if it exceeds 0.1%, it will cause scratches, so it was set in the range of 0.01 to 0.1%. Regarding V, if it is less than 0.03%, the effects of grain refinement and precipitation hardening described above cannot be expected;
If it exceeds, it becomes difficult to handle in steel manufacturing.
The range was 0.03% to 0.3%. Furthermore, Nb (Nb+Ta
If it is less than 0.01%, the effect of crystal grain refinement and precipitation hardening cannot be expected, and the effect of suppressing crystal grain coarsening during quenching heating cannot be sufficiently obtained, and if it exceeds 0.3%, Carbide (NbC) is formed in the form of stringers during lumping, and this does not become a solution during normal blooming and is difficult to dissolve during subsequent heat treatment, reducing the spring properties of the product, so the range is 0.01 to 0.3%. And so. B (Boron): B is an effective element for increasing the hardenability of steel, and is added depending on the purpose of use to obtain the necessary spring characteristics, but if it is less than 0.0005%, it will not have the above effects. is not obtained, and the above-mentioned effects do not increase even if the content exceeds 0.01%, so the content was set in the range of 0.0005 to 0.01%. [S] (sulfur): S is an element that impairs the fatigue strength of springs, and the lower the S content, the more reliable the spring will be, so its upper limit is regulated depending on the purpose of use, etc. It's good. If it is 0.010% or less, there will be almost no harm caused by S, so it was set at 0.010% or less. [O] (Oxygen): O generates oxide-based inclusions, which tend to become the starting point of fatigue failure, so it is best to regulate its content depending on the purpose of use, etc. In this case,
If it is 0.0015% or less, it is unlikely to become a starting point for fatigue fracture, so it is set to 0.0015% or less. Rolling conditions: In order to further improve the fatigue resistance and fatigue strength of springs made of steel with such component contents, by selecting specific rolling conditions, the grain size can be increased to No. 9 or higher. shall be. here,
The reason why the grain size was made to be No. 9 or higher by selecting specific rolling conditions was because if the grain size was less than No. 9, sufficient fatigue resistance and fatigue strength could not be obtained. This is because even if the number is 9 or higher, if the number is 9 or higher depending on the specific rolling conditions shown below, the sag resistance and fatigue strength are further improved. The rolling conditions selected in the present invention include heating temperatures of 930 to 980 for the specific steel shown above.
℃, the finish rolling temperature is 900℃ or less, the reduction rate in finish rolling is 5% or more, and the cooling rate after rolling to the Ar ₁ transformation point is 30℃/min or more. Heating temperature: 930 to 980°C Conventionally, in high-Si steel with a Si content of 1.0% or more, the heating temperature is set to 1000°C or more. This is because high-Si steel is inherently prone to decarburization in the low-temperature heating region (the α region is wide), and the rate of scale (oxide) formation exceeds the decarburization rate, making it more likely that decarburization will occur (α + γ). This is to prevent a large decarburized layer from being formed as a result of slowly cooling the region, which would greatly reduce the fatigue resistance of the spring. Furthermore, even for steels containing Nb or V, heating temperatures of 1000°C or higher have traditionally been adopted due to concerns about decarburization. This is also
This is because Nb and V, which have been dissolved in solid solution due to high-temperature heating, precipitate as NbC, VC, or a composite compound of these during the rolling process, and this precipitation effect can improve the resistance to settling to a certain extent. There is. In contrast to such conventional techniques, in the present invention, the precipitated material in rolled materials such as billets can be
We found that NbC and VC act as resistance to decarburization when heating the material. It was also found that such carbides contribute to preventing coarsening of crystal grains. Therefore, in the present invention, it is possible to lower the heating temperature than the conventional heating temperature, and if the heating temperature is lower than 930°C, the fact that the spring steel is high carbon steel also affects the heating temperature. This is because the load during rolling increases, which may reduce rolling efficiency or breakage of the rolls. If the temperature is higher than 980°C, some Nb and V carbides will melt into the matrix and cause problems during billet heating. This is because the initial crystal grains may become coarse. Finish rolling temperature: 900℃ or less, Reduction rate in finish rolling: 5% or more The rolling temperature and reduction rate in the final rolling roll are set to effectively suppress recrystallization and growth of recrystallized grains after hot rolling. In order to achieve this, it is effective to set the finish rolling temperature to 900° C. or less and the rolling reduction in finish rolling to 5% or more, so these finish rolling conditions were set. Cooling rate to Ar ₁ transformation point after rolling: 30℃/min
The cooling rate to the Ar ₁ transformation point after rolling should be set to suppress recrystallization after rolling, because if it is too slow, it will not be possible to suppress recrystallization, and a decarburized layer will remain that is harmful to use. In order to prevent decarburization, it is necessary to set the temperature at 30°C/min or higher. Next, examples of the present invention will be described together with comparative examples. Steel with the chemical composition shown in the table below was melted, cast, and bloomed to form billets. Next, rolling was started with the billet heating temperature set to the value shown in the same table, and rolling was carried out using the final rolling roll at the finish rolling temperature and rolling pressure shown in the same table. After finishing rolling, the billet was cooled to the Ar ₁ transformation point shown in the table. A steel wire for springs was manufactured by cooling at a high speed. Thereafter, the grain size of each spring steel wire was measured according to JIS regulations, and the same results shown in the table were obtained. Next, the static fatigue and fatigue strength of each spring steel wire were measured. At this time, the static settiness is determined by setting a coil spring with a spring constant of 3 that has been tempered so that the stress at sett is 180Kgf/mm ² under a load of 120Kgf, and pressurizing it with a load of 110Kgf for 150 hours. Measured by static settling (residual shear strain). The results are also shown in the table. On the other hand, the fatigue strength was measured using an Ono rotary bending fatigue tester after tempering each spring steel wire to have a hardness of H _R C45 to 48. The results are also shown in the table.

【table】

[Table] As is clear from the results shown in the table, all samples Nos. 1 to 7, which did not satisfy the rolling conditions specified in the present invention and had a grain size of 9 or less, had large residual shear strains. , it can be seen that the fatigue strength is low. On the other hand, samples No.8 to
32, it can be seen that the residual shear strain is small and the fatigue strength is high. And especially [S],
Sample Nos. 15 to 20 and Sample No. 2 of the present invention whose [O] content was regulated to 0.010% or less and 0.0015% or less, respectively.
Those with numbers 27 to 32 had even higher fatigue strength. Furthermore, when B was added, the fatigue strength could be increased, and in particular, the performance and reliability of springs with large wire diameters could be improved. The depth of the decarburized layer was approximately 0.05 mm in all cases. On the other hand, the attached drawings show changes in fatigue strength due to changes in grain size for Samples Nos. 1 and 7, and show that by heat treatment alone, the grain size can only be adjusted to 10 at most, and fatigue The strength is also low. On the other hand, when controlled rolling was carried out, it was possible to adjust the grain size to as low as 12, and it was also possible to considerably increase the fatigue strength. It was also confirmed that even if the grain size is the same, there is a considerable difference in fatigue strength between controlled rolling and non-controlled rolling. As explained above, in the method for producing high-strength and high-toughness spring steel according to the present invention, C, Si,
Contains a specific amount of Mn and Cr in combination, and
One or more of Al, V, and Nb are added, and B is further added depending on the purpose of use, and [S] and [O] are added according to the purpose of use to further improve fatigue strength.
By regulating the content and further selecting specific rolling conditions for steel with such specific components, the grain size of the steel was made to be No. 9 or higher.
A high-strength, high-toughness spring steel with significantly improved fatigue strength as well as fatigue resistance is obtained, and has the excellent effect of reducing weight when used as a suspension spring material for vehicles.

[Brief explanation of the drawing]

The attached drawing is a graph showing an example of the results of examining changes in fatigue strength due to changes in grain size, together with the presence or absence of controlled rolling.

Claims (1)

[Claims] 1% by weight, C: 0.40-0.75%, Si: 1.0-2.5
%, Mn: 0.5-1.0%, Cr: 0.1-1.0%, and
Al: 0.01~0.1%, V: 0.03~0.3%, Nb: 0.01~
Steel containing 0.3% of one or more of the above, with the balance consisting of Fe and impurities is heated to 930-980°C and hot-rolled, and the finish rolling temperature is reduced to 900°C or less and the rolling is performed in the finish rolling. It is characterized by obtaining a high-strength, high-toughness spring steel with a grain size of No. 9 or more by setting the rolling ratio to 5% or more and cooling rate to the Ar ₁ transformation point after rolling to 30°C/min or more. A method for manufacturing high-strength, high-toughness spring steel. 2 Weight%: C: 0.40-0.75%, Si: 1.0-2.5
%, Mn: 0.5-1.0%, Cr: 0.1-1.0%, and
Al: 0.01~0.1%, V: 0.03~0.3%, Nb: 0.01~
Contains one or more of 0.3% of
Steel consisting of Fe and impurities is heated to 930-980°C and hot rolled, the finish rolling temperature is set to 900°C or less, the rolling reduction in finish rolling is set to 5% or more, and the temperature is reduced to Ar ₁ transformation point after rolling. cooling rate of 30℃/
A method for producing a high-strength, high-toughness spring steel, characterized in that a high-strength, high-toughness spring steel having a crystal grain size of No. 9 or higher is obtained by adjusting the grain size to be at least min. 3 In weight%, C: 0.40-0.75%, Si: 1.0-2.5
%, Mn: 0.5-1.0%, Cr: 0.1-1.0%, and
Al: 0.01~0.1%, V: 0.03~0.3%, Nb: 0.01~
A steel containing 0.3% of one or two or more kinds, further containing 0.0005 to 0.01% of B, and the balance consisting of Fe and impurities is heated to 930 to 980°C and hot rolled, and the finish rolling temperature is set to 900°C. ℃ or less, the rolling reduction in finish rolling is 5% or more, and the cooling rate to the Ar ₁ transformation point after rolling is 30℃/min or more, resulting in high strength and toughness with a grain size of No. 9 or more. A method for producing high-strength and high-toughness spring steel, characterized by obtaining spring steel. 4 In weight%, C: 0.40-0.75%, Si: 1.0-2.5
%, Mn: 0.5-1.0%, Cr: 0.1-1.0%, and
Al: 0.01~0.1%, V: 0.03~0.3%, Nb: 0.01~
Contains 0.3% of one or more types, further contains B: 0.0005 to 0.01%, [S]≦0.010%, [O]
≦0.0015%, and the remaining Fe and impurity steel is heated to 930 to 980°C and hot rolled, the finish rolling temperature is 900°C or less, and the rolling reduction in finish rolling is 5% or more. A high-strength, high-toughness spring characterized by obtaining a high-strength, high-toughness spring steel having a grain size of No. 9 or higher by cooling the cooling rate to the Ar ₁ transformation point after rolling at 30°C/min or higher. Manufacturing method for industrial steel.