JPH0717944B2 - Manufacturing method of bainite steel sheet with excellent spring characteristics - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a bainite steel sheet having a bainite structure and a spring limit value of 100 kg / mm ² or more and excellent spring characteristics.

(Conventional technology and its problems) Since the bainite steel sheet (band) obtained by isothermally transforming high-carbon steel has low yield strength, it has high bendability,
It has excellent drawability and bulging properties, can be formed into complicated shapes, and has excellent properties such as wear resistance, so it is widely used in a wide range of fields such as machine structural parts, office equipment parts, electrical parts or automobile parts. There is. However, its proof strength is low, so that it is not suitable for applications where spring characteristics are particularly required.

Therefore, although hardened and tempered hardened steel strips are used for such applications, they have poor yielding due to their high yield strength, and their springback during molding is large, making it difficult to obtain product shapes. It has drawbacks.

On the other hand, as a strip steel with excellent fatigue strength and small fatigue after repeated use, there is a manufacturing method utilizing a manufacturing technique of patenting → cold drawing → blueing used in the manufacture of piano wires. 57-164928.

However, in order to obtain high strength with this manufacturing method, it is necessary to work harden at a high cold working rate of 50 to 80% after patenting treatment, which results in poor formability and different mechanical properties. There is a problem that the directionality becomes large.

An object of the present invention is to provide a method for producing a steel sheet (strip) having high strength and excellent spring characteristics while ensuring good cold formability.

(Means for Solving Problems) As a result of intensive studies in view of the above-mentioned points, the inventors of the present invention have limited the steel composition and the isothermal transformation treatment condition, the cold workability and the reheating condition. We obtained the knowledge to solve the technical problems.

That is, according to the present invention, after the austenitizing treatment,
Bainite quenching in a temperature range of Ms point to 550 ° C of the component steel, a steel sheet having a bainite steel sheet with a metal structure is subjected to cold rolling at a temperature of 150 ° C or lower at a reduction rate of 5 to 50%, if necessary, Then, by reheating to 200 to 400 ° C, the spring characteristics are remarkably improved.

(Structure of the invention) The present invention is, by weight%, C: 0.50 to 1.20%, Si: less than 0.5%, Mn: 0.30 to 2.0%, and austenitizing a steel containing other unavoidable impurity elements, The present invention provides a method for manufacturing a bainite steel sheet having good spring properties, which comprises quenching bainite in a temperature range of Ms point to 550 ° C to complete transformation to form a bainite structure, and then reheating to 200 to 400 ° C.

Further, the present invention is, by weight%, C: 0.50 to 1.20%, Si: less than 0.5%, Mn: 0.30 to 2.0%, after austenitizing a steel containing other unavoidable impurity elements, the Ms point of the steel to 550 Bainite quenching in the temperature range of ℃ to complete the transformation to form a bainite structure,
Further, the present invention provides a method for producing a bainite steel sheet having good spring properties, which comprises cold rolling at a temperature of 150 ° C or lower at a rolling reduction of 5 to 50% and then reheating to 200 to 400 ° C.

In addition, the present invention is wt%, C: 0.50 to 1.20%, Si: less than 0.5%, Mn: 0.30 to 2.0%, Cr: 0.10 to 1.0%, after austenitizing a steel sheet composed of other unavoidable impurity elements, Provided is a method for producing a bainite steel sheet having good spring properties, which comprises bainite quenching in a temperature range of Ms point to 550 ° C of the steel to complete transformation to form a bainite structure, and then reheating to 200 to 400 ° C. .

Further, the present invention is wt%, C: 0.50 ~ 1.20%, Si: less than 0.5%, Mn: 0.30 ~ 2.0%, Cr: 0.1 ~ 1.0%, after austenitizing the steel sheet consisting of other unavoidable impurity elements, Bainite is quenched in the temperature range of Ms point to 550 ° C of the steel to complete the transformation to form a bainite structure, and further cold rolled at a rolling reduction of 5 to 50% at a temperature of 150 ° C or less, and then 200 to Provided is a method for producing a bainite steel sheet having good spring properties, which comprises reheating to 400 ° C.

The reasons for limiting the steel composition and manufacturing conditions in the present invention will be described below.

C: C must be added in an amount of 0.50% or more to obtain the spring limit value of bainite steel sheet of 100 kgf / mm ² or more, and if added in excess of 1.20%, the material strength by bainite treatment becomes high, and the subsequent cold working Rolling becomes difficult, so that range
It was limited to 0.50 to 1.20%.

Si: Si is added as a deoxidizer, but if 0.50% or more is added, B-based inclusions increase, so 0.50% was made less.

Mn: Mn is an element that increases hardenability, and if Mn is less than 0.30%, Mn
During bainite quenching in the temperature range from s point to 550 ° C, pearlite is generated and the spring limit value decreases. Further, if it exceeds 2.0%, if added, the bainite transformation end time is significantly increased and the manufacturability is deteriorated, so the content is limited to 0.30 to 2.0%.

Cr: Cr facilitates the bainite treatment, suppresses the precipitation of pearlite during bainite quenching, and is effective in preventing a decrease in the spring limit value. For that purpose, at least 0.1% is necessary, but 1.0% If it is added in excess of 10%, the bainite transformation end time will be prolonged, so the content was limited to 1.0% or less.

Inevitable impurities: The steel of the present invention contains P and S as inevitable impurities.

Isothermal transformation temperature: After heating the steel strip to the austenitizing temperature and immediately isothermally treating it in a salt bath furnace or lead bath, a bainite structure can be obtained, but at low temperatures below the Ms temperature of the steel Martensite is generated, and cracks may occur during the subsequent cold rolling. Also, the isothermal transformation temperature is 55
When the temperature exceeds 0 ° C, a pearlite structure is formed, and not only high strength cannot be obtained, but also good cold workability peculiar to bainite structure cannot be obtained. Furthermore, if the isothermal treatment temperature exceeds 550 ° C and becomes a pearlite structure, the degree of increase in strength after the subsequent cold rolling is low and high strength cannot be achieved. Limited to the temperature range.

Cold rolling: Cold rolling of a bainitic steel after isothermal transformation is very effective for strengthening, and the spring limit value is remarkably increased by subsequent reheating to a temperature of 200 to 400 ° C. When cold working, it is necessary to perform at least 5% rolling reduction. However, if the cold working ratio exceeds 50%, edge cracks will occur at the edge of the steel strip during cold rolling, and the rolling mill The cold rolling ratio is 5 because it brings disadvantages from the economical aspect such as an increase in load.
Limited to ~ 50%.

In addition, without cold rolling (cold rolling rate: 0%) 200 to 4
Since the spring limit value is also increased by reheating to 00 ° C., it is only necessary to select the cold rolling rate that gives the required spring limit value depending on the application.

Furthermore, if the material temperature during cold rolling exceeds 150 ° C, aging occurs during rolling, cold rollability also deteriorates, and a high spring limit value cannot be obtained during subsequent heating at 200 to 400 ° C. Therefore, the material temperature during rolling was limited so as not to exceed 150 ° C.

Reheating temperature After the isothermal transformation treatment, when the steel sheet having a bainite structure and the steel sheet subjected to cold working are reheated to a temperature range of 200 to 400 ° C, improvement in strength and spring limit value due to strain age hardening is recognized, Especially, the improvement of the spring limit value is remarkable.

However, if the heating temperature is less than 200 ° C, the effect is small,
Since the spring limit value drops sharply above 400 ℃, the heating temperature was limited to the range of 200-400 ℃.

(Specific Disclosure of the Invention) Hereinafter, the present invention will be described more specifically with reference to Examples.

Example 1 Plate thickness having the chemical composition of Table 1 produced by a conventional process
Using a 1.0 mm SK5 polished special steel strip, after austenitizing at 850 ° C for 10 minutes, it was immediately quenched in a lead bath maintained at a temperature of 500 ° C and subjected to an isothermal transformation treatment for 10 minutes to obtain a bainite steel sheet.

Then, it was reheated in a muffle furnace maintained at 300 ° C for 10 minutes for aging treatment.

The characteristic values obtained are shown in Table 2.

Compared with the conventional bainite steel sheet, the target spring limit value of 100 kgf / mm ² or more was obtained by reheating at 300 ° C.
A significant improvement in the spring limit value is recognized.

Furthermore, FIG. 1 shows the change in the spring limit value when the steel sheet having a bainite structure by the isothermal transformation treatment was reheated in the temperature range of 150 to 450 ° C. for 30 minutes and aged.

Target 100kgf in reheating temperature range of 200-400 ℃
A spring limit of over / mm ² is obtained.

Example 2 Using a SK5 polished special steel strip having a chemical composition shown in Table 3 and a thickness of 1.0 mm manufactured by a conventional process, and immediately after austenitizing at 850 ° C. for 10 minutes, a temperature of 450 to 600 ° C. Quenching was carried out in the salt bath furnace held for 1 hour, and the isothermal transformation treatment was carried out by holding for 10 minutes.

Then, the respective steel strips having different isothermal transformation temperatures were cold-rolled to 0.8 mm at a temperature of 80 ° C., and subsequently reheated in a muffle furnace kept at 300 ° C. for 10 minutes for aging treatment.

The characteristic values obtained are shown in Table 4.

When the cold rolling ratio and the reheating conditions are the same, the lower the isothermal transformation temperature, the higher the spring limit value and the tensile strength, so that good spring properties are exhibited. However, in the case of the pearlite structure with an isothermal transformation temperature of 600 ° C shown in No. 6, the spring limit value is considerably lower than that of the bainite structure.

Example 3 Using an SK5 polished special steel strip having a chemical composition of Table 3 and a thickness of 1.0 mm, after austenitizing at 850 ° C. for 10 minutes,
Immediately, it was quenched in a salt bath maintained at a temperature of 450 ° C and held for 10 minutes to carry out an isothermal transformation treatment.

Then, when cold rolling these steels, the material temperature of each pass is cooled to 80 ° C or less and rolled,
The cold-rollability was compared with the case where the sample was heated and rolled in an oil tank kept at ℃. The results are shown in FIG. 80
When cold-rolled at a temperature of ℃ or below, edge cracks occur at the edge of the steel strip at a cold rolling rate of 60%, but when heated to 160 ℃ it becomes 50
Ear cracking occurred at a cold rolling rate of%.

Further, Table 5 shows the properties of the bainite steel sheet cold-rolled by 45% at each cold rolling temperature after being reheated for 10 minutes in a muffle furnace kept at 300 ° C. Samples cold-rolled at a temperature of 160 ° C age-harden during cold-rolling and overage in subsequent reheat treatment, reducing the spring limit.

Example 4 Plate thickness with the chemical composition of Table 6 produced in a conventional process
Using a continuous processing furnace with a heating zone, a lead bath, and a cooling zone, 1.0 mm of each polished special steel strip was austenitized at 860 ° C for 7 minutes and immediately quenched into a lead bath maintained at 450 ° C. An isothermal transformation process of holding for a minute was performed, and then rapidly cooled to room temperature.

Subsequently, these steels were rolled with a rolling oil to a material temperature of 80 ° C or less in each pass, rolled at a cold rolling ratio of 20% to a plate thickness of 0.8 mm, and then kept at 300 ° C in a tunnel furnace. The sample was subjected to an aging treatment of reheating for 10 minutes.

The characteristic values obtained are shown in Table 7.

No. 9 (S35C), which has a lower C content than the method of the present invention, has a spring limit value lower than the target 100 kgf / mm ² , and Mn and Cr have higher values than those of the present method, No. 13 (Mn steel) and No. Bainite transformation of 14 (Cr steel) was not completed by the isothermal transformation treatment at 450 ° C for 5 minutes, and it became a two-phase structure including the martensite structure after cooling, and the strength became extremely high. It can be seen that cracks in the ears occur and the cold rolling property is significantly deteriorated.

Furthermore, in order to completely complete the bainite transformation of the No. 13 and No. 14 materials with high Mn and Cr contents by the isothermal transformation treatment, the treatment time was 18 minutes for No. 13 (Mn steel), and No. 14 ( For Cr steel), it has to be extended up to 30 minutes each, and manufacturability is greatly reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of heating temperature on the spring limit value. FIG. 2 is a diagram showing the effect of cold rolling temperature on the hardness after cold rolling.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 38/04 38/38

Claims (4)

[Claims] 1. A steel containing a weight ratio of C: 0.50 to 1.20%, Si: less than 0.5%, Mn: 0.30 to 2.0%, and a balance of Fe and unavoidable impurities, and then Ms of the steel. A method of manufacturing bainite steel sheets with excellent spring properties, which consists of quenching bainite in the temperature range of points to 550 ° C to complete the transformation to form a bainite structure and then reheating to 200 to 400 ° C. 2. A steel containing a weight ratio of C: 0.50 to 1.20%, Si: less than 0.5%, Mn: 0.30 to 2.0% and the balance Fe and inevitable impurities, and then Ms of the steel. Bainite is quenched in the temperature range of points to 550 ° C to complete the transformation to form a bainite structure, which is then cold-rolled at a temperature of 150 ° C or less at a reduction rate of 5 to 50%, and then re-rolled to 200 to 400 ° C. A method for manufacturing a bainite steel sheet with excellent spring properties, which consists of heating. 3. By weight%, C: 0.50 to 1.20%, Si: less than 0.5%, Mn: 0.30 to 2.0%, Cr: 0.10 to 1.0%, austenitizing steel with the balance Fe and unavoidable impurities. After that, a bainite steel sheet with excellent spring characteristics is formed by quenching bainite in the temperature range of the Ms point to 550 ° C of the steel to complete the transformation to form a bainite structure and then reheating to 200 to 400 ° C. . 4. An austenitizing process of steel containing C: 0.50 to 1.20%, Si: less than 0.5%, Mn: 0.30 to 2.0%, Cr: 0.10 to 1.0%, and the balance Fe and unavoidable impurities. After that, bainite quenching was performed in the temperature range of Ms point to 550 ° C of the steel to complete the transformation to form a bainite structure, and further cold rolling was performed at a temperature of 150 ° C or less at a reduction rate of 5 to 50%, A method for producing bainite steel sheets with excellent spring properties that consists of reheating to 200-400 ℃.