JPH0637691B2 - High carbon stainless steel having high strength and high ductility and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention has high strength and high ductility, and high strength is obtained by the formation of work-induced martensite after work forming, and hardening by quenching is also possible. TECHNICAL FIELD The present invention relates to high carbon stainless steel and a method for manufacturing the same. In particular, large press-formed products such as bathtubs and kitchen sinks, members such as various bolts that undergo heavy working such as cold and warm forged products, and even high-strength materials that can be used as materials for wafer slicers It relates to carbon Cr-based stainless steel plates and rods.

(B) Conventional technology Conventionally, high-carbon martensitic stainless steel such as SUS 440 steel has been generally used as stainless steel having high hardness and excellent wear resistance. Conventionally, this steel has been subjected to spheroidizing annealing for the purpose of softening and processed into a ferrite + Cr carbide structure (Stainless Steel Handbook,
Supervised by Masayoshi Hasegawa, published in 1973, P. 679-68
0). However, since the contents of C and Cr are high and the amount of precipitation of Cr carbides is large, even if spheroidizing annealing is performed, at most 20-
Only about 30% elongation can be obtained. Therefore, it has only been applied to relatively simple shaped members such as various blades, gauges, bearings, cams, bushes and rollers that have been conventionally formed by punching, drawing or cutting (stainless steel handbook, edited by Masayoshi Hasegawa). , 1973, p. 380). Moreover, since high-carbon stainless steel easily produces coarse carbides having a grain size of more than 10 μm, there is a high risk of fracture during the drawing and drawing of these carbides. Therefore, the area reduction rate in one wire drawing is low, and in order to obtain a high area reduction, it is the current situation to perform intermediate annealing several times during the wire drawing. Further, due to the above reasons, cold and warm workability is remarkably inferior, and forging and processing of headers are usually carried out hot (Handbook of Stainless Steel, supervised by Masayoshi Hasegawa, published in 1973, P.376).

On the other hand, austenitic stainless steel typified by SUS 304 steel has been conventionally used as a stainless steel having relatively high strength and high ductility and workability. However, although it has a high ductility of elongation of 40 to 60%, the tensile strength remains at most 60 to 70 kg / mm ² , which is not a sufficiently satisfactory level as a strength member. Therefore, as a conventional method for increasing the strength, a method of performing cold working such as temper rolling on metastable austenitic stainless steel to perform work hardening and generate work-induced martensite (for example, iron and steel 67 (1981) ) S619), a low temperature (for example, 200 to 550) at which the martensite phase does not disappear after temper rolling.
A method of strengthening the martensite phase by performing an aging treatment at (° C.) (for example, Journal of Japan Institute of Metals 21 (1957) P.583) is known. Ultra thin plates for wafer liners are manufactured using this technique. However, the strength obtained by these methods is not at a sufficiently satisfactory level.

Therefore, precipitation hardening type stainless steel was developed in which precipitation hardening elements such as Ti, Al and Cu were added singly or in combination to give strength by precipitation hardening (Stainless Steel Handbook, edited by Masayoshi Hasegawa, published in 1973, P.483-502).
Some of these have a relatively high ductility with an elongation of 20 to 40% and a tensile strength of 150 to 200 kg / mm ² , but the hardness and wear resistance after hardening are comparable to those of martensitic stainless steel. And is inferior.

(C) Problems to be Solved by the Invention Although many steels satisfying any one of high corrosion resistance, high strength, high ductility, high hardness and wear resistance are found as described above, they satisfy all of them. Conventionally, there was no steel.
The present invention has high corrosion resistance, high strength, high ductility and further high strength can be obtained by forming work-induced martensite after forming, and a steel capable of high hardness and high wear resistance by quenching treatment and The manufacturing method is disclosed.

(D) Means for Solving the Problem and Its Action Since it is essential to increase the C content in order to improve the hardness of the steel material, the workability of the high carbon content martensitic stainless steel was improved. As described above, the high carbon content martensitic stainless steel is usually processed in the state of the ferrite structure, but the present invention has been accomplished aiming at the processing in the state of the austenite structure.

Conventionally, a large amount of austenite stabilizing elements Ni and Mn have been considered as a means of forming an austenite structure at room temperature, but since the Ms point is always below room temperature, hardening by quenching treatment is impossible and subzero The strength of the martensite phase generated by the treatment or the transformation induced by the transformation was low and was not at a sufficiently satisfactory level. Therefore,
The present inventors considered the use of C for generating a metastable austenite phase at normal temperature, strengthening the work-induced martensite phase, and imparting quench hardening ability. As a result, the inventors invented a stainless steel in which a larger amount of C was solid-solved than before, and which was austenitized at room temperature.

In the steel of the present invention, Cr is 10.0 to 20.0% by weight and C is 0.5.
It is a steel containing ~ 1.5% by weight and containing 80% or more austenite phase. This steel is 0.2 after heating above 1150 ℃
Obtained by cooling at ° C / s. Cr amount is 16.0
Steel containing 18.0 wt% and 0.60 to 1.20 wt% C has been conventionally standardized as SUS 440 steel, and is a Kochi component system. However, the conventionally known steel has a ferrite + Cr carbide structure or a martensite structure at room temperature, and has a structure distinctly different from the steel of the present invention. Conventionally known steel is aimed at quenching hardening and is not only aimed to reduce the austenite phase with low hardness as much as possible, but it is necessary to suppress coarsening of austenite grains in the quenching process in order to secure toughness after the quenching process. I have to. Therefore, it is essential that the quenching treatment is performed at a heating temperature of 1100 ° C. or less, and the amount of austenite after quenching is also 20% or less.

The steel of the present invention and its manufacturing method are metallurgical material designs based on the knowledge that high ductility can be obtained when a large amount of austenite is obtained and strong working is possible, and the components themselves include conventionally known steels. However, the austenite structure that was conventionally repelled was 80% or more, and it was repelled conventionally.
There is novelty in performing solution treatment at 1150 ° C or higher.
Moreover, since a high ductility that is not comparable to that of the conventional steel of the same composition is obtained, the inventive step is sufficiently recognized.

The point of the steel of the present invention is that it is completely different from the amount of retained austenite that has been conventionally recognized, and that substantially the entire amount is made into an austenite phase. Further, the point of the method of the present invention is to perform solution treatment at a temperature much higher than the temperature used in the conventional quenching treatment.

FIG. 1 shows the relationship between the amount of austenite and elongation in a martensitic stainless steel having a Cr content of 17% by weight and a C content of 0.7% by weight. From this result, it was revealed that the ductility was drastically improved when the austenite phase was 80% or more. It is considered that the mechanism for improving the ductility is due to the martensitic transformation-induced plasticity generated by applying the deformation to the austenite phase.

The amount of austenite at room temperature is closely related to the heating temperature and cooling rate in solution treatment. Figure 2 shows
It is a figure showing the influence of the heating temperature and the cooling rate of solution treatment on the amount of austenite of the martensitic stainless steel with a Cr content of 17% by weight and a C content of 0.7% by weight at room temperature. 0.2 ℃ / s by heating above 1150 ℃
It was clarified that the amount of austenite at room temperature was 80% or more when cooled at the above cooling rate. If the heating temperature is lower than 1150 ° C, the Ms point becomes high because the amount of solid solution C is low, and martensitic transformation occurs in the cooling process, and the amount of austenite at room temperature becomes less than 80%. Also, the heating temperature is 11
If the cooling rate is less than 0.2 ° C / s even at 50 ° C or higher,
Once solid-soluted C precipitates as carbides and the amount of solid-solved C decreases, the Ms point similarly rises, and the amount of austenite becomes less than 80% due to martensitic transformation.

FIG. 3 shows a work strengthening curve in the case of cold rolling a high carbon content martensitic stainless steel containing 17% by weight of Cr, 0.7% by weight of C and 90% of a metastable austenite phase. With a cold rolling rate of 5% or more, it has a hardness of at least the minimum level required for a material for a wafer slicer. Moreover, it has been revealed that the cold rolling ratio is 25% or more and the strength is higher than that which can be achieved by the conventional technique.

From the above findings, the ductility of martensitic stainless steel was significantly improved, and it became possible to manufacture a stainless steel having high strength and high ductility and capable of being quenched.

Next, the reasons for limitation of the present invention will be described.

The lower limit of the C content is 0.5% by weight, which is the minimum necessary for lowering the Ms point by high temperature solution treatment and increasing the amount of normal temperature austenite to 80% or more. However, if it exceeds 1.5% by weight, ductility is remarkably reduced due to the formation of huge carbides and an increase in the amount of carbide grain boundary precipitation, so the upper limit was made 1.5% by weight.

The Cr content has a lower limit of 10.0% by weight, which is the minimum necessary for obtaining the corrosion resistance required for stainless steel. But,
If it exceeds 20.0% by weight, large carbides and intergranular carbides are likely to be formed and ductility is remarkably reduced.
The upper limit was 0% by weight.

The solution heating temperature of the present invention or the coiling temperature after hot rolling is set to 11
The reason why the temperature is 50 ° C. or higher is that it is the minimum temperature required to form a solid solution in the steel in the amount of C necessary for lowering the Ms point and making the amount of normal temperature austenite 80% or more. However, if the temperature exceeds the solidus temperature, it will be melted, so the upper limit was made the solidus temperature.

Furthermore, the cooling rate after solution heat treatment or hot rolling is 0.2
If the cooling rate is lower than this, the C dissolved in the steel during the cooling is re-precipitated as Cr carbide and the amount of the C dissolved decreases, so that the Ms point becomes high and the martensitic transformation occurs. This is because the resulting ductility is significantly reduced.

The cold rolling rate of the temper rolling is set to 5% or more because the metastable austenite phase is transformed into the process-induced martensite by the temper rolling and the tensile strength required as a material for wafer slicer is 130 kg / mm ² or more. This is because it is the minimum cold rolling rate required for.

(E) Example Steel having the components shown in Table 1 was melted in a 150 kg vacuum melting furnace, then hot rolled and spheroidized, and then the plate thickness was 1.5 mm.
The cold-rolled steel sheet was heat-treated under various conditions shown in Table 2. JIS 13 B test pieces were collected from these materials,
Similarly, a tensile test was conducted at the tensile temperature shown in Table 2 to examine the tensile strength and the total elongation.

As a result, as shown in Table 2, Sample N of the present invention
It is apparent that those of o.1 to 9 have a very good material with a tensile strength of 90 kg / mm ² or more and a total elongation of 20 to 100%. On the other hand, Sample Nos. 10 to 14 have a ferrite + Cr carbide structure because of the conventional spheroidizing heat treatment, and have low tensile strength and total elongation. Sample No. 15 has a low heating temperature, and Samples Nos. 17 to 18 have a low C content, so that the Ms point is high and martensitic transformation occurs during cooling and the total elongation is extremely low. Further, Sample No. 16 has a ferrite + Cr carbide structure due to a slow cooling rate, and has low tensile strength and total elongation.

Next, steel with the components shown in Table 1 was melted and hot-rolled to a diameter of 6 mm.
After finishing the wire coil, wind it under the conditions shown in Table 3,
Heating or cooling was performed. Further, after winding under the conventional hot rolling conditions, subjected to a spherical processable treatment at 750 ° C. for 5 hours, a solution treatment was performed under the conditions shown in Table 3. Tensile test pieces were sampled from these materials, and a tensile test and a compression test were also performed at the processing temperatures shown in Table 3 to examine the total elongation and the cold forgeability. The evaluation of the cold forgeability was determined by the presence or absence of cracks when a compression test was conducted on a φ5 mm × 7.5 mm test piece with an upsetting ratio of 50%.
The mark indicates that cracking occurred.

As a result, as shown in Table 3, the sample Nos. 1 to 14 which are the method of the present invention have a very good ductility with a total elongation of 30 to 95% and a good forgeability although the fluctuations are observed depending on the processing temperature. It is clear that it is superior to. On the other hand, sample N
Since Nos. 15 to 17 and No. 19 have a low solution temperature, they have a martensite structure and have extremely low elongation.
In addition, since the sample No. 18 has a slow cooling rate after winding, it has a ferrite + Cr carbide structure during cooling, so the elongation is 2
It is as low as 0 to 25% and inferior in forgeability. Sample Nos. 20 to 23 produced by the conventional method have a ferrite + Cr carbide structure and have low elongation and poor forgeability.

Further, the hot rolled coil of steel having the components shown in Table 1 was subjected to spheroidizing annealing and pickling, and then cold rolled annealing was repeated to perform solution treatment under the heat treatment conditions shown in Table 4 and temper rolling. gave. A JIS 13 B test piece was sampled from this material, and the tensile strength was examined.

As a result, as shown in Table 4, it is clear that all of Sample Nos. 1 to 11 produced by the method of the present invention can obtain the tensile strength of 130 kg / mm ² or more required for the wafer slicer material. . On the other hand, Sample Nos. 12 to 15 having a low solution heat treatment temperature have a martensite structure after the solution heat treatment, and therefore cracking occurs at a cold rolling ratio of the temper rolling of 5% or less and cold rolling becomes impossible. Sample N with slow cooling rate after solution treatment
In Nos. 16 to 19, austenite was transformed into ferrite during cooling, so that the tensile strength after temper rolling was low.

From the above examples, it is clear that the present invention works extremely effectively in close association of the components and heat treatment conditions and has an excellent strength-ductility balance.

(G) Effect of the invention As is clear from the above examples, according to the present invention, the tensile strength is high, 90 kg / mm ² or more, and the ductility is high. It is possible to manufacture stainless steel which has high strength and can be hardened by quenching. With this technology, conventional martensite stainless steel, which is mainly used for small parts, can be used for large-scale press forming such as bathtubs and kitchen sinks, as well as for cold and warm forgings such as various bolts. It has become possible to apply it to applied parts.

Further, according to the present invention, Cr, which was conventionally considered impossible,
It has become possible to apply it as a raw material for wafer slicers of stainless steel. In particular, by performing temper rolling at a cold rolling rate of 25% or more, the maximum tensile strength of the past was 200 kg.
It has become possible to manufacture ultra-thin stainless steel for high-strength wafer slicers exceeding / mm ² . As a result, it is possible to cut a wafer of Si or GaAs ingot with high precision, which makes it possible to improve the cutting yield and to manufacture an extremely thin wafer, which greatly contributes to the industry.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship of elongation of austenite amount in a martensitic stainless steel having a Cr content of 17% by weight and a C content of 0.7% by weight. FIG. 2 is a diagram showing the relationship between the austenite amount and the heating temperature and cooling rate of the solution treatment in the martensitic stainless steel having a Cr content of 17% by weight and a C content of 0.7% by weight. . FIG. 3 is a diagram showing a work strengthening curve by cold rolling of martensitic stainless steel having a metastable austenite phase.

Claims (6)

[Claims] 1. Cr: 10.0-20.0% by weight, C: 0.
A high carbon stainless steel having high strength and high ductility, which comprises 5 to 1.5% by weight and has an austenite phase of 80% or more at room temperature. 2. Cr: 10.0 to 20.0% by weight, C: 0.
After high-carbon stainless steel containing 5 to 1.5% by weight is solution heated to above 1150 ℃ and below the solidus temperature, cooling rate is 0.2 ℃
/ S or higher cooling method for producing high-carbon stainless steel having high strength and high ductility. 3. Cr: 10.0-20.0% by weight, C: 0.
High strength characterized in that after high-carbon stainless steel containing 5 to 1.5 wt% is hot-rolled, it is cooled at a coiling cooling rate of 0.2 ° C / s or more at 1150 ° C or more and solidus temperature or less. A method for producing high carbon stainless steel having high ductility. 4. Cr: 10.0-20.0% by weight, C: 0.
A high-carbon stainless steel containing 5 to 1.5% by weight is hot-rolled, then solution heat-treated at a temperature of 1150 ° C or higher and a solidus temperature or lower, and cooled at a cooling rate of 0.2 ° C / s or more. A method for producing high carbon stainless steel having high strength and high ductility. 5. Cr: 10.0-20.0% by weight, C: 0.
Production of high carbon stainless steel with high strength and high ductility, characterized by performing temper rolling with a cold rolling ratio of 5% or more on a steel sheet containing 5 to 1.5% by weight and having an austenite phase of 80% or more at room temperature. Method. 6. Cr: 10.0-20.0% by weight, C: 0.
High carbon stainless steel sheet containing 5 to 1.5% by weight at 1150 ° C
Solution heating below solidus temperature, cooling rate 0.2 ℃ /
A method for producing a high-carbon stainless steel having high strength and high ductility, which comprises performing temper rolling at a cold rolling rate of 5% or more after cooling at s or more.