JP2003253340A - Method for producing low carbon martensitic stainless steel sheet with excellent corrosion resistance and machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a low carbon martensitic stainless steel sheet for a disc brake, which has excellent corrosion resistance of a material before quenching and excellent machinability after quenching. SOLUTION: C: 0.03 to 0.10 mass%, Si: 0.5 mass%
Hereinafter, Mn: 1.0 to 2.5 mass%, Cr: more than 10.0 to 15.0 mass%,
Ni: 1.0 mass% or less, Cu: 0.5 mass% or less, N: 0.003 or more
0.10 mass%, the remainder is hot rolled sheet annealing of a steel sheet comprising Fe and inevitable impurities, soaking temperature T (° C), soaking time t (Hr), 650 ≤ T ≤ 950, t ≧ −T / 80 + 15
The cooling rate from the soaking temperature to 500 ° C is set to 20 ° C / Hr or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low carbon martensitic stainless steel sheet having excellent corrosion resistance and machinability, and more particularly to a method for producing a material suitable for use as a disc brake of a motorcycle. .

[0002]

2. Description of the Prior Art Disc brakes for two-wheeled vehicles are different from those for four-wheeled vehicles and have a structure visible from the outside. Therefore, a stainless steel plate is used from the viewpoint of designability. This stainless steel plate for disc brakes is required to have not only corrosion resistance but also toughness and wear resistance as the most important properties. The wear resistance increases as the hardness increases, but on the other hand, the toughness decreases. for that reason,
This stainless steel plate has the balance with toughness as the most important issue, and further, the matching property with the brake pad,
Considering the weight and performance of the mounted motorcycle, the Hv300
Adjusted to a hardness range of ~ 400.

Conventionally, 13Cr-high carbon martensitic stainless steels such as SUS420Jl and SU have been used for disc brakes.
The steel plate of S420J2 was used by being hardened and tempered to be adjusted to the above hardness range. However, in this case, since the heat treatments of quenching and tempering are required twice, the manufacturing burden is large. In order to avoid such a problem, the conventional techniques are disclosed in JP-A-57-198249 and JP-A-60-10.
As disclosed in Japanese Patent No. 6951, many low carbon martensitic stainless steel sheets, which have an appropriate hardness only by quenching and do not require tempering, are used.

[0004]

By the way, the low carbon martensitic stainless steel sheet used for the disc brake is either a hot rolled steel sheet or annealed (hot rolled sheet annealed), and the surface condition remains scaled. Alternatively, the scale is removed by shot blasting, grinder or pickling before shipment. Unsurprisingly, scaled steel sheets are cheaper than descaled ones. However, the scaled steel plate has poor corrosion resistance, and therefore, there is a problem in that a coiled material or a sheet-cut material causes rust due to rain wet or dew condensation during shipping transportation or storage in a warehouse.

Further, the disc brake is manufactured by mechanically grinding and polishing the blade surface of the material steel plate. Therefore, when the rust is mild, the rust is removed by the above-mentioned processing, so that there is no problem in the characteristics of the disc brake, but the occurrence of rust itself reduces the commercial value of the material and is not preferable. However, in the case of severe rust, pitting-like pitting corrosion occurs, and if it remains after grinding and polishing, it becomes a starting point of cracking during use of the brake, which is a serious risk factor. Therefore, improving the corrosion resistance before quenching (before grinding) is important not only in terms of appearance but also in the function of the disc brake.

Further, the above-mentioned grinding / polishing process is generally
It is performed after adjusting to an appropriate hardness by heat treatment. Therefore, in order to suppress the manufacturing cost, machinability after heat treatment is also an important required characteristic. This machinability is dominated by the effect of hardness, but in the case of scaled steel plates, the tool slips on the scale attached to the surface layer, and the tool clogs on the peeled scale, resulting in machinability. There was a problem of making it worse.

An object of the present invention is to provide excellent corrosion resistance before quenching treatment, improve the machinability after quenching treatment, and reduce the load of grinding / polishing treatment. It is to propose an advantageous manufacturing method of a martensitic stainless steel sheet.

[0008]

What is important in the present invention is to improve the corrosion resistance before quenching, that is, to suppress rusting during the period from the production of the material to the quenching of the disc brake manufacturing process. Improvement of machinability after quenching treatment, that is, improvement of grinding / polishing property after quenching treatment in the disc brake manufacturing process.

The rust before quenching is due to the so-called crevice corrosion caused by the infiltration of water due to rain wetting, condensation, etc. between the scale and the base metal, and the occurrence of porous red rust further promotes this corrosion. To do. On the other hand, the machinability after the quenching treatment is significantly hindered by the slip of the tool due to scale and clogging. Therefore, from the viewpoint of corrosion resistance and machinability, it is preferable to use a material from which scale has been removed, but using a material from which scale has been removed causes an increase in material cost.

The present inventors have examined the relationship between the prior art, problems and scales because both of these problems are remarkable in steel plates with scales that are shipped without removing the scales. As a result, the above-mentioned problem that the conventional technology has, even without removing the scale, can be solved by appropriately controlling the conditions of hot-rolled sheet annealing performed after hot-rolling and improving the properties of the generated scale. Then, they have completed the present invention.

That is, according to the present invention, the corrosion resistance before quenching is improved by densifying the scale produced by hot-rolled sheet annealing to make it difficult to retain the moisture from the atmosphere, and the workability after quenching is improved. The property is based on the finding that the above-mentioned scale is improved by forming the scale thickly during annealing and facilitating peeling by rapid cooling in the subsequent quenching treatment.

The present invention developed on the basis of the above concept, C: 0.03 to 0.10 mass%, Si: 0.5 mass% or less, Mn:
1.0 to 2.5 mass%, Cr: over 10.0 to 15.0 mass%, Ni: 1.0 mass
s% or less, Cu: 0.5 mass% or less, N: 0.003 to 0.10 mass%
When the soaking temperature T (° C.) and the soaking time are t (Hr) for the annealing of the hot rolled steel sheet containing Fe and the balance being Fe and unavoidable impurities, 650 ≦ T ≦ 950, and Satisfying the condition of the equation (1), further, the cooling rate from T ℃ to 500 ℃ is 20 ℃ /
It is a method for producing a low carbon martensitic stainless steel sheet excellent in corrosion resistance and machinability, which is characterized in that it is Hr or less. t ≧ −T / a + b (1) where a and b are constants

In the above manufacturing method of the present invention,
The rate of temperature rise during annealing of the hot rolled steel sheet is preferably 20 ° C./Hr or more.

Further, in the above manufacturing method of the present invention, the constants a and b in the above formula (1) are preferably a = 80 and b = 15, and a = 50 and b = 23. More preferable.

The present invention, in addition to the above component composition,
Further, Ti: 0.01 to 0.5 mass%, V: 0.01 to 0.5 mass%,
One or more selected from Nb: 0.01 to 1.0 mass% and Zr: 0.01 to 1.0 mass%, and / or B: 0.0002
~ 0.0010mass%, Mo: 0.01-0.5mass%, Co: 0.01-0.5
It is preferable to contain one or more selected from mass%.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, as described above, hot-rolled sheet annealing is an essential step, but the present inventors investigate the relationship between this hot-rolled sheet annealing condition and corrosion resistance and machinability. I experimented. The experiment will be described in detail below. (Experiment 1) In this experiment, C: 0.05 mass%, Si: 0.3 mass
s%, Mn: 1.55mass%, Cr: 12.5mass%, N: 0.04mass
%, Cu: 0.2mass%, Ni: 0.6mass% as the basic components of the steel A was melted to form a 200mm thick slab, and after heating to 1150 ℃, hot rolling end temperature: 970 ℃, coiling temperature: 770 ℃ A hot-rolled sheet rolled to a thickness of 5 mm by hot rolling was used as a test material. This hot-rolled sheet was subjected to hot-rolled sheet annealing in which the soaking temperature was changed to 600 to 1000 ° C and the soaking time was changed in the range of 2 to 18 hours. In the above annealing, the temperature rising rate up to a predetermined soaking temperature is 35 ° C / Hr, and the cooling rate from the soaking temperature to 500 ° C or lower is 12
℃ / Hr, annealing atmosphere is hydrogen 95vol% + nitrogen 5vo
It was conducted under the conditions of 1% and dew point of -30 ° C.

The scale-peelability and machinability of the obtained hot-rolled annealed plate after the quenching treatment were investigated. The scale peeling test is performed by charging a 50 mm wide × 200 mm long test piece equipped with a thermocouple into an electric furnace set at 1200 ° C, water cooling immediately after the test piece temperature reaches 1000 ° C, and drying. Then, wire brushing was performed and the scale peeling area ratio was measured. The machinability was evaluated by the time required for polishing the surface of the test piece after the quenching treatment with a polishing machine under a constant load to completely remove the scale of the surface layer and to give a new surface.

FIG. 1 shows a soaking temperature T in hot-rolled sheet annealing.
It shows the effect of (° C.) and the soaking time t (Hr) on the scale releasability after quenching. From this figure, the inventors have found that in order to improve the machinability and reduce the load of the grinding / polishing process, the soaking temperature T (° C.) should be increased or the soaking time t (Hr ) Is required to be appropriately long, and the relationship between the peeling area ratio and the soaking temperature T (° C.) and the soaking time t (Hr) is a linear expression such as the following expression (1). We found that it can be formulated by. t ≧ −T / a + b (1) where a and b are constants. Further, the improvement of machinability is confirmed by these constants in the range of a: 30 to 100, b: 10 to 30 and further in the range of a: 30 to 60, b: 20 to 30, It has become clear that machinability can be improved.

When the above formula (1) is applied to FIG. 1, the soaking temperature T (° C.) and the soaking time t (Hr) satisfy T ≧ 650 ° C. and t ≧ −T / 80 + 15. In this case, a scale peeling area ratio of 60% or more is obtained, and further, T ≧ 650 ° C. and t ≧
-If T / 50 + 23 is satisfied, the scale peeling area ratio is 80%
As described above, more excellent peelability is obtained. Therefore, it is understood that the hot rolled sheet annealing conditions may be appropriately determined based on the above formulas in the component composition and manufacturing conditions of Experiment 1. Further, FIG. 2 shows the relationship between the scale peeling area ratio and the time required for polishing. As the scale peeling area ratio increases, the grinding processing time shortens,
It can be seen that the machinability is improved.

(Experiment 2) A steel having the composition shown in Table 1 was melted into a slab having a thickness of 200 mm, heated to 1120 ° C., and then hot rolled at a hot rolling finish temperature of 900 ° C. and a coiling temperature of 710 ° C. Was carried out to obtain a hot rolled plate having a plate thickness of 4 mm. These hot rolled sheets were subjected to hot rolled sheet annealing under various conditions shown in Table 2. Here, the soaking temperature and soaking time of the hot-rolled sheet annealing are set to the conditions that a scale peeling area ratio of 60% or more after the quenching treatment, which is clarified in Experiment 1, is obtained, and the cooling after the annealing is The cooling rate from the soaking temperature T (° C) to 500 ° C was variously changed. The atmosphere during annealing is 85 vol% hydrogen + 15 vol% nitrogen, dew point -10
℃ was made.

The hot rolled annealed sheet thus obtained was examined for corrosion resistance and scale releasability after quenching. For the corrosion resistance test, a salt spray test (5 mass% salt water, 35 ° C, sample width 70 mm × length 150 mm size) was carried out for 96 hours according to JIS Z 2371, and the area ratio of red rust after the test was completed (flow rust was also observed. (Including). The scale peelability test was performed in the same manner as in Experiment 1.

In Table 2, the measurement results of the area ratio of rust generation and the area ratio of scale peeling are shown together. The scale peeling area ratios are all good at 60% or more. In addition, FIG.
3 shows the relationship between the rust occurrence area ratio and the cooling rate in Table 2. From this figure, the rust occurrence area ratio greatly depends on the cooling rate, and when the cooling rate is gradually cooled to 20 ° C / Hr or less, the rust occurrence area ratio becomes 30% or less, indicating that the corrosion resistance is good. Recognize.

As is clear from Tables 1 and 2 and FIGS. 1 to 3 as a result of the above Experiments 1 and 2, the annealing conditions of the hot rolled sheet were as follows:
When soaking temperature T (° C) and soaking time are t (Hr), 650 ≤
T ≦ 950 and satisfying the condition of the above-mentioned formula (1), and further, setting the cooling rate from T ° C. to 500 ° C. to 20 ° C./Hr or less controls the property of the scale, and the above-mentioned problems It was found to be an important requirement for solving the problem.

[0024]

[Table 1]

[0025]

[Table 2]

Next, the component composition of the raw material on which the present invention is applied will be described. C: 0.03 to 0.10 mass% C is an element effective in increasing the hardness of martensite after quenching and improving wear resistance. C content is 0.03
If the content is less than mass%, a suitable hardness as a disc brake cannot be obtained, and if the content of C exceeds 0.10 mass%, the hardness becomes too high only by quenching (without tempering treatment). Therefore, the range of C needs to be 0.03 to 0.10 mass% in order to obtain the proper hardness of the disc brake only by quenching.

Si: 0.5 mass% or less Si is an element that stabilizes the ferrite phase, and if it is contained excessively, not only the quenching hardness is lowered but also the toughness is adversely affected, so the upper limit is 0.5 mass%. And

Mn: 1.0 to 2.5 mass% Mn is an element that suppresses the formation of a ferrite phase at high temperatures, and by containing 1.0 mass% or more, the quenching temperature dependency of hardness is reduced, and stable quenching hardness is obtained. Can be obtained. However, if added excessively, the descaling property is deteriorated and the surface properties are adversely affected, and the corrosion resistance is also adversely affected by the precipitation of MnS, so the upper limit is made 2.5 mass%.

Cr: over 10.0 to 15.0 mass% Cr must be contained in excess of 10.0 mass% in order to impart corrosion resistance. However, if Cr is contained excessively, a ferrite phase appears in the quenching temperature range, and proper hardness cannot be stably obtained, so the upper limit is set to 15.0 mass%.

Ni: 1.0 mass% or less Ni, like Mn, is an element that suppresses the formation of a ferrite phase at high temperatures, and has the effect of stabilizing the quenching hardness. In the present invention, since these effects are sufficiently obtained by the addition of Mn, it is not necessary to specify the lower limit of the Ni addition amount, and the mixing level is set in the steelmaking process. On the other hand, Ni is an expensive element, so the upper limit is 1.0 mass from the viewpoint of economy.
%.

Cu: 0.5 mass% or less Cu, like Mn, is an element that suppresses the formation of a ferrite phase at high temperatures, and has an effect of stabilizing quenching hardness.
In the present invention, since these effects are obtained by adding Mn, the lower limit of the Cu amount is not particularly specified. On the other hand, if Cu is contained excessively, surface cracking occurs during hot rolling, and surface eaves are easily generated. Since this surface eaves remains until the final product and causes a decrease in yield, and is also an expensive element, its upper limit is set to 0.5 mass%.

N: 0.003 to 0.10 mass% N, like C, is an element effective in increasing the martensite hardness after quenching and improving the wear resistance. Further, it has an effect of forming a nitride and suppressing softening of martensite. However, if the content is large, the quenching hardness becomes excessively high, and conversely, if the content is too small, the sufficient softening suppressing effect cannot be obtained. Therefore, the content range is set to 0.003 to 0.10 mass%.

The low carbon martensitic stainless steel sheet of the present invention may further contain the following elements. Ti: 0.01 to 0.5 mass%, V: 0.01 to 0.5 mass%, Nb: 0.01
~ 1.0mass%, Zr: 0.01 ~ 1.0mass%, these elements form carbonitrides, and therefore have the effect of suppressing the precipitation of Cr carbonitrides that cause the Cr deficient layer and improving the corrosion resistance. . It also has the effect of suppressing softening of martensite. However, if the addition amount is too small, these effects cannot be obtained, and conversely, even if added in excess, the effects are saturated. Therefore, these elements are Ti: 0.
01-0.5mass%, V: 0.01-0.5mass%, Nb: 0.01-1.0m
Ass% and Zr: 0.01 to 1.0 mass% can be added.

B: 0.0002 to 0.0010 mass% B enhances the hardenability and is effective in obtaining a stable hardened hardness. On the other hand, excessive B forms a compound having a low melting point with Fe and Cr, and causes hot cracking in the continuous casting and hot rolling steps. Therefore, B can be added with 0.0010 mass% as the upper limit. Further, in order to exert the effect of stabilizing the quenching hardness, B is preferably contained in an amount of 0.0002 mass% or more.

Mo: 0.01 to 0.5 mass% Mo has the effect of increasing the temper softening resistance of martensite and improving the heat resistance. On the other hand, when Mo is excessively contained, it stabilizes the ferrite phase and lowers the quenching hardness. Therefore, 0.5 mass% can be added with the upper limit. Further, in order to exert the above heat resistance improving effect, it is preferable to add Mo in an amount of 0.01 mass% or more.

Co: 0.01 to 0.5 mass% Co has the effect of suppressing the formation of a ferrite phase at high temperatures and stabilizing the hardness after quenching. It also has the effect of improving toughness. To obtain these effects, addition of 0.01 mass% or more is preferable. However, even if added in excess, the effect is saturated, so the upper limit is preferably 0.5 mass%.

Next, the most important hot rolled sheet annealing conditions in the manufacturing process of the present invention will be described. If the heating rate of the hot-rolled sheet annealing is slow, coarse carbonitrides are formed, so the carbonitrides are less likely to be a solution during the quenching treatment, which causes variations in the quenching hardness. Particularly in recent years, the quenching treatment is often performed in a short time in a short time by high-frequency heating or electric heating from the viewpoint of productivity, so that it is necessary to suppress the formation of coarse carbonitride. Therefore, the average rate of temperature rise from room temperature to soaking temperature in hot-rolled sheet annealing is preferably 20 ° C./Hr or more.

In the hot-rolled sheet annealing, in order to grow the scale produced during hot-rolling thicker and improve the machinability after quenching, the soaking temperature T (° C) and the soaking time t (Hr) Need to be properly controlled. That is, in order to improve machinability, it is effective to satisfy the condition of the following formula (1) obtained from the above experimental results. t ≧ −T / a + b (1) where a and b are constants. When the soaking temperature T (° C.) and the soaking time t (Hr) do not satisfy the above conditions, the amount of scale produced is small and the effect of improving machinability is small. But,
If hot-rolled sheet annealing is performed at high temperatures, energy costs will rise, and in addition, equipment cost will increase because annealing equipment with excellent soaking and heat insulating properties is required. Therefore, the upper limit of the annealing temperature is 950 ° C.

In the subsequent cooling after soaking and holding, if the cooling rate is too fast, the residual stress between the base metal and the scale becomes large, the adhesion of the scale deteriorates, and eventually the corrosion resistance deteriorates. Therefore, it is important that the cooling after soaking is slow cooling to relax the residual stress between the base steel and the scale and form a delicate and strong scale. In order to improve the adhesion of the scale after annealing the hot-rolled sheet and ensure the corrosion resistance, it is necessary to slow the cooling rate to 20 ° C / Hr or less.

In the annealing of the hot rolled sheet, the material to be annealed is generally annealed in a coil state with a hot rolled scale. Therefore, the atmosphere gas during annealing does not need to be strictly controlled because the circulation of the atmosphere gas is suppressed except for the outermost circumference and the innermost circumference of the coil.
% Or more, nitrogen 20 vol% or less, and dew point 0 ° C. or less.

[0041]

[Example] Steels having the chemical compositions shown in Table 3 were melted to 20
After making a steel slab with a thickness of 0 mm, it is heated to 1180 ° C and hot-rolled to a rolling finish temperature of 980 ° C and a winding temperature of 800 ° C.
A hot rolled sheet having a sheet thickness of 4.5 mm was used. For this hot rolled sheet, Table 4
The hot rolled sheet was annealed under various conditions as shown in. At this time, the atmosphere gas is hydrogen 90 vol% + nitrogen 10 vol%, dew point -5 ℃
And The obtained hot rolled annealed sheet was examined for corrosion resistance and scale releasability after quenching treatment. Corrosion resistance test is JIS
A salt spray test based on Z 2371 (5 mass% salt water, 35 ° C., sample width 70 mm × length 150 mm size) was carried out for 96 hours, and the area ratio of red rust generation (including flow rust) after the test was evaluated. In the scale peelability test, a test piece with a width of 50 mm and a length of 200 mm equipped with a thermocouple was placed in an electric furnace set at 1200 ° C, and water cooling was performed immediately after the temperature of the test piece reached 1000 ° C. Then, after drying, wire brushing was carried out to measure the area ratio of scale peeling.

[0042]

[Table 3]

In Table 4, the area ratio of rust after the corrosion resistance test of the hot rolled annealed plate and the area ratio of scale peeling after the quenching treatment are shown together. When hot-rolled sheet annealing was carried out under the conditions compatible with the present invention, the rust occurrence area ratio in the corrosion resistance test was 30% or less, and the scale peeling area ratio after quenching showed a value of 60% or more. Therefore, it can be seen that a low-carbon martensitic stainless steel plate suitable for disc brake applications has been obtained. On the other hand, when hot-rolled sheet annealing is performed under a condition outside the preferred range of the present invention, only a steel sheet having either or both of corrosion resistance after annealing and machinability after quenching is inferior is obtained. Absent.

[0044]

[Table 4]

[0045]

As described above, according to the present invention,
Low-carbon martensite for disc brakes that not only improves corrosion resistance before quenching in the manufacturing process of disc brakes, but also improves machinability after quenching and reduces the load of grinding and polishing. It becomes possible to manufacture system stainless steel sheets.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of soaking temperature and soaking time of hot-rolled sheet annealing on scale releasability.

FIG. 2 is a diagram showing a relationship between a scale peeling area ratio and a polishing treatment time.

FIG. 3 is a diagram showing a relationship between a cooling rate of hot-rolled sheet annealing and a rust generation area ratio.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Osamu Furu             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. (72) Inventor Yoshihiro Yazawa             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. F-term (reference) 4K037 EA02 EA05 EA10 EA12 EA13                       EA15 EA16 EA17 EA18 EA19                       EA20 EA27 EA28 EA31 EA32                       EA35 FF02 FF03

Claims (6)

[Claims] 1. C: 0.03 to 0.10 mass%, Si: 0.5 mass% or less, Mn: 1.0 to 2.5 mass%, Cr: more than 10.0 to 15.0 mass%,
Ni: 1.0 mass% or less, Cu: 0.5 mass% or less, N: 0.003
When the soaking temperature T (° C) and the soaking time are t (Hr), 650 ≤ T ≤ 950, with the balance being 0.10 mass% and the balance being Fe and unavoidable impurities. And below (1)
A method for producing a low-carbon martensitic stainless steel sheet excellent in corrosion resistance and machinability, characterized in that the cooling rate from T ° C to 500 ° C is 20 ° C / Hr or less, which satisfies the condition of the formula. Note t ≧ −T / a + b (1) where a and b are constants 2. The temperature rising rate during annealing of the hot rolled steel sheet is 20.
C / Hr or more, The manufacturing method of the low carbon martensitic stainless steel plate of Claim 1 characterized by the above-mentioned. 3. The method for producing a low carbon martensitic stainless steel sheet according to claim 1, wherein a = 80 and b = 15 in the above formula (1). 4. The method for producing a low carbon martensitic stainless steel sheet according to claim 1, wherein a = 50 and b = 23 in the above formula (1). 5. In addition to the above component composition, Ti: 0.01-
0.5mass%, V: 0.01 to 0.5mass%, Nb: 0.01 to 1.0mass
%, Zr: 0.01 to 1.0 mass%, or one or more selected from the group consisting of 0.01 to 1.0 mass%, the method for producing a low carbon martensitic stainless steel sheet according to any one of claims 1 to 4. 6. In addition to the above component composition, B: 0.0002
~ 0.0010mass%, Mo: 0.01-0.5mass%, Co: 0.01-0.5
The method for producing a low-carbon martensitic stainless steel sheet according to any one of claims 1 to 5, comprising one or more selected from mass%.