JP2001158942A - Martensitic stainless steel capable of easily softening and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce steel capable of easily executing softening and also capable of obtaining sufficient quenching hardness at the time of quenching heat treatment after the working and to provide a method for producing the same steely by inline heat treatment in a continuous hot rolling line. SOLUTION: This steel is composed of 0.10 to 0.12% C, <=0.2% Si, <=0.4% Mn, <=0.3% Ni, 11 to 13% Cr, <=0.025% N, and the balance Fe with impurities. In hot rolling, the steel is subjected to forced working at 850 to 980 deg.C within 10 sec at the reduction of area of <=65%, is thereafter cooled at a cooling rate of air cooling or more and is subjected to inline heat treatment at >=700 deg.C to complete its transformation into ferrite. In the inline heat treatment, preferably, the same is slowly cooled from the point of time in which its temperature reaches 760 to 800 deg.C to a temperature of 720 to 750 deg.C so as to arrive within 1 to 10 min and is thereafter held at 720 to 750 deg.C till the passing time from the start of the slow cooling reaches 15 to 60 min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SUS410 martensitic stainless steel which can be soft-annealed easily and can obtain good quench hardness during hardening heat treatment after processing and a method for producing the same. Things.

[0002]

2. Description of the Related Art Since stainless steel screws and pins require high strength, martensitic stainless steels such as SUS410 are often used, and are processed by cold forging using wire or steel wire as a material. Thereafter, a hardening heat treatment is performed to obtain a product. In order to facilitate the working, the wire is manufactured by performing soft annealing after hot rolling, and the steel wire is manufactured by performing soft annealing on the wire and then soft annealing again. That is, the steel
At first glance, a seemingly reciprocal characteristic is required, which is as soft as possible when shipped as a wire or a steel wire, and as hard as possible after being subjected to a hardening heat treatment after final processing.

[0003] Unlike ordinary ordinary steel, martensitic stainless steel is hardened by martensitic transformation even if the cooling process after hot rolling is not rapid cooling, so that cold working becomes difficult. Therefore, conventionally, a coil rolled by hot rolling in a continuous wire rod rolling line is subjected to Ac _{3 in a} batch type furnace.
After heating to a high temperature above the temperature and transforming to austenite, the steel is gradually cooled at a very low cooling rate to completely transform the austenitic phase (hereinafter referred to as γ phase) into ferrite phase + carbide (hereinafter referred to as α + C), ie, martensite. Soft annealing to prevent transformation was required.

[0004] This softening annealing usually requires about 10 hours, and conventionally, in-line heat treatment in a continuous wire rolling line has not been possible. For ordinary steel, austenitic stainless steel, and ferritic stainless steel, a technology has been developed that completes heat treatment in a few minutes in-line.Therefore, the problem of the heat treatment cost of martensitic stainless steel is higher than these steel types. there were.

To solve this problem, Japanese Patent Application Laid-Open No. 11-1525
No. 15 proposes a technique for realizing softening annealing of martensitic stainless steel by in-line heat treatment in a continuous wire rolling line. The content is 0.06
In hot rolling of a martensitic stainless steel wire containing 0.40% C and 10-16% Cr, 75%
After subjecting to strong processing with a surface reduction rate of 65% or more at 0 to 950 ° C within 10 seconds, forced cooling is performed, and constant temperature transformation heat treatment is performed in-line without cooling to 600 ° C or less.
The heat treatment is preferably performed at 700 to 800 ° C. for 10 to 40 minutes. With this technology, γ →
Complete transformation of α + C was completed, and the production of martensitic stainless steel wire by in-line heat treatment became possible.

[0006]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 11-15 / 1999
The technology proposed in US Pat.
It is indispensable to carry out a strong working at a surface reduction rate of 65% or more at 50 to 950 ° C. within 10 seconds. On the other hand, in a normal continuous wire rod rolling line, 1000 ° C. is used in order to lower the deformation resistance of the steel material to facilitate rolling and prevent hot working cracks.
In many cases, rolling is performed at a degree. Therefore, in the technology of the above publication, it is necessary to lower the hot rolling temperature than usual,
In addition to requiring special cooling equipment, there is a problem that the rolling load increases due to the strong working after cooling.

Therefore, the problem to be solved by the present invention is as follows:
In a SUS410 martensitic stainless steel processed into a screw, a pin, etc., it is possible to provide a component steel in which softening annealing can be easily performed and sufficient hardening hardness can be obtained at the time of hardening heat treatment after processing. Another object of the present invention is to provide a method for producing a steel wire by in-line heat treatment in a continuous hot rolling line, without requiring special cooling equipment in finish rolling.

[0008]

Means for Solving the Problems The steel of the present invention for solving the above-mentioned problems is as follows: C: 0.10 to 0.12%, Si: 0.2% or less, Mn: 0.4% by weight. %, Ni: 0.3% or less, Cr: 11 to 13%, N: 0.025% or less, and is a martensitic stainless steel easy to soften and annealing, characterized by the balance of Fe and impurities.

[0009] The method of the present invention for solving the above-mentioned problems is to provide the above-mentioned martensitic stainless steel of the present invention in hot rolling at a temperature range of 850 to 980 ° C within 10 seconds within a surface reduction rate of 65% or more. After performing strong working, cooling is performed at a cooling rate equal to or higher than air cooling, and further, inline heat treatment is performed in a temperature range of 700 ° C. or more to complete transformation to ferrite, thereby producing a martensitic stainless steel. is there. Then, in the method of the present invention, upon the in-line heat treatment, the temperature is gradually cooled from the time when the temperature reaches 760 to 800 ° C. to the temperature range of 720 to 750 ° C. within 1 minute or more and within 10 minutes. In the temperature range of ° C., it is preferable that the temperature is maintained until the elapsed time from the start of slow cooling becomes 15 to 60 minutes.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the components of the steel of the present invention will be described. The present inventors have studied by changing the components of ordinary SUS410. By reducing Mn and N compared to conventional materials, the γ → α + C transformation is promoted,
Combined with the effects of other components, the wire rod is sufficiently good in cold workability, and a wire with good hardness can be obtained even during hardening heat treatment after cold working, after strong working in hot rolling and after rolling. It was clarified that it can be manufactured by in-line heat treatment in the cooling process.

The wire made of the steel of the present invention can be produced by a conventional continuous hot rolling line having an in-line heat treatment facility. In the wire rod rolling equipment, when hot rolling the above stainless steel billet of the present invention, it is subjected to forcible cooling after being subjected to strong working with a finishing mill, and is developed in a non-concentric ring shape on a conveyor by a laying head, and transported. While in-line heat treatment is performed to form a coil using a concentrator.

The reasons for limiting the content of each element are as follows. C is an element necessary for securing quenching hardness, and requires 0.10% or more. On the other hand, if the C content increases, the tensile strength after softening annealing increases, the toughness decreases, and there is a risk of cracking during cold working. Therefore, the upper limit was set to 0.12%. Since Si is an element that greatly increases the tensile strength after α + C transformation by solid solution strengthening, its content must be reduced as much as possible, and the upper limit is made 0.2%.

Mn is an element that greatly delays the transformation of γ → α + C. If the Mn content exceeds 0.4%, promotion of the softening heat treatment, which is the object of the present invention, cannot be performed.
%. Cr is a basic element for imparting corrosion resistance as stainless steel, and the lower limit is set to 11% in order to obtain this effect. On the other hand, Cr is a ferrite-forming element and generates delta ferrite during rolling to deteriorate hot workability and hardenability. Therefore, the upper limit is set to 13%.

Ni, like Mn, is an element that greatly delays the γ → α + C transformation, so its content must be reduced as much as possible, and the upper limit is made 0.3%. N is an element that improves the hardenability to some extent, but is an element that delays the γ → α + C transformation. Therefore, the upper limit is 0.02
5%.

[0015] The martensitic stainless steel of the present invention having the above-mentioned composition system can not only complete softening annealing by in-line heat treatment described below, but also uses a batch annealing furnace as in the case of softening annealing a drawn steel wire. Also in this case, there is an advantage that soft annealing can be easily performed. The effect is
Although not so large that batch annealing can be omitted during hot rolling, the cooling rate can be increased to about several times in slow cooling after austenite transformation, and the time required for heat treatment can be reduced.

Next, the method of the present invention will be described. JP 11
As described in JP-A-152515, the method of the present invention realizes in-line heat treatment by accumulating strain by performing strong working in finish rolling and promoting isothermal transformation of γ → α + C. On the other hand, the present invention relates to a method disclosed in
The finish rolling temperature is increased so that the transformation can be completed in a short time even when milder strain is applied than in JP-A-2515.

In hot rolling, it is only necessary to perform a strong working at a temperature reduction of 65% or more within 10 seconds in a temperature range of 850 to 980 ° C., and it is not necessary to add a special cooling facility. Further, there is no problem that the rolling load is particularly increased. The strong working is performed by a finish rolling mill, but the rolling temperature at which the strong working is performed is particularly higher than 940 ° C. to 980 ° C. depending on the condition of the rolling mill and the rolled material.
950 ° C. to 980 ° C., more preferably. After rolling, sufficient strain is accumulated in the rolled material by performing forced cooling at a cooling rate higher than air cooling in the cooling trough on the exit side of the finishing mill, and γ → α + C transformation is promoted by the subsequent in-line heat treatment. .

After the finish rolling, the steel sheet is forcibly cooled and then developed by a laying head in a non-concentric ring shape on a conveyor, passed through a holding furnace or a heating furnace while being conveyed, and subjected to in-line heat treatment. In the in-line heat treatment, the γ → α + C transformation is completed in a temperature range of 700 ° C. or more, and the formation of a martensite phase is prevented. Thereafter, it may be dropped into a concentrator to form a coil, and then cooled to room temperature by cooling or water cooling.

In the in-line heat treatment in the method of the present invention, 720 hours after reaching a temperature range of 760 to 800 ° C.
After slowly cooling to reach a temperature range of ７750 ° C. within 1 minute or more and within 10 minutes, the temperature is maintained in a temperature range of 720 ° C. to 750 ° C. until the time elapsed from the start of slow cooling becomes 15 to 60 minutes. Is preferred. These preferred conditions are effective for completing the γ → α + C transformation of the steel of the present invention in a short time and sufficiently softening the structure after the transformation.

The present inventors have conducted intensive studies on the effect of the retention temperature on the γ → α + C transformation, and have reached the following conclusion. That is, in the SUS410 system, γ → α + C
At the time of transformation, carbide is first precipitated from the γ phase, and the carbide becomes a transformation nucleus, and α + C transformation starts to reach complete transformation. This transformation is completed in the shortest time if it is maintained at a transformation nose temperature, that is, a temperature slightly higher than 700 ° C. On the other hand, the subcooling degree increases as the retention temperature is lowered and approaches the nose temperature, so that many transformation nuclei are generated, and the transformed structure becomes fine grains, and the transformed α + C structure becomes relatively hard. . Due to the pinning effect of the precipitated carbide, this fine grain structure does not grow and remains fine unless the carbide is re-dissolved in the γ phase by a high-temperature heat treatment at _one or more Ac points.

As a measure for solving this trade-off, the present invention preferably employs a two-step heat treatment pattern in which, at the start of the stabilization, the stabilization is performed in the α + C transformation region as high as possible, and thereafter, the stabilization is performed in the transformation nose region. They revealed. By maintaining the temperature in a high temperature range of 760 to 800 ° C. at the start of the retention, the number of precipitated transformation nuclei can be suppressed and a coarse α-structure can be obtained. On the other hand, after that, 720-750 ° C
By maintaining the temperature in the range of, the temperature is maintained in the transformation nose region, and the γ → α + C transformation is promoted.

Hereinafter, the reasons for setting the preferable heat treatment conditions will be described. To achieve the two-step heat treatment described above, the heat treatment start temperature must be in the α + C transformation region,
The temperature was set to 00 ° C. or less. On the other hand, on the low temperature side, the temperature was set to 760 ° C. or higher because a large number of precipitation transformation nuclei were generated to form a fine grain structure. Next, since the heat treatment end temperature must be in the vicinity of the transformation nose, the steel of the present invention was hot-rolled at the above-mentioned finishing temperature, and was set to 720 to 750 ° C. before and after the transformation nose when the strain was accumulated.

The time from the high-temperature region to the low-temperature region is set to 1 minute or more because the time until the transformation nucleus precipitates must be in the high-temperature region. If the time exceeds 10 minutes, the transformation completion time will be further delayed. After reaching the low temperature range,
In the temperature range of 750 ° C, the elapsed time from the start of slow cooling is 15
By holding for up to 60 minutes, the soft annealing can be completed in a time at a level that can be maintained on a normal conveyor without providing special equipment.

In this process, the wire material which is developed in a non-concentric ring shape on the conveyor and is being conveyed is charged into a holding furnace or a heating furnace at a temperature range of 760 to 800 ° C., and is heated at 720 to 750 ° C.
, And gradually heat-treated at the final temperature for the above-mentioned time. When the martensitic stainless steel wire obtained by the method of the present invention is processed into products such as screws and pins by cold forging,
It has good workability equal to or higher than that of a conventional batch-annealed material, and can obtain sufficiently good hardness in a hardening treatment after working.

[0025]

EXAMPLES A martensitic stainless steel billet having the components shown in Table 1 was hot-rolled in a continuous wire rod rolling line.
A wire was manufactured by performing in-line heat treatment. In the finishing mill, strong processing was carried out, the concentric ring was developed on the conveyor, and during transportation, the product was charged into a holding furnace and subjected to in-line heat treatment, and dropped into a concentrator to form a coil product. Table 2 shows finish rolling conditions and in-line heat treatment conditions. Table 1
The symbol * in the figure is outside the range of the components of the steel of the present invention. The asterisks in Table 2 deviate from the conditions of the method of the present invention, and the # marks deviate from the preferable conditions in the method of the present invention.

After hot rolling, the bundle was immediately converged, and batch annealing was performed in a coil shape. Table 3 shows finish rolling conditions and batch heat treatment conditions. Table 4 shows the results of the material survey of the obtained wires.
Shown in Quenching was performed by quenching after holding at 1050 ° C. for 10 minutes in a vacuum.

In Table 4, the present invention example No. 1 to No. No. 4 is an example in which the inventive steels A to D in Table 1 were treated under the preferred conditions a of the inventive method shown in Table 2, and all were good in both cold workability and hardenability. Comparative Example No. 5-No. 11
Is a comparison of the comparative steels E to K in Table 1 with the present invention conditions a in Table 2.
This is an example in which processing is performed. No. Sample No. 5 was poor in quenching hardness due to insufficient C content. No. In No. 6, the C content was excessive, so that the toughness was poor and the tensile strength after the heat treatment was too high, and there was a problem in cold workability. No. 7 has an excessive amount of Si,
The tensile strength was too high and there was a problem in cold workability.

No. 8 is γ → α + C due to an excessive amount of Mn.
The transformation was not completed and there was a problem in cold workability. No. 9 is C
The hardness after quenching was poor due to the excessive amount of r. No.
No. 10 has an excessive amount of Ni. In No. 11, since the N content was excessive, the transformation of γ → α + C was not completed in any case, and the tensile strength was too high to cause a problem in cold workability. Conventional example No. 1
2 is an example in which the conventional steel L in Table 1 was treated under the conditions a of the present invention in Table 2 as well. Since the amounts of Si and Mn are excessive,
The transformation of γ → α + C was not completed, and the tensile strength was too high to cause a problem in cold workability.

Next, for steel C of the present invention shown in Table 1, the results obtained when the hot rolling conditions and the in-line heat treatment conditions were changed are shown in Table 4.
No. 13-No. 24. Conditions are b to n in Table 2.
It is. No. of the present invention example. 13 and No. 14 is N
o. As in the case of No. 3, it was an example of treatment under the preferred conditions of the method of the present invention in Table 2. In each case, softening annealing was completed in 20 minutes, and both cold workability and hardenability were good. No. of the comparative example. In No. 15, the finish rolling temperature was too high. In No. 16, the area reduction rate was too low. No. 17 takes too long to roll,
In each case, the accumulation of strain was insufficient, so that the transformation was not completed even after the holding heat treatment for 20 minutes, and the tensile strength was too high.

In the example of the present invention, Sample No. 18 was within the conditions of the method of the present invention, but deviated from the preferable conditions and the temperature reached was high. Therefore, although transformation was completed in 20 minutes, softening was insufficient and a slightly hard structure was obtained. However, depending on the application, it can be sufficiently cold worked. No. of the comparative example. No. 19 is No. 1
Since the finish rolling temperature was higher than 8 and the strain accumulation was slightly insufficient, the transformation was not completed in 20 minutes even if the finish rolling was performed at 970 ° C., and a longer heat treatment time was required. No. 20
The cooling start temperature was too high, and the transformation was not completed in 20 minutes.

In Example No. of the present invention, 21 was within the conditions of the method of the present invention, but deviated from the preferred conditions and the slow cooling start temperature was low. Therefore, the transformation was completed early, but the α structure after the transformation became a fine grained and slightly hard structure. However, depending on the application, it can be sufficiently cold worked. No. of the comparative example. In No. 22, the ultimate temperature was too low. In No. 23, the heat treatment time was too short. 2
Sample No. 4 was subjected to a heat treatment at a temperature of less than 700 ° C., which is outside the conditions of the method of the present invention.

Next, the result in the case of batch annealing is expressed as N
o. 25 to 27. No. 25 shows the steel C of the present invention in Table 1.
Is an example of processing under the p conditions in Table 3, and the slow cooling rate is 180 ° C.
At / hr, the transformation was completed and softened. On the other hand, Table 3
No. processed under the condition r of No. 26 is a slow cooling rate of 180 ° C. /
The transformation was not completed in hr. Also, N processed under the condition s
o. No. 27 required a slow cooling rate of 90 ° C./hr to complete the transformation.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

As described above, the martensitic stainless steel of the present invention can more efficiently perform softening annealing for obtaining workability in processing products such as screws and pins by cold forging or the like than conventional materials. On the other hand, good hardness can be obtained even in a hardening treatment after processing. Further, according to the method of the present invention, in a continuous wire rod rolling line, a strong rolling is performed without providing a special cooling facility in a finishing rolling mill to accumulate strain in the rolled material, and γ → α +
By completing the C transformation, a wire having good workability equivalent to or higher than that of the conventional off-line heat-treated material is obtained. After the obtained wire is processed into screws or pins, it is hardened by hardening heat treatment such as quenching. Good hardness is obtained.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Yoshitaka Nakamura 3434 Shimada, Hikari-shi, Nippon Steel Corporation Inside the Hikari Works (72) Inventor Akiyoshi Yoshizawa 3434 Shimada, Hikari-shi, Hikari-shi Nippon Steel Corporation F term in the Hikari Works (reference) 4K032 BA02 CB02 CD05 CF02

Claims (3)

[Claims] 1. In weight%, C: 0.10 to 0.12%, Si: 0.2% or less, Mn: 0.4% or less, Ni: 0.3% or less, Cr: 11 to 13 %, N: 0.025% or less, martensitic stainless steel easy to soften and annealing, characterized by being composed of the balance of Fe and impurities. 2. After subjecting the martensitic stainless steel according to claim 1 to hot rolling at a temperature range of 850 to 980 ° C. within 10 seconds and subjecting it to strong working with a surface reduction rate of 65% or more, the steel is cooled by air or more. Cool at the cooling rate, and then 700 ° C
A method for producing a martensitic stainless steel, comprising performing inline heat treatment in the above temperature range to complete transformation to ferrite. 3. The method according to claim 2, wherein said in-line heat treatment is performed for at least one minute from a time when a temperature range of 760 to 800 ° C. is reached to a temperature range of 720 to 750 ° C.
After slowly cooling to reach 0 minutes or less, 720-750 ° C
A method for producing martensitic stainless steel, wherein the temperature is maintained until the elapsed time from the start of slow cooling becomes 15 to 60 minutes in the above temperature range.