JPH0751737B2 - Cr-based stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a Cr-based stainless steel used for welding twice or more at the stage of forming processing, and has strength and toughness even if it is welded twice or more. And a Cr-based stainless steel that does not deteriorate in corrosion resistance.

FIG. 2 is a diagram showing an example of a bicycle rim forming process.
(A) The figure shows the coiled steel sheet 1 which is rewound and supplied in the direction of the arrow for forming. This steel sheet is bent at its left and right ends as shown in FIG. Next, the rim material is cut and formed into a ring shape as shown in (c), and the cut surfaces 3 and 3'are butt-welded to each other. Where seam welded part 2-1
(Dotted line) is the part welded once, but the seam welded part 2-2 (shaded line) is to be welded twice, so
In the seam weld 2-2 and its vicinity, strength, toughness,
Corrosion resistance will be impaired.

[Prior Art] Generally, when Cr-based stainless steel is welded, the strength, toughness, and corrosion resistance of the welded portion and the heat-affected zone are likely to be impaired. As a technique for improving the weldability of Cr-based stainless steel, JP-A-51-13463 is disclosed for martensitic stainless steel, and JP-B-55-47102 is disclosed for ferritic stainless steel.
JP-B No. 55-47103 and JP-A No. 60-23856 are known.

However, each of these steels is an improvement of the material of Cr-based stainless steel for applications where welding is performed once and is not planned to be welded more than once. We do not guarantee the material suitable for Cr-based stainless steel used for welding.

[Problems to be Solved by the Invention] For example, the seam welded portion 2-1 (dotted line) in FIG. 2 rarely rusts during use, but the seam welded portion 2-2 (shaded line) is welded twice. ) Has the problem that it often rusts during use.

The present invention solves such a conventional problem, and the first purpose thereof is little deterioration of the material even if welding is performed twice or more.
A second object of the present invention is to provide a Cr-based stainless steel, and since a high-strength martensitic stainless steel is desirable as a bicycle rim material in order to reduce the weight of the bicycle, it is welded more than once. The purpose of the present invention is to provide a high-strength martensite-based bicycle rim material that does not deteriorate in quality. Furthermore, a third object of the present invention is to use a soft Cr-based bicycle rim material for performing a large forming process as a bicycle wheel material. Since stainless steel is desired, it is an object of the present invention to provide a soft Cr-based stainless steel for a bicycle wheel that does not deteriorate in material even if it is welded more than once.

[Means for Solving Problems] In the present invention, (1) by weight, C <0.035%, N <0.02%, Si: 0.1 to 0.5%, Mn: 0.1 to 0.8%, Cr: 11 to 17%, Ni: 0.8-3.0% Al: 0.005-0.03%, Nb: 0.05-0.35%, Cu: 0.05-0.8% and C + N <0.05%, Nb / (C + N): 2.5-7 Cr-based stainless steel with a CRE value of 5 to 20 and a balance of Fe and inevitable impurities, which is used for welding more than once.

CRE = 2Cr + 3Si + 5Al-80 (C + N) -6Ni-4Mn-5Cu + 5.6Nb, and (2) the component described in (1) above, which has a proof stress of 60 kgf /
It is a Cr-based stainless steel with a size of ² mm ² or more and is used for welding twice or more to form a bicycle rim. (3) The composition according to (1) above, with a yield strength of 30 kgf /
It is a Cr-based stainless steel with a width of ² mm or more and an elongation of 25% or more that is welded twice or more to form a bicycle wheel.

For example, when the steel of the composition of the present invention having a plate thickness of 0.6 mm is heated to 950 to 1000 ° C. and air-cooled, a Cr-based stainless steel with a tough and massive martensitic structure is obtained, but this steel plate has a yield strength of 60 kgf / High strength steel plate with mm ² or more and elongation of about 10
% Cr is suitable for use as a bicycle rim because it is
It becomes system stainless steel.

Further, for example, steel of the composition of the present invention having a plate thickness of 9 mm is 670 to 780.
When annealed at a temperature of ℃ and air-cooled, a Cr-based stainless steel sheet with a structure mainly composed of ferrite can be obtained, but this steel sheet has a yield strength of 30 kgf / mm ² or more and an elongation of 25% or more and is soft and has good workability. Therefore, it becomes a Cr-based stainless steel plate suitable for use as an automobile wheel.

[Operation] (1) When Cr-based stainless steel is welded, weld metal and Ac
_The heat-affected zone heated to _one or more points becomes the γ phase, and martensite and ferrite are generated from the γ phase during cooling.
When ferrite is generated, the solid solution limit of C and N is small, so excess C and N are discharged.
Since C and N combine with Cr and mainly precipitate at the grain boundaries, the Cr concentration near the grain boundaries decreases and the corrosion resistance deteriorates. on the other hand,
When martensite is generated from the γ phase, the supersaturation degree of C and N is large in martensite, and thus it is difficult for surplus C and N precipitates to form, and therefore corrosion resistance in the vicinity of grain boundaries is less likely to deteriorate. Therefore, in the present invention, the composition of the steel is selected within a range where martensite is easily generated after welding. Also,
When the C and N of the steel are lowered, it is more difficult to precipitate the surplus C and N when martensite is formed, and therefore the corrosion resistance is less deteriorated. Therefore, the steel composition keeps C and N in a low range. However, at this time, since C and N are elements that promote the formation of martensite, the content of martensite-forming elements commensurate with forming a martensite phase in excess of the necessary amount in addition to lowering C and N It is necessary to add, and in the present invention, the main component is Ni, and it is possible to partially replace it with Mn or Cu in the case of supplementing or additionally improving other performance.

(2) Since the heat-affected zone that was in the temperature range of Ac ₁ or less during welding does not become the γ phase, the steel with a ferritic structure remains ferrite before welding, and the steel with a martensitic structure before welding. Is tempered from martensite to ferrite. (However, at this time, the lower limit temperature of the tempering range is determined by the decomposition rate of martensite,
Although it depends on welding conditions, plate thickness, composition, etc., it is approximately 500 ° C. ) At this time, C and N become excessive,
In the present invention, Nb is contained in the steel in a weight ratio of 2.5 to 7 times that of (C + N), so that virtually no carbides or nitrides of Cr are newly formed at the grain boundaries, and therefore the heat effect The corrosion resistance of the part does not deteriorate. Also, C is usually Nb
When N and N are fixed to improve the intergranular corrosion resistance, the necessary Nb amount is set to be higher than the stoichiometric ratio. However, in the present invention, as described above, it is not higher than the stoichiometric ratio. A small amount of addition is sufficient. In addition, unlike Ti, Nb is unlikely to produce harmful oxides during welding, so a more sound weld is obtained.

When the steel of the present invention having a ferritic structure is welded more than once, martensite is generated in the weld metal part and a part of the heat-affected zone in the first welding, but good corrosion resistance is maintained by the action (1). In addition, the heat-affected zone where martensite is not generated does not deteriorate in corrosion resistance due to the action (2). In the second and subsequent welding, a new martensite part and a part where the martensite is tempered are newly generated, but the former part is the action (1), and the latter part is the action (2). Corrosion resistance does not deteriorate. Further, when the steel of the composition of the present invention having a martensitic structure is welded twice or more, martensite is tempered in a part of the heat-affected zone in the first welding,
Corrosion resistance does not deteriorate due to the action (2).
In the second and subsequent welding, a part where martensite is newly tempered and a part where martensite is converted again are generated, but the former part is due to the action (2) and the latter part is due to the action (1). Corrosion resistance does not deteriorate. The reasons for limiting the components of the steel of the present invention will be described below.

Both C and N significantly deteriorate the toughness, workability, and intergranular corrosion resistance of the weld. In the present invention, a martensite phase is formed to suppress this harmful effect, and further deterioration of corrosion resistance and toughness due to decomposition of the martensite phase due to thermal influence is suppressed by addition of a trace amount of Nb. It is a thing. Therefore, from this meaning, C is 0.035%, N
0.02% is the upper limit, and the total amount of C and N must be 0.05% or less.

Si is a decarburizing element, and in the present invention, it is necessary to add 0.1% or more from the viewpoint of soundness of the welded portion, conversely, addition of a large amount significantly reduces toughness and workability, It has the effect of suppressing the formation of martensite. Therefore, in the present invention, it should be 0.5% or less.

Mn is a deoxidizing element like Si and in the present invention, 0.1% or more is contained. Further, Mn is an element that promotes the formation of martensite like Ni, and although part of the action of Ni can be supplemented by Mn, in the present invention because the addition of a large amount of Mn reduces hot workability. Has an upper limit of 0.8%.

Cr is a basic element indispensable for ensuring the corrosion resistance of stainless steel, and in the case of the present invention, it requires 11% or more. However, when Cr is contained in a large amount, the formation amount of the martensite phase gradually decreases with the addition amount, and in the present invention,
If it exceeds 17%, it becomes difficult to form sufficient martensite even if other elements are adjusted. Therefore, 17
% Is the upper limit.

Ni is an important additive element in the present invention, and is essential for forming martensite, further improving the toughness of the welded portion, increasing the strength of the material, and also improving the corrosion resistance. In the present invention, at least 0.8% is required to form an austenite phase (which becomes a martensite phase after cooling) in which C and N should be concentrated at high temperatures. The higher the amount, the higher the weld toughness and corrosion resistance. However, when the content exceeds 3.0%, the amount of retained austenite increases, which causes work-induced transformation to deteriorate the workability, and causes a problem that the susceptibility to stress corrosion remarkably increases. Therefore, the optimum Ni addition range is 0.8 to 3.0%.

Al is a strong deoxidizing element and the lower limit is made 0.005% in order to maintain the cleanliness of steel. However, Al has a strong effect of suppressing the formation of martensite, and also has the effect of accelerating the rate of transformation into ferrite with the cooling of austenite and the transformation of martensite into ferrite, and is an element that is very easily oxidized. Therefore, if contained excessively, the soundness of the welded portion may be impaired. In the present invention, the upper limit is 0.03%.

Nb, like Ni, is an important additional element in the present invention. That is, in the present invention, toughness of the welded portion, workability, C, N which deteriorates corrosion resistance is lowered, and martensite is formed by further adding Ni to form C, N as a solid solution therein, C, It suppresses the harmful effects of N, and at the same time improves the mechanical properties of the material, especially strength and toughness, by low-C martensite, but when it is again affected by heat, decomposition of martensite occurs and , N will have adverse effects. Nb has the effect of deteriorating the corrosion resistance of the heat-affected zone due to the decomposition of martensite, lowering the toughness and preventing softening, and is effective even if added in a relatively small amount. FIG. 1 shows an example of the effect.

That is, the amount of Nb is determined relative to the amounts of C and N, and the lower limit of Nb / (C + N) is 2.5, but if it exceeds 7,
The effect tends to be saturated, the material at the time of hot rolling annealing is hardened, the toughness of the welded portion is lowered, and the weld crack susceptibility is also increased.

Further, in the present invention, the lower limit absolute amount for obtaining the above-mentioned effect of Nb is 0.05%. However, Nb has a detrimental effect on heat workability as the amount of Nb increases, and further, it has a harmful effect of increasing susceptibility to welding cracks. Therefore, Nb needs to be suppressed to 0.35% or less.

Cu is an element effective in improving rust resistance, but since it significantly deteriorates hot workability, its addition amount is set to 0.05 in the present invention.
~ 0.8%

Further, in the present invention, it is necessary to have the above-mentioned components and satisfy the CRE value of 5 to 20 calculated by the following formula CRE = 2Cr + 3Si + 5Al-80 (C + N) -6Ni-4Mn-5Cu + 5.6Nb. .

That is, the CRE value calculated by the above formula is an index indicating the ease of forming martensite, the smaller the CRE value, the more the amount of martensite in the weld, but in the present invention to some extent as described above. It is necessary to secure the amount of martensite in the weld, and for this, the CRE value must be 20 or less. On the other hand, the smaller the CRE value, the more stable the austenite phase at high temperature and the larger the amount of martensite in the resulting weld.However, if the CRE value is too small, the martensite structure becomes coarser, and rather the toughness of the weld is increased. The tendency to spoil is revealed. Therefore, the CRE value should be 5 or more.

Then, in the stainless steel having the composition system within the range of the present invention satisfying the CRE value of 5 to 20, the properties and the amount of martensite in the welded portion maintain a proper range,
Not only sufficient toughness, workability, and corrosion resistance such as intergranular corrosion resistance are ensured, but also these various characteristics deteriorate even when subjected to welding heat such that martensite is tempered again. There is no.

Further, in the steel of the present invention, it is possible to form a ferrite single-phase structure, a ferrite + martensite dual-phase structure, and a martensite structure by heat treatment, but as shown in Table 1, any structure can be used. Even if it exists, it has excellent toughness and good workability.

All of these properties, the properties of the martensite formed, are those that are affected by the amount, in the steel of the present invention, the amount of martensite can be sufficiently obtained by reducing the C, N amount. Therefore, the amount of C, N absorbed in it is also sufficient to ensure corrosion resistance, and the martensite phase itself is not brittle and needle-shaped, and even when mixed with other ferrite phases, it becomes a microstructure. However, even when this is sufficiently tempered to become a ferrite single phase, the fineness of the structure is maintained. Therefore, as described above, excellent toughness and workability can be achieved with any structure.

Next, the effects of the present invention will be more specifically described with reference to Examples. To explain.

[Example 1] 25 kg of the steel having the components shown in Table 2 was vacuum-melted and 100 mm square × 300 mm
After casting into a steel ingot of about l, it was hot-rolled at 1180 ° C, 1 hr heating, 7 passes, finishing temperature 900 ° C to obtain a 9 mm thick hot-rolled steel sheet. The hot workability was judged from the state of cracks that occurred at the ends (ears) of the hot rolled sheet at this time, and the results shown in Table 3 were obtained. That is, as shown in the hot-rolled crack column of Table 3, steels having components other than comparative steel P exhibited good hot workability.

Next, all steel is annealed at 700 ℃ for 3hr, then thickness 8mm, width 40mm
The bending test piece of No. 1 was sampled from the l direction, and 180 ° bending of 1 tR was performed at 0 ° C. to evaluate the workability. The results are shown in the hot rolled sheet bending column of Table 3. The steels of the present invention all showed good results, but the comparative steels K, L, and U cracked. 1300 ° C
After 10 min.WQ, use the same bending test piece at room temperature 90 °, 1
When subjected to tR bending, as shown in Table 3, comparative steel K,
Cracks occurred in L, P, U and V. Also, the results of these 1-direction tensile tests for hot rolled annealed sheets As shown in the column of Hot 700 ° C annealed material in Table 3, comparative steels K, L,
Elongation is 25% or less for U and V, yield strength is 30 kg for comparative steels M, R, S and T
It is less than f / mm ² and both are insufficient. That is, it was difficult to clear the balance of yield strength and elongation by adjusting the annealing temperature and time in the steel types of these comparative materials.

Furthermore, all steel grades were annealed at 700 ℃ for 3hr, then machined into a steel plate with a thickness of 6mm, voltage 20-24V, current 130-140A, welding speed 25cm / m.
A Charpy test piece with a 2 mm-V notch and a 5 mm subsize was sampled from the Depo. and HAZ of the welded TIG. The results are shown in Table 3 Tig + Named part vTrs
(° C). The steels of the present invention show a transition temperature of 0 ° C or lower in both Depo. And HAZ, but the comparative steels K, L, M, P, Q, R, S, T, U and V are D
The transition temperature of either epo. or HAZ was above 0 ° C. On the other hand, it is clear that the steel according to the invention has a very good toughness.

In addition, the welds obtained under the above-mentioned conditions of each steel except Steel No. L were tested by a sulfuric acid-copper sulfate test according to JIS-G0575 to evaluate the corrosion resistance of the welds. The result is the third
Shown in the table. Comparative steels M, R, S and V produced significant intergranular corrosion in the welds in this test, while the other steels were sound.

Next, match the TIG.names of each steel except the steel number L in the direction of the arrow as shown in FIG.
18mm, upset margin 7.1mm, flash butt welding with flash time 15sec., Made a test piece that was affected twice by heat from welding, and made it corrosion-resistant by sulfuric acid-copper sulfate test according to JIS-G0575. An evaluation was made. The results are shown in Table 3. Similar to the previous test, the comparative steels M, R, S and V showed severe deterioration in corrosion resistance over the entire weld zone.
N, O, P and T are severely deteriorated in corrosion resistance on the bead of TIG. Name which is the first welding and on the heat-affected zone of the flash butt of the second welding. No defect is recognized.

Next, the hot rolled annealed plate described above is cold rolled to a thickness of 1.5 mm, and then 1000 ° C. × 10 min. After heating, it was air-cooled and subjected to a tensile test to measure the yield strength. The results are shown in Table 3. Comparative steel
All other steel types except R, S and V have a yield strength (PS) of 6
0k / mm ² or more, but comparative steels R, S and V have a proof stress of 50kg / m
The amount of martensite formed was 30% or less at m ² or less. Further, regarding R, S, and V, no range was found in which the yield strength was> 60 kgf / mm ^{2 in} the same heat treatment in the temperature range of 700 to 1300 ° C. (However, in the as-cold rolled state, the yield strength was remarkably high at approximately 80-100 kg / mm ^2, but the elongation was extremely low at approximately 3%).

Further, for the steels B, F, H of the present invention and the comparative steels K, O, T, the cold-rolled sheet having a thickness of 1.5 mm was heated at 700 ° C. for 10 min. 0.5 μm Al ₂ O
A so-called salt spray test in which 5% NaCl solution was sprayed at 35 ° C. for 240 hours was performed for finish polishing to ₃ and the rust resistance was evaluated. The results are shown in Table 3, and red rust was noticeable in K, O, and T of the comparative steels.

As is clear from the above examples, the steel according to the present invention has good workability, toughness, and corrosion resistance even in the welded portion, and especially 2
Corrosion resistance does not deteriorate even in the parts that have been welded more than once.

[Example 2] As shown in Table 4, 500 kg of melted steel Nos. W to Z were subjected to 1150 ° C for 1 hr.
After heating, hot rolling to a thickness of 6.5 mm at a finishing temperature of 900 ° C
℃, 3hr air-cooled, pickled, machined 6mm thick x 28
A 0 mm square plate was prepared. These plates are butted against each other in the l direction, with a flash allowance of 17 mm and a flash time of 1
Flash butt welding was performed for 2.6 sec and an upset margin of 6.5 mm. Furthermore, the front and back surfaces of the joint obtained in this way were precision machined to be smooth and pulled by 15% in the direction of 1 (right angle to the flash butt welding line) with a strain amount, but the ring diameter was 200 mm on one side. Place a Viton O-ring (diameter: 5 mm) on which a transparent acrylic plate with a thickness of 10 mm is placed, and a diameter of 20 mm surrounded by the test piece, acrylic plate, and O-ring.
Evaluating the soundness of the flash butt weld by leaving a space of 0 mm and a height of 5 mm as a vacuum of ~ 10 ^-3 Torr and leaving it for 72 hours. I did it for 20 bodies each.

In the steel of the present invention W, none leaked after 72 hours, but in the comparative steel Z, 4 out of 20 leaked completely, and in the steel of the present invention, a sounder weld portion was obtained. It is clear.

[Example 3] 500 kg of steel having the components shown in Table 4 was melted, hot-rolled, annealed, pickled, and cold-rolled to form a 0.7 mm thick steel strip, and then steel numbers W and X were set to 1000 ° C. Steel plate Y is finally heat-treated at 850 ° C and has a cross-sectional shape as shown in Fig. 4 with a diameter of 63.5 mm (25 mm
Inch) WO type seam rim for bicycle was created. still,
At this time, the material exhibited proof stress, strength and elongation at break as shown in Table 5. These rims were subjected to an impact test at room temperature centering on the flash butt weld, and the impact absorption energy of each rim was measured for 60 pieces. In addition, the rim is cut into 14 cm long strings around the flash butt welded part of each rim, and a tensile test is performed in the tangential direction of the central part of the string to determine the tensile strength of the flash butt welded part in the rim shape. Measure 60 each Decided Furthermore, the tensile test of this flash butt welded portion was performed on each of the 60 pieces after the salt spray test for 144 hours, and 60 pieces were similarly performed. The results are shown in Table 5. It is clear that the rim made of the steel W of the present invention has a high impact value and a high tensile strength, and the strength reduction after corrosion is extremely small, and that a bicycle rim having extremely excellent performance can be obtained as compared with those of X and Y. Is.

[Effects of the Invention] As is clear from the above examples, according to the present invention, the toughness, workability, and corrosion resistance of the welded portion are good, and there is no deterioration of these characteristics even when subjected to two or more weldings. At the same time, when it is used as a bicycle rim or an automobile wheel, it becomes a healthier welded part, and Cr-based stainless steel showing particularly excellent performances is obtained, and the industrial effect is extremely remarkable.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of the correlation between the effect of adding Nb on the corrosion resistance and the amount of C + N for the site where double welding is assumed. FIG. 2 is a schematic view showing a bicycle rim forming process, and FIG. 3 is a perspective view showing a flash butt welding joint production procedure of a steel plate shown in an embodiment. FIG. 4 is a perspective view showing an example of a sectional shape of a bicycle rim. 1: Coiled steel plate, thin steel strip, 2: Seam weld, 2-1 (dotted line): Seam welded once, 2-2 (diagonal line):
Seam welds welded twice, 3, 3 ': cut surface (butt weld surface).

Claims (3)

[Claims] 1. In weight% C <0.035% N <0.02% Si: 0.1 to 0.5% Mn: 0.1 to 0.8% Cr: 11 to 17% Ni: 0.8 to 3.0% Al: 0.005 to 0.03% Nb: 0.05 to 0.35% Cu: 0.05 to 0.8% and C + N <0.05%, Nb / (C + N): 2.5 to 7, and the CRE value represented by the following formula is 5 to 20, and the balance is Fe and inevitable impurities. Cr-based stainless steel used for applications where it is welded more than once. CRE = 2Cr + 3Si + 5Al-80 (C + N) -6Ni-4Mn-5Cu + 5.6Nb 2. Weight% C <0.035% N <0.02% Si: 0.1 to 0.5% Mn: 0.1 to 0.8% Cr: 11 to 17% Ni: 0.8 to 3.0% Al: 0.005 to 0.03% Nb: 0.05 to 0.35% Cu: 0.05 to 0.8% and C + N <0.05%, Nb / (C + N): 2.5 to 7, and the CRE value represented by the following formula is 5 to 20, and the balance is Fe and unavoidable. It is a component consisting of impurities, and the proof stress is 60 kgf / mm ² or more, 2
Cr-based stainless steel that is used to weld bicycle rims more than once. CRE = 2Cr + 3Si + 5Al-80 (C + N) -6Ni-4Mn-5Cu + 5.6Nb 3. Weight% C <0.035% N <0.02% Si: 0.1-0.5% Mn: 0.1-0.8% Cr: 11-17% Ni: 0.8-3.0% Al: 0.005-0.03% Nb: 0.05- 0.35% Cu: 0.05-0.8% and C + N <0.05%, Nb / (C + N): 2.5-7, and the CRE value represented by the following formula is 5-20, and the balance is unavoidable impurities. It has a yield strength of 30 kgf / mm ² or more and an elongation of 25%.
With the above, Cr-based stainless steel used for welding as a bicycle wheel more than once. CRE = 2Cr + 3Si + 5Al-80 (C + N) -6Ni-4Mn-5Cu + 5.6Nb