CA2044639C

CA2044639C - Method of making steel for springs

Info

Publication number: CA2044639C
Application number: CA002044639A
Authority: CA
Inventors: Tsunetoshi Suzaki; Tomoyoshi Iwao; Teruo Tanaka; Toshiro Yamada
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1990-06-19
Filing date: 1991-06-14
Publication date: 2001-08-28
Anticipated expiration: 2011-06-14
Also published as: EP0462779B1; EP0462779A2; DE69121982D1; KR930012177B1; AU633737B2; DE69121982T2; EP0462779A3; KR920000959A; AU7837391A; CA2044639A1

Abstract

Steel material containing by weight from 0.4 % to 0.8 % carbon, from 0.5 % to 2.5 % silicon, from 0.3 %
to 2.0 % manganese, from 0.1 % to 1.5 % chromium, and from 0.1 % to 0.5 % molybdenum is hot-rolled to form a plate. The hot-rolled plate 1 is annealed and cold-rolled at a rolling reduction between 10 % and 80 %. The cold-rolled plate is heated at a temperature above Ac3 critical point for a time sufficient to austenitize carbide and annealed.

Description

1 2~4~6~9 TITLE OF THE INVENTION
Method of Making Steel for Springs BACKGROUND OF THE INVENTION
The present invention relates to a method of making steel for springs such as a diaphragm spring provided in a clutch of a motor vehicle.
In recent years, the environm~antal temperature of the spring used in a machine increases with increase of the output power of the machine. :For example, clutch torque of the clutch of the motor vehicle is increased due to increase of the engine power of a motor vehicle such as a four-wheel drive vehicle. As a result, the environmental temperature of the clutch increases up to 250-~350°C from 150°C which is a maximum temperature of a conventional motor vehicle.
The diaphragm spring is made of carbon tool steel such as SK5 {Japanese Industrial Standard). However, the spring of carbon steel relaxes quickly, namely it becomes inoperative when the temperature thereof increases to the above described high environmental temperature.
It is known that if silicon content of steel is increased, endurance of the spring, that is a property of the spring resisting heat without settling, z5 increases. However, conventional ;steel including a large silicon content is liable to relax at a high temperature.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of making steel having a property resisting high temperature, whereby a spring made of the steel withstands the relaxation thereof at high tempE~rature.
Another object of the present invention is to provide steel which may be quickly quenched at a low temperature, thereby preventing the steel from reducing in endurance.
The inventors found that steel having excellent endurance against the relaxation in spring at high temperature could be made by properly controlling solid solution and precipitation of carbide in the steel including carbon (C), silicon (Si), Manganese (Mn)., chromium (Cr), molybdenum (Mo) and others.
According to the present invention, the method for making steel comprises hot-rolling steel material consisting essentially by weight of from 0.4% to 0.80 carbon, from 0.50 to 2.50 silicon, from 0.3% to 2.Oo manganese, from 0.1% to 1.5a chromium, from 0.1% to 0.50 molybdenum, from 0% to 0.50 vanadium, from Oo to 0.5%
niobium, from 0% to 0.0200 aluminum and remaining iron and inevitable impurities to form a plate, annealing the hot-rolled plate, cold-rolling the annealed hot-rolled plate at rolling reduction of 10% to 80%, annealing the 20~~~~~

cold-rolled plate at a temperature below Acl critical point, heating the annealed cold-rolled plate at a temperature above Ac3 critical point for a time sufficient to austenitize carbide, cooling the heated cold-rolled plate at a speed higher than a lower critical cooling speed, heating the cooled cold-rolled plate for a time necessary for precipitating carbide and then cooling it to a room temperature.
The lower critical cooling speed is a speed above which the austenite is fully transformed to the martensite.
In the last heating process, molybdenum carbide is finely precipitated, thereby preventing the dislocation migration which causes the relaxation of the spring at high temperature. The heating is performed at a temperature between 450°C and 600°C: for a time sufficient to precipitate the carbide.
The silicon content and chromium content are selected so as to satisfy the equation:
_~<4xSi(~)-lOxCr(~)<5.
The heating of the cooled cold',-rolled plate is performed so as to provide an annealed hardness between HV400 and HV550.
Furthermore, the annealing of the cold-rolled plate is performed at a temperature between 550°C and ~0~4fi~9 730°C, thereby providing carbide having an average grain diameter less than 2y~m.
The other object and features of this invention will become understood from the following description with reference to the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
The figure is a graph showing relationship between heating temperature and hardness of steel.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Quantity of each component, preparing condition and reason for numerical limitation of the component are described hereinafter.
(Carbon) Carbon is effective in increasing the strength of steel. In order to obtain a strength necessary for the the spring; carbon content over 0.4 ~ by weight must be included. However, if carbon is included in excess of 0.8 ~, quenching crack and reduction of toughness of steel occur. Therefore the carbon of 0.4 ~ to 0.8 ~ by weight is included in the steel.
(Silicon) In the method of the present invention, the material is tempered at high temperature. Silicon is added to prevent the strength from reducing due to the high temperature tempering. It is necessary to add silicon over 0.5 ~ by weight. If ~cilicon content exceeds 2.5 %, internal oxidation <~nd decarburization which are unfavorable to the spring occur, and graphitization is enhanced in the hot rolling and annealing.
(Manganese) Manganese is effective in deo:~idizing steel and in increasing the hardenability of thE= steel, if the manganese is included over 0.3 % by weight. If manganese content exceeds 2.0 %, the toughness of steel reduces exceedingly after quenching and tempering.
(Chromium) Chromium acts to restrict the graphitization and the internal oxidation which are enhanced by silicon, and is effective in increasing the hardenability as is effected by manganese, if chromium content is included in excess of 0.1 % by weight. If chromium content exceeds 1.5 %, the toughness of the steel reduces after quenching and tempering.
Moreover, Si content and Cr content are determined so as to satisfy the following equation, thereby preventing decarburization and grad?hitization.
-7<4xSi(%)-lOxCr(%)<5 (Molybdenum) The molybdenum included in the steel of the present invention forms carbide in the steel after the cold rolling and annealing thereof.. The carbon becomes solid solution in austenite when t:he steel is heated over the Ac3 critical point. Consequently, the austenite is transformed into mart~ensite after quenching, and carbide separates finely upon tempering at high temperature, thereby remarkably increasing endurance withstanding against relaxation. In order to obtain such an effect, it is necessary to include molybdenum over 0.1 ~ and below 0.5 ~ by weight. If molybdenum content exceeds 0.5 ~, .a large amount of carbide remains without becoming solid solution in austenite when the steel is heated above the Ac3 critical point.
(Vanadium, Niobium) The vanadium and niobium included in the steel of the present invention become carbide after the cold rolling and annealing thereof. Remaining vanadium and niobium without becoming solid solution in austenite act to prevent austenite grain from growing. On the other hand, solid solution of vanadium and niobium in austenite are in solid solution in martensite when quenching, and precipitate finely <~s carbide when tempering, thereby enhancing endurance withstanding against relaxation. In order to aittain these effects, vanadium and niobium over 0.05 ~ a:re necessary. If the content exceeds 0.5 $, quantity of un.dissolved carbide in austenite increases when the stf~el is heated above 2044G~9 Ac3 point, thereby reducing fatigue strength of the steel.
(Aluminum) The spring is fatigued by repeated bending or twisting. Existence of hard inclusions such as aluminum aggravates th fatigue. In order to reduce the influence of the hard inclusion, aluminum content is limited below 0.020 weight percent.
Manufacturing conditions are described hereinafter.
In the cold rolling, when rolling reduction is smaller than 10 ~, the grain size of carbide becomes coarse when annealed below the critical point Acl.
Consequently, a long time is required for transforming the carbide to austenite when heated above the Ac3 critical point, which causes an increase in decarburization and hence spring characteristic is deteriorated. When the rolling reduction is larger than 80 ~, work hardening due to t:he cold rolling is 2~ remarkably increased, causing deformation such as edge crack. Therefore, an upper limit .is 80 ~.
If the annealing after the cold rolling is performed at a temperature above 7.30°C (Ac1 critical point), spheroidized grain of carbide becomes coarse.
Consequently, it takes a long time to transform the carbide to austenite, resulting in increase of 2~44G~9 decarburization causing deterioration of spring characteristic. Therefore, the annealing after the cold rolling is carried out at a temperature below the Acl point. If the annealing temperature is lower than 550°C, the hardness increases, so that the formability of the material reduces. Therefore, the annealing temperature is between 550°C and 730°C.
If the average grain diameter of carbide after the annealing is less than 2~m, carbide is easily dissolved austenite at quenching. Therefore, it is necessary into maintain the average grain diameter of carbide to a value smaller than 2~m for effectively performing the quenching.
In order to increase the strength of the steel made by the cold rolling and annealing to a value necessary for the spring, the strip is heated at a temperature higher than the critical point Ac3 for a time sufficient for austenitizing 'the spheroidal carbide, after which cooled at a speed higher than a lower critical cooling speed, namely quenching.
Thereafter, the strip is heated at a temperature between 450°C and 600°C for a time to precipitate fine carbide and cooled to a room temperature (that is tempering). At the quenching, the parent material is austenitized by heating it over thE~ Ac3 point, and then carbon and other elements are dissolved to martensite 20~4~~9 by cooling at a speed higher than the lower critical cooling speed. By tempering the material at a temperature higher than X50°C, carbide of Mo, V and Nb is finely precipitated from the martensite, thereby increasing the endurance withstanding against the relaxation. If the tempering is carried out at a higher temperature than 600°C, a carbide of Mo, V and Nb becomes coarse which can not prevent the dislocation migration. In addition, the strength of the steel largely reduces. Therefore, the tempering is performed at a temperature below 600°C.
Example 1 Table 1 shows contents of steels. In the table, A
to F are steels of the present invention, and G to L
are comparative steels.
Each of the steels A to F is :made into a hot-rolled plate of 3.5 mmt by ordinary hot rolling and then the plate is annealed and cold rolled at a rolling reduction between 5 ~ and 90 ~. T:hereafter, the steel is annealed at 700°C below the Acl paint for 10 hours, and is soaked at 900°C above the Ac3 point for a period necessary to provide remaining carlbide ratio below 1 ~
by weight. Thereafter, the steel :is quenched into oil.
Table 2 shows results of tests for edge crack and depth of decarburization. When the rolling reduction exceeds 80 ~, edge crack occurs. :If the rolling l0 2044639 reduction is smaller than 10 ~, carbide becomes coarse.
Consequently, it takes a long tims~ to dissolve carbide into austenite, so that the depth of decarburization remarkably increases.

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~5 ;n ~p~4639 Example 2 Each of the steels A to F is made into a hot-rolled plate of 3.5 mmt by ordinary hot rolling and annealed and cold rolled at rolling reduction of 35 to form a cold rolled plate of 2.3 mm. Thereafter, the steel is annealed once at 700°C fo:r 10 hours, and is heated at a temperature between 850°C and 900°C for 10 minutes. Thereafter, the steel is quenched into oil and tempered at a temperature between 420°C and 63U°C
for 30 minutes.
A relaxation test was perform<=d in order to estimate endurance against relaxat_~on. The test was carried out at 350°C, initial 1.0 '~ strain, holding time of 12 hours. Load reduction after the test was regarded as relaxation rate.
Table 3 shows the result of the relaxation test.
Since comparative example G is smaller than the present invention in carbon content, comparative example 1 is smaller in silicon content, comparative example J is in manganese content, and K is in chromium content, each of these steels has low strength so that the relaxation rate thereof is high. Although the comparative example H has a large carbon content, the z-elaxation rate is not largely reduced. Since the comparative example L
has not molybdenum, the carbide of which is effective to increase the endurance, relaxation rate is very high. Although each of comparative examples A', D' and F' has the same ingredient content as the present invention, the tempering temperature is out of the range of the present invention. Consequently, the relaxation rate is not largely reduced.
To the contrary each steel according to the present invention has a very low relaxation rate comparing with the comparative examples, which means that the steel has a high endurance withstanding against the relaxation.

TABLE
QuenchingTempering HardnessRelaxation Steel Rate Temp. Temp. (C) ( H V (%) (C) ) 900 480 496 16. 2 900 520 475 15. 1 present A -- ----~ --- -----900 560 452 14. 4 in~~ention 8 5 0 5 2 0 4 6 2 1 5. 7 B 900 560 470 13. 5 C 900 520 479 1 5. 7 900 480 51 3 14. 2 900 520 492 1 3. 4 ---- --_-. --~ -----900 560 468 1 2. 6 ' 850 560 452 1 3. 2 E 900 560 453 14. 1 F 900 580 472 1 1. 1 G 900 520 348 40. 2 H 900 560 473 20. 1 compara- -- ----- ----. .__.- .~_---I 880 520 394 25. 2 tlVe --- ----.- --.--_ .--..~ -...._--J 900 520 442 18. 2 example -- _.--- ----_ -_.- ---._ K 900 560 421 21. 7 L 880 520 427 32. 5 900 420 567 21. 5 A ' _.--- ----_ --- ----900 630 41 3 1 9. 2 compara- -- .---- ----_ --- -.--.._ 900 420 591 20. 1 tive D ' ---- ----_ --._ ----900 630 426 18. 1 example -- ---- ---._ -._- --._-900 420 625 19. 5 F ' ---- ----_ --- _.---900 630 513 17. 3 16 2044G~9 Example 3 Alphabets A to G in Table 4 are steels of the present invention and H to L are comparative steels.
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Each of the steels in the table was hot-rolled to provide a hot-rolled plate having a thickness of 3.5 mmt, and then annealing the hot-rolled plate. The plate was cold rolled at rolling reduction of 35 ~ to prepare a cold-rolled plate of 2.3 mmt thickness. The cold-rolled plate was annealed at a temperature between 650°C and 750°C for 10 hours to provide a test piece.
Hardenability test was performed in such a manner that the test piece was rapidly heated to 850°C at the rate of 140°C/sec, heated from 850°C to a test temperature between 900°C and 1100°C at the rate of 30°C/sec, and then rapidly cooled immediately after the heating without taking a holding time. The hardenability was estimated by the hardness of the test piece after the quenching. Results of the test are shown in the attached figure.
As is seen from the graph, the test piece A having ingredient contents according to the present invention has an average grain diameter of carbide less than 2~m when annealed at 650°C and 700°C. Even if the test piece A is heated to the lowest temperature 900°C, the hardness becomes the higher value. However, if it is annealed at 750°C so that the average grain diameter exceeds 2~m, the hardness does not reach the highest value unless the quenching temperature is elevated up to 950°C.

19 ~04463~
The comparative example H has a SICR value of -7.42 out of the range of the present invention.
Consequently a large amount of chromium remains in carbide after the annealing. Accordingly, the steel must be heated up to 1000°C in order to obtain the higher hardness, although the average grain diameter is smaller than 2um.
From the comparing, it will be seen that the carbide is rapidly dissolved into austenite at a lower temperature in accordance with the present invention.
Fatigue test and relaxation tc=st, piece are estimated as follows. The cold-ro:Lled plate having 2.3 mm thickness is annealed at 680°C :Eor 10 hours, and then heated at 900°C and quenched. Thereafter, a plurality of the plates are tempered at various temperatures for 30 minutes.
The fatigue test was performed in alternating plane bending fatigue. The result of the test is shown in Table 5.

TABLE
Tempering Hardness Test Temp. Fatigue Strength Steel Temp. (C (HV) ( C ) (kgf/mm2) ) .A 5 8 0 4 5 4 ---.-- --------I 5 8 0 4 4 8 ---__ ._------From the table, it will be seen that although the steel A of the present invention has a hardness appraximately equal to the comparative example I, the steel A is superior to the comparative example I in fatigue strength. This is caused by the fact that the aluminum content of the steel A is less than 0.020 weight percent, which means hard inclusion causing fatigue fracture is small. The steel G has the same fatigue characteristic as steel A.
The comparative steel J has a small Cr content compared with Si content, so that ~SICR value is 7.50 out of the range of the present invention, producing graphite at annealing. In addition, since a long time was required for austenitization, decarburization increased. As a result, the fatigue characteristic is inferior to the steels A and G.
The endurance withstanding against relaxation was estimated by the relaxation test. Table 6 shows test results.

Quenching Tempering Hardness Relaxation Steel Rate Temp. (C) Temp. (C) (1-IV) (/) 580 454 1 2. 4 540 490 1 2. 2 580 461 13. 1 540 494 1 3. 1 C 900 520 4'71 1 6. 3 540 469 1 2. 0 D 9 0 0 --,_ ---- ----510 496 11. 9 -- - ,._. --~.~ --- .-..-__ E 900 520 435 1'7. 3 F 900 560 455 1 2. 5 550 452 1 1. 5 G 9 0 0 --_ --- ----,-500 G>05 1 1. 3 850 430 450 36. 7 K --.-_ --.T- -~.~ -~,.-850 400 4'75 38. 5 L 900 550 x':59 3 1. 5 M 880 490 x:65 25. 2 Example 4 In table 7, A to G are steels of the present invention, H to L are comparative steels.

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Each of the steels in the tab_Le was hot rolled to provide a hot-rolled plate having a thickness of 3.5 mmt, and then annealing the hot-rolled plate. The plate was cold rolled at rolling reduction of 35 ~ to prepare a cold-rolled plate of 2.3 mmt thickness. The cold-rolled plate was annealed at Ei80°C for 10 hours, and then heated at a temperature between 850°C and 900°C for 10 minutes and quenched into oil. All plates were tempered at various temperatures for 30 minutes.
The fatigue test was performed in alternating plane bending fatigue. The result of the test is shown in Table 8.

Quenching TemperingHardnessTesting Fatigue Steel Strength Temp. (C Temp. ( H V Temp. (C (kgf/mm2) ) (G ) ) - ) 5 4 0 4 6 3 2. 5 5 1 I

.A 900 480 508 2'.5 58 I

- note: I represents present invention steels, while II represents comparative steels.

From the table, it will be seen that although the steel E of the present invention has a hardness approximately equal to the comparative example I, the steel E is superior to the comparative example I in fatigue strength because of small aluminum content.
Even if contents of ingredients are within the range of the present invention, thc~ fatigue strength reduces if the annealed hardness e:~ceeds HV550.
The endurance withstanding ag<~inst settling was estimated by the relaxation test. Test temperature was 350°C, initial strain 1.0 ~, and holding time 12 hours.
Table 9 shows test results.

Quenching TemperingHardnessRelaxation Steel Rate Notes Temp. (C Temp. ( H V ( % ) ) (C ) ) 480 496 16. 2 A 9 0 0 5 2 0 4 7 5 Tl 5.

560 452 14, 4 present 850 520 462 ~1 5.

B 9 0 0 5 6 0 4 7 0 1 3 .

invention C 9 0 0 5 2 0 4 7 0 1 5. 7 440 509 16. 1 D 9 0 0 4 8 0 4 9 1 1 5 .

5 2 0 4 6 5 T1 5 .

1 6 .

900 580 453 ~~ 2~.

9 0 0 5 8 0 4 7 2 ~l 1 .

G 9 0 0 5 2 0 4 4 6 7. 3 , 900 520 348 40. 2 '-'-"-'-'- comparative 8 8 0 5 2 0 3 9 4 2 5 .

example 880 520 42? ~~2. 5 I
850 430 450 36. 7 A 900 650 391 22. 1 -- ---- ----comparative 9 0 0 6 2 0 3 8 2 2 3 .

example 900 660 392 23. 2 II
G 900 620 378 2~5. 1 Comparative steels H and J have small C content and Si content, and hence they have high relaxation rates, respectively. Since comparative steel K has no Mo, it has a high relaxation rate. Even if each of steels A, D, E and G has the contents of the present invention, the relaxation rate is not largely reduced if tempering temperature increases and hardness is lower than annealed hardness HV400, as shown in comparative examples IT.
While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth. in the appended claims.

Claims

1. A method for making steel comprising:
hot-rolling steel material consisting essentially by weight of from 0.4% to 0.8% carbon, from 0.5% to 2.5%
silicon, from 0.3% to 2.0% manganese, from 0.1% to 1.5%
chromium, from 0.1 to 0.5% molybdenum, from 0% to 0.5%
vanadium, from 0% to 0.5% niobium, from 0% to 0.020%
aluminum and remaining iron and inevitable impurities to form a plate;
annealing the hot-rolled plate;
cold-rolling the annealed hot-rolled plate at a rolling reduction between 10% and 80%;
annealing the cold-rolled plate at a temperatures below Ac1 critical point;
heating the annealed cold-rolled plate at a temperature above Ac3 critical point for a time sufficient to austenitize carbide;
cooling the heated cold-rolled plate;
heating the cooled cold-rolled plate for a time or precipitating carbide and then cooling it to room temperature.

2. The method according to claim 1 wherein the steel material further contains at least one of vanadium, from 0.05% to 0.5% by weight and niobium from 0.05% to 0.5% by weight.

3. The method according to claim 1 wherein the steel material further includes aluminum less than 0.020 %
by weight.

4. The method according to claim 1 wherein the heating of the cooled cold-rolled plate is performed at a temperature between 450°C and 600°C.

5. The method according to claim 1 wherein silicon content and chromium content are selected so as to satisfy the equation:
-7 <= 4 x Si(%) - 10 x Cr(%) <= 5

6. The method according to claim 1 wherein the cooling of the heated cold-rolled plate is performed at a speed higher than a lower critical cooling speed.

7. The method according to claim 3 wherein the heating of the cooled cold-rolled plate is performed so as to provide an annealed hardness between HV400 and HV550.

8. The method according to claim 5 wherein the annealing of the cold-rolled plate is performed at a temperature between 550°C and 730°C, thereby providing carbide having an average grain diameter less than 2µm.