US3904446A

US3904446A - Process of making high strength cold rolled steel having excellent bake-hardening properties

Info

Publication number: US3904446A
Application number: US487653A
Authority: US
Inventors: Kuniki Uchida; Kenzi Araki; Hirosi Narita; Shiro Fukunaka; Takao Kurihara
Original assignee: Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1973-07-12
Filing date: 1974-07-11
Publication date: 1975-09-09
Anticipated expiration: 1992-09-09
Also published as: JPS5441983B2; JPS5026714A; FR2236945A1; GB1435460A; DE2433665B2; DE2433665A1; FR2236945B1

Abstract

A high tension steel of which the yield point is more increased by the ensuring heat treatment after press-forming than that of the properties in which the tensile strength is above 40 to 80 Kg/mm2, is made by the following steps; 1. THE CHEMICAL COMPOSITION OF STEEL IS SUBSTANTIALLY CONTROLLED AT THE RANGE OF 0.04 TO 0.12% C and 0.1 to 1.60% Mn, 2. THE ABOVE STEEL IS HOT-ROLLED WITH A FINISHING TEMPERATURE OF MORE THAN 800*C and a coiling temperature of less than 700*C, 3. after the above steel strip cold-reduced with the ordinary requirements is continuously heated up to 700* to 900*C and then rapidly cooled by a jet of water, said strip is reheated up to 180* to 400*C and held for 2 to 300 seconds at said temperature to partially remain solution carbon in steel.

Description

United States Patent Uchida et a1. Sept. 9, 1975 1 PROCESS OF MAKING HIGH STRENGTH 3,671,336 6/1972 Korchynsky ct al 148/12 F COLD ROLLED STEEL HAVING 3,787,250 1/1974 Korchynsky ct a1 148/12 F EXCELLENT BAKE-HARDENING PROPERTIES Inventors: Kuniki Uchida; Kenzi Araki; Hirosi Narita; Shiro Fukunaka; Takao Kurihara, all of Yokohama, Japan [73] Assignee:

Tokyo, Japan 22 Filed:

[] Foreign Application Priority Data July 12, 1973 Japan 48-77865 [52] US. Cl. 148/123; 148/12 F [51] Int. Cl C21d 1/00 [58] Field of Search 148/12 R, 12 C, 12 F, 12.3

[56] References Cited UNITED STATES PATENTS 3,666,452 5/1972 Korchynsky et a1.... 148/12 F 3,666,570 5/1972 Korchynsky ct al 148/12 F Aging Treatment T/me (second) July 11, 1974 Appl. No.: 487,653

1: 7771's invent/ans scape Nippon Kokan Kabushiki Kaisha,

Preferable scope numem/s; JYP (Kg/mm Primary Examiner-W. Stallard Attorney, Agent, or FirmFlynn & Frishauf [5 7] ABSTRACT A high tension steel of which the yield point is more increased by the ensuring heat treatment after pressforming than that of the properties in which the tensile strength is above 40 to Kglmm is made by the following steps;

1. the chemical composition of steel is substantially controlled at the range of 0.04 to 0.12% C and 0.1 to 1.60% Mn,

2. the above steel is hot-rolled with a finishing temperature of more than 800C and a coiling temperature of less than 700C,

3. after the above steel strip cold-reduced with the ordinary requirements is continuously heated up to 700 to 900C and then rapidly cooled by a jet of water, said strip is reheated up to 180 to 400C and held for 2 to 300 seconds at said temperature to partially remain solution carbon in steel.

5 Claims, 1 Drawing Figure Aging Treatment Temperature 1C} PATENTEBSEP 91915 1 3,904,446

, k /5- e E g /0- [:I] This inventions scope [:3 Preferable scope numerals,- dyPlkg/mm ging Treatment Temperature (C) PROCESS F MAKING HIGH STRENGTH COLD ROLLED STEEL HAVING EXCELLENT BAKE-HARDENING PROPERTIES The present invention relates to a process of making an inexpensive high tension cold-reduced steel, and more in particular a process of offering cold-reduced steel excellent in accelerated aging properties wherein remarkable improvement is strength is seen by maintaining the tensile strength of 40 to 80 Kg/mm and particularly, further increasing its yield point by the coming heat treatment after press forming in actual use.

'i-ieretofore, it was well known in the art that the de velopment of cold-reduced steel had been oriented toward a so-called soft steel having low yield point. In the process of pursuing safety in vehicles, however, particularly in passenger cars, the recent demand is ori ented toward the cold reduced steel with high tensile strength. Various other proposals have been made and put into practice with this point in mind, and they may roughly be classified into the following;

i. That which performs a complete recrystallization by using special elements as Ti, Nb, Cu, Ni, etc.

2. That which controls the annealing requirements and partially retains the cold working structure.

3. That which re-cold-reduces the steel having completed recrystallization and tries to increase strength by work hardening.

4. That which aims at improving strength by quenching the low carbon steel.

It is true that these processes are defective in one way or the other. For instance, the process (1) is defective in its high cost, while the process (2) causes dispersions in the longitudinal direction as the differences in the annealing temperature occur in the inner and the outer peripheries of the coil. The process (3) does improve the stength, but its elongation is not improved proportionately, thus leaving a problem unsolved. On the other hand, the process (4) is suitable for obtaining inexpensive high tension cold-reduced steel and various proposals have been made recently in sucession on said process, namely in Japanese Pat. Nos. 40-3020, 469541 and 469542.

The art disclosed in Japanese Pat. No. 40-3020 is well known as a BISRA process (in England). The art heats up the coldreduced strip to 740 to 850C, quenches the strip down to 150 to 250C, coils the same and then performs an overaging treatment through selfannealing of said coil. The steel manufactured by said process has no accelerated aging property, i.e., AA property, caused by the heat treatment such as baking the coat after press forming, which causes an excessive lowering of the strength. Accordingly, it becomes necessary to further increase C content if the strength is to be enhanced. At the same time, the continuous process is interrupted by the above mentioned self-annealing, consequently decreasing the productivity.

Those arts disclosed in Japanese Pat. Nos. 46954l and 46-9542 are called the INLAND process (In America). The former heats up the coil comprising fine cementite or grannular cementite and ferrite to more than the A;, point in order to make perfect austenite structure not containing terminated pearlite, and then quenching uniformly to obtain a structure substantially comprising rnartensite. The latter heats up the coil from the A to the A point to obtain partial austenitization, and then uniformly quenches the same to obtain the mixed structure of ferrite and martensite. The steels made by these processes are defective in that their strength is lowered by about 15 Kg/mm by the coating and its baking after press forming, and that it has a poor ductility compared to its strength. This means that the steel is very difficult to handle since it is hard when being subjected to press forming and then it becomes soft when finished.

On the contrary, the steels excellent in the above AA property, which said strength is improved by the heat treatment after press forming, were proposed by the INLAND Steel Corporation. (Blast Furnace and Steel Plant, March 1971. pp. 149 to 153). The art adds about ppm nitrogen specially at the time of the initial steel-making stage. Such a steel is cold-reduced by the ordinary method, annealed by batch type annealing and then shipped. The users are expected to perform suitable press forming and a heat treatment to improve the strength. Such an improvement in strength is attributable to the aging of said nitrogen as above discussed, but it is true that such an effect is unfortunately limited automatically so that the initial strength is as low as 40 to 50 Kg/mm in respect of its tensile strength while said AA effect is also as low as 5 to 6 Kg/mm thus imposing limitations in the applicable field of use. It may be said, therefore, that there is no process available today in the art which would obtain stable AA effects at the yield point of about 9 Kg/mm at the baking stage after press forming.

The present invention was developed specifically to offer a solution comprising a method for overcoming the aforementioned difficulties and it is mainly characterized in that the manufacture is carried out by a con tinuous annealing process. Heating and soaking in this art is selected from among the range of 700 to 900C at which the solution treating of cold-reduced steel is achieved, and the steel is subjected to quenching by a jet stream of water from the above temperature to the room temperature, then said steel is reheated to to 400C and is given the aging treatment at the said latter temperature. This aging treatment is not for completing the precipitation of C in the steel, but for causing partial remaining solution carbon therein. The steel is then cooled down to the room temperature and then coiled. After tempering, the tensile strength of 40 to 80 Kg/mm is stably obtained. When the steel sheet thus obtained is subjected to the heat treatment for baking coat of said steel after press forming, its yield strength is improved by about 7 to 11 Kg/mm in comparison with the initial strength before press-forming.

Thus, an object of this invention is to provide a process for manufacturing high tension steel sheet imparting safety to vehicles when used therein by improving of the accelerated aging property of said sheet.

Another object of this invention is to provide an inexpensive manufacturing process in a continuous annealing furnace of a high tension steel sheet excellent in accelerated aging property without addition of any special element.

Other objects and advantages will be apparent from the following description and with the accompanied drawing in which;

The FIGURE is a graph showing the variation of accelerated aging property dependent to the pre-heating requirements of travelling strip.

The steel to which the present invention may be applied consists of substantially 0.04 to 0.12% C and 0.10

o 1.60% Mn. There is no need for addition of special :lements. Any type of furnace for manufacturing steel znown in the art today may be used for this invention 1nd any preferred processes after steel manufacture nay be employed including ingot making blooming, :ontinuous casting, etc. After scarfing the steel is leated up to about 1,250C or more, and is hot rolled [t the finishing temperature of 800C or more and a :oiling temperature of 700C or less. Ordinary cold re lucing process may be employed after pickling. The lnnealing in the ensuing step is a complete continuous mnealing process. In this case, said strip is heated up to to 900C and is maintained for 10 to 120 seconds at said temperature. The heating temperature should )referably be A to 850C and should be controlled rigirously since it will affect the strength of the final prodnot very much.

C in ssteel steel thus made into solution in the above nentioned heating and soaking process. The following uenching from said temperature, should be performed [5 rapidly as possible. Therefore the use of a jet stream )f water is recommended since C in steel will leave as .olution carbon mentioned above. The travelling strip hus quenched down to the room temperature is re ieated up to 150 to 400C, and more preferably to 180 to 300C and is to be maintained for some suitable ength of time. The optimum period of maintaining deaends on the reheating temperature, and more in paricular the reheating temperature selected from among he range of 150 to 180C will take 15 to 300 seconds; [80 to 300C, 4 to 300 sec. or more preferably 20 to 120 sec.; and 300 to 400C, 2 to 200 see. This type of ow temperature aging treatment will not cause the lbove-mentioned solution carbon in steel to precipitate :ompletely but to remain partially in solution state. The .ravelling strip is then forcibly cooled down to the room vemperature, coiled and tempered. The steel sheets .hus obtained will easily have the following properties:

40 to 80 Kg/mm 30 to 60 Kg/mm (kg/mm elongation more than 73.

The value of TS (kg/mm' E] is used as an index .0 evaluate the balance between the two since the in- :reased stength usually lowers the elongation which 'epresents ductility. The present invention steel thus :reated will bring a remarkable improvement in the ;trength, particularly in the yield point of at least 7 (g/mm when given the heat treatment of 100 to 200C after being pressed and formed into a shape in- .ended. This naturally is attributable to the aging efects of the solution carbon partially left by the abovenentioned low temperature aging treatment. In other vords, shaping is easily performed at the time of press 'orming because of the comparatively low yield point, :onsequently the excellent pressed shape of goods is naintained as it is, and the above-mentioned C aging :ffect becomes apparent in the baking process for said goods because of the heat used ordinarily performed to he goods thus shaped. This is none other but the object )f the present invention, and the safety achieved when luch a steel is used for passenger cars and the like, is lnrivaled.

The present invention process is further elaborated in 'espect of its respective steps.

Tensile strength:

Yield strength:

Tensile strength C in steel is an element which plays an important part in the present invention. Its lower limit is set at 0.04 percent in view of the stable operation of the converter used in most steel making stages and of the need to maintain required strength for steel. The upper limit was set at 0.12 percent in view of the press-formability and weldability. But more concretely, the upper limit should be selected from within the above range depending upon the levels of the required strength of steel. The lower limit of Mn content is set at 0.10 percent because of the red shortness. The upper limit thereof was set at 1.60 percent in view of the stable operations for the ingot making. Actually, Mn content is selected from within the range of 0.10 to 1.60 percent as is the case for determination of C content.

No undesirable effects are seen if Si and A1 are maintained at 0.2 percent and 0.02 percent or less respectively in view of the deoxidation control. No specific limit is placed on P or S and they may be present at an ordinary level.

The hot rolling conditions for obtaining a uniform hot rolled structure should be such that the finishing temperature is more than 800C and preferably 830C or more. The coiling temperature may be on the same level as those for ordinary cold reduced sheets, that is below 700C or 500 to 650C on an industrial level. No special problems are existent if normal pickling and cold rolling after the hot rolling are performed.

The present invention is most remarkably characterized in its annealing process. The said annealing process is performed in one continuous line starting with paying-off of the cold rolled coils and finishing with coiling of annealed strip.

The heating requirements should be such that the strip must be heated up to a temperature where a great amount of solution carbon is formed in order to obtain the required strength since the hardening mechanism of the present invention steel depends greatly on the solution carbon in steel, fine precipitation therefrom and hardening thereby. That is to say, the heating-temperature should be at least 700C or more and more than the A point practically. On the other hand, the raising of said heating temperature in this way will increase quenching structure of martensite system thus increasing the strength of steel. However, this at the same time causes lowering of elongation since the differences of hardness between the ferrite matrix and the quenching structure as the second phase particles become larger. That is why there should be an upper limit imposed on the heating temperature such as 900C at most and 850C industrial-wise.

The minimum period of time for holding the travelling strip at the said heating temperature should be such that it will be sufficient for the cold rolled strip to complete recrystallization and for C in steel to become solution state. That is the reason for the lower limit of 10 seconds. However, the longer the holding time becomes, the more the softening acceleration caused by the grain grown after recrystallization occurs which at the same time requires extending of heating-soaking zone in the continuous treating line and lowering of the line speed. Thus the upper limit is placed at seconds or less.

The travelling strip thus heated and soaked is quenched down to the room temperature by a jet stream of water. In this case, the jetting of water is used to speed up said cooling and display its full cooling effect irrespective of place, i.e., in the air or under water. That is to say, when the travelling strip heated is quenched into water, there is instantaneously produced a boiling steam film on the surface of said strip which damages the thermal conductivity and slows 6 On the other hand, too high a reheating temperature will soften a part of ferrite in the two phases structure wherein ferrite matrix and quenched structure coexist, thus causing inevitable lowering of steel strength. In addown the cooling speed excessively. The jet stream of dition, the differences in hardness between ferrite and water in this invention is the most useful in eliminating quenched structure become remarkable so that there such a steam film and the applicants have proven occurs no recovery of elongation is spite of the lowered through experiments that a high speed cooling of more tensile strength and the said tensile strength elongathan 3,000C/sec. is easily obtained. On the other hand, tion" index becomes lower. High temperature reheatit is confirmed that a cooling speed obtained by mere 10 ing will also cause supersaturated solution carbon to 7 water, oil, salt bath or metal bath is only less than precipitate in while at said aging treatment stage so that l,000C/sec. The reason for adopting such ahigh speed no AA effects can be anticipated. Accordingly, the cooling rate in the present invention is because the 50- upper limit of the reheating temperature is set at lution carbon obtained through the process of heating 400C. There are no advantages obtained at the temand soaking is to held there at room temperature. The perature exceeding 400C. For practical purposes, the quenching to the room temperature as above menmost preferable temperature is set at 180 to 300C, tioned has determinative influences on the low temper- The optimum range of the holding time for such reheatature aging treatment which is the next step. ing temperatures varies depending upon said temperaln reheating as the successive process of low temper ture. If the said reheating temperature is set within the ature aging treatment as mentioned above, the precipirange of 150 to 180C, the time should preferably be tation nucleus for time carbide of the solution carbon in 15 to 300 seconds; if within 180 to 300C, 4 to 300 steel is first formed. Fine carbide thus formed is quite seconds; if 300 to 400C, 2 to 200 seconds respecuseful for improving the strength, but will act to curb tively. If the said low temperature aging treatment is the lowering of strength caused by said solution carbon performed with such temperature at time requirements to a minimum. This is the first reason why the strip then all the problems that have been discussed heretomust be quenched down to the room temperature. The fore will be dissolved, and it becomes easily feasible to second reason is that if the steel was left as it was after cause some part of the solution carbon to remain in this being quenched to the room temperature, the strength stage. By such a process, the lowering of tensile at this stage due to solution hardening of C is extremely strength elongation index caused by the increased high. However, it is unavoidable that the above-men- 3O differences in hardness of the said two phase structure tioned solution carbon is precipitated and invites the is possible to be avoided and sufficient AA effects are lowering of strength by coating-baking treatment ordiexpected. In actual practice, when a holding time for narily performed after press forming (usually for 10 to 20 to 120 seconds as against a reheating temperature of 20 minutes at 100 to 200C. In other words, it be- 180 to 300C is selected, the maximum AA properties, comes most difficult to handle the steel since the e.g., AYP is possible to be secured, which is recomstrength is high at the time it is subjected to press-formmended as the most suitable range. The graph in the ac ing and becomes lower after press-forming. For this companying drawing shows the fluctuation at this very reason, a low temperature aging treatment is perstage. It is seen at said graph that AA properties (in this formed by reheating the travelling strip that has been case AYP Kg/mm of at least 7 Kg/mm are stably obquenched down to the room temperature. In this stage, tained when the temperature X time is selected from said solution carbon should be precipitated to a degree within the above range. It is possible to easily obtain the so as not to cause lowering of said strength at the time value of AYP of 1 1 Kg/mm with optimum conditions. of coating-baking for formed goods. More concretely, In the said graph, AYP shows the increase in yield point the conditions for heating, soaking and cooling thereafof the steels which have been subjected to the coatingter should be optimumly provided for so that not all the baking process after press forming as versus those solution carbon be precipitated by reheating, but some being just before said press forming. remain in solution state. There are no special restrictions imposed on the In the above-mentioned low temperature aging treatcooling requirements after the above mentioned low merit, a good product with excellent AA property is obtemperature aging treatment has been performed. tained when the reheating temperature is low since the However, if forced air cooling should be emloyed betensile strength will become higher and the yield cause of the practical reasons, it is quite easy to cool strength lower as well as good shapability at the press the strip down to a temperature suitable for tempering forming stage. However, if the temperature is too low, in case that the temper rolling mill was connected intethere will occur no precipitation of solution carbon and grally following the cooling step to the continuous line. the lowering of strength becomes unavoidable since The strip is coiled off after temper rolling and is said solution carbon is precipitated by the heat treatshipped out of the plant. If the said temper rolling mill ment in the coating-baking stage. Accordingly, the were not to be attached, then a normal line of coiling minumum reheating temperature is made at the temthe strip and then temper rolling the same is employed. perature where the solution carbon is precipitated in The following table shows the comparisons of the part. The lower limit of such a temperature is C. 60 properties of this invention steels and those of other Concretely speaking, the temperature for treatment type steels. calls for above C for obtaining stable results in view of the baking temperature after press forming.

Composition Heat treating requirements Steel C Mn Object Heating Quenching Reheating Baking l 0.07 0.39 Influence 800C 1min In jet water 250C lmin 180C 30min -continued Composition Heat treating requirements Steel C Mn Object Heating Quenching Reheating Baking 2 of In still water 3 cooling In 250C lead bath 250C lhr method 4 Influence of 690C 1min In jet of water 250C 1min 5 max. heating 750C 1min 6 temperature 800C lmin a 7 850C 1min 8 920C 1min 9 Influence of 800C 1min Non l reheating 100C 1min 1 I temperature 250C 1min 12 320C lmin 13 500C 1min l4 Influence of 250CXl0sec l reheating 250C 1min 16 time 250CX4min 17 250C 10min 18 0.07 0.39 Influence of 250C 1min 19 0.10 0.45 composition 20 0.15 0.37 21 0.07 0.58 a 22 H L03 ,1 ,1 I, I, a 23 H 52 I, H I I,

( the present invention steels) After low temp. aging 1% skin pass) After baking AA efiects T.S(Kg/mm") Y.P(Kg/mrn T.S(Kg/mm El(%) T.S+El Y.P T.S A Y.P A T.S

Notes:

The requirements of manufacture other than those listed above are the same for all the steels and are as follows. 1. Hot rolling requirements Finishing temperature: 850C. coiling temperaturez600C Finished thickness of strip: 2.8mm

2. Cold rolling requirements Final sheet thiekne 1.2mm

3. Baking requirements after press forming are the same as those generally used in autombile manufacturing companies.

@Those compared with those after baking.

1n the above table, the steels l to 3 were investigated in respect of the impacts of the cooling methods. The steel 1 was quenched in a jet water stream in accordance with the present invention. The steel 2 was quenched in still water and the steel 3 was quenched by BISRA method. The steel of the present invention shows the values of YP: 34.2 Kg/mm TS: 44.7 Kglmm El: 30.9 percent at the time of shipping out of the plant after skin pass; and the values of YP: 44.5 Kg/mm TS: 45.5 Kg/mm after coatingbaking treatment following press-forming indicating a remarkable improvement of AYP, namely by 10.3 Kg/mm This more than explains the excellent AA effects of the presant invention and the reason for recommending the ex- :ellent quenching effects due to the jet water stream. lhe fact that TS El index in the present invention ;teel l is as high as 75.6 compared to 66.5 of the steel 2 and 68.8 of the steel 3 also suggests excellent press formability.

The steels 4 to 8 were investigated in respect of the impact of the maximum heating temperatures, and the steels 4 and 8 are comparison steels while the steels 5, 6 and 7 are the present invention steels. The maximum heating temperature is lower for the steel 4 than for the present invention steel while that for the steel 8 is higher conversely. The difference is clearly manifested in the mechanical properties of the steel. YP El index for comparison steels are all below 73 while those for the present invention steels exceed 73. This indicates that undesirable influences are seen in the press formability if the maximum heating temperature is outside the range of the present invention. This at the same time affects AA effects as illustrated by the fact that the present invention steels all show excellent AYP values of more than 9 Kg/mm while the comparison steels 9 show quite insufficient values of 5.0 Kg/mm and 7.7 Kg/mm respectively. This fact tells that the desired ob jects are hardly achieved if the heating requirements are outside the range of the present invention even if the treatment following said quenching are the same. Thus, the heating temperature should be selected from among the range of 700 to 900C, and more preferably from A point to 850C.

The steels 9 to 13 were investigated in respect of the influences of reheating temperatures, and steels 9 to 10 and 13 are the comparison steels while steels l 1 and 12 are the present invention steels. On these steels, the steel 9 was not given any reheating treatment, but shows remarkable lower value of TS, that is ATS is as low as 20.1 Kg/mm exemplifying a typical steel which is hard at the time of press forming and becomes soft as goods. Since the reheating temperature is 100C for the steel 10, TS at the time of press forming is as high as 58.2 Kg/mm However, TS remarkably lowers to 47.5 Kg/mm i.e., ATS, to l0.5 Kg/mm by the coating-baking after press forming and the steel is not a well balanced steel even if its AYP showed 10.1 Kglmm Thus, this steel 10 cannot be called one having good AA effects. Whereas, the steels 1 1 and 12 of the present invention maintain TS value as at the time of press forming while AYP rises to 10.9 Kg/mm and 7.9 Kg/mm respectively and TS El index also show good values of 75.6 and 73.4 respectively. It is evident that the steels are well balanced and satisfactory. Thus, it is understood from the above going that the reheating temperature should be selected from within the range of 150 to 400C and preferably 180 to 300C.

The steels 14 to 17 were investigated in respect of the influences of a holding time at the reheating temperature. Of these steels, the steel 17 is the comparison steel while the steels 14 to 16 are the present invention steels. As shown in the table, the present invention steels 14 to 16 show well balanced properties and have sufficient AA effects. On the other hand, the comparison steel 17 with a long treatment time of 10 minutes have a low YP value of 4.0 Kg/mm thus displaying absence of AA effects. The holding time at reheating temperature is to be selected from within the range of 2 to 300 seconds, corresponding to the particular reheating temperature being employed.

The steels 18 to 20 were investigated in respect of the influences of the compositions, and the steel 20 is the comparison steel while the steels 18 to 19 are the present invention steels. The steel 20 has been subjected to identical treating requirements, except for its higher C content by 0.15 percent than the present invention steels. As is seen from the table, it has a low TS El index of 71.5. This clearly will present difficulties in stable formability and in weldabilities. Accordingly, the steel should be without doubt avoided even if it presents satisfactory AA effects after baking. On the other hand, the steels within the range of the present invention compositon all show excellent TS El index and AA effects and are most satisfactory. Therefore, it becomes known that C content in steel should be within 0.12 to 0.04 percent range and should be varied depending upon the level of the strength required.

The steels 21 to 23 are the present invention steels and were investigated in respect of Mn influences. They show that the strength improved by Mn increase does not necessarily invite the lowering of elongation and does not invite the deterioration in TS El index. Thus, Mn is an effective element in the present invention since it doesnt act to deteriorate the mechanical properties but improves the strength directly and its amount is selected according to the level of the strength required as in the case of determination of C content.

As has been explained in the above references being made to numerous examples, the fundamental technical differences as well as the remarkable effectiveness of the present invention method over the conventional BISRA process of semi-continuous process and the IN- LAND Steel process of batch type annealing aiming at AA effects by nitrogen in steel exist in; (1) the differences between quenching of the strip to coiling temperature in the lead bath in BISRA method and quenching down the strip to the room temperature in the jet water stream in the present invention method, (2) the differences of precipitation of solution carbon by self annealing at the said coiling temperature in BISRA process and the retaining of solution carbon caused by a reheating treatment in the present invention, and (3) the differences of the action of nitrogen content in the IN LAND Steel type batch annealing where no aging effect caused by C content is possible and the controlling process solution carbon in the full continuous annealing process of the present invention (wherein N content is an unavoidable component in the steel making).

We claim:

1. In a process of making high strength coldrolled steel sheet comprising a steel-making stage, a hot-rolling stage, a cold-reducing stage and a full continuous annealing stage, an improved process for obtaining a steel having excellent accelerated aging properties characterized in that:

l. at the above steel making stage, C content and Mn content in steel are controlled within the ranges of 0.04 to 0.12 percent and 0.1 to 1.60 percent respectively,

2. at the above hot-rolling stage, a finishing temperature of more than 800C and a coiling temperature of less than 700C are maintained,

3. at the full continuous annealing stage,

a. a travelling strip is heated up within the range of 700 to 900C and is held at the above temperature for to 120 seconds,

b. said heated and held strip is rapidly cooled by a jet stream of water to room temperature,

0. said rapidly cooled strip is reheated to the range of 150 to 400C and held there for a period of time selected from the range of 2 to 300 seconds corresponding to the above reheating temperature, and

d. cooling and coiling the strip obtained in (c).

2. The process of claim 1, wherein said holding time at the reheating step of the full continuous annealing stage is from to 300 seconds with said reheating temperature of 150 to 180C.

3. The improved process as set forth in claim 1 wherein said holding time at the reheating step of the full continuous annealing stage is selected from the range of 4 to 300 seconds in case that said reheating temperature is set at the range of 180 to 300C.

4. The improved process as set forth in claim 1 wherein said holding time at the reheating step of the full continuous annealing stage is selected from the range of 2 to 200 seconds in case that said reheating temperature is set at the range of 300 to 400C.

5. The improved process as set forth in claim 1 wherein said holding time at the reheating step of full continuous annealing stage is selected from the range ature is set at the range of 180 to 300C.

of to 120 seconds in case that said reheating temper- UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. ,446

DATED September 9, 1975 INVENTOR(S) 1 KUNIKI UCHIDA et al It is certified that error appears in the above-identified patent and that said Letters Patent 0 are hereby corrected as shown below:

Column 1, line 10: before "strength", replace "is" with in Column 3, line 19: delete "ssteel";

after "steel", insert is 0 Columns 5-6, Table: under "Quenching", after "jet",

insert of Column 10, line 24: Before "solution", insert of Eng'ned and Scaled this second D3) Of March 1976 [SEAL] O Attest:

RUTH c. MASON c. MARSHALL DANN g Arresting Officer I Commissioner ofParents and Trademarks e E l I- .h...

Claims

1. IN A PROCESS OF MAKING HIGH STRENGTH COLD-ROLLED STEEL SHEET COMPRISING A STEEL-MAKING STAGE, A HOT-ROLLINGSTAGE, A COLD-REDUCING STAGE AND A FULL CONTINUOUS ANNEALING STAGE, AN IMPROVED PROCESS FOR OBTAINING A STEEL HAVING EXCELLENT ACCELERATED AGING PROPERTIES CHARACTERIZED IN THAT:

1. AT THE ABOVE STEEL MAKING STAGE, CCONTENT AND MN CONTENT IN STEEL ARE CONTROLLED WITHIN THE RANGES OF 0.04 TO 0.12 PERCENT AND 0.1 TO 1.60 PERCENT RESPECTIVELY,

2. AT THE ABOVEHOT-ROLLING STAGE, A FINISHING TEMPERATURE OF MORE THAN 800*C AND A COILING TEMPERATURE OF LESS THAN 700*C ARE MAINTAINED,

2. at the above hot-rolling stage, a finishing temperature of more than 800*C and a coiling temperature of less than 700*C are maintained,

2. The process of claim 1, wherein said holding time at the reheating step of the full continuous annealing stage is from 15 to 300 seconds with said reheating temperature of 150* to 180*C.

3. at the full continuous annealing stage, a. a travelling strip is heated up within the range of 700* to 900*C and is held at the above temperature for 10 to 120 seconds, b. said heated and held strip is rapidly cooled by a jet stream of water to room temperature, c. said rapidly cooled strip is reheated to the range of 150* to 400*C and held there for a period of time selected from the range of 2 to 300 seconds corresponding to the above reheating temperature, and d. cooling and coiling the strip obtained in (c).

3. The improved process as set forth in claim 1 wherein said holding time at the reheating step of the full continuous annealing stage is selected from the range of 4 to 300 seconds in case that said reheating temperature is set at the range of 180* to 300*C.

3. AT THE FULL CONTINUOUS ANNEALING STAGE, A. A TRAVELLING STRIP IS HEATED UP WITHIN THE RANGE OF 700* TO 900*C AND IS HELD AT THE ABOVE TEMPERATURE FOR 10 TO 120 SECONDS, B. SAID HEATED AND HELD STRIP IS RAPIDLY COOLED BY A JET STREAM OF WATER TO ROOM TEMPERATURE, C. SAID RAPIDLY COOLED STRIPSIS REHEATED TO THE RANGE OF 150* TO 400*C AND HELD THERE FOR A PERIOD OF TIME SELECTED FROM THE RANGE OF 2 TO 300 SECONDSCORRESPONDING TO THE ABOVEREHEATING TEMPERATURE, AND D. COOLING AND COILING THE STRIP OBTAINED IN (C).

4. The improved process as set forth in claim 1 wherein said holding time at the reheating step of the full continuous annealing stage is selected from the range of 2 to 200 seconds in case that said reheating temperature is set at the range of 300* to 400*C.

5. The improved process as set forth in claim 1 wherein said holding time at the reheating step of full continuous annealing stage is selected from the range of 20 to 120 seconds in case that said reheating temperature is set at the range of 180* to 300*C.