US3668016A - Process for producing cold-rolled steel plate high in the cold-formability - Google Patents

Abstract

COLD-ROLLED STEEL HAVING EXCELLENT COLD-FORMABILITY IS MADE BY POURING INTO A MOLD A MOLTEN STEEL HAVING A COMPOSITION OF C$0.07 WT. PERCENT, 0.04 TO 0.20 WT. PERCENT MN, 0.004 TO 0.020 WT. PERCENT S, THE RATIO OF MN TO S BEING AT LEAST 7, NOT MORE THAN 0.0030% N, AND THE BALANCE IRON AND IMPURITIES. THE MOLTEN STEEL IS ALLOWED TO RIM IN THE MOLD FOR A TIME INTERVAL. THEN THE CORE OF THE MOLTEN STEEL IS KILLED BY ADDING AL TO THE MOLTEN STEEL AFTER THE TIME INTERVAL, SUCH THAT THE ACID-SOLUBLE AL IN THE REMAINING MOLTEN STEEL IS MORE THAN 0.010% TO

OBTAIN AN INGOT. THE INGOT IS THEN SUBJECTED TO HOTROLLING, COLD-ROLLING RECRYSTALLIZING AND ANNEALING.

Description

June 6, 1972 MINEO SHIMIZU ETA!- 3,553,016

PROCESS FOR PRODUCING COLD-ROLLED STEEL PLATE'HIGH I IN THE CQLD-FORMABILITY Fued March 5, 1969 3 Shuts-Shut 1 O I.8 O

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UQMMWNXMM PM ATTORNEYS June 6, 1972 MINEO SHIMIZU ETA!- 3,663,016

PROCESS FOR PRODUCING COLD-ROLLED STEEL PLATE HIGH IN THE COLD-FORMABILITY Flled Maren 5, 1969 3 Sheets-Shoot 2 FIG. 2

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INVENTORS M in ac Sh I'm in Hirosh/ Ta/rechi H/rvyu/r/ Kajio/ra M/rww awa/Iamda ATTORNEYS J1me 1972 MINEO SHIMIZU ETAL 3,668,016

PROCESS FOR rnonucme com-momma sum:- PLATE men 4 IN THE cow-romsxm'mr Flled March 5. 1969 .3 Shots-53! s INVENTORS MINEO SHIMIZ'U HIROSHI TAKECHI HIROYUKI KA M-INORU KAWAl-IARADA ATTORNEYS United States Patent Ofiice 3,668,016 Patented June 6, 1972 3,668,015 PROCESS FOR PRODUCING COLD-ROLLED STEEL PLATE HIGH IN THE COLD-FORMABILITY Mineo Shirnizu, Hiroshi Takechi, Hiroyuki Kajioka, and Minoru Kawaharada, Kitakyushn, Japan, assignors to Nippon Steel Corporation, Tokyo, Japan Filed Mar. 3, 1969, Ser. No. 803,669 Claims priority, application Japan, Mar. 2, 1968, 43/ 13,459 Int. Cl. 322d 25/06 US. Cl. 148-2 1 Claim ABSTRACT OF THE DISCLQSURE Cold-rolled steel having excellent cold-formability is made by pouring into a mold a molten steel having a composition of C0.07 wt. percent, 0.04 to 0.20 wt. percent Mn, 0.004 to 0.020 wt. percent S, the ratio of Mn to S being at least 7, not more than 0.0030% N, and the balance iron and impurities. The molten steel is allowed to rim in the mold for a time interval. Then the core of the molten steel is killed by adding Al to the molten steel after the time interval, such that the acid-soluble Al in the remaining molten steel is more than 0.010% to obtain an ingot. The ingot is then subjected to hotrolling, cold-rolling recrystallizing and annealing.

This invention relates to a cold-rolled steel plate having few surface flaws and excellent cold-formability and a process for producing the same.

In the case of forming a cold-rolled steel plate by a pressing operation, it is required that the steel plate should be nonageable and should have good deep-drawability and extrudability depending on the use.

If a cold-rolled steel plate shipped from a rolling mill after being tempered and rolled ages before it is subjected to a press working, there will be produced a nonuniform irregular pattern called a stretcher strain when it is pressed, which will remarkably impair the appearance of the formed product and will reduce the press-workability. Therefore, it is requisite that the material should be nonageable.

The main cause of such ageing phenomenon is considered to be N solid-dissolved in the steel and can be prevented by adding an element which will combine with N. Among such additive elements, cheap Al is used in most cases. An aluminum-killed steel containing more than 0.02% acid-soluble Al (which shall be referred to as sol. Al hereinafter) is typical thereof.

However, in the case of producing such steel plate, if required Al is added into a ladle or into a mold when pouring a molten steel, A1 will mainly gather near the skin of an ingot, whereby not only the surface properties of the ingot will be deteriorated, but also the surface flaws of the rolled product will be produced and the yield of the product will be remarkably reduced.

As a measure of preventing such surface flaws it is already known to add Al immediately or several minutes after a rimmed steel is poured into a mold. (The thus produced steel shall be called a rim-stabilized steel here.) In this ingotting method, however, it is necessary to regulate the contents of C and Mn and the like from the necessity of causing a strong rimming action to take place at the time of pouring a molten steel, but on the other hand, the cold-formability (the deep-drawability and extrudability here) of the product must be deteriorated by this regulation. Therefore, this contradiction presents a serious problem in the industry.

The present invention seeks to solve the above described problem and has for an object to provide a rimstabilized steel which has favorable surface properties and excellent cold-forrnability.

Other objects of the present invention will be clear from the following description and accompanying drawings.

In the accompanying drawings:

FIG. 1 shows influences of the Mn content on the '1 value of the final product when only the Mn content was varied; I

FIG. 2 shows influences of the absolute values and combination of the Mn and S contents on the F and n values of the final product when both the Mn and S contents were varied;

. FIG. 3 is a photograph of a corner sample of an ingot made by a conventional method and having an Mn content of 0.30%;

FIG. 4 is a photograph of a corner sample of an ingot according to the present invention.

The present invention shall be described more particularly in the following.

In producing a rim-stabilized steel, according to an ordinary ingotting method wherein Al is added immediately or several minutes after a molten steel to be made a rimmed steel is poured, it is possible to prevent A1 0 from mainly gathering near the skin of an ingot and the surface defect from being caused by A1 0 In this case the thickness and soundness of the rim layer coagulated before Al is added are important in relation to the surface flaws. It is considered that a range of 0.07 to 0.10% C is desirable to obtain a sound rim layer.

If the above described ingotting method is applied to a molten steel of less than 0.07% C, the rimming action is weak and there remain many small bubbles as shown in the photograph (steel with 0.050% C, 0.30% Mn and 0.018% S) in FIG. 3, whereby the soundness of the rim layer is remarkably reduced. The bubbles distributed near the ingot skin are subjected to an oxidation during the soaking heating and cannot be pressed into contact, causing thereby the formation of surface flaws of the rolled product.

However, from the view point of the formability of the final product, 0 content should be as low as possible and is required to be below 0.07%

In order to solve such contradiction, the present inventors have discovered that, when the C content is reduced, only the Mn value in the ladle is related with the removal of the generated bubbles and that, when the Mn content is held to a low value which is not conceivable in the usual conventional cold-rolled steel plate, that is, less than 0.2%, the bubbles near the skin of the ingot (steel with 0.052% C, 0.17% Mn and 0.016% S) vanish as shown in the photograph in FIG. 4, thus the problem of the surface flaws in the product may be solved. I

Then, the influence of the reduction of the ladle Mu value to less than 0.20% on the formability of the product. becomes an important problem.

FIG. 1 shows the relation between the F value and the Mn content of a product manufactured from an ingot of rim-stabilized steel of 0.006% S, said steel ingot being prepared by melting so that the Mn content thereof may be varied, but other chemical components not intentionally varied, by subjecting said ingot to following steps: blooming, hot-rolling, cold-rolling at a reduction rate of 70% and to recrystallization annealing at 700 C. for 4 hours.

The F value, which is also called a plastic strain ratio, is a mechanical characteristic represented by the ratio of the width strain to the thickness strain of a tension test piece and is known to show a very favorable correlation with the deep-drawability of a metal plate. The

F value is a mean value in each direction in the plane of the plate.

The larger the F value, the higher the deep-drawability. It has a value of about 1.3 in an ordinary rimmed steel or about 1.6 in a higher aluminum-killed steel.

As is clear from FIG. 1, there is recognized a definite relation between the F value and Mn content. When the content is about 0.10%, the F value will be the maximum.

As described later, unless a sufiicient amount of Mn is contained in relation to S, the material will break or crack due to the phenomenon of red-hot brittleness during the hot-rolling.

The F value of the materials on the low Mn side in FIG. 1 is that converted from a measured value for a final product obtained from a material partly cracked in hot-rolling as a mother material, said measured value being obtained by quantitatively measuring the crystal surface parallel with the plate plane of the final product by X-rays. Because the sample to be used for the X-ray measurement is large even though it is smaller than that needed for a mechanical test, even a. partly cracked material can be used as a sample, while avoiding the cracked part. Further, as is described in detail, for instance, in the Japan Metal Society Journal, vol. 29, No. 4, 1965, the relation between the value measured with X-rays and the F value measured from the mechanical test is very close. In such ordinary steel plates, as for example, the current Japanese Industrial Standard G 3310 (1965) SPC 3 (steel plates to be deep-drawn), the Mn content is defined to be 0.25 to 0.45%. However, from the results in FIG. 1, it. can be said to be advantageous to the F value to further reduce the Mn content.

However, the present inventors have simultaneously discovered that the F value is greatly influenced not only by the Mn content but also by the S content. That is to say, FIG. 2 shows the relationships of the F value and 11 value with the Mn and S contents, when treating in the same manner as in the case of the material of FIG. 1, 57 rim-stabilized steel ingots obtained by various combinations within ranges of 0.02 to 0.40% Mn and 0.003 to 0.030% S. It is clearly recognized from this graph that the F value and n value are influenced by both Mn and S. In the graph, the F value and the n value are represented respectively by round and square marks on the same material. However, in this graph, the position of the round mark indicates the right position. The square mark is drawn to show the n value is placed beside the corresponding round mark. The round mark having no square mark therebeside represents that the material was broken due to red-hot brittleness and the n value could not be measured.

The n value so called here is also called a work hardening index and is known to have a good relation with the extrudability of the metal plate. The n value corresponds to the index n when the true stress (a)strain (e) curve obtained by the tension test of the material is approximately a'=Ce (wherein C is a constant) and is generally determined by the average gradient of the log. c'10g. c curve in the strain range of to The larger this value, the higher the extrudability. It is about 0.22 in an ordinary rimmed steel or aluminum-killed steel.

As is described later, the steel of the present invention is made so that the sol. A1 contained in the steel may be more than 0.01%. However, this is about the same as in an ordinary aluminum-killed steel and is suflicient to make the steel nonageable.

However, in an ordinary aluminum-killed steel, when the acid-soluble N (sol. N) contained in the steel is about the same as in the steel of the present invention, that is, 20 to p.p.m., it is very difiicult to obtain an F value of more than 1.5. On the contrary, it is a great feature of the present invention that a favorable F value reaching 2.0 can be obtained by reducing the Mn content and balancing Mn and S.

From the results shown in FIG. 2 it is concluded that for the Mn content a range of 0.04 to 0.20% (preferably 0.08 to 0.18%) and for the S content a range of 0.004 to 0.020% (preferably 0.008 to 0.018%) should be selected, if an F value exceeding 1.6 which is an approximate average level of the F value of an aluminumkilled steel, that is, a typical high grade deep-drawable steel plate, and an n value of more than 0.22, which is an approximate average level of the n value of the same, are set as targets to be attained. When Mn and S were contained in these ranges, an F value of 1.7 to 2.2 and n value of 0.26 to 0.34 of the product were obtained. At present, conventional steels of the same kind contain Mn and S in the ranges of about 0.30 to 0.40% and 0.010 to 0.025% respectively and have an F value of 1.4 to 1.8 and an 11 value of 0.20 to 0.25, which clearly demonstrates the superiority of the present method, when compared with the above-mentioned values of the steel of the present invention.

On the other hand, in metallurgy there is phenomenon called a red-hot embrittlement, in which an embrittlement of a steel is caused when it is hot-rolled. This is considered to be caused by the fact that S in the steel is reticulately deposited as FeS around an initial crystal. As a countermeasure for preventing this phenomenon there has been widely adopted a practice of adding Mn in an amount matching S.

Because of the segregation of S in an ingot there has been heretofore empirically carried out the steps of adding Mn in an amount such that the ratio of Mn to S; 10 in the case of a rimmed steel. However, in the case of a rim-stabilized steel, the segregation of S is so little and 0 content in the steel is so low that the value can be lower than in a rimmed steel.

It is found also from the results in FIG. 2 that, when the ratio of Mn to S is at least 7 the material is not broken.

In collective consideration of the above-mentioned, it is evident that the Mn and S contents for obtaining sound cold-rolled steel plates high in the F and n values are to be found in the range hatched with diagonal lines in the graph.

The range hatched with diagonal lines in FIG. 2 shall be called the designated range of FIG. 2 hereinafter.

C content has an influence on the formability, as abovementioned, and should be as low as possible. Therefore, the C content as a ladle composition is made 0.07%. However, in consideration of the economy of the steel producing operation and the surface quality of the product, the C content as a ladle composition is made more than 0.03%. In order to improve the workability of rimmed steel, such steel has been decarburized by an open coil annealing system. But, it is a feature of the method of the present invention that, without subjecting the steel of the present invention to such a decarburization treatment, a very high press-formability can be obtained.

Further, it is to be noted that a conventional aluminumkilled steel usually contains more than 0.020% acid-soluble Al. This is, of course, an amount too large for the putrhprge of making the steel plate nonageable by combining W1 However, this amount is based on the idea of simultaneously elevating the F value by forming AlN in an op timum amount. Therefore, in the case of the present invention in which the F value is increased by the balance of Mn and S, and Al is added only for the purpose of obtaining the nonageability of steel, the content of more than 0.010% acid-soluble Al in the product is sufiicient for the said purpose.

A method of carrying out the present invention shall be described in the following.

A molten steel made in a converter or open-hearth furnace and having a ladle composition of C 0.07% and Mn and S within the designated range of FIG. 2 is toppoured or bottom-poured so as to be a rimmed steel. In pouring it, some shot Al is used in response to the state of the rimming action. After a desired coagulated layer (rim layer) is obtained, Al is added into the molten steel in the mold so that the sol. Al in the remaining molten steel may be more than 0.02% and the molten steel is coagulated. A cold-rolled steel plate is made from it through respective blooming, hot-rolling, cold-rolling and annealing steps by a conventional process. That is to say, it is hot-rolled at a temperature above the Ar point, is coldrolled at a reduction rate in a range of 50 to 90% and is then annealed at a temperature above the recrystallizing temperature.

In the case of manufacturing the steel of the present invention Al is added after a desired rim layer has been obtained when making a rimmed steel from the molten steel, which prevents the formation of any surface fiaw to be caused by A1 partially gathering near the ingot skin by the sound rim layer which is first obtained by making Mn 0.20% as described above.

The amount of Al to be added difi'ers depending on the property of the molten steel, the pouring condition and the time used for adding Al. However, in case sol. Al is 0.02%, it is necessary to add more than 0.7 kg./ton of Al. The time of adding Al is somewhat different depending on such pouring method as top-pouring or bottom-pouring and on the pouring rate but is generally determined by taking into consideration both the thickness of the rim layer required when rolling an ingot and the refloating of A1 0 series impurities.

As to the amount and time of adding Al they are not particularly specified in the present invention, because they are strongly influenced by Working conditions.

The thus produced product contains C in the range of 0.06 to 0.02% and Mn and S in the ranges substantially not varied, but still such that the ratio of Mn to S 57, that is, in the designated range of FIG. 2. Therefore, the object is attained thereby. Even if C is made less than 0.02%, the product cannot be expected to improve in quality. Therefore, C is defined to be than 0.02%.

EXAMPLE 9 tons of a molten steel of 0.06% C, 0.14% Mn and 0.010% S made in a converter were bottom-poured into a downwardly expanding flat mold and 0.9 kg./ton of Al was added thereto immediately before the completion of the pouring to produce an ingot. This ingot was hot-rolled, was then cold-rolled to be 0.8 mm. thick at a reduction rate of 68% and was annealed at 710 C. for 5 hours to obtain a cold-rolled steel plate of 0.050% C, 0.14% Mn, 0.01% S, 0.038% acid-soluble Al and 0.014% P, the rest being Fe and impurities. The surface test result and coldformability in this case as compared with those in a conventional method are shown in Table 1. The conventional method here designates a treatment of a rim-stabilized ingot containing 0.07% C, 0.31% Mn and 0.015% S by the same process as is mentioned above. By the following comparison the effect of the method of the present invention can clearly be demonstrated.

What is claimed is:

1. A process for producing cold-rolled steel plates having excellent cold-formability comprising the steps of pouring into a mold a molten steel made in a converter or open-hearth furnace and consisting essentially of a composition of C 50.07 Wt. percent, 0.04 to 0.20 wt. percent Mn, 0.004 to 0.020 wt. percent S, the ratio of Mn to S being at least 7, not more than 0.0030% N, and the balance iron and impurities; allowing said molten steel to rim for a time interval in said mold; then killing the core of said molten steel by adding Al to the molten steel after said time interval, so that the acid-soluble AI in the remaining molten steel may be more than 0.010% to obtain an ingot; and subjecting the ingot to hot-rolling, cold-rolling, recrystallizing and annealing.

References Cited UNITED STATES PATENTS 3,239,390 3/1966 Matsukura et al. 14812 3,496,032 2/1970 Shimizu et al. l4812 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R.

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