US3806373A

US3806373A - Process for producing cold-rolled steel plates high in the cold-formability

Info

Publication number: US3806373A
Application number: US00330651A
Authority: US
Inventors: M Shimizu; H Takechi; H Kajioka; M Kawaharada
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-03-02
Filing date: 1973-02-08
Publication date: 1974-04-23
Anticipated expiration: 1991-04-23

Abstract

PROCESS FOR PRODUCING COLD-ROLLED STEEL PLATE HAVING GOODSURFACE PROPERTIES AND EXCELLENT COLD-FORMABILITY FROM A RIMMED STEEL OR CAPPED STEEL CONSISTING OF LESS THAN 0.10%C, LESS THAN 0.4% MN AND LESS THAN 0.02% S, THE RATIO OF MN TO S BEING MORE THAN 10, AND THE REST BEING IRON AND IMPURITIES BY SUBJECTING AN INGOT TO HOTROLLING WITH A TEMPERATURE AT THE FINISH WHICH IS ABOVE THE AR3, TRANSFORMATION POINT, COILING AT A TEMPERATURE BELOW 570*C., AND COLD-ROLLIG, AND FINALLY TO RECRYSTALLIZATION ANNEALING BY HEATING IN A NON-DECARBURIZING ATMOSPHERE AT AN INCREASING TEMPERATUREE AT A RATE OF FROM 5-50* C./HR.

Description

United States Patent Ser. No. 330,651

Claims priority, appgc/atiog Japan, Mar. 2, 1968,

Int. Cl. mid 25/06 US. Cl. 148-2 Claims ABSTRACT OF THE DISCLOSURE Process for producing cold-rolled steel plate having good surface properties and excellent cold-formabihty from a rimmed steel or capped steel consisting of less than 0.10% C, less than 0.4% Mn and less than 0.02% S, the ratio of Mn to S being more than 10, and the rest being iron and impurities by subjecting an ingot to hotrolling with a temperature at the finish which is above the Ar transformation point, coiling at a temperature below 570 C., and cold-rolling, and finally to recrystallization annealing by heating in a non-decarburizing atmosphere at an increasing temperature at a rate of from 550 C./hr.

This application is a continuationdn-part of application Ser. No. 803,611, filed Mar. 3, 1969, now abandoned.

This invention relates to a process for producing coldrolled steel plates having few surface flaws and excellent cold-formability (deep-drawability and extrudability in the present invention).

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

In making an ingot from a molten steel when producing a cold-rolled steel plate, the steel is metallurgically largely classified into a killed steel, semi-killed steel or rimmed steel according to its deoxidized state. The killed steel is a steel which has been fully deoxidized and has excellent properties, but it has a drawback that it iscostly. 0n the other hand, rimmed steel has much oxygen in the molten steel so that it may combine with C simultaneously present in the molten steel and a rimming action may be caused by CO gas evolved in thev mold. Rimmed steel is inferior in its properties to killed steel, butis so cheap that it is used in large quantities. H

Further, steel in which a part of the rimming action is mechanically or chemicallycontrolled is called capped steel, which is close to rimmed steel in its properties and price. In order to increase the rimming action and to obtain a sound rim layer, it is considered desirable that 0.07 to 0.10% C be contained in the molten steel.,On the other hand, C is an element detrimental to the coldformability of the product and from this viewpoint it is desirable that it be lower.

However, if C in the molten steel is reduced to less than 0.07%, the rimming action is weak, and many small bubbles remain in the ingot whereby the soundness of the rim layer is greatly reduced. These bubbles distributed near the skin of the ingot are subjected to oxidation during the subsequent soaking heating, and can not be pressed out, causing thereby the formation of surface flaws in the rolled product.

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The present invention seeks to solve these contradicting problems of the surface properties and cold-formability at one stroke and has as an object to provide a steel plate in which both of these properties are excellent.

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

In the accompanying drawing:

FIG. 1 shows the influences of the absolute amounts and combinations of various Mn and S contents in a rimmed steel and capped steel on the F value and n value of the final product.

The present invention will be explained more particularly in the following.

The present inventors have discovered that for a steel which hasa C ladle content less than 0.07% so that the product formed therefrom has good cold-formability, but which steel has insufficient rimming action, resulting in inferior surface properties of the product, the bubbles near the ingot skin'can be greatly reduced, the problem of the surface flaws in the product can be eliminated and the cold-formability can be further greatly improved. This can be done by reducing the MN in said steel to a value so low as to be inconceivable from the standpoint of common sense in any conventional cold-rolled steel plate, that is, less than 0.02%. With regard to a steel which has a high C ladle content such as 0.07 to 0.10% and therefore has a sufficient rimming action, and consequently no problem exists with respect to the surface properties, but which steel is inferior with respect to cold-forrnability, the cold-formability of such steel can be made far superior to any conventional product of today by regulating the Mn and S contents so that they are within the hereinafter described range and by processing the steel, i.e. hot rolling, cold rolling, coiling and annealing under the proper conditions.

The deep-drawability and extrudability of a cold-rolled steel plate are represented usually respectively by an F value and an n value.

The F value, which is also called the 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 or F 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.

The n value so called here is also called a work hardening index and is known to have a good correlation with the extrudability of the metal plate. The n value corresponds to the index n when the true stress (u)-stra.in (e) curveobtained by the tension test of the material is approximately 0=Ce (wherein C is a constant) and is generally determined by the average gradient of the log. -log. 2 curve in the strain range of 10 to 20%. The larger this value, the higher the extrudability. It is about 0.22 in an ordinary rimmed steel.

FIG. 1 shows the relations between the F value and n value for the Mn and S contents of products manufactured from 43 kinds of ingots of rimmed steel and capped steel, said ingots having combinations of the Mn content and S content within the ranges of 0.02 to 0.40% and 0.003 to 0.030% respectively, with chemical components other than Mn and S remaining unchanged. The products were made subjecting said ingots to the following steps: blooming, hot-rolling, cold-rolling at a reduction rate of 70%, coiling, and lastly to recrystallization annealing in a neutral atmosphere at 700 C. for 4 hours. 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 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 drawn to show the n value is placed beside the corresponding round mark.

On' the other hand, as described later, unless Mnis present in an amount ofmore than times'S, the material will breakor crack due to the phenomenon of redhot brittleness during the hot-rolling.

The i 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 amaterial partly 'cracked in 'l'hot rollin g' as a mother material, saidfmeasu'red value being obtained by quantitatively measuring'the crystal sur face parallel with the plate plane. of the final product by X-rays. Because the sample to be used'for the X '-"'ray meas- .',u'r ement is large" enough even though it is smaller than that needed for a mechanicaltest.even'apartly cracked material can be used as'a sample, while avoiding the icracked part. Further, as is described in detail, for instance, in the Japan Metal Society'Iournal, Vol.29, No.4,

given for the sample broken by 0.08 to 0.18%) and for an S content a range. of 0.004 to 0.020% (preferably 0.008 to 0.018% should be selected if an i value exceeding 1.3, which is an approximate average level of the F value for a rimmed steel or for a capped steel of a rimmed steel or for a capped steel, and an n value of more than 0.22, which is an approximate average level of the n value of the same, are set astargets to be attained. When Mn and S were contained in these ranges, and the conditions under which the steel was made were as described hereinafter, an F value of 1.3 to 2.0 and nvalues of 0.26 to 0.32 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 7 value of 1.1 to 1.4 and an n value of 0.20 to 0.24, 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 a phenomenon called red-hot embrittlement, in which embrittlement of the 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 the S.

Because of the segregation of S in an ingot, there has been heretofore empirically carried out the step of adding Mn in an amount such that the ratio of Mn to S210 in the case of a rimmed steel or a capped steell' However, if this relation should be applied to the optimum range in FIG. .1, the ratio of Mn, to'S will be within'the part hatched with diagonal lines. In fact, it is evident from this'graph that any sample containing Mn and S Withinthis' hatched range has a very favorable cold-formability. The range hatched with diagonal lines shall be called the designated range of FIG. 1 hereinafter.

C in an amount of 0.l0% is adopted as a ladle composition of an ordinaryrimmed steel or capped steel. It has already been attempted by various means to improve the cold-formability by adding a special element to an ordinary rimmed steel. However, according to the results of the research of the present inventors, unless a special producing step is taken, the additionof V and B must, on'the contrary, reduce the 7 value. Therefore, if the steel of the present invention is to be made nonageable, these elements must not be added. Further, in order to improve the workability, the recrystallizing annealing is often carried out in a decarburizing atmosphere. However,

in the steel of the present invention, it is possible to obtain a very good workability even without carrying out such a specialtreatment. This is very advantageous to the producing cost.

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 C0.l0% and Mn and S within the designated range of FIG. 1 is top-poured or bottom-poured so as to be a rimmed steel. In this case, it may be made a capped steel by mechanically and chemically controlling the rimming action. A coldrolled steel plate is made from the thus obtained ingot through respective blooming, hot-rolling, cold rolling and recrystallizing annealing steps by a conventional process. The steel is hot-rolled at a temperature such that the temperature at the finish of the hot-rolling is above the Ar point, and is cold-rolled at a reduction rate in a range of 50 to By keeping the finishing temperature for the hot rolling above the Ar;, transformation point, the development of crystals of steel which have random crystal orientation and have a ()-orientation harmful to deep drawability is prevented.

The steel must then be cooled to below 570 C. and coiled whereby Mn and S contained in the steel can be held in solid solution, and they are prevented from being precipitated. The cooling can be by conventional water spray means. Then the steel which has been thus hotrolled, cooled and-coiled, is cold-rolled and then subjected to a recrystallization annealing. In the annealing step, the steel is slowly heated at a heating rate of 5-50 C./hr. in a non-decarburizing atmosphere. The heating must be to a temperature above 500 to 550 C., and should be no higher than the Ar transformation point, about 860 C. The higher the temperature within this range, the better for the formation of a (111) texture. If the steel is left in the coiled condition for the annealing, it is preferred that the upper limit of the temperature not exceed 750 0., since sticking may occur in the coil. The most preferred temperature is 700 to 710 C. The time at the upper limit should be at least 10 minutes, but if subsequent grain growth is desired, a longer time should be used. Usually 4 hours gives a satisfactory result.

Applicants have found that when the steel contains the components and is subjected to the sequence of treating steps under the conditions described above, the manganese and sulfur compound which is precipitated is predominantly B-Mn/S in finely divided form. Because the Mn/S is predominantly present in the [3 form, the collective structure is given a (111)-orientation, which is desirable for enhancing the deep drawability of the steel. Some a-Mn/S will be precipitated, but in amounts which are insufiicient to adversely affect the properties of the steel as compared with the effect produced by the amount of 5-Mn/S.-

Applicants have further found that the fl-Mn/S, which has a zinc blend ore type crystal structure closely resembling wurtzite ore type structure, will be precipitated in inclusions which have a size below 0.5, and that these are combined with crystals of the metal into crystals having a (111)-orientation which contribute to the improvement of the 7- value of the steel, whereby an 7 value of more than 1.8 to1.9 can be obtained.

When components are present in amounts other than as described above and the steps are not carried out as described, a type Mn/ S is precipitated, which has a rock salt type structure which is detrimental to the 7 value.

EXAMPLE 13 tons of a molten steel of 0.07% C, 0.12% Mn and 0.009% S made in a converter were top-poured into a downwardly expanding flat mold to produce a rimmed or capped ingot. This ingot was hot-rolled, was then coldrolled with a reduction rate of 70% and was annealed in a neutral atmosphere at 700 C. for 4 hours to form a cold-rolled steel plate 0.8 mm. thick. Its chemical composition was as follows: for the rimmed steel, 0.04% C, 0.11% Mn, 0.007% Si, 0.009% S and 0.013% P, the rest being Fe and unavoidable impurities, and for the capped steel, 0.051% C and the others substantially the same as in the rimmed steel. The surface tests results and cold-formability in this case as compared with those in steel with a conventional composition and made by a conventionalmethod are shown in the table.

The conventional composition and method here designates a treatment of a rimmed steel ingot containing 0.07% C, 0.33% Mn and 0.018% S by the same process as is described above. By the following comparison the effect of the method of the present invention can be clearly demonstrated.

(In the method of the present invention, the upper line shows the results of the rimmed steel and the lower line shows the results of the capped steel.)

What is claimed is:

1. A process for producing cold-rolled steel plates having excellent cold formability, comprising the steps of ingoting a molten steel made in a converter or openhearth furnace and having a composition of C0.10%, less than 0.2% Mn and less than 0.02% S, the ratio of Mn to S being greater than 10, and the balance iron and impurities, as a rimmed steel or capped steel, hotrolling the ingot, cooling the hot-rolled steel to a temperature no higher than 570 C. and coiling the steel, coldrolling the steel at a reduction rate of about 70% and then subjecting the cold-rolled steel to a recrystallization an-- nealing by heating the steel at a rate of 5-50 C./hr. up to a temperature in a range of from about 500 C. to the Ar;; point for a period of from 10 min. to 4 hours.

2. A process as claimed in claim 1 wherein said molten steel has C in an amount of 0.10 to 0.070%, Mn in an amount of 0.04 to 0.20%, and S in an amount of 0.004 to 0.020%, and the balance being Fe and impurities.

3. A process as claimed in claim 1 wherein said molten steel has C in an amount less than 0.070%, Mn in an amount of 0.04 to 0.20%, and S is an amount of 0.004 to 0.020%, and the balance Fe and impurities.

4. A process as claimed in claim 1 wherein said recrystallization step comprises heating the steel to 700 C. for about 4 hours.

5. Cold rolled steel having excellent cold formability, said steel being produced by the process comprising the steps of ingoting a molten steel made in a converter or open-hearth furnace and having a composition of C0.10%, less than 0.2% Mn and less than 0.02% S, the ratio of Mn to S being greater than 10%, and the balance iron and impurities, as a rimmed steel or capped steel, hot-rolling the ingot, cooling the hot-rolled steel to a temperature no higher than 570 C. and coiling the steel, cold-rolling the steel at a reduction rate of about and then subjecting the cold-rolled steel to a recrystallization annealing by heating the steel at a rate of 5-50" C./hr. up to a temperature in a range of from about 500 C. to the Ar point for a period of from 10 min. to 4 hours.

References Cited UNITED STATES PATENTS 3,668,016 6/1972 Shimizu et al 148-2 2,878,151 3/1959 Beall et al. 14812 RICHARD O. DEAN, Primary Examiner US. Cl. X.R.