US 3784418 A
Cold-rolled strip and sheets are made from a special steel alloy containing chromium and molybdenum by subjecting hot-rolled strip formed from the alloy to multi-stage annealing at controlled temperatures and thereafter cold rolling and annealing after each cold rolling step.
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United States Patent [1 1 Randak et a1.
[ 1 Jan. 8, 1974 PROCESS FOR THE MANUFACTURE OF COLD-ROLLED SHEETS FROM A RUST-RESISTANT, FERRITIC STEEL ALLOY CONTAINING CHROMIUM AND MOLYBDENUM [751 Inventors: Alfred Randak; Karl Michel, both of HuttentalGeisweid, Germany  Assignee: Stahlwerke Sudwestfalen AG,
Huttental-Geisweid, Germany  Filed: Nov. 3, 1970  Appl. No.: 86,610
 Foreign Application Priority Data 1969 QE mE HYT'fliiflfilllil?  US. Cl. 148/12  Int. Cl C2111 9/48  Field Of SE3ICII..... 148/12, 12.1, 12.3,
rolling step. I d
 References Cited UNITED STATES PATENTS 2,851,384 9/1958 Waxweiler 148/12 3,067,072 12/1962 Leffingwell et al.. 148/12 3,139,358 6/1964 Graziano 148/12 3,490,956 1/1970 Wilton 75/126 R Primary ExaminerW. W. Stallar'd AttorneyBurgess, Dinklage & Sprung 57 ABSTRACT 13 Claims, No Drawings 1 PROCESS FOR THE MANUFACTURE OF COLD-ROLLED SHEETS FROM A RUST-RESISTANT, FERRITIC STEEL ALLOY CONTAINING CHROMIUM AND MOLYBDENUM BACKGROUND This invention relates to a process for the manufacture of cold-rolled strips and sheets from corrosionresistant, ferritic chromium steel with molybdenum added, having improved deep drawing characteristics similar to austenitic steel.
Deep drawing ability is considered as one of the importantcriteria in judging flat products. In the case of fine sheets, the steels used consists substantially of unalloyed ferritic steels, ferritic chromium steels and austenitic chromium nickel steels.
The austenitic chromium nickel steels and the lowcarbon unalloyed steels have excellent deep drawing characteristics, it being a rule that a reduction of the strength characteristics, especially the yield point, results in an improvement in deep drawing ability.
It is also known, however, that the mechanical-technological properties of flat sheet-like products are impaired by chromium alloying. Rust-resistant steels of the quality X 8 Cr 17 according to DIN Preliminary Standard 17440 have substantially poorer deep drawing ability than unalloyed steel types (Blech No. l l, 1965, page 627).
Although the above-mentioned X 8 Cr 17 grade of steel has a high corrosion resistance, it is necessary, where stronger corrosive attack is involved, to alloy about 1 percent molybdenum with the [7 percent chro mium. This molybdenum content gives this steel an improved resistance to attack by reducing and chloridecontainingcorrosive media, with which it comes in contact, for example, when it is used in atomobile parts exposed to road salt, industrial atmospheres, and the like. However, its more widespread use isimpeded by the fact that its deep drawing ability is even poorer than that of the 17 percent chromium steel.
In case of high corrosive and deep drawing stresses, therefore, herebefore it has been necessary to have re course to the austenitic chromium nickel steels. As previously mentioned, austenitic chromium nickel steels are superior to the ferritic chromium and chromiummolybodenum steels in regard to deep drawing qualities, but chromium nickel steels are substantially more expensive. In addition, there is the disadvantage of the so-called yellowishness, so that these steels have no gained popularity for automotive parts in particular;
Austenitic types of steel have been develped to relieve the cost situation, which makes it possible to reduce the nickel content by partially replacing this element with manganese and/or nitrogen. These steels are not widely used, either, however, because they are still too expensive andthe problem of yellowishness is stillencountered in them. Examples of such steels are AlSl 201 and 202.
It is known that the yield point and tensile strength can be lowered by'reducing the amounts of carbon and nitrogen present (Metallkunde," 1963, pp. 724 sqq.). This method has not become popular, either because an extensive reduction of the carbon content, such as might be expected to result in an indirect improvement in the deep drawing quality, results in an undesirable lengthening of the refining time with a resultant greater 2 loss of chromium, shorter furnace lining life, and the like.
A diminution of the carbon content in ferritic chromium steels by the so-called vacuum refining process is technically possible, but has not. gained popularity in the manufacture of strip, since the capacities needed for this purpose are not available. Furthermore, the use of this process entails an increase in cost.
ln view of the numerous and sometimes very different processes of sheet forming, it has proven to be difficult to find a universal measuring method for every possible type of cold forming, The so-called roundel ratio or maximum drawing ratio has established itself as a very meaningful index of the deep drawing ability. This parameter is defined by In the case of cylindrical drawing, this means the ratio of the starting diameter D of the sheet steel roundel to the diameter d of the finished drawn specimen (Maschinenmarkt, Vol. 74, 1968, No. 31, p. 561). In other words, the greater the maximum drawing ratio is, the greater is the depth of the cup that is drawn. Since the roundel diameter is an indication of the roundel area that is to be formed, if the roundel diameter is increased by about 10 percent, for example, about 20 percent more roundel area has to be formed, resulting in a corresponding increase in the cup depth. In practice, therefore, the object is to achieve the highest possible drawing ratios in one draw.
The maximum drawing ratio thus permits comparison of the deep drawing characteristics of various materials. Under the experimental conditions described in the literature cited above, thegreatest possible roundel ratio that can be achieved in ferritic steels with their poor formingqualities is approximately 1.6 in a single draw, while the austenitic steels are much better, with values of 22 maximum.
It is technically possible, of course, to achieve forms by a plurality of draws, but in such cases annealing has to be performed after each draw, followed by'pickling, so that there are financiallimits to this procedure.
Furthermore, the loss of surface quality due to the repeated annealing would necessitate more work in fin ishing the workpieces.
It is obvious that, in the case of suchstandardized testing procedures, if the material characteristics are constant, the roundel ratio is dependent upon the shape of the punch, the radii, the drawing oil, and other such factors. In the actual comparison of various types of steel, all of the drawing conditions must, of course, be constant. The deep drawing tests herein were performed on a hydraulic drawing press with a rated ca pacity of metric tons. A flat-faced punch was used, whihc had a diameter of 100 mm. The punch [edge] radus amounted to 10 mm, and the drawing die [edge] radius was 4 mm. A
With this experimental set-up, maximum drawing ratios ranging from 1.96 to 2.03 for the chromium molybdenum steel X 6 CrMo 17, were obtained while chromium nickel steel produced ratios of 2.16 to 2.25. As already explained, this is a considerable difference because, under these conditions, chromium nickel steels permit the forming of an approximately 20 percent greater roundel surface without cracking; consequently substantially deeper cups can, be drawn than with chromium steel. It is known, therefore, that the deep drawing of ferritic steels involves an appreciable expenditure on multiple draws, annealing between draws, and surface finishing operations, since, in many cases, chromium nickel steels cannot be substituted for the reasons given earlier.
SUMMARY The present invention involves a new process whereby the deep drawing ability of ferritic chromium steels containing molybdenum is so substantially im proved that it is similar to the austenitic chromium nickel steels. That the fieldof application of this type of steel is thus substantially expanded.
The improvement in deep drawing qualities is brought about by, according to the invention, by the application of the following steps:
a. For themanufacture of the strips and sheets according to this invention the following steel composition is used, containing:
0.005 0.12% carbon 1.0% silicon 0.3 3.0% manganese 15.0 20.0% chromium 0.15 3.0% nickel 0.5 1.5% molybdenum 0.05 0.20% nitrogen 0 0.10% aluminum Balance: iron and impurities, and this composition gives an F factor of 3.3 to 6.0 if the following equation is applied:
F ['80 %C) 1300 %N) 150 %Ni) 25 %Mn) 8 %Cu)]/[3.6 %Cr) 9 %Si) 7.6 (%Mo) 75 %Al)] b. These steels are subjected, in the form of the hotrolled strip, to a multi-stage box annealing in the range of 970 to 600C, the temperature being lowered stepwise by at least 30C in each step, and
c. Annealing is performed between the individual cold rolling steps and after the final step at temperatures of 820 to 650C, with holding times of seconds to 10 minutes. followed by cooling.
All percentages given herein are by weight unless indicated otherwise.
DESCRIPTION Extensive experiments have shown that the steel composition used in this invention in conjunction with special annealing measures in the hot-rolled state and during the cold working process make possible a great improvement over the art as regards the deep drawing capacity of this type of steel.
The steel composition used in this invention comes partially within the scope of the molybdenum alloy 17 percent chromium steels of the following composition:
Cr Approx. 17%
Mo Approx. 1.0%
The standards for this type of steel composition varies as follows:
German Preliminary Standard DIN 17440:
M0 0.9 1.2% Balance: iron and impurities. USA Standard A.l.S.l. Type 434:
BaL: iron and impurities.
Known chromium steels contain, in their regular commercial form, a maximum of about 0.30% nickel, about 0.03% nitrogen, and a maximum of 0.20% coper. The aluminum content runs up to 0.05%. On the basis of these common alloying admistures, factors of F 2.5 are obtained when the above equation is applied.
However, it is necessary according to this invention to achieve F values of 3.3 to 6.0. This is achieved by increasing the normal contents of carbon, nickel, manganese, copper, and especially nitrogen used in alloying such steels. The decided improvement in deep drawing qualities which is achieved by this in combination with the other two measures described herein is unexpected. Whereas the state of the art teaches improving deep drawing qualities by diminishing the amounts of carbon and nitrogen present, an impressive improvement over the prior-art figures is achieved unexpectdely by increasing the quantity certain components previously thought to be harmful, and by the simultaneous application of the two special annealing measures described herein.
According one embodiment of the process, it is advantageous if the F factor computed according to the above equation equals 3.4 to 5.3. It is furthermore advantageous to select the alloy composition such that F is between 3.5 and 4.5, and preferably between 3.7 and 4.3.
The box annealing of hot rolled strips and sheets of chromium steels and chromium molybdenum steels, which is designed to eliminate the linear arrangement of the ferrite grains and carbides produced as a result of the preceding hot rolling process, it customarily performed under inert gas in the 850 to 800C range (Bander Bleche Rohre," Duesseldorf, 2, 1963, p.61 The box annealing of hot rolled strip is normally performed in one step. Multi-step annealing processes can be used if desired.
A multi-step hot rolled strip annealing process for ferritic chromium steels prevents the occurrence of the so-called riffle structure and improves the mechanical-technological characteristics (German Auslegeschrift No. 1,222,520).
The solution of the problem to which the invention is addressed is achieved by the precise selection of the steel composition through the alloying quotients and the simultaneous use of stepped box annealing of the hot rolled strip and intermediate annealing after one of the cold-rolling steps in some cases and, in any case, final annealing after the last cold-rolling step.
The possibility of raising the [3 value in 17% chromium steels above the normal level has not previously been possible.
In a number of publications other multiple annealing processes have become known, which are intended to eliminate the linear arrangement of the hot-rolled strip structure in ferritic chromium steel to which no molybdenum is added. This improvement is achieved by a more or less long annealing of the hot-rolled strip, preferably in a temperature range between 950 and 1,100C, followed by cooling to room temperature, and
followed again by annealing in 'thetemperature range of around 805C which is customary for ferritic chromium steels. The publications in question are U.S. Pat. Nos. 2,772,992, 2,808,353 and 3,139,358. From these patents is itnot possible to learn how the [3 value can be raised above the normal level in the case of 17% chromium molybdenum steels.
The multi-step hot-rolled strip annealing prescribed according to theinvention, in the form of three-step annea1ing,consists of the followingprocess steps: 1
1. Annealing for 30 min to hours at 970 800C.
2. Coolinginfurnace by at least C to a temperature of 850 600C.
3. Annealing of 5 to hours at 850 to 600C.
The preferred temperatures are 920 850C for the first processstep, and 740 650C for the third step. According to an additional emodiment of the invention, the mu1ti-step-hot-ro11ed strip annealing can consistof a continuous five-step treating process entailing the following steps:
l. Annealing for3O min to 15 hours at 970 -800C.
2. Cooling iri furnace by about 30C to a temperature processes described in detail above, other hot-rolled strip annealing processes involving greater numbers of steps come withinthe scope of the teaching of the invention. Nevertheless, the upper limit of 970C, the minimum of30C for each step in the temperature reduction, and the lower temperature limit of 600C must be observed.
These measures are novel'for 17% chromium molybdenum steels and so advantageousin their effect that s the longer boxannealing time is amply compensated.
The annealed andpickled. hot-rolled strip can be cold-rolled by known methods.
Cold-rolling hardens the chromium molybdenum steel and thereforeithas to be annealedprior to cold forming or deep drawing operations. The technical expert is familiar with the fact that higher temperatures result in *better ductility in the finished strips, butloss of surface quality places limits on the temperature that canbe used. Temperatures of 850C are rarely exceeded." To protect the surface the steel strips are annealed in a continuous furnace and then pickled in chemical baths.
Since relativelyhigh strip speeds must be used for reasons ofoutput arid cost (e.g., ata strip thickness of 1 mm and a furnace length of about 30 m, the speed is greater than 15 m/min, resulting in an annealing; time of 1 Ssec), the temperature has to be raised faster and the holding time has to be shortened accordingly in order to to assure perfect annealing. Usually, therefore, short periods are sufficient, depending on the strip thickness, to transformthe strip to a ductile state, al-
though in the case of chromium molybdenum steels temperatures of 850 820C are necessary in orderto achieve the state of optimum formability at short annealing times (e.g., max. 10 min for a strip 5 mm thick).
Surprisingly, the process of the invention succeeds with substantially lower temperatures. Contrary to the technical teaching that has been known and described above, this steel has to be annealed at temperatures of 780 to 700Cin order to have the maximum ductility and outstanding deep-drawingcharacteristics.
An additional embodiment of the process calls for a preferred annealing range of 770 720C. Temperatures of 750 730C are also possible. It is sufficient that the strip beheld at these temperatures for 5 seconds to. 10 minutes. It is therefore advantageous to perform this annealingin continuous furnace, although stationary annealing process can also be used.
It is surprising in this invention that, in spite of the fact that it runs counter to known procedures and re sults in severalrespects, it results in an improvement in deep drawing characteristics that goes beyond the state of theart. The involves especially in alloying the elements carbon and nitrogen, and in the appreciable lowering of the intermediate and/or final annealing temperatures below the customary levels.
Nevertheless, the combination of these measures with the special box annealing process produces a new effect, which consists in animprovement of deep drawing ability in 17% chromium molybdenum steels to an extent unknown hitherto.
The improvement in deep drawing properties is also produced when the final annealing is performed such that 1 to 10% transformation structrures are present in addition to ferrite in the finished strip. This effect, with hfi slsshnkal ass isjf i arimis k P to 666" vfiien steels having a 'y to 60 transformation are heated so high and so long that the 7 region is present at least partially, and are then cooled in such a manner thatthe state of equilibrium the structure is forestalled.
This individual measure is described in a similar manner in German Pat. No. 1,188,109, and serves in that patent to eliminate the so-called flow lines sensitivity. Whereas it is necessary in this patent to operate with relatively high temperatures, the combination of the intermediate and/or final annealing (according to German Pat. No. 1,188,109) with the present invention has the advantage that the effect of the elimination of low figures occursat temperatures as low as the lower limit or below the customary final annealing temperature. In addition, a further improvement ofthe deep drawing qualities is possible. And furthermore, the dressing operation is eliminated which is normally necessary in the porduction of ferritic chromium molybdenum steels and this has a very noticeable positive effect as regards costs, since this dressing operation requires special rolling trains equipped with rolling and grinding devices and the like.
With the process of the invention it is possible to produce ferritic chromium molybdenum steels whose deep-drawing characteristics are comparable to the substantially more expensive. austenitic chromium nickel steels. Using the experimental deep-drawing press described above, it has been possible easily to achieve maximum drawing ratios of approximately 2.25 with numerous stripsand sheets manufactured according to the invention;
The invention is further described in the following examples:
CONTROL 1 The Prior Art A melt of the following chemical composition: C 0.05%
Mo 1.0% N 0.031%
was prepared in an electric furnace, cast into slabs, and rolled in a roughing mill to a strip 4 mm thick.
The application of the formula given above showed the F factor to be 1.136. The hot-rolled strips were annealed in a stack in the box annealing furnace for a period of 34 hours at 840C, then pickled, and then coldrolled to 1.5 mm thickness. Then the material was annealed at 840C for about 20 sec and pickled. Afterward, the strips were rolled again to a final thickness of 0.6 mm, treated for about 15 sec at 850C in the continuous annealing furnace and then pickled. After the continuous annealing process the strips were sensitive, in intermediate and final thickness, to the occurrence of the flaw known as flow figures. The material therefore had to be re-rolled twice in final thickness in a dressing machine.
Flow-figure susceptibility was found to be absent in the final examination for release. A deep drawing test of the material on the press described above produced B values of 2.0 in the 1.5 mm thickness, and in the finished state the maximum drawing ratio was B 2.02.
Another pickled hot-rolled strip from the same melt was cold-rolled to 0.6 mm without any intermediate annealing treatment, then annealed for 15 sec at 850C and pickled. On account of its susceptibility to flow figures, this strip also had to be dressed in two passes, and in the finished state it had a maximum drawing ratio B 2.03
CONTROL 1 The Invention A melt of the following composition:
Mo 0.87% N 0.096%
was rolled, as in control 1, to 4 hot-rolled strips of a thickness of 4 mm. The use of the equation showed an F factor of 4.103. Two sets of two each of these hotrolled strips were subjected to a three-step box annealing procedure as follows:
Run 2.]: The strips were annealed for 3 hours at 880C, then cooled in the furnace by 170C to 710C and held at this temperature for 20 hours. The total annealing time amounted to 41 hours.
Two more sets of two hot-rolled strips from the same melt were treated by a five-step box annealing procedure as follows:
Run 2.2: These strips were annealed for 2.5 hours at 900C; after furnace cooling by C down to 740C they were held at the latter temperature for 6 hours, furnace-cooled again by 40C down to 700C and held at this temperature for 15 hours. The total annealing time was 43 hours.
All of the hot-rolled strips were pickled. One strip 2.11 and strip 2.21 were cold rolled to 1.5 mm thickness and, in the roll-hard state, they were divided into two halves. The one half 2.1 1 1 and 2.21 l was annealed for about 20 sec at 760C, then rolled down to a final thickness of 0.6 mm and divided into two halves.
The second hot-rolled strip -2. l2 and 2.22 was coldrolled directly to 0.6 mm and also divided into two strips while in the roll-hard state. The one half of the cold-rolled strip in each case, in the 0.6 mm thickness, was given a final annealing for about 15 sec at 760C. Examination of the material in the finished state showed a ferritic structure and flow-figure susceptibility in all strips of the 1.5 and 0.6 mm thicknesses. The finished strips therefore had to be dressed in two passes. Then the flow-figure susceptibility was eliminated.
Testing for deep drawing ability showed maximum draw ratios B between 2.21 and 2.23 for the strips in the 1.5 mm thickness. In all of the finished strips of the 0.6 mm thickness, maximum draw ratios B of 2.24 to 2.26 were found after direct rolling and also after twostep rolling through the intermediate thickness.
The strip halves in the intermediate and final thicknesses which had been cut in the roll-hard state, Nos. 2.112, 2.12, and 2.122, 2.222, 2.1112 and 2,2112, were subjected to another separate annealing. The strips 2.112 and 2.212 of the 1.5 mm thickness were annealed 25 sec at 810C and cold-rolled to 0.6 mm; all strips of the 0.6 mm thickness were treated at 810C in the continuous annealing furnace and force-cooled. After pickling, all of the strips in the finished state showed transformation structure along with 3 to 6% ferrite. All strips were free of flow figures and therefore they could be cut into plates and shipped to fabricators without any additional dressing procedure. In this state, maximum draw ratios B of 2.24 to 2.27 were found.
EXAMPLE 2 The Invention A melt of the chemical composition Mo 0.85% N 0.06%
was made, as described in Example 1, into cold-rolled strip of the 0.6 mm thickness, some of it through the 1.5 mm intermediate thickness and some of it by direct rolling. The alloying quotient (F) of this melt was 4.169. In the case of the separate final annealing and forced cooling which was used in part, the making of this melt into cold-rolled strip differed only in that, to eliminate the dressing procedure, the strips were annealed at a temperature of 800C with the same holding times as in Example 1, and after this separate annealing and forced cooling they were found to have 3 to 5% transformation structrue.
The rest of the strips or portions of strip prepared according to the invention, which had been treated to create transformation structure, were subjected according to the invention to a final annealing in the 730-740C range, cooled normally, and were ferritic as well as susceptible to flow-figure formation, so that they then had to be dressed.
What is claimed is:
1. Process for the manufacture of cold-rolled strips and sheets from corrosion-resistant ferritic chromium steel with molybdenum added which comprises forming a hot-rolled strip from an alloy having the composition:
0.005 0.12% Carbon 0-l.0% Silicon 0.3-3.0% Manganese l5.0-% Chromium 0.l5-3.0% Nickel 0.5l.5% Molybdenum 0.05-0.20% Nitrogen 0-0.50% Copper 0-0.l0% Aluminum Balance: iron and impurities, said alloy having an F factor equal to 3.3 6.0 calculated using the equation F= [8O %C) 1300 %N) 150 %Ni)+( (%Mo) 75 %Al)] annealing the hot-rolled strip in a plurality of steps below 970C and above 600C, the temperature being lowered step-wise but at least C during each annealing step, cold-rolling said strip and after each coldrolling step, annealing at temperatures of 820 650C and at holding time of 5 sec to 10 min. and cooling the annealed material.
2. Process of claim 1, wherein said alloy has an F factor between 3.4 and 5.3.
3. Process of claim I, wherein said alloy has an F factor between 3.5 and 4.5.
4. Process of claim 1, wherein said alloy has an F fac- 10 tor between 3.7 and 4.3.
5. Process of claim 1, wherein before cold rolling a three-step annealing process consisting of the following steps is carried out:
a. Annealing for 30 min to 15 hours at 970800C.
b. Cooling by at least 30C to a temperature of 850 600C. c. Annealing for 5 to 40 hours at 850 600C. 6. Process of claim 5, wherein step (a) is carried out at a temperature between 850 and 920C. and step (c) is carried out at temperaturesbetween 740 and 650C. 7. Process of claim 1, wherein before cold rolling a five-step hot-rolled strip annealing process consisting of the following steps is carried out:
a. Annealing for 30 min to 15 hours at 970 800C. b. Cooling by at least 30C to a temperature of 850 c. Annealing for l to 40 hours at 850 700C.
d. Cooling by at least 30C to a temperature of 750 e. Annealing for 5 to 40 hours at 750 600C.
8. Process of claim 7, wherein step (a) is carried out at temperatures between 920 and 850C, step (c) is carried out at temperatures between 800 and 720C, and step (e) is carried out at temperatures between 730 and 650C.
9. Process of claim 1, wherein annealing after at least one cold-rolling step is carried out at a temperature between 780 and 700C.
10. Process of claim 9, wherein the annealing is carried out at a temperature between 770 and 720C.
11. Process of claim 9, wherein the annealing is carried out at a temperature between 750 and 730C.
12. Process of claim 1, wherein crharacterized the annealing after at least one cold-rolling step is held in the 820650C range for a period of time and is cooled in a manner such'that after cooling, 1-l0% transformation structure is present in the endproduct.
13. Process of claim 1, wherein the annealing after at least one cold-rolling step is carried out in continuous furnaces after at least one cold-rolling pass.