SE406089B - SOFT COAL STEEL AND WAY TO PRODUCE THIS - Google Patents

Description

7306999-9 use of niobium, tantalum, vanadium, boron, or titanium as carbide and / or nitride-forming elements in order to obtain non-aging unsealed steel with good deep drawing properties. Reference may be made to U.S. Patent Nos. 2,999,749 (E.R. Saunders et al.), 3102831 (N.F. Tisdale) and 3183078 (T. Ohtake et al.) To summarize previous work in the art.

U.S. Patent No. 3,521,210 (M. Shimizu et al.) Discloses a process for producing cold rolled steel which is said to be non-aging and has excellent deep drawing properties. The steel contains from more than 0,001% to less than 0,020% carbon, less than 0,45% manganese, less than 0,015% oxygen, less than 0,007% nitrogen, from more than 0,02% to less than 0,5% titanium (with the exception of titanium present as titanium oxides), and the rest iron. The steel may contain sulfur in amounts of less than 0.05% and small amounts of aluminum. Titanium must be present in amounts greater than four times the carbon content. The method involves hot rolling the material at a temperature higher than 780 ° C, cold rolling with a reduction of more than 30%, and annealing at a temperature of 650-1OC0 ° C. Continuous annealing is stated to give better properties in the product.

U.S. Patent No. 3,607,456 (J.L. Forand, Jr.) discloses a steel which is attributed to having excellent deep drawing properties in the cold rolled and annealed state and an .ASTM grain size of 6.0-9.0. The steel consists mainly of 0.020% maximum carbon, 0.60% maximum manganese, 0.010% maximum nitrogen, 0.015% maximum oxygen, 0.15-0.30% titanium, with the rest mainly consisting of iron.

A maximum of 0.03% sulfur may be present, and aluminum may be present in small amounts. The weight ratio of titanium to the sum of the carbon and nitrogen contents must be at least 7: 1. The product is produced by hot rolling, finishing at a temperature above s43 ° C, cooling and hasping within a temperature range of 4a2-649 ° c, cold rolling with 50-85% reduction, and batch annealing within a temperature range of 843 ° C alpha. gamma conversion temperature.

British Patent Specification 1_192794 (in the name of Nippon Kokan KK) describes a method of producing mild carbon steel which is attributed to being substantially non-aging and having good deep drawing properties, which comprises that the carbon content of an unsealed molten steel is reduced to less than 0.02 % by vacuum degassing, addition of carbide formers, forming of cold-rolled sheets, and annealing of the sheets at 700-950 ° C. The carbide former is titanium, vanadium, niobium, tantalum, zirconium, uranium, hafnium or thorium, and must be quenched in sufficient quantity to reduce the dissolved carbon content at the annealing temperature to less than 0.002%. In the case of titanium, the content must be more than four times the carbon content.

It is clear from the above-mentioned patents that titanium has long been considered a highly effective element for eliminating aging and preventing floating figures in mild carbon steel. However, the titanium-treated steels, manufactured in accordance with previous technical processes, have rather low tensile strength in the cold-rolled and annealed state. This will be seen from the data given below, according to which the average yield strength of titanium-treated steels typical of the prior art is about 303 MN / m2 in the cold-rolled and annealed state.

The Forand patent discloses the addition of an excess of titanium, at least a portion of which will go into solid solution.

This is evident from the claim for a minimum of 0.15% titanium in Forand.

U.S. Patent No. 3,102,831 (NF Tisdale) discloses a process for producing sealed, semi-sealed or unsealed steel containing from about 0.005% to about 0.050% niobium according to which ingots, slabs or rods are heated to a temperature above 1260 ° C, hot rolled with a completion temperature of 843-95S ° C, quenched to below 649 ° C, and then allowed to cool in air at a normal rate. The steels contain 0.02-0.50% carbon, 0.005-0.5% silicon, 0.15-1.6% manganese, 0.005-0.050% niobium, phosphorus and sulfur in residues, with the rest consisting of iron.

U.S. Patent No. 2999,749 (ER Saunders et al.) Discloses a process for preparing non-aging unsealed steels which involves adding to a molten steel additives containing at least 25% manganese and at least one of niobium, tantalum, vanadium and boron. in an amount sufficient to bind with the present nitrogen. Small amounts of a deoxidant, such as zirconium, titanium, beryllium, magnesium, aluminum, calcium, silicon and / or barium, may be incorporated into the additive.

The description of a niobium-treated steel, initially mentioned by the inventors of the present invention, provides a material and method which provides a wide range of properties in either the hot-rolled or cold-rolled conditions, resulting in the niobium addition in precipitation hardening effects which results in reduced extensibility. unless the carbon content has been reduced to a low level and the hot-rolled strip is finished and reeled at a high temperature. Furthermore, the product is relatively expensive due to the relatively high niobium to carbon ratio.

A main object of the present invention is to provide a non-aging mild carbon steel which has substantially no floating figures and which in the hot-rolled state exhibits high extensibility, good formability and low yield strength substantially independent of the coiling temperature and a "total carbon content, which steel in the cold-rolled condition has excellent elongation at break values and high breaking-to-yield strength ratios and which in the cold-rolled continuously annealed and hot-dip coated metal exhibits high elongation at break values and has a high average plastic elongation ratio.Another object is to produce a steel with the above properties with a significant reduction in the niobium and titanium or niobium and zirconium additives compared to the amount of each element required to achieve comparable properties if used alone.

The objects of the present invention are achieved by providing a vacuum degassed and deoxidized mild carbon steel to which niobium and titanium or zirconium additives are made in accordance with the following relation: When titanium and niobium are used, the amount of titanium must be equal to or less than 4 x weight percent carbon + 3.43 x weight percent nitrogen, with the exception of titanium as titanium oxides. This can be expressed as: (i) Ti - 5 * - m <1 atf "T 'T' 'd lg - 1 e un an ag Or 1. SOm J fl- Oxl er, C + 14 N where 12 is the atomic weight of C and 14 is the atomic weight of N. The titanium to carbon ratio is thus equal to or less than 4: 1, with the exception of titanium such as titanium oxides and titanium nitrides.The amount of niobium must be: (2a) greater than 0,025% by weight of __ 'l '.i ___. = i1.

C + äå N 1 or the amount of niobium must be: (Zb) greater than 0,025% by weight + 7§75 Weight% Ctotal - _ (weighted Ti-anls weightmn (4) 7306999-9 s F Ti 4 if C + 22 N <.ï 14 provided [vika ctèycal _ Weight * Ti; 3-43 Weight * Nš-I fl kooa to 0.004 weight c.

In (2b) above, it should be observed + = gas that the factor following 0.025% by weight represents the amount of niobium required to bind with the part of the total carbon which is not already bound with titanium. As will be shown below, the precipitation hardening effect of niobium carbides is avoided if less than 0.003-0.004% by weight of carbon is so bound.

When zirconium and niobium are used, the amount of zirconium must be equal to or less than 7.6% by weight of carbon + 6.51xwt% nitrogen, with the exception of zirconium such as zirconium oxides and zirconium sulphides. This can be expressed as: (a) --_- Z ”E -MÉ C + yy N íf with the exception of Zr as Zr oxides and Zr sulfides, where 12 is the atomic weight of C and 14 is the atomic weight of N. The amount of niobium must be: (4a) greater than 0,025% by weight if Zr _ 7,6 lg 1 C + 14 N or the amount of niobium must be: (4b) greater than 0,025 weight + 'z fl sß-iktee ctotal _ J Gm L c + -1- 2-N 1 14 provided [fold c _ Weight * "'76 excess weight * Ni '<0.003: in 0.004 weight c. 7 7306999-9 FFI (4b) above it should be understood that the factor following 0.025% by weight represents the amount of niobium required to bind with a portion of the total carbon not already bound with zirconium. Weight% for Zr excludes Zr oxides and Zr sulfides. If the compositions of the alloys comply with the requirements set out above in (1), (2a) or (2b), or in (3), (4a) or (4b), the steels will have the following properties: In the hot-rolled state, the precipitation hardening is eliminated the effects observed in nine-barreled steels. In this connection, it should be noted that precipitation hardening is associated with niobium carbide formation in niobium-treated steels. It has now been found that the addition of titanium or zirconium in combination with niobium results in a preferential formation of titanium or zirconium carbides rather than niobium carbides. The hot-rolled thin bar will thus have properties essentially independent of the spinning temperature used in hot-rolling and essentially independent of total carbon content. The properties of principle of interest are: the absence of floating figures, the presence of which causes: undesirable banding breaks or interruptions; good formability and ductility combined with low yield strengths, high breaking conditions to yield strength and good extensibility.

While steels treated with titanium alone will have comparable properties in the hot rolled state, the steels of the present invention require significantly less titanium than those containing titanium alone. Since the titanium yield is relatively low (usually 60-70%), it becomes clear that less total loss occurs at the lower levels of addition required by the present invention, resulting in lower cost. When the steel is cold-rolled and batch annealed, the addition of titanium or zirconium in combination with niobium results in elongation at break values superior to those of steel-containing niobium only and average values of plastic elongation-equivalent equivalent-for steel-containing niobium only. The properties are characterized by: the absence of floating figures in the annealed state; high rm values, resulting in extra deep drawing quality; high breaking-to-yield strength ratios; excellent elongation at break The steel of the present invention has a finer grain size than steels treated with titanium alone. This is advantageous for certain applications of cold-rolled and batch-annealed materials, for example the avoidance of orange surface on drawn parts where the appearance is of great importance, such as for chrome-plated parts which require a surface finish of jewelery quality.

With the steel as cold-rolled and continuously annealed, or continuously annealed and by hot doping metal coated, the addition of titanium or zirconium in combination with niobium results in elongation at break markedly superior to steels containing niob alone only, and average values of plastic elongation ratios superior to those of steels. solely. The properties are characterized by: 7 absence of floating figures in the annealed state; high rm values, resulting in extra deep drawing quality; high breaking strength to yield strength ratios; excellent elongation at break. In its broadest range, the steel according to the present invention has the following composition in the ingot stage or the stage of hot-rolled strip, all percentages expressing% by weight: niobium 4-> 0, o2s-o, 12% titanium about 0.015-0.12% , with the exception of Ti or as Ti oxides zirconium about 0.028-0.18%, with the exception of Zr as Zr oxides and sulphides carbon f about 0.002-0.020% nitrogen 0% - about 0.008% manganese 0% - about 0.60% sulfur 0% - approx. 0.035% oxygen (total) AO% - approx. 0.010% aluminum (total) 0% - approx. 0.045% phosphorus residual content silicon residual content residual iron and impurities In the above-mentioned steel, all nitrogen is bound as titanium or zirconium nitrides, and all carbon exceeding 0.003-0.004% is bound as titanium or zirconium carbides. When zirconium is used, all sulfur is bound as zirconium sulphide.

The composition range in the cold rolled and annealed state will be substantially the same as indicated above for the ingot or hot rolled strip. However, it should be noted that when the material will be subjected to treatment conditions which tend to cause nitrogen uptake (e.g. annealing of cold rolled strip in tightly wound belt rings in a hydrogen nitrogen atmosphere) it is within the scope of the present invention that add sufficient excess of titanium or zirconium to the molten charge to clear away the anticipated nitrogen uptake and thus prevent any significant formation of free nitrogen in the finished product.

The steel according to the invention is prepared by melting a batch of steel in any conventional manner with a maximum carbon content of about 0.05%, vacuum degassing of the steel to a carbon content of about 0.020% maximum, an oxygen content of about 0.010% maximum, and a nitrogen content of about 0.008% maximum, addition of titanium or zirconium in an amount calculated to be sufficient to react with all the carbon, nitrogen and oxygen (plus sulfur in the case of zirconium), addition of niobium in an amount sufficient to react more than 0.025% niobium in solid solution in the hot-rolled state, as determined by analysis of sheet metal at room temperature.

The degassed steel is then cast into ingots or strand cast, solidified, hot rolled to strip thickness terminating at ordinary temperatures of about 816 ° C to about 927 ° C, and reeled in accordance with standard practice. The hot-rolled product will then normally be pickled and cold-rolled to completion, and subjected to a finished annealing at ce 7os ° c to 7sa ° c in a batch annealing, or up to 900 ° C strip temperature at a continuous annealing.

The degassing step involves deoxidation by adding enough aluminum to eliminate excessive gas evolution before adding niobium and titanium or zirconium. Silicon or titanium could also substitute aluminum at this stage as a deoxidant.

It will be appreciated that the present invention differs from the aforementioned Shimizu et al. U.S. Pat. No. 3,521,210 in that it requires titanium (or zirconium) in combination with niobium, with a titanium content equal to or less than four times the carbon content plus 3.43 times the nitrogen content. The patent holders describe a composition containing 0.001-0.020% carbon and 0.02-0.5% titanium (with the exception of titanium as oxides), with the titanium content greater than four times the carbon content. The Shimizu patent does not disclose any steels within the scope of the invention, in 9 g the Wilka titanium content is equal to or less than four times the carbon content plus 3.43 times the nitrogen content. The aforementioned Forand U.S. Pat. No. 3,607,456 requires a minimum of 0.15% titanium in a steel having a maximum carbon content of 0.02% and a maximum nitrogen content of 0.010%, with the titanium content being a minimum of seven times the carbon plus nitrogen contents. In contrast, the present invention requires titanium (or zirconium) and niobium, with a maximum titanium content of 0.12% and equal to or less than four times the carbon content plus 3.43 times the nitrogen content.

While the above composition is given in wide ranges, preferred and more preferred ranges, resulting in optimal combination of properties, are as follows, with all percentages expressing% by weight: in Preferred More Preferred Niobium> 0.025-0.060%> 0.025 ~ 0.040% Titanium (excluding Ti 0.015-0.061% 0.015-0.045% as Ti oxides) or Zirconium (excluding 0.028-0.12% 0.028-0.085% Zr as Zr sulphides and oxides) Carbon o, oo2-o , o10% o, oo2_-o, oo6% Nitrogen o, ooz-o, oo6% o, oo, 2-o, oo6% Manganese 0% - 0.35% 0% - 0.35% sulfur 0% - 0 .02% 0% - 0,01% Acid (total) 0% - 0,004% 0% - 0,004% Aluminum (total) o, o ~ 1s-o, o2o% o, o1s-o, o2o% Phosphorus 0% - 0.010% 0% - 0.010% Silicon 0% - 0.015% 0% - 0, C15% Iron and impurities remained Preferably, in the above alloys, when titanium was used Ti 22 C + 14 N, 2 1 with the exception of Ti as Ti oxides; and when zirconium was used: Zr _ 7.6 was _ 1 C + 14 N with the exception of Zr as Zr oxides and sulfides. 7306999-9 'F - If zirconium is added in addition to 7.6 x weight percent carbon + 6.51 x weight percent nitrogen, it will bind with sulfur in the weight ratio of 1.42 zirconium z sulfur, whether or not enough manganese is present to bind with sulfur.

Since the present invention is intended to subject the molten charge to vacuum deoxidation and substantially complete deoxidation with aluminum or titanium, the amount of zirconia formed will be negligible. Although titanium and zirconium have essentially equivalent functions when added with niobium, from what has been said above, it will be apparent that there are some differences. It has been discovered that, unlike niobium, titanium and zirconium do not produce a precipitation hardening effect. On the other hand, titanium has only a very slight effect in slowing down recrystallization, while zirconium has a strong effect in slowing down recrystallization, comparable to that of niobium. In 7 Zirconium removes carbon, nitrogen and sulfur in the presence of niobium, manganese and aluminum. Titanium behaves similarly with respect to carbon and nitrogen. Titanium is a stronger carbide former than niobium. However, both titanium and zirconium react preferentially with nitrogen before carbon.

In the preferred composition ranges shown above, exemplifying compositions may be calculated according to formulas (1) and (2a) or (2b), or (3) and (4a) or (4b), which will exhibit the desired properties; As an illustration of this, the following table layout for titanium and niobium additives in which total carbon, nitrogen and niobium weight percentages are given; The percentage by weight of titanium includes the amount available to form carbides and nitrides, but excludes titanium as titanium oxides. 7306999-9 11 Required- For 0.003% N For 0.004% N For 0.005% N For row% Cb required% Ti required% Ti required 7G Ti 96 C 0.03 0.0225 0.0260 0.0295 0.003 0.04 0 , 0173 _ 0.0208 0.0243 0.003 0.05 0.0122 0.015? 0.0192 0.003 0.06 0.0070 0.01C5 0.0140 0.001 0.03 0.0345 '0.0380 0.0415 0.006 0.04 0.0293 0.0328 0.0363 0.006 0.05 0.0241 0.0276 0.0311 0.006 0.06 o, o1a9 o, o'224 o, o259 0.006 0.03 0.0456 0.0500 0.0535 0.009 0.04 0.04 0.0413 0.0448 0.0483 0.009 0, 0.0361 0.0396 0.0431 0.009 0.06 0.0309 0.0344 0 0.0379 0.009 The relationship of composition, and in particular the amount of unbound niobium, with the properties of a titanium and niobium-treated steel with varying carbon contents were examined. The carbon content of a steel roll ingot was increased from the base to the top of the ingot using a carbonaceous sink box compound after tapping.

Analysis and properties at varying carbon levels are given in Table I.

.T f MWS flfi NNM al wq MWM flfl mx Gwšmo HMN al etc. muæ f m> m um fi oxwnnu .Smšunwooum n f O fl ummco al w u flfl of Mao f w o. In mmww Nah 0 .l Amšmum mo: mwcmu flfi m oo .bcoum n fiflfl m f ocomo et al mum caøu WMM fl m> NNM> mon wh fi mnm xw mmamcmx al mxm M o wmoå o w f ood ææmoå ww al qo: Å o | ämoooå 2 o æwoâ o o oomoö wm fi oå på o o xxwßood l o æamoå Noooå o womoö wm fl qo omÄ o | ... unowooxo o: ooâ o omoâ o oomoå wm fl oå w fl à 2 .. m mmvoå xæmå Nm fi oö kwmoïo väom fl oå xmwoä. wæwozo umøcdb umncsn UQZ pmm UH. z fi o 02.1 »Eu fi ßææ Hm: E w Z U E. nä: men: mšo fi Emm Lä fi Eom _: öm _ 2 .:» | 0 Nmß Umm .u 02 lwx É. ... ä. o vag IQ .... mš fi mtå fi es? 3. “wow Umm fl mš v fl nnmwf n fl mxlm fl nou. H læuonmq H H fi wnme 7306999-9 7306999-9 1? F It should be noted that the highest carbon sample, where there was no unbound titanium and niobium but 0.0017% unbound carbon, showed a significant number of floating figures, both in the hot-rolled and cold-rolled and annealed states. In contrast, the sample containing 0.0076% carbon, with 0.028% niobium in solid solution, showed no floating figures in either the hot-rolled or cold-rolled and annealed states, and showed a marked increase in transverse value. In this connection, it can be explained that the absolute rm value for this sample would be about 2 if it had been subjected to cold rolling in the factory and annealing.

The magnitude of the individual r values is not significant, but the differences between the first and the last two samples show the elimination of floating figures and the marked increase in r values, resulting from the presence of more than 0.025% niobium in solid solution.

In Table I, the distribution of titanium as TiN and TiC and niobium as NbC was derived as follows: 3i-1T1N = -4-71 = ¿° f- ° -3.43 r .. 4790 ~ cšlmc = -ï = ä .- = 4, o mb. 929.1 -C-1Nbc = -ï = -2- = 7.7s Ti as TiN = 3.43 x% N Ti as TiC = 4.0 x% C .Nb as NbC 7.75 x (% Ct ° number - % C as TiC) The effect of coiling temperature and carbon content on the hot-rolled properties of niobium and titanium-treated steels has been investigated. Partial analyzes and mechanical properties of a number of melts are given in Table II. For comparison, several melts of only niobium-treated steel are also included. 7306999-9 V Table II Hasp- 0.5%% pre- _ _lings- O, O, elongation Melt Nb Ti CN tegp rS 2 B 2 at 5 cm C hN / m MN / m feed length 800556 0.066 0.076 0.0022 0 , 0053 704 170 324 45.2 800555 0.12 0.064 0.0038 0.0054 726 165 341 44.3 1254431 0.051 0.081 0.0055 0.0031 649 164 330 43.5 2260116 0.060 0.076 0.0068 0.0038 649 179 344 45.5 1254284 0.056 0.078 0.0104 0.0034 704 196 344 40.0 1254431 0.051 0.081 0.018 0.0031 649 196 356 39.5 800146 0.098 0 0.0028 0.0050 649 206 371 39.0 5967 0 , 11 0 0.004O 0.005 588 242 346 39.5 0.11 0 0.0040 0.005 707 223 336 40.5 290378 0.135 0 0.008 0.0058 499 344 458 28.0 0.135 0 0.008 0.0058 593 322 436 28, 0 0.135 0 0.008 0.0058 649 279 425 32.5 0.135 0 0.008 0.0058 704 216 352 40.0 In all the melts shown above, the percentage occurrence of floating figures of the hot-rolled thin bar was equal to zero. The oxygen content of all melts was typical of vacuum degassed material and averaged about 0.003%.

Niobium and titanium-treated steels and niobium-treated steels in Table II are listed in the order of increasing carbon content. It should be noted that the carbon contents in the range 0.0022-0.018% and the spinning temperatures in the range 649-726 ° C had very little effect on the tensile strength and elongation properties of hot rolled niobium and titanium treated steels. In contrast, in a niobium-treated steel with a carbon content above about 0.005%, low coiling temperature causes precipitation hardening. At lower carbon levels, however, the spinning temperature has little effect on the properties of the hot-rolled niobium-treated steel.

Partial analyzes and properties of cold-rolled and batch-annealed niobium and titanium-treated steels according to the present invention are set forth in Table III. A nominal cold reduction of 60% was performed in all samples. 7306999-9 1; V Table III Sample- 0, 5% 76 pre- position in og eyes long-band- 2 2 ning xx Melt Nb Ti CN ring MN / m MN / m at rm cm measure 800555 0.12 0.002 0.0035 0.0053 T (1 length1 163 308 48.0 1.94 & cross) (Ti des- 0.12 0.064 0.003B 0.0038 T (length 156 320 43.1 1.84 oxidized) & cross) 0% temper *** 600556 0.066 0.078 o, 0024 0.0050 F 131 305 48.0 2.03 0% temperxäx 0.069 - 0.0025 _ M 125 290 48.3 1.94 0.067 0.075 0.0020 0.0056 T 131 294 50.0 2 .09 210644 0.064 0.051 0.009 0, o0s2 4T (1d Length 161 309 43.8 1.96 (string- 0.064 0.051 0.009 0.0052 4F (lengthD 128 312 46.5 1.99 cast) (ladle analysis) 0% temper *** 1254431 gàO51 0.091 0.0055 0.0031 T 168 311 48.0 1.95 0.5% temper “1254431 0¿251 0.081 0.0058 0.0031 T 169 303 49.0 2.03 0.7 % temper f '_ -go .0093 _ F 205 308 48.0 1.94 1254284 g * g56 0.078 0.0086 0.0034 3T 168 309 46.5 1.93 0.7% temper _ -9 0.0104 _ 1F 176 310 47.0 1.05 2260778 0.041 0.049 0.005 0.0029 F 151 400 46.0 1.92 (skänk- 1 analysis) * xx T 125 291 49.0 2.07 0% temper 3T 134 301 48 .0 2.05 I Tensile Limit extension =: X * BE 9G !! 0% in all samples T = band tail (ingot base) F = band front (ingot top) M = band center rm = 1/4 ¿E (longitudinal joint + r (transverse joint) + 2r (diagonal joint7 "temper" in both tables III, IV and VI indicates a cold rolling, also called "skin pass", after 60% cold reduction. "0.7% temp" thus indicates that the cold rolled steel was subjected to a further reduction in thickness of 0.7%. "0% temper" indicates that it cold-rolled steel was not subjected to any further reduction in thickness, ie no leather pass.

Partial analyzes and properties of cold-rolled, continuously annealed and hot-dip galvanized niobium and titanium-treated steels according to the invention are shown in Table IV. From a comparative point of view, several cold-rolled, continuously annealed and hot-dip galvanized niobium-treated steels are also included.

It can be seen from Table IV that niobium and titanium-treated steels according to the invention have values for elongation at break (% elongation at 5 cm measuring length) and rm which are superior to those of niobium-treated steels. 7 Composition and properties of a niobium and zirconium- Table Iv Sample 0, 5%% precedence in oš oh long-band 2 2 Melting Nb Ti CN ring MN / m MN / m at rm cm measures 800555 0, 12 0.062 0.0035 0.0053 1T 195 322 43.5 -1.94 (Ti de-oxidized) * xx 4T 172 309 44.0 2.17 0.7% temper 800556 0.066 0.076 0.002 0.0050 21 138 295 46.5 2.42 0 7% thump *** 51 169 295 46.0 2.11 2260113 0.051 0.070 0.0066 _ F 124 285 49.5 2.16 0%, _ temper *** 0.004S - T 116 290 47.0 2.18 1254279 0.056 0.075 _ 11 123 293 45.0 2.06 0% temper *** 29 129 296 47.5 2.10 2250618 0.028 0.038 0.004 0.0042 F 131 292 43 1.92 (skänkanaåxå) 0% temper 490376 0.10 0 0.007 0.005 M 183 328 39.5 1.80 0% temperäxx .400854 0.12 0 0.008 0.0035 3F 158 328 38.5 1.75 0% temper *** '400854 0.120 0 0.008 i0.0035 28 216 326 42.0 1.80 1% cemper ***. 21 240 333 41.5 1.76 400853 0.11 0 0.008 0.0056 1M 219 326 40.5 1.79 1% temper *** 2M 224 327 40.5 1.73 3M 230 332 41.0 1, 69 *** see page 15 Flow figures = 0% in all samples Pure melt is given in Tables V and VI respectively. j7 7306999-9 Table V Product and Al test mode Nb Zr CNOS Mn (total) Hot rolled F 0.066 0.044 O, Û077 0.0072 0.0045 0.021 0.3 0.06 TO 067 0 048 0.0053 U, O060 0.0062 0.021 0.3 0.06 7 i 7 fïnïinïšäj F 0.066 0.05 o, uoba o, oos7 o, oo74 0.021 0.3 o, oe 9 9 T o, oe4 0.05 o, oo39 o, ooe1 o, o1o o, o19 0.3 o, os gal and galvanized Table VI O'5%% elongation- Sample- OÉ 2 0% 2 ning at rm% flow- Product position MN / m MN / m 5 cm measuring figures Hot rolled 292 400 35 , 8 - 1,0 T 206 354 41.5 - O Continuous. annealed and E 125 308 46.5 1.80 O galvanized T 119 306 45.5 1.72 O 0% temper Cold rolled and Batch 2F 145 319 43.0 1.97 0 annealed xx * 2M 181 328 43.5 - 0 0.3% temper *** see page 15 It can be observed that the front sample of the hot-rolled thin bar showed 1.0% floating figures. The correlation between mechanical properties and the calculated distribution of carbon, nitrogen, oxygen and sulfur among the elements niobium, zirconium, manganese and aluminum confirms the theory of the present invention. This can be demonstrated as follows: year. S 91 22 C 1 ZrC -T: ~ EE. 91 22 N J. ZrN = -4-14 Nb 12 i Nbç, âêiâl _ = 7.5 = 6.51 750699949 18 F Hot rolled Front 7 Tail 'o, o44 zr as zrm <0 ooevw) 0 039 zr as zrn ( 0 ooeou) 0.044 Zrtotal,% l% Qå_§§ as ZrC (0: 0012C) _ 4 Zr remained of N (0.000SN) as AlN, total 0.0045 0 as Al2O3 0.0062 0 as A1 03 0.021 s as mns - 0.021 s as Mnš 0.0596 Nb as NbC (0.0077C) 0.032 Nb as NbC (0.0041C) ° * ° 66 Nbtotal '9 # 9É1 Nbtotal 0'0o64 Nbickebundet O'O35 Nbickebundet% yPE = 1.0% % YPE = 0 60% lab.-cold rolled 60% lab.-cold rolled and lab.-annealed rm = 1.57 and lab.-annealed rm = 1.67 Continuously annealed and galvanized Front Tail 0.044 Zr as ZrN (0.0067N ) 0.0397 Zr as ZrN (0.0061N) 0.006 Zr as ZrC (0.0008C) 0.0103 Zr as ZrC (0.0013C) ° = ° 5 Zrtotai ° '° 5 Zrtøtai 0.0074 0 as Al2O3 0.010 O as Al 2 O 3 0.021 S as MnS in 0.019 S as MnS 0.039 Nb as Nbc (o, oosoc) 0.020 Nb total 'total o'027 Nbickebundet O'044 Nbickebundet% YPE = O% YPE = O rm = 1.8 rm = 1, 72 From the calculations shown above, it is clear that when the quantity of unbound niobium is less than 0.025% by weight (hot-rolled front sample) niobium and zirconium-treated steel have floating figures and a relatively low rm value. In all other samples, in which unbound niobium ranges from 0.027% to 0.044%, the product has no floating figures and thus is non-aging.

In the cold-rolled and annealed samples where the grain size was measured, it was found to be between grain sizes 8 and 10 according to ASTM. : An examination of the recrystallization response of the niobium and titanium treated steels of the present invention in comparison with steels containing titanium alone showed that the presence of niobium in solid solution significantly increases the recrystallization temperature as compared to titanium in solid solution. Table VII compares two melts containing titanium alone and two melts containing titanium and niobium, and it should be noted that an increase in the titanium content without any niobium present had no effect on the recrystallization temperature, while progressively increasing niobium contents increased recrystallization temperature. In all cases, recrystallization occurs through the formation of recrystallized grains distributed randomly throughout the cold-worked matrix. There was no evidence of progressive inward recrystallization from the sheet metal surfaces, which typically occur in niobium-treated steel.

In all melts according to Table VII, the RB hardness values were less than 40 after completion of recrystallization. Accordingly, there is an absence of a precipitation hardening effect in the steels according to the invention.

Table VII Recrystallization response Temp ° C Melt ta ï-Nb% Ti annealed '1 h 800553 0 0.12 593 - 100% non-recrystallized 621 - start of recrystallization, random 649 - almost complete recrystallization 677' - 100% recrystallization 800552 0 0 , 593 - 100% non-recrystallized 621 - start of recrystallization 649 - approx. 60% recrystallization 677 - 100% recrystallization 800556 0.067 0.075 621 - 100% non-recrystallized 649 - start of recrystallization, random 677 - approx. 50% recrystallization 704 - 100555 0.12 0.063 649 - 100% non-recrystallized, 677 - start of recrystallization, random 704 - approx. 80% recrystallization 732 - 100% recrystallization Modifications can be made without departing from the inventive concept. While, for example, the specific examples show niobium and titanium-treated steels, or niobium and zirconium-treated steels, it is clear that mixtures of titanium and zirconium can be added together with niobium. In this case, the calculation of the respective proportions of titanium and zirconium becomes somewhat more complicated.

Claims (4)

7306999-9 F Patent claims Mild carbon steel having essentially no floating figures in the hot-rolled and cold-rolled and annealed state, characterized in that it consists essentially of 0,002-, 020% carbon, 0% to 0,60% manganese, from larger than 0.25% to 0.12% niobium, at least one of titanium and zirconium, titanium currently present 0.015_0.12%, virconium present present 0.028-0.18%, 0% to 0.008% nitrogen, 0 % to 0.010% total oxygen, 0% to 0.035% sulfur, 0% to 0.045% total aluminum, residues of phosphorus and sulfur, with the remainder consisting of iron and impurities, the titanium content being equal to or less than four times the weight. % carbon plus 3.43 times weight% nitrogen excluding titanium as titanium oxide, zirconium content is equal to or less than 7.6 times weight% carbon plus 6.51 times weight% nitrogen excluding zirconium such as zirconia and zirconia, wherein there is more than 0.025% niobium in unbound form, and not more than 0.004% carbon bound with niobium. Steel according to claim 1, characterized in that it consists essentially of 0.002-0.010% carbon, 0% to 0.35% manganese, from greater than 0.025% to 0.060% niobium, at least one of titanium and zirconium, wherein titanium is 0.015-0.061%, zirconium 0.028-0.12%, with 0.002 -0.06% nitrogen, 0% to 0.04% total oxygen, A0% to 0.020% sulfur, 0% to 0.010% phosphorus, 0.015 -0.020% total aluminum, 0% to 0.015% silicon. Steel according to claim 1, characterized in that it consists essentially of 0.002-0.006% carbon, 0% to 0.35% manganese, from greater than 0.025% to 0.040% niobium, at least one of titanium and zirconium, wherein titanium is 0.015-0.045%, zirconium is 0.028-0.085%, with 0.002-0.006% nitrogen, 0% to 0.004% total oxygen, 0% to 0.01% sulfur, 0% to 0.010% phosphorus, 0.015-0.020% total aluminum, 0% to 0.1% kisei. A method of producing a steel according to any one or more of claims 1-3, characterized in that a batch of steel having a maximum carbon content of 0.05% is melted, vacuum degassing the steel to obtain a melt having a carbon content of 0.020% maximum, a total oxygen content of 0.010% maximum, a nitrogen content of 0.008% maximum, 0% to 0.60% manganese, 0% to 0.035% sulfur, with the remainder of iron and impurities, desolides by adding a des- ~ oxidant selected from a class consisting of aluminum, titanium and silicon, adds at least one of titanium and zirconium, titanium being used in an amount sufficient to obtain a titanium content in the hot-rolled state within a range of 0.015-0.12%, and zirconium when used is added in an amount sufficient to obtain a zirconium content in the hot-rolled state within a range of 0.028-0.18%, adding niobium in an amount sufficient to provide more than 0.025% niobium in solid solution in the warm water as determined by sheet metal analysis at room temperature, cast and solidify said steels, hot rollers to strip thickness having a final temperature of at least 816 ° C, and reels, the titanium content being equal to or less than 4 times the weight of carbon plus 3.43 times by weight of nitrogen with the exception of titanium as titanium oxide, the zirconium content is equal to or less than 7.6 times by weight of carbon plus 6.51 times by weight of nitrogen with the exception of zirconium as zirconium sulphide and zirconia, and not more than 0.004% carbon bound with niobium. PUBLICATIONS MADE: Germany 2 109 431