CS196442B2 - Low-carbon steel resistant against aging - Google Patents

Abstract

Non-ageing steel contains (%) 0.0025-0.015 C, 0.02-0.3 Nb, 0.1-0.6 Mn, =0.035 S, =0.01 O2, =0.012 N, =0.045 Al. The Nb/C ratio is between 8:1 and 12:1, such taht all the Nb is present as NbC. The Mn/S ratio is 7:1 to reduce the number of harmful inclusions. N is stabilised either by Al or Ti prior to the addition of Nb to the melt. The steel has higher ductility and formability than current non-ageing steels, and is suitable for most deep drawing operations.

Description

Author of the invention ELIAS JAMES ANDREW, MIDDLETOWN and at the same time HOOK ROLL-IN · EARL · ,. DAYTON. (Sp. St. A.) 'Patentee. (54) Non-aging low carbon steel

The invention is. concerns low carbon non - aging QQeli p. niobium, not showing. elongation at yield point in the annealed state it has. excellent surface properties and is. essentially free of non-metallic inclusions. with a wide range of mechanical properties, and a production method. of this steel.

Steel according to. invention. is particularly suitable in sheet form. k. deep drawing and. stretching and. hodí se'pro. . plating methods and. for. production. with ceramic. enamels.

U.S. Pat. No. 3,183, C, 7 * 8, issued. May 11, 1965,. of T. 'Ohtake and. spol.,. describes the method. production of non-aging steel for enamelling with good ductility. . According to this method it produces. molten steel containing. % by weight. less. than. 0.04%: carbon and. her . the other composition is otherwise comparable to conventional non-restrained steel, except for the preferred manganese content at most. 0.5%; molten. the steel is vacuumed to reduce the content. carbon to less than. 0.02%, less than 0.02%. sulfur and 0.002 to 0.007% nitrogen and aluminum and titanium are added. in an amount sufficient to bind carbon,. nitrogen and sulfur present in the steel.

French patent. No. 1,511,529,. of 18. December. 1967 companies'. . Yawata. Iron and. Steél Co. Ltd., describes a method similar to that of U.S. Pat

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by za. cold . rolled, sheet metal. Sh 'good deep drawing. and good. · .. According to this French ’. patent. the molten steel is evacuated with the addition of aluminum as a deoxidation. reagents,. to give a degassed steel containing in%. %, less than 0.020% carbon and less than 0.015%. Titanium is added in mass. 4: 1 ratios. to carbon and degassed. . the steel is then. cast, hot rolled. at. A finishing temperature exceeding 780 ° C. (1053 K). rolled for. cold at a reduction ratio greater than. %. and finally calcined at 650-1000 ° C (923-1273 K). .... ... ........

U.. low carbon steel. by. of the invention; is the average ratio of the permanent ratio. . extension. r normalized quantity, calculated as

r = = r / i. [r (longitudinal) -г-r (transverse) Ч-4.2r (diagonal)]. .

Considering. . disadvantages of increased costs resulting from practical. the addition of up to four times the amount of titanium. than theoretically. for example,. not reached vacuum degassed, titanium processed steel market acceptance in. a comparison with the restless and soothed steels to the deep. campaign, protaho196442

• for use · for coating and enamelling.

Abrahamson · a · spol. in the Transactions Metallurgical Society · of. ΑΙΜΕ, Vol. 218, December 1960, pp. 1101 to 1104, that niobium and zirconium substantially retard the rate of recrystallization during annealing of the cold-rolled material compared to other additive elements such as titanium and chromium. These findings were based on one-hour annealing tests increasing. temperatures during each annealing. No practical gains or advantages have yet been drawn from this knowledge.

This drawback is eliminated by non-aging low carbon steel, without yield strength, with good surface properties and free of inclusions, in the shape of hot or cold rolled strips of non-plating or metallized according to the invention, which consists in that it consists of. 0.002 and 0.15% by weight of carbon, from 0.025 to 0.30% by weight of niobium, from 0.05 to 0.60% by weight of manganese, from trace to 0.035% by weight of sulfur in. in the form of manganese sulphide, from traces up to 0.010% by weight of oxygen, from traces up to 0.012% by weight of nitrogen, from traces up to 0.08%. % by weight of aluminum, with a trace of up to 0.010% by weight of phosphorus and with a trace of up to · 0.015% by weight of silicon, the remainder being iron. wherein at least 0.025% by weight of the niobium is in an uncomplexed form when determining the amount of uncomplexed niobium present in the hot rolling condition at room temperature.

The invention also relates to a process for producing low carbon steel, in which the steel with a maximum content of 0.05% by weight is melted. degassed to a 'maximum carbon content' of 0,015 · '%, to a maximum oxygen content of 0,010%. and to a maximum nitrogen content of 0,012%, cast and left to solidify, heat rolling to a strip thickness of the sheet, and rolled into coils at a temperature above; 817 ^O C '· · which is characterized in that the niobium additive to a degassed · · · steel into the mold. Before it solidifies. · '·

The process according to the invention yields a low-carbon, non-aging steel having almost no yield strength and free of critical roughing in both the hot-rolled state and the hot-rolled state. · Za. Cold and annealed, which does not have the disadvantages of earlier titanium steels and, moreover, it exhibits to a large extent crystalline oriéntaei cube. on· . edge and good elongation values and a relatively fine grain that is stable over a wide temperature range. In addition, this material can be produced with a wide range of properties as in the rolled state. hot and hot. cold. The process of the invention includes processes for preparing fused steel having a maximum carbon content of about 0.05% and a sufficient amount of manganese; To bind almost all the sulfur present in the steel; vacuum degassing · steel · up to · · carbon content · max.

0,015 '. %, oxygen max. about 0.010% and nitrogen max. about 0.012%; niobium additive · in an amount sufficient to at least reduce the recrystallization speed of the steel after its solidification; casting and solidification of degassed steel; hot-rolling steel up to strip thickness, finishing temperature of about 815 to 928 ° C, and coiling at coils at a temperature of about 815 ° C or less. The heat rolling product is highly desirable for some applications in coils or annealed. As a rule, after hot rolling the product is pickled and cold calibrated to the final dimension, po. followed by final annealing at a temperature and a dwell time selected according to the required strength and ductility of the finished strip or sheet,

The hot-rolled product may be used in rolls or subjected to final annealing in the temperature range of 732 to 928 degrees Celsius. The cold rolled product is generally annealed at a temperature of 538. up to 871 degrees Celsius. In any case, the annealing can be carried out either in batches, or continuously or as a secondary but necessary re-plating operation. immersion and may last from a few seconds to about 16 '. hours. To achieve maximum hardness and strength in the hot-rolled product, the coil temperature should be in the range of about 505 to 705 ° C, and for the cold-rolled product, the final temperature. annealing is in the range of about 53.8 ° to 760 ° C. If, on the contrary, the lowest hardness and high ductility of the hot-rolled product is to be achieved. 1a, the coil temperature should be between 705 to 815 ° C and the final annealing temperature of the cold-rolled product should be in the range of about 760 to 871 ° C.

It has been found that niobium has unexpectedly better properties than titanium, both for production and for the product, for some important reasons. ’

For example, users have found that the low recrystallization rate of the niobium-containing and cold-rolled steels of the present invention makes it possible to achieve a wide range of mechanical properties if certain manufacturing precautions are observed. The recrystallization of the cold rolled steel structure of the invention is completely different from any other low carbon steel. . ·

In the production process according to the invention, the sulfur is bound to manganese and therefore the manganese content is preferably kept in a weight ratio of 7: 1 to sulfur. To bind oxygen. and the nitrogen may be added, aluminum, in which case it is preferred that the weight ratio of aluminum to oxygen is 1.2: 1, while the ratio of aluminum to nitrogen is preferably 2: 1. Because there is a sufficient amount of aluminum and manganese to effectively bind sulfur, oxygen and nitrogen, and because it has niobium. less affinity for oxygen, sulfur and nitrogen than titanium at set temperatures, then almost all niobium added at degassing and after addition of aluminum is to. available for reaction with carbon. The result is greater efficiency, and can be. achieve a niobium recovery of 75 to 95%,. . Aluminum can be omitted because it can. be replaced by another nitride forming element such as titanium. If you. the nitride-forming element is nitrogen. . bound to niobium. If. to anneal tight coils in a nitrogen atmosphere · - hydrogen, add aluminum, because steel · receives · nitrogen from. an annealing atmosphere which is bound to niobium,. if there was no sufficient excess of aluminum to produce a product having an annealed yield strength in the annealed yield state if nitriding were to occur. free nitrogen was present in the steel. · If annealing in open coils is to be carried out, it is not necessary to observe this precaution. .

Using niobium. instead of ‘titanium. and addition of a sufficient amount. . of aluminum. to bind. oxygen and. nitrogen. a. . presence, sufficient quantity. of manganese '. k. . binding. Sulfur is obtained. material. p. surface properties better than steel. with titanium, and non-metallic inclusions. se v. basically 'exclude when. processing according to. . of the invention by passing into slag. It is well known that titanium steels contain difficult steels. . many inclusions and poor quality. surface. ..

Steel according to the invention. has. constantly higher. ratio. permanent! elongation. . Than titanium steels similarly treated. It was found to be high. ratios. permanent ratio. the extensions are obtained. if the niobium. add in quantity. larger than what is needed to bind carbon. and .free. nitrogen, .tj. when. is .niob .. present in the * platform rolled. hot in free state · ...

Steel. according to the invention,. molded or continuously rolled can be hot rolled by conventional methods in normal equipment, while ensuring low operating costs and reducing the cost of new equipment.

Atomic scale. The niobium is 92.91 and the theoretical stoichiometric ratio for complete reaction with carbon o. atomic weight 12.01, present in the steel. is therefore about 7.75: 1. Titanium has. atomic weight 47.90 and the theoretical stoichiometric ratio of titanium to carbon is therefore about 4: 1. The niobium to carbon ratio was found to be 10: 1, or. preferably 12: 1, produces a material that is not aging at all and exhibits no yield strength. Niobium to carbon ratio of 8: 1 can create a marginal material. stability, which. may exhibit some yield strength under certain annealing conditions. . However, steel showing a certain yield strength may be subjected to standard light cold rolling which eliminates the yield strength,. and material. it will not be aging due to its low carbon content. .Other. would be such. The material could be decarburized after cold rolling either by special operation or by final recrystallization annealing to achieve complete stability. Steel having a niobium to carbon ratio of less than 8: 1 is. therefore, it is considered to be within the scope of the invention, e.g. when we find that in practice, a titanium to carbon ratio of 6: 1 is required. considering to its compatibility with other elements and due to its low yield, nevertheless,. that. the theoretical stoichiometric ratio is 4 ': 1, with a significant niobium activity above. titanium. ;

Although the high cost of niobium. will be on. at first glance, it appears to be disadvantageous for its use in low carbon steel applications. as . coating, enamelling, etc., have been found by users,. that. the use of niobium is achieved. 'reduction. production, cost, deletion of some operations, reduced scrap and higher. yield, which costs ·· for the additive ..nobium and vacuum degassing more than replaces.

The invention is carried out as follows: steel. is melted in a Martin furnace, in basic oxygen converters. or in an electric furnace, the composition of the melt having. correspond to a typical but non-limiting composition. Melts for. restless or soothed deep - drawing. steel. with 0.02 to 0.05% carbon; 0.35% manganese; 0,020% of sulfur,. 0.001 to 0.010% nitrogen. % by weight and the remainder substantially iron. . Rendered. the steel is decarburized. vacuum degassing · v. a conventional device, preferably. bubbling. argon, ^x which helps to remove impurities. and prevents layered temperature distribution · S '. advantage is. . adds some aluminum before degassing to prevent excessive gas evolution. Other deoxidizing agents, such as silicon, may be added in small amounts.

The rest of aluminum. is added preferably. during vacuum degassing but after decarburization,

It is not desirable to add more aluminum. . than the amount needed to merge. with nitrogen. and oxygen,. because it could adversely affect. . quality of the finished product. Presence of excessive aluminum. .in the product. it may prevent the removal of brittleness. hot to hot. may occur although hot brittleness is faced by. that is. ensures sufficient. quantity. Manganese,. that would basically. completely bound the sulfur present in the steel. . For this reason, the manganese to sulfur ratio should be maintained. 7: 1, however. higher manganese content. can be tolerated and not affected. adverse end properties. ’‘

Niobium is added. preferably aluminum. during. degassing or pan or mold if:. its proper distribution should be guaranteed. . ‘

The niobium to carbon ratio is chosen to be 12: 1 to. se. guaranteed complete and permanent carbon removal by forming niobium carbide. However, higher ratios of niobium to potassium may be used. resistance of grain orientation. and to get. the desired mechanical properties of the finished product.

Silicon is preferably not added, but small amounts can be tolerated. Other elements in normal residual amounts. can.

also tolerate. .

The degassed steel should preferably have this chemical composition and the composition in. %. weight196442. The 7 of the final product will also be essentially the same:

carbon 0.005 to 0.010% niobium 0.08 to 0.12% manganese 0.10 to 0.35% sulfur up to 0.02% oxygen up to 0.004% nitrogen up to 0.006% aluminum. 0.015 to 0.020% phosphorus up to 0.010% silicon up to 0.015%

The remainder is essentially iron except for the initial impurities.

The degassed and processed steel can then be cast into the ingot molds or continuously cast by conventional methods.

If the steel needs to be continuously rolled, the ingots are attenuated to the thickness of the platforms, reheated if necessary, rolled to the strip thickness and wound into coils.

The normal finishing temperature of the coil is 815 to 928 ° C and is not critical to the process of the invention. However, the finishing temperature below about 815 ° C results in higher power requirements and is more difficult to achieve the desired thickness. Finishing temperatures above 928 degrees Celsius require higher rolling speeds, and thicker and warmer blanks get through the finishing mills.

Preferably, rapid cooling to a coiling temperature of about 594 to 705 ° C is used, although higher or lower coiling temperatures can be used. Generally, winding at a higher temperature, i.e. up to 815 ° C, results in a lower hardness of the product, while winding at lower temperatures below 505 degrees Celsius results in harder products. Cooling to such low winding temperatures is difficult to achieve on existing plants. In connection with winding or as an alternative to winding at relatively high temperature, continuous annealing or batch annealing can be performed at a temperature of up to about 955 ° C to provide a hot-rolled product with the lowest hardness and ductility.

The wound material is then pickled and cold rolled to a final caliber, without intermediate annealing, as in conventional practice. The reduction in cold rolling may be of the order of 60 to 70%. Higher degree of cold reduction up to 90% will result in higher permanent elongation ratios. ·· - · - ·,

The cold rolled strip is then subjected to a final annealing in a protective atmosphere, and this annealing may be continuous or half-dosed.

Obviously, the hot-rolled or flat-rolled strip can be a commercial blank and its properties depend on the composition of the steel and the coiling temperature of the coils, i.e. the cooling rate from the finishing temperature to the coiling temperature of the coils and the degree of annealing. in a compact luminaire while cooling it slowly. Unlike conventional low carbon or titanium treated steels, a hot-rolled product can be prepared with a wide range of mechanical properties ranging from high strength and hardness to medium to low strength accompanied by high ductility. The ratio of permanent elongation to all will generally be 1.0, such as ц. hot rolled low carbon steel.

The process of the invention comprises quenching the hot-rolled material from a finishing temperature of 815 to 928 ° C to a lower temperature at a rate sufficient to cause precipitation of the carbides in finely dispersed form. The steel product of the invention is fully stable due to the niobium additive and is particularly suitable for deep drawing or drawing, enameling, metallization and other applications where good ductility is required, with no grain coarsening, aging and yield strength.

199442

IQ

SUBJECT OF THE INVENTION

Claims (3)

1. Non-aging, low-carbon steel, non-yielding, with good surface properties and free of inclusions, in the form of hot or cold rolled strips or sheets, optionally metallised, characterized by a thickness of 0,002 to 0,015% by weight of carbon , from 0.025 to 0 _Л 30% by weight of niobium, from 0.05 to 0.60% by weight of manganese, traces up to 0.035% by weight of sulfur in the form of manganese sulfide, traces up to 0.010% by weight of oxygen, traces up to 0.012% % by weight of nitrogen, traces of up to 0.08% by weight of aluminum, traces of up to 0.010% by weight of phosphorus, and traces of up to 0.015% by weight of silicon, the remainder being iron and wherein at least 0.025% by weight of niobium is determining the amount of uncomplexed niooib present in the hot rolling condition at room temperature. 2. Low carbon steel as defined in item 1, characterized in that all carbon is in the form of niobium carbide. - 3. The process for producing low carbon steel according to items 1 and 2, wherein the steel is fused with a maximum content of 0.05% by weight of carbon, degassed to a maximum carbon content of 0.015% / o, a maximum oxygen content of 0.010% and a maximum nitrogen content. It is poured and allowed to solidify, hot rolled to strip and sheet thickness, and rolled into coils at. ^For temperatures above 817 · C, wherein the niobium additive to a degassed steel into the mold prior to solidification.