BRPI0605644B1 - hot rolled seamless tube and hot rolled seamless tube manufacturing process for use on cargo vehicle suspension axles - Google Patents

Abstract

manufacture of high mechanical strength hot-rolled seamless tube for use on cargo vehicle suspension axles and high mechanical strength hot-rolled seamless tube for use on cargo vehicle suspension axles the chemical composition developed to ensure High mechanical strength with good toughness and good weldability in seamless steel pipes, in hot rolled condition and standardized in line or continuous furnace, provides lower cost of the production process (previously only possible in tempered and tempering condition), besides positively affect lead time. the seamless tube manufacturing process from the production of charcoal pig iron, steel making in ld converter, secondary furnace metallurgy, vacuum degassing, continuous casting with electromagnetic stirring, hot rolling in mandrel continuous rolling mill ( continuous mandrel mill or plug mill, with continuous or in-line standardization around continuous, ensures the metallurgical quality and final properties necessary to meet the mechanical stresses to which the shafts are exposed.

Description

Invention Patent Descriptive Report for "HOT LAMINATED SEAMLESS TUBE MANUFACTURING PROCESS, AND HOT LAMINATED SEAMLESS TUBE FOR USE ON LOAD VEHICLE SUSPENSION AXLES".

[001] The present patent deals with seamless tubes that had their chemical composition and manufacturing process specially designed to guarantee high mechanical resistance in the laminated state, combined with good toughness and good weldability. This development aims to reduce the costs with heat treatments and the "lead time", guaranteeing, already in the laminated state, a set of mechanical properties until then obtained only through tempering and tempering.

[002] The good performance of an axis begins with a well-designed engineering design, capable of incorporating the demands arising from use. Then, these requests must be translated into requirements for the selection of materials and manufacturing processes. The main characteristic of the quality of a suspension project as a whole is the fatigue resistance of the component, other design details, such as stress concentrating points, welding of auxiliary components, selected materials, should also be considered. (dimensions and mechanical properties) and the processes involved in manufacturing (forging, machining, welding, etc.).

[003] The careful selection of materials contributes decisively to obtain the good resistance to fatigue of the component. The main quality characteristics indirectly responsible for fatigue strength are: mechanical strength (mainly the limit of tensile strength) and toughness. In addition, these characteristics must be obtained with an alloy that guarantees good weldability. [004] The technological alternatives available and generally used in 30 tubular components for suspensions, are summarized in the table in Fig. 1. Of these, the alternatives with high tensile strength and good toughness stand out (SAE 1030 and BS 150M19 ) which also have a maximum carbon equivalent of 0.59%, according to the llW formula. Currently, in tube rolling, this set of characteristics specified for these two steels is obtained only through tempering and tempering.

[005] To obtain such quality characteristics in the laminated state, the alloy and the manufacturing process must be designed with a focus on grain refining.

[006] This hardening mechanism is the only one capable of, at the same time, improving tensile strength and toughness. This foundation has been successfully applied to flat rolling, in which steels microalloyed to Ti, Nb, V and B are subjected to a well-defined thermomechanical processing in order to finally control the microstructure (grain size, constituents, etc.) and the sub-structure (precipitation, sub-grains, etc.).

[007] Based on the above, a chemical composition and process was developed for the production of seamless steel tubes, in compliance with the same quality characteristics presented by the 150M19 steel in Fig. 1, except for the maximum limits for the tensile strength and hardness.

[008] The objectives of the invention are achieved by a process of manufacturing seamless hot-rolled tube for use in suspension axles for cargo vehicles, comprising the following steps: - fabrication of steel in LD converter from pig iron produced in charcoal blast furnaces, comprising: - the addition of nitrogen inputs to incorporate nitrogen into the primary steel, and - the carrying out of a submerged blow using at least one of argon and nitrogen - secondary refining in a pot oven, which it includes adjusting the alloy composition, metallurgical treatment with calcium-silicon and argon bubbling, - vacuum degassing, - continuous casting with electromagnetic stirring, - hot rolling in continuous rolling with mandrels or in automatic rolling, and - normalization in-line, [009] that the in-line standardization step comprises cooling the tube in a bed of res intermediate cooling until its complete transformation and then the reheating of the tube on an oscillating threshold at a temperature of 930 ° C to 950 ° C, [0010] where - leaving the oven the tube is unscrewed and processed in the calibrator laminator or drawing laminator reducer; - then the tube is cooled in bed and sawn to the specified length.

[0011] The objectives of the invention are further achieved by a seamless hot-rolled tube for use on cargo vehicle suspension shafts, produced by the process as described here, the tube is made of a steel alloy with high mechanical strength comprising contents: 0.10 to 0.26% carbon, 1.20 to 2.00% manganese, maximum 0.025% phosphorus, maximum 0.010% sulfur, 0.30 to 0.50% silicon, maximum 0.20% nickel, maximum 0.20% chromium, maximum 0.10% molybdenum, 0.05 to 0.20% vanadium, maximum 0.20% copper, maximum 0.010 % aluminum and at least 0.0060% nitrogen, [0012] the remainder being iron, the elements nickel, chromium, molybdenum and copper being obligatorily present in the alloy, [0013] and the tube has a carbon content CequW equivalent between 0.56% to 0.57%, Minimum yield limit of 520 MPa, Resistance limit of 720 to 800 MPa, Elongation (50mm) minimum of 18%, Hardness d and 220 to 250 HB, Energy absorbed in an impact test Charpy minimum of 40J and individual minimum of 40J.

[0014] The characteristics that must be met are: Ceq iw <0.59%, Flow Limit> 510 MPa (for wall thickness <13mm) or> 480 MPa (for wall thickness> 13mm), Resistance Limit between 630 and 820 MPa, Elongation (50mm)> 18%, Hardness between 187 and 260 HB, Energy Absorbed in Charpy impact test (specimen 10x10x55mm, V notch with 2mm, temperature of -20 ° C): average> 40J and individual> 30J.

[0015] The chemical composition of the product developed is shown in the table in Fig. 2. In the design of this chemical composition, each element was carefully adjusted according to the criteria described below: the main hardening mechanism used in this alloy is the precipitation of Vanadium carbonitrides on ferrite. This precipitation occurs preferably after the transformation, since the precipitation in the interphase (austenite -> ferrite) is hindered by the low transformation temperature of this alloy, approximately 730 ° C.

[0016] The literature shows that Vanadium levels close to 0.12% are sufficient to maximize this mechanism. It also shows that more important than the Vanadium content is the availability of interstitial elements, Carbon and Nitrogen. These elements contribute to the tensile strength at the rate of 15MPa / 0.01% C and 5MPa / 0.001% N. It is important to note that the carbon content must be limited due to its negative effect on weldability and toughness.

[0017] Chemical elements, such as Aluminum, Niobium, Titanium and Boron, should have their contents kept as low as possible, for two reasons: to maximize the availability of interstitial elements, increasing the driving force for precipitation in the ferrite. This increases the rate of nucleation of the precipitates and increases the efficiency of this mechanism; and, avoid the coprecipitation of Vanadium carbonitrides on the most stable precipitates formed before the finishing lamination.

[0018] The contents of the elements, such as Sulfur and Phosphorus, should also be as low as possible, due to their negative effects on toughness, associated with microinclusion and microsegregation, respectively. The other elements, such as Manganese, Silicon, Nickel, Chromium, Molybdenum and Copper, contribute to the increase of the mechanical resistance by different hardening mechanisms. These elements thus form the basis of the alloy, that is, they define the proportion of microconstituents - ferrite, perlite and bainite - present after cooling at rates typical of the in-line product.

[0019] Another fundamental mechanism for obtaining the set of properties mentioned above, is the refining of the grain. This mechanism is the only one that contributes, at the same time, to the increase of the mechanical resistance and to the increase of the toughness. Grain refining can be achieved by a combination of alloy design and process - thermal or thermomechanical treatments. For the product described here, in-line normalization was adopted as the processing route, necessary for refining the grain that will guarantee the specified toughness. This route is capable of promoting a decrease in the ductile-brittle transition temperature of approximately 55 ° C, as shown in Fig. 3, for the first prototype.

[0020] From the alloy and process designs defined above, two prototype runs were cast and rolled for the evaluation of the final mechanical properties. The two races have the same basis regarding chemical composition, varying only the content of Carbon and, consequently, the Equivalent Carbon, CeqIIW. The first race has an Equivalent Carbon of 0.508% and the second, 0.566%. The tubes were laminated with an external diameter of 127.0 mm and wall thicknesses of 11.7, 13.0, 16.0 and 19.0 mm. A more detailed description of the processes involved in the manufacture of these tubes will be provided later.

[0021] The results obtained from the two runs are presented in the tables of Figs. 4 to 8: Flow Limit, Tensile Strength Limit, Elongation, Hardness and Impact. Obviously, the first run meets only 480 MPa yield limit (wall thickness> 13 mm). This run is also presented here as a reference for defining the working range in the preparation of steel to meet the specifications for wall thicknesses <13 mm. For this type of alloy, the process dispersion for Carbon Equivalent is 0.042%, for a 99.7% confidence level.

[0022] Within the developed chemical composition, the microstructure in the laminated state with in-line or out-of-line normalization, with typical cooling speeds of tubes for shafts, is composed of ferrite, perlite and a small amount of bainite. The typical microstructures of this material are shown in Fig. 9 and 10 for gauges 127.0 x 11.7 and 127.0 x 19.0 mm, respectively.

[0023] The steel used in this product comes from an LD converter from pig iron produced in blast furnaces with charcoal. O

LD converter is equipped with submerged blowing and allows the use of Argon and Nitrogen throughout the processing. This stage is completely controlled by computer through static and dynamic models with its own development. Continuous temperature measurements during blowing and calculation of the addition of alloys are part of this system. This routine guarantees the product the necessary low levels of phosphorus and sulfur. In this stage, nitrogen inputs are used for the incorporation of Nitrogen into primary steel, in addition to other operational practices with the same objective.

[0024] Then, the steel is subjected to secondary refining in a pan oven, where adjustments are made to the chemical composition, as well as metallurgical treatment with calcium-silicon. The addition of alloys, the treatment with Calcium-Silicon, the bubbling of Argon and the removal of samples, are done in this equipment in a fully automated way. Through this process, steel production is guaranteed in a narrow range of composition, with a view to meeting product quality. With the result of bubbling inert gas and the use of synthetic slags, the sulfur content can reach levels below 0.003%. The secondary refining in a pan oven also improves the micro-purity, allows a better distribution of the alloying elements and a better adjustment of the temperature.

[0025] After secondary metallurgy in a pan oven, the steel is sent to continuous casting, in a 4-shaft machine, where it is subjected to a simultaneous cooling and gutting process. In this process, round bars with diameters of 180, 194 or 230 mm and lengths between 6 and 12 m are obtained. The machine is equipped with double electromagnetic stirring that provides a better quality of the ingot material in terms of segregation of elements and porosity in the central region. A last Nitrogen incorporation technique is also used: the cooling of the gate valve springs in the pan with nitrogen gas.

[0026] The bars produced at the steelworks are cut into blocks of suitable length before being used in hot-rolled pipe mills. The tubes are laminated via Continuous Lamination with mandrels, to produce tubes with an external diameter between 26.7 to 177.8 mm (3/4 to 7 inches), or via Automatic Lamination (MilI Plug), for tubes with a diameter between 168.3 and 365.1 mm (6 to 14 inches).

[0027] In Continuous Lamination the block is reheated in a rotary hearth oven at a temperature of 1280 ° C and the oven time is controlled by an optimization system with its own research and development. After reheating, the block is drilled in the oblique laminator and the tube obtained from this process goes to the reducing laminator in order to standardize the outside diameter for the subsequent step. The tube goes to the continuous laminator with mandrels, where a wall thickness close to that of the final product is obtained. The tube of this laminer goes to the intermediate cooling bed, for the realization of the normalization in line. This is made possible by cooling the material until its complete transformation before being heated again to a temperature close to 950 ° C, in an oscillating hearth oven. Then, the tube is stripped and processed in the drawing reducer laminator, where the final product dimensions are obtained. The tube is cooled in bed and sawed to the specified length. A tracking system supervises the entire operation and records approximately 400 parameters of the hot rolling process.

[0028] In Automatic Lamination the block is reheated in an oscillating hearth furnace at approximately 1280 ° C and then it is drilled in the oblique laminator. After drilling, the tube is transported to the laminator with mandrels (Plug MilI), where the wall thickness is very close to that specified for the product. Then, the tube of this process undergoes a smoothing with expansion of the outside diameter in the smoothing laminator (Reeler Mill). The tube of this process goes to the intermediate cooling bed for the realization of the standardization in line, through the cooling of the material until its complete transformation. Then, the tube will be reheated in the oscillating hearth oven, at a temperature of approximately 930 ° C. Leaving the oven, the tube is stripped and passes through the calibrator laminator to obtain the final dimensions. The tube is then cooled in a bed and sawn to the specified length. A tracking system supervises the operation throughout the hot rolling process.

Claims (7)

1. Process for the manufacture of seamless hot-rolled tube for use in suspension axles for cargo vehicles, comprising the following steps: - manufacture of steel in LD converter from pig iron produced in blast furnaces with charcoal, comprising: - the addition of nitrogen inputs to incorporate nitrogen into the primary steel, and - the submerged blowing with the use of at least one of argon and nitrogen - secondary refining in a pot oven, which comprises adjusting the alloy composition , metallurgical treatment with Calcium-Silicon and argon bubbling, - vacuum degassing, - continuous casting with electromagnetic stirring, - hot rolling in continuous rolling with mandrels or in automatic rolling, and - in-line normalization, characterized by the fact that the normalization step in line to understand the cooling of the tube in an intermediate cooling bed until the its complete transformation and then the reheating of the tube in an oscillating sill at a temperature of 930 ° C to 950 ° C, in which - when leaving the oven, the tube is stripped and processed in the calibrating laminator or the drawing reducer laminator; - then the tube is cooled in bed and sawn to the specified length. 2. Seamless tube manufacturing process according to claim 1, characterized by the fact that it also comprises a step of nitrogen incorporation by cooling the pot's valves-drawer with gaseous nitrogen. 3. Seamless tube manufacturing process according to either claim 1 or claim 2, characterized by the fact that in the continuous casting step with electromagnetic stirring, refrigeration and gutting are carried out simultaneously. 4. Seamless tube manufacturing process according to any one of claims 1 to 3, characterized by the fact that in the continuous rolling step, the blocks formed in the casting step are heated to a temperature of 1280 ° C in a sill and then drilled. 5. Hot-rolled seamless tube for use on cargo vehicle suspension axles, produced by the process as defined in claim 1, the tube being made of a steel alloy with high mechanical strength characterized by,% by mass, understand the contents of: 0.10 to 0.26% carbon, 1.20 to 2.00% manganese, maximum 0.025% phosphorus, maximum 0.010% sulfur, 0.30 to 0.50% silicon, at most 0.20% nickel, at most 0.20% chromium, at most 0.20% copper, at most 0.10% molybdenum, 0.05 to 0.20% vanadium, at maximum 0.010% aluminum, minimum 0.0060% nitrogen, and the remainder iron, with the elements nickel, chromium, molybdenum and copper necessarily present in the alloy; and the tube has a Ceqiiw equivalent carbon content between 0.56% to 0.57%, minimum yield limit of 520 MPa, resistance limit of 720 to 800 MPa, minimum elongation (50mm) of 18%, hardness of 220 at 250 HB, Energy absorbed in impact test Charpy minimum minimum of 40 J and minimum individual of 40 J. 6. Hot-rolled seamless tube, according to claim 5, characterized by the fact that the metallic alloy comprises contents of 0.10 to 0.26% carbon, 1.20 to 2.00% manganese, in the maximum 0.025% phosphorus, maximum 0.010% sulfur, 0.30 to 0.50% silicon, maximum 0.20% nickel, maximum 0.20% chromium, maximum 0.08% molybdenum 0.05 to 0.20% vanadium, maximum 0.20% copper, maximum 0.010% aluminum and minimum 0.0060% nitrogen. 7. Hot-rolled seamless tube according to either of claims 5 or 6, characterized by the fact that the metal alloy is produced from pig iron.