A hot worked and annealed graphitic steel for parts machining and manufacturing method thereof

Abstract

The invention deals with a hot worked and annealed graphitic steel product having a composition comprising in weight percent: 0.70 % ≤ C ≤ 1.1 %, 0.35 % ≤ Mn ≤ 0.80 %, 1.0 % ≤ Si ≤ 3.0 %, 0.010 % ≤ Ti ≤ 0.090 %, 0.001 % ≤ Al ≤ 0.09 %, P ≤ 0.090 %, S ≤ 0.2, 0.0090 ≤ N ≤ 0.0150 and comprising optionally one or more of the following elements, by weight percent: 0 % ≤ B ≤ 0.006 %, 0 % ≤ Cr ≤ 1 %, 0 % ≤ Mo ≤ 0.5 %, 0 % ≤ V ≤ 0.2 %, 0 % ≤ Nb ≤ 0.1 %, 0 % ≤ Ni ≤ 3.0 %, 0 % ≤ Co ≤ 3.0 %, 0 % ≤ Cu ≤ 3.0 %, 0 % ≤ Sn ≤ 0.1 %, 0 % ≤ Ce ≤ 0.1 %, 0 % ≤ Mg ≤ 0.1 %, 0 % ≤ Zr ≤ 0.1 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel having a microstructure comprising 1 to 10% in area fraction of free graphite particles, up to 5% optional cementite, the balance being ferrite.

Description

A hot worked and annealed graphitic steel for parts machining and manufacturing method thereof

[0001] The present invention relates to a hot worked and annealed graphitic steel suitable for manufacturing of machine parts.

[0001] Steels for machine parts can be used to manufacture many parts for machines such as tools, rolls, rings shafts or gears that can undergo manufacturing operations with smaller chip size such as forming, shearing, cutting, molding and other metal removal process. Such parts can be used for automotive and electronic equipment, where complex shapes and size precision are required.

[0002] Free-cutting steels generally contain high amount of various metallic (Pb, Bi) as well as non-metallic inclusions (MnS) for machinability improvement. But the processing of such grades is however difficult to manage in reason of lead fumes that can be detrimental for the operator’s health. Moreover, additional improvements are needed in the reduction of tool wear and in the energy consumption during the machining of the steel bars.

[0003] There is therefore a need to develop a steel that fulfill those requirements without using lead in its composition.

[0004] Another purpose of the present invention is also to make available a method for the manufacturing of steel parts that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts.

[0005] This object is achieved by providing a steel product according to claim 1 . The steel product can also comprise characteristics of claims 2 to 3, taken alone or in combination. Another object is achieved by providing the method according to claim 4. Another aspect is achieved by providing machine parts according to claim 5.

[0006] Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

[0007] Carbon is present in the steel of present invention from 0.70% to 1.1 % in weight percent. Carbon is an element necessary for generating graphite to improve tool life and to secure strength after quenching. When the carbon content is less than 0.70% by weight, the effect of improving the machinability is insufficient, and the distribution of graphite grains is uneven even whengraphitization is completed. On the other hand, when the content exceeds 1.1 % by weight, there is a concern that the the chemical composition leads to a too low solidification temperature with detrimental consequences on casting ability and productivity by using continuous casting technology.

[0008] Manganese is added in the present steel from 0.35% to 0.8% in weight percent. Manganese is an element that is necessary because it is present as MnS or forms a complex with an oxide by combining with S and oxygen and serves as a nucleating site for producing graphite. Manganese sulfides also contributes to improvement of machinability. However, if the amount of Mn is above 0.8%, graphitization is significantly inhibited as its completion is delayed and strength and hardness may increase to much and thereby deteriorate machinability. The preferred limit for the presence of Manganese is from 0.35% to 0.65% and more preferably from 0.40% to 0.55%. In a preferred embodiment, the Mn / S weight ratio of the grade is ranging from 2 to 5.

[0009] Silicon is present in the steel of present invention from 1 % to 3% in weight percent. Silicon is a necessary component as a deoxidizer in manufacturing of molten steel and is added because it is a graphitization promoting element. It indeed has a small bonding force with carbon atoms that destabilizes cementite so that carbon can be precipitated as graphite. To precipitate a sufficient amount of graphite during annealing treatment, meaning that the degree of graphitization will be high, it is necessary to add at least 1 % of silicon. However, when the amount of Silicon exceeds 3%, the chemical composition leads to a too low solidification temperature with detrimental consequences on casting ability and productivity by using continuous casting technology. In addition, hardness increases due to solid solution strengthening effects, and thus abrasion of tools is accelerated during cutting, embrittlement is induced in accordance with an increase in non-metallic inclusions, and excessive decarbonization may be induced during hot working (like rolling for example) and may also causes deterioration in cold workability. The preferred limit for the presence of Silicon is from 1 .4% to 2.8% and more preferably from 1 .9% to 2.7%.

[0010] Titanium is present in the steel from 0.01 % to 0.09% in weight percent. Titanium, like boron, aluminum, and the like, binds to nitrogen and a nitride such as TiN is formed. The nitrides act as nuclei for formation of graphite grains duringisothermal annealing heat treatment. TiN is crystalized before formation of austenite is completed since its formation temperature is higher than those of AIN and BN, and thus TiN is uniformly distributed in grain boundaries and inside grains of austenite. Thus, graphite grains generated using TiN as nuclei also finely and uniformly distributed. To obtain such effects, the Ti content is preferably controlled to 0.01 wt. % or more. On the contrary, when the Ti content is above 0.09 wt. %, carbon required for graphite formation is consumed due to formation of a coarse carbonitride, and thus graphitization may be inhibited. Thus, it is preferable to control the Ti content from 0.01 to 0.06 wt. % in the present invention.

[0011] Aluminium is an element that is present in the steel of present invention from 0.001 % to 0.09% in weight percent. Aluminium is an element that promotes graphitization next to silicon. This is because aluminium destabilizes cementite when present in solid solution form. In the present invention, in order to exhibit such an effect, it is included in an amount of 0.001% by weight or more. On the other hand, if the content is excessive, the effect is not only saturated, but also can cause nozzle clogging during casting, and AIN is generated at the austenite grain boundary, so that graphite using it as a nucleus is non-uniform ly distributed at the grain boundary. Therefore, the upper limit of the aluminium content is 0.09% by weight and preferably 0.009% by weight.

[0012] Boron is an optional element that can be present in the steel up to 0.006% in weight percent. Boron binds to nitrogen contained in steels to form BN. BN promotes graphitization by acting as nuclei for formation of graphite grains. The B content is preferably controlled to 0.001 wt. % or more to obtain such effects. On the contrary, when the B content is above 0.006 wt. %, the BN content is excessive in grain boundaries of austenite causing non-uniform distribution of graphite grains after graphitization annealing and also the grain boundaries are weakened and considerably reduce hot rollability. Thus, it is preferable to control the B content from 0.001 to 0.006 wt. % in the present invention and preferably from 0.003 to 0.006% by weight.

[0013] Phosphorus content of the steel of present invention is limited to 0.09% in weight percent. Phosphorus is an impurity inevitably contained in steels. Although machinability may be increased to some extent by weakening grainboundaries of steels, phosphorus increases hardness of ferrite due to considerable solid solution strengthening effects, reduces toughness and delayed fracture resistance of steel materials and causes surface defects. It is therefore preferable to control the P content to be as low as possible. Although it is advantageous to control the P content to 0 in theory, P is inevitably contained during a manufacturing process. Therefore, it is important to control an upper limit of the P content which is 0.09 wt. % in the present invention.

[0014] Sulphur is contained from 0 % to 0.2% in weight percent and can contribute to improving machinability by forming MnS inclusions. When under free form, not linked to manganese, it can also inhibit graphitization of carbon in steels, reduce toughness, and induce mechanical anisotropy due to MnS stretched by rolling, it is preferable to control the S content to an upper limit of 0.2 wt. % in the present invention. In a preferred embodiment, the sulphur content is in the range of 0.01 to 0.2%, or preferably from 0.05 to 0.2% or even better from 0.07 to 0.18%

[0015] Nitrogen is present in an amount from 0.0090 to 0.0150% in weight percent in steel of present invention. Nitrogen binds with other elements to form nitrides such as TiN, which is mainly formed in grain boundaries of austenite. Graphite grains are uniformly distributed by using such nitrides as nuclei during graphitization heat treatment. To this end, the N content is preferably 0.010 wt. % or more in the present invention. However, when the N content is above 0.0150 wt. %, nitrides are formed in such an amount that graphite grains may be non-uniform ly distributed. Alternatively, nitrogen may not bind to a nitride-forming element but can present in a solid solution state in the steel to increase strength and stabilizing cementite, thereby delaying graphitization. Therefore, it is preferable to control the N content from 0.0090% to 0.015 wt. % in the present invention.

[0016] Chromium is an optional element that may be present up to 1 % in weight percent in the steel of the present invention. Chromium can be added to ensure hardenability. However, if added in excess of 1 %, graphitization is significantly inhibited, so the upper limit was set to 1 %. The preferred limit for the presence of Chromium is up to 0.8% and more preferably up to 0.6%.

[0017] Molybdenum is an optional element and may be present up to 0.5% in weight percent in the present invention. Molybdenum can be added to imparthardenability and hardness to steel by forming Molybdenum based carbides and also delays the appearance of Bainite which promotes the formation of ferrite. However, the addition of Molybdenum increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.5%. The preferred limit for molybdenum content is up to 0.4% and more preferably up to 0.2%.

[0018] Vanadium is an optional element for the present invention and its content is up to 0.2% in weight percent. Vanadium is effective in enhancing the strength of steel by precipitation strengthening especially by forming carbides or carbonitrides. Upper limit is kept at 0.2% due to economic reasons.

[0019] Niobium is an optional element in the steel of the present invention and can be added up to 0.1% in weight percent to form carbo-nitrides imparting strength by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbo-nitrides and by retarding the recrystallization during heating process. Thus, finer microstructure formed at the end of the holding temperature and as a consequence after the complete austenitization lead to the hardening of the product. However, Niobium content above 0.1 % is not economically interesting as well as forms coarser precipitates which are detrimental for the fatigue properties of the steel. Moreover, when the content of niobium is 0.1 % or more it is also detrimental for steel hot ductility resulting in difficulties during steel casting and rolling.

[0020] Other optional elements that can be added to the steel of the present invention are nickel, cobalt, and copper have a small bonding force with carbon atoms, destabilize cementite, promote graphitization, are effective in enhancing hardenability, and securing strength. Whenever added, those elements contents are limited to 3.0%, and are preferably done to reach a minimum content of 0.05% in weight percent.

[0021] Other elements such as Tin, Cerium, Magnesium or Zirconium can be added individually or in combination in the following proportions by weight: Tin < 0.1 %, Cerium < 0.1 %, Magnesium < 0.1 %, and Zirconium < 0.1 %. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. In a preferred embodiment, Mg is limited to less than 0.005% by weight.

[0022] The remainder of the composition of the steel consists in iron and inevitable impurities resulting from processing. In particular, the steel of present invention is free from elements such as Lead (Pb).

[0023] After graphitization is performed, the microstructure of the steel consists in ferrite, free graphite particles and optional minor contents of cementite.

[0024] In particular, the steels newly developed in the frame of the invention contain graphite particles that are self-lubricating and can replace lead inclusions.

[0025] Ferrite is the matrix phase of the steel of present invention and is present in an amount of 85% to 99%, in area fraction, in the steel. Such ferrite may comprise polygonal ferrite, lath ferrite, acicular ferrite, plate ferrite or epitaxial ferrite. The presence of ferrite in the present invention provides the steel of the invention with relevant mechanical properties for machining purpose. Indeed, the hardness of ferrite is less than the one of other kind of microstructures like pearlite, bainite, martensite and cementite. Ferrite is less abrasive against the cutting tool during machining, and consequently the wear of the cutting tool is reduced during machining. In addition, lower mechanical properties achieved thanks to the majority ferrite microstructure ensures lower energy consumption during machining.

[0026] The steel of the present invention can contain optional cementite up to 5% in area fraction. Such phase is obtained after transformation of the initial microstructure of the steel that it mainly made of pearlite. During the graphitization annealing, the cementite is turned into ferrite and graphite, but part of it can remain in the final microstructure of the part. This residual cementite is limited to 5% in area fraction as it is detrimental for the in-use properties of the graphitized steel, like machinability.

[0027] Graphite is present in the steel of present invention as particles in an amount of 1 % to 10% in area fraction. The graphitization rate of the steel according to the present invention may be preferably 98% or more, more preferably 99% or more, and most preferably 99.5% or more. In the frame of the invention, graphitization rate is defined as the weight percentage of carbon that is transformed into the graphite particles. Most of the remaining carbon that is not found in graphite particles is found in residual cementite. For machining purpose, cementite is more harmful for the cutting tool than ferrite. Therefore, it isimportant to lower as much as possible the amount of residual cementite and consequently to maximize the graphitization rate. The graphite particles are finely and homogenously distributed in the ferrite matrix of the steel

[0028] A steel for machine parts according to the invention can be produced by any suitable manufacturing process, with the stipulated process parameters explained hereinafter.

[0029] A preferred method is described below but this example does not limit the scope of the invention and the aspects upon which the examples are based. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible ways in which the various aspects of the present invention may be put into practice.

[0030] A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done in any form such as ingots or blooms or billets which is capable of being manufactured or processed into a machine part that can have, for example, a cross section from 1 mm x 1mm to 400mm x 400mm.

[0031] For example, the steel having the above-described chemical composition is casted into a billet and then rolled in form of a bar or a bar in coil or wire rod. This steel product can act as a semi-finished product for further mechanical operations. Multiple rolling or bright drawing or multiple dies drawing steps may be performed to obtain the desired semi-finished product.

[0032] In order to prepare for the steel to be manufactured as a part for the machine, the semi-finished product can be used directly at a high temperature after rolling or may be first cooled to room temperature and then reheated for manufacturing the parts.

[0033] The semi-finished product is reheated from a temperature in the range from Ac3 to Ac3 +400° C, preferably from Ac3+30°C and Ac3 +400°C where it is held during 5 seconds to 10800 seconds to ensure homogenous temperature across the cross section of the semi-finished product as well as to ensure 100% austenite is formed.

[0034] If the reheating temperature of the semi-finished product is lower than Ac3, excessive load is imposed on parts of mechanical operation such as dies during the forming operation or milling tool during tapering and, further, the temperatureof the steel may also decrease below the Ferrite transformation start temperature that will lead to the ferrite formation in the final product which is detrimental for fatigue and mechanical properties. Additionally, the metallurgical transformation under strain can lead to significant change in the obtained microstructure for a given cooling rate or a given chemical composition. As a result, the obtained microstructure will be completely different from the targeted one and so the mechanical properties. Therefore, the temperature of the semi-finished product is preferably sufficiently high so that all the mechanical operations are performed and completed in the 100% austenitic temperature range. Reheating at temperatures above Ac3 +400°C must be avoided because they are industrially expensive and can lead to the occurrence of liquid areas that will affect the forming and tapering of the steel.

[0035] Then the semi-finished product can be subjected to at least one mechanical working operation from Ac3 and Ac3 +400° C. Mechanical operation may comprise rolling tapering, forming, cutting, or any other suitable mechanical operation or manufacturing procedure that is required to form the part of machine from semi-finished product. The preferred temperature for all the mechanical working operations is from Ac3 +30° C and Ac3 +400° C and more preferably from Ac3 +50° C to Ac3 +300° C.

[0036] The final mechanical operation temperature must be kept above Ac3, to have a structure that is favorable to recrystallization and mechanical manufacturing.

[0037] The semi-finished product is cooled to room temperature after mechanical at at an average cooling rate below 50°C / s and preferably below 40°C / s.

[0038] Thereafter the machine part is subjected to a graphitizing annealing wherein it is heated to a soaking temperature from 600°C to 750°C with a heating rate from 10°C / h to 200°C / h.

[0039] Then the machine part is held at the soaking temperature during 1 hour to 48 hours to ensure adequate graphitization and cooled to room temperature at a cooling rate below 100°C / s and preferably below 75°C / s.

[0040] In a second embodiment, the hot worked steel is directly subjected to a graphitizing annealing wherein it is heated to a soaking temperature from 600°C to 750°C with a heating rate from 10°C / h to 200°C / h.

[0041] Then the steel is held at the soaking temperature during 1 hour to 48 hours to ensure adequate graphitization and cooled to room temperature at a cooling rate below 100°C / s and preferably below 75°C / s.

[0042] The machine part is then manufactured by any suitable forming process including cold forging, drawing or machining.

[0043] In both embodiments, the machine parts can be submitted to a final heat treatment like a quench and tempering process for strengthening and / or to a high frequency induction hardening.

[0044] EXAMPLES

[0045] The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only and will display the advantageous features of the present invention.

[0046] Some steel samples with the compositions gathered in Table 1 were prepared.

[0047] Table 1underlined values: not according to the invention.

[0048] After being cast in 160x160 mm billet format, the billets were cooled (natural air cooling) down to room temperature. The billets were then reheated at 1150°C and rolled into 26.5 mm coils. For R1 and 11 , the coils were then heated at 100°C / h up to 750°C and annealed during 16h for graphitization.

[0049] Finally, all steel coils were cold drawn in 25 mm bright bars.

[0050] The microstructures of 11 and I2 were observed and are according to the invention, I2 containing no residual cementite whereas 11 contains some, below 5% area fraction.

[0051] Tool wear tests were then carried out in facing on bar operation by using high speed steel cutting tool. The cutting operation studied is a single point turning,with a High-Speed Steel WKE45 cutting tool. For the cutting tool, a hardness of 65 HRC is required. The cutting tool is originally a square shape 8 x 8 mm bar, length 100 mm, that is ground with the following angles: rake angle: 0°, primary flank angle: 6°; secondary flank angle: 6°, entering angle: 90°, lead angle: -1 °, following the international standard NF ISO 3002-1.

[0052] For the tool wear test, the facing parameters are : feed rate = 0.1 mm / revolution and depth of cut = 3 mm. The cutting test is carried out by using flood emulsion (8% oil and 92% water). The machining is regularly stopped to measure the flank tool wear. The tool wear test is finished when a flank tool wear of 1500 pm is reached.

[0053] In addition, two reference free-cutting steel grades 11 SMn30 and 11SMnPb30 were also characterized following the same protocol for comparison.

[0054] Results from tool wear test are summarized in the table 2 below. Table 2

[0055] The results of the tool wear tests show that the steels according to the invention, which contain no lead, have a better behaviour than the reference steels.

Claims

CLAIMS1. A hot worked and annealed graphitic steel product having a composition comprising in weight percent:0.70 % < C < 1.1 %0.35 % < Mn < 0.80 %1 .0 % < Si < 3.0 %0.010 % < Ti < 0.090 %0.001 % < Al < 0.09 %P < 0.090 %S < 0.20.0090 < N < 0.0150 and comprising optionally one or more of the following elements, by weight percent:0 % < B < 0.006 %0 % < Cr < 1 %0 % < Mo < 0.5 %0 % < V < 0.2 %0 % < Nb < 0.1 %0 % < Ni < 3.0 %0 % < Co < 3.0 %0 % < Cu < 3.0 %0 % < Sn < 0.1 %0 % < Ce < 0.1 %0 % < Mg < 0.1 %0 % < Zr < 0.1 % the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel having a microstructure comprising 1 to 10% in area fraction of free graphite particles, up to 5% optional cementite, the balance being ferrite.

2. A hot worked and annealed graphitic steel product according to claim 1 , comprising aluminium in a range of 0.001 to 0.03 in weight percent.

3. A hot worked and annealed graphitic steel product according to claim 1 or2, comprising boron in a range of 0.001 to 0.002 in weight percent.

4. A method for manufacturing a hot worked and annealed graphitic steel product according to any one of claims 1 to 3 comprising the following steps:- providing a semi-finished steel product having a composition according to anyone of claims 1 to 3,- reheating said product to a temperature in the range from Ac3 to Ac3 +00° C, where it is held during 5 seconds to 10800 seconds,- subjecting said reheated product to at least one mechanical working operation at a temperature in the range from Ac3 to Ac3 +400° C,- cooling said product down to room temperature at at an average cooling rate below 50°C / s,- subjecting said product to a graphitizing annealing wherein it is heated to a soaking temperature from 600°C to 750°C with a heating rate from 10°C / h to 200°C / h,- holding said product at the soaking temperature during 1 hour to 48 hours and cooling it to room temperature at a cooling rate below 100°C / s.

5. A machine part made of a hot worked and annealed graphitic steel product according to claims 1 to 3 or obtained by the method according to claim 4.

Images