EP3330400A1 - Stahl für federn von hoher festigkeit und härtbarkeit - Google Patents

Stahl für federn von hoher festigkeit und härtbarkeit Download PDF

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Publication number
EP3330400A1
EP3330400A1 EP15795207.8A EP15795207A EP3330400A1 EP 3330400 A1 EP3330400 A1 EP 3330400A1 EP 15795207 A EP15795207 A EP 15795207A EP 3330400 A1 EP3330400 A1 EP 3330400A1
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EP
European Patent Office
Prior art keywords
steel
strength
steels
leaf spring
quench
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP15795207.8A
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English (en)
French (fr)
Inventor
Roberto Elvira Eguizabal
Jacinto ALBARRÁN SANZ
Juan José LARAUDOGOITIA ELORTEGUI
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sidenor Investigacion y Desarrollo SA
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Sidenor Investigacion y Desarrollo SA
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Publication date
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Publication of EP3330400A1 publication Critical patent/EP3330400A1/de
Withdrawn legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten

Definitions

  • the present invention relates to a high-strength and high-quench hardenability leaf spring steel, with a long fatigue life, which can be applied in the iron and steel industry, which allows being used for metal structures in the construction sector.
  • Said parts are particularly suitable in the automotive industry, for example, for manufacturing leaf springs or other suspension supporting elements for industrial vehicle suspension systems.
  • the invention allows obtaining leaf spring steel, from a chemical composition and by means of a metallurgical process, that has high mechanical strength while at the same time it has a long fatigue life and strength, in addition to having optimal quench hardenability.
  • leaf spring steels are their elastic behavior, such that they become deformed in response to an external stress and recover their original shape when that stress disappears.
  • leaf spring steels must have very high tensile strength and yield strength values to assure the elastic behavior of the element. Furthermore, these properties must be maintained over time, preventing permanent plastic deformations from occurring.
  • leaf spring steels have to have a good fatigue response, considering the cyclical service demands of suspension elements; good corrosion resistance given that, due to the location thereof in the vehicle and despite incorporating paint or other anticorrosion protection, they are exposed to atmospheric inclemencies and/or to impacts that degrade their protective layer; low hydrogen embrittlement sensitivity, which accelerates the progression of small cracks, and moderate toughness at room temperature and at a low temperature, given that they can be used in environments with sub-zero temperatures.
  • leaf spring and spring steels classified as such in international standards which have a tensile strength ranging between 1300 MPa and 2000 MPa. These values, however, depend considerably on the thickness of the part and the highest values are only obtained in springs that are not very thick, in general coil springs manufactured from cold and hot formed wire rod.
  • Leaf spring steels are hot formed at a temperature between 950°C and 1150°C, after a heating process that can be from gas combustion, induction and other means and that usually is done in the presence of an oxidizing atmosphere for a more or less prolonged time, for example between 30-75 minutes for each end. These processes deteriorate the steel surface either due to decarburization, surface scale formation, which negatively affects the fatigue behavior of the part.
  • Patent no. WO-2011/074600-A1 describes a method for obtaining steel for leaf springs with a Ti alloy and Ti/N ratio ⁇ 10 which prevents bainite from occurring and, with it and a hot shot peening treatment of the component, improves the fatigue life for leaf springs with a Vickers hardness greater than 510 HV.
  • JPH-08295984-A describes a steel with a low Mn content for leaf springs with high toughness and delayed fracture strength for tensile strengths of 1650 MPa.
  • Chinese patent application no. CN-102586687-A describes a leaf spring steel with a strength of up to 1750 MPa and high-quench hardenability, but having reduced ductility even for tempering treatment at 500°C.
  • Chinese patent no. CN-102634735-A describes a high-strength alloy steel, with an addition of Cu between 0.50% and 0.80% and of rare earth elements between 0.02% and 0.07%, reaching a mechanical strength ⁇ 2000 MPa for springs having a diameter ⁇ 16 mm.
  • Patent no. WO-2012/063620-A1 relates to the invention of an Si-Mn spring steel having high corrosion resistance to be applied to cold- or hot-formed coil springs, as well as to the high-strength spring manufactured from this steel.
  • Patent no. WO-2010/110041-A1 solves the problem of the lack of ductility of high-strength steels by means of austempering an Si-Mn-Cr steel which generates a high-strength mixed bainite and retained austenite microstructure having good ductility.
  • Patent no. WO-2008/102573-A1 improves the toughness of high-strength steel by means of adding high Si and a suitable balance of tempering temperatures and times preventing the transformation of ⁇ carbides into cementite.
  • the present invention relates to a leaf spring steel, in which an optimal combination of two opposing mechanical properties, high tensile strength, with strength values of at least 1800 MPa, and high ductility, with elongation values >9% and area reduction values >30%, has been obtained as a result of several investigations.
  • the invention allows obtaining leaf spring steel from a novel chemical composition and a specific metallurgical process, having optimized and adjusted resistance to decarburization, high mechanical strength while at the same time having good ductility, in addition to having optimal quench hardenability, which is important, for example, for the complete transformation of austenite into martensite in very thick parts.
  • the heat treatment applied to the steel has a very important effect on the mechanical features of the end component, i.e., the part with the initial chemical composition is subjected to a specific method of quench hardening and tempering, which must be performed under specific time and temperature conditions.
  • the inventors have found a synergistic effect between a novel combination of chemical elements and a method for obtaining said steel which contemplates a specific heat treatment, achieving a high strength and ductility leaf spring steel for quench hardening and tempering, in addition to good hot forming ability and good quench hardenability.
  • alloy elements are used in alloy steels to improve tensile strength, tempering resistance, toughness or other characteristics, but not with the concentrations by weight indicated, not with the proposed combination of elements, or for obtaining the properties described above which allow use thereof in the mentioned applications.
  • Carbon is an indispensable element for obtaining high strength and hardness after the quench hardening and tempering treatment. Below 0.45% carbon, the strength that is obtained is insufficient. On the other hand, above 0.58%, the toughness of the steel markedly decreases and hydrogen embrittlement may be favored.
  • the optimal range is between 0.47% and 0.55%
  • Silicon is one of the most important hardeners of ferrite by solid solution. Given that it dissolves in ferrite and not in cementite, it tends to inhibit carbide precipitation, and therefore shifts the temperature of the tempering brittleness area to higher temperatures, preventing hydrogen embrittlement and improving corrosion fatigue strength. Furthermore, silicon is a powerful deoxidizer and is used as such in refining processes in iron and steel processes. Likewise it increases quench hardenability and hardens the ferrite matrix of the martensite substituting the iron atoms of the lattice.
  • silicon contributes to shifting the ductile-fragile transition temperature to higher temperatures, such that if the silicon content is very high, the steel can be embrittled to room temperature, such that the upper limit is established at 2.25% to assure an acceptable toughness, 2.00% to assure an acceptable decarburization in quench hardening and tempering treatments and 1.75% to assure acceptable decarburization in rolling and re-rolling process of the leaf spring, depending on the process used.
  • silicon tends to combine with O 2 and greater decarburization takes place, whereby it would need to be limited to 1.75% or even 1.50%.
  • decarburization is lower and the upper limit can be left at 2.00%. Silicon improves the mechanical properties of the steel.
  • Manganese is an indispensable element for assuring the required quench hardenability in steel for leaf springs. Furthermore, it reduces the transformation temperature, which allows obtaining a fine-grain crystalline structure, which at the same time allows increasing strength and improving toughness. It also prevents the harmful effect of the sulfur, combining with said sulfur to form MnS. On the other hand, an excessive content can favor the occurrence of quench hardening cracks, such that the optimal content is between 0.65% and 1.20%, even being able to be limited between 0.80-1.10%.
  • Chromium is an indispensable element for assuring the required quench hardenability in steel for leaf springs. If it is below 0.65%, the quench hardenability may not be enough and unwanted structures may occur in the core of the part. A high chromium content increases the risk of quench hardening cracks. Chromium carbides also act like local electrodes on the surface of the steel by increasing pitting corrosion and reducing the corrosion fatigue strength. The upper limit is therefore established at 1.50%. The best combination of properties is obtained for a range consisting of 0.80% to 1.25%Cr.
  • Molybdenum has a strong, quench hardenability-favoring effect, in turn being a strong carbide-forming element, providing a notable effect of secondary hardening during tempering.
  • molybdenum improves pitting corrosion resistance and prevents tempering brittleness by preventing phosphorus precipitation at the grain boundary. Nevertheless, in high contents, the alloy cost is excessive and economically unacceptable, such that the preferred range is between 0.01% and 0.40%. The best ratio between cost and characteristic is obtained between 0.10% and 0.30% of molybdenum.
  • Vanadium is a microalloying element that contributes to refining grain size and causes intense precipitation hardening, and it greatly increases quench hardenability when it remains in solid solution. Vanadium precipitates are hydrogen nucleators, such that they fix the hydrogen in corrosive environments and improve the delayed fracture strength induced by hydrogen. However, with a very high vanadium content, precipitates coalesce and their effect can become harmful. Therefore, the vanadium content must be between 0.01% and 0.40%, a content between 0.05% and 0.30% being preferable.
  • the steel proposed by the invention may comprise, in addition, at least one of the following elements or a combination thereof, with a weight percentage:
  • Phosphorus hardens steel and segregates at austenite grain boundaries, drastically reducing the toughness of the steel. Furthermore, it favors hydrogen embrittlement and delayed fracture.
  • the phosphorus content is limited to less than 0.040% in order to limit its adverse effect, a content less than 0.020% being desirable.
  • manganese sulfides form inclusions that deform longitudinally in the forging or rolling direction and considerably deteriorate the transverse mechanical properties and the fatigue behavior.
  • a minimum amount of sulfur is required to improve the machinability of the steel and the forming thereof by machining. Therefore, the sulfur content is restricted to less than 0.040%, a content less than 0.015% being preferable.
  • Adding copper prevents decarburization of steel and improves corrosion resistance in a manner similar to nickel by inhibiting the growth of corrosion-induced pitting.
  • a high copper content impairs hot ductility of steel, such that the upper limit of copper is established at 0.50%.
  • the maximum copper content must be limited to 0.30%.
  • Aluminum is an element that acts as a powerful deoxidizer during the steel manufacturing process. Aluminum forms aluminum nitrides which contribute to controlling the austenitic grain size during heat treatments and heating prior to hot forming processes. Nevertheless, it forms very hard oxides which are very prejudicial for fatigue life, such that the upper limit thereof is established at less than 0.050%.
  • Niobium is a microalloying element having effects that are similar to those of vanadium in controlling grain size and in precipitation-induced hardening of steel, such that it contributes to increasing mechanical strength and to improving toughness. Furthermore, niobium precipitates fix the hydrogen attacking the steel in corrosive environments, improving delayed fracture strength. Content above 0.100%, nevertheless, causes coarsening of the precipitates, which is prejudicial for the mechanical properties. The optimal niobium content is established between 0.001% and 0.100%.
  • the titanium is an effective austenitic grain size controller at a high temperature, typically at hot forging temperatures. However, given its affinity for nitrogen, it forms titanium nitrides at temperatures close to those of liquid steel, which transforms its precipitates into extremely hard inclusions that are harmful for fatigue life.
  • the titanium content in the steel is limited to a maximum of 0.050% to limit excessive titanium nitride coarsening.
  • Nitrogen combines with Ti, Nb, Al and V to form nitrides, the precipitation temperatures of which depend on the respective content of the different elements and on constant characteristics. With a suitable size, those nitrides exert a pinning effect on the austenitic grain, controlling its size at high temperature and preventing the coalescence and growth thereof. However, if the nitrogen content or the microalloying element content is very high, precipitation occurs at a high temperature, and the precipitates become coarse, becoming ineffective for controlling the grain and prejudicial for fatigue life. Therefore, the nitrogen content in the steel is limited to 0.004% to 0.020%.
  • spring steels having a standard composition and process for truck leaf spring applications which have been subjected to a conventional quench hardening and tempering process did not come to have the required and previously discussed mechanical properties due to fact that the degree of inclusion cleanliness was lower and the balance of microalloying elements was inadequate with respect to the optimized process that the steel of the invention has.
  • a preferred composition of the steel proposed by the invention comprises a weight percentage:
  • the steel can additionally comprise at least one of the following elements, or a combination thereof, by weight:
  • This entire method of manufacturing steel allows achieving low sulfur levels, below 0.015% by weight, and low phosphorus levels, below 0.020% by weight, in addition to a low inclusion level.
  • CCT Continuous Cooling Transformation
  • the solidification products are subsequently transformed when hot by means of a process consisting of heating at a temperature greater than 1100°C and a series of consecutive deformations by means of hot forging or rolling until obtaining an intermediate product having the suitable section, shape and microstructure.
  • the invention contemplates performing a method whereby said part of steel is obtainable.
  • the method for obtaining parts of said steel comprises a hot forming process, with prior heating at a temperature greater than 950°C which allows providing the steel with sufficient ductility when hot, to confer to the part of steel a shape similar to that of the end component. After being shaped, the part is left to be air cooled.
  • the method subsequently contemplates a quench hardening process which is performed with austenization at a temperature greater than 800°C, followed by final forming operations and then subsequent cooling, for example, in oil.
  • the steel of the invention has a balance of alloy elements that allows obtaining 100% martensite, even in the thicker sections, without increasing the risk of cracks resulting from quench hardening appearing due to the stress produced during cooling.
  • the method then comprises a tempering process, which is carried out at a temperature greater than 300°C for at least one hour, thus achieving the adjustment of the hardness and toughness of the material, in addition to preventing decreases in resilience, which are associated with the brittleness phenomenon of tempering.
  • the method comprises carrying out a shot peening process by applying or not applying stress to the component at a temperature between 0°C and 400°C, usually at room temperature, to generate in surface regions of the component residual compressive stresses improving its fatigue behavior.
  • a surface coating or painting that improves the corrosion behavior of the component can additionally be applied.
  • the method for obtaining parts of steel comprises the following steps:
  • Table 2 includes the ideal critical diameter values according to data tabulated in the ASTM A255-02 standard for each of the compositions and steels described in Table 1.
  • Illustration 1 shows the Jominy quench hardenability curve diagram obtained for each of steels A-H.
  • Steels C, D and H show virtually flat curves, whereas drops in hardness occur in steels A, B, E, F and G at distances from the quench hardened end equal to or less than 40 mm. /*/*]
  • steels A, B, C, D, E, F and G do not attain strength of 1800 MPa, maintaining a minimum area reduction ⁇ 30%.
  • Steels B, C and E have low silicon contents, such that they do not attain 2000 MPa for any tempering temperature.
  • steels A, D and F do not achieve desired ductility levels despite having a high silicon content and exceeding 1800 MPa for tempering temperatures equal to or less than 400°C, because the alloying element combination is not the suitable combination for attaining the required mechanical features.
  • steel H which has a chemical composition within the limits object of the invention, i.e., it is the steel proposed by the invention, it has been found that after being subjected to a quench hardening and tempering treatment, consisting of quench hardening plus tempering, said steel attains the required mechanical features and its quench hardenability is high.
  • the steel of the invention has a moderate decarburization similar to steels commonly used in leaf spring, which allows the processing thereof by hot forming without deteriorating the surface quality of the steel bar.
  • Figures 2, 3 and 4 show the surface of steels B, H and A, respectively, after heating at 960°C for 110 minutes.
  • Figures 5 , 6 and 7 show the decarburized layer of the same steels B, H and A after heating at 1030°C for 35 minutes, followed up by heating at 960°C for 45 minutes.
  • the first treatment is similar to the heating prior to the re-rolling of the leaf spring and the second treatment is similar to the austenization process prior to quench hardening.
  • Steel A with 1.94% Si, as can be seen in Figure 7 , shows a very thick decarburized layer (0.20-0.25 mm) that negatively influences the performances of the part.
  • Steel H located within the limits of the invention, has the quench hardening necessary to assure 100% martensite in thick sections, without generating excessive stress during quench hardening giving rise to cracks. Likewise, it acquires a resistance of 1800 MPa through thermal quench hardening and tempering treatment, maintaining a minimal area reduction ⁇ 30%. All this by maintaining a resistance to decarburization that is sufficient so as not to lose surface mechanical properties.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
  • Heat Treatment Of Steel (AREA)
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EP15795207.8A 2015-07-28 2015-07-28 Stahl für federn von hoher festigkeit und härtbarkeit Withdrawn EP3330400A1 (de)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/ES2015/070582 WO2017017290A1 (es) 2015-07-28 2015-07-28 Acero para ballestas de alta resistencia y templabilidad

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EP3330400A1 true EP3330400A1 (de) 2018-06-06

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JPS6086245A (ja) * 1983-10-17 1985-05-15 Daido Steel Co Ltd ばね用鋼
JP2842579B2 (ja) 1991-10-02 1999-01-06 株式会社 神戸製鋼所 疲労強度の優れた高強度ばね用鋼
JP3255296B2 (ja) 1992-02-03 2002-02-12 大同特殊鋼株式会社 高強度ばね用鋼およびその製造方法
JP3031816B2 (ja) * 1994-04-04 2000-04-10 三菱製鋼株式会社 低脱炭性ばね用鋼
JPH08170152A (ja) * 1994-12-16 1996-07-02 Kobe Steel Ltd 疲労特性の優れたばね
JPH08295984A (ja) 1995-04-25 1996-11-12 Aichi Steel Works Ltd 耐遅れ破壊性に優れた板ばね用鋼
JP3577411B2 (ja) 1997-05-12 2004-10-13 新日本製鐵株式会社 高靭性ばね鋼
US20070256765A1 (en) 2004-08-26 2007-11-08 Kazuyoshi Kimura High Strength Spring Steel, High Strength Springs and Manufacturing Method Thereof
JP5064060B2 (ja) 2007-02-22 2012-10-31 新日本製鐵株式会社 高強度ばね用鋼線及び高強度ばね並びにそれらの製造方法
JP4927899B2 (ja) 2009-03-25 2012-05-09 日本発條株式会社 ばね用鋼およびその製造方法並びにばね
JP5520591B2 (ja) * 2009-12-18 2014-06-11 愛知製鋼株式会社 高疲労強度板ばね用鋼及び板ばね部品
JP5250609B2 (ja) 2010-11-11 2013-07-31 日本発條株式会社 高強度ばね用鋼、高強度ばねの製造方法及び高強度ばね
CN102586687A (zh) 2012-01-09 2012-07-18 东风汽车悬架弹簧有限公司 一种高强度及高淬透性的弹簧钢材料
CN102634735B (zh) 2012-04-09 2013-11-27 广州市奥赛钢线科技有限公司 一种汽车悬架用弹簧钢及其制备方法和应用

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