EP3061837A1 - Blank bainite long product and method for producing the same - Google Patents

Abstract

A bright bainitic long product has a weight fraction of 0.16 to 0.26% carbon, 0.60 to 1.20% silicon, 1.20 to 1.70% manganese, 0.70 to 1.60% chromium, up to 0.20% nickel, up to 0.30% molybdenum, up to 0.03% sulfur, up to 0.01% aluminum, up to 0.03% phosphorus, up to 0.25% copper, up to 0.001% boron, up to 0.01% titanium, up to 0.01% vanadium. up to 0.01% niobium, up to 0.015% nitrogen and up to 0.01% oxygen bound in oxidic inclusions, the remainder iron as well as common impurities. The following structural components are present: 60 to 80% bainite, up to 30% martensite, up to 8% ferrite and up to 15% retained austenite. The tensile test according to ISO 6892-1 gives: Rp0.2 = 950 to 1'400 MPa, Rm = 1'150 to 1'400 MPa, 0.87 <Rp0.2 / Rm <0.99 and A5 = 9.0 to 17.0%.

Description

Technical area

The invention relates to a bare bainitic long product according to the preamble of claim 1 and uses thereof and a process for its preparation.

State of the art

Downsizing tendencies or demands for higher performance with lower environmental impact in the automotive industry lead to the use of new materials with higher strength.

Bainitic steels cooled continuously from the forming heat offer similar strength and toughness properties under static load as heat-treated steels with a hardened structure. However, while tempered steels soften under dynamic load, the bainitic steels are solidified. Due to the TRIP effect (deformation-induced transformation of austenite into martensite), bainitic steels also behave more favorably with overloads (voltage peaks). This manifests itself in a higher time stability.

With the same initial strength, the fatigue properties (fatigue strength, fatigue strength, notch sensitivity, service life) of bainitic steels are therefore clearly superior to tempered steels.

For example, the use of bainitic materials with a tensile strength of about 1,300 MPa would increase the performance of common rails or injectors and thus of the entire internal combustion engine. The WO 2009/090155 A1 describes such an application for a bainitic-ferritic material without TRIP effect.

Despite the advantages, these innovative steels have so far only been able to assert themselves in isolated cases. The high strength brings with it new challenges for parts production. In cutting processes, the cutting forces and the temperatures on the tool increase. This leads to a shorter tool life or to a lower productivity. By optimizing tool geometry and machining parameters, losses in productivity can often be partially compensated (see, eg Biermann, D .; Hartmann, H. Challenges in the machining of high-strength steels. In: Steel and Casting Machining 2013 - Innovative Machining Processes for Economic Production, 12.11.-13.11. 2013, Kassel, ISBN 978-3-942980-33-3, pp. 129-138 The inverse correlation between machinability and strength still remains.

There is therefore a need for machined high strength bainitic steels. This applies in particular to bright steel products, which are used in a drawn version for cutting component production.

EP 2103704 A1 describes such a product. By the inventive production process can be dispensed with the addition of aluminum and titanium. This avoids the formation of abrasive oxide inclusions. A high sulfur content of 0.04 to 0.25% additionally improves the machinability. The sulfur forms with manganese sulfide inclusions, which on the one hand facilitate the chip breaking and on the other hand form a protective layer on the tool. Tool life and productivity are increased. At the same time, however, the manganese sulfides are also material weaknesses, which can cause cracks under dynamic load. This reduces the life expectancy of dynamically loaded components. Therefore, this steel can not be used for highly stressed safety components.

EP 1426453 A1 describes a forging of a bainitic steel, which should be suitable for machining. The good machinability is in In this case, too, only achieved by alloying with sulfur, tellurium, selenium, bismuth or lead (see there claims 6 and 7). Both the environmental compatibility and the suitability for highly stressed components is thereby limited.

A bright bainitic long product with a bainitic microstructure that can be machined well in machining is currently not yet available for use in dynamically stressed components.

Presentation of the invention

The object of the invention is to provide a bright bainitic long product, with which in particular the above disadvantages are avoided. Further objects of the invention relate to uses of the long product according to the invention. Yet another object of the invention is to provide a process for producing a bare bainitic long product.

The term "bright long product" refers to a drawn, cold-formed long product according to DIN EN 10'277-1.

These objects are achieved by the defined in claim 1 bare bainitic long product, the uses defined in claims 5 and 6 and the method defined in claim 7.

The following percentages by weight (%) or parts per million (ppm) refer to parts by weight unless expressly stated otherwise.

• 0.16 bis 0.26 % Kohlenstoff,
• 0.60 bis 1.20% Silizium,
• 1.20 bis 1.70% Mangan,
• 0.70 bis 1.60% Chrom,
• bis zu 0.20% Nickel,
• bis zu 0.30% Molybdän,
• bis zu 0.03% Schwefel,
• bis zu 0.01% Aluminium,
• bis zu 0.03% Phosphor,
• bis zu 0.25% Kupfer,
• bis zu 0.001% Bor,
• bis zu 0.01% Titan,
• bis zu 0.01 % Vanadium
• bis zu 0.01% Niob
• bis zu 0.015% Stickstoff und
• bis zu 0.01% in oxidischen Einschlüssen gebundener Sauerstoff
• der Rest Eisen sowie stahlübliche Verunreinigungen, wobei folgende Gefügebestandteile vorliegen
• 60 bis 80% Bainit,
• bis 30% Martensit,
• bis zu 8 % Ferrit und
• bis 15% Restaustenit und wobei der Zugversuch nach ISO 6892-1 ergibt:
  • Rp0.2 = 950 bis 1'400 MPa
  • Rm = 1'150 bis 1'400 MPa
  • 0.87 < Rp0.2/Rm < 0.99
  • A5 = 9.0 bis 17.0%.

The inventive bare bainitic long product has a weight fraction of

• 0.16 to 0.26% carbon,
• 0.60 to 1.20% silicon,
• 1.20 to 1.70% manganese,
• 0.70 to 1.60% chromium,
• up to 0.20% nickel,
• up to 0.30% molybdenum,
• up to 0.03% sulfur,
• up to 0.01% aluminum,
• up to 0.03% phosphorus,
• up to 0.25% copper,
• up to 0.001% boron,
• up to 0.01% titanium,
• up to 0.01% vanadium
• up to 0.01% niobium
• up to 0.015% nitrogen and
• Up to 0.01% oxygen bound in oxidic inclusions
• the remainder iron and common steel impurities, the following structural constituents being present
• 60 to 80% bainite,
• up to 30% martensite,
• up to 8% ferrite and
• up to 15% residual austenite and the tensile test according to ISO 6892-1 gives:
  • Rp0.2 = 950 to 1400 MPa
  • Rm = 1'150 to 1'400 MPa
  • 0.87 <Rp0.2 / Rm <0.99
  • A5 = 9.0 to 17.0%.

• Herstellen einer Stahllegierung mit einem Gewichtsanteil von
  • 0.16 bis 0.26 % Kohlenstoff,
  • 0.60 bis 1.20% Silizium,
  • 1.20 bis 1.70% Mangan,
  • 0.70 bis 1.60% Chrom,
  • bis zu 0.20% Nickel,
  • bis zu 0.30% Molybdän,
  • bis zu 0.03% Schwefel,
  • bis zu 0.01% Aluminium,
  • bis zu 0.03% Phosphor,
  • bis zu 0.25% Kupfer,
  • bis zu 0.001% Bor,
  • bis zu 0.01% Titan,
  • bis zu 0.01 % Vanadium
  • bis zu 0.01% Niob
  • bis zu 0.015% Stickstoff und
  • bis zu 0.01% in oxidischen Einschlüssen gebundener Sauerstoff,
  • der Rest Eisen sowie stahlübliche Verunreinigungen,
  und Vergiessen zu einer Form vordefinierten Formates ("Stahlherstellung");
• Wiedererwärmen des Einheitsformats, Warmwalzen zu Draht oder Stab und Abkühlen auf eine Temperatur unter 400°C mit einer Abkühlrate von 0.1 bis 8.0 K/s ("Walzen");
• chemische oder mechanische Entfernung der äusseren Eisenoxidhaut ("Entzundern");
• Ziehen des Walzstahls durch ein Werkzeug mit vordefiniertem Innenprofil ("Ziehen") bei einer Temperatur von 25 bis 600°C, wobei es sich vorzugsweise um ein Rund- oder Sechskantprofil handelt;
• mechanisches Richten zur Erzeugung einer geeigneten Geradheit ("Richten");
• Entspannen bei einer Temperatur von 250 bis 600°C ("Entspannen").

The production method according to the invention comprises the following steps:

• Producing a steel alloy with a weight fraction of
  • 0.16 to 0.26% carbon,
  • 0.60 to 1.20% silicon,
  • 1.20 to 1.70% manganese,
  • 0.70 to 1.60% chromium,
  • up to 0.20% nickel,
  • up to 0.30% molybdenum,
  • up to 0.03% sulfur,
  • up to 0.01% aluminum,
  • up to 0.03% phosphorus,
  • up to 0.25% copper,
  • up to 0.001% boron,
  • up to 0.01% titanium,
  • up to 0.01% vanadium
  • up to 0.01% niobium
  • up to 0.015% nitrogen and
  • up to 0.01% oxygen bound in oxidic inclusions,
  • the remainder iron and common impurities,
  and casting into a form of predefined format ("steelmaking");
• Reheating the unitary format, hot rolling into wire or rod and cooling to a temperature below 400 ° C with a cooling rate of 0.1 to 8.0 K / s ("rolling");
• chemical or mechanical removal of the outer iron oxide skin ("descaling");
• Pulling the rolled steel through a tool with a predefined inner profile ("pulling") at a temperature of 25 to 600 ° C, which is preferably a round or hexagonal profile;
• mechanical straightening to produce a suitable straightness ("straightening");
• Relax at a temperature of 250 to 600 ° C ("relax").

While hot-formed bainitic products (forging, bar steel, etc.) have a low yield strength Rp0.2 and thus a low Rp0.2 / Rm ratio (in the examples of EP 1425453 If the Rp0.2 / Rm ratio is between 0.6 and 0.7), this can be achieved by the combination of pulling and subsequent relaxation treatment can be set almost any Rp0.2 / Rm ratios between 0.6 and 1.0.

It has surprisingly been found that the steel composition according to the invention can be machined significantly better at a constant predetermined tensile strength if the yield ratio Rp0.2 / Rm is above 0.87. The significantly improved machinability manifests itself in significantly longer tool life.

In the bare bainitic long product produced according to the invention, the alloying components are selected such that, at conventional cooling rates from 0.1 to 8.0 K / s, a bainitic-martensitic microstructure always results with a tensile strength level of 900 to 1,200 MPa, without costly alloying elements and / or or special equipment for accelerated cooling from the rolling heat must be used. In order to be able to adjust the properties according to the invention, the product must be drawn in the temperature range between room temperature and 600 ° C. and then tempered in a temperature range of 250 and 600 ° C.

• Im Ziehprozess werden bei Abzügen über 10% (=Querschnittsreduktion) Streckgrenzenverhältnisse Rp0.2/Rm von 0.98 - 1.00 eingestellt. Beim anschliessenden Anlassen sinkt das Streckgrenzenverhältnis Rp0.2/Rm, wobei es erfindungsgemäss auf über 0.87 zu halten ist. Bei Anlasstemperaturen über 600°C fällt das Streckgrenzenverhältnis Rp0.2/Rm in der Regel unter diesem Wert.

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• In the drawing process yield ratios Rp0.2 / Rm of 0.98 - 1.00 are set for deductions over 10% (= reduction in cross section). During the subsequent tempering, the yield ratio Rp0.2 / Rm decreases, and according to the invention it is to be maintained above 0.87. At tempering temperatures above 600 ° C, the yield ratio Rp0.2 / Rm usually falls below this value.

The lower limit of the carbon content to 0.16% in combination with manganese and chromium ensures that only small amounts of ferrite are present in the microstructure. Ferrite levels above 5% affect both the strength level and the impact strength of the product.

The upper limit of the carbon to 0.26% ensures that the tensile strength does not rise above 1400 MPa. Higher strength values degrade machinability in the downstream drawing or machining process. Higher carbon contents also promote the formation of carbides, which adversely affects ductility.

Silicon suppresses the formation of hard and in machining abrasive Fe ₃ C precipitates (cementite). The chosen silicon concentration allows a one-hour annealing treatment at 400 ° C without coarse cementite precipitates can form (based on the description of the carbide-free bainite in WO 96/22396 ). Since silicon is an efficient solid solution hardener in bainite, its content must be limited to 1.2% by weight so as not to exceed the maximum desired tensile strength of 1400 MPa.

The lower limit in manganese to 1.20% by weight and chromium to 0.70% by weight ensures that a bainitic structure can be produced from the forming heat when the air cools down.

If the manganese content is too high, the manganese segregations are pronounced and the microstructure becomes very inhomogeneous. For this reason, the manganese content must be limited to 1.70 wt .-%.

Chromium constricts the bainite area, which favors the formation of martensite. For this reason, the chromium content must be limited to 1.60 wt .-%.

If the steel is to be tempered in the temperature range between 250 and 600 ° C, molybdenum can be alloyed to suppress possible temper embrittlement. The precipitation of iron carbides at the primary grain boundaries and a related loss of toughness can thus be prevented. For reasons of cost, the molybdenum content should be as low as necessary: up to 0.3% by weight of molybdenum.

Nickel improves notched impact strength and therefore has a negative impact on machinability. Therefore, the nickel content is limited to 0.30 wt .-%.

Sulfur is a steel pest. It forms manganese sulfide precipitates and weakens the microstructure. This has a negative effect on the fatigue strength of dynamically loaded components. For this reason, the sulfur content was limited to 0.03 wt .-%.

The addition of aluminum is not mandatory for the production of the product according to the invention. If a subsequent heat treatment of the product, e.g. case hardening is necessary to set a wear-resistant surface, then austenite grain stability can be ensured by adding aluminum. In this case, aluminum contents of 0.02 to 0.04 wt.% Are common.

Phosphorus is a steel pest. It goes to the Austenitkorngrenzen and weakens the structure. This has a negative effect on the fatigue strength of dynamically loaded components. For this reason, the phosphorus content was limited to 0.03 wt .-%.

Copper is a steel pest. At high copper contents, red brittleness occurs in hot forming. For this reason, the copper content is limited to 0.25 wt .-%.

Hard oxide or nitride inclusions have a negative effect on the machinability above a certain size. In particular titanium (titanium oxides and coarse titanium carbonitrides) and aluminum (hard Al ₂ O ₃ compounds) deteriorate both the machinability and the life of dynamically loaded components. For niobium and vanadium, a negative effect must also be assumed. The microalloying elements titanium, aluminum, niobium and vanadium are therefore limited to 0.01% by weight.

Boron is usually alloyed with bainitic steels to suppress the formation of ferrite. For the boron to be effective in this sense, the nitrogen present in the steel must be bound with titanium. Since the addition of titanium is to be avoided, this alloy concept can not be used here. Boron will therefore be present only in traces to 0.001 wt .-%.

No aluminum is added to the bare bainitic long product produced according to the invention. In combination with a high silicon content and a low calcium addition at the end of the metallurgical treatment according to claim 2 oxide inclusions should be set with an Al ₂ O ₃ content of <50%. Preferably, the metallurgical treatment is carried out so that soft, glassy silicate inclusions are formed with the following relative proportions by weight: 20 to 50% CaO, 25 to 65% SiO ₂ and less than 30% Al ₂ O ₃ . The tool life of the tools used in machining is then significantly extended.

Nitrogen reaches the austenite grain boundaries and weakens the microstructure. This has a negative effect on the fatigue strength of dynamically loaded components. For this reason, the nitrogen content was limited to 0.0150% by weight.

Before further processing of the bare bainitic long product, a heat treatment up to 2 hours at 300 to 600 ° C according to claim 8 may be useful. Tensions introduced by strain hardening are thereby reduced.

Ways to carry out the invention

Fig. 1

die Ergebnisse für die Werkzeugstandzeit bei zwei Werten der Werkstofffestigkeit Rm in Abhängigkeit des Streckgrenzenverhältnisses Rp0.2/Rm.

An embodiment of the invention will be described in more detail below with reference to the drawing, in which:

Fig. 1

the results for the tool life at two values of the material strength Rm as a function of the yield ratio Rp0.2 / Rm.

In one embodiment, a steel melt "steel 1" was cast and then rolled into bar steel. The molten steel was produced by the electrical steel process with a secondary metallurgical treatment on a ladle and subsequent casting to 150 × 150 mm ² sticks in a continuous casting plant. The billets were then reheated in a walking beam oven to 1'150 to 1'200 ° C and then rolled into bar steel in diameter 043 mm. The cooling of the rods after rolling was carried out in air (cooling rate about 1.0 K / s).

In the bright steel production, the rods were shot peened to remove the iron oxide skin. Thereafter, the rods were either drawn at room temperature or in the temperature range between 350 and 450 ° C by a multi-stage die to the final dimension of 040 mm and directed in a two-roll straightening machine. The final conductive heating at 350 resp. 500 ° C was used to reduce deformation-induced surface tensions.

The machinability of the manufactured variants was tested in a standard test at the ISF in Dortmund. The test specimens used were double-turned and one-sided center bored rods with a length of 190 mm. The tests were carried out under the type Becher Avantin 3309 emulsion with a concentration of about 6-8%. The turning tool (CNMG120404-PF4215) was selected to allow the standard steel 42CrMo4 + QT (at Rm ≈ 1'000 MPa) to have an industrially common machining time of 18 minutes without violating a wear _{mark width criterion} of V _Bmax ≤ 300 μm. At a feed f of 0.3 mm / rev and a cutting depth a _p of 1 mm, the cutting speed was 200 m / min

As the machinability decreases with increasing material strength Rm (the tool life is shortened), a comparative test only makes sense with constant material strength. However, given a given steel composition, it is not possible to adjust any combination of tensile strength Rm and yield ratio Rp0.2 / Rm by varying the production and heat treatment parameters. For this reason, the manufactured variants were compared with the steel 2 and with reference steels known from the literature steel 3 and steel 4, which had also been characterized at the ISF in Dortmund by the same test method.

Table 1 shows the steel compositions used for this comparison. The composition of the existing oxide inclusions was analyzed by scanning electron microscope with EDX. The average composition (inclusions measured at 2222) in the normalized ternary system CaO-Al ₂ O ₃ -SiO ₂ was CaO = 46.3%, Al ₂ O ₃ = 27.9%, SiO ₂ = 25.8%.

The structural components of steel 1 were determined quantitatively in the scanning electron microscope (for bainite and ferrite) and in the X-ray diffractometer (for retained austenite). Within the measuring accuracy the differences of the three variants are small. The results are shown in Table 2 for the various heat treatments. For steel 2 there is a 100% compensation structure (tempered martensite). The steels 3 and 4, similar to steel 1, have a mixed structure of bainite, martensite and retained austenite. <u> Table 1 </ u> steel1 Steel2 Stahl3 steel4 Wt .-% Wt .-% Wt .-% Wt .-% C 12:17 12:51 12:22 12:17 Si 1.20 12:26 12:49 1:46 Mn 1:49 0.67 1.72 1:52 P 0015 0016 0019 0013 S 0015 0007 0013 0017 Cr 1.21 1:06 1:58 1:32 Not a word 12:28 12:20 12:30 12:06 Ni 12:07 12:18 12:19 12:16 al 0005 0029 0005 0016 B - - - 0.0021 Cu 12:11 12:22 0015 12:13 Nb - - 0095 0029 Ti - - 0032 0029 N 0.0099 0.0070 0.0258 0.0113 bainit martensite austenite ferrite % % % % Pulled at RT Relaxed at 350 ° C 71 ± 3 23 ± 5 3 ± 3 3 ± 1 Pulled at 400 ° C Relaxed at 350 ° C 72 ± 3 18 ± 5 7 ± 3 3 ± 1 Pulled at 400 ° C Relaxed at 500 ° C 76 ± 3 11 ± 5 9 ± 3 4 ± 1

Table 3 shows the tool life results for a selected material strength of Rm = 1,230 ± 30 MPa. The correlation between yield ratio Rp0.2 / Rm and tool life was surprisingly found. Increasing the Rp0.2 / Rm ratio more than doubled tool life. <u> Table 3 </ u> A5 rm Rp0.2 / Rm Service life (V _B = 300 μm) % MPa MPa min Steel 1 * Drawn at RT Relaxed at 350 ° C 11.6 1234 0.94 * 12 Steel 1 * Pulled at 400 ° C Relaxed at 350 ° C 10.5 1231 0.99 * 20 Steel 2 14.1 1202 0.85 8th Steel 3 14.4 1253 0.7 6.5 * According to the invention

For a review of this relationship at Rm = 1'165 MPa, only two steel variants were available (Table 4). Since the wear is slower due to the lower strength, the criterion for the wear mark width was reduced to 200 μm. The comparison shows that even with a Rp0.2 / Rm ratio of 0.88 significantly better tool life is achieved. <u> Table 4 </ u> A5 rm Rp0.2 / Rm Service life (V _B = 200 μm) % MPa MPa min Steel 1 * Pulled at 400 ° C Relaxed at 500 ° C 16.2 1163 0.88 * 18 Steel 4 12.5 1167 0.67 9 * According to the invention

The results from Tables 2 and 3 are in FIG. 1 summarized.

Claims (7)

A bright bainitic long product with a weight fraction of 0.16 to 0.26% carbon, 0.60 to 1.20% silicon, 1.20 to 1.70% manganese, 0.70 to 1.60% chromium, up to 0.20% nickel, up to 0.30% molybdenum, up to 0.03% sulfur, up to 0.01% aluminum, up to 0.03% phosphorus, up to 0.25% copper, up to 0.001% boron, up to 0.01% titanium, up to 0.01% vanadium up to 0.01% niobium up to 0.015% nitrogen and up to 0.01% oxygen bound in oxidic inclusions, the remainder iron and common impurities, the following structural constituents are present: 60 to 80% bainite, up to 30% martensite, up to 8% ferrite and up to 15% retained austenite and wherein the tensile test gives ISO 6892-1: Rp0.2 = 950 to 1400 MPa Rm = 1'150 to 1'400 MPa 0.87 <Rp0.2 / Rm <0.99 A5 = 9.0 to 17.0%. The bare bainitic long product of claim 1, wherein Rp0.2 / Rm = 0.87 to 0.94. The bare bainitic long product according to claim 1 or 2, characterized in that it contains oxide inclusions with less than 50 wt .-% Al ₂ O ₃ . The bare bainitic long product according to claim 3, wherein the oxide inclusions have the following relative parts by weight: 20 to 50% CaO, 25 to 65% SiO ₂ and less than 30% Al ₂ O ₃ . Use of a bare bainitsichen long product according to any one of claims 1 to 4 for machining. Use of a bare long baintischen product according to any one of claims 1 to 4 for the production of dynamically loaded automotive components, in particular in the field of diesel injection. A process for producing a bare bainitic long product according to any one of claims 1 to 4, comprising the following steps: - Manufacture of a steel alloy with a weight fraction of 0.16 to 0.26% carbon, 0.60 to 1.20% silicon, 1.20 to 1.70% manganese, 0.70 to 1.60% chromium, up to 0.20% nickel, up to 0.30% molybdenum, up to 0.03% sulfur, up to 0.01% aluminum, up to 0.03% phosphorus, up to 0.25% copper, up to 0.001% boron, up to 0.01% titanium, up to 0.01% vanadium up to 0.01% niobium up to 0.015% nitrogen and up to 0.01% oxygen bound in oxidic inclusions, the remainder iron and common impurities in the steel,
and casting into a form of predefined format; - reheating the unitary format, hot rolling to wire or rod and cooling to a temperature below 400 ° C with a cooling rate of 0.1 to 8.0 K / s; - chemical or mechanical removal of the outer iron oxide skin; - Pulling the rolled steel through a tool with predefined internal profile at a temperature of 25 to 600 ° C; - mechanical straightening; - Relax at a temperature of 250 to 600 ° C.

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