EP2103704B1 - Hot-rolled long product and method for its manufacture - Google Patents

Description

Technical area

The invention relates to a hot-rolled long product according to the preamble of claim 1 and a method for its production.

State of the art

In order to be able to produce steel components with high strength and high toughness, tempered steels are generally used. With tempered steels it is possible to achieve tensile strengths of more than 1 000 MPa with simultaneous necking of more than 45%. The necessary heat treatment (heating, quenching, tempering) is cost-intensive and polluting. If carried out on the finished component, expensive post-processing (straightening, grinding) may be necessary due to distortion. Pre-tempered steels have significant disadvantages in machining (long chips, low tool life). These processing disadvantages can be somewhat reduced by the addition of a maximum of 0.04 wt.% Sulfur. Higher levels of sulfur degrade the manufacturability and microscopic purity of these Al-alloyed steels.

In order to avoid austenitic grain coarsening during the necessary heat treatment, the tempered steels are alloyed with at least 0.015% aluminum. During steel production, hard and abrasive Al ₂ O ₃ -containing oxide inclusions are produced during the cutting process, which have a negative effect on tool life. In order to achieve good machinability, these inclusions must be converted into less abrasive calcium aluminate inclusions by adding calcium in a complex metallurgical process.

As an alternative to tempered steels, ferritic-martensitic dual-phase steels were developed. The structure of these steels is via a thermomechanical Treatment achieved during hot rolling. With these steels, good toughness properties can only be set as long as the stored martensite islands remain small enough. The achievable tensile strength is thereby limited to below 1'000 MPa.

Another development is the direct-hardening soft-martensitic steels. A disadvantage of these steels is that the required martensitic structure is achieved only by accelerated cooling with high quenching rate from the forming heat. For this reason, this method is mainly used in thin-walled parts (forgings, pipes) its application. For products with medium or large expansion, the set structure is unacceptably inhomogeneous across the cross section. For the production of hot-rolled long products such as wire rod and steel bars in conventional dimensions, these steels are therefore not suitable.

Another development direction is taken with the AFP (precipitation-hardening ferritic-pearlitic) steels. By a controlled cooling from the forming heat carbonitrides of the elements titanium, vanadium and niobium are eliminated. These then lead due to the dispersion hardening to a higher strength of the base material. Compared to tempered steels, they have a low yield strength and low toughness. For use in the field of high loads, they are therefore unsuitable. A controlled adjustment of the waste products requires tight analysis specifications for the steel and precise control of the cooling from the forming heat.

Recent developments show that very good combinations of properties with complex phase steels can be achieved even with air cooling directly from the forming heat. These steels usually have a bainitic-martensitic structure with residual amounts of ferrite and retained austenite.

First applications of complex phase steels are found today in the production of tubes of flat steel as well as in the production of rails.

Steels for pipe production must be characterized in particular by good toughness and weldability. For this to be achieved, a low carbon content of less than 0.13 wt% is required. The desired high-strength, tough structure is achieved by accelerated cooling from the rolling heat. In the temperature range of 800 to 500 ° C (range of conversion) cooling rates of 10 to 40 K / s are used. The structure of these steels then consists of allotriomorphic ferrite and bainite (at least 20%). The low carbon content of the accelerated cooling guarantees the avoidance of high martensite content, which enables good toughness properties. The tensile strength is thereby limited to below 1'000 MPa.

In particular, the wear and fatigue strength play an important role in the production of rail steel. In WO 96/22396 describes the production of a bainitic rail with conventional continuous cooling from the rolling heat. To achieve the desired wear resistance, the formation of coarse cementite particles must be suppressed ("carbide-free bainite"). This can be done by the addition of silicon. The kinetics of cementite excretion is thereby slowed down. These steels are not suitable for machining.

US 2003 0084 965 discloses a machinable steel with 0.1-0.6 C, 0.01-2.0 Si, and 0.005-0.20 s.

The in CN 1477226 bainitic-martensitic steel (C = 0.15 to 0.34%) described achieves a tensile strength of over 900 MPa. Manganese levels in excess of 1.8% are foreseen. Although this high manganese content facilitates the adjustment of the bainitic structure at conventional cooling rates for hot rolling. At the same time, however, it leads to difficult to control seeding problems, which manifest themselves in undesirable martenseats. The mechanical-technological properties are therefore subject to unacceptable fluctuations for conventional hot-rolled long products. The machining is severely impaired by the irregularly present martensite parts.

In EP 0845544 (C ≤ 0.12%) describes a microalloyed bainitic steel which has a tensile strength of more than 1000 MPa at room temperature. To achieve these properties, the steel is austenitized again after rolling and then quenched at a cooling rate of 17 to 150 K / s. These cooling rates are well above the air cooled long products in conventional rolling mills.

DE 102005052069 describes a B / Ti-alloyed bainitic-martensitic steel for hot-rolled long products. The required N content requires an additional degassing treatment. The described steel is suitable for thin wire dimensions with air cooling or for thicker wire dimensions with accelerated cooling. Compared to ferritic-pearlitic tempered steels, significantly lower machinability in machining can be expected. The alloy concept limits (because of the formation of titanium carbosulfides) the use of machining additives such as sulfur. For this reason, the economic use is limited to the massive forming.

An easily machinable bainitic-martensitic complex phase steel for the production of air-cooled conventionally hot-rolled long products in a size range of 5.0 to 70 mm is not yet available. The material concept must be designed in such a way that the dimensional differences in the cooling rate of approx. 0.1 to 8.0 K / s do not lead to any significant fluctuations in the mechanical-technological properties of the final product.

Presentation of the invention

The object of the invention is to provide an improved hot-rolled long product, with which in particular the above disadvantages are avoided. Another object of the invention is to provide a process for producing a hot-rolled long product.

These objects are achieved by the defined in claim 1 hot rolled long product and defined in claim 6 manufacturing process.

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

• 0.20 bis 0.25% Kohlenstoff,
• 0.90 bis 1.35% Silizium,
• bis zu 0.20% Nickel,
• 0.1 bis 0.5% Molybdän,
• 0.04 bis 0.25% Schwefel,
• bis zu 0.01% Aluminium,
• bis zu 0.035% Phosphor,
• bis zu 0.0008% Bor,
• bis zu 0.02% Titan,
• bis zu 0.3% Blei,
• bis zu 0.3% Wismut,
• bis zu 1.93% Mangan
• bis zu 4.0% Chrom
• bis zu 0.02% Stickstoff und
• bis zu 0.01% in oxidischen Einschlüssen gebundener Sauerstoff mit einem Rest an Eisen sowie unvermeidbaren Verunreinigungen auf, wobei

(Mangangehalt - 1.72 Schwefelgehalt) < 1.50% und
Chromgehalt + (Mangangehalt - 1.72 Schwefelgehalt) > 2.6 Gew.% sein soll und folgende Gefügebestandteile vorliegen:

• 50 bis 90% Bainit,
• bis 50% Martensit,
• bis zu 10% Ferrit und
• bis zu 10% Restaustenit.

The inventive hot-rolled long product has a weight fraction of

• 0.20 to 0.25% carbon,
• 0.90 to 1.35% silicon,
• up to 0.20% nickel,
• 0.1 to 0.5% molybdenum,
• 0.04 to 0.25% sulfur,
• up to 0.01% aluminum,
• up to 0.035% phosphorus,
• up to 0.0008% boron,
• up to 0.02% titanium,
• up to 0.3% lead,
• up to 0.3% bismuth,
• up to 1.93% manganese
• up to 4.0% chrome
• up to 0.02% nitrogen and
• up to 0.01% oxygen bound in oxide inclusions with a balance of iron and unavoidable impurities, wherein

(Manganese content - 1.72 sulfur content) <1.50% and
Chromium content + (manganese content - 1.72 sulfur content)> 2.6% by weight should be and the following structural components are present:

• 50 to 90% bainite,
• up to 50% martensite,
• up to 10% ferrite and
• up to 10% retained austenite.

In the product produced according to the invention, the alloying components are selected so that at usual cooling rates from the rolling heat of 0.1 to 8.0 K / s a bainitic-martensitic microstructure always results with a tensile strength level of 1'000 to 1400 MPa, without costly alloying elements and / or or special equipment for accelerated cooling from the rolling heat must be used.

The lower limit of the carbon content to 0.20% ensures, in combination with manganese and chromium, that only small amounts of ferrite are present in the microstructure. Ferrite levels above 10% affect both the strength level and impact strength of the product.

The upper limit of the carbon to 0.25% 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 affects the carbon activity and slows down the precipitation of carbides. The selected silicon concentration allows a one-hour annealing treatment at 400 ° C without degrading the ductility due to carbide precipitations (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 1.35% in order not to exceed the maximum desired tensile strength of 1400 MPa.

If the manganese content is too high, the manganese segregations are pronounced and the microstructure becomes very inhomogeneous. For this reason, the "free", ie not bound in manganese sulfides, manganese content (≈ total manganese content - 1.72 Sulfur content) to 1.50%.

The so determined manganese content is not sufficient to achieve a bainitic-martensitic structure after air cooling from the rolling heat. The product must also contain so much chromium that chromium content + (manganese content - 1.72 sulfur content)> 2.6% by weight applies. Together with a carbon content of at least 0.20%, a bainitic-martensitic microstructure with <10% ferrite is ensured.

Molybdenum is said to prevent the precipitation of iron carbides at the primary grain boundaries and associated loss of toughness. For cost reasons, the molybdenum content should be as low as necessary: 0.1 to 0.5% molybdenum.

To achieve a significant improvement in machinability, the steel should contain at least 0.04%, preferably 0.12 to 0.17% sulfur. The sulfur combines with manganese to form manganese sulfide precipitates, thus improving both chip breaking and tool life. Since these precipitates also reduce the transverse toughness of the hot-rolled long product, the addition of sulfur should be limited to 0.25%.

No aluminum was added to the product prepared according to the invention. In order to avoid the formation of hard, abrasive oxide inclusions of the corundum type, the aluminum content should be limited to 0.01%. In combination with the 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, 35 to 65% SiO ₂ and less than 25% Al ₂ O ₃ . The tool life of the tools used in machining is then significantly extended.

The good machinability of the hot-rolled long product produced according to the invention can be further improved according to claim 3 or 4 by the addition of 0.05 to 0.3% lead or 0.05 to 0.3% bismuth.

For the production according to the invention of the hot-rolled long product, the austenite grain size before the structural transformation and the cooling rate during structural transformation in a temperature range between 800 and 500 ° C. are of crucial importance. A fine austenite grain leads to a finer final structure with better toughness values. For this reason, the austenite grain after the last forming step according to claim 7 should not be greater than 50μm. The cooling rates should be between 0.1 and 8.0 K / s. The upper value is given by the possibilities of conventional cooling of accelerated air. The lower limit of 0.1 K / s is to ensure that no ferrite> 10% occur. Large bar dimensions that cool down inside the bar much slower than 0.1 K / s can not be produced with this technology.

Before the further processing of the hot-rolled long product, a heat treatment for 0.5 to 2 hours at 300 to 500 ° C according to claim 8 may be useful. The high silicon content of the product delays the rearrangement of carbon atoms in the microstructure. This is desirable to suppress the formation of coarse carbide precipitates. On the other hand, also in connection with carbon known aging processes that begin immediately after the hot rolling, slowed down. In particular, the maximum ductility of the structure sets in only after a few weeks. In cases where the rolled long product is to be further processed immediately, a heat treatment is therefore recommended.

Ways to carry out the invention

Fig. 1

Gefügebilder nach 200-facher Vergrösserung (Ätzmittel: HNO₃ 2%-ig), für (a) 22 mm Stabstahl, (b) 52 mm Stabstahl;

Fig. 2

eine schematische Darstellung der Entnahme der B8x40 mm- Zugproben;

Fig. 3

dem Verlauf der Vickers Härte über den Querschnitt eines 22 mm und eines 52 mm Stabs (von der Oberfläche bis zum Kern).

Exemplary embodiments of the invention will be described in greater detail below with reference to the drawings, in which:

Fig. 1

Micrographs after 200-fold magnification (etchant: HNO ₃ 2% strength), for (a) 22 mm bar steel, (b) 52 mm bar steel;

Fig. 2

a schematic representation of the removal of the B8x40 mm tensile specimens;

Fig. 3

the progression of Vickers hardness across the cross section of a 22 mm and a 52 mm rod (from the surface to the core).

In the context of one embodiment, a molten steel was poured and then rolled into bar steel in various dimensions. 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 to bar steel in the dimensions 22 (cooling rate is about 1.5 K / s) and 52 mm (cooling rate is about 0.4 K / s) rolled. The cooling of the rods after rolling was carried out in air.

The steel was made 0.22% to carbon 0.94% silicon 00:07% nickel 0.14% to molybdenum 00:15% sulfur 0.003% aluminum 0.012% phosphorus <0.001% boron 0.011% titanium <0.003% lead <0.003% bismuth 0.013% nitrogen 1.60% manganese 1:34% Manganese - 1.72 Sulfur 1:54% chrome 2.88% Chromium + (manganese - 1.72 sulfur) and other impurities caused by melting.

The high sulfur content of 0.15% ensures good chip breaking and improves tool life. The low aluminum content suppresses the formation of hard, abrasive, clay-containing oxide inclusions.

The metallographic micrographs at 200x magnification are in the Fig. 1 shown. The microstructure is a very fine mixed structure. The bainite and martensite fractions could not be reliably quantified. However, the pictures as well as the obtained strength level show that the structure consists primarily (>> 50%) of bainite. The structure of the 52 mm rod is slightly coarser than the structure of the 22 mm rod due to the low cooling rate from the rolling heat. In the vicinity of manganese sulphides (which can serve as nucleating sites for ferrite formation), isolated ferrite grains can be recognized. The ferrite content is extremely low (<< 10%). The determination of the residual austenite quantity in the X-ray diffractometer showed 5.1 ± 0.45% for the 22 mm rod and 4.4 ± 1.34% for the 52 mm rod.

Since the samples were taken for the tensile tests immediately after hot working, they were stored for one hour at 300 ° C under inert gas to accelerate the natural aging before the tensile test. Despite the different cooling conditions from the rolling heat at 22 and 53 mm bars the strength values for the steel produced according to the invention are within a range of 100 MPa (Table 1). Table 1: Strength values 22 mm 52 mm Rp0.2 886 MPa 842 MPa rm 1168 MPa 1,064 MPa A5 14.2% 08.11%

For 52 mm steel bars, tensile specimens were used at various points (see Fig. 2 ) to demonstrate the uniformity of the properties can. The results are shown in Table 2 below. Table 2: Results of tensile tests Distance from core 5 mm 13 mm 20 mm Rp0.2 777 MPa 842 MPa 862 MPa rm 1029 MPa 1064 MPa 1071 MPa A5 10.04% 08.11% 09.12%

The high uniformity of hardness across the bar cross-section was confirmed for a 22 mm and a 52 mm bar steel on non-outsourced samples by HV1 measurements ( Fig. 3 ). Due to the faster cooling rate, the hardness or strength of the 22 mm is slightly higher than that of the 52 mm steel bar.

A one-hour aging of the 52 mm bar steel samples at 300, 400 and 500 ° C showed no significant change in the mechanical properties (determined here on a B8x40 mm sample taken from R / 2): Table 3: Mechanical properties after aging Outsourcing for 1 hour 300 ° C 400 ° C 500 ° C Rp0.2 842 MPa 878 MPa 815 MPa rm 1064 MPa 1068 MPa 1124 MPa A5 08.11% 04.13% 12.0%

The above data show that the mechanical properties of the product produced according to the invention are almost constant over a large dimensional range. A tensile strength of> 1 000 MPa typical for tempered steels is achieved with a simultaneously good elongation at break of> 11% without the necessary tempering treatment. The reduced aluminum content and the increased sulfur content compared to the tempered steels ensure a significantly better machinability.

Claims (8)

1. A hot-rolled long product with weight fractions of
   0.20 to 0.25% carbon,
   0.90 to 1.35% silicon,
   up to 0.20% nickel,
   up to 0.5% molybdenum,
   0.04 to 0.25% sulfur,
   up to 0.01% aluminum,
   up to 0.035% phosphorus,
   up to 0.0008% boron,
   up to 0.02% titanium,
   up to 0.3% lead,
   up to 0.3% bismuth,
   up to 1.93% manganese
   up to 4.0% chromium
   up to 0.02% nitrogen and
   up to 0.01% oxygen bound in oxidic inclusions, with a residue of iron as well as further admixtures usually contained in steel, wherein:

   (manganese content - 1.72 sulfur content) is < 1.50 % and

   chromium content + (manganese content - 1.72 sulfur content) is > 2.6 wt.-%.

   with the following structural constituents:

   50 to 90% bainite,

   to 50% martensite,

   up to 10% ferrite and

   up to 10% residual austenite.
2. The hot-rolled long product according to claim 1, characterized in that it contains oxidic inclusions with less than 50 wt.-% Al₂O₃, preferably with oxidic inclusions having the following relative weight fractions: 20 to 50% CaO, 35 to 65% SiO₂ and less than 25% Al₂O₃.
3. The hot-rolled long product according to claim 1 or 2, with a lead content of 0.05 to 0.3 wt.-%.
4. The hot-rolled long product according to any one of claims 1 to 3, with a bismuth content of 0.05 to 0.3 wt.%.
5. The hot-rolled long product according to any one of claims 1 to 4, with a drawing strength Rm of 1'000 to 1'400 MPa.
6. Use of a long product according to any one of claims 1 to 5 for a chip removing machining process.
7. A method for manufacturing of a hot-rolled long product according to any one of claims 1 to 5, wherein:

   - the average austenite particle size after the last hot forming step is less than 50 µm;

   - the cooling down from the forming heat under static or circulating air occurs in such a way that the temperature range between 800 and 500°C is passed with a cooling rate of 0.1 to 8.0 K/s.
8. The method for manufacturing of a hot-rolled long product according to claim 7, wherein the aging of the steel structure after hot-rolling is accelerated by a subsequent additional heat treatment for 0.5 to 2 hours at 300 to 500°C.

Images