CN1764734B - Production of steels for high-strength components with good low-temperature toughness and the use of this type of steel - Google Patents

Abstract

The invention relates to a high tensile steel that has excellent ductile fracture values J integral even at low temperatures so that the danger of a fracture of the structural component produced from said steel is reduced to a minimum even under unfavorable and very hard working conditions. The inventive steel comprises (in % by weight) 0.08 - 0.25 % C, 0.10 - 0.30 % Si, 0.80 - 1.60 % Mn, = 0.020% P, = 0.015 % S, the sum of P and S content being = 0.030 %, 0.40 - 0.80 % Cr, 0.30 - 0.50 % Mo, 0.70 - 1.20 % Ni, 0.020 - 0.060 % Al, 0.007 - 0.018 % N, = 0.15 % V, = 0.07 % Nb, the sum of V and Nbcontent being = 0.020 %, and the remainder iron and unavoidable impurities. The inventive steel is especially suitable for producing high tensile chains.

Description

Production of steels for high-strength components with good low-temperature toughness and the use of this type of steel

technical field

The present invention relates to a steel for producing high strength parts with good low temperature toughness. For example, this type of steel can be used to make the stops or clamps needed to secure and clamp loads. In particular, these steels are processed to form hot-rolled bars, rolled wire or bright steel, which in turn are made into welded round steel chains.

Background technique

The requirements for the aforementioned types of steel are set out in DIN17115. In addition to good deformability and equally good suitability for welding, the steel must also have good strength and toughness properties in order to meet the requirements imposed due to the stresses that actually occur.

The high-quality steels 23MnNiCrMo53 and 23MnNiCrMo54, which are known for this purpose and specified in DIN17115, respectively comprise (by weight): 0.20 to 0.26% of carbon, = 0.25% of silicon, 1.10 to 1.40% of manganese, each 0.020% phosphorus and sulfur, not to exceed 0.035% total phosphorus and sulfur, including, if desired, 0.020 to 0.050% aluminum, up to 0.014% nitrogen and 0.40 to 0.60% chromium. 0.20 to 0.30% molybdenum and 0.70 to 0.90% nickel are also added to 23MnNiCrMo52 steel, while 0.50 to 0.60% molybdenum and 0.90 to 1.10% nickel are also added to 23MnNiCrMo54 steel.

In the open text of Chinese patent CN-1281906, another kind of chain steel for fixing or mooring ships or drilling platforms is disclosed. An abstract of this publication is available in the WPINDEX database, which discloses that such known steels comprise (by weight percent): 0.25 to 0.35% carbon, 0.15 to 0.30% silicon, 1.45 to 1.75% manganese, 0.90 to 1.40% chromium, 1.00 to 1.20% nickel, 0.45 to 0.65% molybdenum, 0.02 to 0.06% niobium, 0.020 to 0.05% aluminum, up to 0.020% phosphorus, up to 0.15% sulfur, up to 0.20% copper, up to 0.03% tin, up to 0.01% antimony, up to 0.04% arsenic, up to 0.005% boron, up to 0.009% nitrogen, up to 0.0020% oxygen, up to 0.0002% hydrogen, the rest is iron and unavoidable impurities, and the carbon equivalent must also be greater than 1.4.

Practical experience has shown that although steels are known to fulfill the requirements for strength and toughness at ambient temperatures, at lower temperatures problems arise, especially with regard to toughness.

Contents of the invention

It is therefore an object of the present invention to provide a high-strength steel which has good toughness even at low temperatures, so that even under unfavorable and difficult operating conditions, the risk of fracture of all parts made of this steel will be reduced to a minimum. lowest. Beneficial uses of this steel will also be detailed here.

With regard to this steel, according to the invention, this object is achieved due to the fact that according to the invention a steel for high strength components with good low temperature toughness is produced having the following composition (in weight percentages):

Carbon: 0.08 to 0.25%;

Silicon: 0.10 to 0.30%;

Manganese: 0.80 to 1.60%;

Phosphorus: ≤0.020%;

Sulfur: ≤0.015%;

The total content of phosphorus and sulfur ≤ 0.030%;

Chromium: 0.40 to 0.80%;

Molybdenum: 0.30 to 0.50%;

Nickel: 0.70 to 1.20%;

Aluminum: 0.020 to 0.060%;

Nitrogen: 0.007 to 0.018%;

Vanadium: ≤0.15%;

Niobium: ≤0.07%;

The total content of vanadium and niobium is ≥0.020%, and the rest is iron and unavoidable impurities.

In the case of steels according to the invention, various alloy compositions can be selected in order to obtain a property curve which best meets all requirements. This will be achieved by the minimum values set by the invention for the content of chromium, nickel and nitrogen and the content of niobium and vanadium. Particularly high toughness, good hardenability and improved hardness retention during tempering as well as particularly fine grain structures can be obtained if the content ranges of these alloying elements set by the invention are observed. The steel according to the invention also has high cold formability and high strength in the finished state. It is also characterized by high notched impact toughness and low fracture appearance transition temperature (fracture appearance transition temperature), which allows brittle fracture to occur only at temperatures significantly lower than the brittle fracture temperature of steel known in the prior art .

A carbon content in the range of 0.08 to 0.25% by weight ensures that the steel of the invention has good low temperature resistance. Particularly positive effects in this respect are produced if the carbon content is in the range from 0.16 to 0.23% by weight.

The good hardenability and hardness retention during tempering of the steel according to the invention are achieved by limiting the chromium content in the range of 0.40 to 0.80% by weight in combination with the molybdenum content in the range of 0.30 to 0.50% by weight. This degree of certainty in achieving the bonding effect can be increased by adjusting the chromium content to be in the range of 0.40 to 0.65% by weight and the molybdenum content to be in the range of 0.35 to 0.50% by weight.

A nickel content in the weight range of 0.70 to 1.20%, especially 0.75 to 1.00%, brings about good low-temperature toughness, which is particularly prominent in the steel according to the invention.

An aluminum content in the weight range of 0.020 to 0.060%, especially 0.020 to 0.045%, and a nitrogen content in the weight range of 0.007 to 0.018%, especially 0.007 to 0.015%, give the steel according to the invention a particularly fine-grained structure.

Finally, although the vanadium content is limited to a maximum of 0.15% by weight and that of niobium to a maximum of 0.07% by weight, the fact that the steel of the invention comprises a total of at least 0.02% by weight of niobium and vanadium ensures that the required fine grain structure. In this respect it was surprisingly found that this effect occurs particularly reliably if the steel according to the invention is free of vanadium. Therefore, according to a preferred solution, none of the steels according to the invention contain vanadium, or it is only present as an unavoidable impurity.

Fine grains remain stable even during tempering-quenching treatments. Therefore, the austenite grain size of the finished steel of the present invention is generally smaller than ASTM10. The fineness of the steel structure of the present invention is obviously greater than that of the known steel. According to the standard of DIN17115, the known steel needs to have an austenite grain size of ASTM5.

The present invention thus provides a steel which has good toughness even at low temperatures. Due to its favorable combination of properties, even under unfavorable, difficult operating conditions, the risk of fracture of components manufactured from the steel of the invention is minimized.

The steel of the invention is preferably processed as rolled steel. The purpose of this processing is to maintain the finest grain structure possible for the steel of the invention through each processing step. This includes not only processing steps during heating and rolling, but also annealing treatments before and after component creation. According to the invention, the heating and rolling conditions can be selected so that, although diffusion processes occur during heating, excessively high rolling temperatures can be avoided in order to suppress the formation of coarse grains. The temperature during further deformation can also be selected to maintain a desired fine grain structure by controlled energy extraction during thermal deformation. Accelerated heat removal immediately after the final deformation step, in the sense of "freezing" the finally acquired structural state, can prevent undesirable precipitation processes that would otherwise lead to a loss of hardness and toughness. Alternatively, the relatively low material strength of the steel in the hot-rolled state can be obtained by prolonged heat treatment to produce the desired precipitated state of carbonitrides in relation to the size and distribution of the steel formed into individual components Required for the cold forming process.

Due to the unique property spectrum of the inventive steel it is particularly suitable for the production of high strength components by cold forming followed by a tempering-quenching process. For example, these components may be means for carrying, pulling, lifting, transporting or securing loads of the highest strength class. These articles which may be collectively referred to as "braking and clamping devices" include, for example, attachment points, hooks, clips, ferrules, chains, joints, stop elements, rockers, struts, arbors and ratchet clamps, connecting rings and the like.

Connections of structural elements with good service characteristics can also be made from the steel according to the invention. These structural elements are, for example, bolts or other connecting or force-transmitting elements such as screws, clamps, struts or the like.

One field of application for which the steel of the invention is particularly suitable is the manufacture of chains. A chain made of steel of the composition according to the invention can reliably withstand heavy loads even at very low temperatures without any risk of breaking or similar breakage. Thus, round steel chains, in particular welded round steel chains, which can satisfy even the most demanding requirements, can be produced from the steel of the invention.

Components made of steel according to the invention generally have a strength of at least 1200 MPa, in particular more than 1550 MPa, 1600 MPa or 1650 MPa. It should be emphasized at this point that at a strength of at least 1550 MPa, the fracture morphology transition temperature FATT of components made of steel according to the invention is usually at most -60°C. This limit temperature is significantly lower than the fracture morphology transition temperature of known steels.

Also notable are the notched impact working values, which are generally greater than 45 J for components made of steels according to the invention and the technical crack initiation toughness J _IC of the individual components at -60° C. is greater than 170 N/mm, in particular greater than 185 N /mm. Crack initiation toughness J _IC is a value defined in ASTM1820, which is used to evaluate the deformation fracture tendency of steel materials.

The high toughness of the steel of the invention can also be seen, the elongation at break of parts made of this type of steel being higher than 28%.

Detailed ways

Hereinafter, the present invention will be described in more detail with reference to an example.

A steel includes (by weight percentage)

0.19% carbon;

0.20% silicon;

1.31% manganese;

0.005% phosphorus;

0.010% sulfur;

The total content of phosphorus and sulfur = 0.015%;

0.45% chromium;

0.37% molybdenum;

0.88% nickel;

0.400% aluminum;

0.008% nitrogen;

0.01% vanadium;

0.06% niobium;

(total content of vanadium and niobium = 0.07%),

The rest is iron and unavoidable impurities,

This steel is melted and processed to form rolled steel. In order to ensure the finest possible grain structure of the resulting product after hot rolling, the rolling temperature is kept low during the hot rolling process. In addition, the rolled product is cooled between each rolling step in order to dissipate the heat generated by the thermoforming itself. Immediately after hot rolling, the resulting hot rolled product is quenched in order to freeze the fine grain structure that the steel assumes as it leaves the hot rolling pass, so that this structure will also be reliably maintained in subsequent processing steps.

After hot rolling and a long heat treatment to set a beneficial strength for subsequent cold forming, the rolled steel is shaped to form links which, once assembled, are closed by welding.

The fine grain structure of the chain made in this way is ASTM11, the strength is 1270N/mm ² , and the fracture morphology transition temperature FATT measured under this strength is -70°C. Their notched impact working value at a test temperature of -60°C was 557J and their elongation at break was 28%.

In the accompanying drawing, the change trend of the ductile fracture value J of the steel of the present invention is plotted against the crack expansion REW with a standard initial crack length a/w of 0.4 at a temperature of -60°C. It can be seen that the crack initiation toughness J _IC is 185 N/mm ² at the onset of technically relevant stable crack expansion.

Claims (23)

1. A steel for producing high-strength parts with good low-temperature toughness, which has the following components (by weight): Carbon: 0.08 to 0.25%; Silicon: 0.10 to 0.30%; Manganese: 0.80 to 1.60%; Phosphorus: ≤0.020%; Sulfur: ≤0.015%; The total content of phosphorus and sulfur ≤ 0.030%; Chromium: 0.40 to 0.80%; Molybdenum: 0.30 to 0.50%; Nickel: 0.70 to 1.20%; Aluminum: 0.020 to 0.060%; Nitrogen: 0.007 to 0.018%; Vanadium: ≤0.15%; Niobium: ≤0.07%; The total content of vanadium and niobium is ≥0.020%, and the rest is iron and unavoidable impurities. 2. Steel according to claim 1, characterized in that the carbon content is in the range of 0.16% to 0.23% by weight. 3. Steel according to claim 1, characterized in that the manganese content is in the range of 1.00% to 1.35% by weight. 4. Steel according to claim 1, characterized in that the chromium content is in the range of 0.40% to 0.65% by weight. 5. Steel according to claim 1, characterized in that the molybdenum content is in the range of 0.35% to 0.50% by weight. 6. Steel according to claim 1, characterized in that the nickel content is in the range of 0.75% to 1.00% by weight. 7. Steel according to claim 1, characterized in that the aluminum content is in the range of 0.020% to 0.045% by weight. 8. Steel according to claim 1, characterized in that the nitrogen content is in the range of 0.007% to 0.015% by weight. 9. Steel according to claim 1, characterized in that it has an austenite grain size smaller than ASTM 10. 10. Use of steel according to any one of the preceding claims for the production of high-strength components by cold forming followed by quenching and tempering. 11. Use according to claim 10, characterized in that the part is a device for carrying, dragging, lifting, transporting or securing a load. 12. Use according to claim 10, characterized in that the component is a device for the connection of structural elements. 13. Use according to any one of claims 10 to 12, characterized in that the part is a chain. 14. Use according to claim 13, characterized in that the chain is a round steel chain. 15. Use according to claim 13, characterized in that the chain is welded. 16. Use according to any one of claims 10 to 12, characterized in that the part has a strength of at least 1200 MPa. 17. Use according to claim 16, characterized in that said strength is at least 1550 MPa. 18. Use according to claim 16, characterized in that said strength is at least 1600 MPa. 19. Use according to any one of claims 10 to 12, characterized in that at a strength of at least 1550 MPa, the fracture morphology transition temperature FATT of the part is at most -60°C. 20. Use according to any one of claims 10 to 12, characterized in that the notched impact energy of the component is greater than 45J. 21. Use according to any one of claims 10 to 12, characterized in that the fracture toughness J _IC of the component is greater than 170 N/mm ² . 22. Use according to claim 21, characterized in that the fracture toughness J _IC is greater than 185 N/mm ² . 23. Use according to any one of claims 10 to 12, characterized in that the component has an elongation at break higher than 28%.