DE102008041391A1 - High strength bolt has bainite structure produced by austempering which extends across whole cross-section of the bolt and increases its tensile strength - Google Patents

Abstract

High strength bolt (2) has a bainite structure produced by austempering. This extends across the whole cross-section of the bolt and gives it a tensile strength of at least 1400 N/mm 2>. Independent claims are included for: (A) bolted joints comprising the bolt and components (3, 4) held together by it; (B) a method of making the bolt comprising austempering to produce the desired bainite structure; and (C) a method for assembling the bolted joint in which the bolt is first tightened above its elastic limit, producing an increased elastic limit, and then tightening to this new elastic limit.

Description

TECHNICAL FIELD OF THE INVENTION

The The invention relates to a high-strength screw, a screw connection with a high strength screw and a component, a method for Making a high strength screw and a method of manufacturing a screw connection with a high-strength screw and a Component in which the screw is produced by Zwischenstufenvergüten Has bainite structure.

Under High strength screws have been used in the prior art screws the property classes 8.8, 10.9 and 12.9 understood.

STATE OF THE ART

Out Kübler, Mages "Handbook of high-strength screws", 1st edition, 1986, page 225, bullet point 8.3.3 Intergrading is known as a tempering process for steels. In the case of intermediate stage tempering, the part to be tempered is treated, starting from the hardening temperature in a hot bath, until the conversion process with subsequent cooling in the air or in the water. As a result, the subsequent tempering of the part is unnecessary. The temperature of the warm bath is adjusted so that neither perlite nor martensite is formed.

From the British patent GB 1 492 720 is a high-strength screw with a produced by Zwischenstufenvergüten bainite microstructure known. The screw has at least two zones transverse to its longitudinal axis, which are arranged coaxially to the longitudinal axis. The first zone forms the surface of the screw and consists of bainite, while the second zone has ferrite perlite in the core of the screw. The screw may have a carbon content of 0.15 or 0.18 weight percent.

From the European patent application EP 1 728 883 A1 is a high-strength screw with a strength of 1200 N / mm ² or more known. The screw has a carbon content of 0.5 or more percent by weight. The wire serving as a starting material is reshaped into a screw and then tempered at a temperature range of 100 ° C to 500 ° C. The screw may have a bainite structure in a small part.

From the SAE publication 2003-01-1179 "Development of 1600 N / mm² class ultra-high strength bolts" is a high-strength screw with a tensile strength of 1600 N / mm ² known. The screw is made of a high carbon steel, more specifically a steel with a carbon content of 0.8 to 0.85%.

From the SAE Publication 2007-01-1003 "New bainitic steels for high strength automotive components" For example, a steel with a low carbon content and a bainite structure is known. An application example is a long shaft ball pin ("steering rod").

Out a presentation by ArcelorMittal on October 19, 2007 to the applicant in Homberg / Ohm is a high-strength screw with a bainite structure known that without a tempering treatment from a bainitschen Steel is produced.

From the Taiwanese patent TW 553 786 is a screw with a strength class 14.99 known. Such a screw is still from the TW 1 253 656 known.

A method for reducing tolerances of the prestressing forces at the yield point in a screw is known from the German patent application DE 103 26 898 A1 known. The screw can be thermally tempered. The screw is plasticized to a defined stretch-biasing force above a mating limit, wherein the stretching takes place with a tensile force resulting from a stress state composed of tensile and torsional stress. To realize the defined tensile force, the prestressing force is measured exactly in a stretching device and the change in length of each individual screw is checked to ensure that in each case a plastic deformation has taken place during pre-stretching.

OBJECT OF THE INVENTION

Of the Invention is based on the object, a high-strength screw, a Screw connection with a high-strength screw and a component, a method for producing a high-strength screw and a Method for producing a screw connection with a high-strength To provide screw and a component in which the screw a particularly high tensile strength with a very good ductility has.

SOLUTION

The The object of the invention is in accordance with the invention the features of the independent claims 1, 10, 11 and 12 solved.

DESCRIPTION OF THE INVENTION

The invention relates to a high-strength screw, which has a bainite structure produced by intermediate stage hardening, which essentially extends over the entire cross-section of the screw stretches, wherein the screw has a tensile strength R _m of at least 1400 N / mm ² .

The invention further relates to a screw connection with a high-strength screw and a component described above and to a method for producing the screw connection. In this case, the method has the steps of first tightening the screw connection beyond which the yield point R _{eS1 results} , resulting in an increased yield strength R _{eS2 of} the screw, and second tightening of the screw connection at least up to the increased yield strength R _{eS 2} for producing the screw connection.

The The invention also relates to a method for producing an above described high-strength screw.

The inventive high-strength screw has due of the bainite structure produced by interstage tempering a very high breaking strength with a very high ductility on. This high ductility or toughness distinguishes the bainite structure is essentially of a martensite structure, which in the prior art by hardening in a known manner is generated with subsequent tempering. In the intermediate stage the hardening takes place instead by a rapid cooling from the austenite phase by an isothermal microstructure transformation in the bainite level. The part, in particular the screw, lingers, at an isothermal temperature in the salt bath until the microstructure transformation of the Austenite to bainite over the entire cross section completed is. The required in the martensitic hardening Starting process canceled. This is also the tendency for Hardness reduced.

With Bainitizing eliminates the direct structural transformation to martensite and thus the rugged lattice transformation with high lattice stresses. This means that the parts with bainite structure by extremely high strength values in combination with maximum toughness characteristics distinguished.

Alloyed steels with very good hardenability and at the same time good formability are suitable as starting material for the screw with bainite structure. On the other hand, the steels preferred from the literature for bainitizing have a relatively high carbon content and are only limitedly formable. Steels with a carbon content of less than 0.5%, in particular from about 0.3% to 0.4%, are preferred for the screws according to the invention. A preferred concrete starting material is an alloyed stainless steel, in particular 34CrNiMo6 (material No. 1.6582). This has a very good cold workability. The particular suitability of the material results from the time-temperature conversion diagram ("ZTU diagram"). Especially materials with a small upper critical cooling rate or those materials which are characterized in the ZTU diagram by a position of the bainite transformation line ("bainite transformation nose") at high times are suitable for the bainitic treatment method according to the invention. As a result, the extremely high tensile strength R _m of at least 1400 N / mm ^{2 can be achieved} with very good ductility. The breaking elongation A is at a strength of about 1600 N / mm ² at between about 10 to 18%, preferably between about 12 to 14%. The equivalent elongation at break is only about 8% for screws of strength class 12.9 of the prior art. The Brucheinschnürung Z is in the inventive screws at a strength of about 1600 N / mm ² at about 50% or more.

Farther It is advantageous that in the new compensation process no special requirements regarding the structure or heat treatment state of the starting material, as is the case in the prior art. To improve the tool cutting times in the cold forming, it may be useful to the starting material of the Screw - d. H. the wire - z. B. pretreated by annealing.

One Example of a concrete application of the high-strength screw is a connecting rod of a motor vehicle.

The particular screws of the invention a yield ratio of less than 90%, in particular 85% and preferably 80%. Below the yield ratio becomes the ratio of the yield strength or the 0.2% proof strength understood to breaking strength. Screws of the prior art usually have a yield ratio from 90% up. Screws of high strength classes become optimal Utilization attracted beyond the yield point, wherein the screw is elastically-plastically deformed. The measure of permissible plastic deformation is due to the ductility the screw determines. In screws according to the invention may be due to the reduced yield ratio a greater plastic deformation when tightening achieved the screw without destroying the screw than is the case in the prior art. Especially preferred new strength classes of the invention Screw are thus 14.8, 15.8, 16.8 and 17.8.

With the new extremely high-strength screw with a strength class of more than 12.9, pickling or phosphating is out of the question with regard to the risk of hydrogen embrittlement. Such methods are commonly used in the State of the art to achieve uniform numbers of friction for coating the screw performed. In phosphating, a crystalline layer, e.g. As zinc phosphate, applied to the screw, which adjusts either directly the desired friction numbers or as a carrier layer for further surface treatments. A disadvantage of phosphating is the use of acidic chemicals during the process. As a result, there is a risk of hydrogen introduction into the screw material with the subsequent risk of hydrogen embrittlement and / or the emergence of pickling pores, which adversely affect the fatigue strength of the screw. This risk increases with the strength. In addition, in the material preferably used according to the invention, problems arise due to its high proportion of alloying elements with the adhesion of the phosphate layer to the base material.

In the screws according to the invention, therefore, other coating or lubricants are used. Preferably, bonded coatings are used which do not have the disadvantages described above. When selecting suitable bonded coatings, the adhesion of the bonded coating to metallic bright surfaces is of crucial importance. It has been found that bonded coatings are particularly suitable with a polymer lubricant, as they also allow in particular uniform friction coefficients in multiple suit. Particularly advantageous and environmentally friendly are water-soluble bonded coatings. MoS, MoS ₂ , graphite, polyethylene or another polymer can furthermore be used as lubricants.

The inventive screw has predominantly a bainitic structure, which by bainitizing was produced. Bainitizing is also used as an intermediate stage designated. Under the "predominant" presence of the bainite structure or of the "essentially over the entire cross-section of the screw-extending bainite structure is to understand first that not only defined Layers (ie only parts) of the cross-section of the screw this Bainite structure have. It is not among them understand that small parts of another structure - z. Martensite or retained austenite - the realization of the Feature of essentially the entire cross section of the Leave screw extending bainite structure. The bainite structure can be made up of both the upper and the lower part of the structure lower bainits or even the granular bainite. In addition, low levels of martensite be targeted, d. H. Shares of less than 15%, shares of about 10% or less, in particular between 8% and 11%, preferably levels between about 3% to 5%. These proportions relate based on the areal proportions of martensite in an average Cross-sectional area of the screw. By the martensite the breaking strength is increased, whereby the Bainitanteil high enough to maintain the desired high ductility to reach.

In the method according to the invention for producing a screw connection with a high-strength screw and a component described above, the tightening of the screw connection takes place in two successive steps, namely a first tightening of the screw connection and a second tightening of the screw connection. When first tightening the screw, the screw is already on or in the component, which is to be connected by means of the screw. So there is no additional measuring or testing device application. The first tightening of the screw is the so-called "training suit". With this training _suit , the screw or the screw connection is tightened beyond the yield point R _eS1 . Due to the subsequent strain hardening results in an increased yield strength residual and increased breaking strength of the screw.

there can be compared to a conventionally tempered one Screw significantly greater uniform elongation be used to advantage because the screw over the yield strength It can be stretched significantly further without the screw constricted in cross section. Both the absolute values like also the tolerances of the tightening parameters, in particular the angle of rotation in an angle suit or a combined torque / angle suit can be chosen significantly larger.

From the training suit results in a clearer sharp-edged expression of the transition from elastic to elastic-plastic deformation. Since reaching the yield point in practice is only slightly delayed, the reference to the 0.2% proof stress R _p0.2 is particularly _appropriate . This means that the first tightening of the screw connection preferably takes place beyond the 0.2% proof stress R _p0.2 and the second tightening of the screw connection takes place at least up to the increased 0.2% proof stress R _p0.2 . This ensures that plastic deformation takes place in each case.

The Tightening the screw can be stretch limit controlled and / or be executed angle controlled. The safest way is the desired limit value (yield strength or 0.2% proof stress) is exceeded achieve a combination of torque and angle controlled suit.

The first tightening the screw is preferably without consideration of Vor tension. This means that the first tightening of the screw takes place without a measurement or generation of a numerically determined - ie defined - preload force. Such accurate adjustment of the biasing force is not required. It is only about to reach the area of elastic-plastic deformation in the training suit, so that the desired work hardening of the screw occurs. For certain connections, such as. B. connecting rods, in which the connection is already bolted for precise, mechanical finishing of the connecting rod for the first time, the training suit can be carried out advantageously and without additional cost already in this step anyway necessary. It is also advantageously exploited that the screw in the final suit in the assembly of the connecting rod the same, increased yield strength and thus also the same bias as in the training suit again.

advantageous Further developments of the invention will become apparent from the claims, the description and the drawings. The in the introduction to the description advantages of features and combinations of several Features are merely exemplary and may be alternative or cumulatively without compelling advantages of embodiments of the invention must be achieved. Other features are the drawings - in particular the illustrated geometries and the relative dimensions of several Components to each other and their relative arrangement and operative connection - to remove. The combination of features of different embodiments the invention or features of different claims is also different from the chosen relationships the claims possible and is hereby stimulated. This also applies to such features, in separate drawings are shown or mentioned in their description. These Features may also be consistent with features of different claims be combined. Likewise, in the claims listed features for further embodiments the invention omitted.

BRIEF DESCRIPTION OF THE FIGURES

in the The invention is described below with reference to the figures preferred embodiments further explained and described.

1 shows a first embodiment of the new screw with a high-strength screw and a component.

2 shows a schematic stress-strain diagram of the new screw in the first and second suit.

3 shows a schematic stress-strain diagram of the new screw compared to conventionally coated screws.

DESCRIPTION OF THE FIGURES

1 shows a first embodiment of a screw connection 1 with a high-strength screw 2 and a first component 3 and a second component 4 , The screw 2 has a head 5 a shaft section 6 and a threaded portion 7 with an external thread 8th on. The first component 3 has a hole 9 and the second component 4 a hole 10 on. The hole 10 has an external thread 8th the screw 2 corresponding internal thread 11 on. About the meshing threads 8th . 11 and the headrest surface 12 of the head 5 the screw 2 is the one for the secure screw connection 1 required clamping force achieved.

At the screw 2 It is an extremely high-strength screw with a tensile strength of at least 1400 N / mm ² . The screw 2 has a bainite structure produced by interstage tempering, which extends essentially over the entire cross section of the screw 2 extends. The screw connection 1 was initially dressed as part of a training suit, with the screw 2 was tightened beyond its yield point. For the final production of the screw connection 1 then took place the second tightening the screw 1 at least up to the resulting from the training suit increased yield strength.

2 shows a schematic stress-strain diagram of the screw according to the invention 1 at the first and second suits. It can be seen that the first tightening takes place beyond the yield strength or the 0.2% proof strength, which results from the cold work hardening of the screw 2 an increased yield strength results. This increased yield strength is then used in the second suit, which in turn takes place into the elastic-plastic deformation area. Furthermore, in 2 clearly recognizable that the curve of the second suit is formed much sharper due to the performed first suit, so that leaving the purely elastic deformation area is better noticeable when tightening.

3 is a schematic stress-strain diagram showing the behavior of the screw connection according to the invention 1 that of a conventionally tempered screw. In this case, the practice-relevant comparative scale is a conventionally tempered screw of a lower strength class (dash-dotted lines), as conventional tempered screws of a strength of more than 1400 N / mm ² in the practice-relevant state of the art does not occur. Nevertheless, a corresponding curve (dashed line) was drawn for the systematic comparison. It is in 3 recognizable that the present invention bainitic tempered screw 2 has an extremely high strength with a very high ductility. This is reflected in the sharp-edged line and the very large uniform expansion.

1

screw

2

screw

3

component

4

component

5

head

6

shank portion

7

threaded portion

8th

external thread

9

drilling

10

drilling

11

inner thread

12

Head support surface

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - GB 1492720 [0004]
• EP 1728883 A1 [0005]
• - TW 553786 [0009]
• - TW 1253656 [0009]
• - DE 10326898 A1 [0010]

Cited non-patent literature

• - Kübler, Mages "Handbook of high-strength screws", 1st edition, 1986, page 225, bullet point 8.3.3 [0003]
• - SAE publication 2003-01-1179 "Development of 1600 N / mm 2 class ultra-high strength bolts" [0006]
• SAE Publication 2007-01-1003 "New Bainitic Steels for High Strength Components for Automotive Parts" [0007]

Claims (15)

High strength screw ( 2 ), characterized in that it comprises a bainite structure produced by interstage tempering, which extends substantially over the entire cross section of the screw ( 2 ) and the screw ( 2 ) has a tensile strength R _m of at least 1400 N / mm ² . High strength screw ( 2 ) according to claim 1, characterized in that it has a tensile strength R _m of between 1400 N / mm ² and 1700 N / mm ² . High strength screw ( 2 ) according to claim 1 or 2, characterized in that it has a carbon content of less than 0.5%, in particular from about 0.2% to 0.4%. High strength screw ( 2 ) according to at least one of claims 1 to 3, characterized in that it consists of alloyed stainless steel, in particular 34CrNiMo6. High strength screw ( 2 ) according to at least one of claims 1 to 4, characterized in that it has a yield ratio of less than 90%, in particular 85% or less. High strength screw ( 2 ) according to at least one of claims 1 to 5, characterized in that the elongation at break A at a strength of 1600 N / mm ^{2 is} about 10-18%, in particular about 12-14%. High strength screw ( 2 ) according to at least one of claims 1 to 6, characterized in that the Brucheinschnürung Z at a strength of 1600 N / mm ^{2 is} greater than about 50%. High strength screw ( 2 ) according to at least one of claims 1 to 7, characterized in that it comprises a coating in the form of a lubricating varnish or lubricant having as a lubricant in particular MoS, MoS ₂ , graphite, polyethylene or another polymer. High strength screw ( 2 ) according to at least one of claims 1 to 8, characterized in that it has a low content of martensite of less than 15%. Screw connection ( 1 ), with a high-strength screw ( 2 ) according to at least one of claims 1 to 9, and a component ( 3 . 4 ) with the high-strength screw ( 2 ) is screwed. Method for producing a high-strength screw ( 2 ), in particular a high-strength screw ( 2 ) according to at least one of claims 1 to 9, characterized by the step of intermediate leveling to produce a substantially over the entire cross section of the screw ( 2 ) extending bainite structure in the screw ( 2 ), so that the screw ( 2 ) has a tensile strength R _m of at least 1400 N / mm ² . Method for producing a screw connection ( 1 ) with a high-strength screw ( 2 ) according to at least one of claims 1 to 9, which has been produced according to claim 11, and a component ( 3 . 4 ), characterized by the steps of first tightening the screw connection ( 1 ) beyond the yield strength R _eS1 , resulting in an increased yield strength R _{eS2 of} the screw ( 2 ), second tightening the screw ( 1 ) at least up to the increased yield strength R _eS2 for the production of the screw connection ( 1 ). A method according to claim 11, characterized in that the first tightening of the screw ( 1 ) beyond the 0.2% proof stress R _p0.2 addition and the second tightening the screw ( 1 ) takes place at least up to the increased 0.2% proof stress R _p0.2 . A method according to claim 12 or 13, characterized in that the tightening of the screw ( 1 ) Stretch limit controlled and / or rotational angle controlled. Method according to at least one of claims 11 to 14, characterized in that the first tightening of the screw ( 1 ) is performed without consideration of the biasing force.