DD272973C2 - PROCESS FOR PREPARING HIGH-STRENGTH STANDARD PARTS WITH IMPROVED PHYSICAL-MECHANICAL PROPERTIES - Google Patents

Abstract

The invention relates to a rational and energy-saving method for producing high-strength standard parts with improved physical-mechanical properties. From carbide / carbonitriding steel microalloyed steel, a cold heading steel of high strength and good crushability is made by utilizing various hardening mechanisms, so that screws of strength class 8.8 and other fasteners, e.g. As bolts, pins, without the otherwise usual Schluszverguetung be made. The degrees of deformation during cold drawing amount to a maximum of 40%. The wire has a uniform fine-grained texture throughout the cross-section. {Steel, micro-alloyed; Standard parts, high strength; Cold heading steel; compressibility; Strength class 8.8, screws; Bolt; Schluszverguetung; fine-grained texture; Cold drawing; Wire; energy saving}

Description

Characteristic of the known technical solutions

Higher-strength standard parts in strength classes 8.8, 10.9 and 12.9 are usually produced from wire rod via the production stages annealing - drawing - compressing - tempering, different variants of the first-mentioned process stages being used. Usually, silicon, manganese and / or chromium alloyed steels having carbon contents between 0.20% and 0.40% are used. Recently, microalloyed steels have also been used. Such steels have improved hardenability and improved uniformity of properties. Boron-alloyed steels (Wire Journal [1974] No. 9, pp. 139-148, Tasaka, K., etc.) have prevailed here.

Disadvantage of all these methods is that the compressed standard parts must be subjected to a subsequent cost and energy-intensive heat treatment in order to achieve the required high strength values. Methods have become known in which this final remuneration is saved. Patent DE-OS 2 635188 describes a process in which a steel of 0.30-0.70% C is used. The steel wire must be patented, pulled and the finished disadvantages tempered at temperatures of 200-400 ° C.

The advancement of the tempering process in the wire production stage is recommended in the patent DE-AS 1 508416 and also practiced (wire 1973 No. 9, p 470/77 or Techn. Message Röchling-Burbach 1975/76). Disadvantage of this method is the still to be carried out heat treatment process.

The combination of the heat treatment with the hot forming process as a way to produce high-strength standard parts is highlighted in the patents DE-OS 3427557 and DE-OS 2620449. In this thermomechanical treatment, the surface layer of the wire is tempered. It has a tempered martensite structure while the core zone remains ferritic / pearlitic. The essential feature of a wire produced in this way is the different hardness distribution over the cross section. The edge zone which is subjected to the greatest strain during the subsequent forming process is the hardest, has the lowest forming capacity and thus causes a very high degree of wear of the forming tools. This disadvantageous effect of the thermal pre-consolidation is not remedied by an additional micro-alloying of the steels (DD-PS 238073). Another type of thermomechanical treatment is the quenching of the wire from the rolling heat to the martensite stage with subsequent inductive annealing treatment. However, this process has production limits with regard to the achievable rolling speed and the wire rod dimension.

To achieve the required property values, a deformation at temperatures between 250 ⁰ C and A _{1 has been} proposed in the following patents: DE-AS 2344027, DE-AS 2334453, US Patent 2767 835 to 2767838. However, the upsetting process is technically difficult to control equipment side more complex and requires an accurate temperature control.

Object of the invention

The object of the invention is to dispense with previously customary non-rational and energy-intensive annealing or heat treatment processes by means of a suitable process in the production of high-strength standard parts of rolled and cold drawn wires by cold heading or compression molding on modern machines.

Explanation of the essence of the invention

The invention is based on the technical object, for high-strength standard parts such as screws, bolts, pins and the like. To develop a chemical composition adapted method, with the o.g. Fasteners with strengths> 800Mpa and improved physical-mechanical properties can be produced. It will be a steel with max. 0.4% C and max. 1.5% Mn, which is microalloyed with carbide or carbonitride-forming elements,

used. As is known, an increase in the yield strength by fine graining and precipitation hardening is achieved by means of microalloying. At a favorable vote, the toughness and thus the cold workability is raised at the same time. The prerequisite for good cold workability is the avoidance of brittle microstructural components and the setting of defined microstructural parameters with a high degree of uniformity.

According to the invention, this object is achieved in that the steel is cooled after the hot rolling on wire or Feinstahlstraßen of usual Endwalztemperaturen up to a temperature just above A ₁ in water cooling sections and then controlled in air so that a uniform, ferritic-pearlitic or ferritic-pearlitic-bainitic structure with less than 40% by volume of perlite and an average particle size of less than 20%.

For the hot-rolled wire, the minimum tensile strength with R _m > 700 MPa and the constriction with Z> 55% are fixed as mechanical properties.

Further processing before cold heading includes descaling and drawing, the degree of deformation being ε40% for the

Ensure compressibility φ> 1.3 (φ = In) should not exceed. Furthermore, the cold work degree ε is so

ho

adjust the strength and constriction after drawing to at least 105% of the parameters required on the standard parts. The production of standard parts can be done on the usual machines by cold heading or compression molding. A subsequent heat treatment is eliminated.

The process according to the invention has the advantage that the standard parts produced do not have to be subjected to an annealing or heat treatment process. The manufacturing process is thus decisively streamlined and the energy costs significantly reduced. The elimination of a manufacturing stage simplifies TLU processes and improves material dispensing. The structure features of the cold-formed parts are retained. They manifest themselves in a higher hardness or strength of the formed material zones and in high thermal stability of the mechanical properties in the temperature range between -40 ⁰ C and 400 ⁰ C.

embodiment

The invention will be explained below with reference to an example. Two C-Mn steels were melted and micro-alloyed using the scrap-iron process. The steels were rolled on a high speed rolling mill to a wire of dimension 08.0 mm. The controlled cooling after the last pass was primarily by water cooling to 800 ⁰ C, at a rolling speed of 32 m / s. The further cooling in air was carried out so that a structure consisting of 33% perlite and 67% ferrite formed. The wire rod was pickled, phosphated and drawn in one go at 07.05 mm. M8 screws made from drawn wire on multi-stage presses meet the requirements of strength class 8.8 (R _n , г 800MPa, A ₅ > 12%) without subsequent heat treatment. The average hardness HV30 is 380 in the head and 282 in the shaft. The strength at 400 ⁰ C is only 9% lower than at room temperature. As a result of aging, the Rp0.2 limit is increased by max. 11.5% raised.

former. Analysis% Mn V B mechanical properties after this = 22% To the Z A ₁₀ after this Fiction 1.06 0.10 - overlapping where Pull Z compress 57.1 14.1 pay contemporary 1.03 0.10 - and controlled with ε = 54.5 C Si Cooling rm 59 rm RmMPa ^A 5% stole 0.22 0.32 0.90 - 0.0037 Rm _MPa Z% A ₁₀ % 916 889 60 - - 1 0.26 0.40 730 64 19.6 970 60 932 - - 2 796 65.7 18.5 comparison 0.19 0.26 650 650 > 800> 12 stole 553 - -

Claims (1)

Process for producing high-strength standard parts with improved physico-mechanical properties with a tensile strength of more than 800 MPa from a carbon-nitride-forming element of micro-alloyed steel with max. 0.4% C and max. 1.5% Mn, characterized in that this steel is cooled following the hot rolling on wire or Feinstahlstraßen of usual Erdwalztemperaturen up to a temperature just above A ₁ in water cooling sections and the subsequent cooling in air sogesteuert is that a uniform ferritic-pearlitic or ferritic-pearlitic-bainitic microstructure with less than 40VoI .-% pearlite and a particle size smaller than 20μιη formed, and the steel is cold drawn with a cross-sectional decrease of less than 40% and processed unvulcanized on cold forging machines to standard parts. Field of application of the invention The invention relates to a technological process for the production of high-strength standard parts such as screws, bolts, pins and the like. In the size range to M 20 with a strength over 800MPa and with special physical-mechanical properties.