RU2724210C1 - Method of increasing mechanical properties of steel ab2-1 in direct laser growing of metal workpieces - Google Patents

Abstract

FIELD: technological processes.SUBSTANCE: invention relates to direct laser growth of metal workpieces from steel AB2-1 with improvement of its mechanical properties. On substrate, placed in working sealed chamber, filled with argon of high purity to excess pressure in range from 2 MPa to 5 MPa with residual oxygen content of not more than 500 ppm, layers of metal powder of steel AB2-1 of fraction from 45 mcm to 200 mcm are successively applied. Powder is supplied by means of transport gas into zone of deposited metal through nozzle of installation for direct laser growth with flow rate of transport gas from 10 l/min to 40 l/min and mass consumption of metal powder from 30 g/min to 100 g/min. Speed of nozzle displacement relative to substrate varies from 15 mm/s to 35 mm/s, pitch of vertical displacement of layers within 0.2 mm to 1 mm, step of transverse displacement of layers in range from 1.4 mm to 2 mm and acting on metal powder by laser beam with power ranging from 2 kW to 3 kW, focused in spot with diameter from 1 mm to 5 mm.EFFECT: obtaining shipbuilding materials with high strength and cold resistance for operation, including in Arctic conditions.1 cl, 3 dwg, 1 tbl

Description

The invention relates to the field of metallurgy, namely the production of shipbuilding materials with high strength and cold resistance. The invention can be used to create materials for the manufacture of products intended for use in the Arctic and in other areas requiring high mechanical properties.

One of the main materials for the construction of critical structures for ships, submarines, drilling rigs and offshore platforms operated in the Arctic is steel grade AB2-1. Steel AB2-1 - high-strength steel of the bainitic-martensitic class, has a combination of high strength and impact strength [1]. A large number of studies are known on the study of bainite, including the kinetics of structure formation, crystallography, microstructural morphology, and the mechanical properties associated with it [2]. A widespread method of manufacturing these steels is smelting followed by rolling.

The prior art from the publication [3] and patent No. 2394108 (publ. 07/10/2010) known "Method for the production of sheets of cold-resistant steel", applicable to steels of type AB2. The method includes obtaining a workpiece, heating to a temperature above Ac3, deformation with regulated compression, cooling. According to it, the deformation is carried out in 3 stages, first preliminary deformation is carried out at a temperature of 950 ÷ 1050 ° C with reductions in the first three passes, 6 ÷ 10% per passage and with a total compression of at least 25%, the workpiece is stiffened and intermediate deformation is carried out at a temperature of Ar3 + 30 ° C with reductions of at least 15% per pass with a total deformation of at least 55%, and then perform the final deformation at a temperature of Ar3-20 ° C with reductions of at least 8-10% per pass with pauses between passes of at least 5 s , with a total reduction of at least 40%, while cooling the sheet after deformation is carried out at a speed of at least 30 ° C / min to a temperature of 400 ° C, then in air.

The disadvantage of this method is the multi-stage process with subsequent heat treatment to obtain the final mechanical properties, as well as obtaining material only in sheet form.

As a prototype, patent No. 2439173 was selected “Method for the production of rolled products from high-strength cold-resistant steel” (publ. 10.01.2012), it is also applicable to AB type steels. The production method includes steel smelting, continuous casting into billets, heating slabs, preliminary and final rolling and accelerated cooling, and after heating, the slabs are pre-rolled with a total deformation of 50-70% in the direction perpendicular to the axis of the slab, and then the final rolling is performed in the region below recrystallization temperature at a temperature of 750-900 ° С with a total deformation of 65-80%, moreover, 30-40% of the total deformation occurs in rolling in the direction perpendicular to the axis of the roll, after which the rolling is rapidly cooled from a temperature of Ar ₃ ± 20 ° С to a temperature 400-600 ° C, and then cooled slowly to a temperature of 20-200 ° C at a speed of 0.05-0.15 deg / s. Steel is smelted in an oxygen converter. After the release of metal, it is processed in a ladle and poured on a continuous casting machine. During out-of-furnace processing of metal in the ladle, final deoxidation, refining, purging with neutral gas and modifying treatment with calcium are carried out. Slabs with a size of 246 × 1550 mm are rolled onto a sheet with a thickness of 24.5 mm at the 5000 single-strand reversing mill.

In this method, the increase in strength properties is achieved due to the implementation at the last stage of the forced thermocyclic processing of the workpiece. The disadvantages of this method are the multi-stage process to obtain the final mechanical properties, as well as obtaining material only in sheet form [4].

The technical result of the proposed method is to increase the mechanical properties of AB2-1 steel during direct laser growing (PLW) of billets and expand the scope of its application in ship and heavily loaded structures due to the possibility of manufacturing billets of any shape and thickness.

To achieve the technical result, a method is proposed for improving the mechanical properties of AB2-1 steel during direct laser growth of metal billets, which consists in the fact that on a substrate placed in a working sealed chamber filled with high-purity argon to an overpressure in the range from 2 MPa to 5 MPa with a residual oxygen content of not more than 500 ppm, layers of metal powder from steel AB2-1 of a fraction from 45 μm to 200 μm are successively applied, feeding it by means of a transport gas into the zone of the deposited metal through the nozzle of a direct laser growth apparatus with a flow rate of transport gas from 10 l / min to 40 l / min and a mass flow of metal powder from 30 g / min to 100 g / min, while changing the speed of movement of the nozzle relative to the substrate in the range from 15 mm / s to 35 mm / s, the step of vertical displacement of the layers in the range from 0.2 mm to 1 mm, the step of the transverse displacement of the layers in the range from 1.4 mm to 2 mm, and affecting the meta laser powder with a power ranging from 2 kW to 3 kW, focused into a spot with a diameter of 1 mm to 5 mm.

The technical result is achieved due to the natural volumetric thermocyclic processing of the workpiece, which is realized by direct laser growing from a metal powder from steel AB2-1 at the proposed mode parameters. Volumetric thermal cycling leads to a bainitic structure in the workpiece material and, as a result, an increase in its mechanical properties. PLV technology allows you to adjust the volume thermal cycling by setting the parameters of the PLV installation operating mode and control the formation of a bainitic structure, which provides a structure with the desired morphology and properties.

A possible implementation of the method is carried out on a technological complex based on a fiber laser. During the growing process, the sealed chamber is filled with high purity argon (at least 99.99%) to an overpressure of 2 MPa. In the chamber after filling with argon, the oxygen content should not exceed 500 ppm. The PLV process is carried out with the following technological parameters: radiation power of 2 kW, nozzle moving speed relative to the substrate 25 mm / s, spot laser beam spot diameter on the surface of the podzhka or previous layer 3 mm, mass flow rate of the supplied powder 30 g / min, flow rate of transport gas 10 l / min In this case, the step of vertical displacement of the nozzle is 0.8 mm, and the step of lateral displacement of the nozzle is 0.2 mm.

The laser beam forms a melt bath on the surface of the substrate or the previous layer, into which a filler powder is fed through the nozzle. The nozzle moves with a given speed forming a single roller. The layer is formed by sequential application of the rollers with their partial overlap in the cross section. After completion of the formation of the layer, the nozzle rises to a given step of vertical displacement and the process is repeated. As a result of applying subsequent layers, the previous layers are reheated, which in the mass of the entire workpiece leads to thermal cycling of the material and the formation of metastable structural components with high mechanical properties. The cooling process takes place in the temperature range from 8 to 46 ° C / s; the predominantly bainitic structure of the workpiece material is formed.

Confirmation of the formation of such a structure and its effect on fracture during mechanical tests is shown in the figures: in FIG. 1 - microstructure of the workpiece material obtained by the proposed method, namely, a) b) is the bainitic structure image from an optical microscope, c) is the structure of upper bainite (WB) image is from a scanning electron microscope, d) is the structure of granular bainite (GB) image from a scanning electron microscope; in FIG. 2 - fractograms of the workpiece obtained by the PLV method, after tensile tests; FIG. 3 - fractograms of the workpiece obtained by the PLV method, after tests for impact bending.

In the process of cooling the deposited layers during the growth of the workpiece, layers close to the substrate have high cooling rates. This is explained by intense heat removal to the massive substrate. The microstructure of these layers includes quenching structures and contains mainly martensite, which can be removed at the stage of separation of the workpiece from the substrate. As you move away from the substrate, the heat sink becomes less intense and, as a result, the cooling time is significantly longer. This leads to a more favorable structure of the deposited layers due to volumetric thermal cycling.

To confirm the claimed method, samples of flat walls were grown on the parameters of the PLW regime indicated in the example of a possible implementation of the method, metallographic studies of thin sections of the samples were carried out, mechanical tests were performed. It was experimentally shown that the metal of the deposited layers manages to cool before surfacing a new layer. Auto-heating from the previous layer affects the cooling rate of the metal of the subsequent layers. A single high-temperature heating does not cause a change in hardness, while re-heating to a lower temperature leads to a decrease in hardness. During the growing process, the hardness changes cyclically within 250-300 HV at a distance in the range of 0.7-0.8 mm, which indicates the constant heating of the previous layers and the ongoing thermal cycling. Moreover, the regions of the obtained flat walls close to the substrate have a higher hardness, which is associated with high cooling rates.

The ongoing thermal cycles favorably affect the mechanical properties of the resulting blanks. Table 1 shows a comparison of the main mechanical properties of AB2-1 grade steels obtained by the traditional method [3] and the PLV method.

As can be seen from table 1, samples of material AB2-1 have hanged mechanical properties in comparison with analogues.

In FIG. 2 shows fractograms of samples obtained by the PLV method after tensile tests. In the fracture of the samples obtained by the PLV method, there are equiaxed pits, the average size of which is 2.41 microns. The nature of the destruction is viscous. The amount of fibrous component in the fracture is 95%. In the kink after shock bending, equiaxed pits were found, the average size of the pits was 2.5 μm (Fig. 3). There are no oxides. The nature of the destruction is viscous. The amount of fibrous component in the fracture is 90%.

The claimed technical solution allows us to solve the problem using the PLV method of products from steels of grade AB2-1 and significantly improve the mechanical properties of the products obtained without subsequent thermal and thermal deformation processing. The advantage of the method from classical methods of obtaining products, such as casting, rolling, is to reduce technological operations, due to the occurrence of thermal cycling in the process of growing and obtaining improved mechanical properties. This method allows to obtain high-precision billets, as well as finished parts of a predetermined shape, thickness and size without using additional processes of thermal, thermal deformation and other types of processing, in contrast to traditional methods of manufacturing products and sheet metal (billets) [5, 6].

List of references:

1. Gorynin I.V., Rybin V.V., Malyshevsky V.A., Khlusova E.I., Nesterova E.V., Orlov V.V., Kalinin G.Yu. Economically alloyed steels with a nano-modified structure for operation in extreme conditions // Problems of Materials Science. - 2008. No. 2 (54). - S.7-19.

2. Malakhov N.V., Motovilina G.D., Khlusova E.I., Kazakov A.A. Structural heterogeneity and methods for its reduction to improve the quality of structural steels // Problems of Materials Science. - 2009. No3 (59). - S. 52-64.

3. Ilyin A.V. Scientific-production complex "Structural steels and functional materials for marine engineering" Catalog of products and services, Section 1. High-strength welded steels, High-strength steels of grades AB, National Research Center "Kurchatov Institute" Central Research Institute of Structural Materials "Prometheus" named I .IN. Gorynina, St. Petersburg, from 25-27. [Electronic resource] access mode: http://www.crism-prometey.ru/about/activities/konstruktsionnyye-stali-i-nanostrukturirovannyye-materialy.pdf (Date of access 09.17.2019).

4. Golosienko S. A., Motovilina G. D., Khlusova E. I. The influence of the structure formed during hardening on the properties of high-strength cold-resistant steel during tempering // Problems of Materials Science. - 2008. No. 1 (53). - S. 32-44.

5. Turichin, G.A., Zemlyakov, E.V., Klimova, O.G., Babkin, K. D. Hydrodynamic instability in high-speed direct laser deposition for additive manufacturing, Physics Procedia, Vol. 83, 2016, pp. 674-683.

6. Fedyukin B.K. Thermocyclic treatment of steels and cast irons. - L .: Leningrad State University, 1977 .-- 144 p.

Claims (1)

A method for direct laser growth of metal billets from steel AB2-1 with an increase in its mechanical properties, characterized in that a substrate placed in a working sealed chamber filled with high-purity argon to an overpressure in the range from 2 MPa to 5 MPa with a residual oxygen content of more than 500 ppm, successively apply layers of metal powder from steel AB2-1 of a fraction from 45 μm to 200 μm by feeding it through a transport gas into the zone of the deposited metal through the nozzle of a direct laser growth unit with a flow rate of transport gas from 10 l / min to 40 l / min and a mass flow rate of metal powder from 30 g / min to 100 g / min, while changing the speed of movement of the nozzle relative to the substrate in the range from 15 mm / s to 35 mm / s, the step of vertical displacement of the nozzle in the range from 0.2 mm up to 1 mm, the step of lateral displacement of the nozzle in the range from 1.4 mm to 2 mm and act on the metal powder with a laser beam with a power ranging from 2 kW to 3 kW, focused into a spot with a diameter of 1 mm to 5 mm.

Images