18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika 

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Fig. 1 - Dissolubility of precipitates NbC, TiC and VN according to carbon content in HSLA steel [5]

DEVELOPMENT OF FORGING TECHNOLOGY FOR MICROALLOYED STEEL FOR OIL INDUSTRY

Pavel PODANÝ a, Michal ZEMKO a, Martin BALCARb

a COMTES FHT, a. s., Průmyslová 995, 334 41 Dobřany, Česká republika, ppodany@comtesfht.cz b ŽĎAS, a.s. Strojírenská 6, 591 71 Žďár nad Sázavou, Česká republika martin.balcar@zdas.cz

Abstract

The article deals with production of the micro alloyed steel forged products for oil industry. Customer requires high toughness in lowered temperatures, guaranteed weldability and also relatively high yield strength. Applied heat of steel has 0,1 of carbon content and 1,5 percent of manganese. Conventional alloying with conventional heat treatment not always lead to consistency in mechanical properties. Application of usual heat treatment (quenching and subsequent tempering) ensures only close compliance of requirements in yield strength. Toughness at -46°C (minimum 45 KV) is usually 20 to 120 KV (considering the transition area), therefore it is very complicated to meet these requirements and ensure the reproducibility of the results by applying conventional heat treatment. New heats with combination of several microalloying elements (Ti, V, Nb ..) were prepared. Numerical simulation in DEFORM 3D software was used for development of new forging technology. This technology consists in controlled forging and finishing at the temperature interval 940 – 800° C. Controlled cooling and subsequent tempering at particular temperature follows. Tempering provides another precipitation of microalloying elements and therefore another increasing of tensile and yield strength. Physical simulation of forging on real specimens was done on unique device with possibilities of temperature and deformation controlling. Subsequent mechanical testing and metallographical examinations proved improvement of mechanical and microstructure properties.

1. INTRODUCTION

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels. They are not considered to be alloy steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition. [1] In order to realize the full strengthening potential of microalloying additions, it is necessary to use a soaking temperature prior to forging that is high enough to

dissolve all vanadium-bearing precipitates. [1] A soaking temperature above 1100 °C (2010 °F) is preferred. For Nb–Ti microalloyed steel the single step austenite reheating temperature at 1150 °C provided better austenite conditioning than the higher reheating temperature at 1240 °C. [2]. According to [3] and [4] complete dissolution of carbonitride precipitates occurs at 1140 °C respectively in interval between 1100 – 1200°C. Illustration about dissolution of NbC, TiC a VN precipitates offers fig. 1

where individual isotherms show dissolubility of precipitates for different carbon content in HSLA steels. [5]

    18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika 

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Further processing after austenitisation should consist of two phase forging. A higher amount of deformation at elevated temperature (ca 1100 °C) facilitates dynamic recrystallization. In this stage a critical amount of strain is required for austenite grain refinement through repeated recrystallization. The second stage should be carried out below 900°C i.e. in below recrystallization temperature (TR) with an intention of making the austenite grain pancaked and to introduce interfacial defects in hot austenite. These interfacial defects in turn lead to an increase in effective grain boundary areas and enhances the nucleation potency of ferrite. Subsequently, the microalloying elements come out of solution with decrease in temperature, which inhibit austenite grain growth by forming microalloying carbides or carbonitrides at these interfacial defects. [6]

Controlled cooling after forging leads to desired microstructure. Subsequent tempering could bring another strengthening due to the precipitation of microalloying elements. Tempering temperature 600°C and holding time 4 hours are used in this experiment to meet customer requirements

2. EXPERIMENT

2.1 Numerical simulation

Numerical simulation in DEFORM 3D software was used for simulation of forging of ingot with dimension Dh = 774mm, Ds = 710mm, Dp = 645mm. Simulation was focused on last step of forging from 500 mm bar diameter to 440 mm bar diameter. Forging takes place in the range of 940 – 800°C temperature interval.

Fig. 2 - Shape, geometry and chosen network of forged bar

Fig. 3 – Temperature distribution during one of the last pass

2.2 Physical simulation

One locality in the forged bar was chosen for further physical simulation. Physical simulator is a unique device with possibilities of temperature and deformation controlling. It allows simulating similar conditions of one locality of real piece from the view of temperature/deformation in the small specimen. The specimen is shape similar to tensile test sample and it is resistive heated and loaded with tension and press. Numerical modelling allows accurate computing of precise deformation and temperature distribution in every locality of real forged product according to your choice. One locality 40 mm below the bar surface was chosen. It is the same locality, where the customer wants to test the mechanical properties.

    18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika 

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Fig. 7 – Numerical simulation of real temperature/deformation distribution in specimen for physical simulation

Fig. 4 – Physical simulator of temperature/ deformation cycles. Device parameters: Maximum force of 250 kN; heating rate of up to 150°C/s; cooling rate of up to 150°C/s; maximum frequency of cyclic loading 30 Hz; maximum forming velocity of 600mm/s.

Fig. 5 – Specimen in physical simulator

Fig. 6 - Specimen after physical simulation

Cooling rate of physical simulator allows simulation of cooling in various environement. There is possible to simulate cooling in the air, quenching in oil and water. Simulation of cooling in the water was chosen to be

the best for this task. Cooling of whole bar to the water is also the most simple for the producer.

Three experimental heats were used for this experiment. Chemical composition was: 0,1 C; 1,2 – 1,4 Mn; 0,12 Si; 0,12 Cr; 0,12 Cu and three combinations of microalloying additions (see table 1).

Table 1 – Chemical composition of experimental steels

Sample (heat) C Mn Si P S Cr Ni Cu Mo V Ti Nb Zr

1 0,10 1,47 0,14 0,004 0,001 0,15 0,09 0,11 0,04 0,11 0,17 0,045 <0.010 2 0,11 1,60 0,16 0,004 0,001 0,15 0,09 0,12 0,03 0,09 0,004 0,049 <0.010 3 0,11 1,46 0,12 0,004 0,001 0,14 0,09 0,12 0,03 0,13 0,004 0,051 0,195

10 mm

 

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    18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika 

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Table 3 – mechanical properties of heat 2 before and after tempering

Sample Rp0,2 Rm KV

mini A2 (A5)

[MPa] [MPa] [J] [%] 2

as quenched 410 614 8,9 45

2 as tempered (600 °C 4 hours)

480 624 9,3 44

3.1.3 Heat 3

Considerably highest increase of tensile and yield strength was recorded on heat 3 due to the precipitation strengthening after tempering. However the notch toughness is not very high, but it is most likely caused by coarse undissolved sharp shaped ZrC(N) particles founded in microstructure .

Table 4 – mechanical properties of heat 3 before and after tempering

Sample Rp0,2 Rm KV

mini A2 (A5)

[MPa] [MPa] [J] [%] 3

as quenched 355 601 3,5 46

3 as tempered (600 °C 4 hours)

482 625 3,3 44

3.2 Microstructure

Microstructure was investigated on light microscope Nikon Epiphot 200 with Nikon 5 MPx CCD camera and NIS elements software 2.3 for image analysis. Detailed images and EDX measurements was done by means of scanning electron microscope JEOL 6380 and INCA X-sigth EDX detector. Microstructure refinement due to full count of deformation cycles is clearly visible on the comparison of microstructures of heat 1 and 2 (see fig. 10 and 11).

Fig. 10 – as tempered state of heat 1 – 100x Fig. 11 – as tempered state of heat 2 – 100x

 

ACKNOWL

This proj

LITERATUR[1] ASM: A

[2] Zrnik JMateri(6 ref.)

[3] MishraVol. 39

[4] PanditVolum

[5] Hulka,Availab

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LEDGMENT

ject OE0800

RE Alloying – Unde

J. : Effect of theals science & e), p. 321-325

a(Pathak), S.K. 9, No. 2, 1998, p

t, A. : Strain ine 53, Issue 11,

K.: Characterisble online: <http

rCN particlesentified by m

Precipitationure

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erstanding the b

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: Investigation p. 253–259

nduced precipita2005, p. 1309-1

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s in microstrmeans of EDX

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