Investigation of the Relations between the Material Properties and the Process Parameters of the Boron Micro-alloyed Quenched and

Tempered Steels

József L�rinczi 1- Gyula Králik 2- Mihály Kovács 3- Ákos Horváth 3 1 DUNAFERR Research Institute – Dunaújváros, Hungary

2 DUNAFERR Co. – Dunaújváros, Hungary 3 DUNAFERR Steelworks Ltd. – Dunaújváros, Hungary

Keywords: hardenability, microalloyed steels, boron steels, case hardening steel grades, quenched and tempered steels, HSLA steels

Abstract. The boron micro-alloyed quenched and tempered steels, contrary to their lower carbon content, are perfectly applicable for the manufacturing of steels having good wear resistance, strength and toughness. The manufacturing properties of steel, thanks to its lower carbon content, are better than that of low alloyed quenched and tempered steels to be replaced by them. The presentation covers the history of the introduction and production of the boron steels used in our country with special view on the experiments of DUNAFERR Company Group. Starting with factual examples, it comparises local and European-made products used for similar purposes regarding the European and national standardisation. Finally, it is alleging the most recent research activities of the company group to wider its production range of boron micro-alloyed quenched and tempered steels.

Introduction

No sooner than the beginning of the 1900’s the boron as steel alloying element has been investigated. It has been realised in the early forties that the boron, feed in small quantities, has significant effect on the quenching of steels, so it can be used to replace the expensive and sometimes strategically important steel alloying element such as Mn, Cr, Mo, V, Ni. The property altering effect of the boron alloying appeared significantly in the low and medium carbon steels. In that time the importance of the complex investigation of the relationship between the alloying elements and the material properties has not been recognised, so one could not distinguish the direct and direct effect of the boron on the hardenability. Developments in the investigation technologies and production technologies in the 70’s have made possible to start scientific research to help to reveal as accurate as possible the effect mechanism of the boron. As a consequence of the above mentioned the property modifying effect of the boron should be discussed steel group by steel group according to this the following boron steels have been developed regarding the demands of the application areas:

� case-hardening steels, � quenched and tempered steels, � high strength, low alloyed (HSLA) steels, � austenitic stainless steels.

Boron alloyed steels have been produced in Hungary from the late seventies. For the application of case hardening boron steels for so-called extended life gearboxes the Csepeli Autógyár (Csepel Automotive Factory) and the Zahnradfabrik (Friedrichshafen) company established a license contract. The needed base material, the so-called ZF-steels were made in the LKM factory. The DUNAFERR Ironworks, ancestor of DUNAFERR, started to develop, for the demands of the market in the 80’s, a boron micro-alloyed, toughening, medium carbon steels mainly for parts exposed to heavy wear in a form of hot rolled plates. The orderers of these steels were mainly the

Materials Science Forum Vols. 414-415 (2003) pp 267-274Online available since 2003/Jan/15 at www.scientific.net© (2003) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.414-415.267

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producers of agricultural machines. The most frequently produced machine parts are the following: discs, plough breasts, groundwork machines used in road construction, etc.

State of the European and national standardisation of the boron micro-alloyed quenched and tempered steels

The European Commission of Iron and Steel Standardisation (ECISS) asked the TC 23 Technical Committee to substitute EURONORM 83-70 „Quenched and tempered steels. Quality specifications” standard by a European standard. During the works on EN 10083 Quenched and tempered steels. Part 1.: Technical delivery conditions of quenched and tempered steels and Part 2.: Unalloyed quality steels, technical delivery conditions they decided to process the standard of boron alloyed quenched and tempered steels as a Part 3. of Standard EN 10083. The weight of the task is well illustrated by the fact that the creators of the EN 10083-3 published in 1995 announced that there are not enough technical data concerning the normative mechanical properties of the toughened state of steels of this standard. This is the reason why the ECISS/TC 23 “Steels for heat treating, alloyed steels and automatic steels – quality specifications” Technical Committee asks the users of the published European standard to collect and give free run of the data those needed to be included in the following edition of the EN 10083-3. For the local manufacturers and users the topicality is increased by the fact that this standard on the way to become Hungarian standard. This standard contains specifications for the manufacturing process, the chemical composition, the hardenability (in case of quenched and tempered steels the hardenability values are only for information purposes), the microstructure and for the quenching and surface stage at delivery. It does not contain specifications for the mechanical properties, these must be agreed upon. The only national example for the product standard mentioned is the DVSZ 200-1989 and its amendments, namely DASZ 200:1996, and DASZ 200:2001. When creating this standard the manufacturer aimed that the products involved should conform the demands of the national further processing units and the end users, as well. In the following we would like to introduce briefly our most recent research activity to develop boron micro-alloyed quenched and tempered steels in our company group.

The aim of the investigation on the boron micro-alloyed quenched and tempered steels

In the first part of our lecture we show the comparative tests of base materials and material properties performed on parts made of boron steel of our own and of foreign origin. In the second part we outline the expedient experimental programme, in which we tried to prognostise the processes passing off during the inferential process technologies. Finally, we outline our aim to develop products by comparing steels conform to the local market demands, included in the European standards and used in international practice.

Comparative tests on parts made of boron steel of our own and of foreign origin

The aim of the tests performed in the DUNAFERR Research Institute is to comparatively analyse the chemical composition, mechanical properties and microstructure of the boron micro-alloyed disc saucer of different origin and to control the conformity to quality requirements of the application area and standard specifications. Of the specimens involved in the tests the number 1. is from a 5 mm thick Spanish plate, the number 2 and 3. are from a DUNAFERR Steelworks Ltd. made DHR 52 steel grade according to DASZ 200:1996 with a thickness of 6,5 mm. One important point of the tests is the way of taking the specimens. The cutting scheme is spaced by 120°, because of the anisotropy of the rolled sheet, as shown in Figure 1. The first digit of the denomination of the specimens indicates the material testing method (respectively: chemical

268 Materials Science, Testing and Informatics I

analysis, hardness test, impact test, metallographical investigations) The shape and the heat treated state of the disc made it impossible and not justified the performance of tensile test. The results of the chemical analysis and the specifications of the DASZ 200:1996 standard is shown in Table 1, the results of the hardness tests is shown in Table 2, while the results of the impact tests are shown in Table 3. The characteristic microphotos of the two material are shown in Figure 2. and 3.

31

21

K1

Plasma cutting

with limited

heat affectedzone-

K2

41

41m

23

43

43m

2242m

42

33v33

32v32

31v

Figure 1 Cutting scheme of the specimens

Table 1 Results of the chemical analysis and the standard values

Specimen or steel grade

denomination

C %

Mn %

Si %

S %

P %

Cr %

Mo %

V %

Ti %

Al %

Bsum %

1 0,27 1,28 0,23 0,004 0,015 0,03 - 0,003 0,020 0,031 0,003

2 0,37 1,38 0,36 0,004 0,016 0,177 0,182 0,003 0,071 0,033 0,003

3 0,37 1,38 0,36 0,004 0,017 0,178 0,184 0,003 0,071 0,032 0,003

30MnB5 0,27

- 0,33 1,15 -1,45

max 0,40

max 0,040

max 0,035

- - - - - 0,0008-0,005

DHR 50 0,28 –0,35

1,20 -1,60

0,20 -0,50

max 0,010

max 0,025

0,20 -0,50

0,10 -0,40

- 0,06-0,10

0,02-0,06

0,0010-0,005

DHR 52 0,36 –0,40

1,20 -1,60

0,20 -0,50

max 0,010

max 0,025

0,10 -0,40

0,10 -0,40

- 0,06-0,10

0,02-0,06

0,0010-0,005

Materials Science Forum Vols. 414-415 269

Table 2 Results of the hardness tests

Specimen Hardness, HRC

denom. 1 2 3 Mean

211 48,5 49,0 50,0 49,2

221 48,5 50,5 49,0 49,3

231 49,5 49,5 50,5 49,8

212 57,0 57,0 57,0 57

222 54,5 56,0 55,0 55,2

232 55,5 58,0 56,5 56,7

213 55,5 57,0 58,5 57

223 53,5 57,0 56,0 55,5

233 58,0 55,5 56,0 56,5

Table 3. Results of the impact tests

Specimen

denom. Impact energy, J

Testing temperature

1 2 3 mean °C

1 17 17 22 18,7 20

Figure 2.

Microscopic photo of specimen No.1 Section number: 411.

Nital; original magnification 500x. Martensitic matrix

Figure 3. Microscopic photo of specimen No.2

Section number: 412. Nital; original magnification 500x.

Martensitic matrix

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One can ascertain that the chemical composition of the base material of Spanish origin differs from the base materials produced by the DUNAFERR Steelworks Ltd. as regards to C, Mn, Si, Cr, Mo and Ti content. Concerning the chemical composition, the Spanish base material is principally similar to 30MnB5 steel grade according to EN 10083-3 standard. Due to the chemical composition of the Spanish specimen the mean value of the hardness measured is 6-7 HRC higher than that of DUNAFERR Steelworks Ltd. made specimens. Because of the great hardness values of this latter mentioned we could only produce V-notched specimens and perform impact test from the Spanish base material. The mean value of the impact energy measured on 20 °C testing temperature is: 18,7 J. No significant difference can be observed between the two martensitic microstructure. The austenitic grain size measured by ASTM E 112:1982 standard is Grade 8 on both sections. This conforms to the specifications given in the DASZ 200:1996 product standard. (at least Grade 7). Contrary the lower hardness value the feedback of the local users concerning the life expectancy of the discs is positive.

Modelling of the process technology of the boron steel aided by experimental and inspection programme

The critical problem of the users of the semi-product is that the original material properties may be changed during the process technologies. For this reason we found it to be important to plan experimental series which is modelling the circumstances of the process technology the best. The inspection introduced in the following is aimed to analyse the mechanical state and the microstructure of imported, boron micro-alloyed steels of different technological state. The specimen taken into inspection has a thickness of 8 mm, are from a Finnish plate, the base material can be identified as either 27MnCrB5-2 or 33MnCrB5-2 according to EN 10083-3 standard. The schematic operational and inspection sequence of the experimental programme is shown in Table 6. The heat treatment given in the experimental programme has been completed in the DUNAFERR Research Institute under laboratory circumstances. The results of the chemical analysis and the specifications of the EN 10083-3 standard is shown in Table 4., the results of the hardness measurements are shown in Table 5., while the results of the impact tests are shown in Table 7. Microstructures conforming to two characteristic technological state (annealed resp. tempered) are shown in Figure 4 and 5, resp. The first digit of the denomination of the specimens indicates the material testing method (respectively: chemical analysis, hardness test, impact test, metallographical investigations)

Table 4 Results of the chemical analysis and the standard values

Specimen or steel grade

denomination

C %

Mn %

Si %

S %

P %

Cr %

Mo %

V %

Ti %

Al %

Bsum %

K1 0,30 1,23 0,23 0,007 0,011 0,36 0,001 0,008 0,038 0,04 0,002

27MnCrB5-2 0,24

-0,30 1,10 -1,40

max 0,40

max 0,040

max 0,035

0,30 -0,60

- - - - 0,0008-0,005

33MnCrB5-2 0,30 -0,36

1,20 -1,50

max 0,40

max 0,040

max 0,035

0,30 -0,60

- - - - 0,0008-0,005

DHR 50 0,28 -0,35

1,20 -1,60

0,20 -0,50

max 0,010

max 0,025

0,20 -0,50

0,10 -0,40

- 0,06 -0,10

0,02 -0,06

0,0010-0,005

DHR 52 0,36 -0,40

1,20 -1,60

0,20 -0,50

max 0,010

max 0,025

0,10 -0,40

0,10 -0,40

- 0,06 -0,10

0,02 -0,06

0,0010-0,005

Materials Science Forum Vols. 414-415 271

Table 5 Results of the hardness tests Specimen Hardness, HRC

serial number

1 2 3 Mean

211 51,5 51,5 50,5 51,2

213 47,5 48,0 47,5 47,7

215 53,5 54,5 53 53,7

217 50,5 50,0 52 50,8

225 48 48 48,5 48,2

227 31,5 30,5 31 31,0

Table 6 Schematic operational and inspection sequence of the experimental programme

Serial number of stage Serial

number Technological parameter,

testing 0. 1. 2. 3. 4. 5. 6. 7. 8.

1 quenching temperature,

°C - 880 880 900 900 880 880 880 880

2 holding, minute - 10-12 10-12 10-12 10-12 10-12 10-12 10-12 10-12

quenching agent - water water oil oil water water water water

HRC hardness

measurements after quenching

- yes - yes - yes - yes -

3 annealing temperature, °C - - - - - 195 195 500 500

4 annealing time, minute - - - - - 60 60 60 60

HRC hardness

measurement, after annealing

- - - - - yes - yes -

5 Inspection of grains yes - - yes - - - yes -

Inspection of

microstructure yes yes - yes - yes - yes -

Remark: 0 means as-rolled state; the serial number of the technological stage is indicated by the

third digit of the specimen denomination.

272 Materials Science, Testing and Informatics I

Table 7 Results of the impact tests

Specimen denomination Impact value, J Testing temperature

1 2 3 mean °C

411, 421, 431 13 13 11 12,3 0

412, 422, 432 12 13 8 11 -20

413, 423, 433 15 18 16 16,3 0

414, 424, 434 7 12 15 11,3 -20

415, 425, 435 21 24 24 23 0

416, 426, 436 21 19 20 20 -20

417, 427, 437 85 92 88 88,3 0

418, 428, 438 85 79 75 79,7 -20

Figure 4. Microscopic photo of the technological-

state material 5 and 6 Section number: 515.

Nital; original magnification 500x Martensitic-bainitic matrix

Figure 5. Microscopic photo of the technological-state

material 7 and 8 Section number: 517. alcoholic picric acid

original magnification 500x. Bainite + granular pearlite

It can be ascertained that the chemical composition of the Finnish base material differs from the specifications of the DASZ 200:1996 standard, concerning all quality, as regards Mo and Ti content The material of the specimen cooled in water from 880 °C austenitization temperature and annealed on 195 °C (technological stage 5 and 6) – considering its hardness – conforms mostly to the DHR 50 steel grade of DASZ 200:1996 standard. At the same time, quenched and oil cooled specimens (technological stage 3 and 4) cannot fulfil the hardness requirements of the DHR 50 even if not annealed. Impact energy measured in toughened stage indicates good toughness. The specimens having great hardness were subjected to un-notched bend impact test. No crack occurred in the specimens, this indicates good toughness. The microstructures of the quenched and water or oil cooled and not annealed specimens (technological stage 1,2,3 and 4), and specimens cooled in water and annealed on 195 °C (technological stage 5 and 6) are martensitic with bainitic islands, while the toughened specimens (technological stage 7 and 8) have a bainitic and granular pearlite.

Materials Science Forum Vols. 414-415 273

The austenitic grain size measured on the section of the base material according to ASTM E 112:1982 standard is Grade 8. In this respect, both the base material and the variants quenched by different way conform to the specifications given in the product standards (DASZ 200:1996 standard specifies Grade 7, EN 10083-3 specifies only Grade 5).

Summary, results of development

We have see previously that there are boron micro-alloyed steels in widespread use because of their high toughness. We could seen, as well, that the steels of foreign origin tested by us are also included in the European standards and that in the development of boron steels the ECISS expedites international co-operation. Based on the above mentioned the experts of the company found it reasonable to widen the product scale of the DUNAFERR with similar products acceptable by the local demands. For the product development which is based with marketing activity the DUNAFERR Steelworks Ltd. started an experiment series to develop new steel grades. Upon the results of this experiment series obtained hitherto it became important to reconsider the workshop product standard. The reconstructed standard (DASZ 200:2001), on the one hand, takes into account the establishment made so far concerning the chemical composition, applying small changes in each grade of the steel group, on the other hand it contains a new steel product. In the meanwhile the steel grade with standard designation DHR 48 has been introduced. Its specified hardness in quenched and annealed state is 46-50 HRC. The characteristics of the specifications on the chemical composition is listed here: � in order to increase toughness we decreased the Ti content, � because of the higher N content of our steels – in order to utilise the boron – we increased the Al

content to balance the increase in the Ti content, � although the molybdenum, similarly to boron, increases the hardenability of the steels, we do

not need to increase the hardenability of our product because of their thickness, so the molybdenum has been left over.

The percentage chemical composition of steel grade DHR 48 is the following:

C: 0,25-0,30 Cr: 0,20-0,45 Mn: 1,10-1,40 Mo: - Si: 0,15-0,40 Al: 0,030-0,060 Pmax: 0,025 Ti: 0,030-0,060 Smax: 0,010 B: 0,0010-0,0050

In case of the earlier steel grades involved in the standard, besides the above mentioned, we decreased the specification concerning the upper limit of the carbon content, as well. With our new and modified-composition product assuring wide user properties we serve our clients.

References

[1] Dr. Romvári Pál, Kocsisné Baán Mária, dr. Tóth László: Adalékok a „Bórral mikroötvözött acélok metallurgiai, fémtani és min� sítési jellegzetességeinek kutatása” c. kiadványhoz; 1990. Published by: Miskolci Egyetem Továbbképzési Iroda

[2] Simon B.: A bóros ötvözés néhány sajátossága; Presentation; XVI National Heat Treating Conference, Székesfehérvár, Hungary 1995

[3] DVSZ 200-1989 [4] DASZ 200:1996 [5] DASZ 200:2001 [6] EN 10083-3

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Materials Science, Testing and Informatics I 10.4028/www.scientific.net/MSF.414-415 Investigation of the Relationships between Material Properties and Processing Parameters of Boron

Micro-Alloyed Quenched and Tempered Steels 10.4028/www.scientific.net/MSF.414-415.267