EP1126042A1

EP1126042A1 - A steel

Info

Publication number: EP1126042A1
Application number: EP01850029A
Authority: EP
Inventors: Rainer Leppänen
Original assignee: Ovako Steel AB
Current assignee: Ovako Steel AB
Priority date: 2000-02-14
Filing date: 2001-02-08
Publication date: 2001-08-22
Also published as: US20010024621A1; JP2001247933A; SE0000458L; CN1309188A; CN1124360C; SE0000458D0; SE515623C2

Abstract

A chain type steel, suitable for the production of bars with a diameter of up to about 160 mm, e.g. to be used for the manufacture of heavy anchor chains, comprising, in weight-%:

C 0.15 - 0.23

Si 0.10 - 0.40

Mn 1.00 - 1.50

P max. 0.025

S max. 0.025

Cr 1.50 - 2.20

Ni 0.80 - 1.50

Mo 0.30 - 0.60

Cu max. 0.30

Al < 0.2

V < 0.2

Nb < 0.2

Ti < 0.2

the balance being Fe.

Description

BACKGROUND OF THE INVENTION

Technical field

The present invention relates to a steel, and more specifically a chain type steel, suitable for the production of bars with a diameter of up to about 160 mm, e.g. to be used for the manufacture of heavy anchor chains.

Prior art

For many years the applicant has delivered bars for the manufacture of heavy anchor chains primarily used for anchoring of oil riggs. Bar dimensions up to 155 mm diameter have been delivered. For the coarse dimensions a very low carbon steel type have been used which causes a number of difficulties in the steel plant, being very aggressive towards the melting equipment. The steel types used for smaller diameter anchor chains being low alloyd steels give unsatisfactory mechanical properties for bigger diameter anchor chains, that is for bar diameters above about 130 mm.
Thus, there is a need for an improved steel for making heavy anchor chains, which behaves better in the steel plant.
In e.g. GB 2 110 239 A, a steel for producing anchor chains is disclosed having the following composition, in wt.%: C 0.03 - 0.07; Si 0.10 - 1; Mn 1.2 - 2.5; Cr 1.8 - 3; Ni 1.5- 3; Mo ≤ 0.5; Nb,V,Ti total 0 - 0.10. This steel is claimed to have a yield point of at least 600 Mpa, a rupture limit of at least 900 Mpa at room temperature and an impact toughness of at least 40 Joule at -20°C. The restrictions of anchor steels for oil riggs in the ocean are becoming even stricter, and there is a demand for a steel with improved characteristics.
Through JP-61276956 is previously known a low alloy chain link steel including chromium and nickel being processed to obtain a tempered martensitic structure. This steel comprises, in wt.%: C 0.20 - 0.30; Si 0.10 - 0.30; Mn 0.70 - 1.70; Cr 0.40 - 0.70; Ni 0.75 - 2.00; Al 0.01 - 0.05; P ≤ 0.03; S ≤ 0.030. This steel is tempered after being quenched or case-hardened by means of carburising so that the microstructure is tempered martensite. At the upper region of the carbon content range, the weldability will deteriorate as well as the toughness, and there is a risk for hardening cracks. The absence of Mo means there is a risk for temper embrittlement. Ni is obviously used to compensate for a low Cr content, which will render this steel quite expensive.
Through JP-52006847B is previously known a high stregnth low alloy steel chain manufactured from steel bars containing, in wt.% C 0.1 - 0.2; Si 0.1 - 0.5; Mn 1.0 - 1.6; Cu 0.1 - 0.5; Ni 0.5 - 1.5; Cr 0.3 - 1.0; Mo 0.2 - 0.8, P < 0.02; S > 0.015, and acid.sol Al 0.02 - 0.1. The starting steel bars have a high tensile strength, improved weldability and good workability, and the steel chain produced is tempered at 550 - 680°C. The low Cr, and the low C content both affect the hardenability, which is deleterious for large diameter anchor chains.

THE INVENTION

Summary of the invention

The object of the invention is to provide a steel with improved mechanical properties and an improved behaviour in the steel plant.
This is obtained with a steel according to the invention, comprising in wieght-%:
C 0.15 - 0.23
Si 0.10 - 0.40
Mn 1.00 - 1.50
P max. 0.025
S max. 0.025
Cr 1.50 - 2.20
Ni 0.80 - 1.50
Mo 0.30 - 0.60
Cu max. 0.30
Al < 0.2
V < 0.2
Nb < 0.2
Ti < 0.2
the balance being Fe.
Embodiments of the steel according to the invention are claimed in the subclaims.

Brief description of the accompanying drawings

Fig. 1 is a graph showing the hardness as a function of the tempering temperature for laboratory melt sample materials.
Fig. 2 is a graph showing the hardness as a function of the depth underneath the surface, for hardened and not tempered samples of said laboratory melt sample materials.
Fig. 3 is a graph showing the hardness as a function of the depth underneath the surface, for hardened and tempered samples of said laboratory melt sample materials.
Fig. 4 is a graph showing the jominy hardenability for said laboratory melt sample materials.
Fig. 5 is a graph showing the jominy hardenability for a full scale melt material.

Description of preferred embodiments of the invention

The steel grade according to the invention is intended for manufacture of so called K4 chain with a diameter up to about 160 mm, and is not aggressive towards the melting equipment, and which steels result in very high qualities of the the finished chain.
For the investigation, sample material was manufactured as laboratory melts with ingot dimensions of 225x225 mms. The respective ingots were forged into bars with a diameter of 140 mms. This gives a reduction rate of about 3, which is insufficient in normal production. This means that the results from normal production will be significantly better than the test results discussed in the following description.
Test samples were produced having two different analyses, MnCrNiMo variant, and MnCrNiMoV, respectively
The steel according to the invention, after through hardening gives a very small difference between surface hardness and hardness at the center.
In table I, the analyses are given for two different steel samples.

Variant C Si Mn P S Cr Ni Mo Cu V Al

MnCrNiMo
No. 129 0.20 0.30 1.26 0.008 0.004 1.80 1.25 0.45 0.19 0.022

MnCrNiMoV
No. 131 0.19 0.24 1.10 0.007 0.003 1.78 1.20 0.33 0.18 0.10 0.024

These two sample steels were analyzed according to the following.

1. Tempering:

Fig. 1 shows the hardness as a function of the tempering temperature, tempering time 1 hour. Hardening temperatures for the respective melts are 850°C for melt No. 129, and 890°C for melt No. 131. Sample size 25x25x25 mm.
As can be seen in Fig. 1 melt No. 129 exhibits a flat curve without breaking points, which makes it less sensitive to variations in tempering temperature fluctuations. For the melt 131 the vanadium gives a strong tempering resistance up to 630°C, but at higher temperatures a steep curve is obtained with an increased sensitivity for temperature variations.

2. Through hardening:

Fig. 2 shows the hardness as a function of the depth underneath the surface of a hardened not tempered sample with a diameter of 140 mm, and
Fig. 3 shows the hardness after tempering at 615°C for the melt No. 129 and at 645°C for melt No. 131.
The hardening temperature for the melt No. 129 was 850°C and for the melt No. 131, 890°C, all being quenched in water.
As is evident from the diagramms the melt No. 129 exhibits the best result of the through hardening both for the not tempered and the tempered sample. The difference in hardness between surface and center is very small.

3. Jominy:

Fig. 4 shows the result of the jominy test. The austenitization temperature has been the same as with the through hardening test according to item 2 above.
The jominy test result corresponds well with the through hardening result according to item 2. Melt No. 129 has the best hardenability.

4. Mechanical properties

Table II below shows the mechanical properties of hardened and tempered bar samples with a diameter of 140 mm. Heat treatment and taking of samples were made according to normal practice for testing of chain material. The melt No. 129 showing the best results in the hardenability testing and tempering tests has been examined at three different tempering temperatures.

Mechanical properties
Melt	Harden. temp °C	Tempering. temp°C	ReI Mpa	Rm Mpa	AS %	Z %	KV,J
							- 40°C	- 20°C	±0°C
129	850	615	822	917	17	63	83	134	138
		590	857	937	16	72	89	101	142
		570	923	992	15	67	108	102	123
131	890	645	896	963	17	64	99	122	126
Demand acc. to DNV	580	860	12	50		50	70

The two melt samples show rather similar results. The lowest allowed tempering temperature for chain K4 is 570°C according to DNV (Det Norske Veritas). As is evident from Table II this demand would not cause any problems, but at the same time does not allow for any substantial reductions of alloy elements.
The impact toughness at -20°C is close to the demand according to DNV, but only an area reduction rate of 3 is made with the melt sample, while castings in the production will have an area reduction rate of about 12, so this feature will be substantially improved.

5. Testing according to DNV approval rules of a full scale production melt.

Charge analysis for the production of 160 mmØ bar:

C Si Mn P S Cr Ni Mo Cu

0.19 0.26 1.19 0.008 0.009 1.75 1.18 0.44 0.14

Al Sn Sb(ppm) As B (ppm) O (ppm) N (ppm)

0.015 0.007 2 0.008 ≤1 9.8 72

Heat treatment sensitivity analysis

Varied austenitization temperature

Austenitisation 30 min, cooling in water at hardening Tempering 610°C, 60 min, cooling in water after tempering

Sample No. Austenitisation temperature °C ReI
Mpa Rm
Mpa A5
% Z
% KV -20°C
J

1 840 890 958 18 70 over 147

2 870 879 957 17 71 over 147

3 910 879 957 18 72 over 147

Heat treatment sensitivity analysis

Varied tempering temperature

Austenitisation 870°C, 30 min, cooling in water at hardening Tempering 60 min, cooling in water after tempering

Sample No.	Tempering temperature °C	ReI Mpa	Rm Mpa	A5 %	Z %	KV -20°C J
6	570	991	1057	16	67	over 147
7	590	925	999	17	69	over 147
2	610	879	957	17	71	over 147
8	630	838	914	20	72	over 147
9	650	782	858	21	73	over 147

Heat treatment sensitivity analysis

Varied tempering time

Austenitisation 870°C, 30 min, cooling in water at hardening Tempering 610°C, cooling in water after tempering

Sample No. Tempering time, min ReI
Mpa Rm
Mpa A5
% Z
% KV -20°C
J

4 30 890 958 18 70 over 147

2 60 879 957 17 71 over 147

5 90 869 941 18 74 over 147

Testing for temper embrittelment

Varied cooling velocity after tempering

Austenitizing 870°C, 30 min, cooling in water at hardening Tempering 610°C, 60 min

Sample No. Cooling after tempering KV -0°C
J KV -20°C
J KV -40°C
J

2 Water 147 147 147

2L >40 min to 300°C 147 147 147

The degree of reduction is about 12 times, which fact explains the big differences in performance compared to the laboratory test materials, having a degree of reduction of only about 3 times, but still being improved compared to the prior art.

Claims

A chain type steel, suitable for the production of bars with a diameter of up to about 160 mm, e.g. to be used for the manufacture of heavy anchor chains, comprising, in weight-%:

C 0.15 - 0.23

Si 0.10 - 0.40

Mn 1.00 - 1.50

P max. 0.025

S max. 0.025

Cr 1.50 - 2.20

Ni 0.80 - 1.50

Mo 0.30 - 0.60

Cu max. 0.30

Al < 0.2

V < 0.2

Nb < 0.2

Ti < 0.2

the balance being Fe.
A chain type steel according to claim 1, comprising, in weight-%:

C 0.19 - 0.21

Si 0.20 - 0.30

Mn 1.15 - 1.25

P max. 0.015

S max. 0.020

Cr 1.65 - 1.75

Ni 1.15 - 1.25

Mo 0.42 - 0.48

Cu max. 0.25

Al < 0.2

V < 0.2

Nb < 0.2

Ti < 0.2

the balance being Fe.
A chain type steel according to claim 2, comprising, in weight-%:

C 0.18 - 0.20

Si 0.20 - 0.30

Mn 1.15 - 1.25

P max. 0.015

S max. 0.020

Cr 1.65 - 1.75

Ni 1.15 - 1.25

Mo 0.30 - 0.36

Cu max. 0.25

V 0.10 - 0.14

Al < 0.2

Nb < 0.2

Ti < 0.2

the balance being Fe.
A chain type steel according to claim 2, being hardened at 850°C, and tempered at 615°C.
A chain type steel according to claim 3, being hardened at 890°C, and tempered at above 630°C, preferably at 645°C.