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, including, 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   &lt;0.2         V   &lt;0.2         Nb   &lt;0.2         Ti   &lt;0.2                                          
 
     the balance being Fe.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    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.  
           [0002]    In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.  
           [0003]    For many years the applicant has produced bars for the manufacture of heavy anchor chains primarily used for anchoring of oil riggs. Bar dimensions up to 155 mm diameter have been produced. For these coarse dimensions a very low carbon steel type has been used which causes a number of difficulties in the steel plant, this steel being very aggressive towards the melting equipment. The steel types used for smaller diameter anchor chains, being low alloyed steels, give unsatisfactory mechanical properties for bigger diameter anchor chains, that is for bar diameters above about 130 mm.  
           [0004]    Thus, there is a need for an improved steel for making heavy anchor chains, which behaves better in the steel plant.  
           [0005]    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.  
           [0006]    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 makes this steel quite expensive.  
           [0007]    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&lt;0.02; S&gt;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.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the invention is to provide a steel with improved properties and an improved behavior in the steel plant.  
           [0009]    According to one aspect, these and other aspects are obtained with a steel according to the invention, 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 &lt; 0.2           V &lt; 0.2           Nb &lt; 0.2           Ti &lt; 0.2                      
 
           [0010]    the balance being Fe. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a graph showing the hardness as a function of the tempering temperature for laboratory melt sample materials.  
         [0012]    [0012]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.  
         [0013]    [0013]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.  
         [0014]    [0014]FIG. 4 is a graph showing the jominy hardenability for said laboratory melt sample materials.  
         [0015]    [0015]FIG. 5 is a graph showing the jominy hardenability for a full scale melt material. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The steel grade according to the invention is well-suited 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.  
         [0017]    For the investigation, sample material was manufactured as laboratory melts with ingot dimensions of 225×225 mm. The respective ingots were forged into bars with a diameter of 140 mm. 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.  
         [0018]    Test samples were produced having two different analyses, MnCrNiMo variant, and MnCrNiMoV, respectively.  
         [0019]    The steel according to the invention, after through hardening, gives a very small difference between surface hardness and hardness at the center.  
         [0020]    In Table I, the analyses are given for two different steel samples.  
                                                           TABLE I                       Variant   C   Si   Mn   P   S   Cr   Ni   Mo   Cu   V   Al                   MnCrNiMo   0.20   0.30   1.26   0.008   0.004   1.80   1.25   0.45   0.19       0.022       No. 129       MnCrNiMoV   0.19   0.24   1.10   0.007   0.003   1.78   1.20   0.33   0.18   0.10   0.024       No. 131                  
 
         [0021]    These two sample steels were analyzed according to the following.  
         [0022]    1. Tempering:  
         [0023]    [0023]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 25×25×25 mm.  
         [0024]    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.  
         [0025]    2. Through hardening:  
         [0026]    [0026]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  
         [0027]    [0027]FIG. 3 shows the hardness after tempering at 615° C. for the melt No.  129  and at 645° C. for melt No.  131 .  
         [0028]    The hardening temperature for the melt No.  129  was 850° C. and for the melt No.  131 , 890° C., all being quenched in water.  
         [0029]    As is evident from the diagramms the melt No.  129  exhibits the best result of the through hardening both for the untempered and the tempered sample. The difference in hardness between surface and center is very small.  
         [0030]    3. Jominy:  
         [0031]    [0031]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.  
         [0032]    The jominy test result corresponds well with the through hardening result according to item 2. Melt No.  129  has the best hardenability.  
         [0033]    4. Mechanical properties  
         [0034]    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.  
                                                                                                                             TABLE II                           Mechanical properties                Harden.   Tempering   ReI   Rm   A5   Z   KV, J            Melt   temp ° C.   temp ° C.   Mpa   Mpa   %   %   −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                  
 
         [0035]    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.  
         [0036]    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 in full scale manufacture.  
         [0037]    5. Testing according to DNV approval rules of a full scale production melt.  
         [0038]    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                  
 
         [0039]    Heat treatment sensitivity analysis  
         [0040]    Varied austenitization temperature  
         [0041]    Austenitisation 30 min, cooling in water at hardening  
         [0042]    Tempering 610° C., 60 min, cooling in water after tempering  
                                                       Sample   Austenitisation   Rel   Rm   A5   Z   KV - 20° C.       No.   temperature ° C.   Mpa   Mpa   %   %   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*                          
 
         [0043]    Heat treatment sensitivity analysis  
         [0044]    Varied tempering temperature  
         [0045]    Austenitisation 870° C., 30 min, cooling in water at hardening  
         [0046]    Tempering 60 min, cooling in water after tempering  
                                                       Sample   Tempering   Rel   Rm   A5   Z   KV - 20° C.       No.   temperature ° C.   Mpa   Mpa   %   %   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*                          
 
         [0047]    Heat treatment sensitivity analysis  
         [0048]    Varied tempering time  
         [0049]    Austenitisation 870° C., 30 min, cooling in water at hardening  
         [0050]    Tempering 610° C., cooling in water after tempering  
                                                       Sample   Tempering   Rel   Rm   A5   Z   KV - 20° C.       No.   time, min   Mpa   Mpa   %   %   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*                          
 
         [0051]    Testing for temper embrittelment  
         [0052]    Varied cooling velocity after tempering  
         [0053]    Austenitizing 870° C., 30 min, cooling in water at hardening  
         [0054]    Tempering 610° C., 60 min  
                                               Sample   Tempering   KV - 0° C.   KV - 20° C.   KV - 40° C.       No.   temperature ° C.   J   J   J                   2   water   147*   147*   147*       2L   &gt;40 min to   147*   147*   147*           300° C.                          
 
         [0055]    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.  
         [0056]    While the present invention has been described by reference to the above-mentioned embodiments, certain modifications and variations will be evident to those of ordinary skill in the art. Therefore the present invention is to be limited only by the scope and spirit of the appended claims.