Steel, product made of said steel, and manufacturing method thereof

Abstract

Disclosed is a steel composition including specified ranges of Ni; Mo; Co; Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C; Co+Mo; Ni+Co+Mo; and traces of Al; Ti; N; Si; Mn; C; S; P; B; H; O; Cr; Cu; W; Zr; Ca; Mg; Nb; V; and Ta in specified ranges; the remainder being iron and impurities. The inclusion population, as observed by image analysis over a polished surface measuring 650 mm2 if hot-formed or hot-rolled; and measuring 800 mm2 if cold-rolled, does not contain non-metallic inclusions of diameter >10 μm, and, in the case of a hot-rolled sheet, does not contain more than four non-metallic inclusions of diameter 5-10 μm over 100 mm2, the observation being performed by image analysis over a polished surface measuring 650 mm2.

1 . Steel, characterised in that its composition, in percentages by weight is as follows:
 10.0%≤Ni≤24.5%, preferably 12.0%≤Ni≤24.5%;   1.0%≤Mo≤12.0%, preferably 2.5%≤Mo≤9.0%;   1.0%≤Co≤25.0%;   20.0%≤Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C≤29.0%, preferably 22.0%≤Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C≤29.0%, more preferably 22.5%≤Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C≤29.0%;   Co+Mo≥20.0%; preferably Co+Mo≥21.0%; more preferably Co+Mo≥22.0%;   Ni+Co+Mo≥29%; preferably Ni+Co+Mo≥41.0%;   traces ≤Al≤4.0%, preferably 0.01%≤Al≤1.0%;   traces ≤Ti≤0.1%;   traces ≤N≤0.0050%;   traces ≤Si≤2.0%; preferably 0.04%≤Si≤2.0%;   traces ≤Mn≤4.0%;   traces ≤C≤0.03%;   traces ≤S≤0.0020%, preferably traces ≤S≤0.0010%;   traces ≤P≤0.005%;   traces ≤B≤0.01%;   traces ≤H≤0.0005%;   traces ≤O≤0.0025%;   traces ≤Cr≤5.0%;   traces ≤Cu≤2.0%;   traces ≤W≤4.0%;   traces ≤Zr≤4.0%;   traces ≤Ca≤0.1%;   traces ≤Mg≤0.1%;   traces ≤Nb≤4.0%;   traces ≤V≤4.0%;   traces ≤Ta≤4.0%;   with the remainder being iron and impurities resulting from the smelting and manufacturing process;   and in that the inclusion population, as observed by means of image analysis over a polished surface measuring 650 mm 2  if the steel is in the form of a component part/work piece that is hot-formed or a hot-rolled sheet; and measuring 800 mm 2  if the steel is in the form of a cold-rolled sheet, does not contain non-metallic inclusions having equivalent diameter greater than 10 μm, preferably does not contain non-metallic inclusions having equivalent diameter greater than 8 m, and, in the case of a hot-rolled sheet, does not contain more than four non-metallic inclusions having equivalent diameter of 5 μm to 10 μm over 100 mm 2 , the observation being performed by means of image analysis over a polished surface measuring 650 mm 2 .   
     
     
         2 . Steel according to  claim 1 , characterised in that 18.0%≤Ni+Mo≤27.0%. 
     
     
         3 . Steel according to  claim 1 , characterised in that Cr is present only in trace amounts resulting from the smelting and manufacturing process. 
     
     
         4 . Steel according to  claim 1 , characterised in that traces ≤Cr<0.10%. 
     
     
         5 . A manufacturing method for the manufacture of a product made of steel, characterised in that:
 a remelting electrode is prepared being made from a steel whose composition is in conformity with  claim 1 ;   the remelting of this electrode is performed by means of a single or multiple remelting process in order to obtain a remelted electrode;   at least one process of hot forming of the remelted electrode is carried out, at a temperature comprised between 1050° C. and 1300° C., in order to obtain a hot-formed sheet or a hot-formed strip;   and optionally a heat treatment process is carried out on the said hot-formed sheet or the said hot-formed strip.   
     
     
         6 . A method according to  claim 5 , characterised in that the said hot-formed sheet or strip, possibly heat treated, may have a tensile strength that is greater than or equal to 1010 MPa, a Young's modulus greater than or equal to 130 GPa, and a uniform elongation greater than or equal to 2%. 
     
     
         7 . A method according to  claim 5 , characterised in that the said sheet that is hot-formed or the said strip that is hot-formed and possibly heat-treated may subsequently be cold-rolled in one or more passes in order to obtain a sheet or a strip having a thickness that is less than or equal to 2 mm, preferably less than or equal to 1 mm. 
     
     
         8 . A method according to  claim 7 , characterised in that the sheet or strip may be subjected to at least one heat treatment process between cold rolling passes and/or after the last cold rolling pass. 
     
     
         9 . A method according to  claim 7 , characterised in that the cumulative rate of cold rolling of the various passes may be at least 30%, preferably at least 40%. 
     
     
         10 . A method according to  claim 5 , characterised in that the said sheet or strip that is hot-rolled or cold-rolled and possibly heat-treated may have a tensile strength that is greater than or equal to 2270 MPa, a yield strength greater than or equal to 2250 MPa, and a uniform elongation greater than or equal to 2%. 
     
     
         11 . A method according to  claim 5 , characterised in that the said sheet or strip that is hot-rolled or cold-rolled and possibly heat-treated may be cut, and subsequently possibly subjected to forming. 
     
     
         12 . A method according to  claim 11 , characterised in that the said sheet or strip that is hot-rolled or cold-rolled and possibly heat-treated, cut and, possibly, subjected to forming, may be subjected to a hardening treatment between 420° C. and 550° C. for a period of 30 min to 2 hrs, preferably at 450° C. to 550° C. for a period of 1 hr to 2 hrs. 
     
     
         13 . A method according to  claim 12 , characterised in that the said sheet or strip that is hot-rolled or cold-rolled, and possibly heat-treated, cut and, possibly, subjected to forming, may be subjected, after its hardening, to a surface treatment process for improving its resistance to dynamic loads. 
     
     
         14 . A method according to  claim 13 , characterised in that the said surface treatment may be a process of carburising, or gas nitriding, or ion nitriding, or carbonitriding, or shot peening. 
     
     
         15 . A method according to  claim 5 , characterised in that the grain size of the hot-rolled sheet or strip that is possibly heat-treated, or of the cold-rolled sheet or strip that is possibly heat-treated, may be 8 ASTM or finer, preferably 10 ASTM or finer. 
     
     
         16 . A product made of steel that has been subjected to a heat forming process and that has possibly been heat-treated, characterised in that the composition thereof, in percentages by weight is as follows:
 10.0%≤Ni≤24.5%, preferably 12.0%≤Ni≤24.5%;   1.0%≤Mo≤12.0%, preferably 2.5%≤Mo≤9.0%;   1.0%≤Co≤25.0%;   20.0%≤Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C≤29.0%, preferably 22.0%≤Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C≤29.0%, more preferably 22.5%≤Mo+Co+Si+Mn+Cu+W+V+Nb+Zr+Ta+Cr+C≤29.0%;   Co+Mo≥20.0%; preferably Co+Mo≥21.0%; more preferably Co+Mo≥22.0%;   Ni+Co+Mo≥29%; preferably Ni+Co+Mo≥41.0%;   traces ≤Al≤4.0%, preferably 0.01%≤Al≤1.0%;   traces ≤Ti≤0.1%;   traces ≤N≤0.0050%;   traces ≤Si≤2.0%; preferably 0.04%≤Si≤2.0%;   traces ≤Mn≤4.0%;   traces ≤C≤0.03%;   traces ≤S≤0.0020%, preferably traces ≤S≤0.0010%;   traces ≤P≤0.005%;   traces ≤B≤0.01%;   traces ≤H≤0.0005%;   traces ≤O≤0.0025%;   traces ≤Cr≤5.0%;   traces ≤Cu≤2.0%;   traces ≤W≤4.0%;   traces ≤Zr≤4.0%;   traces ≤Ca≤0.1%;   traces ≤Mg≤0.1%;   traces ≤Nb≤4.0%;   traces ≤V≤4.0%;   traces ≤Ta≤4.0%;   with the remainder being iron and impurities resulting from the smelting and manufacturing process;   and in that the inclusion population, as observed by means of image analysis over a polished surface measuring 650 mm 2  if the steel is in the form of a component part/work piece that is hot-formed or a hot-rolled sheet, does not contain non-metallic inclusions having equivalent diameter greater than 10 μm, preferably does not contain non-metallic inclusions having equivalent diameter greater than 8 μm, and, in the case of a hot-rolled sheet, does not contain more than four non-metallic inclusions having equivalent diameter of 5 μm to 10 μm over 100 mm 2 , the observation being performed by means of image analysis over a polished surface measuring 650 mm 2 .   
     
     
         17 . A product made of steel that has been subjected to a heat forming process and that has possibly been heat-treated according to  claim 16 , characterised in that 18.0%≤Ni+Mo≤27.0%. 
     
     
         18 . A product made of steel that has been subjected to a heat forming process and that has possibly been heat-treated according to  claim 16 , characterised in that Cr is present only in trace amounts resulting from the smelting and manufacturing process. 
     
     
         19 . A product made of steel that has been subjected to a heat forming process and that has possibly been heat-treated according to  claim 16 , characterised in that traces ≤Cr<0.10%. 
     
     
         20 . A product made of steel that has been subjected to a heat forming process according to one of  claim 17  and that has possibly been heat-treated, characterised in that it has a tensile strength that is greater than or equal to 1010 MPa, a Young's modulus greater than or equal to 130 GPa, and a uniform elongation greater than or equal to 2%.