METHOD OF PRODUCING STEEL PLATE WITH GOOD LOW-TEMPERATURE TOUGHNESS

Abstract: A method of producing steel plate with good low-­ temperature toughness comprises the steps of casting a steel melt consisting of 0.001 - 0.300 wt% C, not more than 0.8 wt% Si, 0.4 - 2.0 wt% Mn, not more than 0.007 wt% Al, 0.0010 - 0.0100 wt% O and the remainder of iron and unavoidable impurities, and subjecting the cast steel either as it is or after rolling at a small reduction ratio to accelerated cooling. In the cooling step, a fine-grained acicular ferrite texture having oxide-system inclusions as the nuclei develops radially and provides the resulting steel plate with excellent low-temperature toughness.

Description: This invention relates to a method of producing tough steel plate that is homogeneous and nonoriented and more particularly to a method for producing such steel plate without reheating following casting, wherein the desired steel plate is obtained simply by casting or by optional rolling at a low reduction ratio after casting. In the production of steel plate by hot rolling there has in recent years been a strong movement toward the realization of cost reduction through the simplification and elimination of production steps. The hot charge rolling method in which the intermediate step of reheating between casting and hot rolling is eliminated is one example of this trend. However, most of the hot charge rolling processes developed to date rely on an ensuing rolling step for the production of fine crystal grains and have not been able to realize the elimination or simplification of the rolling step. In this specification, the term "hot charge rolling" will be used to mean a process in which a slab is subjected to hot rolling following casting or continuous casting. While it is well known to be metallurgically feasible to conduct hot charge rolling of steel plate with respect to mild and ordinary steels for which the low-temperature toughness requirements are not so stringent, application of the hot charge rolling process to, for example, low-temperature steels and high-tensile steels requiring low-temperature toughness results in products with low-temperature toughness that is inferior to that obtained by conventional production processes. The main reason for this is that in the hot charge rolling process the initial austenite grains prior to rolling are extremely large, making it difficult to realize a fine-grained microstructure through rolling. As a way of avoiding this problem it has been proposed to conduct heavy controlled rolling prior to controlled cooling (see, for example, JP-A-57-131 320.) However, the use of this method introduces an additional requirement for time-temperature control and thus greatly impairs the productivity of the rolling process. For overcoming the limitation on the fineness of the microstructure obtainable by rolling there have been proposed methods such as that disclosed in JP-A-61-213 322, which relates to a method wherein oxide inclusions including a composite crystalline phase consisting of either or both of TiO and Ti₂O₃ are used as transformation nuclei. With this method, however, the quality of the steel is greatly affected by the amount of Ti incorporated and, moreover, precise control of the amount of Ti in the refining process becomes difficult. As regards methods which use Ti inclusions as transformation nuclei for realizing a fine-grained microstructure, there are known techniques for attaining high toughness of weld metal or of the heat-affected zone (HAZ) in the weld. In the production of steel plate, for utilization of Ti oxides as transformation nuclei it is necessary for the Ti oxides to be fine and well dispersed. If excessive Ti is added, the residual Ti in solid solution will lead to precipitation hardening and this in turn will impair toughness, particularly at welding heat affected zones and the like. Precise control of Ti content must therefore be carried out at the refining process. As this is not easy, it is difficult to realize stable and efficient production. An object of the invention is to provide a method for stably and efficiently producing steel plate by the hot charge rolling process without the use of Ti inclusions. Another object of the invention is to provide a method for stably and efficiently producing thick steel plate exhibiting superior low-temperature toughness by the hot charge rolling process without the use of Ti inclusions. The above objects are achieved by the method according to the claims. As regards the steel provided in the method according to the invention, C, Si and Mn enhance the strength of the steel and also increase the hardness in the HAZ. They therefore have to be contained in appropriate quantities but care must be taken to prevent their content from becoming too high. From this viewpoint, a steel to be subjected to the method of this invention should contain C at from 0.001 to 0.300%, Si at not more than 0.8% and Mn at from 0.4 to 2.0%. While Al is generally added for the purpose of deoxidization, if contained at more than 0.007%, it will interfere with the formation of oxide inclusions such as (Mn, Si)O that act as nuclei for a fine-grained acicular ferrite. Therefore the Al content is limited to not more than 0.007%. The O content is defined as falling in the range of 0.0010 to 0.0100% so as to ensure the presence of an adequate amount of oxide inclusions without degrading the steel quality by the presence of excess O. While Cu is effective for improving the corrosion resistance and strength of the steel, it promotes hot cracking at excessively high content levels. The content thereof has therefore been defined as not more than 1.5%. As Ni enhances both the strength and low-temperature toughness of steel, it is added to steels which require these properties. However, when the amount of Ni exceeds 10%, the additional effect obtained is not commensurate with increased cost. For this reason, the content thereof has been limited to not more than 10%. Cr, Mo and B enhance the tempering characteristics of steel and in the process according to the present invention have an effect of stabilizing the acicular ferrite microstructure. However, when too much of these elements are present, hot cracking occurs at the time of transformation from the γ phase. Therefore, Cr and Mo are limited to not more than 1% each, while B is limited to not more than 0.0025%. In the present invention, Nb and V contribute to increased steel strength by precipitating out as fine nitrides during cooling following rolling. However, too much of these elements deprives the steel of low-temperature toughness. Therefore, the content of Nb is limited to not more than 0.2% and that of V to not more than 0.5%. As toughness deteriorates markedly when either the Ti or Zr content exceeds 0.05%, the upper limit for Ti and Zr content has been set at 0.05% each. Ca and REM fix S contained in steel and thus work to reduce the MnS content, which is advantageous since MnS has a deleterious effect on the ductility and notch toughness of the steel. They are therefore added for this purpose. However, when present in excessive amounts they lower the cleanliness of the steel and become a cause for internal defects in the steel plate. Therefore, the upper limit of Ca content has been set at 0.008% and that of REM at 0.05%. While the P, S and N content is of no special significance, the lower the content of these elements is the better is the toughness at weld joints (HAZ and the weld metal) of the steel. It is therefore preferable to maintain the content of P and S at not more than 0.025% and that of N at not more than 0.0050%. Explanation will now be made regarding the rolling method and the cooling conditions following rolling in the method according to this invention. In accordance with one aspect of the present invention, molten steel meeting the aforesaid requirements regarding chemical composition is cast in the thickness of the desired product plate, the cast steel is cooled between the liquidus and solidus at a cooling rate (hereinafter referred to as the "solidification rate") of not less than 10°C/min, and following solidification is cooled from 800 to below 600°C at a cooling rate of between 2°C/sec and not more than 50°C/sec. In accordance with another aspect of the present invention, molten steel meeting the aforesaid requirements regarding chemical composition is cast in the thickness of the desired product plate, the cast steel is cooled between the liquidus and solidus at a solidification rate of not less than 10°C/min, the solidified steel is subjected to rolling in the course of cooling at a temperature of not less than 800°C and at a reduction ratio of not more than 1.5, and the rolled steel is cooled from 800 to below 600°C at a cooling rate of between 2°C/sec and not more than 50°C/sec. In the method according to the present invention, use is made of an acicular ferrite microstructure having oxide inclusions as the transformation nuclei. For this it is necessary to precipitate the (Mn, Si)O and other oxide inclusions serving as the transformation nuclei in the form of finely dispersed secondary deoxidization products. The formation of secondary deoxidization products is closely related to the solidification rate. Specifically, the slower the solidification rate, the coarser is the secondary deoxidization product grain site. Moreover, the number of the grains also decreases as the solidification rate becomes slower and at a rate lower than 10°C/min, it becomes difficult to obtain an adequate number. It is therefore necessary to use a solidification rate of not less than 10°C/min. Rolling at a temperature lower than 800°C causes the rolled texture to remain in the γ phase, which is harmful to the formation of the acicular ferrite microstructure. When the rolling is carried out at a reduction ratio of more than 1.5, the γ grains become fine and transformation from the grain boundary predominates, which is also harmful to the formation of the acicular ferrite microstructure. Therefore, rolling either is not carried out (i.e. the steel plate is left as cast) or is carried out at a temperature not lower than 800°C and at a reduction ratio of not more than 1.5. If the cooling rate below 800°C is too fast, the microstructure becomes coarse bainite or martensite, and if it is too slow, ferrite-pearlite microstructure is formed and, as a result, the acicular ferrite aimed at by the present invention cannot be obtained. Thus it is necessary to carry out cooling from 800°C to below 600°C at a rate of not less than 2°C/sec and not more than 50°C/sec. The method of the present invention is capable of providing steels for use in various kinds of steel structures which are used at ambient or lower temperatures, and, specifically, can provide steels for use in line pipes, low-temperature pressurized storage vessels , ships and offshore structures. In the conventional method of producing such steels, the casting has been followed by reheating and rolling, hot charge rolling or quenching/tempering, and then by normalizing, rolling and accelerated cooling. In the method of the present method, the steel is subjected to accelerated cooling immediately after casting or after rolling at a small reduction ratio following casting, whereby a fine-grained acicular ferrite develops radially from oxide inclusions as the nuclei during the cooling step. Thus in the production of thick plate, which is the main application of this invention, not only is the reheating step eliminated from the production processes but the rolling step is also eliminated or simplified. As a result, the casting step and the rolling step, if carried out at all, can be directly connected and/or integrated. The present invention provides steel plate with strength and toughness equal to or better than that produced by conventional methods. Moreover, it enables production of high quality steel plate with no rolling whatsoever or at any rate with much less rolling than is used in the conventional methods. It therefore makes possible a dramatic improvement in productivity and reduction in facility cost. Examples Table 1 shows the chemical composition of steel plates produced from slabs produced by vacuum melting. Table 2 shows the production conditions of steel plates produced according to the invention and of steel plates produced according to the conventional method, and Table 3 shows the properties of plates produced from the same. As will be noted, all of the steels produced by the method of the present invention exhibited better low-temperature toughness than the steels produced by the conventional method.

Claims: A method of producing steel plate comprising acicular ferrite microstructure and exhibiting good low-temperature toughness without reheating the steel following casting, the method comprising the steps of casting a molten steel comprising 0.001 - 0.300 wt% C, not more than 0.8 wt% Si, 0.4 - 2.0 wt% Mn, not more than 0.007 wt% Al and 0.0010 - 0.0100 wt% O, the remainder being iron and unavoidable impurities, cooling the molten steel between the liquidus and solidus thereof at a cooling rate of not less than 10°C/min to disperse and precipitate an oxide inclusion including (Mn,Si)O throughout the steel slab, optionally rolling the cast steel starting from a temperature of not lower than 800°C and at a reduction ratio of not more than 1.5, and cooling the rolled or as-cast steel from 800 to below 600°C at a cooling rate of between 2°C/sec and 50°C/sec. A method of producing steel plate comprising acicular ferrite microstructure and exhibiting good low-temperature toughness without reheating the steel following casting, the method comprising the steps of casting a molten steel comprising 0.001 - 0.300 wt% C, not more than 0.8 wt% Si, 0.4 - 2.0 wt% Mn, not more than 0.007 wt% Al, 0.0010 - 0.0100 wt% O, one or two elements selected from among not more than 1.5 wt% Cu, not more than 10 wt% Ni, not more than 1 wt% Cr, not more than 1 wt% Mo, not more than 0.2 wt% Nb, not more than 0.5 wt% V, not more than 0.05 wt% Ti, not more than 0.0025 wt% B, not more than 0.05 wt% REM, not more than 0.008 wt% Ca and not more than 0.05 wt% Zr, the remainder being iron and unavoidable impurities, cooling the molten steel between the liquidus and solidus thereof at a cooling rate of not less than 10°C/min to disperse and precipitate an oxide inclusion including (Mn,Si)O throughout the steel slab, optionally rolling the cast steel starting from a temperature of not lower than 800°C and at a reduction ratio of not more than 1.5, and cooling the rolled or as-cast steel from 800 to below 600°C at a cooling rate of between 2°C/sec and 50°C/sec. A method of producing steel plate comprising acicular ferrite microstructure and exhibiting good low-temperature toughness without reheating the steel following casting, the method comprising the steps of casting a molten steel comprising 0.001 - 0.300 wt% C, not more than 0.8 wt% Si, 0.4 - 2.0 wt% Mn, not more than 0.007 wt% Al, 0.0010 - 0.0100 wt% O, three elements selected from among not more than 1.5 wt% Cu, not more than 10 wt% Ni, not more than 1 wt% Cr, not more than 1 wt% Mo, not are than 0.2 wt% Nb, not more than 0.5 wt% V, not more than 0.05 wt% Ti, not more than 0.0025 wt% B, not more than 0.05 wt% REM, not more than 0.008 wt% Ca and not more than 0.05% Zr, the three elements being selected in one of the following combinations: (i) Ti, Cu, Ni (ii) B, Cu, Ni (iii) Nb, Ti, Ca (iv) Nb, Ti, V (v) Nb, Ti, B (vi) Ti, Ca, Zr (vii) Nb, B, V (viii) Nb, Ni, Mo (ix) Nb, Cu, Ni, the remainder being iron and unavoidable impurities, cooling the molten steel between the liquidus and solidus thereof at a cooling rate of not less than 10°C/min to disperse and precipitate an oxide inclusion including (Mn,Si)O throughout the steel slab, optionally rolling the cast steel starting from a temperature of not lower than 800°C and at a reduction ratio of not more than 1.5, and cooling the rolled or as-cast steel from 800 to below 600°C at a cooling rate of between 2°C/sec and 50°C/sec. A method of producing steel plate comprising acicular ferrite microstructure and exhibiting good low-temperature toughness without reheating the steel following casting, the method comprising the steps of casting a molten steel comprising 0.001 - 0.300 wt% C, not more than 0.8 wt% Si, 0.4 - 2.0 wt% Mn, not more than 0.007 wt% Al, 0.0010 - 0.0100 wt% O, four elements selected from among not more than 1.5 wt% Cu, not more than 10 wt% Ni, not more than 1 wt% Cr, not more than 1 wt% Mo, not more than 0.2 wt% Nb, not more than 0.5 wt% V, not more than 0.05 wt% Ti, not more than 0.0025 wt% B, not more than 0.05 wt% REM, not more than 0.008 wt% Ca and not more than 0.05% Zr, the four elements being selected in one of the following combinations: (i) Nb, Ti, Ca, B (ii) Nb, Ti, B, V (iii) Nb, Ti, Cu, Ni (iv) Nb, B, Ni, V, the remainder being iron and unavoidable impurities, cooling the molten steel between the liquidus and solidus thereof at a cooling rate of not less than 10°C/min to disperse and precipitate an oxide inclusion including (Mn,Si)O throughout the steel slab, optionally rolling the cast steel starting from a temperature of not lower than 800°C and at a reduction ratio of not more than 1.5, and cooling the rolled or as-cast steel from 800 to below 600°C at a cooling rate of between 2°C/sec and 50°C/sec. A method of producing steel plate comprising acicular ferrite microstructure and exhibiting good low-temperature toughness without reheating the steel following casting, the method comprising the steps of casting a molten steel comprising 0.001 - 0.300 wt% C, not more than 0.8 wt% Si, 0.4 - 2.0 wt% Mn, not more than 0.007 wt% Al, 0.0010 - 0.0100 wt% O, five elements selected from among not more than 1.5 wt% Cu, not more than 10 wt% Ni, not more than 1 wt% Cr, not more than 1 wt% Mo, not more than 0.2 wt% Nb, not more than 0.5 wt% V, not more than 0.05 wt% Ti, not more than 0.0025 wt% B, not more than 0.05 wt% REM, not more than 0.008 wt% Ca and not more than 0.05% Zr, the five elements being selected in one of the following combinations: (i) Nb, Ti, Cu, Ni, Cr (ii) Nb, Ti, Ca, Cu, Ni (iii) Nb, Ti, Cu, Ni, V (iv) Nb, Ti, B, Cu, Ni (v) Cu, Ni, Cr, Mo, V, the remainder being iron and unavoidable impurities, cooling the molten steel between the liquidus and solidus thereof at a cooling rate of not less than 10°C/min to disperse and precipitate an oxide inclusion including (Mn,Si)O throughout the steel slab, optionally rolling the cast steel starting from a temperature of not lower than 800°C and at a reduction ratio of not more than 1.5, and cooling the rolled or as-cast steel from 800 to below 600°C at a cooling rate of between 2°C/sec and 50°C/sec.

Inventors: MURATA, MASAHIKO, NISHIOKA, KIYOSHI, TAMEHIRO, HIROSHI
Applicants: NIPPON STEEL CORPORATION
Application year: 1988
Main classification: C21D8
IPC codes: C21D9, C21D8, C22C38