JPS647135B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a Ni-containing hot-rolled steel strip having high strength and extremely excellent low-temperature toughness as hot-rolled at a low cost. <Industrial Application Fields> Chemical industry equipment such as ethylene plants and coal liquefaction plants, which have been increasing in number in recent years, are equipped with equipment that can withstand use at temperatures below -100°C. For example, ASTM standard A203 Grade D or E steel is required.
3.5% Ni steel, which corresponds to the above, was frequently used. However, recently, there has been a noticeable trend toward larger sizes of the above-mentioned devices, and there is a growing demand for thicker and stronger steel materials, and ways to deal with this are being actively investigated. It has become like that. <Prior Art> Therefore, under the above-mentioned circumstances, the applicant has developed a specific component composition and heat treatment that can ensure high strength and excellent low-temperature toughness even for extremely thick members exceeding, for example, 50 mm. We developed a strong Ni-containing steel obtained by combining the above and proposed it earlier as JP-A No. 53-95120, and it has had good results in use to date. By the way, this type of low-temperature Ni-containing steel sheets, including the one proposed in JP-A No. 53-95120 mentioned above, have generally been manufactured at plate rolling mills, and have not been manufactured at hot rolling mills. There has been little consideration. Furthermore, it was common to heat-treat this thick plate to manufacture it. This is because the desired high strength and low-temperature toughness could not be achieved with hot rolling. The thicker the plate, the more it required well-controlled heat treatment. <Purpose of the Invention> For this reason, the present inventors omitted the heat treatment in the manufacturing process of low-temperature Ni-containing steel sheets as described above, and produced a hot-rolled steel sheet at a hot-rolling factory.
In order to obtain a hot-rolled steel strip that has a brittle fracture surface transition temperature well below -100℃ even when it is a thick steel plate with a thickness of 4.5 mm or more, and a tensile strength of about 50 Kgf/mm ² or more, we have developed a steel composition. As a result of examining the hot rolling conditions and hot rolling conditions from various viewpoints, the following findings were obtained. <Findings> Using Ni-containing steel whose chemical composition has been adjusted to a specific one, and subjecting it to controlled rolling and controlled cooling under specific conditions, this method is unimaginable as a manufacturing method for ordinary hot-rolled steel strip. If the coiling is carried out at a significantly low temperature range that does not cause the hot rolling, a structure consisting of fine grained ferrite and a uniformly dispersed fine grained hard second phase (mainly fine pearlite and bainite) will be obtained. It is possible to obtain a Ni-containing hot-rolled steel strip that has both high strength and excellent low-temperature toughness even as it is. <Structure of the Invention> This invention was made based on the above findings, and includes: C: 0.02 to 0.15% (hereinafter, % representing the component ratio is expressed as weight %), Si: 0.7% or less, Mn: 0.3-1.5%, S: 0.01% or less, Ni: 1.5-3.9%, So. Contains A: 0.01 to 0.10%, and further contains Nb: 0.05% or less, V: 0.10% or less, Ti: 0.15% or less, Cu: 0.5% or less, Cr: 0.5% or less, Mo: 0.3% or less. , Ca: 0.0100% or less, and the balance: Fe and other unavoidable impurities. Cumulative reduction rate at 1100℃ or less: 70% or more, Finishing temperature: 850~ After hot rolling at 650°C, it is immediately quenched at a cooling rate of 5°C/sec or higher, and coiled in a temperature range of 550 to 250°C, resulting in high strength and excellent low-temperature toughness.
This method is characterized by the fact that it produces Ni-containing hot-rolled steel strip at a low cost. Next, in the method of the present invention, the reason for numerically limiting the amount of the steel composition and the hot rolling/coiling conditions as described above will be explained. A Compositional component (a) C The C component has the effect of improving the strength of steel, but if its content is less than 0.02%, the desired strength cannot be secured, while if it exceeds 0.15% Since C content greatly deteriorates the low-temperature toughness of the weld zone, the C content was set at 0.02 to 0.15%. (b) Si The Si component has the effect of increasing the strength of steel even when added in a small amount, but if it is added in excess of 0.7%, toughness will decrease, so the Si content should be 0.7%. % or less. (c) Mn The Mn component has the effect of improving both the strength and toughness of steel, but if its content is less than 0.3%, the desired effect cannot be obtained;
Since weldability deteriorates if the Mn content exceeds 0.3% to 1.5%. (d) S S is an impurity element that combines with Mn to form A-based inclusions and reduces toughness and ductility, so it is preferable to minimize the S content as much as possible. was set at 0.015%. However, it is preferably limited to 0.003% or less. (e) Ni Ni is an essential element for ensuring the strength and toughness of steel. And its content is
If it is less than 1.5%, it will not be possible to ensure low-temperature toughness sufficient to withstand temperatures of around -100°C, which is required for structural members of ethylene plants, etc.; on the other hand, if it is 3.9%
The Ni content was set at 1.5 to 3.9% because Ni content in excess of 1.5% would only increase the price of the steel material and would not result in commensurate property improvement effects. (f) So. A So. Component A is an element necessary for deoxidizing steel, and in order to ensure sufficient deoxidizing effect, it is necessary to deoxidize steel.
It is necessary to contain 0.01% or more. On the other hand, 0.10
If the Sol.Al content exceeds 0.01% to 0.10%, the effect not only becomes saturated but also has a negative effect on toughness. Therefore, the Sol.Al content was set at 0.01 to 0.10%. (g) Nb, V, Ti, Cu, Cr, Mo, and Ca These ingredients have the effect of increasing the strength of steel and improving toughness and ductility, so one or more of these ingredients may be added if necessary. There is, but below:
The reason for limiting the amount of each element added will be explained. Nb, V, and Ti These components each exhibit a significant strength-increasing effect through precipitation hardening, but Nb content is 0.05%, V content is 0.10%, and Ti content is 0.15%. The Nb content should be 0.05% or less, as the effect will be saturated even if it exceeds each.
The V content was determined to be 0.10% or less, and the Ti content was determined to be 0.15% or less. Cu, Cr, and Mo These components are also effective elements for increasing the strength of steel, but if the Cu content exceeds 0.5%, the effect will be saturated, and if the Cr content exceeds 0.5%, the effect will be saturated.
%, the effect will be saturated, and even if the Mo content exceeds 0.3%, the effect will also be saturated. Therefore, the Cu and Cr contents should be 0.5% or less, and the Mo content has been set at 0.3% or less. Ca Ca component is an effective element that improves toughness and ductility by controlling inclusion morphology.
If the Ca content exceeds 0.0100%, the amount of inclusions in the steel will increase, and the toughness and ductility will deteriorate, so the Ca content was set at 0.0100%. Note that although there is no need to particularly limit P as an impurity element in the method of the present invention, it is preferable to reduce it as much as possible. In particular, when the steel material obtained by the method of the present invention is used after being heat treated, it is preferable to sufficiently suppress the P content. B. Hot rolling/coiling conditions (a) Cumulative rolling reduction at temperatures below 1100℃ If the cumulative rolling reduction at temperatures below 1100℃ falls below 70%,
Since the grain refinement of austenite does not proceed much and the desired fine grain structure cannot be achieved, it is difficult to obtain the desired low temperature toughness. Therefore, in the temperature range below 1100℃, 70
% or more. (b) Hot-rolling finishing temperature If the hot-rolling finishing temperature exceeds 850°C, austenite grain refinement will not proceed, making it impossible to obtain the desired fine-grained structure. On the other hand, in the temperature range below 650°C, steel As the hot deformation resistance of
The temperature was set at 850-650℃. In addition, the hot rolling finishing temperature is
When the temperature is below 700°C, it undergoes processing after ferrite transformation, but if the quenching and low-temperature winding are carried out according to the conditions of this invention, the ferrite grains will not become coarse, and rather good low-temperature toughness can be obtained. (This is also clear from FIG. 1, which will be described later). (c) Cooling rate When the cooling rate after hot rolling is slower than 5℃/sec, the steel structure becomes a coarse ferrite/pearlite structure, and the pearlite distribution becomes band-like, resulting in good low-temperature toughness. do not have. In particular, in steels where the toughness of the ferrite base has been greatly improved, such as the Ni-containing steel targeted by the method of this invention, the hard second phase (pearlite and bainite other than ferrite), which is prone to cracking, is etc.) appears to have an extremely large influence on toughness. Therefore, the cooling rate after hot rolling was determined to be 5° C./sec or more. (d) Coiling temperature If the coiling temperature is higher than 550°C, even if the cooling rate after hot rolling is 5°C/sec or more, a coarse ferrite/pearlite structure will result, and the pearlite will deteriorate. The distribution also becomes band-like, making it impossible to obtain good low-temperature toughness. On the other hand, if the material is rapidly cooled at a cooling rate of 5° C./sec or more and is wound at a temperature lower than 250° C., martensitic structure will be mixed in and the toughness will deteriorate as well. Therefore, the winding temperature was set at 550 to 250°C, but preferably 500 to 250°C.
It is recommended to wind in the temperature range of °C. Figure 1 shows 0.06%C-0.22%Si-0.65%Mn-0.005%P, which is the target steel for the method of the present invention.
-0.001%S-3.54%Ni-0.038%Al-0.0040%N
This is a graph showing the influence of hot rolling finishing temperature and coiling temperature on the toughness of steel, where the hot rolling heating temperature is 1200°C in both cases, the finishing plate thickness is 9 mm, and the cooling between coiling after hot rolling. All speeds were investigated within the range of 5 to 35°C/sec. From FIG. 1, it can be seen that the hot rolled steel of the present invention
It can be seen that high strength and excellent low temperature toughness can be combined only when cooling and winding conditions are applied. Next, the present invention will be specifically explained using examples and comparing with comparative examples. <Example> First, steels A to L having the compositions shown in Table 1 were produced by a conventional method. Next, each of these steels was hot rolled under the conditions shown in Table 2 and coiled to produce hot rolled thick steel strips having a thickness of 9 mm. Test pieces were cut out from each of the hot-rolled steel strips obtained in this way and their mechanical properties were examined, and the results shown in Table 2 were also obtained. As is clear from the results shown in Table 2,
All hot-rolled steel strips obtained by the method that satisfies the conditions of the present invention show high strength and excellent low-temperature toughness as hot-rolled, whereas test numbers with a small reduction at 1100°C or less The hot-rolled steel strip obtained by method 1 has poor sharpy impact properties, and the hot-rolled steel strip obtained by test number 2, which has a slow cooling rate after hot rolling, has poor Ni content. Made of low steel material

[Table] (Note 1) “Ratio” indicates “comparison steel”.
(Note 2) * indicates that the conditions are outside the conditions of the present invention.

[Table] (Note) * indicates that the conditions are outside the conditions of the present invention.
It can be seen that the hot rolled steel strip obtained by the method of Test No. 3 was inferior in both strength and Charpy impact properties. Of course, the hot-rolled steel strip obtained by the method of the present invention can be used as it is as hot-rolled to obtain fully satisfactory effects, but if the hot-rolled steel strip is used as a raw material, Of course, it may be used after being subjected to quenching and tempering or normalizing treatment. <Overall Effects> As described above, according to the present invention, it is possible to stably produce Ni-containing steel strips that have high strength and excellent low-temperature toughness as hot-rolled at low cost. It can be used as a material for chemical industrial equipment such as ethylene plants and coal liquefaction plants, and exhibits excellent performance, resulting in industrially useful effects.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of hot rolling finishing temperature and coiling temperature on the toughness of Ni-containing steel.

Claims (1)

[Claims] 1. C: 0.02 to 0.15%, Si: 0.7% or less, Mn: 0.3 to 1.5%, S: 0.015% or less, Ni: 1.5 to 3.9%, So. Steel with a chemical composition consisting of A: 0.01~0.10%, balance: Fe and other unavoidable impurities, is subjected to hot rolling at a temperature of 1100°C or less, a cumulative reduction rate of 70% or more, and a finishing temperature of 850~650°C. A method for producing a hot-rolled steel strip containing low-temperature Ni, the method comprising immediately quenching at a cooling rate of 5° C./sec or more and coiling in a temperature range of 550 to 250° C. 2 In terms of weight percentage, C: 0.02 to 0.15%, Si: 0.7% or less, Mn: 0.3 to 1.5%, S: 0.015% or less, Ni: 1.5 to 3.9%, So. Contains A: 0.01 to 0.10%, and further contains Nb: 0.05% or less, V: 0.10% or less, Ti: 0.15% or less, Cu: 0.5% or less, Cr: 0.5% or less, Mo: 0.3% or less, Ca. : Contains one or more of the following: 0.0100% or less, the remainder: Fe and other unavoidable impurities, Cumulative reduction rate at 1100℃ or less: 70% or more, Finishing temperature: 850-650℃ , after hot rolling, immediately quenching at a cooling rate of 5°C/sec or more, and coiling in a temperature range of 550 to 250°C. Method.