JPH058255B2 - - Google Patents

Description

[Detailed description of the invention]

``Object of the Invention'' The present invention relates to a method for producing ultra-high tensile maraging cold-rolled steel sheets, in which ultra-high tensile maraging cold-rolled steel sheets having a tensile strength of 200 Kgf/mm ² or more are suitably produced by a continuous annealing line. The aim is to provide a method that can produce Industrial Applications Manufacture of ultra-high tensile maraging cold-rolled steel sheets. PRIOR ART Maraging steels with extremely high strength have been known for some time. In other words, this conventional general maraging steel has V, Ti,
It is a steel in which binary or multi-element intermetallic compounds such as Al, Co, Mo, ^etc. The above tensile strength can be obtained. However, in the past, the use of such steel has been limited to special fields such as military, aviation, nuclear power, and ocean development due to its composition and manufacturing cost, and research and development of steel has focused on composition design and design with the premise of use in these special fields. The main factors were strengthening mechanisms and environmental embrittlement. Problems to be solved by the invention Conventional maraging steels as mentioned above are
There are very few cases where its use has been considered in the field of cold-rolled thin steel sheets, which generally requires low cost and versatility, but the above-mentioned characteristics of this maraging steel make it suitable for cold-rolled steel sheets. Needless to say, it is preferable to use it in the field of thin steel sheets. However, due to recent advances in steel manufacturing technology, advances in manufacturing technology such as high purity steel and continuous manufacturing processes, as well as reduced gauge due to increased strength, are factors that enable use in the above thin steel plate field. However, the technical problems that hinder this are as follows. Maraging steel requires solution treatment prior to age hardening treatment, and the solution treatment conditions are heat treatment at 800 to 850℃, preferably 820℃ for about 1 hour. It is difficult to carry out such solution treatment. 450 more to achieve sufficient age hardening.
Aging treatment is required at ~500°C, preferably at 480°C for about 3 hours, which also shows the difficulty of implementation in a continuous heat treatment line. Since the rolled steel strip structure during cold rolling is a martensitic phase, the deformation resistance is high, and conventional rolling techniques inevitably result in large mill loads. By the way, Japanese Patent Publication No. Sho 58-18408 discloses one method for solving these problems.
In other words, the above-mentioned problems are due to the fact that the material design of conventional maraging steel has always aimed at the highest level of material quality, and if equivalency can be guaranteed in terms of application technology, heat treatment is required. It is possible to break through preconceived notions regarding conditions, etc.
Based on this viewpoint, the present invention attempts to manufacture a maraging steel strip using a strip form. However, this technology assumes that the microstructure of the steel is equivalent to that of a material solution-treated at 820°C for 1 hour, and the solution treatment conditions are such that the energy cost is low when performing solution treatment on a continuous heat treatment line. It is not possible to achieve the optimum target or surface quality. "Structure of the Invention" Means for Solving Problems The present invention was created after repeated studies in view of the above-mentioned circumstances, and includes: C: 0.02wt% or less, Si 0.1wt%, Mn 0.2wt %, P0.01wt%, S0.01wt%, N0.01wt%, Ni: 15-25wt%, Co10.0wt%, Mo7.0wt%, Al0.2wt% Ti1.5wt%. or 2 or 3 or more, with the remainder being
Steel consisting of Fe and unavoidable impurities is heated to 1000℃.
The following cumulative rolling reduction ratio should be 60% or less and 950℃
Ultra-high tensile strength, characterized by finishing hot rolling at 900℃ or higher with the following cumulative reduction ratio of 20% or less, coiling at 300 to 600℃, and then recrystallization annealing and solution treatment after cold rolling. This is a method for manufacturing maraging cold-rolled steel sheets. Effect The product according to the present invention as described above achieves effective age hardening due to its component composition, and also achieves age hardening of maraging steel in a continuous annealing line in a short time, reducing energy costs and reducing thermal energy costs. Good material quality and surface quality can be achieved by appropriately selecting the cumulative reduction rate during inter-rolling, finishing temperature, winding temperature, etc. EXAMPLES To further explain the present invention as described above, the present invention is advantageous in terms of cost in an actual continuous heat treatment line such as a continuous annealing line.
Moreover, the purpose is to produce a maraging cold-rolled steel strip that is excellent in terms of material and surface quality.
The steel having the above-described composition is hot-rolled and then cold-rolled as described above, and then subjected to recrystallization annealing and solution treatment using a continuous annealing line.The heat treatment temperature T and heat treatment time t are as follows. Control and process. 825℃T1050℃ t0.5min at T950℃ texp (-T/90.2+9.8) at T1000℃ t1.0min at T1000℃ texp (-T/64.6+15.5) Also, the final final solution as described above Following the oxidation process,
A process in which a steel plate cooled to a temperature of 200°C or less by any cooling method is subjected to age hardening treatment at a temperature of 400°C or more and 600°C or less for 10 minutes or less in a heating and soaking furnace placed in the same pass line. Add as appropriate. To explain the details of this invention,
The present invention aims to perform recrystallization annealing and solution treatment after cold rolling within a short time without impairing the material quality and surface quality of the maraging steel strip to be heat treated. By controlling the microstructure of the steel plate to an optimal state, we succeeded in obtaining the desired material quality with a lower temperature and shorter heat treatment compared to conventional techniques. However, regarding the utilization technology of maraging steel strips, it is most advantageous to perform age hardening treatment after forming in a solution state, and from this point of view, recrystallization annealing and
The quality of the material in the solution treated state must be guaranteed. Therefore, the present inventors determined that in wt% (hereinafter simply referred to as %), C: 0.009%, Si: 0.01%, Mn: 0.05%,
P: 0.005%, S: 0.001%, N: 0.0037%, Ni:
18.1%, Co: 8.2%, Mo: 4.7%, Al: 0.004%,
Various studies were conducted using a steel containing (Steel 1 in Table 1, which will be described later) as a material. That is, the hot-rolled steel strip was first rolled in a hot rolling mill and finished at 970°C to a thickness of 4.0 mm, then allowed to cool to room temperature, and after pickling, the steel strip was cold-rolled to 1.0 mm at a reduction rate of 75%. Softening annealing was performed by varying the recrystallization annealing temperature and time. However, the critical bending radius (R/t: ratio of the critical radius to the plate thickness) is the point at which hair cracking occurs in the bent portion of a material that has been heat-treated at 800°C, 900°C, and 1000°C for different times and then air-cooled to room temperature. ) are arranged in relation to the heat treatment time, and the results are shown in Figure 1. In other words, as heat treatment is carried out at high temperatures and for a long time, the critical bending radius becomes smaller, indicating an improvement in bending formability, but when used as an ultra-high strength steel plate, R/t of 3 or less is practical. It can be said that it is suitable for serving. Further, the results of a detailed study on the temperature of such heat treatment are shown in FIG. That is, the number shown in the circle in FIG. 2 is R/t, and as mentioned above, the range of annealing temperature and time required for bending formability is as follows, with R/t=3 as the boundary. At 950°C or higher, the time is 0.5 minutes or more, but 820°C is the lower limit, and at 850°C, the time is 2 minutes or more. Incidentally, surface texture is an important characteristic when used as a cold-rolled steel sheet, and oxidation of the surface due to the furnace atmosphere poses a problem particularly when heat treatment is performed at a high temperature. However, when performing bright annealing in a continuous heat treatment furnace, it is necessary to make the atmosphere inside the furnace reducing.
Generally, annealing is performed in an atmosphere of nitrogen (N ₂ ) + 5 to 10% hydrogen (H ₂ ). Radiant tube heating furnaces are capable of annealing in such a completely reducing atmosphere, but if operation at temperatures exceeding 850-900°C is required, energy efficiency may deteriorate and Since this is not preferable in terms of deterioration of the radiant tube, etc., a direct-fired heating furnace is used when such high-temperature heating is performed. In the case of this direct-fired heating furnace, heating is possible in a non-oxidizing atmosphere by operating at a low air-fuel ratio at relatively low temperatures, but when heating at around 1000℃, surface oxidation due to combustion waste gas is unavoidable, and continuous heating is possible. In the heat treatment line, it is necessary to install a strong reduction zone or pickling tank after annealing. Therefore, after performing recrystallization annealing and solution treatment at high temperatures, it is necessary to suppress surface oxidation during the heat treatment process as much as possible in order to remove the oxide film in-line or to maintain good surface quality of the final product. . Therefore, the present inventors heat-treated cut samples of 1.0 mmt at various temperatures and times under conditions of an air-fuel ratio of 0.7 to 0.9 in a small direct-fired furnace simulating the actual machine, and then heated them at room temperature in an N ₂ + 5% H ₂ atmosphere. The sample was cooled to a temperature of 100 mL, pickled in a nitric-fluoric acid solution, and its surface properties were evaluated. In other words, 5 items are almost bright annealed, 3 items are those where a uniform oxide film is formed and the surface equivalent to that before heat treatment can be obtained with pickling within 1 min, and 3 items are those where a strong and selective oxide film is formed. Figure 3 shows the results of evaluating the surface of each treated material on a five-point scale, with one point being those that are not only impossible to pickle within 1 minute, but also have surface defects that remain even after complete pickling. The deterioration of the surface quality becomes noticeable after long-term treatment at high temperatures. However, the surface condition that does not pose a problem in practical use is generally 3 or higher, and this range is the range shown in the direction of the hatched arrow.If the heat treatment time is 1 minute, the temperature may be 1000℃, but if the heat treatment time is 10 minutes, Therefore, the limit is 850℃. Furthermore, Figure 4 shows the relationship between processing temperature and time in detail in relation to the surface texture, and Figure 4 shows the allowable temperatures with the above-mentioned score 3 as the boundary. and specific areas of time are shown separately. Furthermore, by combining the results shown in FIG. 4 and the results shown in FIG. 2 described above, the range of recrystallization annealing temperature and heat treatment time that is preferable in terms of formability and surface texture is as shown in FIG. , the range shown in FIG. 5 is the range under the following conditions. 825℃T1050℃ t0.5min atT950℃ texp (-T/90.2+9.8) atT950℃ Lower limit of heat treatment time t1.0min atT1000℃ texp (-T/64.6+15.5) atT1000℃ Upper limit of heat treatment time However, in the present invention Hot rolling conditions are regulated within a specific range in order to obtain excellent material within the above-mentioned range and in consideration of the manufacturing process. That is, first, in the present invention, in hot finish rolling, the cumulative rolling reduction in the temperature range of 1000°C or less is controlled to be 60% or less, and the cumulative rolling reduction in the temperature range of 950°C or less is controlled to 20% or less, and the cumulative rolling reduction in the temperature range of 950°C or lower is Subject to completion of rolling. The steels used in the studies shown in Figures 1 to 5 above were rolled at various cumulative reduction rates below 1000°C, and hot-rolled steel plates finished at 910°C to 4.0 mmt were rolled to 1.0 mmt. After cold rolling, recrystallization annealing and solution treatment were performed at 900°C for 3 minutes. The difference (ΔEl) between the elongation in the rolling direction (El _L ) and the elongation in the perpendicular direction (El _T ) of the steel strip when performing a tensile test using a JIS No. 5 test piece in the solution state is calculated from 1000℃ to 900℃. The results organized by the cumulative rolling reduction rate and the cumulative rolling reduction rate from 950°C to 900°C are shown in Figure 6. i.e. 1000℃
The greater the cumulative reduction rate below, the greater the anisotropy of elongation, and the anisotropy increases particularly when the cumulative reduction rate below 950°C is large. This indicates that the dynamic recovery and recrystallization rate during hot rolling is slow in this steel, so when strong pressure reduction is performed in a relatively low temperature range, austenite grains are elongated in the rolling direction and the texture develops significantly. This tendency is particularly noticeable in the surface layer, resulting in a difference in texture in the thickness direction. The texture in the austenite region is inherited by the martensitic transformation phase upon cooling, and the anisotropy of the crystal structure formed in this way completely disappears during the cold rolling and recrystallization annealing processes. It is presumed that this is due to not doing so. Therefore, in the present invention, as a guideline for anisotropy, ΔEl 1.0% is set as no problem in practice, and therefore the cumulative rolling reduction rate below 1000°C is set to 60% or less, and the cumulative rolling reduction rate between 950 and 900°C is set to 20%. % or less. Next, the winding temperature after hot rolling as described above is also an important factor in the manufacturing process, and in the present invention, this is regulated for the following reasons. First, the lower limit of the winding temperature is 300℃, and if a hot-rolled steel sheet like the one described above is austenitized at 900℃ and then subjected to a tensile test in the temperature range of 700℃ to 100℃ during cooling, The yield strength increases rapidly by 0.2% from around 200℃. This suggests an increase in the winding load in the process, and the reason is that martensitic transformation starts at around 200°C from the continuous cooling transformation (CCT) behavior shown in Figure 7. In addition, in the case of this steel, if the transformation starts on the runout table before winding, the progress of the transformation will be uneven due to the uneven cooling of the runout, and the shape of the plate will be disordered. It is preferable for the plate shape to occur uniformly. That is, from these points of view, the lower limit of the winding temperature was regulated to 300°C. On the other hand, the upper limit of the winding temperature is set at 600°C, but this is mainly done in consideration of descaling properties.
In the case of cold-rolled steel sheets, it is necessary to descale them by pickling or mechanical methods before cold rolling. Therefore, with hot-rolled steel sheets that have formed strong scales, descaling is not only time-consuming but also difficult. Descaling tends to be incomplete, thereby causing serious defects in the surface texture of cold rolled steel sheets. Figure 8 shows a sample in which the above-mentioned steel was rolled in a laboratory-scale hot rolling mill, kept in an atmospheric furnace maintained at 700°C to 100°C for 1 hour, and then rolled up to simulate rolling. The relationship between the pickling time and the equivalent winding temperature when pickling with hydrofluoric acid is shown.
If the temperature is exceeded, the pickling time increases considerably. This is not only disadvantageous in the manufacturing process, but also means a decrease in yield due to an increase in the amount of pickling.From this perspective, the upper limit of the winding temperature was set at 600°C. When the steel according to the present invention is put to practical use as a maraging steel, it is important to exhibit the desired age hardening.
Two or more of the following types, including: In particular, in the present invention, from the viewpoint of component cost, the upper limit of the addition amount that provides effective age hardening is regulated. 15.0% Ni25.0% Co10.0% Mo7.0% Al0.2% Ti1.5% These elements are converted into Ni ₃ Mo,
(Fe, Ni, Co) ₂ Mo, Fl ₃ Mo, Ni ₃ Ti, Fl ₃ Al,
It forms an intermetallic compound of two or more elements such as Ni ₃ (Al, Ti) and contributes to significant hardening.
In the present invention, intermetallic compounds mainly composed of Ni [Ni ₃ Mo, (FeNi, Co), Mo, Ni ₃ Ti, Ni ₃
(Al, Ti)] for the purpose of precipitation strengthening.
Further strengthening can be achieved by adding precipitation strengthening such as Fe ₃ Mo and Fe ₃ Al. Therefore, Ni is an essential element and a lower limit is set, and the lower limit is 15% in order to maintain the precipitation strengthening state in the present invention.
The upper limit is set at 25% because not only does it increase the material cost, but it also makes it impossible to expect precipitation strengthening commensurate with the added amount. Figure 9 shows steel containing 18% Ni as a base and various elements added to it at 480°C after being subjected to the series of treatments according to the present invention described above.
The amount of increase in hardness (HRC) after aging for 10 minutes is shown, and as is clear from this figure, the amount of saturated hardening in approximately short-time aging is reached within the above-mentioned addition range of each element. This is a result of the addition of either a single element or two elements, and it is clear that the amount of hardening can be increased by further adding a combination of elements. Therefore, in the present invention, the necessary maximum addition amount shown in FIG. 9 is taken as the upper limit addition amount of each element. In addition, although not particularly specified in the present invention, Nb, Cr,
It is also effective to add Cu or the like. Furthermore, in the present invention, as the basic component system, C0.02%, Si0.1%,
Mn0.2%, P0.01%, S0.01%, N0.01
%. These are mainly based on consideration of ductility, and by restricting them to the above ranges, significant deterioration in elongation can be avoided. Thus, the present invention enables the production of in-line age-hardened steel strips in a continuous heat treatment line along with the method for producing maraging cold-rolled steel sheets in a solution-treated state based on the above content. In other words, the present inventors found that the age hardening of maraging steel was approximately 40% hardened compared to the solution condition within 3 minutes from the start of aging.
In addition, since hardening of about 50 to 60% was observed in about 10 minutes, we focused on the fact that even short-time overaging in an overaging furnace in a continuous annealing line can increase the strength enough for practical use. FIG. 10 shows the above-mentioned steel after solution treatment under the conditions according to the present invention.
This shows the age hardening behavior when aged at 480℃, and as is clear from this figure, even with the heat treatment time (3 min) that is commonly performed in an overaging furnace in an actual continuous annealing line, the hardening behavior is more than 40%. Hardening was observed, and 50 to 60% hardening was observed in 10 min, which is considered to be possible in an actual overaging treatment furnace. This means that when aged for 3 hours at the same temperature, the hardening rate is 70.
Judging from the fact that it is about ~80%, it can be said that sufficient strength for practical use can be obtained even with short aging. Therefore, in the present invention, after solution treatment, short-time aging is carried out in-line to produce maraging steel strip in a semi-hard state at 400 to 600°C.
Aging treatment is performed at a temperature of 10 minutes or less. Here, if the aging temperature is less than 400℃, aging for less than 10 minutes will not cure more than 30% of the solution state set by the inventors as a guideline for the hardening rate;
This is because at a temperature exceeding 10 minutes, an over-aging phenomenon occurs and the material deteriorates even in a short aging period of 10 minutes or less, and it is judged to be unfavorable for furnace operation. A specific manufacturing example of a product according to the method of the present invention will be described below. First, Table 1 below shows the chemical compositions of the steel according to the present invention and the comparative steel specifically used by the present inventors.

【table】

[Table] Example 1 Steels 1 to 7 in Table 1 melted in a vacuum melting furnace were heated to 1250°C and then hot rolled. The hot rolling conditions according to the present invention are a cumulative reduction rate of 50% at 1000°C or less, a finishing temperature of 920°C and 4.0 mmt, and this test material was cold rolled to 1.0 mmt after pickling and then heated to 900°C.
Recrystallization annealing and solution treatment were performed for ×3 min. Hardness (HRC) and elongation at break (G.
L.: 50mm, GW: 12.5mm) and 480℃×10min
The hardness (HRC) values after aging were as shown in Table 2 below.

[Table] In other words, the inevitable elements (C,
It is understood that if Si, Mn, P, S, N) exceeds the upper limit regulation value according to the present invention, age hardening comparable to the steel according to the present invention can be obtained, but bendability in the solution state deteriorates. be done. Example 2 Steel 1 was melted under the same conditions as Example 1 and subjected to hot rolling, cold rolling, and recrystallization annealing/solution treatment.
The ductile anisotropy (ΔEl) after solution treatment, the critical bending radius (R/t), and the hardness after aging (HRC) of Steel No. 2 and Steels 8 to 14 are shown in Table 3 below.

[Table] In other words, Ni, Co, Mo, added as reinforcing elements,
If Al and Ti exceed the upper limits in the present invention, the hardness after age hardening will increase, but the recrystallization rate will be delayed and the cold rolled structure will not recover sufficiently under the heat treatment conditions specified in the present invention. Therefore, the anisotropy of ductility and the critical bending radius are greater than the steel according to the invention. On the other hand, for Ni, the lower limit value (15
%) or less, a sufficient curing rate cannot be expected. Example 3 The effects of the manufacturing method according to the present invention are detailed in Figures 1 to 10 for Steel 1 in Table 1, but the same applies to other steels, and the results are shown in Table 4 below along with the comparative method. This is also shown.

[Table] "Effects of the Invention" According to the present invention as explained above, ultra-high tensile maraging cold-rolled steel sheets can be produced using a continuous annealing line for thin steel sheets, which improves both the material properties and the surface quality, and also reduces energy consumption. This invention can be manufactured cost-effectively and has great industrial effects.

[Brief explanation of drawings]

The drawings show the technical content of the present invention, and Figure 1 shows the annealing temperature and time for the steel strip after recrystallization annealing to reach the critical bending radius (R/t: ratio to the plate thickness) in the direction perpendicular to rolling. Figure 2 is a diagram showing the range of recrystallization annealing temperature and heat treatment time in the present invention regulated by this limit bending ratio, Figure 3 is the surface oxide film rating after recrystallization annealing Fig. 4 is a chart showing the range of recrystallization annealing temperature and heat treatment time in the present invention regulated by surface texture rating, Fig. 5 is a chart showing the influence of annealing temperature and time on recrystallization annealing in the present invention. Figure 6 is a diagram showing the relationship of the present invention to temperature and time; Figure 6 is a diagram showing the influence of the differential volume reduction during hot rolling on the anisotropy (ΔEl) of ductility after solution treatment; Figure 7 Figure 8 shows the continuous cooling transformation (CCT) behavior of one of the steels used in the present invention. Figure 9 is a diagram illustrating the influence of the amount of added alloying elements on the age hardening rate after solution treatment. Figure 10 is the relationship between aging time and age hardening rate at 480°C for short-time solution treatment materials. This is a chart showing the following.

Claims (1)

[Claims] 1 Contains C: 0.02wt% or less, Si0.1wt%, Mn0.2wt%, P0.01wt%, S0.01wt%, N0.01wt%, and Ni: 15 to 25wt%. , Co 1.00wt%, Mo 7.0wt%, Al 0.2wt%, Ti 1.5wt%, including two or more of these, including Ni, with the balance consisting of Fe and unavoidable impurities. 900 with the cumulative rolling reduction rate below 1000℃ being 60% or less and the cumulative rolling reduction rate below 950℃ being 20% or less.
A method for producing an ultra-high tensile strength maraging cold-rolled steel sheet, which comprises completing hot rolling at a temperature of 300 to 600°C, followed by cold rolling followed by recrystallization annealing and solution treatment. 2 Following the solution treatment process, the steel strip cooled to a temperature below 200°C is heated and heated in the same pass line.
Claim 1: Age hardening treatment in a soaking furnace
A method for producing an ultra-high tensile strength maraging cold-rolled steel sheet as described in 2.