JPH0285323A - Method for continuously annealing ageing-resistant cold-rolled steel sheet - Google Patents

Abstract

PURPOSE:To produce the title press worked steel sheet having excellent ageing resistance and workability by primarily cooling the cold-rolled sheet of low- carbon steel free of Ti and Nb or low-carbon aluminum killed steel in the continuous annealing, then overageing the sheet, and imparting specified elastic strain or plastic strain to the sheet. CONSTITUTION:The cold-rolled sheet of the low-carbon steel free of Ti and Nb as the impurities or low-carbon aluminum killed steel is heated, soaked, and primarily cooled in the continuous annealing. The sheet is overaged so that the thermal activation index defined by the value obtained by integrating the integrand exp[-10100/T] from t=0 to t=t1 becomes >=5.1X10<-6> [T is the overageing temp. in deg.K at time (t), and t1 is the time in sec from the start of overageing to the start of strain impartation]. The sheet is then secondarily cooled from the last half of overageing, strain is imparted at 300-400 deg.C, strain impartation is finished at 150-350 deg.C to impart 0.06-2.30% strain, the strain imparting time is controlled to 2-180min, and elastic strain or plastic strain is imparted.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a cold rolled steel plate with excellent aging resistance and workability, such as one for press working, using convenient continuous annealing equipment. It is.

[Conventional technology]

Due to its excellent workability, soft cold-rolled steel sheets are used in steel sheets that undergo severe forming processes to become final products, mainly for automobiles. However, this workability may deteriorate over time, and this deterioration over time is called aging. Aging property occurs because C and N, which are interstitially dissolved in the steel, fix mobile dislocations introduced in the final step of temper rolling. Aging properties cause deterioration such as an increase in yield point, a decrease in elongation at break, and the occurrence of elongation at yield point. Therefore, even among soft cold-rolled steel sheets, those used for applications that undergo particularly severe forming have a high aging property. It shouldn't be.

Among solid solute C and H, which are the causes of aging, solute N is in a small amount, so by making aluminum killed steel, it is fixed in the form of aluminum nitride, or by adding B, it is fixed in the form of boron nitride. Therefore, aging due to solid solution N can be avoided.

On the other hand, solid solution C has a very small solubility limit in cementite at low temperatures, so if it is cooled over a long period of time as in box annealing, very little will remain. However, box annealing has drawbacks such as requiring a long time, low productivity, and large variations in material quality within the steel strip.In recent years, continuous annealing has been attempted to produce soft cold-rolled steel sheets in a short time. A technology has been developed to do so.

However, in continuous annealing, solid solution C is
remains, resulting in significant C aging, that is, strain aging occurs due to solid solution C fixing mobile dislocations. Therefore,
Many studies have been conducted on methods for reducing solid solution C in producing soft cold-rolled steel sheets with excellent aging resistance and workability by continuous annealing.

Conventional research on methods for reducing solid solution C in continuous annealing methods can be roughly divided into three streams. That is I
There are three types: F steel, rapid cooling/overaging treatment, and strain imparting annealing.

The so-called IF @ (Interstitial F
ree fi) refers to the removal of carbon by vacuum degassing during steelmaking.
It is produced by reducing the amount to about 0 ppm or less, and adding strong carbonitride-forming elements such as Ti and Nb to more than the stoichiometric amount of C and N. IF steel is completely non-aging even with continuous annealing, but the production of IF steel requires special steelmaking equipment and operations as mentioned above, and
The drawback is that the manufacturing cost is high because an expensive alloy is used.

Overaging treatment is a technique that has been researched for the longest time as a method for reducing solid solution C in continuous annealing, and is currently widely applied industrially.

However, with the continuous annealing method that includes overaging treatment that is currently used industrially, it is extremely difficult to ensure complete anti-aging properties and workability at the same time.

Aging property is often indicated by the aging index (AI) or the elongation at yield point after accelerated aging at 100°C for 60 minutes, but in order to be considered non-aging property, an aging index of at least 3 k is required.
gf/- or less, or the elongation at yield point after aging must be 0.4% or less. On the other hand, as described in the paper "Continuous annealing thermal cycle and solid solution C precipitation behavior" published in Japan Kokan Giho No. 99 (1983), page 22, ordinary over-aging treatment is included. The continuous annealing method has a non-aging property comparable to the box annealing method, that is, an aging index of 3 kgf/
− It is not possible to find sufficient measures to ensure that:
There is a problem in that the level of aging index of cold rolled steel sheets currently manufactured industrially by continuous annealing method is generally over 3 kgf/-, excluding IF steel.

As a measure to ensure that the continuous annealing method also has non-aging properties comparable to the box annealing method, a method has been proposed in which the primary cooling after soaking is rapid cooling, and then an overaging treatment is performed.

For example, Japanese Patent Application Laid-Open No. 58-52429 proposes a method in which primary cooling is performed at a rate of 40 to 300°C/second, the rapid cooling is stopped at an overaging treatment temperature, and then an overaging treatment is performed. An example is described for producing a steel plate that exhibits non-aging properties comparable to box annealing, with a post-yield point elongation of 0.1 to 0.3%.

In addition, in Japanese Patent Publication No. 49-1968, primary cooling is
A method is described in which an overaging treatment is performed by rapidly cooling to a temperature of 00° C. or lower and then reheating to an overaging treatment temperature.

Furthermore, Japanese Patent Publication No. 54-13403 proposes a method of performing overaging treatment by rapidly cooling the primary cooling to room temperature at a rate of 200°C/second or more and then reheating to the overaging treatment temperature. In the examples, it is stated that the aging index of the invention steel is approximately the same value as box annealing.

The idea common to these technologies is to increase the amount of supersaturated carbon immediately before the start of overaging treatment by rapid cooling during primary cooling and supercooling to below the overaging treatment temperature, increase the density of cementite precipitation nuclei, and achieve industrial The method is to rapidly precipitate solid solution C as cementite within a short overaging treatment time that can be carried out.

However, to apply these techniques, it is necessary to have cooling equipment, equipment to remove temper color that occurs on the steel strip surface due to rapid cooling, reheating equipment, etc.
There is a problem that continuous annealing equipment is expensive. In the actual operation of continuous annealing equipment, it is rare that only high-grade cold-rolled steel sheets are manufactured that require sophisticated heat treatments such as rapid cooling and reheating overaging treatment, but rather ordinary primary cooling Generally, cold-rolled steel sheets for general use and original sheets for surface treatment, which can be sufficiently produced by over-aging treatment, are also produced using the same continuous annealing equipment. Therefore, it is industrially undesirable to provide expensive equipment for rapid cooling and reheating only for a limited number of high-grade non-aging cold-rolled steel sheets.

Another disadvantage is that additional operating costs such as energy and cooling water are required for reheating and rapid cooling. In addition, extreme rapid cooling and reheating overaging treatment as seen in Japanese Patent Publication No. 49-1968 causes extremely fine cementite to precipitate within the crystal grains during the overaging treatment, so aging resistance improves but , there is a drawback that ductility deteriorates.

Cold rolling of superior materials using relatively convenient continuous annealing equipment! l1
Strain imparting annealing has been studied as a method for producing iI plates.

For example, in Japanese Patent Publication No. 56-25499, strain is applied during the primary cooling process to 530°C after soaking,
A method of cooling it as it is to room temperature, and a method of applying strain 53
A method of performing high-temperature overaging treatment at 0° C. or higher has been proposed, and it is explained that applying strain as a means of reducing solid solution C promotes the precipitation of cementite. However, even if the precipitation of cementite is promoted by applying strain, the solid solubility C decreases below the cementite solid solubility limit at the temperature at which precipitation is completed.
It is thermodynamically impossible to reduce. Therefore, the invention steel of the publication contains solute C in an amount exceeding the cementite solid solubility limit at 530''C, and cannot ensure anti-aging properties comparable to box annealing.

Furthermore, Japanese Patent Publication No. 57-9411 proposes a method of applying strain during heating, but there is no easy means to precipitate a large amount of solid solution C dissolved during soaking. , it is impossible to ensure non-prescription property.

Furthermore, Japanese Patent Publication No. 54-43452 proposes a method in which strain is imparted during the soaking and primary cooling process and then an overaging treatment is performed. However, from the perspective of the low range of solid solute C, the elastic strain or plastic strain imparted before the over-aging treatment will promote the precipitation of cementite, but ultimately the over-aging treatment Solid solution C can only be reduced up to the cementite solid solubility limit at certain temperatures. Therefore, the strain imparting according to the invention described in the above-mentioned publication does not have the effect of ensuring non-aging properties comparable to box annealing. In fact, the aging index shown in the examples described in the above publication is 4.6 to 5.9k.
There is also gf/-, which cannot be said to be non-aging.

As described above, the concept of promoting cementite precipitation and reducing solid solution C by applying elastic strain or plastic strain has been around for a long time. However, the conventional strain imparting annealing technique cannot be put to practical use in ensuring anti-aging properties because the timing of strain imparting is inappropriate or the amount of strain is insufficient.

[Problem to be solved by the invention]

The present invention solves the above-mentioned problems and enables continuous annealing to produce cold-rolled steel sheets with excellent aging resistance and workability, such as those for press working, using simple continuous annealing equipment without deteriorating ductility. , the purpose is to provide a method for improving aging resistance.

[Means to solve the problem]

The gist of the present invention is that when continuously annealing a cold rolled steel strip of low carbon steel or low carbon aluminum killed steel that does not contain either Ti or Nb, following heating, soaking, and primary cooling, the integrand function exp is (10100/T ) (T represents the overaging treatment temperature at time t in Kelvin) from time 1 = 0 to 1 = 1 + (1 + is the time from the start of overaging treatment to the start of strain application) Thermal activation stroke index defined as the value integrated up to (expressed in seconds) is 5.I x 10-6
After carrying out the over-aging treatment as described above, from the latter half of the over-aging zone to the secondary cooling zone, 300 to 400 °C is the strain imparting start temperature, and 150 to 350 °C is the strain imparting end temperature,
The applied strain amount is 0.06 to 2.30%, and 2 to 1
The objective is to apply elastic strain or plastic strain to the steel strip such that the strain application time is 80 seconds.

Hereinafter, the constituent elements of the present invention and the reasons for the numerical limitations will be described.

The present inventors first theoretically investigated the reason why the conventional strain imparting annealing technique cannot be put to practical use in ensuring non-aging properties. As a result, two problems became clear.

The first problem is that even if the application of elastic strain or plastic strain promotes the precipitation of cementite, the amount of solid solute C that ultimately reaches
This means that it is thermodynamically impossible for the cementite solid solubility limit to be lowered at the end temperature of the cementite precipitation treatment. Therefore, in order to suppress aging properties, it is necessary to lower the temperature at which elastic strain or plastic strain is imparted and cementite precipitation treatment is completed. However, if these temperatures are too low, the diffusion rate of C in the ferrite iron will be slow, and the cementite precipitation process will take a long time.

The second problem is that the application of elastic or plastic strain promotes the precipitation of cementite because the application of elastic or plastic strain introduces a large number of lattice defects into the crystal grains, and these lattice defects become solidified as precipitation nuclei. This is because molten C precipitates, and as a result, the precipitated solid solution C forms many fine cementites within the crystal grains, which may deteriorate ductility.

Therefore, the present inventors considered that the timing and temperature at which elastic strain or plastic strain is imparted in the thermal history of continuous annealing are particularly important, and conducted detailed experiments.

As a result, we have discovered a new method for imparting elastic strain or plastic strain during continuous annealing that can solve the above problems and practically suppress aging, and constitute the present invention.

First, the reasons for limiting the timing of applying elastic strain or plastic strain will be described.

If we look at the thermal history of continuous annealing from a metallurgical perspective, it can be roughly divided into the following steps, starting from the entrance of the continuous annealing equipment: heating, soaking, primary cooling, overaging treatment, and secondary cooling. In the present invention, elastic strain or plastic strain is imparted after a certain amount or more of over-aging treatment has progressed.
It is essential to carry out the process from the latter half of the overaging zone to the secondary cooling zone. However, it is not necessary to apply it over the entire length from the latter half of the overaging zone to the secondary cooling zone, and it may be applied to a part thereof. For example, it may be applied to a part of the secondary cooling zone or a part of the latter half of the overaging zone.

Here, the overaging treatment of a certain amount or more means the integrand function ex
p (10100/T) (T represents the overaging treatment temperature at time in Kelvin) from time 1=0 to 1-1. (1. represents the time in seconds from the start of overaging treatment to the start of strain application) The thermal activation process index is defined as 5. IXIO-
'The overage treatment is limited to the above.

As can be seen from the above definition, the thermal activation stroke index is a quantity determined by the thermal history of the over-aging treatment and the time L1 from the start of the over-aging treatment to the start of strain application, and is determined by the physical The meaning is an index of the degree of progress of cementite precipitation during overaging treatment. Thermal activation stroke index is 5. lX1
For reference, the conditions for overaging treatment that result in 0"h or more are exemplified in the case of isothermal overaging treatment. If the overaging treatment temperature is 400°C, the overaging treatment time is 18 seconds or more, 3
If it is 50°C, it will be more than 58 seconds, and if it is 300°C, it will be more than 3 minutes 56 seconds.

Thermal activation stroke index is 5. If it is less than l ductility deteriorates. Thermal activation stroke index is 5. If it is lXl0-6 or more, most of the solid solution C has completed precipitation before strain is applied;
Since the remaining small amount of solid solution C precipitates as nuclei in the lattice defects introduced by elastic strain or plastic strain, the ductility does not deteriorate due to the imparting of strain. Therefore, the thermal activation stroke index is limited to 5.1×10 −h or more.

If the strain application start temperature exceeds 400°C, the solid solubility limit of cementite increases, so the solid solute Cl at the start of strain application becomes excessive, ductility deteriorates, and solid solution C precipitates by the end of secondary cooling. It remains unresolved and the aging process cannot be suppressed. In addition, the strain application start temperature is 3
When the temperature drops below 00°C, the diffusion rate of C slows down, and even with the effect of promoting cementite precipitation by applying elastic strain or plastic strain, solid solution C cannot be precipitated and remains in the short period until the end of secondary cooling. It is not possible to suppress aging. Therefore, the strain application start temperature is limited to 300 to 400°C.

If the strain imparting end temperature exceeds 350° C., the cementite solid solubility limit is large, so that solid solution C remains and aging cannot be suppressed. When the strain application end temperature is lower than 150°C, the diffusion rate of C becomes slow, and even with the effect of promoting cementite precipitation due to elastic or plastic strain application,
Solid solution C cannot be completely precipitated and remains in the short time until the end of secondary cooling, making it impossible to suppress aging properties. Therefore, the temperature at which straining ends is limited to 150 to 350''C.

The amount of elastic strain or plastic strain to be applied is 2.30%.
If the strain exceeds 1, the strain itself deteriorates the ductility and hardens the steel. If the amount of elastic strain or plastic strain is less than 0.06%, lattice defects in the amount necessary to promote cementite precipitation cannot be introduced, and aging cannot be suppressed. Therefore, the amount of elastic strain or plastic strain is 0.06 to 2.30%.
limited to. The range of elastic strain or plastic strain is 0.10~
At 2.00%, the effect of the present invention is particularly remarkable.

The time required to impart elastic strain or plastic strain is 180
If the time exceeds seconds, a long equipment is required or the line speed is extremely reduced, which impedes productivity. When the strain application time is less than 2 seconds, the time required for precipitation of cementite is insufficient, and solid solution C remains. Therefore, the strain application time is 2
~180 seconds.

The method of imparting elastic strain or plastic strain may be rolling, bending/unbending with a leveler roll or hearth roll, stretching with tension, or any other method.

If the material subjected to continuous annealing contains Ti or Nb, solid solution C will precipitate as Ti carbide or Nb carbide before the continuous annealing process, and Ti carbide and Nb carbide will not be present even in the continuous annealing process. Since it does not dissolve easily, it is not possible to further improve the aging resistance even if the strain according to the present invention is applied. Therefore, the material to be subjected to continuous annealing may be a cold rolled steel strip of low carbon steel or low carbon aluminum killed steel that does not contain either Ti or Nb.

In cold rolled steel strips subjected to continuous annealing, if solid solute N remains after annealing, aging will occur due to solid solute N. Therefore, when manufacturing non-aging cold rolled steel sheets, solid solute N should be replaced with A7N. Cold-rolled aluminum killed steel strip that can be fixed as However, even if the cold rolled steel strip contains solute N after annealing, the strain imparting effect of the present invention on the low range of solid solute C is not lost in the slightest. Therefore, if the final product does not necessarily require complete aging resistance, the present invention does not require the use of aluminum-killed steel for the cold-rolled steel strip, for example, low-carbon steel such as St-killed steel or rimmed steel. By applying strain, it is possible to improve aging resistance without deteriorating ductility.

Components other than those mentioned above, hot rolling conditions, and cold rolling conditions do not matter. The present invention applies strain when continuous annealing is applied to materials manufactured using any combination of ingredients, hot rolling conditions, and cold rolling conditions necessary to ensure the material properties other than the aging resistance required for the final product. If this is done, the aging property can be suppressed more than when the strain of the present invention is not applied.

Generally, the steel composition to which the present invention is applied is C50.1%
, SiS2.3%, Mn61%, P≦0.1%, S≦0
．． 03%, u for aluminum killed steel: o, oos
~0.500%.

Regarding the continuous annealing conditions, any method may be used as long as it does not depart from the limited scope of the present invention. For example, the soaking temperature and soaking time can be determined depending on the target product material. The primary cooling can be carried out by any method such as rapid cooling, slow cooling, a combination of slow cooling in the first half and slow cooling in the second half, or a combination of cooling. The overaging treatment temperature and overaging treatment time may be arbitrarily determined as long as the thermal activation stroke index satisfies the range of the present invention. The overaging treatment method is not limited to isothermal overaging treatment, but also the so-called inclined overaging treatment, in which the overaging treatment end temperature is lower than the overaging treatment start temperature, and the copper band temperature is adjusted linearly or along a certain history between them. An overaging treatment may be used to gradually reduce the amount.

〔Example〕

Examples of the present invention are shown in Tables 1 and 2.

Table 1 shows C: 0.045%, AJ: 0.056%, N
: Aluminum killed steel containing 0.0018% and containing no Ti and Nb is melted in a converter, and the slab produced by continuous casting is cooled to around room temperature and reheated to 1050 ° C.
Hot rolled, rolled at 700℃, pickled, cold rolled,
The aging index and elongation at break of a steel strip subjected to continuous annealing and 1% temper rolling are shown together with overaging treatment conditions and strain imparting conditions. Soaking temperature for continuous annealing is 800
The primary cooling rate at 1 °C was approximately 10"C/sec. The primary cooling was stopped when the conditioning temperature reached the overaging temperature, and the overaging treatment was started without intervening appropriate cooling or reheating. Furthermore, as a result of chemical analysis of the cold-rolled steel strip, almost all of the N was precipitated as aluminum nitride, and almost no solid solution N was detected. Granted by controlling.

From Table 1, when the hard iN is completely fixed, the aging index of the steel of the present invention is 3 k without deterioration of ductility.
gf/- or less, and it can be seen that it has non-aging properties comparable to box annealing. Sample numbers 7 and 13 have deteriorated ductility because the strain amount and thermal activation stroke index are outside the range of the present invention. Sample number 6. 8. 9.10
, No. 11.12 has an aging index exceeding 3 kgf/- because the strain application start temperature, strain application end temperature, strain application time, and strain amount are outside the range of the present invention.

Table 2 shows that C: 0.021-0.085%, AJ≦0.
01%.

Steel containing N: 0.0017 to 0.0043% and containing no Ti and Nb is melted in a converter, and a slab that has been cast and bloomed is hot rolled under the conditions listed in the same table and pickled. death,
A steel strip that has been cold rolled and continuously annealed under the conditions listed in the table,
After soaking in continuous annealing, the thermal activation stroke index is approximately 1.0X10
-' The aging index and elongation at break after 1% temper rolling are shown for those with and without strain in the secondary cooling zone after being subjected to over-aging treatment such that the temperature becomes -'. The strain application conditions are: strain application start temperature: 380°C, strain application end temperature: 25°C.
A strain of 1.5% was applied for about 5 seconds from the temperature at which 1 strain was applied at 0°C to the temperature at which the strain was applied.

From Table 2, even when solid solution N is not fixed, the ductility of annealed sheets can be degraded by applying the strain of the present invention in the secondary cooling zone of continuous annealing in steel sheets manufactured under various manufacturing conditions. It can be seen that it is possible to improve only the aging resistance without increasing the aging resistance.

〔Effect of the invention〕

According to the present invention, as is clear from the above examples, there is no need to use expensive continuous annealing equipment such as rapid cooling or reheating overaging treatment, and there is no deterioration of ductility, and simple continuous annealing equipment can be used. , it is possible to improve the aging resistance of cold rolled steel sheets.

As a result, conventionally high-grade non-aging cold rolled steel sheets are box annealed,
-A steel plates are produced by continuous annealing, and to produce high-grade steel plates by continuous annealing, expensive IF steel is used,
I made it knowing that expensive continuous annealing equipment would be essential, but the expensive IF! Also, without using 1iil,
It has become possible to manufacture by continuous annealing without using expensive continuous annealing equipment.

Furthermore, even for intermediate-grade cold-rolled steel sheets that do not necessarily require perfect anti-aging properties, when it becomes necessary to improve aging resistance in response to stricter quality requirements from customers, conventionally, box annealing is used to manufacture them. However, there are cases where a large amount of capital investment is required to extend the line of continuous annealing equipment, such as extending the overaging treatment zone of continuous annealing and adding quenching/reheating overaging treatment equipment. It became possible to deal with this problem by simply modifying the secondary cooling zone.

As a result, the advantages of continuous annealing, such as high productivity, uniform quality, energy saving, labor saving, and short delivery time, can be enjoyed, and together with the fact that expensive continuous annealing equipment and IF steel are not used, the present invention The economic effects are extremely large.

Patent applicant Nippon Steel Corporation

Claims (2)

[Claims] (1) When continuously annealing a cold rolled steel strip of low carbon steel containing neither Ti nor Nb, following heating, soaking, and primary cooling, the integrand function exp [-10100/T] (T represents the overaging treatment temperature at time t in Kelvin) at time t
The thermal activation stroke index defined as the value integrated from = 0 to t = t_1 (t_1 represents the time from the start of overaging treatment to the start of strain application in seconds) is 5.1× 10^-^6
After carrying out the over-aging treatment as described above, from the latter half of the over-aging zone to the secondary cooling zone, 300 to 400 °C is the strain imparting start temperature, and 150 to 350 °C is the strain imparting end temperature,
The applied strain amount is 0.06 to 2.30%, and 2 to 1
A continuous annealing method for an aging-resistant cold-rolled steel sheet, characterized by applying elastic strain or plastic strain to a steel strip for a period of 80 seconds. (2) When continuously annealing a cold rolled steel strip of low carbon aluminum killed steel containing neither Ti nor Nb, following heating, soaking, and primary cooling, the integrand function exp(-10100/T) T represents the overaging treatment temperature at time t in Kelvin) at time t
The thermal activation stroke index defined as the value integrated from = 0 to t = t_1 (t_1 represents the time from the start of overaging treatment to the start of strain application in seconds) is 5.1× 10^-^6
After carrying out the over-aging treatment as described above, from the latter half of the over-aging zone to the secondary cooling zone, 300 to 400 °C is the strain imparting start temperature, and 150 to 350 °C is the strain imparting end temperature,
The applied strain amount is 0.06 to 2.30%, and 2 to 1
A continuous annealing method for an aging-resistant cold-rolled steel sheet, characterized by applying elastic strain or plastic strain to a steel strip for a period of 80 seconds.