JPH0545654B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical field of the invention) The present invention relates to decarburization by vacuum degassing in steel manufacturing,
This relates to a method for manufacturing non-aging cold rolled steel sheets by continuous annealing without using elements such as Ti and Nb. (Prior Art) Due to its good workability, soft cold-rolled steel sheets are used as final products after undergoing severe forming processes, mainly for automobiles. However, this workability may deteriorate over time, and this deterioration over time is called aging. Among soft cold-rolled steel sheets, those used for applications that undergo particularly severe forming should not have this aging property. This aging property is due to the interstitial solid solution of C and N in the steel.
This is because the movable dislocations introduced in the final step of temper rolling are fixed, resulting in deterioration such as an increase in the drop point, a decrease in elongation at break, and the occurrence of elongation at the drop point. Among C and N, which are the causes of this aging property, N can be fixed in the form of aluminum nitride by making aluminum killed steel because it is in a small amount, or can be fixed in the form of boron nitride by adding B. The statute of limitations due to 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 for a long time as in box annealing, almost no residue remains. However, in continuous annealing, solid solution C remains due to cooling in a short time, resulting in large C aging. This solid solution C
In order to reduce this, generally, after continuous annealing, the steel is rapidly cooled to increase the degree of supercooling, and then a cementite precipitation treatment called overaging is performed. This cementite precipitation process consists of a nucleation stage and a growth stage, and since practical steel contains many impurities, it is thought that nucleation occurs through heterogeneous nucleation using impurities as sites. It will be done. It has been often reported that if the cooling rate after annealing is set extremely high, fine cementite is formed within the crystal grains. For example, Tetsu to Hagane 62nd Year (1976) No. 6 624-643
In photo.1.(C) in the paper written on the page,
It has been reported that fine carbides are observed in steel sheets that have been water-cooled from 700°C at 2000°C/s and then over-aged. If the density of carbides is large, the diffusion distance required for their growth will be shortened, and the amount of solid solute carbon will be reduced quickly, but on the other hand, due to precipitation hardening and dispersion hardening caused by these fine carbides, the steel itself will become hard and have low ductility. Become. Therefore, it is necessary to control this intragranular carbide density within a certain appropriate range, but the above report does not take this into account. Also, 2000
Rapid cooling at °C/S has the disadvantage that the shape of the steel sheet collapses due to quenching distortion.Furthermore, such rapid cooling requires water cooling, which means that after cooling to water temperature, the temperature must be raised to the overaging temperature. Problems of thermal energy loss and surface oxidation due to water cooling remain. Regarding the control of precipitation of fine cementite within crystal grains, proposals have been made in JP-A-51-20715 and JP-A-55-44584 that recognize and take into consideration the nucleation stage. As a cementite nucleation treatment, the former is rapidly cooled at a cooling rate of 20°C/S or more after annealing, and then kept in a temperature range of 200 to 350°C for 10
Hold for more than seconds. The latter also includes a temperature range of 250 to 400 degrees Celsius, at least a temperature range of 600 degrees Celsius or less, and a temperature range of 35 degrees Celsius/35 degrees Celsius
Cool at a cooling rate of S or higher and hold at that temperature for 10 seconds or less. However, these conditions are insufficient to control nucleation, and it is particularly difficult to obtain the non-aging steel sheet that the present invention aims at. Aging properties are often indicated by the aging index (AI) or the drop point elongation (YP-El) after accelerated aging at 100℃ for 60 minutes.
To be considered non-prescription, at least 3 AI
Kgf/mm ² or less and YP-El should be 0.4% or less, preferably AI should be 2 Kgf/mm ² or less and YP-El should be 0. On the other hand, in JP-A No. 51-20715, according to its examples, the AI is the smallest, 3.8 Kgf/mm ² in the case of Al-killed steel;
In the case of Al-killed steel as well, in Publication No. 44584, even if YP-El decreases, it is at most 0.5%. These confirm the insufficiency of the cementite nucleation cotonol mentioned above. Under these circumstances, the only continuous annealed steel sheet that can be considered truly soft, non-aging cold-rolled steel sheet is so-called IF steel (Interstitial Free steel). IF steel is made by reducing C to around 50ppm or less through vacuum degassing during steelmaking, and
C, N
This steel is completely unaged;
The production of IF steel requires special steelmaking equipment and operations as described above, and uses expensive alloys, which has the economic disadvantage of high production costs. (Objective of the Invention) The present invention solves the above-mentioned drawbacks and produces manganese sulfide (MnS) during continuous annealing without the need for special steelmaking equipment or treatment, and without using expensive alloys such as Ti and Nb. Manufactured by virtual box annealing by limiting the components and cooling/overaging conditions after continuous annealing to promote cementite nucleation on impurities mainly and obtain an appropriate number of cementite grains. The purpose of the present invention is to provide a method for producing cold-rolled steel sheets with the same softness and non-aging properties as those obtained by continuous annealing. (Structure of the invention) The gist of the invention is C0.01~0.05%, Mu0.05~0.25
% S0.003~0.015%, Al0.005~0.10%, N0.0050% or less, B0.0005~0.0040 as required
%, the balance consisting of Fe and unavoidable impurities, is hot rolled, coiled at a temperature of 650℃ or higher, and then cold rolled,
Next, in performing continuous annealing, after recrystallization annealing at a temperature of 700 to 850°C, rapid cooling from a temperature of 650°C or higher,
Next, the temperature T (°C) is held for 10 to 60 seconds to allow cementite nucleation.
℃ or above and between -70× (logV/1000) ² +350 and −70× (logV/1000) ² +180 (however, v is 1000℃/S or less at the above quenching rate (℃/S)) By setting the temperature to 250 to 400℃, cementite nuclei of 4×10 ⁴ to 2×10 ⁶ pieces/mm ² are generated.
The purpose is to perform a precipitation treatment for 2 to 10 minutes. In other words, we obtained the knowledge that intragrain cementite, which is essential for rapid reduction of solute carbon during continuous annealing, is generated in practical cold-rolled steel sheets by heterogeneous nucleation on impurities, mainly MnS. This means that we have come to know the cooling overaging conditions that control nucleation. Hereinafter, the constituent elements of the present invention will be explained and the reason for limiting the numerical range thereof will be described. Figure 1 shows 0.019%C-0.15%Mn-0.007%S-
Hot rolled 0.006%P-0.037%Al-0.0013%N steel,
After being rolled at 725℃, it was cold rolled, then annealed at 800℃ for 1 minute, cooled to 700℃ at 2℃/S, and then cooled to various temperatures at a cooling rate of 40100400℃/S. This is the result of measuring the number of cementite grains in the material that was held at that temperature for 30 seconds and finally water-quenched. Intragranular cementite was measured by taking a scanning electron micrograph at 3000x magnification after etching to expose cementite, and counting only cementite. In addition, Figure 2 shows a sample that was heated to 350℃ after holding for 30 seconds as described above, then cooled to 250℃ at 20℃/min, then cooled to room temperature by water cooling, and subjected to 1.0% temper rolling. FIG. 2 is a diagram plotting the AI and elongation (El) of the cementite particles against the number of cementite grains in FIG. 1. The plate thickness is 0.8mm. As is clear from Figure 2, as the number of cementite grains increases, AI decreases, but El also deteriorates.
From Figure 2, AI is within 3Kgf/ ^mm2 and El
The number of cementite grains is set as a good range, 4×
10 ⁴ to 2×10 ⁶ pieces/mm ² . By applying this value to Figure 1, the upper limit of the holding temperature T for nucleation is {-70×(logV/1000) ² ×350} (℃), and the lower limit is {-70×(logV/ 1000) ² +180}(℃
) was obtained. If you want more stable and non-aging properties, increase the number of cementite grains to 20×10 ⁴
It is preferable to set the number of particles/mm ² or more, and in that case, the upper limit of T is {-70×(logV/1000) ² ×300} (°C). Furthermore, when the cooling rate is 100℃/S and the holding temperature is 250℃, the holding time is 5 seconds, 10 seconds, 30 seconds, 60 seconds,
When the number of cementite grains was measured at different times of 300 seconds, they were 2.7×10 ⁴ , 6.9×10 ⁴ , 10.2×10 4 , and 10.2×10 ⁴ , respectively.
The results were 12.3×10 ⁴ and 12.1×10 ⁴ pieces/mm ² , and it was observed that there was a tendency for nucleation to be insufficient in less than 10 seconds and saturation in 60 seconds, so the retention time was set to 10 to 60 seconds. Of course, this tendency changes depending on other conditions, but holding for less than 10 seconds is difficult to do stably industrially.
Taking into consideration that the line equipment would become larger if it exceeded 60 seconds, the retention time was set to 10 to 60 seconds. The above are the constant cooling conditions for nucleation and the reasons for their limitations. As a precipitation treatment following nucleation, 250-400℃
Requires ~10 min processing. Below 250℃, even if the number of cementite nuclei is increased and the diffusion distance is shortened,
Due to the temperature dependence of the diffusion coefficient, it takes a long time for carbon atoms to diffuse. Furthermore, when the temperature exceeds 400°C, the equilibrium solid solubility limit of carbon itself becomes large, and even if the precipitation rate is increased, the residual solid solute carbon does not decrease. Considering the temperature dependence of the diffusion coefficient and the equilibrium solid solubility limit of carbon, it is said that it is preferable to conduct the first half of the precipitation treatment at a high temperature of 300 to 400°C and the second half at a low temperature of 250 to 320°C. I can do it. A precipitation treatment time of 2 minutes is required for cementite growth. Also, 10
It saturates in about a minute, and if the time is longer than this,
Considering that continuous annealing would have less significance, the upper limit was set at 10 minutes. The chemical composition of steel requires the following limitations. C needs to be kept relatively low for a low carbon steel, at 0.01 to 0.05%. As is clear from Fig. 2, the present invention uses intragranular cementite to achieve non-aging, but since this intragranular carbide tends to deteriorate El, it reduces the overall ductility. To compensate, the upper limit of C is lowered. In this sense, it is a preferable condition that the upper limit of C is 0.03% and the upper limit of P is less than 0.01%. The lower limit of C is determined in order to increase the degree of supersaturation of C before the start of rapid cooling. In order to obtain more stable intragranular cementite, the C content is preferably 0.015% or more. Mn and S are extremely important, as MnS becomes the main heterogeneous nucleation site for cementite, as described above. The respective lower limits of 0.05% and 0.003% are necessary to secure the amount of MnS, and the upper limits of 0.25% and 0.015%, respectively, are due to the limited solubility of MnS, and above these limits, a moderate amount of MnS is required. This is because a dispersed state cannot be obtained. The present invention is characterized by minimizing carbon aging, so it is necessary to take measures to deal with nitrogen, which also causes large aging deterioration. For this purpose, 0.005% or more of Al is added and N
It is necessary to set N to 0.0050% or less and fix N as AlN. The lower N is, the more desirable it is.
Most preferably it is 0.0020% or less. Also,
In order to firmly fix N as a stable nitride, 0.0005 to 0.0040% of B is added. Coiling conditions are important in hot rolling conditions. This is done in the present invention along with the usual AlN precipitation process.
It is presumed that MnS dispersion treatment is also involved, and for that reason it is necessary to use a high temperature of 650°C or higher. Other hot rolling conditions may be those normally used, but the heating temperature is preferably a low temperature of 1000 to 1150°C in order to prevent coarsening of the hot rolled structure. As is commonly done, cold rolling is carried out at 60
A reduction rate of ~90% is sufficient, but a high pressure reduction of 75% or more is desirable in order to stably obtain a high line feedback value. Next, in continuous annealing, recrystallization annealing is performed at 700 to 850°C. Below 700°C, recrystallization is insufficient and dissolution of carbides is also insufficient, and the degree of carbon supersaturation does not increase no matter how rapidly it is cooled thereafter. Also, 850
When the temperature exceeds ℃, the amount of austenite increases, the texture becomes random, the value decreases, and the crystal grains become coarser. In addition, in order to sufficiently dissolve the carbide, the annealing temperature should be set at 700℃, which has the highest solubility.
It is preferable to slowly cool the temperature to about 5° C./S or less. The annealing time may be 20 seconds to 3 minutes, as is commonly done. After that, cooling is performed under the conditions described above, but the upper limit of the cooling rate is 1000° C./S. This is because if it exceeds this, it becomes difficult to maintain the shape of the steel plate. It is desirable that the cooling is carried out to a holding temperature, but if the cooling is rapid, it becomes difficult to control the cooling to stop at a holding temperature, so there may be supercooling to below the holding temperature.
In that case, the degree of supercooling is preferably as small as possible from the viewpoint of energy saving, and the rate of temperature increase to the holding temperature needs to be as high as 10° C./S or more. The holding temperature shall be 150℃ or higher. Below 150℃,
This method is not applicable because ε carbide is mainly produced and the amount of solid solute C in equilibrium with ε carbide is large. In order to achieve a stable cementite region, the temperature is preferably 200°C or higher. In that case, the lower limit condition −70
×(logV/1000) ² +180 is excluded. The steel manufacturing method may be continuous casting method or ingot method. Further, the rapid cooling means in continuous annealing may be any means such as gas jet cooling, air/water cooling, metal contact cooling, hot water cooling, water cooling, salt bath immersion, etc. Example 1 0.018%C-0.12%Mn-0.006%S-0.005%P
Steel containing -0.043% Al - 0.0015% N was melted in a converter and made into a slab by continuous casting. This slab was heated to 1080°C and then hot rolled. The hot rolling conditions include a finishing temperature of 880°C and a winding temperature of 700°C (some parts are 600°C).
℃). This coil was cold rolled by 80% to a thickness of 0.8 mm, and then continuously annealed. Continuous annealing conditions and mechanical test values after 1% temper rolling are shown in Table 1. Nos. 1 and 4 in Table 1 follow the present invention, but No. 2 has a short precipitation treatment time, Nos. 3 and 5 have nucleation treatment conditions different from each other, and No. 6 has started rapid cooling. The winding temperature of No. 7 is different from that of the present invention. According to the invention, the yield point strength is 18
Kgf/mm ² or less and El 47% or more, which is sufficiently soft and ductile for a 0.8mm plate thickness, and YP-El is 0.2%.
It also has extremely good aging resistance, with an AI of 3Kgf/mm ² or less. On the other hand, the comparative example is far inferior in aging properties.

[Table] Example 2 Steel having the components shown in Table 2 was melted and hot-rolled at a heating temperature of 1050°C, a finishing rolling temperature of 880-895°C, and a coiling temperature of 700-730°C. 0.8mm at 80%
It was made into a thick cold-rolled plate. Subsequently, after performing recrystallization annealing at 800℃ for 1 minute, it was cooled to 700℃ at 3℃/S.
It was cooled uniformly from this temperature to 250°C at 400°C/S and maintained for 30 seconds. Subsequently, the temperature was raised to 350°C in 5 seconds, then cooled to 250°C at a rate of 20°C/min, and then water-cooled, followed by 1.0% temper rolling, and mechanical test values were determined. The results are shown in Table 2. Steel grades A and B according to the present invention exhibit sufficient soft non-aging properties, but steel grades D and E, which have different Mn and S conditions, have insufficient cementite nucleation sites and exhibit large aging properties. In addition, in steel type C containing less carbon, the degree of supersaturation of carbon is insufficient during rapid cooling during continuous annealing, and as a result, large aging deterioration occurs. Also,
Steel type F, which has a high carbon content, has good aging properties, but cannot be said to be soft or highly ductile.

【table】

[Table] (Effects of the Invention) According to the present invention, as is clear from the above examples, a soft non-aging cold rolled steel sheet can be manufactured economically without imposing any burden on steel manufacturing. As a result, high-grade non-aging steel sheets were conventionally box-annealed, and low-grade steels were produced by continuous annealing, and in order to produce high-grade steel by continuous annealing, it was necessary to prepare for the use of expensive IF steel. things are expensive
It is now possible to manufacture by continuous annealing without using IF steel. As a result, the positive points of continuous annealing, namely high productivity, uniform quality, energy saving, labor saving,
It has advantages such as short delivery times and easy production of high-strength steel plates, and combined with the fact that IF steel is not used, the economic effects are extremely large.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the transition of the number of cementite grains when changing the quenching and holding conditions during continuous annealing, and FIG. 2 is a chart showing the relationship between the number of cementite grains, aging index, and elongation.

Claims (1)

[Claims] 1 C0.01~0.05%, Mn0.05~0.25%, S0.003~
A steel consisting of 0.015% Al, 0.005~0.10% N, 0.0050% or less of N, and the balance Fe and unavoidable impurities is hot rolled, coiled at a temperature of 650℃ or higher, and then cold rolled.
Next, in performing continuous annealing, after recrystallization annealing at a temperature of 700 to 850°C, rapid cooling from a temperature of 650°C or higher,
Next, the temperature T (°C) is held for 10 to 60 seconds to allow cementite nucleation.
℃ or higher and -70 x (logV/1000) ² +350 and -70
×(logV/1000) ² +180 (however, V is 1000°C/s or less at the above-mentioned quenching rate (°C/s)), 4×10 ⁴ to 2×10 ⁶ pieces/ ^mm2
The cementite core of 250
A method for producing a non-aging cold rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at ~400°C for 2 to 10 minutes. 2 C0.01~0.05%, Mn0.05~0.25%, S0.003~
0.015%, Al0.005~0.10%, N0.0050% or less,
Steel consisting of B0.0005~0.0040%, balance Fe and unavoidable impurities is hot rolled, coiled at a temperature of 650℃ or higher, then cold rolled,
Next, in performing continuous annealing, after recrystallization annealing at a temperature of 700 to 850°C, rapid cooling from a temperature of 650°C or higher,
Next, the temperature T (°C) is held for 10 to 60 seconds to allow cementite nucleation.
℃ or higher and -70 x (logV/1000) ² +350 and -70
By setting the temperature between (logV/1000) ² +180 (however, V is 1000℃/s or less at the above quenching rate (℃/s)), 4×10 ⁴ to 2×10 ⁶ pieces/mm It produces a cementite nucleus of ² , and then 250 ~
A method for producing a non-aging cold rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at 400°C for 2 to 10 minutes.