JPH0545652B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial application field (technical field to which the invention pertains) The present invention is directed to decarburization by vacuum degassing in steel manufacturing,
This relates to a method for producing non-aging, phosphorus-added, high-strength cold-rolled steel sheets by continuous annealing without using elements such as Ti and Nb. BACKGROUND OF THE INVENTION Due to its good workability, soft cold-rolled steel sheets are used as steel sheets, mainly for automobiles, which are made into final products after undergoing severe forming processes. In recent years, in order to reduce weight and improve dent resistance, this soft cold-rolled steel sheet is being replaced by high-strength steel sheet having a tensile strength of 35 to 45 Kgf/mm ² . Since the same level of workability as conventional products is required, phosphorus is used as an alloying element in this class because it can effectively strengthen steel without relatively deteriorating workability such as the Rankford value (F value). , a new field has emerged: phosphorous-added high-strength cold-rolled steel sheets. However, workability may deteriorate over time.
This deterioration over time is called aging property, and products used in applications that undergo particularly severe molding must not exhibit this aging property. This aging property is caused by the interstitial solid solution of C and N in the steel fixing mobile dislocations introduced in the final process of temper rolling, resulting in an increase in the yield point, a decrease in the elongation at break, and a decrease in the elongation at break. This results in point elongation and other deterioration. Of the C and N that cause this aging property, N is in trace amounts and can be fixed in the form of aluminum nitride by making aluminum killed steel, or fixed in the form of boron nitride by adding B. It can be avoided. On the other hand, the solubility limit of solid solution C in cementite at low temperatures is extremely small, so if it is cooled over time as in box annealing, almost no residue remains. However, in continuous annealing, solid solution C remains because the steel is cooled in a short time, resulting in large C aging. In order to reduce this solid solution C, 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 has been often reported that if the cooling rate after annealing is extremely high, fine cementite is formed within the crystal grains. For example, in the paper described in "Tetsu to Hagane" No. 6, 1976, pages 624-643,
In Photo.1(C), it is reported that fine carbides are observed in a steel plate that has been water-cooled from 700°C at a cooling rate of 2000°C/S and then over-aged. If the density of carbides is large, the diffusion distance required for their growth becomes small, and the reduction of solute carbon progresses rapidly, but on the other hand, the steel itself becomes hard and has low ductility due to precipitation hardening and dispersion hardening caused by these fine carbides. .
Therefore, it is necessary to control this intragranular carbide density within a certain appropriate range, but the above paper does not take this into account. In addition, rapid cooling at 2000℃/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. There remains the problem of thermal energy loss due to water cooling and surface oxidation due to water cooling. Regarding the control of precipitation of fine cementite within crystal grains, the invention described in JP-A No. 51-20715 and the invention described in JP-A No. 55-44584 have been developed by recognizing the nucleation stage and taking this into consideration. be.
As a cementite nucleation treatment, in the former case, after annealing, the temperature is rapidly cooled to 200 to 350℃ at a cooling rate of 20℃/S or more.
temperature range for more than 10 seconds, and in the latter case,
Cool to a temperature of 250 to 400°C to a temperature range of at least 600°C at a cooling rate of 350°C/S or more 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. The statute of limitations is determined by the statute of limitations index (AI) or
Yield point elongation after accelerated aging at 100℃ for 60 minutes (YP−
El), but in order to be considered non-ageable, it must be at least 3Kgf/mm ² or less in AI, and
0.4% or less for YP-El, preferably 2Kgf/mm ² for AI
Must be less than or equal to YP-El and 0. On the other hand, in the invention described in JP-A No. 51-20715, according to its embodiment, the AI is the smallest, 3.8 Kgf/mm ² in the case of Al-killed steel;
Similarly, in the invention described in No. 44584, even if YP-El decreases in the case of Al-killed steel, it is at most 0.5%. These findings support the insufficiency of cementite nucleation control described above. The present inventors have already applied for a mild steel plate on February 7, 1982 in ``Method for producing non-aging cold rolled steel sheet by continuous annealing'' (Koyama, Kato) (Japanese Patent Application Laid-open No. 165321-1981). We learned the conditions for cementite nucleation in
This condition does not apply to phosphorous-added steel. Purpose of the Invention The present invention solves the above-mentioned drawbacks and eliminates the need for special steelmaking equipment or processing, and without using expensive alloys such as Ti and Nb. The composition and cooling/overaging conditions after continuous annealing were limited to obtain an appropriate number of cementite grains, considering that P promotes cementite nucleation on impurities and inhibits this nucleation. By doing so, it is an object of the present invention to provide a method for producing, by continuous annealing, a highly ductile, non-aging, phosphorus-added, high-strength cold-rolled steel sheet that is substantially equivalent to that produced by box annealing. Structure of the Invention The present inventors have discovered that intragranular cementite, which is essential for rapid reduction of solid solute carbon during continuous annealing, occurs in practical cold-rolled steel sheets due to heterogeneous nucleation on impurities mainly consisting of MnS. In addition, they obtained the knowledge that P inhibits this nucleation, and learned the cooling overaging conditions that control the nucleation, leading to the present invention. Therefore, the gist of the present invention is as follows. (1) Contains C 0.01-0.05wt%, Mn 0.05-0.25wt%, P 0.025-0.10wt%, S 0.003-0.015wt%, Al 0.005-0.10wt%, N 0.0050wt% or less, and the balance is Fe and In hot rolling steel containing unavoidable impurities, coiling at a temperature of 650°C or higher, then cold rolling, and then continuous annealing, a temperature of 700°C to
After recrystallization annealing at a temperature of 850℃, it is rapidly cooled from a temperature of 650℃ or higher, and then held at a temperature T (℃) for 10 to 60 seconds to allow cementite nucleation.
150℃ or higher and −70×(logv/1000) ² +320 or less (where v is 1000 at the above rapid cooling rate (℃/s)
℃/s or less), and then
A method for producing a non-aging, high-strength cold-rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at 250-400°C for 2-10 minutes. (2) C 0.01-0.05wt%, Si 0.8wt% or less, Mn 0.05-0.25wt%, P 0.025-0.10wt%, S 0.003-0.015wt%, Al 0.005-0.10wt%, N 0.0050wt% or less When hot-rolling steel containing Fe and unavoidable impurities with the remainder being Fe and coiling at a temperature of 650°C or higher, then cold rolling, and then continuous annealing,
After recrystallization annealing at a temperature of 850℃, it is rapidly cooled from a temperature of 650℃ or higher, and then held at a temperature T (℃) for 10 to 60 seconds to allow cementite nucleation.
150℃ or higher and −70×(logv/1000) ² +320 or less (where v is 1000 at the above rapid cooling rate (℃/s)
℃/s or less), and then
A method for producing a non-aging, high-strength cold-rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at 250-400°C for 2-10 minutes. (3) C 0.01-0.05wt%, Mn 0.05-0.25wt%, P 0.025-0.10wt%, S 0.003-0.015wt%, Al 0.005-0.10wt%, N 0.0050wt% or less, B 0.0005-0.0040wt% When hot rolling steel containing Fe and unavoidable impurities with the balance being Fe and unavoidable impurities, it is rolled at a temperature of 650°C or higher, then cold rolled, and then continuously annealed.
After recrystallization annealing at a temperature of 850℃, it is rapidly cooled from a temperature of 650℃ or higher, and then held at a temperature T (℃) for 10 to 60 seconds to allow cementite nucleation.
150℃ or higher and −70×(logv/1000) ² +320 or less (where v is 1000 at the above rapid cooling rate (℃/s)
℃/s or less), and then
A method for producing a non-aging, high-strength cold-rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at 250-400°C for 2-10 minutes. (4) C 0.01-0.05wt%, Si 0.8wt% or less, Mn 0.05-0.25wt%, P 0.025-0.10wt%, S 0.003-0.015wt%, Al 0.005-0.10wt%, N 0.0050wt% or less, In hot rolling steel containing 0.0005 to 0.0040 wt% of B, with the remainder being Fe and unavoidable impurities, it is coiled at a temperature of 650°C or higher, then cold rolled, and then continuously annealed.
After recrystallization annealing at a temperature of 850°C, it is rapidly cooled from a temperature of 650°C or higher, and then held at a temperature T (°C) for 10 to 60 seconds to allow cementite nucleation.
150℃ or higher and −70×(logv/1000) ² +320 or less (where v is 1000 at the above rapid cooling rate (℃/s)
℃/s or less), and then
A method for producing a non-aging, high-strength cold-rolled steel sheet by continuous annealing, characterized by performing a precipitation treatment at 250-400°C for 2-10 minutes. Below, we will explain the constituent requirements and the reasons for limiting their numerical ranges. Figure 1 shows hot-rolled 725 types of steel shown in Table 1.
After being coiled at ℃, it was cold rolled, then annealed at 800℃ for 1 minute, and then cooled to 700℃ at 2℃/S.
The materials were then cooled to various temperatures at a cooling rate of 100° C./S, held at that temperature for 30 seconds, and finally water-quenched. The results of measuring the number of cementite grains are shown below.
The number of cementite grains was measured by taking a scanning electron micrograph at 3000x magnification after etching to expose the cementite, and counting only the cementite. As a result, the optimal number of cementite nuclei for the present invention is 4×10 ⁴ to 2
It was found that the number of particles was × ¹⁰⁶ / ^mm2 . Note that the percentages explained below are wt%.

[Table] As is clear from Figure 1, phosphorus-added steel types B and C
Compared to steel type A, steel must be kept at a lower temperature in order to obtain the same number of cementite grains. On the other hand, there is little difference between the phosphorus content of 0.041% and 0.089%, except for low temperature retention. It has thus been found that cementite nucleation is inhibited in phosphorous-added steel. The reason for this is not yet clear, but it is thought that phosphorus segregates at the interface between impurities such as MnS and the iron matrix, which are heterogeneous nucleation sites, and this inhibits cementite nucleation. From Figure 1, it was found that in order to obtain the same number of cementite grains with phosphorus-added steel as with low carbon aluminum killed steel, the retention temperature needs to be lowered by approximately 30°C than that of low carbon aluminum killed steel. .
From this, the upper limit of the holding temperature T [-70×(logv/1000) ² +320] (° C.) was obtained. Note that it should preferably be less than [-70×(logv/1000) ² +270] (°C). The retention time was 10 seconds to 60 seconds. If the holding time is less than 10 seconds, it is difficult to perform it industrially stably, and the effect is saturated after 60 seconds, and if it exceeds this, the line equipment becomes large and the industrial significance is lost. The above are the constant cooling conditions for nucleation and the reasons for their limitations. As a precipitation treatment following nucleation, 2 to 400℃ at 250 to 400℃
Requires 10 minutes processing. At temperatures below 250°C, even if the number of cementite nuclei is increased to shorten the diffusion distance, it takes a long time for carbon atoms to diffuse due to the temperature dependence of the diffusion coefficient. Moreover, when the temperature exceeds 400°C, the equilibrium solid solubility limit of carbon itself increases, 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, the first half of the precipitation treatment is at a high temperature of 300 to 400℃, and the second half is at a high temperature of 250℃.
It can be said that the preferred form is to carry out at a low temperature of ~320°C. A precipitation treatment time of 2 minutes is required for cementite growth. Also, it saturates in about 10 minutes,
Furthermore, considering that continuous annealing becomes less meaningful if the time is longer than this, 10 minutes was set as the upper limit. 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%. The present invention uses intragranular cementite to achieve non-aging, but since this intragranular carbide tends to deteriorate El,
In order to compensate for the overall ductility, the upper limit of C is set low at 0.05%. In this sense, it is a preferable condition to set the upper limit of C to 0.03%. The lower limit of C is set to 0.01% in order to increase the degree of supersaturation before the start of rapid cooling. To obtain more stable intragranular cementite, C should be 0.015%.
It is preferable to set it as above. Mn and S are extremely important because MnS becomes the main site for heterogeneous nucleation of 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 the MnS dispersion state is moderate. This is because it is not possible to obtain P is an important element responsible for strength in the present invention. If it is less than 0.025%, sufficient strength cannot be obtained, and if it is less than 0.1%
Exceeding this will make the steel brittle and impair weldability.
Therefore, the addition range of P was set to 0.025 to 0.1%. Furthermore, since Si strengthens steel without significantly affecting cementite nucleation or workability such as value, Si is added in an amount of 0.8% or less as necessary. If it exceeds 0.8%, an oxide film is likely to be formed on the surface even during continuous annealing, and the chemical conversion treatment properties for painting deteriorate. 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 is added.
It is necessary to fix N as AlN at 0.0050% or less. The lower N is, the more desirable it is, 0.0020
% or less is most preferable. Further, in order to firmly fix N as a stable nitride, B is added in an amount of 0.005 to 0.0040%. 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 this reason the temperature needs to be 650°C or higher. Other hot rolling conditions may be those normally used, but the heating temperature is preferably 1000 to 1150°C in order to prevent coarsening of the hot rolled structure. Cold rolling can be carried out at a rolling reduction of 60 to 90% as is usually done, but it is possible to achieve a stable high rank Ford value (
A high pressure of 75% or more is desirable in order to obtain the desired 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 value exceeds 1, the amount of austenite increases, the texture becomes random, the value decreases, and the crystal grains become coarser. In addition, to ensure sufficient dissolution of carbides,
It is preferable to perform slow cooling from the annealing temperature to around 700°C, where the solubility is highest, at a rate of 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 becomes rapid, it becomes difficult to control the cooling to stop at a holding temperature, so supercooling to below the holding temperature may occur. 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. ε below 150℃
This method is not applicable because carbides are mainly produced and the amount of solid solute C in equilibrium with ε carbides is large. In order to aim for a stable cementite region, it is preferable to set the temperature to 200°C or higher. Note that the steel manufacturing method to which the method of the present invention is applied does not matter whether it is a continuous casting method or an 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 Steel having the components shown in Table 2 was melted in a converter and made into a slab by continuous casting. these slabs
It was heated to 1050-1100°C and hot-rolled. The hot rolling conditions are finishing temperature 870-885℃, coiling temperature 600, 700℃,
The temperature was 750℃. This coil was cold rolled by 80% to a thickness of 0.8 mm and then continuously annealed. Continuous annealing conditions and mechanical properties after 1.2% skin pass rolling are shown in Table 3.

[Table] Component codes I to C and E indicate tensile strength 35Kgf/mm ²
Class, code 2 is the component aiming for 40Kgf/mm ^2nd class. As is clear from Table 3, the products according to the present invention have yield point strength and elongation that match the respective tensile strengths, and also have aging properties of 0.2% in YP-El.
Within 3Kgf/mm ² using AI, which is good. On the other hand, the steels under the comparative conditions did not satisfy the aging properties and, like steel No. 3, had a poor balance between El and tensile strength.

[Table] Effects of the Invention According to the present invention, as is clear from the above examples, a non-aging phosphorus-added high-strength cold-rolled steel sheet can be produced economically without putting a burden on steel manufacturing. As a result, high-grade non-aging steel sheets were previously produced by box annealing, and lower-grade steels were produced by continuous annealing, but it is now possible to produce high-grade non-aging steel plates even by continuous annealing. As a result, the advantages of continuous annealing, such as high productivity, uniform quality, energy saving, labor saving, short delivery time, and ease of manufacturing high-strength steel sheets, can be enjoyed, and the economic effects are extremely large.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the retention temperature and the number of cementite grains obtained at that time.

Claims (1)

[Claims] 1 Contains 0.01 to 0.05 wt% of C, 0.05 to 0.25 wt% of Mn, 0.025 to 0.10 wt% of P, 0.003 to 0.015 wt% of S, 0.005 to 0.10 wt% of Al, and 0.0050 wt% or less of N. , the remaining Fe and unavoidable impurities are hot rolled, coiled at a temperature of 650°C or higher, then cold rolled, and then continuously annealed.
After recrystallization annealing at a temperature of 850℃, it is rapidly cooled from a temperature of 650℃ or higher, and then held at a temperature T (℃) for 10 to 60 seconds to allow cementite nucleation.
150℃ or higher and −70×(logv/1000) ² +320 or less (where v is 1000 at the above rapid cooling rate (℃/s)
℃/s or less), and then
A method for producing a non-aging, high-strength cold-rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at 250-400°C for 2-10 minutes. 2 Contains C 0.01-0.05wt%, Si 0.8wt% or less, Mn 0.05-0.25wt%, P 0.025-0.10wt%, S 0.003-0.015wt%, Al 0.005-0.10wt%, N 0.0050wt% or less. , the remaining Fe and unavoidable impurities are hot rolled, coiled at a temperature of 650°C or higher, then cold rolled, and then continuously annealed.
After recrystallization annealing at a temperature of 850℃, it is rapidly cooled from a temperature of 650℃ or higher, and then held at a temperature T (℃) for 10 to 60 seconds to allow cementite nucleation.
150℃ or higher and −70×(logv/1000) ² +320 or less (where v is 1000 at the above rapid cooling rate (℃/s)
℃/s or less), and then
A method for producing a non-aging, high-strength cold-rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at 250-400°C for 2-10 minutes. 3 Contains C 0.01-0.05wt%, Mn 0.05-0.25wt%, P 0.025-0.10wt%, S 0.003-0.015wt%, Al 0.005-0.10wt%, N 0.0050wt% or less, B 0.0005-0.0040wt% However, when steel consisting of the remainder Fe and unavoidable impurities is hot rolled, coiled at a temperature of 650°C or higher, then cold rolled, and then continuously annealed,
After recrystallization annealing at a temperature of 850℃, it is rapidly cooled from a temperature of 650℃ or higher, and then held at a temperature T (℃) for 10 to 60 seconds to allow cementite nucleation.
150℃ or higher and −70×(logv/1000) ² +320 or less (where v is 1000 at the above rapid cooling rate (℃/s)
℃/s or less), and then
A method for producing a non-aging, high-strength cold-rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at 250-400°C for 2-10 minutes. 4 C 0.01-0.05wt%, Si 0.8wt% or less, Mn 0.05-0.25wt%, P 0.025-0.10wt%, S 0.003-0.015wt%, Al 0.005-0.10wt%, N 0.0050wt% or less, B 0.0005 ~0.0040wt%, with the remainder being Fe and unavoidable impurities, is hot rolled, coiled at a temperature of 650°C or higher, then cold rolled, and then continuously annealed.
After recrystallization annealing at a temperature of 850℃, it is rapidly cooled from a temperature of 650℃ or higher, and then held at a temperature T (℃) for 10 to 60 seconds to allow cementite nucleation.
150℃ or higher and −70×(logv/1000) ² +320 or less (where v is 1000 at the above rapid cooling rate (℃/s)
℃/s or less), and then
A method for producing a non-aging, high-strength cold-rolled steel sheet by continuous annealing, characterized by performing precipitation treatment at 250-400°C for 2-10 minutes.