KR100479993B1 - A method for producing a high carbon steel strip with high elongation and hardenability - Google Patents

Abstract

The present invention relates to a method for manufacturing a high carbon steel sheet having excellent ductility and heat treatment characteristics. The present invention relates to a method for producing a high carbon steel sheet, by adjusting graphitization promoting element, graphitization inhibiting element, and B, N, Ti, and appropriately controlling hot rolling conditions and annealing heat treatment conditions. To provide a method of manufacturing high carbon steel sheet having excellent ductility and hardening heat treatment, an object thereof is provided.

The present invention contains, by weight, C: 0.3-0.8%, Si: 0.1-1.5%, Mn: 0.1-0.5%, Al: 0.01-0.1%, N: 0.00065-0.0026%, and B is (0.0005+ 0.77N) ≤B (%) ≤ (0.003+ 0.77N) is added and hot rolled steel with other remaining Fe and unavoidable impurities at finishing rolling temperature of 900 ° C or below and winding temperature of 650 ° C or below. A method of manufacturing a high carbon steel sheet having excellent ductility and heat treatment, characterized in that the steel sheet is manufactured by annealing the produced hot rolled steel sheet at a temperature range of 650 to 710 ° C., and

By weight%, C: 0.3-0.8%, Si: 0.1-1.5%, Mn: 0.1-0.5%, Al: 0.01-0.1%, N: 0.0026-0.015%, Ti is (N-0.0026) / 0.29≤ Ti (%) ≤ (N-0.00065) /0.29, B is added in the range satisfying 0.0005 + 0.77 (N-0.29Ti) ≤B (%) ≤0.003 + 0.77 (N-0.29 Ti), etc. The steel material consisting of the balance Fe and unavoidable impurities is manufactured into a hot rolled steel sheet with a finish rolling temperature of 900 ° C. or lower and a coiling temperature of 650 ° C. or lower, and the prepared hot rolled steel sheet is annealed at a temperature range of 650 to 710 ° C. to ferrite and graphite. The technical gist of the manufacturing method of the high carbon steel strip excellent in ductility and heat processing property characterized by the above-mentioned.

Description

Manufacturing method of high carbon steel sheet with excellent ductility and heat treatment property {A METHOD FOR PRODUCING A HIGH CARBON STEEL STRIP WITH HIGH ELONGATION AND HARDENABILITY}

The present invention relates to a method for manufacturing a high carbon steel sheet applied to parts and the like of people, more specifically, by adjusting the steel components, and appropriately controlled hot rolling conditions and annealing heat treatment conditions, high carbon steel excellent in ductility and hardening heat treatment properties It relates to a method of manufacturing a stand.

The high carbon steel strip is subjected to a series of manufacturing processes such as hot rolling, annealing, forming, and heat treatment (quenching and tempering), which leads to the final product. Because of its high yield strength and low elongation before molding, poor moldability, use has been limited to products requiring high forming properties.

In order to lower the yield strength and improve the elongation of the high carbon steel, many studies have been conducted for a long time. For example, Japanese Patent Laid-Open No. 63-317629 or the like is composed of cementite and ferrite in which high carbon steel is spheroidized. Therefore, attention is paid to high strength and low elongation. By changing to graphite to produce a high carbon steel sheet composed of graphite and ferrite, it was intended to improve the formability by lowering the yield strength and increase the elongation. However, according to this method, cold rolling is required to convert cementite into graphite, and large amounts of Ni and Si, which are graphitization promoting elements, must be added, thereby increasing manufacturing cost and coarsening of the resulting graphite particles. Since the dissolution of graphite does not proceed rapidly during the quenching heat treatment, there is a problem that the quenching heat treatment is deteriorated.

In order to improve this, Japanese Patent Laid-Open No. 7-97659 et al., Added graphite promoting elements B (boron) and Al, and hot-rolled at a temperature higher than the solid solution temperature of BN and AlN, and then 300 to 600 ℃ temperature The AlN is precipitated by heating with annealing and then annealed at 680 to 740 ° C. to bring the size of graphite to 20 μm or less. However, according to this method, the hot rolling temperature is high, so that a large amount of scale defects may occur on the surface of the hot rolled steel sheet, and there is a problem that the post process is complicated and the price is increased.

In this regard, a material that is superior in ductility and quenching heat treatment to existing high carbon steels but does not significantly increase manufacturing cost is desired.

Accordingly, the present inventors have conducted research and experiments to solve the above-mentioned problems of the prior arts, and based on the results, the present invention provides a graphitization promoting element, a graphitization suppressing element, and The purpose of the present invention is to provide a method for producing a high carbon steel sheet having excellent ductility and quenching heat treatment by adjusting B, N, Ti and appropriately controlling hot rolling conditions and annealing heat treatment conditions.

The present invention contains, by weight, C: 0.3-0.8%, Si: 0.1-1.5%, Mn: 0.1-0.5%, Al: 0.01-0.1%, N: 0.00065-0.0026%, and B is (0.0005+ 0.77N) ≤B (%) ≤ (0.003+ 0.77N) is added and hot rolled steel with other remaining Fe and unavoidable impurities at finishing rolling temperature of 900 ° C or below and winding temperature of 650 ° C or below. A method of manufacturing a high carbon steel sheet having excellent ductility and heat treatment, characterized in that the steel sheet is manufactured by annealing the manufactured hot rolled steel sheet at a temperature range of 650 to 710 ° C., and

By weight%, C: 0.3-0.8%, Si: 0.1-1.5%, Mn: 0.1-0.5%, Al: 0.01-0.1%, N: 0.0026-0.015%, Ti is (N-0.0026) / 0.29≤ Ti (%) ≤ (N-0.00065) /0.29, B is added in the range satisfying 0.0005 + 0.77 (N-0.29Ti) ≤B (%) ≤0.003 + 0.77 (N-0.29 Ti), etc. The steel material consisting of the balance Fe and unavoidable impurities is manufactured into a hot rolled steel sheet with a finish rolling temperature of 900 ° C. or lower and a coiling temperature of 650 ° C. or lower, and the prepared hot rolled steel sheet is annealed at a temperature range of 650 to 710 ° C. to ferrite and graphite. It relates to a method for producing a high carbon steel sheet excellent in ductility and heat treatment.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The inventors of the present invention have comprehensively studied the action of the amount of C, Si, Al, Mn, B, Ti, N in the steel, which affects the graphitization of high carbon steel, and the manufacturing conditions such as hot rolling and annealing. It was found that by controlling the manufacturing conditions appropriately, steel having excellent ductility and hardening heat treatment can be produced. In particular, in the present invention, in determining the amount of B, N, Ti addition, based on the following relations.

First, when N content in steel is 0.00065% ≤N≤0.0026%, Ti is not added and B content is a function of N as follows.

[Relationship 1]

B _tot = B _sol + B _BN

(B _tot : total amount of B to be added, B _sol : amount of _{solid solution} B, B _BN : amount of B precipitated into BN)

Here, in order to improve the heat treatment heat treatment by boron addition, the amount of _{solid solution} B ( _sol ) should be at least 0.0005%, but if the amount is more than 0.003%, the effect is saturated and rather brittle occurs. . In other words,

[Relationship 2]

0.0005% ≤B _sol ≤0.003%

And, in order to increase the ductility by the addition of graphite by boron addition, the amount of B (B _BN ) precipitated into _BN should be at least 0.0005% or more. The amount of B used is made into 0.002% or less. In other words,

[Relationship 3]

0.0005% ≤B _BN ≤0.002%

However, since the amount of B precipitated by BN varies depending on the amount of N, the amount of N (N _BN ) for satisfying the relational expression 3 is obtained from the relational expression 4 below.

[Relationship 4]

B _BN = 0.77 N _BN

The amount of N _BN participating in BN formation can be substituted by using Equation 4.

[Relationship 5]

0.00065% ≤N _BN ≤0.0026%

Therefore, the total amount of B addition B _tot is obtained by adding relation 2 and relation 4. In other words,

[Relationship 6]

0.0005% + 0.77N _BN ≤B _tot ≤0.003% + 0.77N _BN

In other words, the total amount of B added in the range of the relationship (5) becomes the relationship (6).

Second, when the amount of N in the steel is 0.0026% <N <0.015%, Ti is added to precipitate N in the steel as TiN, so that the amount of N precipitated as BN in the steel satisfies Equation 5. The amount of Ti added for this purpose is as follows.

First, the relationship between Ti and N for forming TiN precipitates is as follows.

[Relationship 7]

N _TiN = 0.29 Ti

Since N amount (N _BN ) participating in BN formation should satisfy relation 5, some N precipitates TiN by adding Ti among the total N amount (N _tot ), and the remaining N amount to precipitate BN is as follows. Is displayed as:

[Relationship 8]

N _BN = N _tot -0.29 Ti

Substituting Equation 8 into Equation 5 and arranging, the amount of Ti to be added can be determined.

[Relationship 9]

(N _tot -0.0026) /0.29≤Ti≤ (N _tot -0.00065) /0.29

Substituting Equation 8 into Equation 6 yields the total B _tot .

[Relationship 10]

0.0005% + 0.77 (N _tot -0.29Ti) ≤B _tot ≤0.003% + 0.77 (N _tot -0.29Ti)

That is, when the amount of N in steel is 0.0026% or more, the amount of Ti added to reduce N should satisfy the relation 9, and the amount of B added to satisfy the ductility and heat treatment heat treatment should satisfy the relation 10.

Hereinafter, the reason for limitation of the steel component of this invention is demonstrated.

The C is the most important element to increase the hardness of the final product, plays an important role in the conversion of cementite to graphite. The smaller the content of C, the better the ductility. However, if the content is too small, the annealing time takes a long time to graphitize and it is difficult to obtain a high hardness even after heat treatment. Therefore, it is preferable to limit the minimum amount to 0.3% or more. On the other hand, the higher the amount of C, the more the graphitization is promoted. However, if the amount is too large, the amount of precipitated graphite is large, so that cracking may occur during processing in the graphite, so the maximum amount is set to 0.8%.

Si is the most important element for promoting graphitization, and if it is contained in a small amount, it takes a long time to graphitize, so that it is contained 0.1% or more. However, if the amount of Si is too large, graphitization is considerably promoted, but Si is hardly known as an element having a high solid solution strengthening effect, so that ductility is lowered.

Mn is an important element that improves the hardenability during heat treatment to increase the strength of the steel and improves the hot workability, but is also an element that inhibits ductility because it inhibits graphitization. The smaller the Mn, the better the graphitization. However, since the Mn is problematic in strength and hot workability, the content is 0.1% or more. However, if too much, graphitization is suppressed, the upper limit is limited to 0.5%.

Al is an element which promotes graphitization, but if it is added in an amount of 0.005% or less, since Al has little effect of promoting graphitization, it is contained in 0.005% or more. At 0.1% or more, the graphitization promoting effect is almost saturated, and the amount of inclusions in the steel increases, which may lower the ductility of the steel sheet.

N reacts with B to form boron nitride (BN), which acts as a nucleation site of graphite. At this time, if the amount of N is too small, since it is difficult to form BN, it is made to contain 0.00065% or more. On the other hand, if the N content is too large, the amount of BN precipitated was limited to 0.0026% or less because cracking occurs in the slab. On the other hand, when Ti is added, Ti reacts with N to precipitate TiN to reduce the amount of N in the steel, so even if a large amount of N is added, there is no problem. Since there exists a problem that the ductility of a steel plate exists and it exists, it is preferable to restrict N amount to 0.015% or less.

The B reacts with N to precipitate BN, which acts as a nucleation site of graphite, which increases the graphitization rate and finely distributes the graphite, and plays the most important role in enhancing the heat treatment heat treatment in the solid state. do. The effect of the addition of B should be considered together with the amount of N. The minimum amount is more than 0.0005% + 0.77N _BN , the maximum amount is less than 0.03% + 0.77N _BN . If too much B is added, cracks may be generated during slab manufacture. If too little B is used, nucleation of graphite is difficult, and hardening heat treatment is difficult to obtain.

The Ti reacts with nitrogen in the steel to precipitate TiN so that B exists in a solid solution and serves to improve the heat treatment heat treatment. Therefore, the amount of Ti added depends on the amount of N, with an upper limit of (N _tot -0.00065) /0.29 and a lower limit of (N _tot -0.0026) /0.29.

Hereinafter, the manufacturing conditions used by this invention are demonstrated.

Graphitization of high carbon steel is accelerated as the structure becomes finer, so it is important to make the structure finer in the hot rolling process. The microstructure of the hot rolled steel sheet is most dependent on the finish rolling temperature and the winding temperature. The higher the finishing rolling temperature and the winding temperature, the more coarse the structure becomes and the more difficult the graphitization is. Therefore, in order to promote graphitization, the finishing rolling temperature was limited to 900 ° C. or lower and the winding temperature to 650 ° C. or lower.

In addition, the annealing temperature for graphitization is controlled to 650 ~ 710 ℃, because if the temperature is too low, the diffusion rate of carbon is slow, the graphitization progress is slow, if the temperature is too high, carbon is dissolved in austenite This is because graphitization does not occur.

After processing the parts by using the steel sheet manufactured according to the above conditions, and maintained at 800 ~ 900 ℃, which is a normal heat treatment condition for a suitable time, and then transformed to martensite through oil-cooled or water-cooled, and then required by the product Tempering treatment is performed to the hardness to produce high tensile steel with excellent ductility and heat treatment.

Hereinafter, the present invention will be described in detail through examples.

Example 1

In order to examine the effect of the alloy component on the ductility and strength of the high carbon steel, all the steel grades as shown in Table 1 are heated to 1200 ℃ and maintained for 2 hours, the finish rolling temperature is 850 ℃, winding temperature 600 A hot rolled steel sheet having a thickness of 5 mm was prepared at ℃. The hot rolled steel sheet thus prepared was pickled and annealed in a non-oxidizing atmosphere at 680 ° C. for 15 hours. The tensile test was carried out by processing the tensile test specimen from the annealed steel sheet thus prepared, and the results are shown in Table 2.

Steel grade Chemical composition (wt%) C Si Mn Al N B Ti Inventive Steel 1 0.5 0.2 0.2 0.054 0.0021 0.0028 － Inventive Steel 2 0.7 0.2 0.2 0.05 0.0018 0.003 － Invention Steel 3 0.5 0.6 0.2 0.05 0.0016 0.0031 － Inventive Steel 4 0.5 1.0 0.3 0.045 0.002 0.0025 － Inventive Steel 5 0.5 0.2 0.2 0.05 0.005 0.0030 0.01 Comparative Steel 1 0.5 0.2 0.8 0.054 0.002 0.0025 － Comparative Steel 2 0.5 2.0 0.2 0.05 0.002 0.0024 － Comparative Steel 3 0.5 0.2 0.2 0.052 0.002 0 － Comparative Steel 4 0.5 0.2 0.2 0.045 0.002 0.001 － Comparative Steel 5 0.5 0.2 0.2 0.051 0.005 0.002 0.02

Steel grade Mechanical Properties of Annealed Steel Sheet Elongation (%) Yield strength (kg / ㎡) Tensile Strength (kg / ㎡) Inventive Steel 1 44 15 35 Inventive Steel 2 43 14 34 Invention Steel 3 42 17 38 Inventive Steel 4 41 18 40 Inventive Steel 5 44 15 34 Comparative Steel 1 30 39 59 Comparative Steel 2 35 23 45 Comparative Steel 3 32 35 55 Comparative Steel 4 44 15 34 Comparative Steel 5 31 34 53

Looking at the results of Table 2 as follows.

First, it can be seen that the mechanical properties of the invention steel (2) having a larger amount of C than the invention steel (1) are almost the same as the invention steel (1).

Invented steels (1) to (4) and comparative steels (2) examined the influence of Si, it can be seen that as the Si increases, the elongation decreases, the strength increases. The reason for this is that Si promotes graphitization, but Si is an element having a large solid solution strengthening effect. As the amount of Si increases, the hardness of ferrite increases.

Compared with the inventive steel (1), the comparative steel (1) was added a large amount of Mn, it can be seen that as Mn increases, the elongation is reduced and the strength is increased. This is because Mn stabilizes cementite and inhibits graphitization.

Invented steels (1) and comparative steels (3) and (4) examined the effects of B. In comparison steel (3) without B, the elongation is decreased and the strength is considerably increased. . This is because B reacts with N present in the steel and precipitates as BN, and the precipitated BN acts as a nucleation site of graphite, but without B, graphitization does not occur. On the other hand, in the comparative steel 4 to which B was added in small amounts, elongation was increased because BN was formed, but as will be discussed later, quench heat treatment is lowered because B is not in solid solution.

Invention steel (1), invention steel (5), and comparative steel (5) examine the effects of Ti and N in combination. When N is large, like invention steel (5), even if a small amount of Ti is added, Although the amount of graphite increases and the ductility increases, when an excessive amount of Ti is added as in Comparative Steel 5, elongation is low because N precipitates as TiN and BN precipitation is suppressed.

As shown in Table 2, the inventive steel and the comparative steel (4) within the prescribed range of the present invention can be seen that the ductility is very excellent at elongation of 40% or more, yield strength 20kg / mm 2 or less, tensile strength 40kg mm 2 or less.

Example 2

In order to examine the effect of the alloy composition on the ductility and heat treatment characteristics, the steels of Table 1 were heated to 1200 ℃ and maintained for 2 hours, and then the finish rolling temperature was 850 ℃ and the coiling temperature was 600 ℃. A rolled steel sheet was produced. The hot rolled steel sheet thus prepared was pickled and annealed at 680 ° C. for 20 hours in a non-oxidizing atmosphere. Thereafter, the mixture was maintained at 870 占 폚 for 20 minutes, and then quenched and quenched to oil at 60 占 폚. After oil cooling, tempering was performed for 1 hour at 400 ° C., surface hardness was measured, and the results are shown in Table 3.

Steel grade Elongation (%) Hardness (HRC) Employment B amount (wt%) B amount precipitated by BN (wt%) Total amount of B Inventive Steel 1 44 43 0.0012 0.0016 0.0028 Inventive Steel 2 43 47 0.0016 0.0014 0.003 Invention Steel 3 42 44 0.0019 0.0012 0.0031 Inventive Steel 4 41 45 0.0010 0.0015 0.0025 Inventive Steel 5 44 43 0.0014 0.0016 0.0030 Comparative Steel 1 30 44 0.0010 0.0015 0.0025 Comparative Steel 2 35 46 0.0009 0.0015 0.0024 Comparative Steel 3 32 37 0 0 0 Comparative Steel 4 44 36 0 0.001 0.001 Comparative Steel 5 31 44 0.0020 0 0.002

Looking at the results of Table 3 divided by each alloy element as follows.

As can be seen from Table 3, the hardness of the invention steel (2) having a higher C content than that of the invention steel (1) is high. Invented steels (1), (3), (4) and comparative steels (2) to look at the effect of Si, it can be seen that the hardness increases because the hardenability increases as Si increases.

Comparative steel (1) is a steel in which Mn is excessively added, and the hardenability is increased and the hardness is high, but the elongation is low, the ductility is not good.

Invention steel (1), comparative steel (3) and comparative steel (4) is to examine the influence of B, the comparative steel (3) without B is hardened ability is low, the hardness is low, very small amount of B is added However, the hardness of the comparative steel (4) having no solid solution B is low.

The invention steels (1) and (5) and the comparative steel (5) looked at the effects of Ti and N in combination. In the case of high N, the hardness is high because a solid solution of B exists even if a small amount of Ti is added. have.

As shown in Table 3, the invention steel that satisfies the chemical composition specified in the present invention has an elongation of 40% or more and a heat treatment hardness of HRC 40 or more, while at the same time excellent in ductility and heat treatment hardness, comparative steel has a low ductility Or low heat treatment hardness.

In terms of ductility, there is a direct relationship with precipitation of BN, that is, high ductility cannot be obtained because graphite does not precipitate unless BN is precipitated. In addition, the heat treatment hardness is directly related to the amount of solid solution B. In the absence of the amount of solid solution B, the hardening heat treatment property is lowered and the hardness after heat treatment is low.

Example 3

In the effect of hot rolling condition on the ductility of high carbon steel, the finishing rolling temperature and winding temperature are the most important.

First, in order to examine the effect of the finish rolling temperature, the invention steel (1) of Table 1 was heated to 1200 ° C. and maintained for 2 hours, and then the winding temperature was constant at 600 ° C. and the finish rolling temperature was 950, 900, 850. While changing to, 800 ℃, a hot rolled steel sheet having a thickness of 5mm was prepared. The hot rolled steel sheet thus prepared was pickled and annealed at 680 ° C. for 5 hours in a non-oxidizing atmosphere. The tensile test was performed by processing the tensile test specimen from the steel sheet thus manufactured, and the results are shown in FIG. As shown in Fig. 1, when the finish rolling temperature is 900 ° C. or less, the elongation is almost constant, but decreases above 900 ° C., because the finish rolling temperature is high, and the structure becomes coarse. This is because the rate of graphitization is greatly reduced due to the slow decomposition rate of the tight.

Next, in order to examine the effect of the coiling temperature, the invention steel (1) of Table 1 was heated to 1200 ° C. and maintained for 2 hours. A hot rolled steel sheet having a thickness of 5 mm was manufactured while changing to 550 ° C. The hot rolled steel sheet thus prepared was pickled and annealed at 680 ° C. for 5 hours in a non-oxidizing atmosphere. A tensile test was performed on the tensile test piece from the steel sheet thus manufactured, and the results are shown in FIG. 2. As can be seen from Fig. 2, the elongation is almost constant high at the coiling temperature of 650 DEG C or lower, but is decreasing at 650 DEG C or higher. This is because the pearlite structure becomes coarse at the winding temperature of 650 ° C. or higher, the decomposition rate of cementite is slowed, and the graphitization rate is greatly reduced.

 Therefore, in order to obtain an excellent high carbon steel sheet, it can be seen that it is desirable to set the finishing rolling temperature to 900 ° C. or lower and the winding temperature to 650 ° C. or lower.

Example 4

In order to investigate the effect of the annealing temperature on the ductility of high carbon steel, the invention steel (1) of Table 1 was heated to 1200 ° C and maintained for 2 hours, and then the rolling finish temperature was 850 ° C and the winding temperature was 600 ° C. The hot rolled steel sheet was prepared. The hot rolled steel sheet thus prepared was pickled and annealed in a non-oxidizing atmosphere at 590 to 740 ° C. for 15 hours. The tensile test was carried out by processing the tensile test specimen from the steel sheet thus manufactured, and the results are shown in FIG. As shown in FIG. 3, when the annealing temperature is below 650 ° C. or above 710 ° C., the elongation is low, and the annealing temperature has a constant high elongation between 650 and 710 ° C. The reason is that if the annealing temperature is 650 ° C. or lower, the diffusion rate of carbon is slow and graphitization progresses slowly. If the annealing temperature is 710 ° C. or higher, carbon is dissolved in austenite and graphitization hardly occurs.

Therefore, it is preferable that annealing temperature shall be 650-710 degreeC.

As described above, the high carbon steel sheet manufactured by the present invention has excellent ductility before molding, and is easy to mold, and since it is easy to increase hardness and strength through normal heat treatment after molding, it has been difficult to use due to the difficulty of molding parts until now. Applicable to areas that have been limited. Therefore, according to the present invention, there is an effect that can greatly contribute to the development of the parts industry.

1 is a graph showing the change in elongation according to the rolling finish temperature.

2 is a graph showing the change in elongation according to the coiling temperature.

3 is a graph showing a change in elongation according to annealing temperature.

Claims (2)

delete By weight, C: 0.3-0.8%, Si: 0.1-1.5%, Mn: 0.1-0.5%, Al: 0.01-0.1%, N: 0.0026-0.015%, Ti is (N-0.0026) / 0.29≤Ti (%) ≤ (N-0.00065) /0.29, B is added in a range satisfying 0.0005 + 0.77 (N-0.29Ti) ≤B (%) ≤0.003 + 0.77 (N-0.29Ti) The steel material consisting of Fe and impurities is manufactured into a hot rolled steel sheet with a finish rolling temperature of 900 ° C. or lower and a coiling temperature of 650 ° C. or lower, and the resulting hot rolled steel sheet is annealed at a temperature range of 650 to 710 ° C. to form ferrite and graphite. A method of manufacturing high carbon steel sheet having excellent ductility and heat treatment.