KR100742871B1 - A Process for manufacturing High Carbon Containing Steels with Smooth Sheared Planes - Google Patents

Abstract

The present invention relates to the production of high carbon steel strips. An object of the present invention is to provide a high-carbon steel strip having high heat-treatability and good press formability, and especially a beautiful cut surface after processing during fine press processing at high demand.

Method for producing a high carbon steel sheet according to the present invention, by weight ratio, C: 0.35 ~ 0.55%, Si: 0.1 ~ 0.4%, Mn: 0.6 ~ 1.2%, Cr: 0.1 ~ 1.0%, S: 0.008% or less, P: 0.02% or less, B: 0.0005 to 0.005%, Ti: 0.1% or less, Ca: 0.001 to 0.1%, and the steel material consisting of the balance Fe and impurities is heated within 250 minutes at a temperature of 1,200 ° C. or less and hot rolled. to prepare a hot-rolled sheet, and then, the wound hot rolled sheet in the range of 550 ~ 630 ℃, the cooled to yunaeng the wound hot rolled coil the hot-rolled coil is configured to include that the spheroidizing annealing treatment in less than Ac ₁ transformation temperature.

High Carbon Steel, Hot Rolled, Spheroidized, Annealed, Fine Press

Description

A process for manufacturing High Carbon Containing Steels with Smooth Sheared Planes}

The present invention relates to the manufacture of high carbon steel strips, and more particularly, a method for producing a high carbon steel sheet having high heat-treatability and high press formability during fine press processing at a high demand, and particularly having a beautiful cut surface after processing. It is about.

In recent years, most mechanical and automotive parts are manufactured by press forming. Press forming refers to shearing a steel sheet between a punch and a die and pressing it with a punch. Types of press forming can be generally divided into general press forming and fine press forming. In general press forming, the steel sheet is simply placed on a die and punched to the desired part. On the other hand, fine press molding is a technique of fixing a steel sheet with a press molding holder so as not to move, and simultaneously grasp the steel sheet with an upper punch and a lower punch, and gradually pull down. Parts manufactured by normal press forming have a shear surface of about 30% and a fracture surface of about 70%, and the press molding cross section is not good, the parts are not precise, and some deformation occurs during processing. In contrast, parts manufactured by fine press forming have a very beautiful press forming cross section because the shear surface is almost 100%. As a result, the precision of the parts is at the level of the machined parts, and no deformation occurs at all during machining. In addition, since the fine press molding process does not require secondary processing compared to the general press molding, the production process can be omitted. In addition, while making precision parts similar to machining, the productivity is much higher than machining, and has been in the spotlight recently.

As such, the fine press forming process is very different from the general press forming process, the characteristics of the material required for fine press molding is also different. The condition that the fine press molding material should have is uniform and ductile without anisotropy.

There are many types of materials for fine press forming, ranging from low plate steel to high carbon steel. As representative examples thereof, Japanese Patent Application Laid-Open No. 58-104160 and Japanese Patent Laid-Open No. Hei 5-14764 disclose a method for manufacturing a fine press-formed steel sheet using low carbon steel having a relatively low carbon content. However, low carbon steel may have good hardness and low ductility, so it can have excellent fine press formability. However, due to the poor hardening heat treatment property, it is difficult to obtain high hardness and high strength up to the center of the parts required by mechanical parts, and is not suitable for manufacturing thick parts. Do.

On the other hand, Japanese Patent Publication No. 62-2008 discloses a method for producing high-carbon steel for fine press molding in which the carbon content is increased in order to improve the hardening heat treatment property of the raw material and to obtain high strength after heat treatment. However, when the carbon content of the material increases, the amount of pearlite increases, so that the hardness of the material is high and the ductility is low even after spheroidization annealing. Accordingly, the high carbon steel has a problem in that the fine press formability is not only degraded, but also the wear of the fine press forming mold is increased so that the mold is frequently replaced.

Accordingly, the present inventors have repeatedly studied and experimented to obtain a material having both high quenching heat treatment of high carbon steel and excellent fine press formability of low carbon steel, and proposed the present invention based on the results.

An object of the present invention is to control the inclusion and microstructure of high carbon steel to have excellent fine press molding characteristics of low carbon steel level, addition of a small amount of alloying elements, such as Ca and Cr, quench heat treatment and fine press formability at the same time To provide high-carbon steel strips.

Method for producing a high carbon steel sheet according to the present invention for achieving the above object, by weight ratio, C: 0.35 ~ 0.55%, Si: 0.1 ~ 0.4%, Mn: 0.6 ~ 1.2%, Cr: 0.1 ~ 1.0%, S: 0.008% or less, P: 0.02% or less, B: 0.0005 to 0.005%, Ti: 0.1% or less, Ca: 0.001 to 0.1%, and the steel material consisting of the balance Fe and impurities within 250 minutes at a temperature of 1,200 ° C or less After the hot rolling to prepare a hot rolled sheet, and then wound the hot rolled sheet in the range of 550 ~ 630 ℃, the hot rolled coil is cooled by cooling the hot rolled coil is spheroidized annealing below Ac ₁ transformation temperature It is configured to include.

Hereinafter, the present invention will be described in detail.

C contained in the high carbon steel according to the present invention is the most important element for increasing the hardness of the steel, and contains about 0.35 to 0.55% by weight (hereinafter, simply '%'). It is necessary to contain at least about 0.35% of carbon in order to obtain the hardness (HRc: 55 or more) necessary for mechanical parts by hardening and soaking heat treatment. However, if the amount of C in the material is too large, the hardness after spheroidizing annealing becomes high and the ductility decreases, so that the fine press formability is greatly reduced, so that it is preferably about 0.55% or less.

Si is a very important element as a deoxidizer and is contained in about 0.1 to 0.4% of steel. If the amount of Si is too small, since there is little deoxidation effect, it contains 0.1% or more. However, if the amount of Si is too high, decarburization is severe when the steel is reheated, red scale occurs, pickling is difficult, and surface defects are easily generated. Do.

Mn is an important element for improving the hardenability of steel materials, and should be contained at least 0.6% in order to obtain the effect. However, when the content of Mn is increased, band structures in which pearlite and ferrite are distributed in layers are likely to occur, and spheroidization of cementite is suppressed, hardness is increased, and ductility is reduced, so the upper limit of Mn is limited to about 1.2%. desirable.

Cr is an effective element that has less solid solution strengthening effect than Mn and improves hardenability without encouraging the development of band structure. In the present invention, Cr is added at least 0.1% or more mainly for the purpose of suppressing band tissue development and improving hardenability. However, when too much Cr is added, it is preferable to limit the upper limit to about 1.0% because hardness increases and ductility decreases.

S forms an emulsion and is elongated in the rolling direction during hot rolling of the steel to increase anisotropy of the steel sheet, thereby greatly reducing fine press formability. The smaller the S content, the better. However, when the S content exceeds about 0.008%, the fine press formability is greatly reduced, which is not preferable.

The P stabilizes the cementite and prevents spheroidization, and is present in the presence of Mn and segregates to promote the band structure, thereby setting the upper limit to about 0.02%.

B is an important element that segregates at the austenite grain boundary and suppresses the formation of pearlite during the heat treatment of quenching, thereby improving the hardenability, and the hardenability is increased when it is contained at least about 0.0005%. However, when B is contained in an amount of about 0.005% or more, the effect of improving hardenability is small and boron nitride or boron carbide is formed in steel, thereby lowering the ductility.

Ti is added to prevent B from forming oxides or nitrides to lower the curability. In order for B to remain in solid solution and have an effect of hardenability, Ti must be relatively about 0.1% or less. In other words, if the Ti amount is too high, titanium carbonitride is formed and thus the hardness of the matrix becomes high and the ductility decreases, which is undesirable.

The Ca is added to spheroidize the emulsion to reduce the anisotropy of the material. In order to obtain such an effect of adding Ca, at least 0.0005% or more must be contained. However, if the amount of Ca is too high, the amount of Ca added is preferably limited to about 0.1% or less because oxide-based non-metallic inclusions increase to decrease ductility. When Ca is added, it is precipitated as CaS before MnS, so the effect of S on material anisotropy is reduced.

On the other hand, in the present invention, when manufacturing a steel strip using the high carbon steel having the composition as described above, it is important to prevent surface decarburization in the hot rolling process and to refine the structure to suppress the band structure. Surface decarburization of high carbon steel reduces the hardenability of the material, which not only impairs abrasion resistance and durability but also causes deformation during heat treatment. To prevent this, it is desirable to reduce the reheating temperature as much as possible and to reduce the time. For example, surface decarburization of about 1.0% or less with respect to the thickness of the hot rolled sheet does not significantly affect the hardness and deformation of the final heat treatment material. Therefore, in the present invention, hot rolling is performed after heating at most within 250 minutes at a low temperature of about 1,200 ° C. or less so that surface decarburization can be suppressed to 1.0% or less when performing hot rolling.

In addition to the prevention of surface decarburization of the steel, it is necessary to suppress the band structure through the microstructure of the steel in order to improve the spheroidization annealing, the fine press formability, and the ductility and heat treatment heat treatment. That is, the finer the tissue and the more the band tissue is suppressed, the finer and early the spheroidization of cementite is in spheroidizing annealing. As a result, the steel has low strength and high ductility, thereby improving fine press formability and improving hardening heat treatment.

In addition, tissue refinement in hot rolled steel sheet is most dependent on the winding conditions. The higher the coiling temperature, the coarser the tissue and the more difficult the visualization is due to the development of the band structure. However, if the coiling temperature is too low, the structure is fine, but bainite is formed, the strength is too high, and even after spheroidizing annealing, the hardness is high, which causes a great wear of the mold for fine press molding. In the present invention, the coiling temperature is preferably wound in the range of about 550 ~ 630 ℃. If the coiling temperature is higher than 630 ℃, it becomes a coarse structure and the band structure is developed, and at 550 ℃ or less as bainite structure, the strength increases and the hardness is high even after spheroidizing annealing.

In addition, even if the coiling temperature is constant, it affects the pearlite lamellar spacing according to the cooling flow rate in the cooling equipment of the hot rolling process, thereby affecting the press formability after spheroidizing annealing. In the present invention, preferably, the wound hot rolled coil is cooled at a flow rate of about 0.09 to 0.12 m 3 / sec.

Then, in the present invention, the hot rolled coil obtained according to the cooling flow rate of the hot rolling step is subjected to spheroidizing annealing at Ac ₁ transformation temperature or less. When the annealing is performed at the Ac ₁ transformation temperature or less in the spheroidizing annealing process, the size of the spheroidized cementite is coarse or the coarse pearlite is regenerated to lower the fine press formability or the quench heat treatment. In the present invention, spheroidizing annealing is preferably performed in a non-oxidizing atmosphere.

In this manner, fine press forming processing is performed using a spherical annealed steel sheet to obtain a product having a certain shape. At this time, when the fine press molding process using the steel produced by the present invention at the demand price can be very good workability. At the demand price, a fine press-molded product is subjected to quenching heat treatment, followed by a soaking treatment. The quenching heat treatment performed at the demand price is usually performed by maintaining the cementite in solid solution at a temperature of 30 ° C. or higher of the Ac ₃ transformation temperature of the steel, and then obtaining martensite structure through oil cooling.

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

EXAMPLE

Steel having a composition as shown in Table 1 was reheated, hot rolled, wound, cooled, and spheroidized annealing under the same manufacturing conditions as Table 2 to prepare a hot rolled coil having a thickness of 5.0 mm.

division Chemical composition (% by weight) C Si Mn P S Cr B Ti Ca Inventive Steel 1 0.45 0.20 0.72 0.002 0.005 0.10 0.001 0.08 0.004 Inventive Steel 2 0.35 0.20 0.76 0.020 0.008 0.21 0.002 0.08 0.006 Invention Steel 3 0.50 0.20 0.70 0.020 0.005 0.13 0.002 0.08 0.005 Inventive Steel 4 0.55 0.20 0.68 0.020 0.005 0.70 0.002 0.08 0.007 Comparative Steel 1 0.10 0.20 0.90 0.015 0.003 0.20 0.002 0.015 - Comparative Steel 2 0.45 0.21 1.0 0.015 0.004 0.20 0.002 0.016 - Comparative Steel 3 0.20 0.25 2.0 0.016 0.004 0.20 0.002 0.018 - Comparative Steel 4 0.20 0.20 1.0 0.015 0.004 1.50 0.002 0.017 - Comparative Steel 5 0.20 0.23 1.0 0.015 0.010 0.20 0.002 0.016 - Comparative Steel 6 0.20 0.20 1.0 0.015 0.015 0.20 0.002 0.018 0.004 Comparative Steel 7 0.20 0.25 1.0 0.015 0.004 0.20 0.002 0 - Comparative Steel 8 0.20 0.23 1.0 0.015 0.004 0.20 0 0.02 -

The hardness was measured for each hot rolled coil thus manufactured, and the results are shown in Table 2. In addition, by using the hot-rolled coil to produce about 50 car door parts each through a fine press to observe the degree of processing and the results are shown in Table 2. At this time, the degree of processing was evaluated as good if the shear surface of the product after the fine press processing is about 90% or more, good if 90 ~ 80%, and less than 80%.                     

division Manufacture conditions Hardness (Hv) Press formability Reheating Temperature (℃) Working time (min) Winding temperature (℃) Cooling flow rate (㎥ / sec) Spherical annealing furnace (℃) Inventive Example 1 1220 220 590 0.03 710 175 Good Inventive Example 2 1220 250 600 0.06 710 160 Good Inventive Example 3 1220 280 610 0.09 710 178 Great Inventive Example 4 1240 210 630 0.12 710 180 Great Comparative Example 1 1220 210 600 0.18 700 160 Good Comparative Example 2 1220 220 640 0.18 720 185 Good Comparative Example 3 1220 220 580 0.12 750 175 Bad Comparative Example 4 1240 200 540 0.12 750 155 Bad Comparative Example 5 1230 240 600 0.12 720 158 Good Comparative Example 6 1210 250 680 0.18 710 170 Bad Comparative Example 7 1220 260 680 0.12 720 158 Good Comparative Example 8 1230 230 680 0.06 720 162 Good

As shown in Tables 1 and 2, it was found that the hot rolled steel sheet manufactured according to the present invention had good fine press formability after the spheroidizing annealing with a hardness of about 180 Hv or less. This is considered to be because the ferrite and pearlite are finely divided in the case of the steel sheet according to the present invention and the cementite is uniformly distributed after spheroidizing annealing. For reference, in the present invention, the hardness of the raw material became 300 Hv or more when the fine press-worked part was considered for 1 hour at about 400 ° C.

On the contrary, in contrast to the conditions of the present invention, in the comparative example in which the coiling temperature is high or the cooling flow rate is high, the hardness is increased, so that the structure becomes brittle or the press cut surface is poor, and the life of the mold is shortened. On the other hand, in the case of Comparative Examples (1, 5, 6, 8), the press formability is also good and the hardness is not high, but since they are low carbon steel, the wear resistance required by the final product cannot be secured.

 As described above, the high carbon steel hot rolled steel produced according to the present invention has excellent fine press forming characteristics of low carbon steel level, and the hardening heat treatment and fine press formability are excellent at the same time when processing at the final demand. It is very suitable for auto parts.

Claims (2)

By weight ratio, C: 0.35 to 0.55%, Si: 0.1 to 0.4%, Mn: 0.6 to 1.2%, Cr: 0.1 to 1.0%, S: 0.008% or less, P: 0.02% or less, B: 0.0005 to 0.005%, Ti: 0.1% or less, Ca: 0.001 ~ 0.1%, the steel material consisting of the remaining Fe and impurities in a temperature of less than 250 minutes at a temperature of 1,200 ℃ or less and hot-rolled to prepare a hot rolled sheet, and then A method of manufacturing a high carbon steel strip having a beautiful cut surface, which comprises a winding in the range of 550 to 630 ° C., and spheroidizing annealing the coiled hot rolled coil at oil temperature below Ac ₁ transformation temperature. The method of claim 1, wherein the cooling is performed at a flow rate of 0.09 to 0.12 m 3 / sec.