JP3468048B2 - Manufacturing method of high carbon cold rolled steel sheet with excellent formability - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-carbon cold-rolled steel sheet having excellent formability. 2. Description of the Related Art A high carbon thin steel sheet is formed into a predetermined shape by press forming such as punching, bending, drawing, and the like, and then subjected to heat treatment such as quenching and tempering to increase the strength and hardness, thereby improving product quality. Is done. With the expansion of applications, there has recently been an increasing demand for application of high carbon steel sheets to products having complicated shapes such as those obtained by deep drawing. [0003] There is a method in which a low carbon thin steel sheet excellent in deep drawing formability is processed into a product having a complicated shape, and the hardness is increased after processing by a hardening treatment method such as carburizing and quenching. However, hardening methods such as carburizing and quenching are more expensive than ordinary quenching and tempering processes and are not economical. The use of high-carbon steel sheets can easily increase the hardness of products by conventional heat treatment methods, but steels with a high carbon content are represented by ductility and r-value represented by elongation values measured by tensile tests. Due to poor deep drawability, it cannot be processed into a product having a complicated shape. [0004] Carbon steel (S30CM-S75CM) and carbon tool steel (SK2M-
SK7M) hot-rolled steel sheet is subjected to cementite spheroidizing annealing to improve the ductility by changing the pearlite structure in the steel sheet to a structure of spherical cementite and soft ferrite, or to cold-rolling and annealing the hot-rolled steel sheet. To improve the moldability. However, in these methods, moldability,
In particular, the deep drawability cannot be sufficiently improved. [0005] For this reason, there is a demand for a steel that is soft and excellent in formability at the material stage, and excellent in hardenability in which strength and hardness can be easily obtained by simple heat treatment after processing. Japanese Patent Publication No. 4-56088 discloses a method for producing a high-carbon cold-rolled steel sheet having good drawability. This is a production method in which cold rolling and annealing are performed on steel whose chemical components are regulated to a specific range to graphitize cementite in the steel, and then the second cold rolling and recrystallization annealing are performed. Speaking of r value which is an index indicating deep drawability, r
Values are usually only obtained up to levels of about 1.0 or less. According to this method, a cold-rolled steel sheet having a high r-value and a deep drawability comparable to that of a mild steel sheet, which has not been obtained conventionally, can be obtained by cold rolling and annealing a steel sheet obtained by graphitizing cementite. It is supposed to be. However, this method requires a long manufacturing process and is costly. Further, the graphitized steel sheet has low hardenability as compared with a normal high carbon steel sheet, and thus the hardness obtained after the heat treatment is not sufficient. JP-A-8-246051 discloses that a hot-rolled steel sheet having a specific composition is annealed to graphitize at least 50% of the carbon in the steel, and this is subjected to cold rolling and annealing to improve formability. A method for manufacturing a medium carbon steel sheet is disclosed. In this method, although the manufacturing process is simplified as compared with the method disclosed in Japanese Patent Publication No. 4-56088, the improvement of the deep drawability is not sufficient, and the heat treatment after the heat treatment is performed because the steel sheet is a graphitized steel sheet. The hardness obtained is not sufficient. The problem to be solved by the present invention is to have high ductility and r-value and excellent moldability to obtain good hardness by a usual heat treatment method. And a method for producing a high-carbon cold-rolled steel sheet. The gist of the present invention resides in the following method for producing a high-carbon cold-rolled steel sheet having excellent formability. C: 0.1 to 0.65% by weight, S
i: 0.01 to 0.3%, Mn: 0.4 to 2%, so
l. Al: 0.01 to 0.1%, N: 0.002 to 0.
008%, B: 0.0005 to 0.005%, Cr: 0
-0.5%, Mo: 0-0.1%, balance: high-carbon steel consisting of Fe and unavoidable impurities is hot-rolled to 300-
Wound at 520 ° C into a coil, 650 to (A _c1 -10)
High-carbon cold-rolled steel sheet excellent in formability, characterized in that it is annealed at a rolling reduction of 40 to 80% after annealing at 10 to 30 hours at a temperature of 40 to 80% and annealed at 650 to (A _c1 -10) ° C. Manufacturing method. The present inventors have studied various methods for improving the r-value of a high carbon steel having a crystal structure mainly composed of ferrite-cementite, which is easy to obtain hardness after heat treatment. As a result, a high-carbon steel hot-rolled steel sheet having a bainite structure containing a specific range of B is annealed to change its crystal structure to a structure composed of spheroidized cementite and acicular ferrite crystal grains,
It has been found that a high-carbon cold-rolled steel sheet having a high r value can be obtained by subjecting this hot-rolled steel sheet to cold rolling and recrystallization annealing. The reason why a high carbon cold rolled steel sheet having a good r value can be obtained by the above method is not clear, but is presumed as follows. In order to increase the r value used as an index indicating the deep drawability of a steel sheet, the steel sheet must have a recrystallization texture with a high degree of integration in the (111) orientation. (11
1) It is said that recrystallized grains having an orientation are preferentially generated from ferrite crystal grain boundaries of a base material before annealing. Therefore,
In order to increase the r value, it is effective to change the crystal structure of the base steel before cold rolling to such a structure having many ferrite grain boundaries. Bainite has a crystal structure in which cementite is dispersed in acicular ferrite, and has a larger grain boundary area than a normal crystal structure composed of ferrite and pearlite. When a steel sheet having a bainite crystal structure after hot rolling is subjected to spheroidizing annealing, the steel sheet generally has a general spheroidized annealing structure including polygonal ferrite and spheroidized cementite. However, if a high-carbon steel is made to contain B in an appropriate amount, the hot rolling conditions are adjusted, and the steel having a bainite structure is spheroidized and annealed, the effect of the bainite structure remains strongly even after annealing, and the above-described needle-like structure is obtained. A steel sheet having a ferrite structure is obtained. In the steel having the acicular ferrite structure,
There are more ferrite grain boundaries than in a normal polygonal ferrite-structured steel. Therefore, when this is cold-rolled and annealed, it is considered that (111) recrystallized grains increase, the degree of integration in the (111) orientation increases, and a high r value can be obtained. The present invention has been completed based on these newly obtained findings. Embodiments of the present invention will be described below in detail. The chemical composition of steel described below is expressed in% by weight. Chemical composition C of steel: Included to increase the strength of steel and improve hardenability. If the content is less than 0.1%, the strength and hardness of the steel after heat treatment will be insufficient. C content 0.65%
If it exceeds 50, the improvement of the r value becomes insufficient even when the production method of the present invention is applied, and the moldability is not improved. For this reason, the C content is set to 0.1 to 0.65%. From the viewpoint of increasing the hardness of the steel after quenching, the C content is preferably set to 0.2% or more. Si: 0.01% or more is contained in order to improve hardenability. If the Si content is excessive, the steel sheet becomes excessively hard, and the formability is impaired. Therefore, the upper limit of the Si content is set to 0.3%. Mn: S contained as an unavoidable impurity
In order to prevent hot embrittlement due to steel and to increase the hardenability of steel, the content is made 0.4% or more. Since the formability of the steel sheet is impaired as the Mn content increases, the upper limit of the Mn content is set to 2%. Sol. Al: Used as a deoxidizer for molten steel. Al combines with N in the steel to form fine precipitates (A
1N), which has the effect of suppressing coarsening of austenite crystal grains and suppressing deterioration of toughness. To have these effects, sol. 0.01% or more is contained as Al. Even if Al is excessively contained, these effects are saturated and formability is impaired. Therefore, sol. The upper limit of the Al content is set to 0.1%. N: 0.002% or more in order to suppress abnormal grain growth of austenite crystal grains which may occur during annealing and quenching heating, and to suppress deterioration of toughness. However, if N is excessively contained, the ductility of the steel sheet is impaired, so the upper limit is made 0.008%. B: B segregates at the ferrite crystal grain boundaries after hot rolling, and at the time of subsequent annealing, suppresses the movement of the crystal grain boundaries and leaves an acicular ferrite structure that inherits the characteristics of the bainite structure. There is. In order to secure this effect and increase the r-value of the steel sheet after cold rolling and annealing, 0.00
B is contained in an amount of 05% or more. 0.005% B content
If it exceeds 300, the above-mentioned suppressing effect is saturated and the steel may be embrittled. Therefore, the upper limit of the B content is 0.0
05%. The steel used in the manufacturing method of the present invention may be any steel as long as it satisfies the above-mentioned chemical composition. In some cases, the steel may contain the following elements. Cr, Mo: These are not essential, but if it is necessary to further improve the hardenability of steel, 0.01
It is preferable to contain 0.5% of Cr and / or 0.01% to 0.1% of Mo. If the content of these elements is less than the above lower limits, the effect of improving the hardenability by these elements is insufficient. If the content exceeds the above upper limit, the ductility and deep drawability of the steel sheet are impaired. The effect of improving the hardenability is more effective if a plurality of elements are combined and contained than a large amount of one element is contained. Therefore, a combination of Cr and Mo within the above range is effective. It is more effective to add. The other chemical compositions are Fe and inevitable impurities. Processing Conditions A steel having a chemical composition within the above range is smelted by a conventional method in a converter or an electric furnace, and then slab-cast by a continuous casting method or a method of forming into a steel ingot and subsequently performing ingot rolling. It is said. Thereafter, the steel sheet is heated in a slab heating furnace or the heating in the slab heating furnace is omitted, and is rolled by a hot rolling mill to obtain a hot-rolled steel sheet. The hot rolling conditions are not specified, but in order to secure the surface properties and facilitate the rolling, finish rolling is carried out in 8 hours.
Preferably it starts at 00-950 ° C and ends at 750-900 ° C. After finish rolling, it is cooled to a range of 300 to 520 ° C. and wound into a coil. By winding in this temperature range, a steel sheet having a bainite structure can be obtained. When the winding temperature is less than 300 ° C.,
The crystal structure of the resulting hot-rolled steel sheet becomes martensite,
When the winding temperature exceeds 500 ° C., a ferrite-pearlite structure is formed, and in any case, a bainite structure cannot be obtained. After the rolled steel sheet,
The hot rolled sheet is annealed in a temperature range of (A _c1 -10) ° C. for 10 to 30 hours. The purpose of this hot rolled sheet annealing is to soften steel by spheroidizing cementite. If the annealing temperature is lower than 650 ° C., spheroidization of cementite is insufficient. If the annealing temperature exceeds A _c1 , austenite is formed, and the acicular ferrite structure disappears with it, which is not preferable. In order to stably perform annealing at a temperature _lower than A _c1 , the upper limit of the annealing temperature is set to (A _c1 -10).
℃ is good. The soaking time requires at least 10 hours in order to sufficiently perform spheroidization, and if it exceeds 30 hours, spheroidization is saturated, and further annealing impairs economic efficiency. For this reason, the range of the soaking time is set to 10 to 30 hours. In the present invention, the A _c1 point of steel is determined by the following formula based on its chemical composition. The element symbols in the formula represent the content (% by weight) of each element. A _c1 (° C.) = 723-11Mn + 29Si + 17Cr ... The steel sheet subjected to hot-rolled sheet annealing is cold-rolled at a rolling reduction within a range of 30 to 80%. When the cold reduction is less than 30%, the r-value is not improved even if recrystallization annealing is performed due to insufficient formation of the rolling texture. 8 cold reduction
If it exceeds 0%, rolling becomes difficult, and breakage tends to occur during rolling. A preferred rolling reduction range is 40 to 65 to ensure a stable r value and to obtain the stability of production.
%. The cold-rolled steel sheets include 650- (A _c1
-10) Recrystallization annealing is performed in a temperature range of ° C. By this recrystallization annealing, a recrystallization texture preferable for improving deep drawability is formed. When the annealing temperature is lower than 650 ° C., the r-value is not improved due to insufficient growth of crystal grains, and the steel is insufficiently softened, resulting in poor formability. If the annealing temperature exceeds A _c1 , austenite transformation occurs and the recrystallization texture disappears, which is not preferable. In order to stably perform annealing at a temperature _lower than A _c1 , the upper limit of the annealing temperature is set to (A _c1 −
10) The temperature is preferably set to ° C. The annealing time is desirably 20 seconds or more to complete recrystallization and crystal grain growth. As the annealing method, any of box annealing and continuous annealing can be applied. The processing conditions other than those described above are not particularly limited. For example, treatments usually performed when manufacturing a steel sheet, such as descaling of a hot-rolled steel sheet and temper rolling after recrystallization annealing, are performed. It can be applied under normal conditions. EXAMPLES Slabs of various chemical compositions were prepared at 1200 ° C. for 30 minutes.
Heated at 850 ° C for 4 minutes
After rolling, it was cooled under various conditions to 400
It was wound into a coil at ６650 ° C. Table 1 shows the chemical compositions of these steels. [Table 1] After pickling these hot-rolled steel sheets, a steel sheet having a width of 300 mm and a length of 300 mm was cut out from each coil and annealed at 700 ° C. using a box annealing furnace for experiments. The heating rate up to the annealing temperature and the cooling rate after the annealing were both 50 ° C./hr. However, only in one example (test number 17), in order to confirm the influence of annealing conditions, annealing was performed at 750 ° C., which is a coexistence region of ferrite and austenite.
In order to spheroidize the cementite, it was gradually cooled to 650 ° C. at a cooling rate of 10 ° C./hr, and the temperature range of 650 ° C. or less was cooled to room temperature at a cooling rate of 50 ° C./hr. The steel sheet subjected to the hot-rolled sheet annealing was rolled at a rolling reduction of 60% using an experimental cold rolling mill to a thickness of 1.6 m.
m of cold-rolled steel sheet. As a comparative material for verifying the effect of the cold rolling reduction, for steel B, the rolling reduction was 30% and 9%.
Cold rolling at 0% was also performed to obtain cold-rolled steel sheets of 2.8 mm and 0.4 mm in thickness. These cold-rolled steel sheets were subjected to recrystallization annealing in which the surfaces of the steel sheets were degreased and then soaked at 700 ° C. for 15 hours. For comparison, 60% for steel B
Annealing by soaking at 0 ° C. for 15 hours was also performed. A No. 13B test piece specified in JIS-Z-2201 was sampled from the rolling direction of the steel sheet subjected to the recrystallization annealing, and a tensile test was performed. In addition, the width of the recrystallized and annealed steel sheet is 50
mm, a test piece having a length of 50 mm was collected, and
The sample was heated in a heat treatment furnace maintained at 30 ° C. for 30 minutes, quenched into oil at 40 ° C. immediately after completion of the heating, and then the hardness of the test pieces after quenching was measured. Table 2 shows the processing conditions for each steel and the results of the respective characteristic evaluations. [Table 2] As shown in Table 2, steel sheets having a chemical composition within the range specified by the present invention and manufactured under the manufacturing conditions within the range specified by the present invention were all 1.4 or more. It showed a good r-value, a good elongation value and a sufficient quench hardness. On the other hand, in Test Nos. 11 to 16, a good r value could not be obtained because the winding temperature after hot rolling was too high. In test No. 17, in which the hot-rolled sheet annealing temperature was too high in steel B,
The effect of the bainite structure of the hot-rolled steel sheet disappeared due to annealing in the coexisting region of ferrite and austenite, and in Test No. 18 in which the cold rolling reduction was too low, recrystallization was insufficient, and the r value was not preferable in any case. In Test No. 19, in which the cold rolling reduction was too high, the subsequent evaluation was stopped because the cold rolling was difficult. In Test No. 20, the recrystallization annealing temperature was too low, and thus the elongation and the r value were not preferable. In Test Nos. 24 to 28, the r value and elongation value were not improved because the B content was out of the range specified by the present invention for all steels.
In Test Nos. 21 to 23 in which the content of C, Si or Mn was too high, the improvement of the elongation value and the r value was insufficient. The high-carbon cold-rolled steel sheet produced by the production method of the present invention has good ductility and r-value and excellent formability, and is a product of a complicated shape which has been difficult in the past. Processing is possible. Moreover, since the steel sheet is excellent in quenching properties, it can be easily made to have high strength or high hardness by quenching heat treatment under ordinary conditions. Therefore, it is possible to easily obtain a high-strength or high-hardness complex-shaped part which has been difficult in the past.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (1)

(57) [Claims 1] C: 0.1 to 0.65% by weight, S:
i: 0.01 to 0.3%, Mn: 0.4 to 2%, so
l. Al: 0.01 to 0.1%, N: 0.002 to 0.
008%, B: 0.0005 to 0.005%, Cr: 0
-0.5%, Mo: 0-0.1%, balance: Fe and high-carbon steel consisting of unavoidable impurities are hot-rolled,
Wound at 520 ° C into a coil, 650 to (A _c1 -10)
After being annealed at 10 ° C. for 10 to 30 hours, cold-rolled at a rolling reduction of 40 to 80% and annealed at 650 to (A _c1 -10) ° C. Manufacturing method.