KR20130054998A - High-carbon hot-rolled steel sheet having excellent fine blanking properties and process for production thereof - Google Patents

Abstract

Provided are a high carbon hot rolled steel sheet having excellent fine blocking properties in a wound state after hot rolling, and a method of manufacturing the same. In mass%, C: 0.10 to 0.40%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.03% or less, N: 0.01% or less, Al: 0.10% or less, and High carbon having excellent fine blanking properties, characterized in that the balance has a composition composed of Fe and inevitable impurities, and has a microstructure in which the area ratio of the cornerstone ferrite phase occupies the entire structure is 10% or less, and the spheroidization rate of the carbide is 50% or more. Hot rolled steel plate.

Description

High carbon hot rolled steel sheet with fine blanking property and its manufacturing method {HIGH-CARBON HOT-ROLLED STEEL SHEET HAVING EXCELLENT FINE BLANKING PROPERTIES AND PROCESS FOR PRODUCTION THEREOF}

The present invention relates to a high carbon hot rolled steel sheet suitable for use in automobile parts and the like, particularly a high carbon hot rolled steel sheet having excellent fine blanking property (precision punchability) without annealing after hot rolling, and a method of manufacturing the same.

Automobile parts having complex shapes such as gears, missions, and seat recliners are machined into desired shapes by cutting or blanking a high-carbon steel sheet for mechanical structure specified in JIS G 4051, or by further expanding the hole. Then, it is manufactured by performing a quenching tempering process in order to ensure hardness. In recent years, from the viewpoint of improvement of the dimensional accuracy and the simplification of the manufacturing process, a blanking cross section with i) a smooth 100% shear surface is obtained, ii) high dimensional accuracy, and iii) blanking a complex shape once. The process by the fine blanking which has the merit of being able to process by, etc. is employ | adopted.

Fine blanking sets the clearance between the tools very small from about 5 to 10% of the steel sheet thickness to about 2% or less of the steel sheet thickness in the normal blanking, and applies a blank stress to the steel sheet near the tool cutting edge to blank. Way. Therefore, since the high carbon steel plate which is a blanking material undergoes very strict processing, a 100% shear surface does not obtain a smooth blanking cross section, and a crack occurs easily on a blanking surface.

Therefore, a high carbon steel sheet is proposed which does not cause such a problem, that is, excellent in fine blanking property. For example, in Patent Literature 1, in mass%, C: 0.08 to 0.19%, Si: 0.50% or less, Mn: 0.70 to 1.50%, Cr: 0.05 to 0.80%, B: 0.0005 to 0.005%, and Al: 0.08% Disclosed is a steel sheet for precision punching which contains the following and is excellent in complex molding processability and short time rapid heating quenching property, in which the balance is made of Fe and unavoidable impurities.

In Patent Literature 2, in mass%, C: 0.15 to 0.90%, Si: 0.4% or less, Mn: 0.3 to 1.0%, P: 0.03% or less, total Al: 0.10% or less, and the balance substantially has a composition of Fe. Has a structure in which carbides having a spheroidizing ratio of 80% or more and an average particle diameter of 0.4 to 1.0 µm were dispersed in a ferrite matrix, and having an opening angle of 45 degrees and a depth at both sides in the width direction in the central portion of the parallel part in the longitudinal direction of the JIS No. 5 tensile test piece. The high-carbon steel plate excellent in the precision punchability which the tensile test of the tensile strength of 20 mm or more of the tensile test which carried out the tension test using the test piece which put the 2 mm V notch and shows as elongation after fracture | rupture with respect to the gage distance 10 mm of a parallel part longitudinal direction is disclosed. have.

In patent document 3, in mass%, C: 0.15 to 0.45%, Si: 0.25% or less, Mn: 0.3 to 1.2%, P: 0.02% or less, S: 0.02% or less, Al: 0.01 to 0.1%, N: High carbon steel sheet containing 0.008% or less, the remainder having an unavoidable impurity and Fe composition, a ratio of pearlite + cementite of 10% or less, and an average particle diameter of ferrite grains of 10 to 20 µm, and excellent in fine blanking properties. It is.

Patent Document 4 contains, in mass%, C: 0.1 to 0.5%, Si: 0.5% or less, Mn: 0.2 to 1.5%, P: 0.03% or less, and S: 0.02% or less, and the balance Fe and unavoidable impurities And a structure composed mainly of ferrite and carbide, the average particle diameter of the ferrite is 1 to 10 µm, the spheroidization rate of the carbide is 80% or more, and among the carbides, the carbides present in the grain boundary of ferrite Amount Sgb = ｛Son / (Son + Sin)｝ × 100 (Son: Total occupied area of carbides present on the ferrite grain boundary among carbides present per unit area, Sin: Carbide present in ferrite grains among carbides present per unit area) The steel plate excellent in the fine blanking property whose total occupied area) is 40% or more is disclosed.

Among these, the steel sheet described in Patent Literature 1 is a hot rolled steel sheet in a wound state, and the steel sheets in Patent Literatures 2 to 4 are subjected to annealing of the hot rolled steel sheet after winding, or after cold rolling to spheroidization (in Patent Literatures 2 and 4, , 80% or more nodularity).

Japanese Laid-Open Patent Publication No. 59-76861 Japanese Unexamined Patent Publication No. 2000-265240 Japanese Laid-Open Patent Publication 2001-140037 Japanese Unexamined Patent Publication No. 2007-231416

However, in the hot-rolled steel sheet in the wound state described in Patent Literature 1, a 100% shear surface does not obtain a smooth blanking cross section, and cracks tend to occur on the blanking surface, resulting in poor fine blanking. In the steel sheets of Patent Literatures 2 to 4, spheroidization of carbides having a spheroidization ratio of 80% is achieved, so that good fine blanking property can be obtained. It is necessary to anneal the hot rolled steel sheet after winding or after cold rolling, which is expensive. .

An object of the present invention is to provide a high-carbon hot rolled steel sheet in a rolled state without annealing or annealing after cold rolling, that is, having excellent fine blanking property at low cost, and a method of manufacturing the same.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined the high carbon hot rolled sheet steel obtained by the fine blanking property in the wound state, and found out the following.

i) After optimizing the component composition, if the area ratio of the cornerstone ferrite phase occupies the entire structure is 10% or less, and the spheroidization rate of the carbide is 50% or more, excellent fine blanking property is obtained even in a wound state.

ii) Such a microstructure is hot-rolled at a finishing temperature of 800 to 950 ° C., and then cooled to a cooling stop temperature of 450 to 600 ° C. at an average cooling rate of 50 ° C./s or more from a cooling start temperature of 650 ° C. or higher, and 3 s. It is obtained by the method of winding up within.

This invention is made | formed based on such knowledge, In mass%, C: 0.10-0.40%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.03% or less, N: A microstructure containing 0.01% or less, Al: 0.10% or less, the remainder being composed of Fe and unavoidable impurities, the area ratio of the cornerstone ferrite phase occupying the entire structure, and the spheroidization rate of carbide being 50% or more. Provided is a high carbon hot rolled steel sheet excellent in fine blanking property.

In the high carbon hot rolled steel sheet of the present invention, in addition to the above-mentioned composition, it is mass%, and B: 0.0010 to 0.0050%, and satisfying the relationship of 0.50 ≤ (14 [B]) / (10.8 [N]) desirable. However, [B] and [N] represent content (mass%) of B and N, respectively. Furthermore, in mass%, at least one of Ti, Nb, and V: 0.1% or less in total, at least one of Ni, Cr, and Mo: 1.5% or less in total, and at least one of Sb and Sn: in total It is preferable to contain 0.003-0.10% individually or simultaneously.

The high carbon hot rolled steel sheet according to the present invention is obtained by hot rolling a steel having the composition described above at a finishing temperature of 800 to 950 ° C., and then at 450 to 600 ° C. at an average cooling rate of 50 ° C./s or more from a cooling start temperature of 650 ° C. or more. It can manufacture by the method of cooling to a cooling stop temperature, and winding up within 3 s.

In the manufacturing method of this invention, it is preferable to charge the steel plate after winding up to a heat retention apparatus in the wound state.

According to the present invention, it is possible to produce a high-carbon hot rolled steel sheet in a wound state having excellent fine blanking property at low cost. The high carbon hot rolled steel sheet of the present invention is suitable for automobile parts such as gears, missions, seat recliners, and the like.

 EMBODIMENT OF THE INVENTION Below, the high carbon hot rolled sheet steel which is this invention, and its manufacturing method are demonstrated in detail. In addition, "%" which is a content unit of a component shall mean "mass%" unless there is particular notice.

1) Composition

C: 0.10 to 0.40%

If the amount of C is less than 0.1%, the hardness after quenching required as an automotive part cannot be obtained. On the other hand, when the amount of C exceeds 0.40%, the steel sheet becomes hard, and the load on the mold at the time of blanking becomes too large, and the die life for the blanking cannot be secured to an industrially sufficient level. Therefore, the amount of C is made into 0.10 to 0.40%.

Si: 1.0% or less

When the amount of Si exceeds 1.0%, a ferrite phase will harden | cure, fine blanking property will fall, or a surface defect called a red scale will generate | occur | produce. Therefore, the amount of Si is 1.0% or less, Preferably you may be 0.35% or less.

Mn ： 2.0% or less

When Mn amount exceeds 2.0%, a ferrite phase will harden | cure and fine blanking property will fall. Therefore, the amount of Mn is made into 2.0% or less. Moreover, in order to improve hardenability, it is preferable to make Mn amount into 0.2% or more.

P: 0.03% or less

When P amount exceeds 0.03%, it will segregate in a grain boundary etc., and workability, such as fine blanking property, will fall. Therefore, the amount of P is made 0.03% or less, Preferably it is 0.02% or less, but smaller is more preferable.

S: 0.03% or less

When the amount of S exceeds 0.03%, sulfides are formed, and workability, such as fine blanking property, falls, or toughness after quenching falls. Therefore, the amount of S is made into 0.03% or less, Preferably it is 0.02% or less.

N: 0.01% or less

N forms nitrides such as Al and the like and has a function of preventing coarsening of the austenite grains. When the amount of N is excessively greater than 0.01%, workability is deteriorated. Therefore, N amount is 0.01% or less, Preferably you may be 0.005% or less.

Al: 0.10% or less

When the Al amount exceeds 0.10%, not only the cleanliness of the steel is lowered, but the austenite grains are excessively fined upon heating of the quenching treatment, and the formation of a ferrite phase is accelerated during cooling, so that necessary hardness cannot be obtained after quenching. Therefore, Al amount is made into 0.10% or less. In addition, Al has an effect of binding Al to form AlN to prevent coarsening of austenite grains or to effectively exhibit quenchability by B. Therefore, the amount of Al is preferably 0.01% or more.

The balance is made of Fe and unavoidable impurities. For the following reasons, B: 0.0010% to 0.0050%, at least one of Ti, Nb, and V: 0.1% or less in total, and at least one of Ni, Cr, and Mo: 1.5 in total It is preferable to contain 0.003 to 0.10% individually or simultaneously in at least 1 type: in total of% or less and Sb and Sn.

B: 0.0010% to 0.0050%

B is an element which improves quenchability, forms BN during heating of the quenching treatment, suppresses coarsening of austenite grains, and improves toughness after quenching. In order to acquire such an effect, the amount of B needs to be 0.0010% or more. On the other hand, when the amount of B exceeds 0.0050%, the load of hot rolling will become high, operation property will fall, and workability, such as fine blanking property, will also be brought about. Therefore, the amount of B is preferably 0.0010% to 0.0050%, and more preferably 0.0010% to 0.0030%. Moreover, in order to improve the hardenability by B, it is necessary that solid solution B exists at the time of the heating of a quenching process. However, since B has a high affinity with N and tends to form BN during hot rolling or subsequent winding or heating of the quenching treatment, when B is contained, 0.50 ≦ (14 [B]) for N It is necessary to secure employment B by satisfying the relationship of / (10.8 [N]). In addition, [B] and [N] show content (mass%) of B and N, respectively.

At least one of Ti, Nb, and V: 0.1% or less in total

When content of at least 1 sort (s) of Ti, Nb, and V exceeds 0.1% in total, a ferrite phase will harden by precipitation, such as TiC, and will lead to the fall of the blanking die life. Therefore, it is preferable to make content of at least 1 sort (s) of Ti, Nb, and V into 0.1% or less in total. In addition, Ti, Nb, and V easily bond with N to form nitrides, prevent coarsening of the austenite grains during quenching, and improve the quenchability by B, thereby forming BN. Since it also has the effect that it can prevent and reduce the addition amount of B which is necessary, it is more preferable to make content of at least 1 sort (s) of these elements into 0.01% or more in total.

At least one of Ni, Cr, and Mo: 1.5% or less in total

When content of at least 1 sort (s) of Ni, Cr, and Mo exceeds 1.5% in total, a ferrite phase will harden and fine blanking property will fall. Therefore, it is preferable to make content of at least 1 sort (s) of Ni, Cr, and Mo into 1.5% or less in total. In addition, since Ni, Cr, and Mo effectively act to improve the hardenability, the content of at least one of Ni, Cr, and Mo is more preferably 0.1% or more in total.

At least 1 type of Sb and Sn: 0.003 to 0.10% in total

If the heating of the quenching treatment is performed under a condition of mixing the air and controlling the carbon potential at about 900 ° C. for about 1 hour, adsorption phenomenon occurs, and AlN or BN is formed in the surface layer portion of the steel, causing excessive austenite grains. It may be refined or the amount of solid solution B may be remarkably reduced, resulting in a decrease in quenchability. Therefore, the present inventors added Sb and Sn which are known as elements capable of suppressing the absorption of steel, and examined the quenchability, and as a result, if the amount of at least one of Sb and Sn is 0.003% or more in total, the absorption is suppressed. It was found that the quenchability of the surface layer portion can be improved. However, when the amount of at least one of Sb and Sn is added in excess of 0.10% in total, it segregates in the austenite grain boundary during quenching and tempering, thereby significantly reducing the toughness. In addition, these elements also affect the workability of the raw material before the quenching treatment, and if the amount exceeds 0.10% in total, they are segregated at grain boundaries, resulting in degradation of workability such as fine blanking property. Therefore, it is preferable to make content of at least 1 sort (s) of Sb and Sn into 0.003 to 0.10% in total.

2) Microstructure

As a result of investigating the influence of the microstructure on the fine blanking cross-sectional properties, the present inventors have found that 100% shear planes do not obtain smooth cross-sections or cracks occur in the blanking cross-sections. It is evident that this is due to the destroyed cementite. Therefore, in order to obtain excellent fine blanking property, it is effective to suppress the formation of the cornerstone ferrite phase and the pearlite as much as possible, but the area ratio of the cornerstone ferrite phase occupied in the whole structure of the hot rolled steel sheet in the wound state is 10% or less, and spherical carbides are formed. When the microstructure has a saturation rate of 50% or more, a cross section having a 100% shear surface is obtained smoothly, and excellent fine blanking property in which no crack occurs in the cross section is obtained. In addition, the structures other than the saltpeter ferrite phase are pearlite and / or bainite. Inevitably, other structures, such as martensite, may be produced, but if it is 5% or less in area ratio, it does not affect the steel sheet characteristics of the present invention and is acceptable.

Here, the area ratio of each tissue analyzed and calculated | required the tissue photograph image | photographed with the scanning electron microscope using image analysis software.

In addition, the spheroidization rate of carbide calculates the ratio a / b of the maximum diameter (a) and the minimum diameter (b) of each carbide, and is a ratio (%) with respect to the total number of carbides of carbide water whose a / b is three or less. Obtained.

3) Manufacturing conditions

Finishing temperature at the time of hot rolling ： 800-950 degreeC

If the finishing temperature is less than 800 ° C, the cornerstone ferrite phase is likely to be formed during subsequent cooling, and the area ratio of the cornerstone ferrite phase exceeds 10%. On the other hand, when finishing temperature exceeds 950 degreeC, scale thickness will increase, the scale residue after scale peeling and pickling will generate | occur | produce, and the steel plate surface property will deteriorate. Therefore, the finishing temperature at the time of hot rolling shall be 800-950 degreeC.

Cooling condition after hot rolling: It cools to the cooling stop temperature of 450-600 degreeC from the cooling start temperature of 650 degreeC or more at the average cooling rate of 50 degreeC / s or more, and winds up within 3 s. In order to obtain a microstructure having excellent fine blanking properties as described above, the austenitic single phase state is maintained as much as possible after hot rolling, transformed after winding to suppress the formation of the cornerstone ferrite phase, and spheroidization of carbides using transformation heat is performed. It is possible by promoting. To this end, it cools from the cooling start temperature of 650 degreeC or more which is austenite single-phase state to the cooling stop temperature of 450-600 degreeC of temperature range corresponded to the nose of a continuous transformation curve at the average cooling rate of 50 degreeC / s, and winds up within 3 s. Needs to be.

In addition, when the steel sheet after the winding is charged into the heat retention apparatus in the wound state, the steel sheet is further cooled from the temperature at the time of winding, so that the spheroidization of the carbide is promoted to obtain better fine blanking property.

Moreover, it does not need to specifically limit about cooling to the cooling start temperature of 650 degreeC or more after hot rolling, What is necessary is just to adjust suitably according to a manufacturing line. After sufficient recrystallization of the austenite phase in order to reduce anisotropy, it is preferable to set it to air cooling, and in order to stop cooling stably at 450-600 degreeC, it is 650 which is the temperature near the lower limit of the supercooled austenite single phase region. It is preferable to cool to ℃.

To solvent the high carbon steel of the present invention, any of a converter and an electric furnace can be used. In addition, the high-carbon steel thus dissolved is turned into a slab by ingot-fracture rolling or continuous casting. The slab is typically heated and then hot rolled. In addition, in the case of the slab manufactured by continuous casting, you may apply the direct rolling rolling which keeps heat and rolls as it is or for the purpose of suppressing temperature fall. Moreover, when heating a slab and hot rolling, it is preferable to make slab heating temperature into 1280 degrees C or less in order to avoid deterioration of the surface state by a scale. In hot rolling, in order to ensure a finishing temperature, you may heat a to-be-rolled material by heating means, such as a sheet bar heater, during hot rolling.

Example

The hot rolled sheet steel No. which has a plate thickness of 5.0 mm was melt | dissolved in the steel which has a composition of the steel numbers A-K shown in Table 1, and then shown in Table 2. 1-17 were manufactured.

About the hot-rolled steel sheet produced in this way, the area ratio of the cornerstone ferrite phase and the spheroidization rate of carbide were calculated | required by the said method, and the structure other than the cornerstone ferrite phase was also observed by the said method.

Moreover, about fine blanking property, the test piece of 80 mm x 80 mm is extract | collected from the obtained steel plate, and the steel piece of 60 mm x 40 mm is removed using a 110 ton hydraulic press machine: between tool clearance: 0.050 mm (1% of plate thickness) And blanking under the conditions of working force: 8.5 ton and lubrication: with a blanking cross section of 100% shear surface, and no crack in the cross section.

The results are shown in Table 2.

Hot rolled steel sheet No. of the present invention In 1-4, 11, and 12, it turns out that all the fine blanking properties are excellent in (circle). In addition, the steel structure of the example of this invention was a pearlite and / or bainite other than the cornerstone ferrite phase of Table 2.

Claims (7)

In mass%, C: 0.10 to 0.40%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.03% or less, N: 0.01% or less, Al: 0.10% or less, and High carbon having excellent fine blanking properties, characterized in that the balance has a composition composed of Fe and inevitable impurities, and has a microstructure in which the area ratio of the cornerstone ferrite phase occupies the entire structure is 10% or less, and the spheroidization rate of the carbide is 50% or more. Hot rolled steel plate. The method of claim 1,
Furthermore, the high carbon hot rolled steel sheet excellent in the fine blanking property containing B: 0.0010 to 0.0050% by mass%, and satisfy | filling the relationship of 0.50 <= (14 [B]) / (10.8 [N]) However, [B] and [N] represent content (mass%) of B and N, respectively. 3. The method according to claim 1 or 2,
Furthermore, the high carbon hot rolled steel sheet excellent in the fine blanking property characterized by containing at least 1 type of Ti, Nb, and V in mass% in total: 0.1% or less. The method according to any one of claims 1 to 3,
Furthermore, the high carbon hot rolled steel sheet excellent in the fine blanking property characterized by containing at least 1 type: 1.5% or less in total of Ni, Cr, and Mo by mass%. The method according to any one of claims 1 to 4,
Furthermore, the high carbon hot rolled sheet steel excellent in the fine blanking property characterized by containing 0.003-0.10% by mass in total at least 1 type: Sb and Sn by mass%. After hot-rolling the steel which has a composition as described in any one of Claims 1-5 at the finishing temperature of 800-950 degreeC, it is 450-600 at an average cooling rate of 50 degreeC / s or more from the cooling start temperature of 650 degreeC or more. A method for producing a high carbon hot rolled steel sheet excellent in fine blanking characteristics, which is cooled to a cooling stop temperature of ° C and wound up within 3 s. The method according to claim 6,
A method for producing a high carbon hot rolled steel sheet excellent in fine blanking property, wherein the steel sheet after winding is charged into a heat retention apparatus in a wound state.