JP5549450B2 - High carbon hot-rolled steel sheet excellent in fine blanking property and manufacturing method thereof - Google Patents

Description

  The present invention is a high-carbon hot-rolled steel sheet suitable for applications such as automobile parts, in particular, a high-carbon hot-rolled steel sheet that ensures excellent fine blanking property (precision punching property) without annealing after hot rolling and its It relates to a manufacturing method.

  Automotive parts with complex shapes such as gears, missions, and seat recliners are made into the desired shapes by cutting or blanking high-carbon steel plates for machine structures specified in JIS G 4051, or by further expanding holes. After being processed, it is manufactured by quenching and tempering in order to ensure hardness. Recently, from the viewpoint of improving dimensional accuracy and simplifying the manufacturing process, i) a smooth blanking end surface with almost 100% shear surface can be obtained, ii) high dimensional accuracy, and iii) complex shapes once. Fine blanking has been adopted, which has the advantage of being able to process with blanking.

  In fine blanking, the clearance between tools is set extremely small, from about 5 to 10% of the steel sheet thickness in normal blanking to about 2% or less of the steel sheet thickness, and compressive stress is applied to the steel sheet near the tool cutting edge. This is a blanking method. Therefore, since the high carbon steel sheet as the blanking material is subjected to extremely severe processing, a smooth blanking end surface having a 100% shear surface cannot be obtained, and cracks are likely to occur on the blanking surface.

  Therefore, a high carbon steel sheet that does not cause such a problem, that is, excellent in fine blanking properties has been proposed. For example, in Patent Document 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% A steel sheet for precision punching containing the following, with the balance being composed of Fe and inevitable impurities and excellent short-time rapid heating and quenching properties is disclosed. Patent Document 2 includes, in mass%, C: 0.15-0.90%, Si: 0.4% or less, Mn: 0.3-1.0%, P: 0.03% or less, total Al: 0.10% or less, and the balance is substantially Fe. It has a composition with a spheroidization rate of 80% or more and a structure in which carbides with an average particle size of 0.4 to 1.0 μm are dispersed in a ferrite matrix, and it opens on both sides in the longitudinal direction of the parallel part of the JIS No. 5 tensile test piece. Tensile test using a specimen with a V notch with a 45 degree angle and a depth of 2 mm, and the notch tensile elongation expressed as the elongation after fracture for a gauge distance of 10 mm at the center in the longitudinal direction of the parallel part is 20% or more A high carbon steel sheet excellent in precision punchability is disclosed. Patent Document 3 includes mass%, C: 0.15-0.45%, Si: 0.25% or less, Mn: 0.3-1.2%, P: 0.02% or less, S: 0.02% or less, Al: 0.01-0.1%, N : 0.008% or less, the balance is inevitable impurities and Fe composition, the ratio of pearlite + cementite is 10% or less, and the average grain size of ferrite grains is 10 to 20μm. A carbon steel sheet is disclosed. Patent Document 4 includes, in mass%, C: 0.1 to 0.5%, Si: 0.5% or less, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.02% or less, the remaining Fe and inevitable impurities And a structure mainly composed of ferrite and carbide, the ferrite has an average particle size of 1 to 10 μm, the spheroidization rate of the carbide is 80% or more, and among the carbides, ferrite crystals Amount of carbides present at grain boundaries Sgb = {Son / (Son + Sin)} × 100 (Son: total carbide occupied area on ferrite grain boundaries out of carbides present per unit area, Sin: unit A steel plate excellent in fine blanking property is disclosed in which the total occupied area of carbides present in ferrite grains among carbides present per area is 40% or more.

  Among these, the steel sheet described in Patent Document 1 is an as-rolled hot-rolled steel sheet, and the steel sheets of Patent Documents 2 to 4 are annealed hot-rolled steel sheets after winding, or are annealed after cold rolling to form carbides. This is a high carbon steel plate that has been spheroidized (in Patent Documents 2 and 4, the spheroidization rate is 80% or more).

JP 59-76861 A JP 2000-265240 A Japanese Patent Laid-Open No. 2001-140037 JP 2007-231416 A

  However, in the as-rolled hot-rolled steel sheet described in Patent Document 1, a smooth blanking end face having a 100% shear plane cannot be obtained, cracks are easily generated on the blanking face, and the fine blanking property is poor. In the steel sheets of Patent Documents 2 to 4, since the spheroidization of carbide exceeding 80% is achieved, good fine blanking properties can be obtained, but the hot-rolled steel sheet after winding is annealed or cooled. It is necessary to anneal after hot rolling, which is expensive.

The present invention is a hot-rolled steel sheet baked Dong without Rukoto after winding, i.e. at low cost, aims to provide an excellent fine blanking of high carbon hot-rolled steel sheet remains winding having and a manufacturing method thereof And

  As a result of intensive studies on high-carbon hot-rolled steel sheets that provide excellent fine blanking properties while being wound, the present inventors have found the following.

  i) If the microstructure is such that the area ratio of the pro-eutectoid ferrite phase in the entire structure is 10% or less and the spheroidization ratio of carbide is 50% or more after optimizing the component composition, it is excellent even when wound. Fine blanking performance can be obtained.

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

  The present invention has been made 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% Below, N: 0.01% or less, Al: 0.10% or less, the balance is composed of Fe and inevitable impurities, the area ratio of the pro-eutectoid ferrite phase in the entire structure is 10% or less, the carbide spherical Provided is a high carbon hot-rolled steel sheet excellent in fine blanking property, characterized by having a microstructure with a conversion ratio of 50% or more.

  In the high carbon hot-rolled steel sheet of the present invention, in addition to the above composition, B: 0.0010 to 0.0050% is contained in mass%, and the relationship of 0.50 ≦ (14 [B]) / (10.8 [N]) is satisfied. It is preferable to make it. However, [B] and [N] represent B and N contents (% by mass), 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, at least of Sb and Sn 1 type: It is preferable to contain 0.003-0.10% in total individually or simultaneously.

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

  In the production method of the present invention, it is preferable that the steel plate after winding is inserted into the heat retaining device in a wound state.

  According to the present invention, it is possible to produce a high-carbon hot-rolled steel sheet as wound with low cost and excellent fine blanking properties. The high carbon hot-rolled steel sheet of the present invention is suitable for automotive parts such as gears, missions, and seat recliners.

  Below, the high carbon hot-rolled steel sheet and its manufacturing method which are this invention are demonstrated in detail. Note that “%”, which is a unit of content of components, means “mass%” unless otherwise specified.

1) Composition
C: 0.10 ~ 0.40%
If the amount of C is less than 0.1%, the hardness after quenching required for automobile parts cannot be obtained. On the other hand, if the C content exceeds 0.40%, the steel sheet becomes hard, the load on the die during blanking becomes too large, and the blanking die life cannot be secured to an industrially sufficient level. Therefore, the C content is 0.10 to 0.40%.

Si: 1.0% or less
If the Si content exceeds 1.0%, the ferrite phase becomes hard and the fine blanking property is lowered, or a surface defect called red scale occurs. Therefore, the Si content is 1.0% or less, preferably 0.35% or less.

Mn: 2.0% or less
If the amount of Mn exceeds 2.0%, the ferrite phase becomes hard and the fine blanking property decreases. Therefore, the Mn content is 2.0% or less. In order to improve hardenability, the Mn content is preferably 0.2% or more.

P: 0.03% or less
When the amount of P exceeds 0.03%, segregation occurs at grain boundaries and the like, and the workability such as fine blanking is deteriorated. Therefore, the P content is 0.03% or less, preferably 0.02% or less, but the smaller the amount, the more preferable.

S: 0.03% or less
If the amount of S exceeds 0.03%, sulfides are formed, and workability such as fine blanking properties is reduced, and toughness after quenching is reduced. Therefore, the S content is 0.03% or less, preferably 0.02% or less.

N: 0.01% or less
N forms a nitride with Al and the like and has the effect of preventing coarsening of austenite grains. However, if the amount of N exceeds 0.01%, the workability deteriorates. Therefore, the N content is 0.01% or less, preferably 0.005% or less.

Al: 0.10% or less
If the Al content exceeds 0.10%, not only the cleanliness of the steel will decrease, but also the austenite grains will become too fine during the heating of the quenching process, the formation of ferrite phase will be promoted during cooling, and the necessary hardness after quenching will be obtained Disappear. Therefore, the Al content is 0.10% or less. Al has a function of forming AlN by combining with N to prevent coarsening of austenite grains and effectively exhibit the hardenability by B. Therefore, the Al content should be 0.01% or more. Is preferred.

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

B: 0.0010-0.0050%
B is an element that enhances hardenability and also improves the toughness after quenching by forming BN during heating in the quenching process and suppressing coarsening of austenite grains. In order to obtain such effects, the B amount needs to be 0.0010% or more. On the other hand, if the amount of B exceeds 0.0050%, the hot rolling load becomes high and the operability is lowered, and the workability such as fine blanking is lowered. Therefore, the B content is preferably 0.0010 to 0.0050%, and more preferably 0.0010 to 0.0030%. Further, in order to improve the hardenability by B, it is necessary that the solid solution B exists during the heating of the quenching treatment. However, B has a large affinity with N, and it is easy to form BN at the time of hot rolling and subsequent winding or heating in the quenching process. Therefore, when B is contained, 0.50 ≦ (14 [B ]) / (10.8 [N]) It is necessary to secure solid solution B so as to satisfy the relationship. Here, [B] and [N] represent B and N contents (% by mass), respectively.

At least one of Ti, Nb and V: 0.1% or less in total
If the content of at least one of Ti, Nb, and V exceeds 0.1% in total, the ferrite phase becomes hard due to precipitation of TiC and the like, leading to a reduction in the life of the blanking die. Therefore, the content of at least one of Ti, Nb, and V is preferably 0.1% or less in total. Ti, Nb, and V easily bond to N to form nitrides, prevent austenite grains from coarsening during quenching, and prevent formation of BN when improving hardenability with B. Therefore, it is more preferable that the content of at least one of these elements is 0.01% or more in total.

At least one of Ni, Cr, and Mo: 1.5% or less in total
When the content of at least one of Ni, Cr, and Mo exceeds 1.5% in total, the ferrite phase becomes hard and the fine blanking property decreases. Therefore, the content of at least one of Ni, Cr, and Mo is preferably 1.5% or less in total. Since Ni, Cr, and Mo effectively act to improve hardenability, the content of at least one of Ni, Cr, and Mo is more preferably 0.1% or more in total.

At least one of Sb and Sn: 0.003-0.10% in total
When the heating for quenching is performed in an atmosphere where the carbon potential is controlled by mixing air at around 900 ° C for about 1 hour, nitrogen absorption occurs, and AlN and BN are formed in the surface layer of the steel, resulting in austenite grains. May be excessively refined, or the amount of dissolved B may be remarkably reduced and hardenability may be reduced. Therefore, the present inventors added Sb and Sn, which are known as elements capable of suppressing the absorption of steel, and examined the hardenability. The total amount of at least one of Sb and Sn It was found that when the content is 0.003% or more, nitrogen absorption can be suppressed and the hardenability of the surface layer portion can be improved. However, if the total amount of at least one of Sb and Sn exceeds 0.10%, segregation occurs at the austenite grain boundaries during quenching and tempering, and the toughness is significantly reduced. Furthermore, these elements also affect the workability of the raw material before quenching, and when the total amount exceeds 0.10%, segregation occurs at the grain boundaries, leading to a decrease in workability such as fine blanking. Therefore, the content of at least one of Sb and Sn is preferably 0.003 to 0.10% in total.

2) Microstructure When the present inventors investigated the influence of the microstructure on the fine blanking end face properties, it was not possible to obtain a smooth end face of 100% shear face, or cracks occurred on the blanking end face. It was revealed that this was caused by the interface between the pro-eutectoid ferrite phase and pearlite and the broken cementite in the pearlite. Therefore, in order to obtain excellent fine blanking properties, it is effective to suppress the formation of proeutectoid ferrite phase and pearlite as much as possible. If the microstructure has an area ratio of precipitated ferrite phase of 10% or less and a spheroidization rate of carbide of 50% or more, a smooth end face with a 100% shear plane can be obtained, and cracks can occur on the end face. Excellent fine blanking performance can be obtained. The structure other than the pro-eutectoid ferrite phase is pearlite and / or bainite. Inevitably, other structures such as martensite may be generated, but if the area ratio is 5% or less, the steel sheet characteristics of the present invention are not affected and are allowed.

  Here, the area ratio of each tissue was determined by analyzing a tissue photograph taken with a scanning electron microscope using image analysis software.

  Further, the spheroidization rate of the carbide was calculated as a ratio (%) of the number of carbides having a / b of 3 or less to the total number of carbides by calculating the ratio a / b of the maximum diameter a and the minimum diameter b of each carbide. .

3) Manufacturing conditions Finishing temperature during hot rolling: 800-950 ° C
When the finishing temperature is less than 800 ° C., the pro-eutectoid ferrite phase is likely to be formed during the subsequent cooling, and the area ratio of the pro-eutectoid ferrite phase exceeds 10%. On the other hand, when the finishing temperature exceeds 950 ° C., the scale thickness increases, and the scale residue after scale peeling or pickling occurs and the steel sheet surface properties deteriorate. Therefore, the finishing temperature during hot rolling is set to 800 to 950 ° C.

Cooling conditions after hot rolling: 450 at an average cooling rate of 50 ° C / s or more from a cooling start temperature of 650 ° C or more
Cooled to cooling stop temperature of to 600 ° C., the winding take excellent fine blanking of the above is the microstructure obtained within 3s maintains the austenite single-phase state as possible after the hot rolling It is possible to transform after winding to suppress the formation of pro-eutectoid ferrite phase and to promote the spheroidization of carbide using transformation heat. For this purpose, the cooling is started from a cooling start temperature of 650 ° C. or more in the austenite single phase state to a cooling stop temperature of 450 to 600 ° C. corresponding to the nose of the continuous transformation curve at an average cooling rate of 50 ° C./s or more, It is necessary to wind up within 3s.

  In addition, if the steel plate after winding is inserted into the heat retaining device in a wound state, the steel plate is further gradually cooled from the temperature at the time of winding, so that the spheroidization of the carbide is promoted and the finer finer. Ranking is obtained.

  Moreover, it is not necessary to specifically limit cooling to the cooling start temperature of 650 ° C. or higher after hot rolling, and it may be appropriately adjusted according to the production line. In order to sufficiently recrystallize the austenite phase in order to reduce the anisotropy, it is preferable to use air cooling, and in order to stably stop cooling at 450 to 600 ° C., the supercooled austenite single phase region It is preferable to cool to 650 ° C., which is a temperature close to the lower limit.

  To melt the high carbon steel of the present invention, both a converter and an electric furnace can be used. Further, the high carbon steel thus melted is made into a slab by ingot-bundling rolling or continuous casting. The slab is usually heated and then hot rolled. In addition, in the case of the slab manufactured by continuous casting, you may apply the direct feed rolling which heats as it is or keeps heat in order to suppress a temperature fall. Moreover, when heating and rolling a slab, it is preferable to make slab heating temperature 1280 degrees C or less in order to avoid the deterioration of the surface state by a scale. In hot rolling, in order to ensure the finishing temperature, the material to be rolled may be heated by a heating means such as a sheet bar heater during hot rolling.

  Steels having compositions of steel numbers A to K shown in Table 1 were melted, and then hot-rolled steel plates No. 1 to 17 having a thickness of 5.0 mm were produced according to the hot rolling conditions shown in Table 2.

  With respect to the hot-rolled steel sheet thus produced, the area ratio of the pro-eutectoid ferrite phase and the spheroidization ratio of the carbide were determined by the above method, and the structure other than the pro-eutectoid ferrite phase was also observed by the above-described method.

  Also, for fine blanking, 80mm x 80mm test pieces were taken from the obtained steel plate, and using a 110ton hydraulic press machine, a 60mm x 40mm steel piece was removed with a tool clearance of 0.050mm (1 mm of plate thickness). %), Processing force: 8.5 ton, lubrication: blanking was performed, and the blanking end face was 100% shear face and the end face was not cracked.

  The results are shown in Table 2.

  It can be seen that the hot blanking steel sheets Nos. 1 to 4, 11 and 12 of the inventive examples are all excellent in fine blanking property. The steel structures of the examples of the present invention were pearlite and / or bainite except for the pro-eutectoid ferrite phase shown in Table 2.

Claims (7)

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: 0.01% or less, Al: 0.01-0.10 % included, The balance is composed of Fe and unavoidable impurities, and has a microstructure in which the area ratio of the pro-eutectoid ferrite phase in the entire structure is 10% or less and the spheroidization ratio of carbide is 50% or more. fine blanking excellent in winding high-carbon hot-rolled steel sheet remains up to. The fine fiber according to claim 1, further comprising, in mass%, B: 0.0010 to 0.0050%, and satisfying a relationship of 0.50 ≦ (14 [B]) / (10.8 [N]). high-carbon hot-rolled steel sheet remains winding having excellent ranking properties; however, represents a [B], [N], respectively of B, the content of N (mass%). Further, by mass%, containing at least one of Ti, Nb, and V: a total of 0.1% or less, the winding as excellent in fine blanking property according to claim 1 or 2 High carbon hot rolled steel sheet. Furthermore, it contains at least one of Ni, Cr, and Mo in mass%: 0.50 to 1.5 % in total, and has excellent fine blanking properties according to any one of claims 1 to 3. Unrolled high carbon hot rolled steel sheet. Furthermore, the winding excellent in fine blanking property according to any one of claims 1 to 4, characterized by containing at least one of Sb and Sn: 0.003 to 0.10% in total by mass% . Remaining high carbon hot rolled steel sheet. The steel having the composition according to any one of claims 1 to 5 is hot-rolled at a finishing temperature of 800 to 950 ° C, and then 450 ° C at an average cooling rate of 50 ° C / s or more from a cooling start temperature of 650 ° C or more. A method of producing a high carbon hot rolled steel sheet as wound with excellent fine blanking properties, characterized by cooling to a cooling stop temperature of ˜600 ° C. and winding within 3 s. The method for producing a high carbon hot-rolled steel sheet as wound with excellent fine blanking property according to claim 6, wherein the steel sheet after winding is inserted into a heat retaining device in a wound state. .