JP5483914B2 - Carburized annealed steel strip with good punchability and method for producing the same - Google Patents

Description

The present invention relates to a carburized and annealed steel strip having good punchability, which can reduce the “sag” at the time of punching of structural parts and the like, and can reduce the cutting allowance of a product, and a manufacturing method thereof.

  In the manufacturing process of structural parts, after a soft annealed material is punched into a product shape, carburizing and quenching and quenching processes are performed for the purpose of ensuring strength and toughness. Then cut to product dimensions. However, when a soft annealed material is punched, the amount of dripping is large, so that the cutting allowance becomes large and a great amount of work time is required. For this reason, a reduction in punching is required. In Patent Document 1, C: 0.15 to 0.25% steel is used to increase the strength by combining ferrite grain refinement and cold rolling to reduce punching. Further, in Patent Document 2, C: 0.7 to 0.9% steel is used to define the carbide grain size, length, spheroidization rate and hardness after annealing to ensure punchability. However, since the strength is increased by cold rolling or increasing carbon, the properties and die life of the punched end face are not always sufficient.

  On the other hand, the omission of carburizing, which takes a long processing time, is being studied for cost reduction. Carburizing methods include solid carburizing method, liquid carburizing method, gas carburizing method, vacuum carburizing method, etc., all of which are generally heated at 850 ° C or higher, and require a long time at high temperature, requiring enormous energy. It becomes. In addition, expensive carburizing equipment is necessary, and there is a drawback in terms of economy. Further, since it is necessary to make the carburizing gas uniformly contact the surface of the object to be processed, it is limited to the size of the carburizing box, salt bath, heating furnace, etc., and mass production of large parts is difficult.

Japanese Patent Laid-Open No. 2004-292939 Japanese Patent Laid-Open No. 2005-336560 JP 2000-119838 A JP 2001-107217 A JP 2001-115249 A

  Therefore, the present invention reduces the “dripping” at the time of punching when processing into parts by using a method of increasing the surface carbon concentration of the material at the stage of the material (steel strip) before being subjected to punching. An object of the present invention is to provide a steel strip that can reduce the cutting allowance and a steel strip that can omit the carburizing treatment after punching.

The purpose is that the average hardness up to 50 μm depth of the surface layer portion in the plate thickness direction is 170 HV or more, the metal structure is ferrite + pearlite, the surface carbon concentration C _S (mass%) and the average carbon concentration C _{M in} steel. This is achieved by a carburized and annealed steel strip having a good punching property in which the difference (mass%) ΔC = C _S −C _M is 0.1 mass% or more. The component composition of steel is, for example, mass%, C: 0.03 to 0.80%, Si: 0.01 to 1.50%, Mn: 0.20 to 2.0%, P: 0.04. %: S: 0.03% or less, Cr: 0 to 1.50%, T. Al: 0.010 to 0.10%, remaining Fe and inevitable impurities.

In the present invention, as a method for producing such a carburized and annealed steel strip having good punchability, a carburized paint containing 5% by mass or more of a solid carburizing agent is applied to the surface of the steel strip to form a dry paint film, and then the steel The strip is wound in a coil shape so that the contact pressure between adjacent (phase-contacting) plate surfaces sandwiching the coating film is 0.2 MPa or more, and the coil is inserted into a furnace in that state to be inert. Provided is a method of cooling at a temperature of at least (Ac ₁ -100 ° C.) at an average cooling rate of 15 ° C./h or more after heating at 720 to 850 ° C. in a gas atmosphere for 3 hours or more.

The annealed steel strip of the present invention has the following merits.
(1) Since the surface carbon concentration is already high, the “sag” at the time of punching is reduced as compared with the case where a normal annealing material is used.
(2) The center part is soft, and the life of the punching die can be extended as compared with the case where high carbon steel is used.
(3) Since the surface carbon concentration is already high, there is no need to newly carburize after punching. For this reason, carburizing treatment can be omitted and cost reduction can be achieved.
(4) Since it can be provided as a coil product having a high surface carbon concentration, it can be applied to parts having a wide area.
(5) Since the surface can be hardened by ordinary quenching and tempering treatment or induction quenching and tempering, it is not necessary to expose parts to high temperature and long time like carburizing treatment, and austenite grains are also relatively small, so toughness is required. Suitable for parts.

The graph which showed the hardening hardness distribution in a coil about the annealed steel strip of this invention. An example of an optical microscope photograph in which the influence of the contact surface pressure on the cross-sectional structure after annealing is examined.

  In the present invention, the method for increasing the surface carbon concentration is not a gas carburizing method that is generally performed, but is a coating type carburizing in which a carburizing paint containing a solid carburizing agent is continuously applied to the surface of the steel strip and then annealed. .

  The coating type carburizing method is disclosed in Patent Documents 3 to 5 and the like. All of these are methods of applying a carburizing composition (water-based paint in which charcoal is dispersed in an olefin polymer resin) and carburizing at a temperature of 900 to 1200 ° C. in the atmosphere. It is a method of carburizing as it is. However, in these methods, since the temperature is 900 ° C. or higher and heating is performed in the atmosphere, there are problems of generation of oxide scale on the steel strip surface and decarburization of the edge portion. Further, since the carburizing temperature is high, the crystal grains become coarse, and the fatigue characteristics and toughness (impact value) may be reduced.

  In this invention, it is going to implement | achieve at once that ensuring the favorable punching property of a raw material and making it possible to abbreviate | omit the carburizing process after punching. As the technique, a steel strip (steel plate) coated with a carburizing agent is wound in a coil shape and subjected to a normal annealing process, and the normal annealing process also serves as a carburizing process. At that time, by optimizing the contact pressure between the surfaces of the steel strip when wound in a coil shape, the carburizing gas is stably brought into contact with the surface of the steel, and is in a range of 720 to 850 in the normal annealing condition range. Carburization proceeds at a relatively low temperature of ℃. Since the annealing atmosphere is an inert gas, oxidation scale on the surface of the steel strip and decarburization of the edge, which are problems in conventional carburizing in the atmosphere, are avoided, and the carbon concentration is stable in the plate width direction of the steel strip. An annealed steel strip having a high surface layer can be obtained.

Hereinafter, the present invention will be described more specifically.
〔steel〕
The steel as a raw material is targeted for various steel types, and is not particularly limited to carburizing steel. The reason is that, according to the present invention, the carburizing gas generated between adjacent surfaces of the steel strip wound in a coil shape is brought close to the steel surface by optimizing the contact pressure between the surfaces. Even if the carburizing steel is not used, the difference ΔC = C _S −C _M between the surface carbon concentration C _S (% by mass) and the average carbon concentration C _M (% by mass) in the steel is 0.1% by mass. This is because it is possible to increase the surface carbon concentration to be at least%.

Specific examples of steel components include steel types having the following component composition ranges.
By mass%, C: 0.03 to 0.80%, Si: 0.01 to 1.50%, Mn: 0.20 to 2.0%, P: 0.04% or less, S: 0.03 % Or less, Cr: 0 to 1.50%, T. Al: 0.010 to 0.10%, balance Fe and inevitable impurities

[Difference between surface carbon concentration C _S and average carbon concentration C _{M in} steel ΔC = C _S −C _M ]
In the steel having the above C content, when the surface layer portion is carburized so that the ΔC is 0.1% by mass or more, an appropriate hardness as an annealed material is ensured, and a remarkable reduction in punching is achieved. An effect is produced. Moreover, high surface hardness can be obtained after hardening.

Here, the surface carbon concentration C _S is obtained as follows.
First, using a steel sample having a chemical analysis value of C in the range of 0.03 to 0.8% by mass, the detection amount (count) of C characteristic X-rays by EPMA (electron beam microanalyzer), C The relationship between the chemical analysis values of C is examined, and a calibration curve of C in the EPMA is prepared.
The surface layer of the annealed steel strip of the present invention, which is a material to be measured, has a ferrite + pearlite structure, and the surface carbon concentration varies greatly due to the structure. For this reason, the sample cut out from the steel strip is quenched to produce a measurement sample in which the variation in carbon concentration in the surface layer is uniform. Quenching is performed by immersing the sample in a salt bath at 900 ° C., holding the sample temperature for a time approximately matching the salt bath temperature (for example, 60 seconds), and then quenching with water. Thereafter, the scale on the sample surface is removed by pickling, and after degreasing, a nickel plating layer (for example, about 50 μm thick) is formed on the surface. A sample after nickel plating is embedded in a resin, and a cross section parallel to the plate thickness direction is polished, and this is used as a measurement sample. The EPMA performs C line analysis in the thickness direction from the nickel plating layer side of the measurement sample, and obtains the maximum value of C detection amount (count) from the position where nickel plating has been completed to a depth of 200 μm in the thickness direction. This value is converted into C concentration (mass%) by the above calibration curve, and this is defined as the surface carbon concentration C _S.
The average carbon concentration C _M in steel is the C content in the chemical analysis value of steel.

[Solid carburizing agent]
As the solid carburizing agent, which is the main component of the carburizing paint, charcoal powder, graphite, carbon black and the like can be used, and commercially available agents having these as components may be used. The concentration of the solid carburizing agent in the carburized coating is preferably 5 to 20% by mass. When the carburizing agent is less than 5% by mass, the carburizing effect is insufficient, and when the parts produced using the obtained steel strip (steel plate) are subjected to quenching and tempering treatment, the same fatigue as the material produced by ordinary gas carburizing Characteristics and wear characteristics may not be obtained sufficiently. Moreover, even if the coating material containing a carburizing agent exceeding 20 mass% is applied, the carburizing effect is saturated and not effective at an annealing temperature of about 720 to 850 ° C.

The average particle size of the solid carburizing agent is preferably 0.5 μm or less. In general, the smaller the particle size, the more likely it is to uniformly disperse. However, when the particle size becomes too small, the solid carburizing agent particles aggregate, so the average particle size of the solid carburizing agent is adjusted to 0.2 to 0.5 μm. Is more preferable. As this average particle size, a volume-based cumulative average value D ₅₀ obtained by a laser diffraction particle size distribution analyzer can be adopted.

(Carburizing accelerator)
In general, a carburizing accelerator is added to the carburizing paint for the purpose of increasing the pressure of CO ₂ or the like to promote carburizing. The carburizing accelerator is not particularly limited as long as it does not generate a gas such as CO ₂ when added to the base water-based paint and generates a gas only when heated to the carburizing temperature. Although it is selected according to the type of aqueous medium to be used and the method for forming the film, carbonate is preferred. For example, what contains 5 mass% or more of sodium carbonate or barium carbonate becomes a suitable object. However, the effect is saturated even if blended in excess, so it may be, for example, 20% by mass or less.

[Dispersant]
As a dispersant blended in the carburized paint, water-soluble polymers generally used for paints and the like can be used. For example, natural polymer, polyacrylic acid, styrene-maleic acid copolymer, styrene-acrylic acid copolymer, polyvinyl alcohol, polyethylene glycol and the like can be mentioned. In consideration of the properties as a water-based paint, the formation of a film on an object to be processed, and workability, it is preferable to use a powder having an average particle size of about 1 to 3 μm. Moreover, it is preferable to mix | blend these water-soluble polymers in the ratio of 10-40 mass parts with respect to 100 mass parts of water. If it is less than 10 parts by weight, the adhesion of the coating film tends to be insufficient, and if it exceeds 40 parts by mass, the coating film tends to peel off.
Moreover, in order to improve the wettability of a coating material and a steel plate, you may add a small amount of organic solvents, an antifoamer, etc.

〔Preprocessing〕
The steel strip to which the carburized paint is applied may be either a hot-rolled steel strip or a cold-rolled steel strip, but in the case of a hot-rolled steel strip, the oxide scale must be removed through pickling. is there. Before coating, it is preferable to carry out “pretreatment” in which the steel strip surface is washed in advance for the purpose of forming a uniform coating layer. Examples of the cleaning method include alkali degreasing and degreasing with an organic solvent.

[Coating method]
Examples of the method for applying the carburized paint to the surface of the steel strip include a dipping method, a roll method, a spray method, and a bar coating method. You may apply | coat to both surfaces of a steel strip, and may apply a carburizing coating only to one side. In either case, a carburized paint film can be interposed between adjacent steel sheet surfaces when wound in a coil shape. When the application amount of the carburizing paint is applied only to one surface, for example, if the adhesion amount after drying is 40 to 80 g / m ² , the carburizing effect is sufficiently exhibited. In the case of double-sided coating, the coating amount per side can be made smaller than the above amount.

[Dry]
After coating, the coating film is dried to form a film. Drying is intended to evaporate moisture, and heating at about 100 to 130 ° C. × 1 min is sufficient. Heating is carried out under conditions that do not cause boiling or melting of the water-soluble polymer.

[Contact pressure]
The steel strip having the above-described carburized coating dried coating film is wound in a coil shape so that the plate surfaces are in contact with each other with the coating film layer interposed therebetween. As a result of various studies, the inventors conducted stable heat carburization in a normal annealing temperature range of 720 to 850 ° C. when subjected to heat treatment with the contact pressure between adjacent plate surfaces raised to a certain level or more. I found out that it would be feasible. Specifically, it winds up in a coil shape so that the contact pressure between the plate surfaces becomes 0.2 MPa or more. When heat treatment is performed with such a surface pressure applied, the carburizing gas generated by the thermal decomposition of the carburizing agent component (such as charcoal powder) and the carburizing accelerator in the carburized coating film is caused between the plate surfaces of the steel plate. The carburization of the steel sheet surface proceeds efficiently.

Although carburization occurs even when the contact pressure is less than 0.2 MPa, the difference ΔC between the surface carbon concentration C _S (mass%) and the average carbon concentration C _M (mass%) in the steel is stably set to 0.1 mass% or more. It is extremely effective to ensure a contact pressure of 0.2 MPa or more, and more preferably 0.4 MPa or more. The higher the contact pressure, the better the carburizability, but if the contact pressure is excessive, the carburizing agent particles (charcoal, etc.) intervening between the steel plates are pushed into the steel plate surface, which may cause flaws. As a result of various studies, the contact pressure is preferably 20 MPa or less, and more preferably controlled to a range of 10 MPa or less.

  This contact pressure can be adjusted by the tension when the steel strip is wound into a coil. The relationship between the contact pressure and the winding tension may be examined in advance using a pressure sensitive paper, a load cell, a strain gauge, or the like. Collect data. After winding, the entire coil may be tightened with a coil band or the like as necessary so that a contact pressure of 0.2 MPa or more can be maintained.

[Annealing atmosphere]
The steel strip coiled with the carburized coating interposed between the plate surfaces as described above is charged into the furnace in that state and annealed. The annealing atmosphere is an inert gas. As the inert gas, argon gas, nitrogen gas, NX gas (nitrogen-based modified gas) or the like can be applied, and NX gas is preferred industrially. In a reducing gas containing hydrogen, hydrogen and C of the carburizing agent may react to generate CH ₄ and H ₂ O, which may cause decarburization. Further, in the oxidizing gas, the carburizing agent may burn and carburization may not occur, and the steel plate may be oxidized and decarburized. The dew point of the inert gas atmosphere is preferably -40 ° C or lower.

[Annealing conditions]
Heating at 720 ° C or higher for 3 hours or longer is required. If it is less than 720 ° C., the diffusion rate of carbon is extremely slow, and therefore carburization is difficult to proceed effectively. It is also effective to set the temperature to 720 ° C. or higher so that the solid carburizing agent decomposes to produce carburizing gas (for example, a general charcoal decomposition reaction is 700 ° C. or higher). The higher the heating temperature, the faster the carbon diffusion rate and the better the carburizing property. However, in the present invention, using normal annealing equipment, carburizing is performed in a process that also serves as steel annealing. Range. In order to make the temperature higher than that, it is necessary to prepare a carburizing furnace separately, and the advantage of the present invention that carburizing is possible at a relatively low temperature cannot be utilized. The difference ΔC between the surface carbon concentration C _S (mass%) and the average carbon concentration C _M (mass%) in the steel is secured by ensuring a heating holding time (soaking time during which the coil central portion is maintained at the target temperature) for 3 hours or more. Can be made 0.1 mass% or more. Considering the uniformity of the temperature in the coil, it is more preferable to set it to 10 hours or more. However, when heated for a too long time, a phenomenon that the extreme surface layer portion of the steel sheet is decarburized may be seen. As a result of various studies, it is desirable that the heating and holding time be in a range of 30 hours or less.

An annealing structure of ferrite + pearlite can be obtained by setting the cooling rate after heating to 15 ° C./h or more. The surface layer portion having a high carbon concentration at 720 to 850 ° C. is partially in the austenite region, but when the cooling rate is less than 15 ° C./h, the pearlite becomes rough and the cementite becomes spheroidized. Hardness may decrease. In that case, the occurrence of “sag” due to punching may not be sufficiently suppressed. In addition, it is sufficient that the temperature from the heating temperature to the temperature of (Ac ₁ -100 ° C.) is an average cooling rate of 15 ° C./h or more, and the cooling rate in the lower temperature range is arbitrary. The upper limit of the average cooling rate up to the temperature of (Ac ₁ -100 ° C.) is not particularly limited as long as it is not burned, but it is desirable to set it within a range of 30 ° C./h or less for operation.

  Steel having the component composition shown in Table 1 was melted, and a hot rolled steel sheet having a thickness of 4 mm and a width of 920 mm was manufactured using an industrial hot rolling line. Then, it pickled and cold-rolled 50% and finished to a 2.0 mm cold-rolled steel strip.

  The carburized paint uses a general water-soluble polymer (manufactured by Nippon Pure Chemicals Co., Ltd., acrylic acid copolymer; Junron), and charcoal powder (manufactured by Parker Heat Treatment; KG6) as a carburizing agent to a particle size of 0.5 μm or less. Various water-based paints were prepared in which sodium carbonate as a carburizing accelerator was contained in substantially the same amount as the carburizing agent, and the balance was distilled water. The content of the carburizing agent and the carburizing accelerator is a ratio of all components excluding distilled water.

The cold-rolled steel strip is degreased with alkali, and then the carburized paint is continuously applied on one side by a roll method so as to have a dry coating amount of 40 g / m ^2, and then dried by hot air at 100 ° C. for 2 minutes and dried. After forming the coating film, the steel strip was continuously wound into a coil shape. In winding, the contact pressure between adjacent plate surfaces was adjusted by tension control so as to be in various stages in the range of 0 to 20 MPa. The obtained coil was clamped with an iron coil band so that the contact pressure did not decrease.

The coil in the above state was charged into a batch annealing furnace so that the end portions in the width direction of the coil were up and down. The annealing atmosphere is selected from argon, nitrogen, NX (N ₂ -based 3% H ₂ ), hydrogen, and air, and the heating temperature is 700 to 900 ° C., the holding time is ₁ to 30 hours, and the heating temperature is (Ac 1-100 Annealing was performed under various conditions with an average cooling rate of 5 to 300 ° C./h.

A sample was taken from the obtained annealed steel strip and the following investigation was conducted.
[Metallic structure of the surface layer]
The surface layer vicinity of the cross section perpendicular to the rolling direction (C cross section) was observed with an optical microscope, and the metal structure in the region from the surface to a depth of 50 μm was examined. What was a ferrite + pearlite structure was determined to be acceptable (◯ evaluation).

[Surface carbon concentration]
The surface was analyzed by EPMA as described above to determine the surface carbon concentration C _S (mass%).

[Difference from the carbon concentration in the center of the material ΔC]
The difference C _S −C _M between the surface carbon concentration C _S (mass%) and the average carbon concentration C _M (mass%) in the steel was defined as ΔC. When this value was 0.1% by mass or more, it was determined that the carburizing property was good (◯ evaluation).

[Average hardness up to 50μm on the surface]
For the region from the surface of the C cross section to the depth of 50 μm, measure the Vickers hardness of 5 points at 20 μm depth position, 30 μm depth position, and 50 μm depth position, respectively, and average the total measured values (15 points in total) Was the average hardness up to 50 μm on the surface. Those having an average hardness of 170 HV or higher were evaluated as acceptable (◯ evaluation).

[Drop rate of punched material]
The annealed steel strip was punched with a punch diameter of 10 mm and a clearance of 10%, the punched section was photographed to measure the amount of dripping, and the ratio of the amount of drooping to the plate thickness was defined as the droop rate. Dripping rate (%) = (dripping amount / plate thickness) × 100. When the droop rate exceeds 20%, the mold life is remarkably deteriorated, and those having a droop rate of 20% or less were determined to have good punchability (◯ evaluation).
The results are shown in Table 2.

As can be seen from Table 2, in the example of the present invention, the average hardness up to 50 μm depth of the surface layer portion in the plate thickness direction is 170 HV or more, the metal structure is ferrite + pearlite, and the surface carbon concentration C _S (mass %) And the average carbon concentration C _M (% by mass) in steel ΔC = C _S -C _M has a microstructure state of 0.1% by mass or more, and all exhibited good punchability.

In contrast, Nos. 1, 20, 22, and 24, which are comparative examples, are obtained by performing normal annealing without applying a carburized paint. These are low in hardness of the upper layer portion and inferior in punchability. Nos. 2 and 5 were coated with a carburized paint, but the carburization of the surface layer was insufficient due to insufficient contact pressure during annealing, and the punchability was poor. No. 4 had a low heating temperature during annealing, and No. 15 had a too short heating holding time during annealing, so that these were all poorly carburized and inferior in punchability. In No. 17, since the average cooling rate from the heating temperature to the temperature of (Ac ₁ -100 ° C.) was too small, the surface layer portion was in a ferrite + cementite structure state and carburization occurred, but the surface hardness was insufficient. It was inferior to punching. Since No. 18 was annealed in the air, decarburization was remarkable and punchability was poor. Since No. 19 was annealed in hydrogen, some decarburization occurred, and the punchability was not improved.

  With respect to the coil after annealing obtained in No. 3 (example of the present invention) in Table 2, the in-coil distribution of surface hardness after quenching was examined. That is, samples were taken from the steel strip longitudinal direction position of the inner 7 m and the outer 7 m from the No. 3 annealing coil, and 2.0 mm × 30 mm × from the plate width direction central portion and 25 mm position from the width direction end (edge), respectively. A 70 mm quenching test piece was cut out and immersed in a salt bath at 900 ° C. for 1 min, followed by water quenching. The cross-sectional hardness after quenching was examined. FIG. 1 shows the result. It was confirmed that the carburized surface layer of each chair sample had a high hardness, and the characteristic variation was small even in the coil.

  FIG. 2 shows a cross-sectional optical microscopic structure of a coil having a contact pressure of 0 MPa and a 4 MPa when annealing was performed at 720 ° C. for 15 hours in an argon atmosphere using the steel A in Table 1 in the same manner as in Example 1. Illustrate. In the case where contact pressure was applied (right photo), carburization progressed, the amount of carbide in the surface layer (the part that looks black) increased, and the austenite ratio in the surface layer during annealing increased, so ferrite + pearlite Organization has been obtained.

Claims (2)

By mass%, C: 0.03 to 0.80%, Si: 0.01 to 1.50%, Mn: 0.20 to 2.0%, P: 0.04% or less, S: 0.03 %: Cr: 0 to 1.50%, T. Al: 0.010 to 0.10%, the balance Fe and an inevitable impurity component composition, and the average up to 50 μm depth of the surface layer portion in the plate thickness direction The hardness is 170 HV or more, the metal structure is ferrite + pearlite, and the difference ΔC = C _S −C _M between the surface carbon concentration C _S (mass%) and the average carbon concentration C _M (mass%) in steel is 0 . A carburized and annealed steel strip with good punchability of 1% by mass or more. After applying a carburizing paint containing 5% by mass or more of a solid carburizing agent to the surface of the steel strip to form a dry coating film, the contact pressure between adjacent plate surfaces of the steel strip with the coating film interposed therebetween is 0.2 MPa or more. The coil is wound into a coil shape so as to become, and the coil is charged into the furnace in that state, heated at 720 to 850 ° C. for 3 hours or more in an inert gas atmosphere, and then cooled to an average temperature of at least (Ac ₁ -100 ° C.). It cooled at a rate 15 ° C. / h or more, a manufacturing method of a good carburized annealing steel strip punching of claim 1.