JPH0913142A - Graphite precipitated hot rolled steel sheet excellent in bending workability and heat treatability and its production - Google Patents

Abstract

PURPOSE: To produce a graphite precipitated hot rolled steel sheet excellent in bending workability and heat treatability by subjecting a steel stock in which the contents of C, Mn and B are specified to hot rolling and annealing under specified conditions. CONSTITUTION: A steel stock contg., by weight, 0.1 to 1.0% C, 0.05 to 0.3% Mn, 3 to 50ppm B and N; >=three times the content of B by <=200ppm, also contg., at need, one or more kinds among <=3.0% Si, <=3.0% Ni, <=1.0% Al and <=1.0% Cu, and the balance Fe with inevitable impurities is prepd. This steel stock is rolled under the conditions of 930 to 1000 deg.C temp. and >=10% draft and is thereafter annealed at 500 deg.C to the Ac1 to refine graphite so as to regulate the maximum graphite diameter to about <=5μm and the length between the nearest graphite grains to about >=5μm. Thus, the graphite precipitated hot rolled steel sheet having excellent bending workability and high strength can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a graphite-precipitated hot-rolled steel sheet excellent in bending workability and heat treatment property and a method for producing the same. More specifically, it is excellent in bending workability and has a high strength by heat treatment after working. And a graphite precipitation hot-rolled steel sheet (hereinafter referred to as a graphite steel sheet) that provides wear resistance and a method for manufacturing the same.

[0002]

2. Description of the Related Art Many mechanical parts have been subjected to heat treatment after forming to obtain strength and wear resistance. For such applications, high carbon steel has been used because of the need for strength and wear resistance after heat treatment. . However, since the forming method has changed from hot to cold due to the need to improve productivity and suppress decarburization, it has become necessary to improve the cold workability of conventional high carbon steel. Therefore,
The present inventors have found that the workability is improved when the microstructure of high carbon steel is a structure in which graphite is precipitated in ferrite, and that the graphite in ferrite needs to be made fine in order to enhance the heat treatment property of the steel. And that for that reason, the addition of B is particularly effective, and to produce a hot-rolled steel sheet excellent in bending workability and heat-treatability by annealing B-containing steel after hot rolling About Japanese Patent Application No. Sho 63-258931 and Japanese Patent Application No. Sho 63-32164
No. 0 was proposed.

A technique for manufacturing a steel material utilizing a structure in which graphite is precipitated in ferrite is disclosed in, for example, Japanese Patent Laid-Open No. 63-63.
No. 3,176,629, which is proposed as C,
A method in which a steel containing an appropriate amount of Mn, Al, B, and N is hot-rolled, cold-rolled, and then annealed to form a structure mainly composed of ferrite and graphite, which requires cold-rolling. There is no mention of any technique for controlling the graphite distribution.

[0004]

SUMMARY OF THE INVENTION The present invention is intended to solve the above problems, and specifically, in order to improve the bending workability while ensuring the quenching hardness necessary for the B-added graphite steel, It is an object of the present invention to propose a steel in which graphite is refined so that the maximum graphite diameter is 5 μm or less and the length between the closest graphite particles is 5 μm or more, and a manufacturing method thereof.

[0005]

That is, according to the present invention, C: 0.1 to 1.0% by weight and Mn: 0.05 to 0.3 by weight.
%, B: 3 to 50 ppm, N: 200 ppm or less and containing 3 times or more of the amount of B, the balance being Fe and inevitable impurities, and C: 0.1% by weight.
~ 1.0%, Mn: 0.05-1.0%, B: 3-50
ppm, N: 200 ppm or less and containing 3 times or more of the amount of B, and Si: 3.0% or less, Ni: 3.0% or less,
Al: 1.0% or less, Cu: 1.0% or less 1
Characterized in that it contains at least seeds and the balance is Fe and unavoidable impurities, and that a steel material containing the above-mentioned specific chemical components is rolled under conditions of a temperature of 930 to 1000 ° C. and a rolling reduction of 10% or more. After that, annealing is performed at a temperature of 500 ° C. to Ac ₁ .

[0006]

The structure of the means of the present invention will be described in detail below together with its function.

The present inventors have conducted research on improving bendability of steel having a structure in which graphite is precipitated in ferrite. As a result, the maximum graphite grain size and the closest graphite inter-graph length in FIG. As shown in the graph showing the relationship, in order to further improve bending workability, graphite in ferrite has a graphite distance of 5 μm or less and a closest graphite distance of 5 μm.
It has been found that the above is necessary. In addition, the above-mentioned Japanese Patent Application No. 63-25 proposed by the present inventors.
Although the specification of 8931 describes that the number of graphite is at least 1000 pieces / mm ² or more by adding B, that is, the graphite particle size is 5 μm or less, the graphite particle size distance is 5 μm or more. There is no description of a control technique for distribution in, and it has not been known in the past.

[0008] Therefore, as a result of further research and development, the inventors have found that it is effective to control the amounts of B and N added and finishing conditions, and the present invention was established based on this finding. It is a thing. According to the study by the present inventors, B
The bending workability of the added graphite steel is low relative to the strength, because the individual graphite grains are sufficiently fine but the inter-graphite lengths are short or continuous, so that it seems that there are large precipitates. It is thought that this is because cracks occur from that point.

Therefore, in order to improve the bending workability of the graphite steel, it is sufficient to separate the graphite particles into intervals, but in order to separate the intervals, the amount of N added in the steel is set to 3 of the amount of B added.
Double or more, and rolling temperature 930 to 1000
After rolling under conditions of ℃ and rolling reduction of 10% or more,
It was found that annealing should be performed in the temperature range of 500 ° C to Ac ₁ . Although this mechanism is not always clear,
It is considered as follows.

(1) In B-added graphite steel, graphite is precipitated by using BN as a precipitation nucleus site, that is, graphite corresponds to the distribution of BN.

(2) On the other hand, since BN is usually precipitated at a number of several thousand pieces / mm ² or more, the number of individual graphite particles is 5 μm or less at the C content up to 1.0% which is the range of the present invention. .

(3) BN starts precipitation at a temperature of 1000 ° C. or lower when 10% or more of rolling occurs, but 930
Above 0 ° C, diffusion is fast and the driving of precipitation is small, so the individual size is large, but the number is small. That is, B
The closest spacing of N becomes wider. On the other hand, when the temperature is lower than 930 ° C., the diffusion is slow and the driving force for precipitation is large, so the individual size is small, but the number is large. That is, the interval between BNs should be narrow.

(4) Therefore, if rolling is performed at 930 to 1000 ° C. with a reduction rate of 10% or more and BN precipitation is completed within this range, the intervals between BNs are wide, and the graphites using BN as precipitation sites are also mutually separated. It will be distributed apart, and the bending workability of steel will also improve. That is, the scope of the present invention is BN
Is considered to give the conditions under which all the added B precipitates in a temperature range in which they are appropriately separated. However, B
If the amount of N that does not precipitate as N increases, N will need to be annealed for a long time in order to distribute into cementite and stabilize cementite, promote graphitization sufficiently and improve bendability sufficiently. , 200 ppm or less is required.

Next, the reasons for limiting each constituent of the present invention will be described.

C: C is added in an appropriate amount in order to enhance the strength and wear resistance after heat treatment, but if the C content is less than 0.1%, sufficient strength and wear resistance cannot be obtained. If it exceeds 0%, hot rolling becomes extremely difficult, so the lower limit is set to 0.
1% and the upper limit was 1.0%.

Mn: Mn is necessary for fixing S in steel to make it a clean steel and for ensuring hardenability. Since the effect is insufficient, the lower limit was made 0.05%. Mn
If the amount exceeds 1.0%, graphitization will not proceed, so the upper limit was made 1.0%. Further, Mn is a graphitization-inhibiting element, and if the amount of Mn exceeds 0.3%, graphitization becomes difficult to proceed sufficiently. Therefore, to promote graphitization, Si, Ni, A
1, addition of Cu is preferable.

B, N: B precipitates as BN, acts as a precipitation site for graphite, and is necessary for making the graphite particles have a diameter of 5 μm or less. The effect does not occur when the amount of B is less than 3 ppm and is saturated even when it exceeds 50 ppm, so the lower limit is 3 pp.
m and the upper limit was 50 ppm. As shown in Table 2, when the N content is less than 3 times the B content, the graphite grains are connected and precipitated even if proper rolling is performed, and as shown in Table 4, bending workability deteriorates. The amount of N was 3 times or more than the amount of B.

Si, Ni, Al, Cu: These elements are graphitization-promoting elements, and are elements that strengthen the solid solution of steel and are added in appropriate amounts as necessary. Since the graphitization promoting effect and the solid solution strengthening effect are saturated even if added in excess of the above range, the upper limit of each additive element is Si: 3.0% or less, Ni: 3.0% or less, Al:
1.0% or less and Cu: 1.0% or less.

Rolling conditions and annealing conditions: If the steel has chemical components within the range of the present invention, by performing appropriate rolling and annealing, a structure in which graphite having a diameter of 5 μm or less is precipitated in ferrite is formed and bending workability is obtained. Excellent steel. Here, the rolling conditions change the distribution of the graphite precipitation sites BN and affect the graphite distribution. As shown in Table 5, the rolling temperature is 930 ° C.
If it is less than 100%, graphite is connected and deposited, and bending workability is low.
Even if the rolling temperature is 930 ° C. or higher, the effect is the same when the rolling reduction is less than 10%. That is, for good bending, it is desirable to carry out rolling at a rolling temperature of 930 to 1000 ° C. and a rolling reduction of 10% or more. However, in addition to rolling under these conditions, rolling under other conditions is not limited. For example, after rolling at 1100 ° C. and a rolling reduction of 80%, 930 to 100
It is also possible to perform rolling at 0 ° C. with a reduction rate of 30%, and then perform rolling at a temperature below that temperature with a reduction rate of 80%.

Next, the reasons for limiting the annealing conditions will be described.

Regarding the annealing temperature, when the temperature exceeds the Ac ₁ temperature, austenite is generated during heating, and barite is generated after annealing, which hinders softening. Further, the annealing temperature is 5
If the temperature is less than 00 ° C, graphitization will not proceed sufficiently,
The annealing temperature was in the range of 500 ° C. to Ac ₁ temperature.
In order to accelerate the graphitization, it is desirable that the temperature is just below the Ac ₁ temperature. The annealing time is determined according to the need for softening, but it is preferably 1 hour or more because graphitization is difficult to proceed with annealing for less than 1 hour. However, in order to promote spheroidization of cementite before this treatment, Ac ₁ -A
It is possible to hold the c ₃ temperature for a suitable time.

After melting the above component steel in a converter or an electric furnace,
By rolling under the above rolling conditions and then annealing, a structure in which graphite with a diameter of 5 μm or less is precipitated in the steel is obtained, and the same C
Compared with the spheroidized steel of a certain amount of steel, the steel sheet is soft and excellent in bending workability, and the heat treatment property is similar or higher. Similarly, it is also excellent in workability such as drawing and overhanging.
Further, the steel sheet is excellent in free-cutting property of the precipitated graphite due to the chip breaking action and vibration damping property due to the difference in elastic modulus between graphite and ferrite.

[0023]

EXAMPLES The present invention will be described below with reference to examples. First, the test methods for the test materials and symbols in the table will be described.

(1) As for the structure, a steel having the chemical composition shown in Table 2 was melted, rolled and annealed to deposit 80% or more of the added C amount as graphite, and then mirror-finished by buff polishing. The material was observed with an optical microscope at a magnification of 400 times or more without corrosion, and the case where the closest length of the graphite particles was 5 μm or more and the diameter of the graphite particles was 5 μm or less was evaluated as good texture: ◯. In the other cases, it was rated as bad: x. However, in the case where 80% or more of graphitization did not occur due to annealing, the structure defect was x regardless of the distribution of graphite.

(2) The bending workability was measured by melting the steel having the chemical composition shown in Table 2 and then rolling and annealing the test material having a plate thickness of 4.0 mm in accordance with JIS and perpendicular to the rolling direction. The test piece cut out in the direction was subjected to a 180 degree close contact bending test, and depending on whether or not bending could be performed, the bending (workability) was good and the bending (workability) was bad.

(3) Heat treatment was performed at 870 ° C. for 1 hour at a temperature of 870 ° C. for the test material obtained by melting steel having the chemical composition shown in Table 2, followed by rolling and annealing.
After holding for 2 minutes and quenching with 100 ° C. oil, heat treatment is good when a hardness equal to or higher than that determined by the amount of C as shown in Table 1 is obtained: ◯, when it is less than that, heat treatment is poor : It was set as ×.

[0027]

[Table 1]

The present invention will be described below based on examples.

Samples having the chemical components shown in Table 2 were melted in a converter, and then rolled and annealed under the conditions shown in Table 3. The results are shown in Table 4. As is clear from Table 4, the sample having the chemical composition within the range of the present invention was a steel having good bending workability and heat treatment property with the maximum graphite grain size and the closest graphite interval, whereas Si , Ni, Al and Cu were not added, and the Mn content exceeded 0.3%. 2
1, comparative sample No. 1 having a B content of less than 3 ppm. 22 and the comparative example sample No. in which the amount of N exceeded 200 ppm. In No. 27, graphitization did not proceed sufficiently and bending workability was poor.

Further, the sample No. of Comparative Example in which the amount of N is less than 3 times the amount of B In Nos. 6 and 7, the interproximal graphite length was 5 μm or less, and bending workability was poor.

Further, sample No. having a chemical composition within the scope of the present invention. 4 was melted in a converter, rolled at a rolling reduction of 0 to 50% at 910 to 1000 ° C., and then annealed under the conditions shown in Table 3. Table 5 shows the results. As is clear from Table 5, the sample rolled within the range of the present invention was a steel excellent in bending workability and heat treatment together with the maximum graphite grain size and the closest graphite interval, whereas The comparative sample out of the rolling condition range of the invention has a closest graphite interval of 5
It was less than μm and bending workability was poor.

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

As described above, the steel of the present invention is C,
Mn or C, Mn, Si, Ni, Al, Cu, B is contained in an appropriate amount, and the N content is 3 times or more of the B content, and the steel containing these is appropriately rolled and annealed. Is characterized in that graphite having a diameter of 5 μm or less is not connected to each other in the ferrite to form a precipitated structure.

Therefore, unlike the conventional spheroidized high carbon steel, the steel of the present invention is not so hard as cementite but is precipitated with soft graphite in ferrite, so that it is soft and can be worked by punching, etc. Since they are separated from each other, it is possible to sufficiently perform severe processing such as bending. Further, since the graphite is fine, it is also excellent in heat treatment after molding. If the steel of the present invention is used as a material for a complex-shaped part that requires molding, carburizing and carburizing after molding using, for example, low carbon steel, it is possible to omit carburizing, carburizing and other treatment steps. It is possible to improve productivity and reduce costs.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the maximum graphite particle size and the closest graphite distance.

Claims (3)

[Claims] 1. C: 0.1-1.0% by weight%, Mn:
0.05-0.3%, B: 3-50ppm, N: 200
A graphite-precipitated hot-rolled steel sheet excellent in bending workability and heat treatment, characterized in that it is contained in an amount of not more than ppm and not less than 3 times the amount of B, and the balance is Fe and inevitable impurities. 2. C: 0.1 to 1.0% by weight, Mn:
0.05-1.0%, B: 3-50 ppm, N: 200
ppm or less and containing 3 times or more of the amount of B, and Si:
3.0% or less, Ni: 3.0% or less, Al: 1.0% or less, Cu: 1.0% or less, and the balance is Fe and inevitable impurities. A graphite-precipitated hot-rolled steel sheet excellent in bending workability and heat treatment property. 3. A steel material containing the chemical component according to claim 1 or 2 is rolled at a temperature of 930 to 1000 ° C. and a rolling reduction of 10% or more, and then annealed at a temperature of 500 ° C. to Ac _1. A method for producing a graphite-deposited hot-rolled steel sheet excellent in bending workability and heat treatment property, which is characterized by the above.