JPS639578B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a steel with improved ductility and workability due to having a graphite phase, and a method for producing the same. In general, carbon steel used for small containers, vehicle parts such as passenger cars, wire bolts, etc. usually has a mixed structure of ferrite phase and cementite or pearlite phase. were often subjected to various processing such as press working, deep drawing, or cold drawing. For this reason, various studies have been carried out aimed at further improving the workability of carbon steel. Improved workability, such as "Reducing it from the conventional 0.02% (hereinafter, % representing the component ratio is expressed as weight %) to around 0.01% reduces A-based inclusions and improves elongation in the direction perpendicular to rolling." A powerful report has been made that will lead to this, and it has been incorporated into actual operations with good results. On the other hand, it has become known that the cementite present in steel is a factor that inhibits the workability of low-carbon steel, but reducing this content is undesirable, such as reducing the strength of the steel and increasing costs. Because of the results obtained, subsequent research aimed to improve ductility and workability by changing its dispersion state and shape, rather than reducing the amount of cementite itself. Ta. An example is spheroidizing annealing. However, as mentioned above, despite continued research on reducing the S content and devising ways to improve the dispersion state and shape of cementite,
A carbon steel material that still has sufficient ductility and workability.
In particular, it has not been possible to realize a steel material with a good balance between these properties and strength. From the above-mentioned viewpoint, the present inventors first attempted to manufacture a carbon steel material that has further improved ductility and workability while ensuring the necessary strength without requiring any particularly troublesome processes or equipment. In the process of basic research primarily aimed at finding ways to improve the workability of carbon steel, as long as there is cementite, which is hard and brittle and therefore tends to generate voids during plastic working, causing a decline in workability, Even if measures are taken to change the distribution and shape of carbon steel, there is a limit to the effect of improving the workability of steel in actual operations. In order to make this happen, it is necessary to eliminate the cementite phase itself. In order to eliminate the cementite phase, the most likely method in actual operation is to allow the C component that makes up cementite to precipitate as graphite. If a carbon steel material has a structure that has a graphite phase instead of a cementite phase, graphite is softer and more extensible than cementite, so it easily deforms and generates voids when subjected to plastic working. We have come to the conclusion that this may be due to the characteristic of not having to do anything. However, no matter where we search, there are no reports on the conditions and methods for forming a graphite phase in carbon steel.
As a result of repeated research and searching for a means to produce carbon steel with a graphite phase on an industrial scale, we found that (a) Even if carbon steel has a low C content, P
(b) In such a low-P, low-S carbon steel, if the Mn content in the steel is also limited, (c) In this way, both the P and S contents in the carbon steel are reduced to limit the value of P (%) × S (%), and If the Mn content is also limited, the second phase in α-iron can be sufficiently changed from cementite to graphite in an industrially applicable production process. Carbon steel with a graphite phase exhibits extremely good ductility and excellent workability, as typified by elongation, and if the dispersion state of graphite noise, which is the second phase, is appropriately controlled, the material can be (e) In addition, high ductility steel materials with a graphite phase as described above can be processed into a desired shape and used as is. Of course,
If high strength is required, heating to a high temperature after processing and applying heat treatment such as quenching and tempering will result in
The graphite phase disappears, and the C plays a role in significantly strengthening the steel material as martensite, bennite, or pearlite. That is,
Carbon steel materials with a graphite phase exhibit good workability when subjected to deep drawing, stretching, bending, hole expansion, cutting, cutting, punching, etc. )
(f) When manufacturing steel materials with a graphite phase, the P, S and Mn contents must be kept at specified values. It is possible to achieve sufficient results by simply hot working carbon steel whose composition is controlled as follows, cold working it, and then annealing it at a temperature of around 600°C for 2 hours or more. As shown in (a) to (f) above, if a working degree of 5% or more is added, the formation of graphite becomes easier in combination with subsequent annealing, and as a result, a carbon steel material with good workability can be realized. I gained knowledge. However, it is not yet clear why limiting P and S to below specific values in carbon steel contributes to the promotion of graphite phase formation, but it is believed that graphite phase occurs in voids existing in defects etc. in the α-iron structure. Considering this fact, limiting P and S promotes the formation of graphite phase because when the amounts of P and S are large, P and S atoms gather in the voids formed during cold working and fill them. It can be inferred that this is because, on the other hand, when the amounts of P and S are small, these voids become extremely stable, and a sufficient location for graphite phase precipitation is secured. . In any case, this invention was made based on the above knowledge, and the first invention is as follows: steel, C: 0.26 to 0.95%, Mn: 0.20% or less, sol/Al: 0.02 to 0.30% or less, Contains N: 0.006% or less, Si: 1.0% or less, P: 0.010% or less, S: 0.010% or less, and satisfies the formula P (%) × S (%) ≦ 10 × 10 ^-6 , Fe and other unavoidable impurities: and other unavoidable impurities: and other unavoidable impurities. The second invention has the following: C: 0.26 to 0.95%, Mn: 0.20% or less, sol/Al: 0.02 to 0.30% or less, N: 0.006% or less, Si: 1.0% or less, P: 0.010%. Hereinafter, a steel containing S: 0.010% or less, satisfying the formula P (%) × S (%) ≦ 10 × 10 ^-6 , Fe and other unavoidable impurities: the remainder, is heated. After processing, cold processing is applied to a working degree of 5% or more, and then 500 to 750
By annealing at a temperature of 2 hours or more at a temperature of .degree. C., the steel is characterized in that a steel having a structure mainly composed of ferrite and graphite phases and having excellent ductility and workability can be obtained. Here, a structure mainly consisting of a ferrite phase and a graphite phase is different from a conventional structure consisting of a ferrite phase and a cementite phase. , meaning that the volume of graphite is greater than the volume of cementite, and this means that if a steel plate is etched with picric acid and observed under an optical microscope, graphite is painted black, while cementite is black-rimmed. Since it can be observed as a white outline, it can be easily determined. Next, in this invention, the reason why the chemical component ratio of the steel and the manufacturing conditions of the steel material are numerically limited as described above will be explained. A Chemical composition of steel (a) C C component is an element that needs to be contained in a specified amount or more in order to form a graphite phase and improve the ductility and workability of steel, but its content is 0.26%. If the content is less than 0.95%, it is possible to form graphite, but it requires a lot of time and the processability improvement effect of the present invention is small.On the other hand, if the content exceeds 0.95%, the formation of graphite becomes Although it is easier, the processability is significantly lowered, so the C content is set at 0.26 to 0.95%. (b) Mn Since Mn is an element that inhibits the formation of graphite phase, the smaller the amount, the better the results. Especially the content
If it exceeds 0.20%, it becomes difficult to form graphite, so the Mn content is
It was set at 0.20% or less. (c) sol/al The sol/Al component has the effect of cleaning the steel and making it easier to form graphite, and its addition is essential in the steel of the present invention and its production. If the content is less than 0.02%, the desired effect cannot be obtained, while if the content exceeds 0.30%, it becomes difficult to melt the steel.
The sol/Al content was set at 0.02-0.30%. (d) It is desirable that the N N component be as small as possible. If the content exceeds 0.006%, the amount of AlN increases and the elongation decreases, so the N content was set at 0.006% or less. (e) Si Similar to Al, the Si component has the effect of cleaning steel and facilitating the formation of graphite, and is added as necessary. But 1.0%
The Si content was set at 1.0% because the steel tends to become brittle if the Si content exceeds 1.0%. (f) P and S Both P and S are 0.01 like normal steel.
%, the structure of the steel material obtained by industrial manufacturing means will be mainly composed of ferrite and cementite (or pearlite), and no graphite precipitation will occur. However, if the content of each is 0.010% or less and the product of P content (%) and S content (%) is 10 × 10 ^-6 or less, even industrial steel manufacturing methods can be used. Since it is possible to precipitate Aite, the contents of P and S are each set to 0.010% or less, and are limited to values that satisfy the formula P (%) x S (%) ≦10 x 10 ^-6 . As mentioned above, the reason why P and S atoms inhibit graphite precipitation is not clear, but since graphite is formed in voids such as grain boundaries of ferrite structure, it is likely that P and S atoms exist in large quantities. This seems to be because such voids are then filled with P and S atoms before graphite is precipitated, and graphite precipitation is suppressed. And P and S
The content exceeds 0.010% each, or P
If the value of (%) × S (%) exceeds 10 × 10 ^-6 ,
This makes graphite precipitation extremely difficult. B. Manufacturing conditions for steel materials In order to industrially and efficiently manufacture the highly ductile and highly workable steel materials of the present invention, it is advantageous to sequentially perform hot working, cold working, and annealing after melting the steel. be. Hot working is performed for the purpose of making the material suitable for cold working. Although the conditions at this time do not have a great effect on the presence or absence of graphite precipitation, they do affect the precipitation distribution of graphite. Note that the process before hot working does not have a large effect on workability, so hot working may be performed directly from molten steel, or the heating temperature may be lowered from the viewpoint of energy saving, followed by hot rolling. It's okay. Scale deposited during hot working is usually removed before cold working. Cold working is performed to facilitate the formation of graphite during the subsequent annealing step. The degree of processing is 5
If it is less than %, the generation of voids will be insufficient and it will be difficult to form graphite. Therefore, it was set at 5% or more. Although there is no particular upper limit to the degree of processing, a processing rate of 95% or less, which is usually carried out, is industrially advantageous. After cold working, the obtained steel material is often subjected to appropriate degreasing treatment, etc., as necessary, and then annealed. During the annealing process, a graphite phase is formed in place of the cementite phase. If the annealing temperature is less than 500°C, it will take a long time to precipitate graphite, making it unsuitable for industrial use.On the other hand, if the annealing temperature exceeds 750°C, the annealing will simply take more energy and will not be effective in precipitating graphite. The temperature was set at 500-750°C. The holding time in the annealing treatment was determined to be 2 hours or more because a holding time of less than 2 hours is insufficient for cementite to decompose and precipitate graphite. Note that it is desirable that the heating rate for annealing be slow. Generally, annealed steel is made into a product after surface roughness, flatness, and mechanical properties are adjusted. Next, the present invention will be explained using examples and comparing with comparative examples. Example 1 First, C: 0.35-0.40%, Mn: 0.06-0.09%,
Sol・Al: 0.02~0.04%, N: 0.0040~0.0055%,
Contains Si: 0.01%, P and S are each 0.0002
~0.15% and 0.0002~0.035%, with the remainder consisting of Fe and other unavoidable impurities. Steel is melted using a conventional method to form a slab, then slab heating temperature: 1200
℃, finishing temperature: 850℃, and coiling temperature: 450℃ to a thickness of 2.4 mm, pickling, and then cold rolling to a thickness of 1.2 mm at a reduction rate of 50%. After degreasing each obtained cold-rolled steel plate, it was heated at 650℃ for 8
After time box annealing and skin pass rolling with an elongation rate of 0.3%, a JIS No. 5 tensile test and microstructural observation were performed. The results are shown in FIG. In Figure 1, the ● mark indicates a structure that is composed mainly of ferrite and graphite phases, the 〇 mark indicates a structure that consists mainly of ferrite and cementite phases, and the 〓 mark indicates a structure composed mainly of ferrite and cementite phases. and graphite were distributed in almost the same amount, and the elongation value in the tensile test was 〓
The mark indicates 30% or more, the 〓 mark indicates 25% or more, and the 〇 mark indicates less than 25%. As is clear from the results shown in Figure 1,
It can be seen that only when the P and S contents in the steel are limited within the range of the present invention can a structure mainly consisting of ferrite and graphite phases be obtained, and the elongation is better than that of conventional steels. Example 2 C: 0.40%, Mn: 0.16%, sol・Al: 0.085%,
N: 0.0055%, Si: 0.25%, P: 0.0009%, S:
0.0020%, P (%) × S (%) = 1.8 × 10 ^-6 (%) ² , Fe
and other unavoidable impurities: A steel having a composition consisting of the following was melted and hot-worked into a 32 mmφ steel bar at a heating temperature of 1150°C and a finishing temperature of 930°C, and air-cooled. After pickling, the same steel bar E-1 and this 31mm
Steel bar E-2 cold-worked to a round shape (area reduction rate) of 6.2% and to a 28mm round shape (area reduction rate)
At 630℃ for steel bar E-3 cold worked at 23%
Soaking annealing was performed for 0.5 to 16 hours, and the mechanical properties, critical compressibility, and microstructure were observed. For comparison, steel with normal composition (C: 0.36%,
Mn: 0.7%, sol・Al: 0.04%, N: 0.0050%,
Si: 0.10%, P: 0.020%, S: 0.010%, P (%)
A similar investigation was conducted regarding ×S (%) = 200 × 10 ^-6 (%) ² , remaining: Fe and unavoidable impurities). Table 1 is a table of mechanical properties, critical compressibility, and microstructure comparing the method of the present invention and the comparative method. Mechanical properties are shown by tensile test values using JIS No. 4 test pieces taken from the center of steel bars of each diameter, and critical compression ratios are shown by compression tests of 32 mm diameter x 48 mm length test pieces taken from steel bars of each diameter. The original height of the test specimen and the height at the time when a crack appeared due to compression are

【table】

[Table] Refers to a structure consisting mainly of graphite and containing cementite.
The difference is expressed as a percentage (%) of the original height. From Table 1, steel A corresponding to the steel composition of the present invention was processed at a cold working degree of 6.2 or 23%, and then at 630°C for 2 hours.
It can be seen that when the steel is heated and soaked for more than an hour, a structure with high elongation, reduction of area, and compression limit ratio and a structure mainly composed of ferrite and graphite can be obtained. Example 3 First, steels A to I having the chemical compositions shown in Table 2 were melted and subjected to the following conditions: slab heating temperature: 1150°C, finishing temperature: 830-890°C, coiling temperature: 500-600°C. Hot rolling was carried out to obtain hot rolled steel sheets of various thicknesses. Next, each obtained steel plate was pickled to remove scale, and then cold rolled to a thickness of 1.0 mm, followed by box annealing under the soaking conditions shown in Table 3.
After temper rolling was performed at an elongation rate of 1.0%, a tensile test and microstructure observation were performed. The results are also shown in Table 3. Note that when the carbon content changes, the strength of the steel changes significantly and the elongation also changes, so the evaluation of ductility is based on tensile strength and

【table】

[Table] (Note) * indicates that it is outside the scope of the present invention.

[Table] Means a structure that is mainly composed of cementite and contains cementite.
Evaluation was made based on the magnitude of the elongation product. From the results shown in Table 3, test numbers 16 to 24
The cold-rolled steel sheet of the present invention obtained by the method described above has a structure mainly composed of ferrite phase and graphite phase, and exhibits a strength x elongation value of 1000 or more, with good elongation, ductility, and That is, it is clear that the workability is good. That is, tensile strength: approximately 70Kg
Steel A with a tensile strength of around f/mm ² shows an elongation of 20% or more, and even steel C with a high carbon content and a tensile strength of about 85 kgf/mm ² shows an elongation of 19%. Test numbers for these steel plates each exhibiting a structure having a ferrite phase and a cementite phase.
When compared with the steel plates obtained by the methods shown in Nos. 26 to 28, it is clear that the elongation is very good. On the other hand, the comparative steel plate obtained in test number 25 has a high C content, and the comparative steel plate obtained in test number 26 has a low Mn content.
It can be seen that in the comparative steel sheet obtained in Test No. 27, the N amount was outside the range of the present invention, so graphite was not formed and the elongation was low. In addition, the comparison steel sheet obtained in Test No. 28 has a higher amount of P and S than the present invention, so there is no formation of graphite and the elongation is low. As described above, according to the present invention, it is possible to obtain a steel plate having the necessary strength and extremely excellent ductility and workability at a relatively low cost, so that it can be used in pressing, bending, cutting, drawing, and cutting. It can be easily applied to cold and warm processing such as machining, and produces industrially useful effects such as obtaining good results.

[Brief explanation of the drawing]

FIG. 1 is a drawing showing the relationship between the P and S contents and the microstructure and elongation of a cold rolled steel sheet.

Claims (1)

[Claims] 1. In terms of weight percentage, C: 0.26 to 0.95%, Mn: 0.20% or less, sol/Al: 0.02 to 0.30%, N: 0.006% or less, Si: 1.0% or less, P: 0.010% or less , S: 0.010% or less, and satisfies the formula P (%) × S (%) ≦ 10 × 10 ^-6 , Fe and other unavoidable impurities: the remainder, and consists of a ferrite phase and graphite Steel with excellent ductility and workability, characterized by having a structure mainly composed of phases. 2 Weight percentage: C: 0.26-0.95%, Mn: 0.20% or less, sol/Al: 0.02-0.30%, N: 0.006% or less, Si: 1.0% or less, P: 0.010% or less, S: 0.010% or less , and satisfies the formula P (%) × S (%) ≦ 10 × 10 ^-6 , Fe and other unavoidable impurities: remaining, after hot working steel, the working degree :Cold working of 5% or more, then 500~750
A method for producing a steel having excellent ductility and workability and having a structure mainly composed of a ferrite phase and a graphite phase, the method comprising annealing at a temperature of °C for 2 hours or more.