KR100847045B1 - A high carbon steel sheet superior in yield strength and cold working and manufacturing method thereof - Google Patents

Abstract

By weight%, carbon (C): 0.75-0.95%, silicon (Si): 1.8% or less, manganese (Mn): 0.1-1.5%, chromium (Cr): 0.1-1.0%, phosphorus (P): 0.02% or less , And sulfur (S): 0.02% or less and provide a high carbon steel sheet composed of the balance Fe and other unavoidable impurities.

High carbon steel sheet, spring steel sheet, yield strength, cold rolling

Description

High carbon steel sheet with excellent yield strength and cold rolling and manufacturing method thereof {A HIGH CARBON STEEL SHEET SUPERIOR IN YIELD STRENGTH AND COLD WORKING AND MANUFACTURING METHOD THEREOF}

1 is a microstructure of a high carbon steel sheet according to one embodiment of the present invention.

Figure 2 is a microstructure of a high carbon steel sheet of another embodiment of the present invention.

The present invention relates to a high carbon steel sheet and a method for manufacturing the same, and more particularly, to a high carbon steel sheet excellent in yield strength and cold rolling properties and a method for manufacturing the same.

As safety demands on automobiles increase, so does the demand for increased durability of the springs. The durability of the spring is most pronounced when the spring is driven within the elastic deformation zone and thus is greatly affected by the yield strength of the spring material. Therefore, in order to improve the durability of the spring, it is necessary to increase the strength of the spring steel sheet material. In order to meet the trend of higher strength of the spring steel, there is an urgent need for a material having more improved yield strength and rolling property than the existing spring steel.

Looking at the prior art in this field, conventional spring steel sheet is advantageous in pressing because it is composed of a pearlite structure, but has a limit in improving the strength due to work hardening. In order to overcome this weakness, a method using the base bainite or tempered martensite as a hardened structure has been devised. However, there is a problem in that the base structure lacks rolling properties, so that cracking or breaking occurs during the rolling, and thus cold rolling under high pressure, which is essential for spring manufacture, cannot be applied.

In order to solve the above problems, a high carbon steel sheet having high strength and high rolling property is provided.

High-carbon steel sheet according to an embodiment of the present invention, in weight% of carbon (C): 0.75-0.95%, silicon (Si): 1.8% or less, manganese (Mn): 0.1-1.5%, chromium (Cr): 0.1 to 1.0%, phosphorus (P): 0.02% or less, and sulfur (S): 0.02% or less, and consist of balance Fe and other unavoidable impurities. It may further comprise at least one element selected from the group consisting of 0.05 to 0.25% of V, Nb, Mo, Ti, W, Cu and 30 to 120 ppm nitrogen.

In addition, in the microstructure of the steel sheet, the volume fraction of the fine pearlite and upper bainite phase having an interlayer spacing between layered cementite of 0.2 μm or less may be 90% or more, and the sum of the volume fractions of the retained austenite and martensite may be 5% or less. have.

On the other hand, the method for producing a high carbon steel sheet according to an embodiment of the present invention, i) by weight% of carbon (C): 0.75-0.95%, silicon (Si): 1.8% or less, manganese (Mn): 0.1-1.5 %, Chromium (Cr): 0.1 to 1.0%, phosphorus (P): 0.02% or less, sulfur (S): 0.02% or less, and 0.05 to 0.25% of V, Nb, Mo, Ti, W, Cu and 30 to Preparing a steel comprising at least one element selected from the group consisting of 120 ppm nitrogen and consisting of the balance Fe and other unavoidable impurities; ii) forming the steel by spheroidizing cementite and ferrite through hot rolling, cold rolling and annealing Step of manufacturing a steel sheet having a, iii) heating the steel sheet to a temperature range of 800 to 1100 ℃, iii) ossampling the heated steel sheet so that the temperature of the oscillating solder bath is 450 to 510 ℃, 70 seconds or more Steps.

When explaining the reason for limiting the chemical composition of the high carbon steel sheet according to the embodiments of the present invention as follows.

First, the content of carbon (C) is 0.75 to 0.95%. Thus, the reason for limiting the content of carbon (C) is that when the carbon content is less than 0.75%, it is difficult to secure the hardness increase due to quenching, that is, excellent durability. In addition, when carbon (C) exceeds 0.95%, formation of residual austenite is easy. In addition, the possibility of cracking due to stress organic transformation during cold rolling is high, and the toughness and fatigue characteristics of the steel sheet are deteriorated.

The content of silicon (Si) is made 1.8% or less. As the content of silicon (Si) increases, the strength and resistance to permanent deformation of the steel sheet increases. However, in the case of more than 1.8%, the resistance to permanent deformation is rather reduced, and the possibility of decarburization increases during heat treatment. In addition, an increase in scale defects results in a decrease in surface quality.

The content of manganese (Mn) is 0.1 to 1.5%. The reason for limiting the content of manganese (Mn) is that if the content of manganese (Mn) is less than 0.1% of iron sulfide (FeS) combined with sulfur (S) and iron (Fe) inevitably contained during the steel manufacturing process. Redness brittleness occurs by formation. On the other hand, when manganese (Mn) exceeds 1.5%, toughness falls. In addition, since the hardenability becomes larger than necessary, it is necessary to slow down the traveling speed of the steel sheet required to obtain the required microstructure. Therefore, the productivity of the steel sheet is lowered.

The content of chromium (Cr) is made 0.1 to 1.0% or less. Chromium (Cr), like manganese (Mn), improves the hardenability and strength of steel, and is known as an element having anti-carburization and graphitization prevention effects. When the content of chromium (Cr) is less than 0.1%, sufficient hardenability and decarburization effect cannot be obtained. On the other hand, chromium (Cr) is 1.0%

If it exceeds, the hardenability becomes larger than necessary.

The content of sulfur (S) is made 0.02% or less. If the sulfur (S) content is more than 0.02%, the grain boundary segregation occurs, and the toughness of the steel is lowered.

The content of phosphorus (P) is made 0.02% or less. If the content of phosphorus (P) exceeds 0.02%, it will segregate at the grain boundary and the toughness of the steel will be lowered.

Molybdenum (Mo), navydium (Nb), titanium (Ti), vanadium (V), and tungsten (W) combine with carbon (C) or nitrogen (N) in the steel to cause precipitation hardening and copper (Cu) Alone causes precipitation hardening. The content of one or more selected from molybdenum (Mo), navydium (Nb), titanium (Ti), vanadium (V), tungsten (W) and copper (Cu) is 0.05 to 0.25%. These elements contribute to the strengthening of the steel through single or multiple precipitation hardening. However, when added above a certain amount, the precipitation hardening effect tends to be saturated, and the curing ability is improved and the rolling property is lowered. In addition, when the elements are less than 0.05%, the precipitation strengthening effect is lowered. On the other hand, when the elements are more than 0.25%, the brittleness of the steel is increased during hot rolling.

The content of nitrogen (N) is 30 to 120 ppm. When nitrogen (N) is less than 30 ppm, the amount of precipitation of carbonitride is not sufficient, so that the improvement of strength and resistance to permanent deformation of the steel is insignificant. On the other hand, when the nitrogen (N) exceeds 120ppm, the effect of precipitation strengthening is saturated, and the precipitate is supersaturated in the matrix and the toughness of the steel is lowered.

Hereinafter, a method of manufacturing a high carbon steel sheet according to an embodiment of the present invention will be described.

First, in weight percent, carbon (C): 0.75-0.95%, silicon (Si): 1.8% or less, manganese (Mn): 0.1-1.5%, chromium (Cr): 0.1-1.0%, phosphorus (P): 0.02 % Or less, sulfur (S): 0.02% or less, and 0.05 to 0.25% of one or more elements selected from the group consisting of V, Nb, Mo, Ti, W, Cu and 30 to 120 ppm nitrogen, the balance Fe and Manufacture steels made of other unavoidable impurities. The reason for limiting the chemical composition of such steel materials is as described above, so the description thereof is omitted here.

Next, a steel sheet is hot rolled, cold rolled, and annealed to prepare a steel sheet having a structure of spheroidized cementite and ferrite, and the prepared steel sheet is heated to a temperature range of 800 to 1100 ° C. If the heating temperature of the steel sheet is less than 800 ° C., since the spheroidized cementite is not sufficiently dissolved in the quenching process, the strength of the material after heat treatment is lowered. On the other hand, if the heating temperature of the steel sheet exceeds 1100 ℃ decarburization increases, the grain size of the austenitic structure is increased. Therefore, the hardenability is increased more than necessary, it is difficult to obtain a microstructure.

Next, the steel sheet is ostempered for 70 seconds or more, so that the temperature of an ostempering brazing tank becomes 450-510 degreeC. If the holding temperature at the time of ostempering is less than 450 degreeC, the lower bainite or tempered martensite which is hardened structure will form, and rolling property will be inferior. Therefore, cracks or fractures occur during the rolling of the steel sheet. On the other hand, when the holding temperature at the time of ostampering exceeds 510 ℃, the microstructure of the pearlite is formed, it is difficult to improve the strength through the work hardening. In addition, when austempering in less than 70 seconds, the austenite structure generated during the hardening heat treatment cannot be transformed in the lead bath, is transformed into martensite, or remains as retained austenite. Therefore, during cold rolling, the tissues transform into stress-organic martensite and thus act as a cause of crack formation, thereby degrading ductility.

Thus, by adjusting the conditions of ostempering, it is possible to obtain a high strength steel sheet having a fine structure excellent in initial strength and rolling properties.

Hereinafter, the present invention will be described in more detail through experiments. The following experiment is for illustrating the present invention, but the present invention is not limited thereto.

Experimental Example

After rolling the spheroidized heat-treated high carbon steel sheet for high strength spring having the above-mentioned composition into a sheet of 1.5 mm thick, it was heated at a quenching heat treatment temperature of 750-1200 ° C. for 2 minutes, and then ace bath was maintained at 300-600 ° C. Tempering heat treatment is performed. The microstructure of the steel sheet subjected to the osmosis heat treatment was observed and the yield strength and the rolling property were measured. The results are shown in Table 1 below.

Cold rolling is performed by cutting the sheet of 1.5mm thick heat treated under the above conditions into 100mm × 150mm squares, and then rolling it at the edge of 5% every time in cold rolling mill. The number of times up to the point at which significant cracks were easily identified was measured. This can be converted into a cumulative reduction ratio. The cumulative rolling rate was measured as the ratio of the final thickness to the initial thickness. The cumulative rolling rate until the rolling just before the occurrence of cracking was marked according to the number of cold rolling. In some experiments, the experiment was stopped after cumulative reduction of up to about 80%. This was confirmed that the target 80% cumulative pressure reduction is possible, because the error in the thickness control of less than 0.3t on the performance of the mill equipment used in this experiment is likely to increase.

Table 1 below describes the manufacturing conditions, such as the oscillating heat treatment conditions and hardening heat treatment conditions for producing a high-carbon steel sheet for high strength spring.

division Hardening  Heat treatment Temperature (℃) Os Tempering Temperature (℃) Retention time (seconds) surrender burglar( MPa ) Cumulative reduction ratio (%) Microstructure Comparative Example 1 750 480 80 860 80.6 Upper Bainite + Pearlite + Soluble Cementite Example 1 850 500 80 1001 79.6 Fine pearlite + some upper bainite Example 2 950 480 80 1026 80.6 Upper bainite + some fine pearlite Example 3 1050 480 80 1015 80.6 Upper bainite + some fine pearlite Comparative Example 2 1200 480 80 1120 72.2 Upper bainite + fine pearlite + martensite + pole surface part ferrite (decarburization) Comparative Example 3 950 300 80 1554 33.6 Lower bainite + martensite + residual austenite Comparative Example 4 950 400 50 1217 58.1 Lower bainite + martensite + residual austenite Comparative Example 5 950 500 50 989 73.6  Fine Pearlite + Martensite Comparative Example 6 950 600 80 820 80.6 Pearlite

Referring to Table 1, when the quenching heat treatment temperature is 800 ° C. or lower (Comparative Example 1), the matrix structure is inversely transformed, but the dissolution of the spheroidized cementite is not completed, and thus the cementite structure that is not dissolved in the material remains after the osmosis heat treatment. . In addition, since the known carbon concentration is insufficient, the transformation curve is pulled, so that pearlite is generated in the section before being introduced into the lead bath even when the ostempering heat treatment is performed at the same cooling rate. In addition, since the carbide concentration in the bainite is low, the strength after the heat treatment is lowered. When the quenching heat treatment temperature is 1100 ° C. or more (Comparative Example 2), the transformation curve is pushed back due to the increase of the austenite grain size. Thus, a small amount of martensite is produced from the austenite remaining when cooled after passing through the bath. Also, due to decarburization at the polar surface, ferrite may be observed. For this reason, rolling property falls compared with Examples 1-3. When the holding temperature is changed, the holding temperature is low (Comparative Examples 3 and 4), but the yield strength after heat treatment is increased, but the rolling property is lowered due to the influence of lower bainite and martensite structure. Even in the case where the holding time in the lead bath is insufficient (Comparative Examples 4 and 5), since the retained austenite structure remains or martensite structure is generated while cooling after passing through the lead bath, the rolling property is slightly reduced compared with the example. On the other hand, when the Osstem holding temperature (lead bath temperature) is higher than 510 ° C (Comparative Example 6: High-carbon steel sheet for existing springs), there is no problem in rolling property, but the yield strength is low after heat treatment. There is a limit to yield strength. On the other hand, in the embodiment according to the present invention, as shown in the table, the yield strength was increased after the heat treatment while maintaining the rolling properties. As shown in FIGS. 1 and 2, the internal microstructure is composed of 90% or more of fine pearlite having an upper bainite or pearlite interlayer spacing of 0.2 μm or less, and a martensite or stress organic martensite transformation acting as a cracking source during rolling. This is because the residual austenite that generates cracks through is limited to 5% or less.

1 is a photograph of a microstructure of a high carbon steel sheet according to an embodiment of the present invention with a scanning electronic microscope (SEM).

Figure 2 is a photograph of the microstructure of the high carbon steel sheet according to another embodiment of the present invention.

As mentioned above, although preferred embodiment of this invention was described, this invention is not limited to this, It can be variously modified and implemented in a claim, a detailed description of an invention, and the range of an accompanying drawing. And of course, this also belongs to the scope of the present invention.

High carbon steel sheet according to an embodiment of the present invention, is prepared by including a step of ostamping for more than 70 seconds so that the temperature of the oscillating lead bath is in the 450 ~ 510 ℃ range. The sum of the volume of the fine pearlite and upper bainite phases having an interlaminar spacing between layered cementite of 0.2 μm or less is therefore 90% or more, and the sum of the volumes of residual austenite and martensite phases is 5% or less.

In addition, since the high carbon steel sheet according to the embodiment of the present invention has such a phase ratio, both the yield strength and the cold rolling are excellent.

In particular, when manufacturing a plate-like spring using a high carbon steel sheet according to an embodiment of the present invention, it is possible to improve the yield strength of the spring. Therefore, a highly durable spring can be manufactured.

Claims (4)

By weight%, carbon (C): 0.75-0.95%, silicon (Si): 1.8% or less, manganese (Mn): 0.1-1.5%, chromium (Cr): 0.1-1.0%, phosphorus (P): 0.02% or less Sulfur (S): 0.02% or less, and 0.05 to 0.25% of V, Nb, Mo, Ti, W, Cu, and at least one element selected from the group consisting of 30 to 120 ppm nitrogen, the balance Fe and other unavoidable Made of impurities, A high carbon steel sheet having a volume fraction of at least 90% of the fine pearlite and upper bainite phases having an interlayer spacing between layered cementite in the microstructure of not more than 0.2 µm and a sum of the volume fractions of retained austenite and martensite of 5% or less. delete delete By weight%, carbon (C): 0.75-0.95%, silicon (Si): 1.8% or less, manganese (Mn): 0.1-1.5%, chromium (Cr): 0.1-1.0%, phosphorus (P): 0.02% or less Sulfur (S): 0.02% or less, and 0.05 to 0.25% of V, Nb, Mo, Ti, W, Cu and 30 to 120 ppm of nitrogen, one or more elements selected from the group consisting of balance Fe and other unavoidable Manufacturing a steel material consisting of impurities, Manufacturing a steel sheet having a structure of spheroidized cementite and ferrite through hot rolling, cold rolling, and annealing of the steel; Heating the steel sheet to a temperature range of 800 to 1100 ° C., and The heated steel sheet was subjected to an ostempering lead bath at a temperature of 450 to 510 ° C., followed by ostempering for 70 seconds or more, whereby the volume fraction of the fine pearlite and upper bainite phases having an interlayer spacing between layered cementite in the microstructure was 0.2 μm or less. And the sum of the volume fractions of retained austenite and martensite is 5% or less. Method for producing a high carbon steel sheet comprising a.

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