JP5391589B2 - Steel sheet excellent in cold workability and method for producing the same - Google Patents

Description

  The present invention has a yield strength YS of less than 210 MPa, a total elongation El of 49% or more, and a yield ratio YR (= YS / TS × 100; TS is tensile strength), which is particularly suitable for leaf springs, claws of tillers, and the like. The present invention relates to a steel sheet excellent in cold workability of less than% and a method for producing the same.

  The claws of the cultivator are required to have wear resistance against earth, sand, mud, clay, etc., and therefore, a steel plate equivalent to the spring steel SUP6 specified in JIS G 4801 is used as the material. This steel sheet is made of steel melted in a component range equivalent to SUP6 by an electric furnace or a converter to form a bloom or billet by casting or split rolling, and after heating, it is 6 to 12 mm thick and 50 to 50 mm wide by a hot rolling mill Manufactured as a steel plate (flat bar) with a size of 100mm and length of 6-10m. And the nail of the tiller cuts out a piece of 200-250mm in length from this steel sheet, after hot working into a predetermined shape, heated to austenite region to give wear resistance, subjected to quenching treatment, Manufactured by tempering to give impact resistance.

In recent years, from the viewpoint of energy saving and labor saving, it has been studied to replace the above hot working with cold working, Patent Document 1 includes cold workability and toughness as a material for nails of a tiller, Furthermore, a steel plate excellent in hardenability has been proposed. This steel plate is C: 0.40-0.80 wt%, Si: 0.20-2.00 wt%, Mn: more than 0.50 to 1.50 wt%, Al: 0.001-0.150 wt%, P: 0.018 wt% or less, S: 0.010 wt% or less N: 0.0050 wt% or less, or Ti: 0.10 wt% or less, Nb: 0.05 wt% or less, Zr: 0.050 wt% or less, and B: 0.01 wt% or less. Steel is manufactured by hot rolling at a finishing temperature of 500-800 ° C, winding, and annealing at 500-750 ° C for 1-200 hours. The main structure is ferrite and graphite, and the graphitization rate is 40% or more. As a result, softening is performed so that the TS becomes 60 kgf / mm ² or less, and excellent cold workability and toughness are provided. Further, the graphite particle size is refined to 10 μm or less to provide excellent hardenability.
Japanese Patent Publication No.8-30241

  Recently, however, the requirements for cold workability, especially shape freezing properties of steel sheets as materials have become increasingly severe, and the steel sheets with excellent cold workability described in Patent Document 1 cannot sufficiently meet such requirements. There is a need for steel sheets with lower YS, higher El, and lower YR.

  In view of such demands, an object of the present invention is to provide a steel sheet excellent in cold workability having a YS of less than 210 MPa, an El of 49% or more, and a YR of less than 50%, and a method for producing the same.

  As a result of repeated studies on conditions under which YS can be less than 210 MPa, El can be 49% or more, and YR can be less than 50%, the steel structure can be made to include a structure containing ferrite, graphite, and cementite. It has been found that it is effective to make the volume fraction of the cementite and graphite present in the ferrite grains 15% or less after making the volume fraction of graphite occupied, that is, the graphitization ratio 40% or more.

  The present invention has been made based on such findings, and in mass%, C: 0.40 to 0.80%, Si: 0.20 to 2.00%, Mn: more than 0.50 and not more than 1.50%, Al: 0.001 to 0.150%, P: 0.018% or less, S: 0.010% or less, N: 0.0050% or less, having a component composition consisting of the balance Fe and inevitable impurities, having a structure containing ferrite, graphite, and cementite, and in the entire structure The total volume ratio of ferrite, graphite, and cementite is 95% or more, and the volume ratio of graphite (graphitization ratio) is 40% or more in the entire graphite and cementite. Provided is a steel sheet excellent in cold workability, characterized in that the total volume fraction of cementite is 15% or less.

  The steel sheet of the present invention further contains, in mass%, at least one selected from Ti: 0.10% or less, Nb: 0.05% or less, Zr: 0.050% or less, B: 0.01% or less. Is preferred.

  The steel sheet of the present invention is a hot-rolled sheet obtained by hot rolling a steel having the above component composition at a finishing temperature of 800 to 950 ° C., and the hot-rolled sheet after the hot rolling has a temperature of 50 ° C./s or more. After cooling to a cooling stop temperature of 600 ° C. or lower at an average cooling rate, it can be produced by a method of winding at a winding temperature of 550 ° C. or lower, and annealing the hot rolled sheet after the winding at an annealing temperature of 720 ° C. or lower.

  According to the present invention, it is possible to produce a steel sheet having excellent cold workability with YS of less than 210 MPa, El of 49% or more, and YR of less than 50%. Since the steel sheet of the present invention has a very low YS, it is not only excellent in cold workability, particularly shape freezing, but also effective in extending the life of tools used for cold work. Further, since YR is low, TS is high compared to YS, and it can be said that it is a preferable material from the viewpoint of securing the strength of the non-quenched portion even for applications in which partial quenching is performed after cold working.

  Moreover, the steel plate of the present invention can be applied as a raw material for uses such as the above-mentioned tiller claws, applications that require good cold workability and wear resistance after quenching, such as tools and automobile parts. .

  Below, the steel plate excellent in the cold workability which is this invention, and its manufacturing method are demonstrated in detail. Note that “%” representing the amount of a component means “% by mass” unless otherwise specified.

1) Component composition
C: 0.40 ~ 0.80%
C is an indispensable element for ensuring hardenability and imparting excellent wear resistance to the nails of a tiller and the like. Therefore, the C amount needs to be 0.40% or more. On the other hand, if the amount of C exceeds 0.80%, the effect of improving the hardenability is almost saturated, but the impact resistance is deteriorated and the base of the nail may be damaged during the tilling work. Therefore, the C content is 0.40 to 0.80%.

Si: 0.20-2.00%
Si enables high strength that cannot be achieved only by quenching strengthening with C by solid solution strengthening, improves wear resistance, promotes graphitization of cementite, and achieves low YS, high El, low YR, Improve cold workability. Therefore, the Si content needs to be 0.20% or more. On the other hand, when the amount of Si exceeds 2.00%, the above effect is saturated, and the manufacturing cost is increased. Therefore, the Si content is 0.20 to 2.00%.

Mn: More than 0.50 and less than 1.50%
Mn is an element that improves hardenability, and is particularly effective in preventing quenching distortion because it significantly lowers the critical cooling rate during the quenching process. Therefore, the amount of Mn needs to exceed 0.50%. On the other hand, if the amount of Mn exceeds 1.50%, the susceptibility to cooling cracking after casting of steel increases and surface defects are likely to occur. Therefore, the Mn content is more than 0.50% and not more than 1.50%.

Al: 0.001 to 0.150%
Al combines with solid solution N to form AlN, and has the effect of excluding the action of stabilizing N cementite, which will be described later, and inhibiting the formation of graphite. Therefore, the Al amount needs to be 0.001% or more, preferably 0.005% or more. On the other hand, if the Al content exceeds 0.150%, the hardenability is hindered. Therefore, the Al content is 0.001 to 0.150%, preferably 0.005 to 0.150%.

P: 0.018% or less
If the amount of P exceeds 0.018%, the hardenability and cold workability deteriorate due to the grain boundary segregation of P, and cementite is stabilized and graphite formation is inhibited. Therefore, the P content is 0.018% or less.

S: 0.010% or less
When the amount of S exceeds 0.01%, nonmetallic inclusions such as MnS are formed and cold workability is lowered, and cementite is stabilized and graphite formation is inhibited. Therefore, the S content is 0.010% or less.

N: 0.0050% or less
When the N content exceeds 0.0050%, cementite is stabilized and graphite formation is inhibited. Therefore, the N content is 0.0050% or less.

  The balance is Fe and inevitable impurities, but contains at least one selected from Ti: 0.10% or less, Nb: 0.05% or less, Zr: 0.050% or less, B: 0.01% or less for the following reasons It is preferred that

Ti: 0.10% or less
Ti is a nitride-forming element, and fixes solute N harmful to graphite formation as nitride. Therefore, the Ti content is preferably 0.01% or more, and more preferably 0.02% or more. However, when it exceeds 0.10%, the effect is saturated, and the manufacturing cost is increased. Therefore, the Ti content is 0.10% or less.

Nb: 0.05% or less
Nb, like Ti, is a nitride-forming element, and fixes solute N harmful to graphite formation as nitride. For this reason, the Nb content is preferably 0.01% or more, but if it exceeds 0.05%, the effect is saturated and the manufacturing cost increases. Therefore, the Nb content is 0.05% or less.

Zr: 0.050% or less
Zr, like Ti and Nb, is a nitride-forming element, and fixes solute N harmful to graphite formation as nitride. Therefore, the Zr content is preferably 0.005% or more, and more preferably 0.01% or more. However, when it exceeds 0.050%, the effect is saturated and the manufacturing cost is increased. Therefore, the Zr content is 0.050% or less.

B: 0.01% or less
B, like Ti, Nb and Zr, is a nitride-forming element, and fixes solute N harmful to graphite formation as nitride. For this reason, the B content is preferably 0.0005% or more. However, if it exceeds 0.01%, the effect is saturated and the manufacturing cost increases. Therefore, the B content is 0.01% or less.

2) Structure In order to increase the softness and high ductility of the steel sheet and improve the cold workability, as described in Patent Document 1, the structure contains ferrite, graphite, and cementite, and the ferrite occupies the entire structure. The total volume ratio of graphite and cementite must be 95% or more, and the graphitization ratio must be 40% or more. At this time, in the present invention, the case where the graphitization rate is 100%, that is, when all cementite is graphitized is obtained because the same effect is obtained. When the total volume ratio of ferrite, graphite, and cementite is less than 95%, that is, when the volume ratio of other phases such as pearlite and bainite exceeds 5%, cold workability is deteriorated. Moreover, if the graphitization rate is less than 40%, excellent cold workability cannot be obtained.

Here, the volume ratios of ferrite, graphite, and cementite were determined as follows. That is, after polishing the thickness 1/4 position of the thickness cross section in the rolling direction of the steel sheet, it corrodes with Nital, and with an optical microscope, observe 5 places per field of view at a magnification of 400 times, 10 fields (50 places in total), These images are subjected to image analysis processing by the image analysis software “Image Pro Plus ver.4.0” manufactured by Media Cybernetics, to determine the areas of ferrite, graphite, and cementite, and the ratio (area ratio) of the total observation area to ferrite, The volume ratios of graphite and cementite were used. The ratio (area ratio) of the area of graphite (Sgr) to the sum of the area of graphite (Sgr) and the area of cementite (Scm) was defined as the volume ratio of graphite (graphitization ratio). That is, the graphitization rate (%) can be expressed by the following formula.
Graphitization rate = {Sgr / (Sgr + Scm)} x 100
However, by controlling the total volume ratio and graphitization ratio of ferrite, graphite, and cementite, YS of less than 210 MPa, El of 49% or more, and YR of less than 50% are not necessarily obtained. . In order to achieve this target, the total volume fraction of cementite and graphite present in the ferrite grains needs to be 15% or less, preferably 10% or less.

  The present inventors have made various studies in order to achieve low YS and high El. An example of study is shown below. Steel slab consisting of C: 0.62%, Si: 1.68%, Mn: 0.88%, P: 0.004%, S: 0.0013%, Al: 0.008%, N: 0.0013%, balance Fe and inevitable impurities heated to 1150 ℃ After that, 5 passes of rough rolling, 7 passes of finish rolling at a finishing temperature of 870 ° C. to form a hot rolled sheet with a plate thickness of 8.0 mm, winding at a winding temperature of 520 ° C., pickling, 720 ° C. And 50hr batch annealing. At this time, in order to change the amount and distribution state of cementite and graphite, the temperature range from the finish rolling to the coiling temperature is changed by changing the average cooling rate from air cooling (5 ° C / s) to 200 ° C / s. did. Then, the structure and tensile property values (YS, El, YR) were investigated as follows.

Structure: Same as above, the thickness 1/4 position of the cross section in the rolling direction was polished and subjected to nital corrosion, then observed with an optical microscope over 10 fields (total of 50 fields) at a magnification of 400 times at 5 sections. Using the image analysis software described above, cementite and graphite existing on the ferrite grain boundary and cementite and graphite existing on the ferrite grain are identified, the occupied area S _on of cementite and graphite existing on the ferrite grain boundary, and ferrite measuring the occupied area S _in of cementite and graphite present in the grain, determine the area ratio of cementite and graphite present in the ferrite grain from the following equation, there it ferrite grains to the total cementite and graphite cementite And the volume ratio V (%) of graphite. That is, V (%) can be expressed by the following formula.
V = {S _in / (S _on + S _in )} × 100
Here, the cementite grains or graphite grains having at least a portion existing on the ferrite grain boundary are the same as the cementite grains or graphite grains existing on the ferrite grain boundary. The area of cementite or graphite grains not having a portion present on the ferrite grain boundary was measured as the area occupied by cementite grains or graphite grains present in the ferrite grains.

  Tensile test: JIS No. 5 tensile test specimens were collected along the rolling direction and subjected to a tensile test to obtain YS, El, and YR (= YS / TS × 100). In addition, a tensile test was performed twice for each condition to obtain an average value, which was used as a characteristic value of the steel sheet.

  FIG. 1 shows the relationship between the volume fraction V and YR of cementite and graphite present in the ferrite grains. As shown in Fig. 1, when V is 15% or less, YR of less than 50% is obtained, and in this case, YS of less than 210MPa and El of 49% or more are also obtained. It turns out that it is very excellent in property.

  As a result of various studies based on the above studies, the inventors have determined that the total volume fraction of cementite and graphite present in ferrite grains is 15% or less in order to ensure excellent cold workability. It has been found that more preferably 10% or less. The details of the reason why good cold workability can be obtained by defining the structure in this way are unknown, but it is considered that the difference in distribution of cementite and graphite in grain boundaries and grains is involved. . That is, since the diffusion rate of carbon is faster in the ferrite grain boundary than in the ferrite grain, coarsening of the cohesion is promoted more than in the ferrite grain, and cementite and graphite on the ferrite grain boundary are coarser than those in the ferrite grain. The interval between each cementite grain and graphite grain tends to be widened. For this reason, it is considered that good cold workability can be obtained when the ratio of cementite and graphite existing in ferrite grains is reduced and the ratio of ferrite grains at the ferrite grain boundaries is increased.

3) Production conditions The following are preferred production conditions for the steel sheet of the present invention. In addition, the manufacturing method of the steel plate of this invention is not limited to the following.

Finishing temperature during hot rolling: 800-950 ° C
When the finish temperature during hot rolling is less than 800 ° C, the increase in rolling load becomes significant. When the finish temperature exceeds 950 ° C, the scale to be produced becomes thick and the pickling property decreases, and a decarburized layer is formed on the steel sheet surface layer. Therefore, the temperature is set to 800 to 950 ° C.

Average cooling rate after hot rolling: 50 ° C / s or higher If the steel sheet after hot rolling is immediately cooled to the cooling stop temperature described later at an average cooling rate of 50 ° C / s or higher, it is hot rolled into austenite. The introduced rolling strain tends to remain in the structure after transformation, resulting in an increase in dislocation density. As a result, the number of nucleation sites for graphitization increases, and it becomes easy to form graphite with dislocations as nuclei during annealing. Moreover, if it is cooled at an average cooling rate of 50 ° C./s or more, the formation of pro-eutectoid ferrite is suppressed and ferrite and cementite are finely precipitated. For this reason, C is likely to diffuse into the ferrite grain boundaries during annealing after winding, which promotes the agglomeration, coarsening, and graphitization of cementite on the ferrite grain boundaries, and decreases the cementite and graphite in the ferrite grains. As described above, it is possible to improve cold workability such as low YS. From the above, the average cooling rate is 50 ° C./s or higher, preferably 80 ° C./s or higher. The upper limit of the average cooling rate is not particularly limited, but is preferably set to 200 ° C./s or less in order to suppress the deterioration of the shape of the steel plate and ensure the shape of the steel plate.

Cooling stop temperature in cooling after hot rolling: 600 ° C or less The minimum temperature that needs to be cooled at the cooling rate as described above, that is, when the cooling stop temperature exceeds 600 ° C, it is first precipitated during cooling until winding While ferrite is generated, pearlite is generated, and cementite and graphite present in the ferrite grains are increased during annealing after winding, resulting in a decrease in cold workability such as an increase in YS. Is 550 ° C or less. The lower limit of the cooling stop temperature is not particularly required, but is preferably 200 ° C. or higher in order to ensure the shape of the steel sheet.

Winding temperature: 550 ° C or less The hot-rolled sheet after cooling is immediately wound, but at that time, if the winding temperature exceeds 550 ° C, pearlite is generated, and cementite and graphite existing in the ferrite grains are annealed during annealing. Increased, resulting in a decrease in cold workability such as an increase in YS. Therefore, the coiling temperature is 550 ° C or lower. In order to sufficiently obtain the cooling effect after the hot rolling described above, the winding temperature is preferably lower than the cooling stop temperature. Further, since the shape of the hot-rolled sheet is likely to deteriorate, the coiling temperature is preferably 200 ° C. or higher and more preferably 450 ° C. or higher when ensuring the shape of the steel plate.

Annealing temperature: 720 ° C. or less The hot-rolled sheet after winding is subjected to annealing to remove the scale by pickling or the like and then to promote the spheroidization and graphitization of cementite and to soften it. At that time, if the annealing temperature exceeds 720 ° C., pearlite is generated during cooling, resulting in a decrease in cold workability and the like. Further, if the annealing temperature is less than 600 ° C., there is a tendency that the cementite and graphite present in the ferrite grains increase, leading to an increase in YS and the like, so the annealing temperature is preferably 600 ° C. or higher.

  Note that the annealing time is not particularly limited, but to form graphite and reduce cementite and graphite in the ferrite grains to be 8 hours or more, and the ferrite grains are excessively coarsened, Since there is a possibility of causing a decrease in El, it is preferably 100 hours or less.

  To melt the steel of the present invention, both a converter and an electric furnace can be used. The steel thus melted is made into a slab by ingot-bundling rolling or continuous casting. The slab is usually heated (reheated) and then hot rolled. In addition, in the case of the slab manufactured by continuous casting, you may apply direct feed rolling which rolls as it is or in order to suppress a temperature fall. When the slab is reheated and hot-rolled, the slab heating temperature is preferably 1280 ° C. or lower in order to avoid deterioration of the surface state due to scale. Hot rolling can be performed only by finish rolling, omitting rough 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. The thickness of the hot-rolled sheet is not particularly limited as long as the production conditions of the present invention can be maintained, but is preferably 1.0 to 10.0 mm. The hot-rolled sheet is subjected to hot-rolled sheet annealing after removing the surface scale by pickling or shot blasting. The annealed steel sheet can be temper-rolled as necessary.

  Slabs of steel Nos. A to U having the composition shown in Table 1 were heated to 1250 ° C., hot-rolled under the hot rolling conditions shown in Table 2, and pickled, and then annealed under the annealing conditions shown in Table 2 as well. Thus, steel plates No. 1 to 32 having a plate thickness of 8.0 mm were produced. Then, the graphitization rate, the volume fraction V of cementite and graphite existing in the ferrite grains occupying the entire cementite and graphite, and the tensile property value were determined by the above method.

  The results are shown in Table 3. It can be seen that all the steel sheets of the examples of the present invention have YS of less than 210 MPa, El of 49% or more, and YR of less than 50%, and are excellent in cold workability, particularly shape freezing property. As shown in Table 3, the structure of the steel sheet of the present invention was substantially composed of ferrite, cementite, and graphite, and it was confirmed that the total volume ratio of these was 95% or more. Moreover, it was confirmed that the hardenability was not a problem in the steel sheet of the present invention.

It is a figure which shows the relationship between the volume fraction V and YR of the cementite and graphite which exist in a ferrite grain.

Claims (3)

In mass%, C: 0.40-0.80%, Si: 0.20-2.00%, Mn: more than 0.50 1.50% or less, Al: 0.001-0.150%, P: 0.018% or less, S: 0.010% or less, N: 0.0050% or less And having a composition comprising the balance Fe and inevitable impurities, having a structure containing ferrite, graphite, and cementite, and the total volume ratio of ferrite, graphite, and cementite in the entire structure is 95% or more, graphite the volume ratio of graphite to the total cementite (graphitization ratio) is 40% state, and are a total of 14% or less of graphite and cementite volume fraction of that present in ferrite grains to the total graphite and cementite, the yield strength YS Is a steel sheet with excellent cold workability, characterized in that is less than 210 MPa, total elongation El is 49% or more, and yield ratio YR is less than 50% .   Furthermore, it has a component composition containing at least one selected from mass%, Ti: 0.10% or less, Nb: 0.05% or less, Zr: 0.050% or less, B: 0.01% or less. 2. A steel sheet excellent in cold workability according to claim 1. The steel having the component composition according to claim 1 or 2 is hot-rolled by hot rolling at a finishing temperature of 800 to 950 ° C, and the hot-rolled plate after the hot rolling is at least 50 ° C / s. After cooling to a cooling stop temperature of 550 ° C. or lower at an average cooling rate, winding at a winding temperature of 550 ° C. or lower, and annealing the hot-rolled sheet after the winding at 50 ° C. or lower at an annealing temperature of 720 ° C. or lower The total volume ratio of ferrite, graphite, and cementite in the entire structure is 95% or more, and the volume ratio of graphite in the entire graphite and cementite (graphitization ratio) 40% or more, the total volume ratio of graphite and cementite in the ferrite grains occupying the entire graphite and cementite is 14% or less, the yield strength YS is less than 210 MPa, the total elongation El is 49% or more, the yield ratio YR But A method for producing a steel sheet having excellent cold workability of less than 50% .

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