JP4966316B2 - Steel wire rod excellent in cold workability and hardenability, and manufacturing method thereof - Google Patents

Description

  The present invention relates to a method of manufacturing a steel wire used for a material for cold working, and more specifically, the strength of the material is significantly low and cold working is easy even if the spheroidizing heat treatment is omitted in the wire state. On the other hand, the present invention relates to a steel wire rod excellent in cold workability and hardenability, which can greatly increase the strength by quenching, and a method for producing the same.

  In general, since cold work has higher productivity than hot work or cutting, it is widely used as a method for efficiently producing machine parts such as bolts, nuts, and screws, automobile parts, and the like.

  Since the cold working as described above is forging the material as it is without any special heating process or cutting process, the steel material used for the cold working has physical properties suitable for cold working. Is required. That is, it is required that the material has a low tensile strength so that it can be sufficiently deformed even by a low external force, and that it is required to have excellent ductility so that the material is not broken even during cold and deep processing.

  However, as described above, parts produced by cold working are mainly machine parts such as bolts, nuts, and screws, and automobile parts, and thus are required to have high strength.

  In other words, an excellent cold work material is required to have low strength and high ductility before cold work, but the final product must have high strength. Since the two properties cannot be satisfied at the same time, it is necessary to perform appropriate processing for each stage to satisfy the required physical properties. Usually, a cold-worked part having the shape of the final product undergoes a normal quenching process to increase strength. Although the strength of the parts increases by quenching, an element that can enhance the hardenability is contained in the steel material in order to satisfy a desired strength standard.

  Conventionally, in order to improve the hardenability of steel, a method of containing a large amount of carbon inside the steel material has been used. Carbon is a typical hardenability-enhancing element, and the strength of parts is greatly improved by quenching heat treatment.

  However, a steel material containing a large amount of carbon has a high hardness and strength and is not suitable for use in cold forging as it is without being subjected to a quenching treatment, and therefore requires special treatment. That is, when carbon is dissolved in the steel material, the strength of the steel material increases as a result of the solid solution strengthening of the steel material. As a result, the tool life is reduced during cold working and the material is not ductile. Problems such as cracks occur.

  Therefore, when using a steel material containing a large amount of carbon for cold working, carbon is precipitated in the form of spherical cementite in order to prevent the workability from being reduced by carbon during cold working. Therefore, it is necessary to precede the step of suppressing the solid solution of carbon and causing solid solution strengthening. The step of precipitating the spherical cementite is called spheroidizing heat treatment.

  Therefore, in order to process steel parts containing a large amount of carbon into high-strength parts, the process of spheroidizing heat treatment-cold working-quenching process is finally performed, and the toughness of steel parts is finally improved. In some cases, incineration heat treatment is performed.

  However, since the heat treatment time of several hours to several tens of hours is required for the spheroidizing heat treatment, there is a possibility that the productivity may be lowered and the manufacturing cost may be increased. It is preferable to omit it.

  In order to solve the above problems, Patent Document 1 includes at least one of B: 0.0055% or less and Zr: 0.035% or less by weight%, and N: 0.0005-0. .0070% and B, Zr, and N satisfy −0.001 ≦ [N] −1.3 [B] −0.15 [Zr] ≦ 0.0020, so that the ferrite crystal grains can be refined. The present invention proposes a method for manufacturing a steel wire rod that suppresses an increase in deformation resistance in a room temperature or a heat generation region, which is characterized by suppressing the above.

  In the case of this technique, B or Zr is added to fix solute C and solute N, thereby suppressing the refinement of ferrite crystal grains and suppressing an increase in deformation resistance during cold working. However, the most important thing in the cold working of steel materials in which the spheroidizing heat treatment is omitted is the tensile strength of the material before the cold working, but this invention has a problem that this point is overlooked. ing. That is, before cold working, the lower the tensile strength of the material, the lower the deformation energy, which is advantageous for processing, but there is a limit to reducing the strength of the material only by suppressing ferrite crystal grains and refinement, and as a result When cold working without performing spheroidizing heat treatment, there is a problem that the tool life is reduced and cracks occur in the material.

Japanese Unexamined Patent Publication No. 2001-303189

  The present invention is for solving the above-mentioned problems, and has a sufficiently low strength without performing a spheroidizing heat treatment, which is advantageous for cold working, and the strength during the subsequent quenching process is greatly improved. It is an object of the present invention to provide a steel wire rod excellent in cold workability and hardenability and a method for producing such a steel wire rod.

  In order to achieve the above object, the steel wire rod of the present invention includes C: 0.1 to 0.4% by weight, Si: 0.3 to 1.5% by weight, Mn: 0.3 to 1.7% by weight, P: 0.015 wt% or less, S: 0.015 wt% or less, Cr: 0.05 to 1.7 wt%, Al: 0.05 wt% or less, B: 0.001 to 0.005 wt% Ti: 0.01 to 0.05% by weight, N: 0.015% by weight or less, remaining Fe and other inevitable impurities.

  At this time, it is preferable that the atomic weight ratio Ti / N of Ti and N is 1.39 or more and the weight-based content ratio B / Cr of B and Cr is 0.04 or less.

  The steel wire preferably has an internal structure made of ferrite and pearlite.

  At this time, it is effective that the ferrite fraction in the internal structure is 50% or more.

  Moreover, it is preferable that the sum of the fraction of a bainite and a martensite is 0.5% or less among the said internal structures.

In order to ensure the cold workability of the steel wire rod, the relational expression TS (MPa) = 258 + 959 * [C] + 112 * [Si] + 111 * [Mn] + 5 * [Cr] + 439 * [Ti] -0. 7 * [Ferrite fraction]
It is preferable that the tensile strength represented by 590MPa or less.

  The production method of the present invention for producing the steel wire having the above advantageous effects is as follows: C: 0.1 to 0.4% by weight, Si: 0.3 to 1.5% by weight, Mn: 0.3 1.7% by weight, P: 0.015% by weight or less, S: 0.015% by weight or less, Cr: 0.05-1.7% by weight, Al: 0.05% by weight or less, B: 0.005% or less. After heating a steel slab consisting of 001 to 0.005 wt%, Ti: 0.01 to 0.05 wt%, N: 0.015 wt% or less, the balance Fe and other inevitable impurities at 1000 to 1100 ° C, It rolls and it cools to the temperature of 500 degrees C or less with the cooling rate of 0.1-5 degrees C / sec.

  And at the time of the said rolling, it is good for finish rolling completion | finish temperature to be 850 degrees C or less.

Moreover, when the atomic weight ratio Ti / N of Ti and N is 1.39 or more and the weight-based content ratio B / Cr of B and Cr is 0.04 or less, the advantageous effect of the steel wire rod of the present invention is ensured. It is particularly effective.

The manufactured steel wire has the following relation: TS (MPa) = 258 + 959 * [C] + 112 * [Si] + 111 * [Mn] + 5 * [Cr] + 439 * [Ti] −0.7 * [ferrite content rate]
It is preferable that the tensile strength represented by is 590 MPa or less.

  As described above, according to the present invention, it is possible to provide a steel part excellent in cold pressure composition and a method for manufacturing the same even if the heat treatment step for cold working is omitted.

  The steel parts produced according to the present invention can be used in various fields where high strength is required, such as machine parts, automobile parts, and materials for building structures.

  The present invention is described in detail below.

  Prior to the production of the steel wire of the present invention, the inventor of the present invention should have a tensile strength of 590 MPa or less as a preferred physical property of the steel wire intended in the present invention and an increase in strength by quenching of 100 MPa or more. We tried to establish a steel wire that satisfies the above conditions and a method for manufacturing the same.

(Composition of steel wire)
The steel wire provided by the present invention needs to contain each component element in the range specified below in order to improve cold workability and hardenability. Unless otherwise indicated, the content of each component element described below indicates% by weight. In order to improve cold forgeability without performing spheroidizing heat treatment in the wire state, the present inventors have low strength and can have high tensile strength after quenching and tempering. In order to do so, it has been found that it is important to have a composition that can greatly improve the hardenability of the steel.

  Therefore, as shown in the graph of FIG. 1, in order to confirm the effect of improving the hardenability with respect to the steel component system containing a plurality of main elements, the result of the preliminary test by heating at 880 ° C. and oil cooling. It has been found that the hardenability of the steel component boundary in which C—Si—Mn—Cr—B is contained in the main elements is the best. The hardenability shown in the graph is an index indicating how much the hardness of the part subjected to quenching and tempering has increased by the hardness of the wire before quenching.

C: 0.1 to 0.4% by weight
If carbon (C) is added excessively, the cold workability of steel will decrease, so it needs to be added in an appropriate range. That is, when the carbon content exceeds 0.4% by weight, the pearlite fraction becomes 50% or more, and the cold workability is reduced. On the other hand, if the carbon content is too low and less than 0.1% by weight, the hardenability is reduced, and the strength and fatigue strength of the material of the final product are deteriorated. Therefore, the carbon content is preferably 0.1 to 0.4% by weight.

Si: 0.3 to 1.5% by weight
Silicon (Si) usually performs deoxidation in the steelmaking process, and plays a role of ensuring product strength at the product stage. However, if the Si content is 1.5% by weight or more, the deformation resistance is greatly increased during cold working, and the cold pressure composition is drastically dropped. Then, after quenching and tempering, it is difficult to obtain the strength required for the final product, and it is difficult for the ferrite fraction in the steel material after rolling to be 50% or more. It is preferable to set to 0.3 to 1.5% by weight.

Mn: 0.3 to 1.7% by weight
Manganese (Mn) increases the hardenability of the steel and increases the strength of the steel without reducing impact toughness. If the addition amount is 0.3% by weight or less, it is difficult to obtain such an effect. If the addition amount is 1.7% by weight or more, after hot rolling, the wire tensile strength increases excessively and the cooling is reduced. In order to reduce the intermediate pressure composition, the addition range of manganese is preferably limited to 0.3 to 1.7% by weight.

P: 0.015% by weight or less Phosphorus (P) is segregated at grain boundaries and decreases the toughness and hydrogen fracture resistance of the steel. Therefore, the content is preferably limited to 0.015% by weight or less.

S: 0.015 wt% or less Sulfur (S) is segregated at the grain boundaries and decreases the toughness and hydrogen fracture resistance of the steel. Therefore, the content is preferably limited to 0.015 wt% or less.

Cr: 0.05 to 1.7% by weight
Chromium (Cr) is an element that acts to increase the strength of steel by suppressing the rapid softening of martensite so that a stable martensite structure can be obtained after quenching by increasing the hardenability of steel. If the content is small, it is difficult to obtain such an effect. If the content is large, the effect is saturated. Therefore, the chromium content may be set to 0.05 to 1.7% by weight. preferable.

Al: 0.05% by weight or less Aluminum (Al) is an element useful for deoxidation, so it can be added up to 0.05% or less.

B: 0.001 to 0.005% by weight
Boron (B) is an element that greatly improves the hardenability when added in a small amount. Therefore, hardenability can be obtained in the same steel even if the carbon content is reduced. In the case of addition of 0.005% by weight or more or 0.001% by weight or less, the hardenability is drastically lowered. Therefore, the amount of boron is preferably set to 0.001 to 0.005% by weight.

Ti: 0.01 to 0.05% by weight
Titanium (Ti) is an element necessary for fixing nitrogen and securing the hardenability effect of boron, and can suppress austenite crystal grain growth and suppress fatigue failure of the product when it is overheated during quenching. In order to exhibit such an effect, it is necessary to contain at least 0.01% by weight or more. On the other hand, when the content exceeds 0.05% by weight, a problem occurs in Ti-based precipitates and wire strength increase due to solid solution and cold pressure composition. Therefore, the titanium content is preferably 0.01 to 0.05%.

N: 0.015 wt% or less Nitrogen is an element that easily reacts with boron to form BN. Therefore, the nitrogen content should be as low as possible, and if it exceeds 0.015% by weight, it is difficult to obtain a sufficient quenching effect.

Ti / N: 1.39 or more (atomic ratio, omitted below)
As described above, nitrogen is an element that binds to boron and reduces the effective boron content (B _eff ) to impair the hardenability of the steel wire, so it is preferably contained as little as possible. However, since nitrogen is not an element that can be easily removed in the steelmaking stage, it is effective to reduce the nitrogen activity by other methods. That is, titanium can be cited as an element having an affinity for nitrogen, but when the titanium is contained together with nitrogen, TiN or the like can be formed to reduce the activity of nitrogen. When the activity of nitrogen is reduced, the driving force for forming BN is weakened. As a result, the effective boron amount is increased, which is effective in improving the hardenability of the steel wire. As described above, in order to decrease the activity of nitrogen and increase the effective boron amount, the Ti / N ratio needs to be 1.39 or more. If Ti / N is less than 1.39, a sufficient nitrogen fixing effect does not appear and the effect of improving the hardenability becomes insufficient.

B / Cr: 0.04 or less (weight ratio, hereinafter omitted)
The inventors of the present invention have studied the factors for improving the hardenability of the steel wire, and as a result, the hardenability is improved when boron is included together with chromium rather than when it is included alone. It turns out that the effect is excellent. The relationship between chromium and boron is preferably 0.04 or less in terms of B / Cr ratio.

(Organization)
The steel wire material targeted by the present invention preferably contains 50% or more of ferrite in the area fraction. That is, if the structure other than ferrite is contained by 50% or more, the strength of the steel material increases and the workability deteriorates. In particular, martensite and bainite preferably have a sum of area fractions of 0.5% or less. This is because when a hard structure such as martensite and bainite is formed at the stage of the steel wire rod, the workability of the wire rod is remarkably lowered. Therefore, the preferred internal structure of the steel wire provided by the present invention is a structure mainly containing ferrite, pearlite, etc., and the area ratio of ferrite is 50% or more, and the area ratio of martensite and bainite including the remaining pearlite. The sum is an organization of 0.5% or less.

(Production method)
As described above, in order to obtain the microstructure of 50% or more of ferrite and 0.5% or less of bainite + martensite and the remaining pearlite as intended in the present invention, the production conditions for the wire must be as follows: Goodbye. The heating temperature of the steel slab is in the range of 1000 to 1100 ° C., which is the normal heating temperature of the wire, and the final rolling temperature is 850 after final rolling in order to refine the austenite crystal grains of the material being rolled. It must be hot rolled below ℃. The rolled material needs to be cooled to 500 ° C. at 0.1 to 5 ° C./sec in order to promote ferrite nucleation from the refined crystal grains and increase the ferrite fraction during cooling of the wire. .

Billet reheating temperature: 1000-1100 ° C
The reheating temperature of the steel slab is preferably in the range of 1000 to 1100 ° C. This is the same condition as a normal wire heating operation.

Wire cooling rate: 0.01 to 5 ° C./sec
After the steel slab is hot-rolled in the shape of a wire, the wire cooling rate during cooling is preferably 0.01 to 5 ° C./sec. That is, as described above, the steel parts of the present invention are manufactured by cold working, and if the wire strength before cold working is too high, the deformation resistance increases and the life of the die decreases. there's a possibility that. Therefore, it is preferable that the wire is gradually cooled by a cooling pattern as much as possible. If the wire cooling rate is 5 ° C./sec or more, the strength of the wire becomes high as described above, which is disadvantageous during cold working, and the reason why the lower limit of the cooling rate is limited to 0.01 ° C./sec is a reality. This is because it is difficult to cool the steel material at a lower cooling rate.

Cooling stop temperature of wire: 500 ° C. or less When cooling the wire, controlled cooling must be performed at a cooling rate limited to 500 ° C. or less. When the temperature of the wire becomes 500 ° C. or less, there is no further change in the internal structure or increase in strength, and if the cooling condition is continuously maintained, the productivity deteriorates, so the controlled cooling stops at 500 ° C. or less. .

  It is more preferable that the finish rolling temperature of the wire is rolled at a temperature of 850 ° C. or less in the method for producing a wire as described above. The reason for limiting the finish rolling temperature to the above range is to obtain fine crystal grains, and when the finish rolling is performed at a temperature exceeding 850 ° C., the crystal grains become coarse. When the crystal grains are coarse, the location of ferrite nucleation decreases in the subsequent cooling process, the ferrite fraction increases, and the martensite and bainite fractions increase.

  When producing a wire under all the above conditions, a wire having a tensile strength of 590 MPa or less can be obtained.

(Example)
Hereinafter, the present invention will be described more specifically based on the following examples. However, the following examples are for explaining the present invention in more detail, and are not intended to limit the scope of rights of the present invention.

  Table 1 shows the components of Examples and Comparative Examples. In all cases, P and S were controlled and manufactured so as to be 0.02% by weight or less during the steelmaking operation.

  In Table 1 above, Examples 1 to 7 show those having compositions that satisfy all of the conditions defined in the present invention.

  However, in the case of Comparative Example 1, the C and Si contents deviate from the specified values, and the case where Cr, B, Ti, etc. that are advantageously added in the present invention are not added is shown. Comparative Example 2 shows the case where the Ti content exceeds the upper limit, Comparative Example 3 and Comparative Example 5 show the case where the Si content does not reach the provisions of the present invention, and Comparative Example 4 shows that the Si and Ti contents are Indicates a case that falls outside the standard. And the comparative example 6 shows the case where Cr is not added, and the comparative example 7 shows the case where B and Ti are not added.

  Wires were produced from the materials having the components shown in Table 1 under the conditions shown in Table 2.

  Under the above conditions, Comparative Examples 1 to 6 which do not correspond to the composition of the present invention were all manufactured by the manufacturing method of the present invention, and the effects on the composition were compared. However, Comparative Example 1 also compared the effect of the cooling stop temperature when the cooling stop temperature was set to a temperature higher than 500 ° C. defined in the present invention.

  Moreover, in order to observe the influence of manufacturing conditions also in an Example, Example 4 and Example 5 showed the result tested about the case where finish rolling temperature is very high.

  Examples 1 to 3 and Examples 6 and 7 show manufacturing methods that satisfy all of the conditions defined in the present invention.

  Table 3 shows the results of physical property analysis on the wire manufactured under the above conditions and the results of quench forging and tempering after cold forging without special spheroidizing heat treatment using the wire and processing into a bolt shape. . Quenching conditions and tempering conditions are not particularly limited in the present invention, and may be carried out in accordance with normal bolt quenching and tempering conditions.

  As can be seen from Table 3 above, in the case of Examples 1 to 3 and Examples 6 and 7, which satisfy all of the composition defined in the present invention and the production conditions of the present invention, the ferrite fraction exceeds 50%. The bainite + martensite fraction has a structure of less than 0.5%. In such a case, the tensile strengths of the wires are all 550 MPa or less, and the number of bolts that can be manufactured with one die exceeds 99000 times, and the physical properties suitable for cold forging as it is without special spheroidizing heat treatment. Have.

  However, in the case of Example 4 and Example 5 in which the finish rolling temperature of the wire was high even if the composition of the present invention was satisfied, the fraction of ferrite decreased due to coarse crystal grains, and the fraction of bainite + martensite increased. It showed the problem that cold forgeability decreased.

  And in the case of the comparative example 1 thru | or the comparative example 7 which does not satisfy | fill the composition of this invention, although it manufactured according to all the manufacturing conditions prescribed | regulated by this invention, a martensite and a bainite are formed in large quantities inside, and a wire rod It can be seen that it is difficult to carry out cold forging as it is because the strength of is higher than 600 MPa. Further, when cold working using these, it is found that the life of the die is remarkably reduced, and it is understood that the increase in strength after quenching and tempering is not so high.

  From the above results, it can be seen that the cold pressure composition of the wire increases as the ferrite fraction increases and as the sum of the fractions of martensite and bainite decreases. It was confirmed that the proper ferrite fraction capable of securing the cold pressure composition was 50% or more, and the sum of the fractions of bainite and martensite was 0.5% or more.

  FIG. 2 compares the life of dies of a standard steel called SWRCH45 used for cold forging after spheroidizing heat treatment in Example 1 manufactured according to the present invention and steel developed for normal cold forging. The results are shown in the graph. As can be seen from the graph, the result of Example 1 manufactured according to the present invention compared to the steel subjected to the spheroidizing heat treatment is shown to be substantially the same level, and thus the steel wire of the present invention and the manufacturing method thereof are excellent. I was able to confirm.

  Moreover, in order to observe the influence of each composition in FIG. 1, the result of having compared the hardenability according to design component system was shown. In FIG. 1, C-Mn represents Comparative Example 1, C-Mn-B represents Comparative Example 6, C-Mn-Cr represents Comparative Example 7, and C-Mn-Cr-B represents Example 2. As can be seen from FIG. 1, it can be seen that the hardenability is the best in Example 2.

  As a result of observing the influence of the tensile strength of the wire and the steel composition by combining the above results, the present inventors were able to obtain a regression equation such as the following relational expression.

  TS (MPa) = 258 + 959 * [C] + 112 * [Si] + 111 * [Mn] + 5 * [Cr] + 439 * [Ti] −0.7 * [ferrite fraction]

  When the above regression equation is used, the tensile strength at the wire stage can be predicted to some extent in advance, so that the composition and the production method can be selected in advance and used for the production of the wire. Therefore, it is effective for producing a wire material of 590 MPa or less, which is the object of the present invention. From the above regression equation, it is necessary to limit the contents of C, Si, Mn, Cr, Ti, etc. in order to reduce the tensile strength of the wire. However, when such elements are limited, the hardenability is not ensured, and thus there is a contradiction that it is difficult to ensure the strength after bolt processing. In order to solve such a problem, the element selected in the present invention is B, and this element does not increase the tensile strength of the wire and is a useful element for securing the strength of the bolt.

It is the graph which showed the result of having compared the life of the cold die of invention steel and conventional steel. It is the graph which showed the result of having compared the hardenability of invention steel and comparative steel.

Claims (6)

C: 0.1 to 0.4 wt%, Si: 0.3 to 1.5 wt%, Mn: 0.3 to 1.7 wt%, P: 0.015 wt% or less, S: 0.015 % By weight, Cr: 0.05 to 1.7% by weight, Al: 0.05% by weight or less, B: 0.001 to 0.005% by weight, Ti: 0.01 to 0.05% by weight, N : 0.015 wt% or less, Ri Do balance of Fe and other unavoidable impurities, wherein at Ti and N in atomic weight ratio Ti / N is 1.39 or more, by weight content ratio B / Cr of B and Cr is 0. A steel wire having excellent cold workability and hardenability , having an internal structure composed of ferrite and pearlite and having a ferrite fraction of 50% or more in the internal structure . The steel wire rod excellent in cold workability and hardenability according to claim 1 , wherein the sum of the fractions of bainite and martensite in the internal structure is 0.5% or less. Relational expression TS (MPa) = 258 + 959 * [C] + 112 * [Si] + 111 * [Mn] + 5 * [Cr] + 439 * [Ti] −0.7 * [ferrite fraction]
The steel wire rod excellent in cold workability and hardenability according to claim 2 , wherein the tensile strength represented by the formula (1) is 590 MPa or less. C: 0.1 to 0.4 wt%, Si: 0.3 to 1.5 wt%, Mn: 0.3 to 1.7 wt%, P: 0.015 wt% or less, S: 0.015 % By weight, Cr: 0.05 to 1.7% by weight, Al: 0.05% by weight or less, B: 0.001 to 0.005% by weight, Ti: 0.01 to 0.05% by weight, N : 0.015 wt% or less, Ri Do balance of Fe and other unavoidable impurities, wherein at Ti and N in atomic weight ratio Ti / N is 1.39 or more, by weight content ratio B / Cr of B and Cr is 0. Cold-workability and quenching characterized by heating a steel slab of 04 or less at 1000 to 1100 ° C., then rolling and cooling to a temperature of 500 ° C. or less at a cooling rate of 0.1 to 5 ° C./sec. A method of manufacturing steel wires with excellent properties. The method for producing a steel wire rod excellent in cold workability and hardenability according to claim 4 , wherein the finish rolling finish temperature is 850 ° C. or lower during the rolling. The manufactured steel wire has a relational expression TS (MPa) = 258 + 959 * [C] + 112 * [Si] + 111 * [Mn] + 5 * [Cr] + 439 * [Ti] −0.7 * [ferrite fraction]
The method for producing a steel wire rod excellent in cold workability and hardenability according to claim 4 or 5 , wherein the tensile strength represented by the formula (1) is 590 MPa or less.

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