Patent Application
20250101554
The present disclosure relates to an electric resistance welded steel pipe for an automotive part, and a method of producing an automotive part.
Techniques for producing an automotive part such as a rack bar, using a steel pipe as a material, have been conventionally known (see, for example, Patent Documents 1 to 5).
Patent Documents 6 and 7 disclose electric resistance welded steel pipes for an automotive part or a mechanical structure, which are for use in cold working.
Patent Document 8 discloses a high-carbon steel pipe having a structure in which cementite particles are finely dispersed in a ferrite phase as a base phase, as a high-carbon steel pipe for an automotive part.
When it is intended to produce an automotive part (such as a rack bar), by subjecting an electric resistance welded steel pipe for an automotive part to cold working using a mold, and then to quenching, the electric resistance welded steel pipe for an automotive part may be required to have an excellent cold workability and mold life.
Further, a moderate hardness after quenching (namely, a hardness that is not too high and not too low) may be required for the electric resistance welded steel pipe for an automotive part, from the viewpoint of ensuring the hardness and machine processability of the automotive part to be produced.
The expression “to have an excellent mold life” as used herein means being able to increase the life of the mold to be used when subjecting the electric resistance welded steel pipe for an automotive part to cold working.
An object of the present disclosure is to provide an electric resistance welded steel pipe for an automotive part, which has an excellent cold workability and mold life, and a moderate hardness after quenching, as well as a method of producing an automotive part, for producing the automotive part using the electric resistance welded steel pipe for an automotive part.
Means for solving the above-described problems encompass the following aspects.
The present disclosure provides an electric resistance welded steel pipe for an automotive part, which has an excellent cold workability and mold life, and a moderate hardness after quenching, as well as a method of producing an automotive part, for producing the automotive part using the electric resistance welded steel pipe for an automotive part.
In the present disclosure, any numerical range indicated using the expression “from * to” represents a range in which numerical values described before and after the “to” are included in the range as a lower limit value and an upper limit value.
In the present disclosure, the symbol “%” indicating the content of a component (element) refers to “% by mass”.
In the present disclosure, the content of C (carbon) may be referred to as “C content”. The same may apply to the contents of other elements.
Further, in the present disclosure, the definition of the term “step” includes not only an independent step, but also a step which is not clearly distinguishable from another step, as long as the intended purpose of the step is achieved.
In the present disclosure, the term “base metal portion” refers to a portion other than the electric resistance welded portion and a heat affected zone, of the electric resistance welded steel pipe. The “heat affected zone (hereinafter, also referred to as “HAZ”)” as used herein refers to a zone which is present in the vicinity of the electric resistance welded portion and which has been affected by heat caused by electric resistance welding and a seam heat treatment.
In the present disclosure, the term “as-rolled electric resistance welded steel pipe” refers to an electric resistance welded steel pipe that has not been subjected to a heat treatment other than a seam heat treatment, after pipe-making.
The term “pipe-making” refers to a process of roll-forming a hot-rolled steel sheet that has been uncoiled from a hot coil to form an open pipe, and subjecting the abutting portions of the resulting open pipe to electric resistance welding to form an electric resistance welded portion.
The term “hot coil” refers to a hot-rolled steel sheet that has been produced using a hot strip mill, and coiled in the form of a coil.
The term “roll forming (to roll-form)” refers to subjecting a hot-rolled steel sheet that has been uncoiled from a hot coil to bending continuously, to form the sheet in the form of an open pipe.
The hot-rolled steel sheet produced using a hot strip mill is different from a steel plate produced using a plate mill in that the hot-rolled steel sheet is a continuous steel sheet.
A steel plate cannot be used in roll forming which is a continuous bending, since the steel plate is not a continuous steel sheet.
An electric resistance welded steel pipe is clearly distinguished from a welded steel pipe produced using a steel plate (such as a UOE steel pipe), in the points described above.
The electric resistance welded steel pipe for an automotive part according to the present disclosure (hereinafter, also simply referred to as “electric resistance welded steel pipe”) includes a base metal portion and an electric resistance welded portion,
The electric resistance welded steel pipe according to the present disclosure has an excellent cold workability and mold life, and a moderate hardness after quenching (namely, a hardness that is not too high and not too low).
The fact that the hardness (namely, the hardness before quenching) of the base metal portion is not too high, specifically, that the hardness is 150 Hv or less, contributes to the effects of the cold workability and mold life described above.
The chemical composition of the base metal portion (for example, having a C content of 0.42% or more in order to adequately control the hardness of the base metal portion, and having a B content of 0.0005% or more in order to decrease a difference in hardness, AHv, in a wall thickness direction, which will be described later), and the fact that the base metal portion has a hardness (namely, hardness before quenching) of from 110 to 150 Hv, contribute to the effect of having a moderate hardness after quenching.
A preferred range of the hardness of the electric resistance welded steel pipe that has been quenched, in a case in which the electric resistance welded steel pipe according to the present disclosure is quenched, will be described later.
In the electric resistance welded steel pipe according to the present disclosure, the fact that the base metal portion has a hardness of from 110 to 150 Hv is achieved by subjecting an as-rolled electric resistance welded steel pipe in which the Si content is reduced to 0.20% or less, the Mn content is reduced to 0.70% or less and B is contained in an moderate amount, in the chemical composition of the base metal portion, to a heat treatment under predetermined conditions, without performing a common normalizing (see, for example, production method X to be described later).
The content of each element in the chemical composition of the base metal portion in the electric resistance welded steel pipe according to the present disclosure (hereinafter, also referred to as “chemical composition in the present disclosure”) will be described below.
C is an element effective for increasing the cold workability of the steel structure, and for ensuring the hardness after quenching required for an automotive part.
In a case in which the C content is less than 0.42%, the hardness after quenching may be insufficient. Accordingly, the C content is 0.42% or more. The C content is preferably 0.43% or more.
In a case in which the C content is higher than 0.48%, however, the cold workability may decrease. Further, the hardness after quenching may be too high, possibly resulting in a decrease in toughness. Accordingly, the C content is 0.48% or less. The C content is preferably 0.47% or less.
Si is an element capable of contributing to an improvement in the hardness, and contributing to an improvement in the cold workability by controlling the precipitation of carbides.
In a case in which the Si content is less than 0.01%, it may lead to a significant increase in the cost of refining, and to a failure to sufficiently obtain the effect of adding Si. Accordingly, the Si content is 0.01% or more. The Si content is preferably 0.05% or more.
In a case in which the Si content is higher than 0.20%, however, the hardness may be too high, possibly resulting in a decrease in the cold workability. Accordingly, the Si content is 0.20% or less. The Si content is preferably 0.15% or less.
Mn is an element capable of contributing to an improvement in the hardness, and contributing to an improvement in the cold workability by controlling the precipitation of carbides.
In a case in which the Mn content is less than 0.10%, it may lead to a failure to sufficiently obtain the effect of adding Mn. Accordingly, the Mn content is 0.10% or more. The Mn content is preferably 0.25% or more.
In a case in which the Mn content is higher than 0.70%, however, the hardness may be too high, possibly resulting in a decrease in the cold workability. Accordingly, the Mn content is 0.70% or less. The Mn content is preferably 0.55 or less.
P is an element that can hinder the cold workability or the toughness by segregating at grain boundaries.
In a case in which the P content is higher than 0.030%, a decrease in the cold workability or the toughness may occur. Accordingly, the P content is 0.030% or less. The P content is preferably 0.020% or less.
The P content may be 0%, or may be higher than 0%.
The P content may be 0.0001% or more, from the viewpoint of the production cost.
S is an element that can hinder the cold workability or the toughness by forming MnS.
In a case in which the S content is higher than 0.030%, a decrease in the cold workability or the toughness may occur. Accordingly, the S content is 0.030% or less. The S content is preferably 0.020% or less.
The S content may be 0%, or may be higher than 0%.
The S content may be 0.0001% or more, from the viewpoint of the production cost.
Al is an element effective for deoxidation.
In a case in which the Al content is less than 0.005%, it may lead to a failure to sufficiently obtain the effect of adding Al. Accordingly, the Al content is 0.005% or more.
The Al content is preferably 0.03% or more.
In a case in which the Al content is higher than 0.050%, however, coarse Al oxide may be formed, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the Al content is 0.050% or less. The Al content is preferably 0.040% or less.
Ti is an element that forms TiN and contributes to the refinement of the steel structure, and that reduces the formation of BN to ensure the effect of B to improve the hardenability.
In a case in which the Ti content is less than 0.005%, it may lead to a failure to sufficiently obtain the effect of adding Ti. Accordingly, the Ti content is 0.005% or more. The Ti content is preferably 0.008% or more.
In a case in which the Ti content is higher than 0.040%, however, coarse Ti compounds may be formed, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the Ti content is 0.040% or less. The Ti content is preferably 0.003% or less.
B is an element capable of contributing to an improvement in the hardness by increasing the hardenability.
In a case in which the B content is less than 0.0005%, it may lead to a failure to sufficiently obtain the effect of adding B, possibly resulting in the occurrence of quenching spots. As a result, the difference in hardness, AHv, in the wall thickness direction to be described later may increase.
Accordingly, the B content is 0.0005% or more. The B content is preferably 0.0008% or more.
In a case in which the B content is higher than 0.0050%, however, B compounds may be precipitated at crystal grain boundaries, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the B content is 0.0050% or less. The B content is preferably 0.040% or less.
N is an element capable of contributing to the refinement of the steel structure by forming fine nitrides.
In a case in which the N content is higher than 0.005%, coarse nitrides may be formed, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the N content is 0.005% or less. The N content is preferably 0.003% or less.
The N content may be 0%, or may be higher than 0%.
The N content may be 0.0001% or more, from the viewpoint of the production cost.
O is an element that can hinder the cold workability or the toughness by forming oxide-based inclusions. In a case in which the O content is higher than 0.005%, coarse oxide-based inclusions are formed, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the O content is 0.005% or less. The O content is preferably 0.003% or less.
The O content may be 0%, or may be higher than 0%.
The O content may be 0.0001% or more, from the viewpoint of the production cost.
Ca is an optional element.
Therefore, the Ca content may be 0%, or may be higher than 0%.
Ca is an element capable of contributing to an improvement in the cold workability of the steel structure, by controlling the shape of inclusions to homogenize the steel structure. From the viewpoint of such an effect, the Ca content is preferably 0.0005% or more, and more preferably 0.0008% or more.
In a case in which the Ca content is higher than 0.0050%, however, an excessive amount of inclusions may be formed, possibly resulting in a decrease in the cold workability. Accordingly, the Ca content is preferably 0.0050% or less, and more preferably 0.0035% or less.
Mg is an optional element.
Therefore, the Mg content may be 0%, or may be higher than 0%.
Mg is an element capable of contributing to an improvement in the cold workability of the steel structure, by controlling the shape of inclusions to homogenize the steel structure. From the viewpoint of such an effect, the Mg content is preferably 0.0005% or more, and more preferably 0.0008% or more.
In a case in which the Mg content is higher than 0.005%, however, an excessive amount of inclusions may be formed, possibly resulting in a decrease in the cold workability of the steel structure. Accordingly, the Mg content is preferably 0.0050% or less, and more preferably 0.0035% or less.
REM is an optional element.
Therefore, the REM content may be 0%, or may be higher than 0%.
The term “REM” as used herein refers to a rare earth element(s), namely, at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The “REM content” refers to the total content of the rare earth element(s).
REM is an element capable of contributing to an improvement in the cold workability of the annealed structure, by controlling the shape of inclusions to homogenize the steel structure. From the viewpoint of such an effect, the REM content is preferably 0.0005% or more, and more preferably 0.0008% or more.
In a case in which the REM content is higher than 0.0050%, however, an excessive amount of inclusions may be formed, possibly resulting in a decrease in the cold workability of the steel structure. Accordingly, the REM content is preferably 0.0050% or less, and more preferably 0.0035% or less.
Cr is an optional element.
Therefore, the Cr content may be 0%, or may be higher than 0%.
Cr is an element capable of contributing to an improvement in the strength of the steel structure. From the viewpoint of such an effect, the Cr content is preferably 0.01% or more, and more preferably 0.03% or more.
In a case in which the Cr content is higher than 0.50%, however, coarse Cr compounds may be formed to cause a decrease in the homogeneity of the steel structure, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the Cr content is preferably 0.50% or less, more preferably 0.10% or less, and still more preferably 0.07% or less.
Ni is an optional element.
Therefore, the Ni content may be 0%, or may be higher than 0%.
Ni is an element capable of contributing to an improvement in the strength of the steel structure. From the viewpoint of such an effect, the Ni content is preferably 0.01% or more, and more preferably 0.03% or more.
In a case in which the Ni content is higher than 0.50%, however, Ni may segregate at crystal grain boundaries to cause a decrease in the homogeneity of the steel structure, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the Ni content is preferably 0.50% or less, more preferably 0.10% or less, and still more preferably 0.07% or less.
Cu is an optional element.
Therefore, the Cu content may be 0%, or may be higher than 0%.
Cu is an element capable of contributing to an improvement in the strength of the steel structure. From the viewpoint of such an effect, the Cu content is preferably 0.01% or more, and more preferably 0.03% or more.
In a case in which the Cu content is higher than 0.50%, however, the weldability of the steel may be decreased, possibly resulting in a decrease in the mechanical properties of the electric resistance welded portion. Accordingly, the Cu content is preferably 0.50% or less, more preferably 0.10% or less, and still more preferably 0.07% or less.
Nb is an optional element.
Therefore, the Nb content may be 0%, or may be higher than 0%.
Nb is an element capable of contributing to the refinement and an improvement in the strength of the steel structure. From the viewpoint of such effects, the Nb content is preferably 0.01% or more, and more preferably 0.03% or more.
In a case in which the Nb content is higher than 0.10%, however, coarse Nb compounds may be formed to cause a decrease in the homogeneity of the steel structure, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the Nb content is preferably 0.10% or less, more preferably 0.05% or less, and still more preferably 0.03% or less.
Mo is an optional element.
Therefore, the Mo content may be 0%, or may be higher than 0%.
Mo is an element capable of contributing to an improvement in the strength of the steel structure. From the viewpoint of such an effect, the Mo content is preferably 0.01% or more, and more preferably 0.03% or more.
In a case in which the Mo content is higher than 0.50%, however, the weldability of the steel may decrease, possibly resulting in a decrease in the mechanical properties of the electric resistance welded portion. Accordingly, the Mo content preferably 0.50% or less, and more preferably 0.35% or less.
V is an optional element.
Therefore, the V content may be 0%, or may be higher than 0%.
V is an element capable of contributing to the refinement and an improvement in the strength of the steel structure. From the viewpoint of such effects, the V content is preferably 0.01% or more, and more preferably 0.03% or more.
In a case in which the V content is higher than 0.20%, however, coarse V compounds may be formed to cause a decrease in the homogeneity of the steel structure, possibly resulting in a decrease in the cold workability or the toughness. Accordingly, the V content is preferably 0.20% or less, and more preferably 0.10% or less.
In the chemical composition of the base metal portion, the balance excluding the respective elements described above is Fe and impurities.
The term “impurities” as used herein refers to components which are contained in raw materials (such as ores and scraps), or components which are mixed during production steps and are not intentionally incorporated into the steel.
Examples of the impurities include all elements other than the elements described above. Only one kind, or two or more kinds of elements may be contained as the impurities.
Examples of the impurities include Sb, Sn, W, Co, As, Pb, Bi, and H.
Among the elements described above, Sb, Sn, Co or As can be contained, for example, in a content of 0.1% or less, Pb or Bi can be contained, for example, in a content of 0.005% or less, and H can be contained, for example, in a content of 0.0004% or less, as the impurities.
The contents of other elements need not be particularly controlled, as long as the contents are within usual ranges.
From the viewpoint of the effects of the following elements, the chemical composition of the base metal portion may contain one or more elements selected from the group consisting of:
More preferred ranges of the contents of these elements are as described above.
In the electric resistance welded steel pipe according to the present disclosure, the steel structure of the base metal portion is a ferrite-pearlite mixed structure.
The steel structure of the base metal portion is identified as follows.
A central portion in the wall thickness direction at a base metal 1800 position (namely, a position 180 degrees away from the electric resistance welded portion in a pipe circumferential direction) in a C cross section of the electric resistance welded steel pipe, is taken as an observation surface. This observation surface is observed with an optical microscope at a magnification of 100 times, to identify the steel structure.
In the electric resistance welded steel pipe according to the present disclosure, cementite in the pearlite in the ferrite-pearlite mixed structure may be lamellar cementite, or cementite formed by fragmentation of the lamellar cementite.
In the electric resistance welded steel pipe according to the present disclosure, the base metal portion has a hardness (namely, hardness before quenching) of from 110 to 150 Hv.
In a case in which the hardness before quenching is less than 110 Hv, it may result in a failure to obtain the hardness required for an automotive part, after quenching. Accordingly, the hardness before quenching of the base metal portion is 110 Hv or more, and preferably 120 Hv or more.
In a case in which the hardness before quenching is higher than 150 Hv, however, it may lead to an impairment in the cold workability of the steel structure or a decrease in the mold life. Accordingly, the hardness before quenching of the base metal portion is 150 Hv or less, and preferably 140 Hv or less.
In the present disclosure, the “hardness” (Hv) refers to a Vickers hardness as measured in accordance with JIS Z 2244 (2009) at a test force of 0.98 N.
The hardness (namely, Vickers hardness) of the base metal portion in the electric resistance welded steel pipe according to the present disclosure is determined as follows.
In a C cross section (namely, a cross section perpendicular to the pipe axis direction) of the electric resistance welded steel pipe, in the respective positions that are 90 degrees, 180 degrees and 270 degrees away from the electric resistance welded portion in the circumferential direction in clockwise rotation when the electric resistance welded portion is defined as 0 degrees (namely, “base metal 900 position”, “base metal 1800 position” and “base metal 270° position”, respectively), the positions at a depth of 0.5 mm from the inner surface (three locations in total) of the pipe and the positions at a depth of 0.5 mm from the outer surface (three locations in total) of the pipe are taken as measurement positions (six locations in total).
A Vickers hardness test in accordance with JIS Z 2244 (2009) is carried out at each of the above-described six locations, to obtain the Vickers hardness (Hv) at each position. The test force is set to 0.98 N.
The arithmetic mean value of the thus obtained six Vickers hardness values (measured values) is defined as “the hardness of the base metal portion”.
The size of the electric resistance welded steel pipe according to the present disclosure is not particularly limited.
The electric resistance welded steel pipe according to the present disclosure has an outer diameter of, for example, from 19.0 to 114.3 mm.
In the electric resistance welded steel pipe according to the present disclosure, the value (t/D value) obtained by dividing the wall thickness (t) of the base metal portion by the outer diameter (D) of the electric resistance welded steel pipe is, for example, from 0.02 to 0.30.
In the electric resistance welded steel pipe according to the present disclosure, the wall thickness of the base metal portion is, for example, from 2.0 to 10.0 mm.
<Preferred Hardness after Quenching of Electric Resistance Welded Steel Pipe>
As described above, the electric resistance welded steel pipe according to the present disclosure has a moderate hardness after quenching.
A description will be given below of a preferred range of the hardness of the electric resistance welded steel pipe that has been quenched, in a case in which the electric resistance welded steel pipe according to the present disclosure is quenched.
The electric resistance welded steel pipe according to the present disclosure preferably has a hardness (namely, hardness after quenching) of from 650 to 800 Hv, in a case in which the steel pipe is subjected to quenching under conditions of a heating temperature of 900° C., a heating time of one minute, a cooling rate after heating of 30° C./s, and a cooling attainment temperature at the cooling rate, within a range of from 50° C. to 0° C.
In a case in which the hardness after quenching is 650 Hv or more, it is possible to ensure the hardness and the wear resistance of an automotive part produced by subjecting the electric resistance welded steel pipe according to the present disclosure to cold working and quenching. The hardness after quenching is preferably 700 Hv or more.
In a case in which the hardness after quenching is 800 Hv or less, it is advantageous in machine processability at the time of further machine processing the automotive part produced by subjecting the electric resistance welded steel pipe according to the present disclosure to cold working and quenching. The hardness after quenching is preferably 770 Hv or less.
The hardness after quenching of the electric resistance welded steel pipe according to the present disclosure is determined in the same manner as the above-described method of determining the hardness (namely, hardness before quenching) of the base metal portion in the electric resistance welded steel pipe according to the present disclosure.
From the viewpoint of further reducing strain in the thickness direction, in the electric resistance welded steel pipe after the above-described quenching, an absolute value of a difference between the following maximum value and the following minimum value (hereinafter, also referred to as “Hardness difference in wall thickness direction ΔHv”) is preferably 50 Hv or less. The maximum value and the minimum value are a maximum value and a minimum value respectively among the hardness at a position corresponding to ¼ of the wall thickness from the outer surface in the base metal 180° position of the electric resistance welded steel pipe, the hardness at a position corresponding to ½ of the wall thickness from the outer surface in the base metal 180° position of the electric resistance welded steel pipe, and the hardness at a position corresponding to ¾ of the wall thickness from the outer surface in the base metal 180° position of the electric resistance welded steel pipe.
One example of the method of producing the electric resistance welded steel pipe according to the present disclosure (hereinafter, referred to as “production method X”) will be described below.
The production method X to be described below is a method of producing an electric resistance welded steel pipe of each Example to be described later.
The production method X includes:
According to the production method X, it is possible to produce the electric resistance welded steel pipe according to the present disclosure in which the base metal portion has a hardness of from 110 to 150 Hv, by combining the “chemical composition in the present disclosure” in which the Si content is reduced to 0.20% or less and the Mn content is reduced to 0.70% or less, and the heat treatment under the conditions to be described later.
The respective steps which can be included in the production method X will be described below.
The as-rolled electric resistance welded steel pipe preparation step in the production method X is a step of preparing the above-described as-rolled electric resistance welded steel pipe.
The present step may be a step of merely preparing the above-described as-rolled electric resistance welded steel pipe which has been produced in advance, or may be a step of producing the above-described as-rolled electric resistance welded steel pipe.
The as-rolled electric resistance welded steel pipe can be produced by:
In the preparation step, a seam heat treatment may be performed on the electric resistance welded portion, after forming the electric resistance welded portion.
The production method X includes a heat treatment step of performing a heat treatment on the as-rolled electric resistance welded steel pipe under the following heat treatment conditions.
The fact that the heat treatment temperature (heating temperature) is 720° C. or higher makes it possible to achieve a hardness of the base metal portion of 150 Hv or less, in the electric resistance welded steel pipe to be produced.
The fact that the heat treatment temperature (heating temperature) is 780° C. or lower makes it possible to achieve a hardness of the base metal portion of 150 Hv or less, in the electric resistance welded steel pipe to be produced.
The fact that the heat treatment time (retention time at the heating temperature) is 20 minutes or less makes it possible to achieve a hardness of the base metal portion of 150 Hv or less, in the electric resistance welded steel pipe to be produced.
The heat treatment time (retention time at the heating temperature) is preferably 10 minutes or less.
The fact that the heat treatment time (retention time at the heating temperature) is one minute or more makes it possible to achieve a hardness of the base metal portion of 150 Hv or less, in the electric resistance welded steel pipe to be produced.
The heat treatment time (retention time at the heating temperature) is preferably 5 minutes or more.
The fact that the cooling rate when cooling from the heat treatment temperature to the “heat treatment temperature—100° C.” is 1.0° C./s or less makes it possible to achieve a hardness of the base metal portion of 150 Hv or less, in the electric resistance welded steel pipe to be produced.
The lower limit of the cooling rate when cooling from the heat treatment temperature to the “heat treatment temperature—100° C.” is, for example, 0.01° C./s, but not particularly limited thereto.
The cooling method to be used when cooling from the heat treatment temperature to the “heat treatment temperature—100° C.” may be, for example, air cooling or furnace cooling.
In the heat treatment step, the cooling rate after having reached the “heat treatment temperature—100° C.” by the cooling, may be, for example, air cooling, but not particularly limited thereto.
The production method X may include other steps in addition to the steps described above.
For example, the production method X may include a pipe drawing step of subjecting the electric resistance welded steel pipe to pipe drawing, between the preparation step and the heat treatment step.
Examples of conditions for pipe drawing in the pipe drawing step include a condition in which the area reduction rate is from 20 to 50%.
The method of producing an automotive part according to the present disclosure includes a step of subjecting the above-described electric resistance welded steel pipe according to the present disclosure to cold working and quenching in this order, to obtain the automotive part.
According to the method of producing an automotive part according to the present disclosure, the same effect as the effect provided by the electric resistance welded steel pipe according to the present disclosure can be obtained.
In the method of producing an automotive part according to the present disclosure, tempering may be further performed after the completion of the quenching.
In the method of producing an automotive part according to the present disclosure, the cold working is performed using a dedicated mold.
Since the production of an automotive part is performed using the electric resistance welded steel pipe according to the present disclosure, in the method of producing an automotive part according to the present disclosure, a longer mold life can be achieved.
Examples of the automotive part include rack bars, drive shafts, and transmission shaft stators.
Examples of the present disclosure will be shown below. However, the present disclosure is in no way limited to the following Examples.
Underlines in Table 1 to Table 2 indicate that the corresponding items and values fall outside the scope of the present disclosure, or outside the range of preferred production conditions.
Electric resistance welded steel pipes for an automotive part are produced by the production method X described above.
Details are shown below.
Hot coils each composed of a hot-rolled steel sheet (sheet thickness: from 2.0 to 10.0 mm) having a chemical composition of any one of steel types A to N shown in Table 1 were prepared.
REM in the steel types B, D, G and J is specifically Ce.
In each of Examples and Comparative Examples, an as-rolled electric resistance welded steel pipe having a wall thickness and an outer diameter shown in Table 2 was produced, by: uncoiling the hot-rolled steel sheet from the thus prepared hot coil, and roll-forming the hot-rolled steel sheet uncoiled from the hot coil to form an open pipe; and subjecting the abutting portions of the resulting open pipe to electric resistance welding to form an electric resistance welded portion A.
In each of Examples indicated with “Yes” in the column of “Pipe drawing” in Table 2, the as-rolled electric resistance welded steel pipe was subjected to pipe drawing to achieve an area reduction rate of 23%, and the heat treatment in the subsequent heat treatment step was performed on the as-rolled electric resistance welded steel pipe which had been subjected to the pipe drawing.
In each of other Examples and Comparative Examples, the heat treatment in the subsequent heat treatment step was performed on the as-rolled electric resistance welded steel pipe, without performing the pipe drawing.
The heat treatment under the conditions shown in Table 2 was performed on each of the as-rolled electric resistance welded steel pipes.
In Comparative Example 11, however, the heat treatment was not performed on the as-rolled electric resistance welded steel pipe. Comparative Example 11 is an example simulating the steel pipe disclosed in Patent Document 8 (namely, JP-B No. 5679115) described above.
In Comparative Example 13, the as-rolled electric resistance welded steel pipe was subjected to heating at 900° C. for 10 minutes, air cooling, 23% pipe drawing, heating at 720° C. for 10 minutes, and air cooling. Comparative Example 13 and Comparative Example 12 (in each of which the heating temperature is 900° C.) are examples simulating the steel pipe disclosed in Patent Document 5 (namely, JP-A No. 2004-190086).
In this manner, each electric resistance welded steel pipe for an automotive part was obtained.
When the steel structure of the base metal portion of each electric resistance welded steel pipe for an automotive part was examined by the method described above, the steel structure was a ferrite-pearlite mixed structure in all of Examples and Comparative Examples.
The hardness (hardness before quenching) of the base metal portion in each electric resistance welded steel pipe for an automotive part was measured by the method described above.
The results are shown in Table 2.
Each electric resistance welded steel pipe for an automotive part was subjected to quenching under the conditions of a heating temperature of 900° C., a heating time of one minute, a cooling rate (A) after heating of 30° C./s, and a cooling attainment temperature at the cooling rate (A), within the range of from 50° C. to 0° C.
<Hardness after Quenching>
The hardness after quenching, and the difference in hardness in the wall thickness direction, were measured by the methods described above, for each electric resistance welded steel pipe for an automotive part which had been subjected to the quenching.
The results are shown in Table 2.
H
0.37
I
0.0000
J
0.25
0.80
K
0.50
L
0.35
M
0.86
N
0.002
H
I
J
K
L
M
N
H
I
J
K
L
M
N
645
75
160
163
170
170
165
156
159
638
195
183
165
155
As shown in Table 1 and Table 2, the electric resistance welded steel pipe for an automotive part of each Example in which the chemical composition of the base metal portion is the chemical composition in the present disclosure, in which the steel structure of the base metal portion is a ferrite-pearlite mixed structure and in which the base metal portion has a hardness of from 110 to 150 Hv, had a moderate hardness after quenching.
Since the hardness of the base metal portion is 150 Hv or less, in the electric resistance welded steel pipe for an automotive part of each Example, it is thought that the steel pipe has an excellent cold workability, and is also excellent in the life of the mold to be used for cold working.
In contrast, the results of Comparative Examples were as follows.
In Comparative Example 1 in which the C content is too low in the chemical composition of the base metal portion, the hardness after quenching was insufficient.
In Comparative Example 2 in which the B content is too low in the chemical composition of the base metal portion, the difference in hardness, ΔHv, in the wall thickness direction was excessively large.
In Comparative Example 3 in which the Si content and the Mn content are too high in the chemical composition of the base metal portion, the hardness of the base metal portion was higher than 150 Hv.
In Comparative Example 4 in which the C content is too high, in the chemical composition of the base metal portion, the hardness of the base metal portion was higher than 150 Hv.
In Comparative Examples 5 to 7 in each of which the chemical composition of the base metal portion is the chemical composition in the present disclosure, but the heating temperature in the heat treatment step is too low, the hardness of the base metal portion was higher than 150 Hv.
In Comparative Example 8 in which the Si content is too high in the chemical composition of the base metal portion, the hardness of the base metal portion was higher than 150 Hv.
In Comparative Example 9 in which the Mn content is too high in the chemical composition of the base metal portion, the hardness of the base metal portion was higher than 150 Hv.
In Comparative Example 10 in which the Ti content is too low in the chemical composition of the base metal portion, the hardness after quenching was insufficient.
In Comparative Example 11 in which the chemical composition of the base metal portion is the chemical composition in the present disclosure, but the heat treatment step was not performed, the hardness of the base metal portion was higher than 150 Hv.
In Comparative Example 12 in which the chemical composition of the base metal portion is the chemical composition in the present disclosure, but the heating temperature in the heat treatment step was too high, the hardness of the base metal portion was higher than 150 Hv.
In Comparative Example 13 in which the chemical composition of the base metal portion is the chemical composition in the present disclosure, but the as-rolled electric resistance welded steel pipe was subjected to heating at 900° C. for 10 minutes, air cooling, 23% pipe drawing, heating at 720° C. for 10 minutes and air cooling, the hardness of the base metal portion was higher than 150 Hv.
In Comparative Example 14 in which the chemical composition of the base metal portion is the chemical composition in the present disclosure, but the heating time in the heat treatment step was too long, the hardness of the base metal portion was higher than 150 Hv.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/005451 | 2/10/2022 | WO |
