Hot-rolled steel and steel parts for electric resistance welded pipe, and method for manufacturing same

Abstract

A method for manufacturing steel parts, according to the present invention, comprises the steps of: (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel which comprises, by wt%, greater than 0.38% and less than or equal to 0.55% of C, 0.1-0.4% of Si, 0.3-1.5% of Mn, 0.001-0.05% of Al, 0.05-1.2% of Cr, and the balance of Fe and inevitable impurities, and which meets conditions [1] and / or [2], condition [1] being 0.5% or less of Mo and condition [2] being at least one from among 0.05% or less of Ti, 0.1% or less of V and 0.005% or less of B; (b) forming a pipe from the hot-rolled steel strip by means of electric resistance welding, thereby manufacturing an electric resistance welded pipe; (c) locally heat-treating the weld zone of the electric resistance welded pipe; and (d) quenching the heat-treated electric resistance welded pipe.

Description

Hot-rolled steel and steel parts for pre-welded steel pipes, and manufacturing method thereof

[0001] The present invention relates to a hot-rolled steel material for a welded steel pipe and a steel part, and a method for manufacturing the same, which can improve the durability of a steel part by reducing the difference in hardness between a welded part and a base material part in a welded steel pipe state.

[0002] For electric motor-powered vehicles, much research is being conducted on the materials for the rotor shaft, which is the power transmission mechanism that transmits power from the motor.

[0003] The rotor shaft transmits electrically induced torque from the electric motor to the transmission via a corresponding positive connection. Therefore, it requires high hardness to withstand the loads caused by high torque. Furthermore, considering that the rotor shaft is used in a fatigue environment subjected to repeated stresses, improving its service life is crucial.

[0004] Rotor shafts are widely used as solid parts, and recently, the demand for hollow rotor shafts has been increasing due to the improvement in automobile efficiency and weight reduction.

[0005] Hollow rotor shafts can be manufactured using two methods.

[0006] One method is to use a seamless steel pipe, hollowing out the inside through mechanical processing.

[0007] However, due to the inherent nature of the process, the thickness tolerance is not constant along the circumference. This characteristic leads to increased noise and vibration as the rotational speed increases. Therefore, to improve motor performance, parts must be manufactured with a narrow thickness tolerance.

[0008] Accordingly, a method of manufacturing a hollow rotor shaft using electric resistance welded pipe is being studied.

[0009] Electric resistance welding refers to steel pipe manufactured using electric resistance welding.

[0010] Since the welded steel pipe is manufactured by forming and welding steel plates, it has a smaller thickness tolerance than a seamless steel pipe, and when the welded steel pipe is applied to a hollow rotor shaft, it has the advantage of reducing noise and vibration.

[0011] In this way, in the process of manufacturing a hollow rotor shaft using a pre-welded steel pipe, it is important to manufacture a steel part with overall uniform properties and shape.

[0012] However, since steel parts inevitably create welds (joint lines) during the electric resistance welding process, they will have one or more welds in the longitudinal direction of the steel pipe.

[0013] A weld is formed when two parts of a material are locally melted, then brought into contact and solidified. Through this melting and solidification process, the resulting weld acquires a microstructure and properties distinct from those of the original base material. The greater the difference in properties between the weld and the base material, the more likely it is that cracks will occur during machining of steel parts, significantly reducing their service life.

[0014] Therefore, there is a need to develop steel for hollow rotor shafts that can improve the durability of steel parts while reducing the difference in properties between the weld and base metal parts.

[0015] The purpose of the present invention is to provide a hot-rolled steel material for a welded steel pipe, which can improve the durability of a steel part by reducing the difference in hardness between a welded portion and a base material portion in a welded steel pipe state.

[0016] In addition, an object of the present invention is to provide a steel part using the hot-rolled material for the above-mentioned pre-welded steel pipe, and a method for manufacturing the same.

[0017] The purposes of the present invention are not limited to those mentioned above. Other purposes and advantages of the present invention not mentioned above can be understood through the following description and will be more clearly understood through the embodiments of the present invention. Furthermore, it will be readily apparent that the purposes and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims.

[0018] The hot-rolled steel according to the present invention contains, in weight %, C: more than 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder Fe and unavoidable impurities, and satisfies at least one of the following [1] and [2].

[0019] [1] Mo: 0.5% or less,

[0020] [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these.

[0021] It is characterized by containing 40 to 95 area% of perlite for a total area of ​​100 area%.

[0022] Other microstructures of hot rolled steel may include ferrite, bainite, and martensite.

[0023] Additionally, hot rolled steel can have a Vickers hardness of 150 to 370 Hv.

[0024] Additionally, hot rolled steel can have an elongation of 9 to 30%.

[0025] Additionally, hot rolled steel can have a tensile strength of 400 to 700 MPa.

[0026] A hot-rolled steel according to another embodiment of the present invention is characterized by containing, in wt%, C: more than 0.38% and 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, and the remainder Fe and unavoidable impurities.

[0027] A hot-rolled steel sheet according to another embodiment of the present invention comprises, in weight %, C: 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and is characterized in that it comprises 40 to 95 area % of pearlite with respect to the total 100 area %.

[0028] A hot-rolled steel according to another embodiment of the present invention comprises, in weight %, C: 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and is characterized by a Vickers hardness of 150 to 370 Hv.

[0029] A hot-rolled steel according to another embodiment of the present invention comprises, in weight %, C: more than 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and is characterized by an elongation of 9 to 30%.

[0030] A hot-rolled steel according to another embodiment of the present invention comprises, in weight %, C: 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and is characterized by a tensile strength of 400 to 700 MPa.

[0031] A hot rolled material according to another embodiment of the present invention satisfies at least one of the following [1] and [2],

[0032] [1] Mo: 0.5% or less,

[0033] [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these.

[0034] It is characterized by containing 40 to 95 area% of perlite for a total area of ​​100 area%.

[0035] A hot rolled material according to another embodiment of the present invention satisfies at least one of the following [1] and [2],

[0036] [1] Mo: 0.5% or less,

[0037] [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these.

[0038] It is characterized by a Vickers hardness of 150 to 370 Hv.

[0039] A hot rolled material according to another embodiment of the present invention satisfies at least one of the following [1] and [2],

[0040] [1] Mo: 0.5% or less,

[0041] [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these.

[0042] It is characterized by an elongation rate of 9 to 30%.

[0043] A hot rolled material according to another embodiment of the present invention satisfies at least one of the following [1] and [2],

[0044] [1] Mo: 0.5% or less,

[0045] [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these.

[0046] It is characterized by a tensile strength of 400 to 700 MPa.

[0047] A method for manufacturing a steel part according to the present invention comprises the steps of: (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet containing, in wt%, C: more than 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder Fe and unavoidable impurities, and satisfying at least one of the following [1] and [2];

[0048] [1] Mo: 0.5% or less

[0049] [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these.

[0050] (b) a step of manufacturing a welded steel pipe by forming the hot-rolled steel strip by electric resistance welding; (c) a step of locally heat-treating the welded portion of the welded steel pipe; and (d) a step of quenching the heat-treated welded steel pipe.

[0051] The welded steel pipe of the above step (b) can satisfy the relationship between the hardness difference △H1 between the welded portion and the base material portion and the C content △H1 ≤ 289.61*exp(0.8944*[C]) + 67 Hv.

[0052] (Here, when the thickness of the welded steel pipe is t, the parent material refers to a point located at a distance of 5t in the circumferential direction from the circular cross-section based on the welded section.)

[0053] The pre-welded steel pipe heat-treated in step (c) or the steel component quenched in step (d) may have a hardness difference between the welded portion and the base material portion of △H2, where △H2 ≤ 50 Hv.

[0054] The step (c) above may locally heat-treat the welded portion at a temperature higher than Ar3 for 3 seconds or longer. In addition, the step (c) above may additionally heat-treat the entire pre-welded steel pipe at a temperature higher than Ar3 after locally heat-treating the welded portion.

[0055] The above step (d) can be performed by maintaining the temperature at 820°C or higher for 10 to 10,000 seconds to form a single phase of austenite, and then cooling to a temperature of Mf+100°C or lower at a cooling rate of 10°C / second or higher.

[0056] The Vickers hardness of the steel part quenched in the above step (d) may be 600 Hv or more.

[0057] After the above step (d), the step (e) of tempering the quenched galvanized steel pipe at 150 to 650°C may be further included.

[0058] The Vickers hardness of the tempered steel part in the above step (e) may be 200 Hv or more.

[0059] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet; (b) manufacturing an electric-resistance welded steel pipe by forming the hot-rolled steel strip by electric resistance welding; (c) locally heat-treating a welded portion of the electric-resistance welded steel pipe; and (d) quenching the heat-treated electric-resistance welded steel pipe.

[0060] A method for manufacturing a steel part according to another embodiment of the present invention includes the steps of (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet; (b) manufacturing an electric-resistance welded steel pipe by forming the hot-rolled steel strip by an electric resistance welding method; (c) locally heat-treating a welded portion of the electric-resistance welded steel pipe; and (d) quenching the heat-treated electric-resistance welded steel pipe; wherein the electric-resistance welded steel pipe of step (b) is characterized in that the relationship between the hardness difference △H1 between the welded portion and the base material portion and the C content satisfies △H1 ≤ 289.61*exp(0.8944*[C]) + 67 Hv.

[0061] (Here, when the thickness of the welded steel pipe is t, the parent material refers to a point located at a distance of 5t in the circumferential direction from the circular cross-section based on the welded section.)

[0062] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet; (b) manufacturing an electric resistance welded steel pipe by forming the hot-rolled steel strip by electric resistance welding; (c) locally heat-treating a welded portion of the electric resistance welded steel pipe; and (d) quenching the heat-treated electric resistance welded steel pipe; wherein the electric resistance welded steel pipe heat-treated in step (c) or the steel part quenched in step (d) is characterized in that when a hardness difference between a welded portion and a base material portion is △H2, △H2 ≤ 50 Hv.

[0063] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet; (b) manufacturing an electric resistance welded steel pipe by forming the hot-rolled steel strip by electric resistance welding; (c) locally heat-treating a welded portion of the electric resistance welded steel pipe; and (d) quenching the heat-treated electric resistance welded steel pipe; wherein step (c) is characterized in that the welded portion is locally heat-treated at an Ar3 temperature or higher for 3 seconds or longer.

[0064] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet; (b) manufacturing a welded steel pipe by forming the hot-rolled steel strip by electric resistance welding; (c) locally heat-treating a welded portion of the welded steel pipe; and (d) quenching the heat-treated welded steel pipe; wherein the Vickers hardness of the steel part quenched in step (d) is 600 Hv or more.

[0065] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of: (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel strip containing, by weight %, C: more than 0.38% and 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, and the remainder Fe and unavoidable impurities; (b) manufacturing an electric-resistance welded steel pipe by forming the hot-rolled steel strip by electric resistance welding; (c) locally heat-treating a welded portion of the electric-resistance welded steel pipe; and (d) quenching the heat-treated electric-resistance welded steel pipe.

[0066] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of: (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel strip containing, by weight %, C: more than 0.38% and 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, and the remainder Fe and unavoidable impurities; (b) manufacturing an electric-resistance welded steel pipe by forming the hot-rolled steel strip by an electric-resistance welding method; (c) locally heat-treating a welded portion of the electric-resistance welded steel pipe; And (d) a step of quenching the heat-treated welded steel pipe; and the welded steel pipe of step (b) is characterized in that the relationship between the hardness difference △H1 between the welded portion and the base material portion and the C content satisfies △H1 ≤ 289.61*exp(0.8944*[C]) + 67 Hv.

[0067] (Here, when the thickness of the welded steel pipe is t, the parent material refers to a point located at a distance of 5t in the circumferential direction from the circular cross-section based on the welded section.)

[0068] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of: (a) slitting a hot-rolled steel strip containing, by weight %, C: more than 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, and the remainder Fe and unavoidable impurities to manufacture a hot-rolled steel strip; (b) forming the hot-rolled steel strip by an electric resistance welding method to manufacture a welded steel pipe; (c) locally heat-treating a welded portion of the welded steel pipe; and (d) quenching the heat-treated welded steel pipe; wherein the welded steel pipe heat-treated in step (c) or the steel part quenched in step (d) is characterized in that when a hardness difference between a welded portion and a base material portion is △H2, △H2 ≤ 50 Hv.

[0069] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of: (a) slitting a hot-rolled steel sheet containing, by weight %, C: more than 0.38% and 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, and the remainder Fe and unavoidable impurities to manufacture a hot-rolled steel strip; (b) forming the hot-rolled steel strip by an electric resistance welding method to manufacture a welded steel pipe; (c) locally heat-treating a welded portion of the welded portion of the welded portion; and (d) quenching the heat-treated welded portion of the welded portion; wherein step (c) is characterized in that the welded portion is locally heat-treated at an Ar3 temperature or higher for 3 seconds or longer.

[0070] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of: (a) slitting a hot-rolled steel strip containing, by weight %, C: more than 0.38% and 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, and the remainder Fe and unavoidable impurities to manufacture a hot-rolled steel strip; (b) forming the hot-rolled steel strip by an electric resistance welding method to manufacture a welded steel pipe; (c) locally heat-treating a welded portion of the welded steel pipe; and (d) quenching the heat-treated welded steel pipe; wherein the Vickers hardness of the steel part quenched in step (d) is characterized in that it is 600 Hv or more.

[0071] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of: (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet satisfying at least one of the following [1] and [2];

[0072] [1] Mo: 0.5% or less

[0073] [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these.

[0074] (b) a step of manufacturing a welded steel pipe by forming the hot-rolled steel strip by electric resistance welding; (c) a step of locally heat-treating the welded portion of the welded steel pipe; and (d) a step of quenching the heat-treated welded steel pipe.

[0075] A method for manufacturing a steel part according to another embodiment of the present invention comprises the steps of: (a) manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet satisfying at least one of the following [1] and [2];

[0076] [1] Mo: 0.5% or less

[0077] [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these.

[0078] (b) a step of manufacturing a welded steel pipe by forming the hot-rolled steel strip by an electric resistance welding method; (c) a step of locally heat-treating the welded portion of the welded steel pipe; and (d) a step of quenching the heat-treated welded steel pipe; wherein the hot-rolled steel is characterized in that it satisfies at least one of the following: containing 40 to 95 area% of pearlite with respect to the total 100 area%, having a Vickers hardness of 150 to 370 Hv, having an elongation of 9 to 30%, and having a tensile strength of 400 to 700 MPa.

[0079] The steel component according to the present invention comprises, in weight %, C: more than 0.38% and 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and is characterized by satisfying at least one of the following [1] and [2].

[0080] [1] Mo: 0.5% or less

[0081] [2] At least one of the following: Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less

[0082] The above steel component may be at least 90 area% martensite or at least 90 area% tempered martensite for a total of 100 area%.

[0083] When the hardness difference between the weld and base material of the above steel component is △H2, △H2 ≤ 50 Hv may be achieved.

[0084] A steel component according to another embodiment of the present invention is characterized by containing, in wt%, C: more than 0.38% and 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, and the remainder Fe and unavoidable impurities.

[0085] A steel component according to another embodiment of the present invention comprises, in weight %, C: 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and is characterized in that, with respect to the total 100 area %, martensite is 90 area % or more or tempered martensite is 90 area % or more.

[0086] A steel component according to another embodiment of the present invention comprises, in wt%, C: more than 0.38% and 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and is characterized in that when the hardness difference between the welded portion and the base material portion of the steel component is △H2, △H2 ≤ 50 Hv.

[0087] A steel component according to another embodiment of the present invention satisfies at least one of the following [1] and [2], and is characterized in that it has 90 area% or more of martensite or 90 area% or more of tempered martensite for a total of 100 area%.

[0088] [1] Mo: 0.5% or less

[0089] [2] At least one of the following: Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less

[0090] A steel component according to another embodiment of the present invention satisfies at least one of the following [1] and [2], and is characterized in that when the hardness difference between the welded portion and the base material portion of the steel component is △H2, △H2 ≤ 50 Hv.

[0091] [1] Mo: 0.5% or less

[0092] [2] At least one of the following: Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less

[0093] The steel component according to the present invention has the effect of improving the durability life of the steel component by reducing the microstructural deviation and hardness deviation between the welded portion and the base material portion in the state of the welded steel pipe through steel composition control and normalizing heat treatment in the welded steel pipe.

[0094] Furthermore, steel components can exhibit excellent hardness characteristics exceeding 600 Hv through quenching and heat treatment after manufacturing the pre-welded steel pipe. As a result, the steel components possess a uniform shape and properties, while significantly reducing cracking during the manufacturing process, making them ideal for use in hollow rotor shafts.

[0095] In addition to the effects described above, specific effects of the present invention are described below while explaining specific details for carrying out the invention.

[0096] Figure 1 is a flowchart showing a method for manufacturing a steel part according to the present invention.

[0097] The above-described objects, features, and advantages will be described in detail below with reference to the accompanying drawings, so that those skilled in the art can easily practice the technical idea of ​​the present invention. In describing the present invention, if a detailed description of a known technology related to the present invention is judged to unnecessarily obscure the gist of the present invention, a detailed description thereof will be omitted. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to indicate the same or similar components.

[0098] Hereinafter, hot-rolled steel and steel parts for pre-welded steel pipes according to some embodiments of the present invention, and a method for manufacturing the same will be described.

[0099] In the manufacturing process of a hollow rotor shaft using a pre-welded steel pipe, uniformity of properties in the initial steel pipe state is important in order to manufacture a steel part with uniform properties and shape.

[0100] The weld (joint) that is inevitably created during the electric resistance welding process has a microstructure and properties different from those of the original base material as it goes through the melting and solidification process.

[0101] The base material of the welded steel pipe is basically composed of ferrite and pearlite structures, but immediately after welding, the welded part is composed of martensite or bainite structures, so the hardness difference between the welded part and the base material is the largest.

[0102] The difference in microstructure and hardness between the weld and the base material that occurs at this time is an important factor in processing steel parts, and there is a problem that causes defects in the process of manufacturing steel parts.

[0103] Accordingly, the inventor of the present invention confirmed that the difference in the properties of the welded steel pipe between the joint and the base material is reduced by performing a local normalizing heat treatment on the welded portion immediately after manufacturing the welded steel pipe or by performing a local normalizing heat treatment on the welded portion and then performing a heat treatment on the entire welded steel pipe.

[0104] Local normalizing heat treatment of the weld joint can significantly reduce the hardness difference between the weld and the base metal, as it consists of the same ferrite and pearlite structures as the base metal. Furthermore, by performing subsequent quenching and tempering (QT) heat treatment, it was confirmed that the microstructures of the weld and the base metal of the steel component can be composed of martensite or tempered martensite, resulting in high hardness.

[0105] Therefore, the steel component of the present invention has already reduced the microstructural deviation and hardness deviation between the welded portion and the base material portion in the state of a pre-welded steel pipe, and has a uniform shape and properties, and has a significantly low possibility of cracks occurring during the manufacturing process of the steel component, and has the advantage of being suitable for application to a hollow rotor shaft among automobile components.

[0106] Hot-rolled

[0107] The hot-rolled material according to the present invention may contain, in weight %, C: more than 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder Fe and unavoidable impurities, and may satisfy at least one of the following [1] and [2].

[0108] [1] Mo: 0.5% or less

[0109] [2] At least one of the following: Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less

[0110] C: 0.38% or more to 0.55% or less

[0111] Carbon is an element that stably forms austenite and is the alloying element most influential in enhancing the strength and hardness of steel. It also enhances hardenability, making it the most important element in determining the hardness of martensite after quenching.

[0112] As the amount of carbon added increases, toughness decreases and brittleness increases, so it is desirable to adjust the carbon content according to the application and required level.

[0113] If the carbon content is less than 0.38%, it is difficult to secure the hardness of martensite, making it difficult to secure durability. If the carbon content exceeds 0.55%, the toughness decreases, making it difficult to manufacture steel pipes, and embrittlement may occur during the quenching heat treatment process.

[0114] From this point of view, the carbon content may be between 0.38% and 0.55%, and preferably between 0.39 and 0.54%.

[0115] Specifically, when the carbon content satisfies 0.39 to 0.54%, durability, elongation of hot-rolled material, and toughness can be secured in the final part.

[0116] Si: 0.1 ~ 0.4%

[0117] Silicon has excellent affinity for oxygen and is used as a deoxidizer. It is an element that stably forms ferrite and, when dissolved in ferrite, can improve strength. Conversely, it can impair hot workability and toughness, and degrade scale exfoliation at high temperatures.

[0118] From this point of view, the silicon content may be 0.10 to 0.4%, preferably 0.15 to 0.4%.

[0119] Mn: 0.3 ~ 1.5%

[0120] Manganese combines with the impurity sulfur to improve the purity of steel. Manganese also slightly increases strength and hardness, and enhances the hardenability of steel, helping to form martensite even at low cooling rates. Conversely, high manganese contents can lead to the formation of segregation layers, reducing workability.

[0121] From this point of view, the manganese content may be 0.3 to 1.5%, preferably 0.45 to 1.3%.

[0122] Al: 0.001 ~ 0.05%

[0123] Aluminum acts as a deoxidizer, improving steel cleanliness. It also forms aluminum inclusions at high temperatures, which helps refine the steel's structure. Conversely, adding large amounts of aluminum can lead to a decrease in properties due to inclusions.

[0124] From this point of view, the aluminum content may be 0.001 to 0.05%, preferably 0.001 to 0.04%.

[0125] Cr: 0.05 ~ 1.2%

[0126] Chromium is a ferrite-stabilizing element that, when incorporated into the matrix, enhances hardenability. Conversely, excessive chromium content can lead to brittleness due to high hardenability.

[0127] From this point of view, the chromium content may be 0.05 to 1.2%, preferably 0.05 to 1.05%.

[0128] Mo: 0.5% or less (including 0%)

[0129] Molybdenum not only significantly improves hardenability, but is also remarkably effective at enhancing high-temperature strength. Excessive molybdenum content can lead to brittleness due to high hardenability.

[0130] From this point of view, the molybdenum content may be 0.5% or less, and preferably 0.45% or less.

[0131] Ti: 0.05% or less (including 0%)

[0132] Titanium is an element that forms TiC and TiCN precipitates within hot-rolled steel. This inhibits the growth of austenite grains, thereby increasing the strength of hot-rolled steel sheets. Furthermore, fine precipitates formed during phase transformation during the cooling process after hot rolling significantly enhance the strength of the steel.

[0133] If the titanium content is excessive, it will exist in the form of coarse precipitates rather than fine precipitates in the hot-rolled steel, which may lower the toughness or reduce the performance of heat-treated steel plates or steel pipe parts.

[0134] From this point of view, the titanium content may be 0.05% or less, and preferably 0.03% or less.

[0135] V: 0.1% or less (including 0%)

[0136] Vanadium combines with carbon to form carbides, thereby improving strength and hardness. In particular, fine precipitates formed during the phase transformation process during the cooling process after hot rolling greatly improve the strength of steel.

[0137] If the vanadium content is excessive, the toughness may be reduced due to the formation of coarse precipitates, which may lead to an increase in cost.

[0138] From this perspective, the vanadium content may be less than 0.1%.

[0139] B: 0.005% or less (including 0%)

[0140] Boron helps improve the hardenability of steel by segregating at austenite grain boundaries and delaying pearlite formation. It is advisable to consider the appropriate boron content range to ensure hardenability.

[0141] From this point of view, the boron content may be 0.005% or less, and preferably 0.004% or less.

[0142] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities may be unintentionally incorporated during the normal manufacturing process, and thus cannot be excluded.

[0143] Among the inevitable impurities, nitrogen (N) is an element that stabilizes austenite and forms nitrides. Its content may be 0.01% or less (including 0%), and when it is inevitably included in the manufacturing process, it may be more than 0 and less than 0.01%.

[0144] Since nitrogen can react with boron to form precipitates and offset the effect of adding boron, it is advantageous to lower the nitrogen content to 0.01% or less in order to exhibit the effect of adding effective boron.

[0145] If the nitrogen content exceeds 0.01%, coarse AlN, TiN or TiCN nitrides may be formed, which may act as a starting point for fatigue crack generation when evaluating the durability of steel plates or steel pipes, thereby deteriorating the fatigue durability.

[0146] Among the inevitable impurities, oxygen (O) may also be less than 0.01% (including 0%), and if it is inevitably included during the manufacturing process, it may be more than 0 but less than 0.01%.

[0147] Among the unavoidable impurities, phosphorus (P) and sulfur (S) are impurities that cannot be filtered out during the steelmaking process. Maintaining them in small quantities can improve purity and processability. From this perspective, the respective phosphorus (P) and sulfur (S) contents can be less than 0.03% (including 0%).

[0148] The hot rolled steel may contain 40 area% or more of pearlite, preferably 40 area% to 95 area% of pearlite, with respect to the total 100 area%, and other microstructures may contain 5% or more of ferrite, a small amount of bainite, and a small amount of martensite.

[0149] If the microstructure of the hot-rolled steel is mainly composed of pearlite and ferrite, the increase in strength of the hot-rolled steel sheet due to low-temperature structures such as bainite and martensite can be reduced, which is advantageous for pipe manufacturing. Considering the component contents of the present invention, the hot-rolled steel may contain pearlite at the highest area ratio, followed by ferrite at the next highest area ratio.

[0150] In this way, the hot-rolled material of the present invention contains C: 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, and contains pearlite as the main structure by 40 area% or more, so that the Vickers hardness can be 150 to 370 Hv.

[0151] Hot-rolled steel containing more than 40 area% of pearlite and having a Vickers hardness of 150 to 370 Hv has the advantage of being suitable for manufacturing welded steel pipes.

[0152] If such hot-rolled material is a hard material, it is easily broken during forming of an electric wire-seamed steel pipe. Therefore, it is desirable to soften the hot-rolled material by satisfying an elongation of 9% or more and a yield strength of 700 MPa or less of the hot-rolled material, thereby ensuring the physical properties and workability of an electric wire-seamed steel pipe when manufacturing an electric wire-seamed steel pipe. From this point of view, the elongation of the hot-rolled material may be 9 to 30%, and preferably 10 to 24%. The elongation refers to the rate at which the material stretches without breaking in a tensile test, and in order to secure the formability of the hot-rolled material, the lower limit of the elongation may be 9% or more, and in terms of the material of the hot-rolled material applicable to an electric wire-seamed steel pipe, the upper limit may be 30% or less.

[0153] Additionally, the yield strength of the hot rolled steel may be 400 to 700 MPa, and preferably 450 to 650 MPa.

[0154] Yield strength is the length of a material that increases when force is applied to it and pulled from both sides. Up to a certain point, the material will return to its original size when the force is released. However, if the force exceeds a certain amount and the force is released, the material will not return to its original state and will grow longer. The maximum force that can return to its original state at this time is called the yield strength or yield stress. A material with a high yield strength means that it has a strong ability to resist deformation, while a material with a low yield strength means that it easily succumbs to deformation.

[0155] In the present invention, the pipe formability of the hot-rolled material was secured by imparting softening to the hot-rolled material so as to satisfy the yield strength of the hot-rolled material of 400 to 700 MPa.

[0156] With regard to the measurement method, the microstructure is observed in the cross-section of the material, and can be observed throughout the entire thickness region except for the extreme surface area of ​​the cross-section, and can generally be observed at the 1 / 4t point.

[0157] The Vickers hardness of the weld and base material can be measured over the entire thickness area excluding the extreme surface area of ​​the cross-section in the thickness direction of the material, and can generally be measured at the 1 / 4t point.

[0158] Manufacturing method of welded steel pipe

[0159] The present invention can manufacture steel parts by manufacturing a welded steel pipe using the above-described hot-rolled material, followed by quenching and tempering heat treatment. In the process of manufacturing a welded steel pipe, the difference in microstructure between the base material and the weld is reduced through normalizing heat treatment of the weld, thereby reducing the hardness difference resulting therefrom. This facilitates the manufacture of steel parts and contributes to improving the durability of the steel parts.

[0160] Figure 1 is a flowchart showing a method for manufacturing a steel part according to the present invention.

[0161] Referring to FIG. 1, the method for manufacturing a steel part of the present invention may include a step of manufacturing a hot-rolled steel strip by slitting a hot-rolled steel sheet (S110), a step of manufacturing a welded steel pipe (S120), a step of locally heat-treating a welded portion of the welded steel pipe (S130), and a step of quenching and tempering the heat-treated welded steel pipe (S140).

[0162] Step of manufacturing hot-rolled steel strip by slitting hot-rolled steel (S110)

[0163] The step of manufacturing hot-rolled steel strip by slitting hot-rolled steel may include a homogenization treatment step, a hot rolling step, a cooling step, and a coiling step.

[0164] A steel slab containing, by weight %, C: 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder Fe and unavoidable impurities, and satisfying at least one of [1] Mo: 0.5% or less and [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, can be homogenized by reheating at 1050 to 1300°C for about 2 hours.

[0165] Homogenized steel slabs are hot-rolled to a thickness of approximately 5 to 10 mm at a temperature higher than Ar3. The finishing hot-rolling temperature is preferably Ar3 to 1000°C. If the finishing hot-rolling temperature is lower than Ar3, hot-rolling proceeds in the region where austenite and ferrite coexist, resulting in uneven deformation, poor sheetability, and a risk of sheet fracture due to stress concentration in the soft ferrite phase. On the other hand, if the finishing hot-rolling temperature is excessively high, exceeding 1000°C, there is a risk of poor surface quality due to scale formation.

[0166] In the cooling and coiling step, it is cooled to 500 to 750℃ and coiled.

[0167] Cooling has no special restrictions, but can be performed at an average cooling rate of 80°C / sec or less in order to secure more than 40 area% of pearlite, but is not limited thereto.

[0168] Meanwhile, the coiling temperature is preferably 500 to 750°C, and 620 to 720°C is more preferable in terms of stably forming the target microstructure. If the coiling temperature is lower than 500°C, the strength / hardness of the hot-rolled steel increases due to the formation of low-temperature structures such as bainite and martensite, resulting in poor pipeability. In addition, cooling is uneven in the width direction, making it difficult to obtain a uniform microstructure and causing local overcooling defects. If the deviation in physical properties in the width direction is large, there is a possibility of welding defects occurring when manufacturing a welded steel pipe with the hot-rolled steel.

[0169] On the other hand, if the coiling temperature exceeds 750℃, there is a risk that internal oxidation may occur within the hot-rolled coil or the surface quality may deteriorate due to scale formation.

[0170] Afterwards, hot-rolled steel strip can be manufactured by slitting the hot-rolled steel in coil form.

[0171] Slitting refers to a series of processes for dividing coil-shaped hot-rolled steel along its width to produce narrow coils with the same thickness and length as the original coil-shaped hot-rolled steel, but with a reduced width. For example, slitting can be performed by cutting steel sheets lengthwise using a slitting machine or a width cutter.

[0172] Step for manufacturing welded steel pipe (S120)

[0173] Hot-rolled steel strips can be manufactured into welded steel pipes by using the electric resistance welding (ERW) method.

[0174] In the case of the electric resistance welding method, the base materials can be joined together by melting them using electric resistance without using a separate welding material.

[0175] For example, the electric resistance welding method is a method of joining two surfaces of materials to be welded by bringing them into contact with each other, directly or indirectly passing a welding current using a high-frequency current of 300 to 1000 kHz to heat the two contact surfaces through resistance heating, and then applying pressure (squeeze force).

[0176] The base material of the ERW pipe is basically hot-rolled steel composed of ferrite and pearlite, and when performing ERW welding, the temperature of the material rises near the melting point to around 1400℃, and ERW welding can be completed through a process of rapidly cooling to 200℃ at a speed of 100℃ / sec or more.

[0177] At this time, the weld is composed of martensite or bainite structures, and the hardness difference between the weld and the base material may be large. When the thickness of the pre-welded steel pipe is t, the base material refers to a point located at a distance of 5t in the circumferential direction from the circular cross-section based on the weld.

[0178] Since a large difference in hardness between the weld and base material causes defects in the subsequent steel component manufacturing process, it is important to reduce the hardness difference between the weld and base material from the pre-welded steel pipe state.

[0179] From this perspective, before locally heat treating the weld, the pre-welded steel pipe can satisfy the relationship between the hardness difference △H1 between the weld and the base material and the C content △H1 ≤ 289.61*exp(0.8944*[C]) + 67 Hv.

[0180] Here, when the thickness of the pre-welded steel pipe is t, the parent material refers to a point located at a distance of 5t in the circumferential direction from the circular cross-section based on the welded part. The width of the welded part refers to the width of the weld line that appears in the form of a straight band along the length of the pipe, and can be formed as a narrow area of ​​approximately 6 mm along the circumference of the pre-welded steel pipe.

[0181] △The relationship between H1 and C content satisfies △H1 ≤ 289.61*exp(0.8944*[C]) + 67 Hv, which has a beneficial effect in reducing the hardness deviation between the welded portion and the base material portion in the state of a welded steel pipe.

[0182] A method to confirm the relationship between △H1 and C content in a welded steel pipe is to measure the Vickers hardness across the weld line in the donut-shaped cross-section that appears when the welded steel pipe is cut cross-sectionally.

[0183] Step for local heat treatment of welded joints of welded steel pipes (S130)

[0184] In order to reduce the hardness difference between the weld and base metal from the welded steel pipe state, it is desirable to locally heat treat the weld of the welded steel pipe.

[0185] Typically, pre-welded steel pipes undergo water cooling immediately after welding, resulting in the formation of a low-temperature structure, typically martensite, at the weld, which is significantly higher than the pearlite and ferrite structures of the base material (i.e., hot-rolled steel) outside the weld. This difference in hardness between the weld and the base material can lead to defects in subsequent parts manufacturing processes. Therefore, the difference in hardness can be reduced by either locally heat-treating the weld or by heat-treating the entire pipe after locally heat-treating the weld.

[0186] The process of locally heat treating the welded portion, or heat treating the entire steel pipe after locally heat treating the welded portion, can be performed by normalizing heat treatment followed by slow cooling or air cooling after austenitizing heat treatment.

[0187] Specifically, the weld can be locally heat treated at an Ar3 temperature or higher for 3 seconds or longer. To locally heat treat the weld, the temperature can be raised to 800 to 1000°C, held for about 3 to 100 seconds, cooled with oil, and then cooled at a rate of 0.01°C / sec or higher. Immediately after local heat treatment, the weld is composed of pearlite and ferrite identical to the base material, and the hardness difference between the weld and the base material can be significantly reduced. If the holding time is shorter than about 3 seconds, the heat treatment of the weld may be insufficient, resulting in low-temperature structures remaining, which may result in a large difference in hardness from the base material. Conversely, if the holding time is slower than 100 seconds, there is a risk that grain growth may occur due to under-annealing, resulting in poor toughness.

[0188] After locally heat-treating the welded portion at a temperature above Ar3 for more than 3 seconds, the entire pre-welded steel pipe can be heat-treated at a temperature above Ar3. To heat-treat the entire pre-welded steel pipe, the temperature is raised to 800 to 1100°C and then air-cooled so that the structure of the welded portion and the parent material can be composed of pearlite and ferrite. If the temperature is below 800°C, there is a risk that the heat treatment will be uneven due to insufficient austenite monophasing, and if the heat treatment is performed above 1100°C, there is a risk of a decrease in toughness due to grain growth.

[0189] In this way, if the weld is locally normalized and heat-treated, the weld and base material structures can be configured to be identical, and if the entire welded steel pipe is then heat-treated, the entire welded steel pipe can be controlled to have a uniform microstructure and properties.

[0190] From this point of view, when the difference in hardness between the welded portion and the base material portion of the heat-treated pre-welded steel pipe is △H2, △H2 ≤ 50 Hv may be achieved, and preferably 0 ≤ △H2 ≤ 40 Hv.

[0191] Step of quenching heat-treated pre-welded steel pipe (S140)

[0192] When the above heat-treated pre-welded steel pipe is subjected to quenching and tempering (QT) heat treatment, the structure of the weld and the base material can be composed of martensite or tempered martensite, so the microstructural and hardness deviations between the weld and the base material are reduced, and high hardness can be secured. Accordingly, cracks are rarely generated during the manufacturing process of steel parts, and the service life of the steel parts can be improved.

[0193] The quenching (hardening heat treatment) step can be performed by maintaining the heat-treated preformed steel pipe at a temperature of 820 to 1100°C for 10 to 10,000 seconds to form a single phase of austenite, and then cooling it to a temperature of Mf+100°C or lower at a cooling rate of 1°C / second or higher to manufacture martensitic steel.

[0194] The welded steel pipe can be austenitized by heating it to an austenite single-phase temperature range of preferably 840 to 1100℃ and maintaining it for 100 to 8000 seconds, more preferably 200 to 5000 seconds. If the heating temperature is lower than 820℃, ferrite or pearlite transformation may occur during the cooling process, and uniform martensite may not be generated throughout the entire thickness, making it difficult to secure the target hardness. If the maintaining time is less than 10 seconds at 820℃ or higher, it is difficult to secure a uniform microstructure in the thickness direction, and if it exceeds 10,000 seconds, the problem of increased costs may occur without a heat treatment effect.

[0195] When cooling, the cooling can be performed at an average cooling rate of preferably 1 to 150°C / sec, more preferably 5 to 150°C / sec, to a martensite temperature range, preferably Mf (martensite end temperature) + 100°C or lower, for example, a cooling end temperature such as 100°C, 50°C, or room temperature. If cooling is performed at a rate less than 1°C / sec, ferrite or pearlite transformation occurs during the cooling process, making it difficult to secure the target hardness. In addition, if the cooling end temperature exceeds Mf + 100°C, a bainite structure is formed without sufficiently securing a martensite structure, making it difficult to secure the target hardness likewise.

[0196] When quenched, the steel component may contain more than 90 area% martensite with a high carbon (C) content of greater than 0.38 to 0.55 wt%, preferably a single martensite phase.

[0197] The quenched steel component can exhibit a Vickers hardness of 600 Hv or more at room temperature, and preferably 600 to 750 Hv. Through the quenching process, the hardness properties of the weld and base metal portions of the steel component can be made almost identical.

[0198] After the quenching step, a step of tempering the quenched pre-welded steel pipe (steel part) at 150 to 650°C may be further included.

[0199] The steel part can contain more than 90 area% of tempered martensite through tempering after quenching, and the hardness can be controlled, and the hardness properties of the welded part and the base material part of the steel part can be made almost identical through the quenching and tempering process.

[0200] Tempering can be performed on steel parts that have undergone quenching heat treatment by maintaining them at a temperature range of 150 to 650°C for 10 to 20,000 seconds, preferably 50 to 15,000 seconds, and more preferably 100 to 10,000 seconds. Tempering can improve ductility and toughness by removing defects such as dislocations that occur during the quenching process. When tempering, if the holding time is less than 10 seconds, the effect is minimal, and if it exceeds 20,000 seconds, the problem of increased costs may arise without the heat treatment effect.

[0201] The quenched and tempered steel component may comprise at least 90 area% of tempered martensite with a high carbon (C) content of greater than 0.38 to 0.55 wt%, and preferably may comprise a single phase of tempered martensite.

[0202] And tempered steel parts can secure excellent toughness even if their hardness is slightly lower than that of quenched steel parts.

[0203] The steel component of the present invention can be manufactured by locally heat-treating a welded portion to manufacture a welded steel pipe having no difference in hardness between the welded portion and the base material portion, and then performing quenching heat treatment, and can be manufactured by performing tempering heat treatment as needed, and contains, in weight %, C: more than 0.38% to 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder Fe and unavoidable impurities, and can satisfy at least one of the following [1] and [2].

[0204] [1] Mo: 0.5% or less

[0205] [2] At least one of the following: Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less

[0206] Steel parts with martensite or tempered martensite as their main structure have already reduced microstructural deviation and hardness deviation from the pre-welded steel pipe state, and can be easily manufactured with almost no crack occurrence.

[0207] From this point of view, when the difference in Vickers hardness between the weld and base material of the steel part is △H2, △H2 ≤ 50 Hv may be present, and after quenching, the part may contain a martensite microstructure, and the Vickers hardness may be 600 Hv or more, and after tempering, the part may contain a tempered martensite microstructure, and the Vickers hardness may be 200 Hv or more.

[0208]

[0209] Hereinafter, specific examples of hot-rolled steel pipes, steel parts, and their manufacturing methods will be examined.

[0210] 1. Manufacturing of hot rolled steel and steel parts using the same

[0211] A steel slab having the composition shown in Table 1 below was homogenized by heating at approximately 1200±50℃ for 2 hours, and then rough-rolled and finish-rolled at a temperature higher than A3, and coiled to produce a hot-rolled steel plate specimen. S-Al stands for available aluminum (Sol.Al).

[0212] [Table 1] Chemical composition of steel grades (weight %)

[0213]

[0214] The steel pipe having the composition of Table 1 was subjected to normalizing heat treatment by locally maintaining the welded portion at a temperature higher than Ar3 for about 3 seconds immediately after welding due to the difference in properties between the welded portion and the base material portion, and then cooling to room temperature. The entire steel pipe was then heat treated, maintained at a temperature higher than Ar3 for more than 1 minute, and then cooled to room temperature at a cooling rate of 0.1°C / s or more.

[0215] Accordingly, a welded steel pipe was manufactured with no difference in hardness between the base material and the welded portion through post-welding normalizing heat treatment.

[0216] The hardness of the hot-rolled steel having the above composition is the same as the hardness of the base material, and the hardness and hardness deviation △H1, △H2 of the welded part of the manufactured pre-welded steel pipe and the flattening test results of the pre-welded steel pipe are shown in Table 2 below.

[0217] 1) Elongation and yield strength of hot-rolled steel: Measured at room temperature according to JIS Z 2201 standard.

[0218] 2) Hardness of the parent material and welded portion, immediately after welding, after heat treatment of the welded portion, hardness measurement method: Measured using Vickers hardness at room temperature, measured at 1 / 4t point of the cross-section in the thickness direction of the material.

[0219] 3) Method for measuring crack occurrence after 7 / 8D compression: This is a method for evaluating the soundness of the weld of a pre-welded steel pipe according to the KS D 3517 standard. When the weld of the pre-welded steel pipe was laid down so that it was 90° to the compression direction and compressed with a jig with flat upper and lower parts, the presence or absence of crack occurrence in the weld was judged by visually observing it. The amount of compression was performed until it reached a height of 7 / 8 of the outer diameter D of the steel pipe. A crack was defined as one that was visible to the naked eye, felt deep when touched by hand, and split more than 20㎛ in the depth direction.

[0220] [Table 2]

[0221]

[0222] Looking at Table 2, it can be seen that the hot-rolled steel exhibited elongation of 10 to 24% and yield strength of 487 to 621 MPa in experimental specimens 1 to 10, indicating that softening was imparted to the hot-rolled steel.

[0223] In Experimental Materials 1 to 10, immediately after welding, there is a hardness difference △H1 between the weld and the base material, and it can be confirmed that △H1 ≤ 289.61*exp(0.8944*[C]) + 67 Hv is satisfied before heat treatment. In addition, it is shown that the hardness difference △H2 between the weld and the base material is 50 Hv or less after local heat treatment and overall heat treatment of the weld.

[0224] In particular, experimental materials 1 to 10 showed a reduction in microstructure deviation and hardness deviation from the state of a pre-welded steel pipe, and showed an effect of almost no crack occurrence.

[0225] On the other hand, experimental materials 11 to 13 showed at least one of an elongation of less than 9% and a yield strength of more than 700 MPa, indicating that the material was harder than experimental materials 1 to 10, and it was confirmed that cracks occurred due to microstructural deviation.

[0226] The above-mentioned pre-welded steel pipe was subjected to heat treatment to manufacture steel parts by quenching (maintaining at 880°C for 2,400 seconds to form austenite, then cooling to room temperature at a cooling rate of 100°C / second) and tempering (maintaining at 200°C, 400°C, and 650°C for 3,600 seconds to cool to room temperature).

[0227] The hardness and area fraction of the parent material of the manufactured steel parts are shown in Table 3 below.

[0228] The microstructure area fraction was measured under an optical microscope at 50x magnification.

[0229] [Table 3]

[0230]

[0231] Looking at Table 3, it can be confirmed that experimental materials 1 to 10 exhibited a Vickers hardness of 600 Hv or more after quenching heat treatment and secured a tempered martensite fraction of 90% or more.

[0232] In particular, test materials 1 to 10 exhibited Vickers hardness of 200 to 670 Hv when the tempering temperature was 200 to 650°C, and showed a tendency for the Vickers hardness to decrease slightly as the tempering temperature increased. Specifically, when the tempering temperature was 200 to 230°C, the Vickers hardness was 580 to 670 Hv, when the tempering temperature was 390 to 410°C, the Vickers hardness was 440 to 530 Hv, and when the tempering temperature was 620 to 650°C, the Vickers hardness was 200 to 330 Hv.

[0233] Experimental material 11 showed a tempered martensite fraction of 95% or more, but the Vickers hardness of the steel part after quenching was less than 600 Hv.

[0234] Experimental materials 12 and 13 showed a Vickers hardness of 600 Hv or more after quenching heat treatment, but showed a low microstructure fraction of less than 75%.

[0235] Although the present invention has been described with reference to the drawings exemplified above, it is to be understood that the present invention is not limited to the embodiments and drawings disclosed herein, and that various modifications may be made by those skilled in the art within the scope of the technical idea of ​​the present invention. Furthermore, even if the operational effects according to the configuration of the present invention have not been explicitly described while describing the embodiments of the present invention, it is natural that the effects predictable by the corresponding configuration should also be acknowledged.

Claims

1. In weight%, C: more than 0.38% ~ 0.55% or less, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder including Fe and unavoidable impurities. Satisfying at least one of the following [1] and [2], [1] Mo: 0.5% or less [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these. Hot rolled steel containing 40 to 95 area% of pearlite for a total area of ​​100 area%.

2. In paragraph 1, Other microstructures include hot rolled steel containing ferrite, bainite and martensite.

3. In paragraph 1, Hot rolled steel with Vickers hardness of 150 to 370 Hv.

4. In paragraph 1, Hot rolled steel with an elongation of 9 to 30%.

5. In paragraph 1, Hot rolled steel with a strength of 400 to 700 MPa. 6.(a) A step of manufacturing a hot-rolled steel strip by slitting a hot-rolled product containing, by weight%, C: more than 0.38% and not more than 0.55%, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and satisfying at least one of the following [1] and [2]; [1] Mo: 0.5% or less [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these. (b) a step of manufacturing an electric resistance welded steel pipe by forming the hot rolled steel strip using an electric resistance welding method; (c) a step of locally heat-treating the welded portion of the above-mentioned pre-welded pipe; and (d) a method for manufacturing a steel part, comprising the step of quenching the heat-treated pre-welded steel pipe.

7. In paragraph 6, The pre-welded steel pipe of step (b) above A method for manufacturing a steel part in which the relationship between the hardness difference between the weld and the base material △H1 and the C content satisfies △H1 ≤ 289.61*exp(0.8944*[C]) + 67 Hv. (Here, when the thickness of the welded pipe is t, the parent material refers to a point located at a distance of 5t in the circumferential direction from the circular cross-section based on the welded section.) 8. In paragraph 6, The pre-welded steel pipe heat-treated in step (c) or the steel part quenched in step (d) A method for manufacturing a steel part having △H2 ≤ 50 Hv when the difference in hardness between the weld and the base material is △H2.

9. In paragraph 6, The step (c) above is a method for manufacturing a steel part in which the weld is locally heat-treated at a temperature higher than Ar3 for 3 seconds or longer.

10. In paragraph 6, The step (c) above is a method for manufacturing a steel part in which, after locally heat-treating the welded portion, the entire pre-welded steel pipe is additionally heat-treated at a temperature higher than Ar3.

11. In paragraph 6, The above step (d) is a method for manufacturing a steel part, which comprises maintaining the steel at a temperature of 820°C or higher for 10 to 10,000 seconds to form a single phase of austenite, and then cooling the steel to a temperature of Mf+100°C or lower at a cooling rate of 1°C / second or higher.

12. In paragraph 6, A method for manufacturing a steel part, wherein the Vickers hardness of the steel part quenched in the above step (d) is 600 Hv or more.

13. In paragraph 6, After step (d) above, (e) A method for manufacturing a steel part, further comprising the step of tempering the above-mentioned quenched steel pipe at 150 to 650°C.

14. In paragraph 13, A method for manufacturing a steel part, wherein the Vickers hardness of the steel part tempered in the above step (e) is 200 Hv or higher. A steel part containing 15. wt% of C: more than 0.38% and not more than 0.55%, Si: 0.1 to 0.4%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.05%, Cr: 0.05 to 1.2%, the remainder being Fe and unavoidable impurities, and satisfying at least one of the following [1] and [2]. [1] Mo: 0.5% or less [2] Ti: 0.05% or less, V: 0.1% or less, and B: 0.005% or less, at least one of these 16. In paragraph 15, Steel parts having 90 area% or more of martensite or 90 area% or more of tempered martensite for a total of 100 area%.

17. In paragraph 15, A steel component having a hardness difference between the welded portion and the parent material portion of the above steel component of △H2, wherein △H2 ≤ 50 Hv.

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