JP6697626B1 - Welding method for carbon steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welding method capable of reducing the possibility of causing brittle fracture in a welded portion formed by welding a plurality of carbon steel materials. A carbon steel material welding method includes a welding step (S10) of butt welding a plurality of carbon steel materials. The carbon equivalent Ceq of the carbon steel material is 0.4 or more, and the ideal critical diameter DI is 150 mm or less. This welding method comprises a post-heating step in which the welded portion (40) is reheated to a temperature of Ac1 transformation point or higher before the temperature of the welded portion (40) becomes lower than 100 ° C. And a slow cooling step of cooling at an average cooling rate equal to or lower than an allowable cooling rate (Vmax) in a temperature range up to ° C. [Selection diagram] Figure 1

Description

  The present invention relates to a method for welding carbon steel.

  2. Description of the Related Art Conventionally, various continuous processing lines (for example, pickling, rolling, etc.) are used in the manufacturing process of steel plates (steel strips). In a continuous processing line, coils of a plurality of steel strips are threaded one after another in order to continuously perform a predetermined treatment on the steel strip. When passing a plurality of coils, a process of welding the ends of the two steel strips is performed.

  A joint portion (that is, a welded portion) formed by welding two steel strips is hardened by a heat effect. Depending on the steel type of the steel strip, a simple welding process may significantly harden the weld. For example, in a welded portion obtained by laser welding carbon steel strips together, problems such as weld cracking due to rapid cooling immediately after welding or brittle fracture of the welded portion in the middle of a continuous processing line are likely to occur. Various studies have been conducted on methods for dealing with such problems (for example, Patent Documents 1 and 2).

  Patent Document 1 describes the following technique as a welding method for carbon steel materials containing 0.3 to 1.5% by mass of carbon (C). In this method, a post heat treatment is performed on a welded portion formed by welding carbon steel materials together. Specifically, as a post heat treatment, the welded part is (1-i) reheated to a temperature in the range of 600 to 900 ° C. at a temperature rising rate of 1 ° C./second or more, and then (1-ii). A process of allowing to cool or gradually cool is performed. The treatment of (1-i) suppresses the diffusion of solid solution C in the material structure of the welded portion at the time of reheating after welding, and the treatment of (1-ii) causes the formation of martensite in the welded portion. The amount produced is reduced. As a result, the welded portion after the post heat treatment is softened.

  Further, in Patent Document 2, a welding material containing 0.1% by mass or less of C and 1.22% by mass or less of Cr when laser-welding rolled materials of steel types capable of generating a low temperature transformation structure after laser welding. A technique of laser welding while supplying (adding) a (filler) to a welding target portion is described. Patent Document 2 also describes performing heat treatment as a pretreatment and a posttreatment for laser welding. In this technique, the C content in the welded portion is reduced, and the cooling process of the welded portion after laser welding is relaxed to reduce the amount of low-temperature transformation microstructures such as martensite, thereby reducing the hardness of the welded portion. Is being reduced.

JP 2005-48271 A JP, 2007-175774, A

  However, the hardenability of steel materials is, in fact, greatly affected by the type of steel (that is, chemical composition) and the grain size of the material structure. Therefore, in the technique described in Patent Document 1, the carbon steel material may have very high hardenability depending on the type and amount of alloying elements contained in the carbon steel material. In this case, the post-heat treatment described in Patent Document 1 cannot actually prevent the formation of martensite, and the welded portion may be hard. In a situation where a sufficient time for post-heat treatment cannot be ensured, such as in a continuous treatment line, the above-mentioned problems are more likely to occur.

  In the technique described in Patent Document 2, since it is essential to add a filler having a low C content, it is expected that when the rolled materials are welded to each other, the C equivalent in the weld metal portion will decrease, resulting in softening. On the other hand, the heat-affected zone remains the base metal component, and the rolled material may have very high hardenability depending on the type and amount of alloying elements contained in the carbon steel material as in Patent Document 1. is there. In this case, the generation of martensite in the heat-affected zone cannot be prevented, and the welded portion can be hard. In a situation where a sufficient time for post-heat treatment cannot be ensured, such as in a continuous treatment line, the above-mentioned problems are more likely to occur.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a welding method capable of reducing the possibility of causing brittle fracture in a welded portion formed by welding a plurality of carbon steel materials.

In order to solve the above problems, a method for welding a carbon steel material according to an aspect of the present invention is a method for welding a carbon steel material including a welding step of welding a plurality of carbon steel materials to each other, in the carbon steel material, The carbon equivalent C _eq given by (1) is 0.4 or more, and the ideal critical diameter D _I given by the equation (2) is 150 mm or less, and is generated by the welding process after the welding process. A post-heating step of reheating the welded portion to a temperature of Ac1 transformation point or higher before the welded portion has a temperature of less than 100 ° C., and the welded portion heated in the post-heating step, In the temperature range from the Ac1 transformation point to 500 ° C., a slow cooling step of cooling at an average cooling rate equal to or lower than the allowable cooling rate Vmax (° C./sec) given by the equation (3) is included.
C _eq = C + (Si / 24) + (Mn / 6) + (Cr / 5) + (Mo / 4) + (Ni / 40) + (V / 14) (1)
D _I = (6.99 × C ^0.5 ) × (1 + 0.64Si) × (1 + 4.1Mn) × (1 + 2.83P) × (1-0.62S) × (1 + 2.33Cr) × (1 + 3.14Mo ) × (1 + 0.52Ni) × (1 + 1.5 (0.9−C) B) (2)
Vmax = 900 / D _I ... (3)
Here, as for the value of B in the formula (2) , instead of substituting the mass% concentration of B, when the B content in the carbon steel material is 0.0005 mass% or more, the numerical value 1 is substituted, and When the content is less than 0.0005 mass%, the numerical value 0 is substituted, and for each element symbol in the formula (1) and each element symbol other than B in the formula (2), the mass% concentration of each element Substitute

  According to one aspect of the present invention, it is possible to provide a welding method capable of reducing the possibility of causing brittle fracture in a weld formed by welding a plurality of carbon steel materials.

It is a figure for demonstrating the welding method of the carbon steel material in Embodiment 1 of this invention, <symbol 1001> is a schematic diagram which shows the schematic structure of an example of a continuous processing line, and <symbol 1002> is a welding apparatus. A schematic view showing an example of the configuration, and <1003> is a schematic view showing an enlarged welded portion. It is a flow chart which shows an outline of an example of a welding process in the above-mentioned embodiment. It is a graph which shows the temperature change with time in a welding process for explaining the welding methods of the example of the present invention and the conventional example. 6 is a graph schematically showing a temperature change with time in a welding process, for explaining a method for welding a carbon steel material in a modified example of Embodiment 1 of the present invention. It is a schematic diagram for demonstrating the welding apparatus used in order to implement the welding method of the carbon steel material in Embodiment 2 of this invention. It is a schematic diagram which shows the structure of an Erichsen tester roughly. It is a photograph which shows an example of an Erichsen test result. It is a figure which shows the hardness measurement position in a hardness test.

  Hereinafter, embodiments of the present invention will be described. Note that the following description is for better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. Moreover, in this specification, "AB" has shown that it is A or more and B or less.

  The welding method of the carbon steel material in the present embodiment (hereinafter, may be simply referred to as the main welding method) is, for example, welding at the entrance side of a continuous processing line for continuously performing a predetermined process on a steel strip drawn from a coil. It can be suitably applied to welding processing in an apparatus. This welding device is used for welding different steel strips when a plurality of coils are sequentially passed through while operating a continuous processing line.

<Continuous processing line>
First, an example of a continuous processing line to which the present welding method is applied will be schematically described with reference to FIG. FIG. 1 is a schematic diagram for explaining a method of welding a carbon steel material according to this embodiment. A diagram indicated by reference numeral 1001 in FIG. 1 is a schematic diagram showing a schematic configuration of an example of the continuous processing line L1.

  As indicated by reference numeral 1001 in FIG. 1, the continuous processing line L1 includes a coil payout unit 1, a welding device 2, a looper 3, a steel strip processing line 4, a shearing machine 5, and a winding unit 6. In the continuous processing line L1, a predetermined process is continuously performed on the steel strip flowing from the input side IS toward the output side OS, with the coil payout part 1 as the input side IS and the winding part 6 as the output side OS. It

  This welding method is suitably applied to the welding device 2. Each part of the continuous processing line L1 other than the welding device 2 has a conventionally known structure, and thus detailed description thereof will be omitted, but it will be roughly described as follows. Here, it is assumed that the steel strip is being welded by the welding device 2 immediately after inserting the coil 1A in the coil payout portion 1.

  The steel strip delivered from the coil 1A immediately after replacement in the coil delivery unit 1 is referred to as a trailing steel strip ST1, and precedes the steel strip flowing through the continuous processing line L1 on the delivery side OS side with respect to the welding device 2. It is called steel strip ST2. By the welding device 2, the trailing steel strip ST1 and the preceding steel strip ST2 are butt-welded to each other at their ends. The welding device 2 and the process (welding process S10) performed by the welding device 2 will be described in detail later. A shearing machine may be installed between the coil payout part 1 and the welding device 2 in order to form a butt end surface of the welded part.

  The looper 3 is a device that enables the preceding steel strip ST2 to continue to flow in the continuous processing line L1 while the welding process S10 in the welding device 2 is being performed. The looper 3 is capable of accumulating the preceding steel strip ST2 having a certain length, and can send the preceding steel strip ST2 to the steel strip processing line 4 for a certain time.

  Here, although it depends on the line speed of the continuous processing line L1, that is, the speed of the preceding steel strip ST2 flowing through the continuous processing line L1, there is a limit to the time during which the operation of the continuous processing line L1 can be maintained using the looper 3. Therefore, in the welding process in the welding device 2, it is required to reduce the possibility of brittle fracture in the welded portion, and there is a limit that the allowable time is limited.

  The steel strip processing line 4 is provided with various equipment so as to continuously perform a predetermined processing. The continuous processing line L1 may be various depending on the type of the steel strip processing line 4. For example, the continuous treatment line L1 to which the present welding method is applied includes a continuous pickling line, a continuous rolling line, a continuous pickling / rolling line, a continuous annealing line, a continuous annealing / pickling line, a continuous heat treatment line, and a coil cleaning. Lines, etc.

  The preceding steel strip ST2 flows to the winding section 6 via the shearing machine 5 after the steel strip processing line 4, and is wound as a coil 6A in the winding section 6. When the coil 6A has a predetermined winding amount, the steel strip is cut using the shearing machine 5 to take out the coil 6A. A looper (not shown) may be provided between the steel strip processing line 4 and the shearing machine 5.

(Welding equipment)
Next, a welding apparatus 2 as an example of an apparatus for carrying out the present welding method will be schematically described with reference to the drawings indicated by reference numerals 1002 and 1003 in FIG. The diagram indicated by reference numeral 1002 in FIG. 1 is a schematic diagram showing the configuration of the welding device 2. The diagram indicated by reference numeral 1003 in FIG. 1 is a schematic diagram showing an enlarged welded portion formed by welding.

  As indicated by reference numeral 1002 in FIG. 1, the welding device 2 includes a laser irradiator 21, a preheat heater 22, a postheat heater 23, a cooling adjustment heater 24, and a control unit 20 that controls the operation of each of these units. .. In the present embodiment, the welding device 2 including the preheating heater 22 and the cooling adjustment heater 24 will be described as an example, but the device for performing the present welding method is not limited to this. The preheat heater 22 and the cooling adjustment heater 24 may be installed as needed, and the welding apparatus in another embodiment may not be provided with either the preheating heater 22 or the cooling adjustment heater 24, or Both need not be provided. This also applies to other embodiments described later.

  The welding device 2 in the present embodiment is a laser welding device that performs welding by irradiating a high-density energy beam using a laser irradiation device 21. In laser welding, since the total heat input to the weld is small, the cooling rate of the weld immediately after laser welding is usually relatively fast (that is, the weld is rapidly cooled). The present welding method can be suitably applied to laser welding. The application target of the present welding method is not limited to laser welding, and the present welding method can also be applied to, for example, flash butt welding, arc welding, seam welding (resistance welding), or the like. The welding of the end of the trailing steel strip ST1 and the end of the preceding steel strip ST2 may be butt welding or lap welding. The present welding method can be carried out by imparting a heat history similar to that of a welding step S10 in the present embodiment to be described later to the welded portion, and a specific means for welding (in other words, a specific welding device Form) is not particularly limited.

  The welding device 2 connects the trailing steel strip ST1 and the leading steel strip ST2 so that the end portion of the exit side OS in the trailing steel strip ST1 and the end portion of the entering side IS in the leading steel strip ST2 abut each other. Position. In the present specification, the portions of the trailing steel strip ST1 and the preceding steel strip ST2 that are abutted against each other and that are irradiated with laser from the laser irradiator 21 are referred to as welding target portions 30.

  In the welding device 2, the laser irradiation machine 21, the preheating heater 22, the postheating heater 23, and the cooling adjustment heater 24 all move simultaneously or individually in the welding direction D1, while the trailing steel strip ST1 and the preceding steel strip ST1 are moved. It has a configuration capable of performing welding with ST2. It suffices that the welding device 2 has a configuration capable of performing pre-heat treatment, welding treatment, post-heat treatment, and cooling treatment, and the specific configuration is not particularly limited.

  Known equipment can be used as the laser irradiator 21, and for example, a carbon dioxide gas laser device can be used. The preheating heater 22, the postheating heater 23, and the cooling adjustment heater 24 may be any device that can perform the main welding method described below by locally heating, and the specific device is not particularly limited. Not done. Examples of such a device include a high-frequency induction heating device, a device for conducting electric heating with electrodes, a laser irradiation device, an arc discharge device, and the like. The preheater heater 22, the postheater heater 23, and the cooling adjustment heater 24 may be the same type of device or different types of devices.

  A detailed description of the control content of each part of the welding device 2 by the control unit 20 for carrying out the present welding method will be described later together with the description of the present welding method.

(welded part)
As indicated by reference numeral 1003 in FIG. 1, the welding target portion 30 is irradiated with a laser, and the trailing steel strip ST1 and the preceding steel strip ST2 are laser-welded to form a welded portion 40. The welded portion 40 includes a weld metal portion 41 and a heat affected zone 42. The weld metal portion 41 is a solidified portion in the welding target portion 30 before laser irradiation after the molten steel of the trailing steel strip ST1 and the molten steel of the preceding steel strip ST2 are mixed by the laser irradiation. The heat-affected zone 42 is a portion whose material is changed under the influence of a temperature rise due to laser irradiation.

  Here, the welded portion 40 is locally heat-treated by the post-heater heater 23 or the like, but in this specification, the heat-affected zone 42 is defined as follows. That is, the heat-affected zone 42 is affected by the heat of the laser irradiation (that is, rapidly heated and rapidly cooled), and the preheat heater 22, the postheater 23, or the preheater 22 and the postheater 23, the Ac1 transformation point. The part heated above is defined. Although it is difficult to strictly define the range of the heat-affected zone 42, the temperature rises due to laser irradiation or heating by the preheat heater 22 or the postheat heater 23, and a portion in which austenite is generated in the material structure is formed. It can be said that

<Welding method for high carbon steel>
Hereinafter, the welding method for the high carbon steel material (welding step S10) in the present embodiment will be described in detail with reference to FIGS. 2 and 3.

(High carbon steel)
The high carbon steel material to be welded in the present welding method satisfies both the following condition A and condition B.

Condition A: The carbon equivalent C _eq given by the following formula (1) is 0.4 or more.
C _eq = C + (Si / 24) + (Mn / 6) + (Cr / 5) + (Mo / 4) + (Ni / 40) + (V / 14) (1)
Here, the mass% concentration of each element in the component composition of the high carbon steel material is substituted into each element symbol of the formula (1).

Condition B: The ideal critical diameter D _I given by the following equation (2) is 150 mm or less.
D _I = (6.99 × C ^0.5 ) × (1 + 0.64Si) × (1 + 4.1Mn) × (1 + 2.83P) × (1-0.62S) × (1 + 2.33Cr) × (1 + 3.14Mo ) × (1 + 0.52Ni) × (1 + 1.5 (0.9−C) B) (2)
Here, when the B content in the composition of the high carbon steel is 0.0005 mass% or more, 1 is substituted for the value of B in the formula (2), and when the B content is less than 0.0005 mass%. Substitutes 0. Further, the mass% concentration of each element in the component composition of the high carbon steel material is substituted for each element symbol other than B in the formula (2).

The trailing steel strip ST1 and the preceding steel strip ST2 in the present embodiment are made of a high carbon steel material that satisfies both the above condition A and condition B. Usually, the trailing steel strip ST1 and the preceding steel strip ST2 are of the same steel type. In the case where the trailing steel strip ST1 and the preceding steel strip ST2 are different steel types from each other, of the trailing steel strip ST1 and the preceding steel strip ST2, the one having a relatively large carbon equivalent C _eq should satisfy the above condition A. Good. Further, it is sufficient that the one of the trailing steel strips ST1 and the preceding steel strip ST2, which has the larger ideal critical diameter D _I , satisfies the above condition B.

With a steel material having a carbon equivalent C _eq given by the formula (1) of less than 0.4, there is almost no concern that remarkable hardening and weld cracking will occur in the weld 40 after laser welding. On the other hand, in a steel material having a carbon equivalent C _eq of 0.4 or more given by the formula (1), when the weld 40 is rapidly cooled to, for example, room temperature after laser welding, the weld 40 is significantly hardened by martensitic transformation. .. Then, there is a concern that the generated strain causes weld cracks in the welded portion 40. The present welding method is used for welding a high carbon steel material that satisfies the above condition A.

Further, the ideal critical diameter D _I given by equation (2) is an index indicating the hardenability of high carbon steel is the larger the value of the ideal critical diameter D _I highly hardenability high carbon steel It means that. For a steel material having an ideal critical diameter D _I given by the formula (2) of more than 150 mm, the hardenability is very high, and the welded portion is subjected to the conditions such as the welding step S10 in which the allowable processing time is limited. It is difficult to sufficiently perform a treatment for suppressing hardening of 40. Therefore, the present welding method is used for welding a high carbon steel material that satisfies the above condition B.

  The high carbon steel material to be welded in the present welding method is, for example, a steel strip having a plate thickness of 0.8 mm to 10 mm and a plate width of 400 mm to 1200 mm. The high carbon steel material is not limited to a steel strip and may be a wire rod, a steel bar, or the like.

(Ingredient composition)
Below, the steel composition (component composition) of the high carbon steel material in this embodiment is shown.

(C)
The high carbon steel material in the present embodiment preferably has a C (carbon) content of 0.3 mass% or more and 1.5 mass% or less. C is the most basic alloying element in carbon steel, and the hardness and toughness of the pearlite structure or the hardness and toughness of the martensite structure greatly vary depending on the content thereof. With steel having a C content of less than 0.3% by mass, there is little concern that cracks will occur in the weld even if it is rapidly cooled after welding and martensite is generated.

  On the other hand, if the C content exceeds 1.5% by mass, even if the post-heat treatment of the present invention is applied to the welded portion, the metal structure becomes hard and has low toughness, and there is a concern that it may be broken due to the addition of bending in a continuous processing line. Will increase. Therefore, in the present invention, from the viewpoint of preventing brittle fracture of a weld formed by welding of high carbon steel material, the high carbon steel having a C content of 0.3 mass% or more and 1.5 mass% or less is targeted. ..

  From the viewpoint of preventing brittle fracture of the welded portion, the C content is preferably low and is preferably 1.2% by mass or less.

(Si)
Si (silicon) is an alloying element that acts as a deoxidizer. If the Si content is less than 0.02 mass%, the effect cannot be sufficiently obtained. On the other hand, if Si is added excessively, the ferrite is hardened by the solid solution strengthening action, and the concern about brittle fracture increases. Therefore, when Si is added, the content is set to 2.0 mass% or less. Therefore, the Si content is preferably 2.0 mass% or less, and more preferably 0.05 mass% or more and 1.0 mass% or less.

(Mn)
Mn (manganese) is an alloying element that improves hardenability and is added as necessary. If the Mn content exceeds 2.0% by mass, the steel sheet is hardened and the concern about brittle fracture increases. The Mn content is preferably 2.0% by mass or less, and more preferably 0.2% by mass or more and 1.0% by mass or less.

(P, S)
P (phosphorus) and S (sulfur) are alloying elements that reduce toughness. Therefore, in order to improve toughness, it is preferable to reduce as much as possible. The P content is preferably 0.030 mass% or less, more preferably 0.020 mass% or less. The S content is preferably 0.035 mass% or less, more preferably 0.020 mass% or less.

(Cr)
Cr (chromium) is an element that improves hardenability and increases temper softening resistance, and is added as necessary. However, when a large amount of Cr exceeding 1.8 mass% is contained, even in the present welding method, the welded portion has a hard and low toughness metallographic structure, and there is an increased risk of fracture due to bending in the continuous processing line. .. Therefore, when Cr is added, it is desirable that the Cr content be 1.8 mass% or less. The Cr content is preferably 0.1% by mass or more and 1.6% by mass or less.

(Other elements)
The high carbon steel material in the present embodiment may be a steel type to which the following elements are added for the purpose of improving properties such as hardenability and toughness. As a range that does not impair the formability, Mo is 0.5 mass% or less, Cu is 0.3 mass% or less, Ni is 2.0 mass% or less, Al is 0.1 mass% or less, and V is 0.5 mass%. %, Ti can be added up to 0.5% by mass, Nb up to 0.5% by mass, and B up to 0.0005 to 0.01% by mass.

  The balance other than the above components may be Fe and inevitable impurities. Here, the unavoidable impurities mean components such as O and N that are difficult to remove. These components are inevitably mixed in at the stage of melting the steel slab.

(Welding process)
FIG. 2 is a flowchart showing an outline of an example of the welding step S10 in this embodiment.

  As shown in FIG. 2, the welding step S10 includes a preheating step S1, a welding step S2, a postheating step S3, a first cooling step (slow cooling step) S4, and a second cooling step S5. In addition, in FIG. 2, P1, P2, P3, P4, P41, and P42 indicating the periods corresponding to the respective steps of the preheating step S1 to the second cooling step S5 are referred to in the description using FIG. 3 described later. .

(Preheating step S1)
In the preheating step S1, before performing laser welding (that is, before the welding step S2), the preheating heater 22 heats the welding target portion 30 to a temperature of 200 ° C. or higher and 800 ° C. or lower. As a result, it is possible to reduce the possibility that the welded portion 40 is cooled to a temperature of less than 100 ° C. immediately after laser welding and weld cracking occurs.

  The preheating step S1 is not essential in the method for welding a high carbon steel material according to one aspect of the present invention. However, since the preheating step S1 makes it possible to reduce the cooling rate of the welded portion 40 immediately after laser welding, it is preferable to include the preheating step S1.

  For example, when the preheating step S1 is not performed, the welding device 2 is arranged such that the laser irradiator 21 and the post-heater heater 23 in the welding direction D1 are close to each other, and the welding portion 40 is immediately after laser welding. May be prevented from reaching a temperature of less than 100 ° C. In this case, the welding device 2 may not include the preheater 22.

(Welding process S2)
In the welding process S2, the welding portion 40 is formed by irradiating the welding target portion 30 with a laser along the welding direction D1. The temperature of the welding target portion 30 and its peripheral portion that have been irradiated with the laser rapidly rises and then sharply decreases. Then, the temperature of the welded portion 40 becomes the Ac1 transformation point or lower. At a temperature of 100 ° C. or higher, the material structure of the weld portion 40 immediately after laser welding includes austenite, martensite, a composite structure of austenite and martensite, and the like. Further, if it is rapidly cooled to near room temperature of less than 100 ° C., the welded portion 40 is significantly hardened, and brittle fracture is likely to occur in the middle of the continuous processing line L1 after the welding device 2. In addition, quenching increases the possibility of weld cracking.

(Post-heating step S3)
In the post-heating step S3, the welded portion 40 generated in the welding step S2 is reheated to a temperature of Ac1 transformation point or higher before the temperature of the welded portion 40 becomes lower than 100 ° C. As described above, when the welded portion 40 is cooled to a temperature of less than 100 ° C. immediately after laser welding, the welded portion 40 is affected by the strain associated with the martensitic transformation, and there is a high possibility that weld cracking will occur. This is because.

  In the post-heating step S3, the material structure of the welded portion 40 is changed to an austenite structure by reheating the welded portion 40 to a temperature not lower than the Ac1 transformation point. At this time, the material structure of the welded portion 40 may be a complete austenitic structure, but may be a partial austenitic structure. The "partial austenite structure" means a structure such as austenite + ferrite, austenite + ferrite + granular carbide, austenite + granular carbide, and the like.

  In the post-heating step S3, when the weld 40 is not heated to the Ac1 transformation point or higher, austenite is not generated in the material structure of the weld 40. Therefore, even if the welded portion 40 is cooled after heating, the pearlite structure is not obtained, and the softening of the welded portion 40 becomes insufficient.

  In the post-heating step S3, the welded portion 40 is preferably reheated to a temperature within the range of Ac1 transformation point to 900 ° C. This is because when the weld 40 is heated to a temperature higher than 900 ° C., the surface oxidation in the high carbon steel material in the weld 40 and its periphery becomes remarkable. As a result, peeling of the oxide scale occurs in the middle of the continuous processing line L1, which increases the possibility of scratching or soiling the equipment.

  Further, in the post-heating step S3, it is more preferable to reheat the welded portion 40 to a temperature within the range of Ac1 transformation point or higher and 850 ° C. or lower. This is because when the welded portion 40 is heated to a temperature higher than 850 ° C., the heat input energy required for heating becomes large and the heating time and cooling time become long.

  The relationship between the material structure of the welded portion 40 before being reheated by the post-heating step S3 and the material structure of the welded portion 40 after reheating will be described below.

  In the material structure of the welded portion 40, the portion that was martensite before reheating is tempered during the temperature rise due to reheating and decomposed into ferrite + carbide.

  Here, when the temperature rising rate in the post-heating step S3 is high, the carbides generated by the tempering of martensite are extremely fine. When heated to a temperature equal to or higher than the Ac1 transformation point, a carbide corresponding to the solid solution limit of carbon of austenite at that temperature is instantaneously dissolved in ferrite, and the ferrite and the carbide are transformed into austenite.

  On the other hand, when the rate of temperature rise in the post-heating step S3 is slow, the carbide generated by the tempering of martensite grows with time and the ferrite softens. In this case, immediately after heating to just above the Ac1 transformation point, the carbide partially melts to partially generate austenite, thereby forming austenite + ferrite + granular carbide structure in the material structure of the weld 40. .. By further raising the temperature above the Ac1 transformation point, in the temperature range of the Ac1 transformation point or higher (during heating and cooling), the dissolution of carbides proceeds and the austenite abundance increases.

  In the material structure of the welded portion 40, the portion that was austenite before reheating remains austenite even when heated to a temperature of the Ac1 transformation point or higher when the heating rate is high. On the other hand, when the heating rate is slow, the part of the material structure of the welded portion 40 that was austenite before reheating is decomposed into ferrite + carbide during heating, and the temperature of martensite is increased at a slower rate. Similar changes are shown.

  As described above, in the post-heating step S3, the weld 40 can be softened without providing the soaking and holding time at the temperature of the Ac1 transformation point or higher. However, the soaking and holding at the temperature of the Ac1 transformation point or higher promotes the dissolution of the carbide and the generation of austenite, and therefore does not prevent the softening of the weld 40.

  Therefore, in the post-heating step S3, the welded portion 40 does not need to hold the soaking temperature at a temperature equal to or higher than the Ac1 transformation point, and in this case, the control unit 20 does not set the time for performing the soaking maintenance and the Control is performed. In the post-heating step S3, the welded portion 40 may be soaked and maintained at a temperature equal to or higher than the Ac1 transformation point.

  In the post-heating step S3, the rate of temperature increase during reheating is arbitrary. From the viewpoint of productivity and cost, the rate of temperature increase during reheating is preferably 10 ° C./sec or more and 500 ° C./sec or less. When the rate of temperature increase is less than 10 ° C./sec, the time required for heating becomes long. In order to realize the temperature rising rate exceeding 500 ° C./second, the cost required for heating equipment increases.

(First cooling step S4)
In the first cooling step S4, the temperature range from the Ac1 transformation point to 500 ° C. during cooling after reheating in the post-heating step S3 is equal to or lower than the allowable cooling rate Vmax (° C./sec) given by the equation (3). Cool at the average cooling rate of.
Vmax = 900 / D _I ... (3).

  By the first cooling step S4, the austenite is transformed so that the material structure of the welded portion 40 becomes a structure mainly composed of pearlite or a structure mainly composed of "ferrite + pearlite". Thereby, the welded part 40 after cooling can be softened.

When the average cooling rate in the first cooling step S4 exceeds Vmax (° C./sec) , martensite is generated in the material structure of the weld 40 without completing the pearlite transformation, and the weld 40 is hardened. More likely to.

Here, in the high carbon steel material which does not satisfy the above-mentioned condition B, that is, in the high carbon steel material having a value of the ideal critical diameter D _I of more than 150 mm, Vmax becomes very small. Therefore, the cooling time required for making the material structure of the welded portion 40 a structure mainly composed of pearlite or a structure mainly composed of “ferrite + pearlite” becomes long.

Further, in a high carbon steel material having an ideal critical diameter D _I of more than 150 mm, a pearlite structure cannot be obtained even when cooled at an average cooling rate of Vmax (° C./sec) or less in the first cooling step S4. In this case, the weld 40 after cooling may have a hard material structure containing martensite or bainite.

  Generally, in the continuous processing line L1 (see FIG. 1), there is a limit on the time required for the welding process S10 performed by the welding device 2, and in connection therewith, the time required for the first cooling process S4 is limited. Time can also be limited. For example, the first cooling step S4 is preferably performed for a time equal to or less than the predetermined required time T. Specifically, the required time T is from the time point after the post-heat application in the post-heating step S3 (that is, after the post-heat heater 23 has passed in the width direction of the steel strip) from the welding step S10 in the continuous processing line L1. It is the time until the time when the welded portion 40 first contacts the roll of the next process, in the next process. The required time T is, for example, 60 seconds, although it is difficult to specify a specific value because it varies depending on the configuration of the continuous processing line L1, the strip passing speed, and the like.

  When the value obtained by dividing the temperature difference between the Ac1 transformation point and 500 ° C. by T is the required cooling rate Vmin, the average cooling rate in the first cooling step S4 is preferably Vmin or more and Vmax or less. As a result, the possibility of causing brittle fracture in the welded portion 40 can be reduced while efficiently performing the processing in the continuous processing line L1.

In order to carry out the first cooling step S4 in the present welding method, in one example, the control unit 20 of the welding device 2 controls the temperature range from the Ac1 transformation point to 500 ° C. during cooling of the welded portion 40 after reheating. The cooling adjustment heater 24 may be controlled so that the average cooling rate of the welded portion 40 becomes Vmax (° C./sec) or less.

In the present welding method, in the first cooling step S4, the average cooling rate of the welded portion 40 may be Vmax (° C / sec) or less and is not limited to the above example. For example, the heat input to the welded portion 40 can be controlled by controlling the heating temperature, the heating range, and the like in the optional preheating step S1, and the indispensable welding step S2 and postheating step S3. With such control, the average cooling rate of the welded portion 40 in the first cooling step S4 may be Vmax (° C / sec) or less. Therefore, the cooling adjustment heater 24 only needs to be usable as needed, and the welding device 2 does not have to include the cooling adjustment heater 24.

  The temperature of the welded portion 40 is measured by, for example, a radiation thermometer, and in this case, the temperature specified in the post-heating step S3 and the first cooling step S4 is the maximum of the surface of the welded portion 40 in the sheet passing direction. Is the temperature.

(Second cooling step S5)
Since the pearlite transformation in the material structure of the welded portion 40 is completed in the above-described first cooling step S4, the cooling rate in the second cooling step S5 is not particularly limited. For example, in the second cooling step S5, even if the weld 40 is rapidly cooled, martensite is not generated in the material structure of the weld 40.

  Therefore, according to the present welding method, the time required for the welding step S10 can be further reduced by rapidly cooling the welded portion 40 by using some means in the second cooling step S5.

(Comparison between main welding method and conventional technology)
The comparison between the welding method of the present invention example and the conventional welding method (conventional example) of Patent Document 1 and Patent Document 2 (conventional example) will be schematically described with reference to FIG. FIG. 3 is a graph schematically showing the temperature change with time in the welding step S10 for explaining the welding methods of the present invention example and the conventional example. The diagram indicated by reference numeral 3001 in FIG. 3 is a graph showing the present welding method (example of the present invention). In FIG. 3, the drawing indicated by reference numeral 3002 is a graph showing the welding method of Patent Document 1 (conventional example 1), and the drawing indicated by reference numeral 3003 is a graph showing the welding method of Patent Document 2 (conventional example 2). The graph shown in FIG. 3 schematically shows an example of the result of measuring the time change of temperature at a fixed point on the line of the welding target portion 30 (here, the central portion in the width direction of the steel strip).

(Conventional example 1)
As indicated by reference numeral 3002 in FIG. 3, in the method of Conventional Example 1, preheating before laser welding is not performed (reference numeral C100), and the welded portion is rapidly cooled after laser welding. In the method of Conventional Example 1, there is no regulation regarding the minimum temperature until reheating (reference numeral C101) after quenching, and there is a concern that quench cracking may occur at this stage (C104). Further, the welded portion after laser welding is reheated at a temperature rising rate of 1 ° C./second or more to a temperature in the range of 600 to 900 ° C. (reference C101), soaked and maintained (reference C102), and then, It is allowed to cool or is gradually cooled (reference C103).

  However, when the hardenability is very high due to the composition of the steel material to be welded, even if the material is allowed to cool from the temperature of the Ac1 transformation point or higher (cooling rate of about 20 ° C./second in Patent Document 1), May form martensite and become hard. Therefore, in Conventional Example 1, there is a limit in reducing the possibility of causing brittle fracture in the welded portion.

(Conventional example 2)
As indicated by reference numeral 3003 in FIG. 3, in the method of Conventional Example 2, first, pre-heat treatment for heating the welding target portion to 600 ° C. to 800 ° C. is performed before laser welding. After that, laser welding is performed while supplying and adding a welding material (filler) having a C concentration of 0.1 mass% or less to the welding target portion (reference C200). In the method of Conventional Example 2, there is also a proposal regarding the case where the preheat treatment is not performed (graph shown by a dotted line). In the method of Conventional Example 2, addition of the welding material reduces the hardenability of the weld metal portion. This method reduces the possibility of weld cracking when the weld metal part is rapidly cooled on the assumption that the addition of welding material is essential. However, the heat-affected zone is a portion that has the components of the raw material as it is, and when the welded portion is rapidly cooled to a temperature of 100 ° C. or lower (reference numeral C201), the possibility of weld cracking in the heat-affected zone cannot be reduced.

  Then, the welded portion after laser welding is reheated to a temperature within the range of 800 to 1100 ° C. (reference C202), and then the welded portion is cooled (reference C203).

  However, if the welded portion is reheated to a high temperature, there is a concern that the toughness will decrease due to the coarsening of the crystal grains, and the heating time will become longer and much energy will be required. Further, in the technique of Conventional Example 2, it is recommended to reduce the C concentration in the weld metal portion to about 0.4 mass%. In this case, too much welding material may have to be melted in. Therefore, it is required to use laser equipment with high output and to reduce the welding speed.

  Further, in the method of Conventional Example 2, the cooling rate is not regulated during the period indicated by the reference numeral C203, but the addition of the welding material is indispensable to soften the weld metal portion even if natural cooling is performed. .. However, the heat-affected zone is a portion having the raw material component as it is, and when the steel material to be welded has high hardenability, there is a concern that the heat-affected zone becomes hard when naturally cooled.

(Example of the present invention)
As indicated by reference numeral 3001 in FIG. 3, in the present welding method, the welding step S10 is performed as described above. In particular, in the present welding method, by including the first cooling step S4, the following effects can be obtained without the addition of welding material. That is, the material structure of the welded portion 40 obtained by butt-welding a plurality of high carbon steel materials satisfying the above conditions A and B can be a structure mainly composed of pearlite or a structure mainly composed of "ferrite + pearlite". .. Therefore, the welded portion 40 can be softened. As a result, it is possible to reduce the possibility that brittle fracture will occur in the welded portion 40 at the OS on the exit side of the welding device 2 in the continuous processing line L1.

[Modification]
A modified example of the welding method for high carbon steel according to the present embodiment will be described below with reference to FIG. FIG. 4 is a graph schematically showing a temperature change with time in the welding step S10 for explaining the welding method for the high carbon steel material in the modified example of the present embodiment.

  In this modification, the pre-heating process S1 is not performed, and the subsequent welding process S2 and subsequent processes are performed in the same manner as described in the first embodiment. Also in this case, in the welding step S2 (the period P2 in the figure), if the processing of the post-heating step S3 is started before the temperature of the welded portion 40 after laser welding becomes less than 100 ° C. Good. As a result, it is possible to prevent welding cracks from occurring in the welded portion 40 after laser welding. Then, after the welding step S10, the possibility of causing brittle fracture in the welded portion 40 can be reduced.

[Embodiment 2]
Another embodiment of the present invention will be described below with reference to FIG. For convenience of description, members having the same functions as the members described in the above embodiment are designated by the same reference numerals, and the description thereof will not be repeated. FIG. 5: is a schematic diagram for demonstrating the welding apparatus 2A used in order to implement the welding method of the high carbon steel material in this embodiment.

  As shown in FIG. 5, in the welding method for a high carbon steel material according to the present embodiment, a welding device 2A including a wire feeding unit 25 is used instead of the welding device 2 according to the first embodiment, and the welding process is performed. A wire is added to the welding target portion 30 in S2.

As the wire, it is preferable to use a wire having a C concentration of 0.3 mass% or less. By adding the wire, the composition of the weld metal has a C _eq following C _eq steel (succeeding steel strip ST1 and prior steel strip ST2) to be joined, and the bonding target steel it is preferably adapted to have a D _I value below D _I value. Therefore, it is preferable that the wire has a lower C content than the steel material to be joined and a lower alloy content than the steel material to be joined.

By adding the above wire and performing laser welding, C _eq of the weld metal can be reduced. As a result, it is possible to further soften the weld metal portion 41 of the welded portion 40 and prevent weld cracking. However, in this case, in the heat-affected zone 42, since the components of the high-carbon steel material remain the same, martensite is generated in the heat-affected zone 42 when rapidly cooled after laser welding. Therefore, in the conventional welding method, the heat-affected zone 42 may be hardened to cause brittle fracture in the weld 40.

  On the other hand, according to the welding method for a high carbon steel material in the present embodiment, it is possible to suppress the generation of martensite in the heat affected zone 42 when the wire is added. Therefore, it is possible to reduce the possibility of causing brittle fracture in the welded portion 40 after the welding step S10.

[Additional Notes]
The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention.

  An embodiment of the present invention will be described below.

  Butt welding was performed by using the steel strips of steel types A to N having the components shown in Table 1 and irradiating the welding target portion where the steel strips were butted against each other with a laser. The steel strip has a width of 800 mm and a basic thickness of 3.0 mm. Some steel strips having a thickness of 1.8 mm and 6.0 mm were used. A carbon dioxide laser with a maximum output of 8 kW was used for laser welding. Pre-heat treatment (pre-heating step S1), post-heat treatment (post-heating step S3), and first cooling treatment (first cooling step S4) for controlling the average cooling rate by adjusting the temperature in the temperature range from the Ac1 point to 500 ° C. ) Was carried out using a high frequency induction heating device as required. As the high frequency induction heating device, a device having a maximum output of 25 kW and a heating surface size of 6 × 120 (mm) was used. The welding speed was adjusted between 1 and 10 mpm (m / min).

(Erichsen test)
FIG. 6 is a schematic diagram schematically showing the configuration of the Erichsen tester 50. As shown in FIG. 6, the Erichsen tester 50 includes a punch 51, a plate pressing portion 52, and a die 53. As the punch 51, a body having a diameter of 20 mm and a punch end having a spherical radius of 10 mm was used. The die 53 used had a hole diameter of 40 mm.

The sample material 60 was fixed by the plate pressing part 52 and the die 53 so that the welding line of the welded part of the sample material 60 and the center of the punch 51 were aligned. Then, an Erichsen test was performed, and the presence or absence of embrittlement of the welded portion was evaluated based on the morphology of the cracks as follows.
OK: Ductile fracture (fracture accompanied by large plastic deformation)
NG: Brittle fracture (almost no plastic deformation, fracture in the welding line direction).

  An example of the result of the Erichsen test is shown in FIG. The photograph shown by reference numeral 7001 in FIG. 7 is an example of the welded portion of the evaluation OK in the Erichsen test. The photograph indicated by reference numeral 7002 in FIG. 7 is an example of the welded portion of the evaluation NG in the Erichsen test.

(Hardness test)
FIG. 8 is a diagram showing hardness measurement positions in a hardness test. First, a cross section perpendicular to the welding direction (direction in which the welding line extends) in the welded portion 40 was mirror-polished. Then, the hardness of the central portion P1 in the plate thickness direction of the weld metal portion 41 and the heat affected zone 42 in the cross section was measured at a pitch of 0.2 mm. Specifically, a Vickers hardness meter was used to measure the Vickers hardness at a load of 300 g. The maximum hardness (HV) of the welded portion was determined based on the hardness measurement result.

(Example 1)
Using the steel strips of steel types A, B, and C shown in Table 1, the Erichsen test and the hardness test were performed on the welded portion obtained by laser welding. The results are shown in Table 2.

As shown in Table 2, Example No. which was laser-welded and variously heat-treated under the conditions within the scope of the present invention. In 1-2, 1-4 to 1-10, 1-13 to 1-18, 1-21 and 1-22, the maximum hardness of the welded portion was 400 HV or less, and the evaluation of the Erichsen test was also OK. there were. Even if the pre-heat treatment was not performed before the laser welding, the example No. As shown in 1-2, it is understood that the possibility of brittle fracture in the welded portion can be reduced by setting the average cooling rate in the temperature range from the Ac1 point to 500 ° C. to Vmax (° C./sec) or less.

On the other hand, Comparative Example No. which was laser-welded and various heat-treated under the condition outside the scope of the present invention. In 1-1, 1-3, 1-11, 1-12, 1-19, and 1-20, the maximum hardness of the welded portion exceeded 400 HV, and the evaluation by the Erichsen test was NG. More specifically, in any of the steel types of the steel strips A to C, when the average cooling rate in the temperature range from the Ac1 point to 500 ° C. was made larger than Vmax (° C./sec) , the comparative example No. As shown in 1-1, 1-12, 1-19, 1-20, the welded part after cooling was hardened and the maximum hardness of the welded part exceeded 400 HV.

  When the heating temperature in the post heat treatment is less than the Ac1 point, Comparative Example No. As shown in 1-3, the post-heat treatment did not sufficiently soften the weld, and the maximum hardness of the weld exceeded 400 HV. Comparative Example No. In No. 1-11, since it was rapidly cooled to 40 ° C. immediately after laser welding, weld cracking occurred before the Erichsen test.

(Example 2)
Using the steel strips of steel types D to N shown in Table 1, the Erichsen test and the hardness test were performed on the welded portion obtained by performing the laser welding. When the average cooling rate in the first cooling process is 0.1 ° C./sec, after the post-heat treatment, the movement of the post-heat heater is stopped and the output is adjusted to locally cool the portion of the welded portion. Controlled. Then, the characteristics of the welded portion were evaluated for the portion where the cooling rate was locally controlled in the welded portion. The results are shown in Table 3.

  As shown in Table 3, Example No. which was laser-welded and variously heat-treated under the conditions within the scope of the present invention. In 2-1 to 2-4 and 2-6 to 2-9, the maximum hardness of the welded portion was 400 HV or less, and the evaluation by the Erichsen test was also OK.

On the other hand, Comparative Example No. which was laser-welded and various heat-treated under the condition outside the scope of the present invention. In 2-5 and 2-10 to 2-15, the maximum hardness of the welded portion exceeded 400 HV, and the evaluation by the Erichsen test was NG. More specifically, Comparative Example No. As shown in 2-5, in the steel type H, the value of Vmax is as small as 6.7. Therefore, even if the average cooling rate in the temperature range from the Ac1 point to 500 ° C. is relatively gentle at 10 ° C./sec, the average cooling rate exceeds Vmax (° C./sec) . As a result, the welded part after cooling became hard and the maximum hardness of the welded part exceeded 400 HV.

Further, steel types L~N has exceeded _{D I} value 150 mm (see Table 1), Vmax is very small. Comparative Example No. As shown in 2-11, 2-13, and 2-15, for such steel types, after heating the welded portion after laser welding to a temperature of Ac1 point or higher, the temperature range from the Ac1 point to 500 ° C. It can be seen that even if the average cooling rate is Vmax (° C./sec) or less, it is difficult to sufficiently soften the welded portion.

(Example 3)
Laser welding was performed on steel strips of steel types A, B, and C shown in Table 1 and having steel plate thicknesses of 1.8 mm, 3.0 mm, and 6.0 mm for each steel type. An Erichsen test and a hardness test were performed on the obtained welds. The results are shown in Table 4.

  As shown in Table 4, Example No. which was laser-welded and variously heat-treated under the conditions within the scope of the present invention. In 3-1 to 21-1, the maximum hardness of the welded portion was 400 HV or less and the evaluation of the Erichsen test was OK regardless of the difference in the plate thickness in any of the steel types.

(Example 4)
The Erichsen test and the hardness test were performed on the welded portion obtained by performing the laser welding while using the steel type B shown in Table 1 and adding the welding wire.

  As the welding wire, YGW12 (JIS Z 3312), φ0.9 mm was used. The wire components are shown in Table 5.

  The results are shown in Table 6.

  When laser welding was performed using a welding wire having a low C concentration, the heat-affected zone showed the maximum hardness of the welded part because the composition of steel type B remained unchanged.

Comparative example No. 3 cooled to 30 ° C. before heating in the post heat treatment In No. 4-1, welding cracks were generated in the heat-affected zone before the Erichsen test. In Comparative Example No. 4, the average cooling rate in the temperature range from the Ac1 point to 500 ° C. was 40 ° C./sec, which was faster than Vmax (° C./sec) . In No. 4-2, the heat-affected zone was hardened, the maximum hardness of the weld zone exceeded 400 HV, and the evaluation by the Erichsen test was NG.

  On the other hand, even when laser welding was performed using a welding wire having a low C concentration, laser welding and various heat treatments were performed under the conditions within the scope of the present invention. In 4-3 and 4-4, hardening in the heat-affected zone was suppressed. As a result, the maximum hardness of the welded portion was 400 HV or less, and the Erichsen test was evaluated as OK.

ST1 Trailing steel strip (carbon steel material)
ST2 Leading steel strip (carbon steel)
40 weld

Claims (6)

A method of welding a carbon steel material including a welding step of welding a plurality of carbon steel materials to each other,
In the carbon steel material, the carbon equivalent C _eq given by the formula (1) is 0.4 or more, and the ideal critical diameter D _I given by the formula (2) is 150 mm or less,
After the welding step, a post-heating step of reheating the welded portion generated by the welding step to a temperature of Ac1 transformation point or higher before the welded portion has a temperature of less than 100 ° C.
In the temperature range from the Ac1 transformation point to 500 ° C., the welded part heated in the post-heating step is cooled at an average cooling rate equal to or lower than the allowable cooling rate Vmax (° C./sec) given by the equation (3). A method for welding a carbon steel material, comprising:
C _eq = C + (Si / 24) + (Mn / 6) + (Cr / 5) + (Mo / 4) + (Ni / 40) + (V / 14) (1)
D _I = (6.99 × C ^0.5 ) × (1 + 0.64Si) × (1 + 4.1Mn) × (1 + 2.83P) × (1-0.62S) × (1 + 2.33Cr) × (1 + 3.14Mo ) × (1 + 0.52Ni) × (1 + 1.5 (0.9−C) B) (2)
Vmax = 900 / D _I ... (3)
(here,
Regarding the value of B in the formula (2) , instead of substituting the mass% concentration of B, when the B content in the carbon steel material is 0.0005 mass% or more, the numerical value 1 is substituted, and the B content is If the amount is less than 0.0005% by mass, substitute the numerical value 0,
The mass% concentration of each element is substituted for each element symbol in the above formula (1) and each element symbol other than B in the above formula (2). )   The method for welding a carbon steel material according to claim 1, wherein in the post-heating step, the carbon steel material is reheated to a temperature within a range from the Ac1 transformation point to 850 ° C.   Before the welding step, a preheating step of heating a welding target portion, which is a portion of the carbon steel material that is melted and solidified in the welding step, to a temperature of 200 ° C. or more and 800 ° C. or less is included. The method for welding a carbon steel material according to 1 or 2.   The carbon steel material is, in mass%, C: 0.3% or more and 1.5% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.030% or less, and S: 0. The method for welding a carbon steel material according to any one of claims 1 to 3, which contains 0.035% or less.   The composition of the carbon steel material is, in mass%, Cr: 1.8% or less, Mo: 0.5% or less, Ni: 2.0% or less, V: 0.5% or less, Ti: 0.5% or less. , Nb: 0.5% or less, and B: 0.0005% or more and 0.01% or less, one or more conditions selected from the group consisting of, carbon according to claim 4, characterized in that Welding method for steel materials.   The method for welding a carbon steel material according to claim 1, wherein the welding portion is formed by laser welding in the welding step.