Process for making an electric-resistance-welded steel pipe with high strength

Abstract: An electric-resistance-welded steel pipe with high strength consists essentially of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, the balance Fe and inevitable impurities, and comprises a structure having a martensitic phase ratio of 60 to 100 %, and a tensile strength of 120 to 170 kgf/mm². At least one selected from the group consisting of 0.05 wt.% Nb or less, 1.2 wt.% Cr or less, 0.5 wt.% Mo or less, 0.1 wt.% or less, 0.05 wt.% Ti or less and 0.005 wt.% or less can be added to steel.

Description: The present invention is related to a process for making an electric-resistance welded steel pipe. In particular, the present invention is related to a process for making an electric-resistance welded steel pipe with a high strength. In particular, the present invention relates to a process for making an electric-resistance welded steel pipe with high strength to be used for a door impact bar used for cars, and more particularly, for an impact bar used for the inside of an automobile. In US-A-4 799 971 there is described a method for producing a steel sheet comprising the steps forming flat ferrous metal into a cylindrical shape, welding the edges of the metal to form a tube, heating the tube to an elevated temperature, quenching the heated tube, reheating the tube to another elevated temperature, air cooling the tube, and cold rolling the tube in a sizing mill. In regard to doors for vehicles such as automobiles, a reinforcing material made of steel or the like is applied to the inside of a door to secure safety of men in a car when another car clashes into the side of the car and to lighten car weight. This reinforcement material is referred to as a door impact bar. Japanese Patent Application Laid Open No.197525/90 discloses a reinforcement material of a tube as manufactured or a reinforcement material having a tensile strength of from 60 to 100 kgf/mm 2 after stress relief annealing at a low temperature. It is undesirable, however, to use the prior art reinforcement material as the door impact bar. The reinforcement material having a tensile strength of from 60 to 100 kgf/mm 2 is insufficient for the door impact bar. Even though only this tensile strength is improved, properties effective for the door impact bar cannot be obtained. Thus, it is an object of the present invention to provide a process for making an electric-resistance welded steel pipe with high strength which is desirable as a door impact bar. To obtain this object, according to a first aspect of the present invention, there is provided a process for making an electric-resistance-welded steel pipe with high strength comprising the steps: hot rolling a steel slab to produce a sheet, optionally cold rolling the steel sheet, continuously annealing the obtained steel sheet, and electro-resistance welding the steel sheet to form a pipe, wherein the steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, the balance being Fe and inevitable impurities and has a martensitic phase ratio of 60 to 100%, and the steel pipe has a tensile strength of 120 to 170 kgf/mm 2 . Further, to obtain this object, according to a second aspect of the present invention, there is provided a process for making an electric-resistance-welded steel pipe with high strength comprising the steps: hot rolling a steel slab to produce a sheet, optionally cold rolling the steel sheet, continuously annealing the obtained steel sheet, and electro-resistance welding the sheet to form a pipe, wherein the steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, and at least one element selected from the group consisting of 0.05 wt.% Nb or less, 1.2 wt.% Cr or less, 0.5 wt.% Mo or less, 0.1 wt.% V or less, 0.05 wt.% Ti or less and 0.005 wt.% B or less, the balance being Fe and inevitable impurities and has a martensitic phase ratio of 60 to 100%, and the steel pipe has a tensile strength of 120 to 170 kgf/mm 2 . Overall, there is achieved a steel pipe having a high tensile strength and high maximum load so that the portion having a high hardness at the centre of the electric-resistance welded portion realizes properties desirable as an impact bar. Thus, an impact bar for doors of automobiles and other vehicles can be obtained that is able to secure safety of men in a car and produces an industrially great effect. The above objects and other objects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the appended drawings. Figure 1 is a schematic illustration describing a three-point bending test wherein properties of an impact bar made of an electric-resistance-welded steel pipe of the present invention were tested; and Figure 2 is a graphical presentation showing a distribution of hardnesses of an impact bar adjacent to a seam welded portion of the impact bar made of the electric-resistance-welded steel pipe of the present invention. The reason for limiting the composition of steel as outlined above will now be described as follows: C is an element which produces a martensite and increasing hardness of the martensite. C is an element indispensable for obtaining the strength which is an object of the present invention. When the content of C is less than 0.05 wt.%, a predetermined strength cannot be obtained. When the content of C exceeds 0.30 wt.%, a slab is liable to be cracked, an impact resistance of a final product is lowered or weldability of the final product in CO 2 gas shielded are welding is lowered. The content of 0.05 to 0.30 wt.% C is desirable. Mn is an element increasing a hardenability of an austenite. The content of 0.6 wt.% Mn is required to control an amount of martensite and to obtain a predetermined strength of a slab. When the content of Mn exceeds 2.0 wt.%, an effect is suturated in the case of carrying out a continuos annealing of steel by means of a rapid water quenching. Conversely, such content has a bad influence on a bending workability during manufacturing of a pipe. The content of 0.6 to 2.0 wt.% Mn is desirable. Si is a deoxidizing element. Si contributes to an increase of mechanical properties of a steel sheet and increases a ductility of the steel sheet. When the content of Si is less than 0.03 wt.%, a predetermined mechanical properties or ductility cannot be obtained. However, when the content of Si exceeds 0.7 wt.%, spattering is often generated during electric-resistance welding, which lowers mechanical properties and an impact resistance of an electric-resistance-welded pipe. The content of 0.03 to 0.70 wt.% Si is desirable. C, Mn and Si are elements indispensable for an electric-resistance-welded steel pipe of the present invention, and the balance elements are Fe and impurities. Quality of an electric-resistance-welded portion of a pipe is improved by the above-mentioned range of elements. In the present invention, however, at least one element selected from the group consisting of Nb, Cr, Mo, V, Ti and B can be added to steel within a predetermined range as the case may be required. The properties of each of the elements are effectively utilized for purposes. Nb contributes to making finer martensite structure of steel and to improving hardenability of steel. Cr, Mo, V, Ti and B are elements increasing the hardenability. When the content of Nb exceeds 0.05 wt.%, the spatterring is liable to be generated during electric-resistance welding, which lowers the mechanical properties of an electric-resistance welded portion. 0.05 wt.% is determined to be the upper limit of the content of Nb. When the content of Cr exceeds 1.2 wt.%, the content of Mo exceeds 0.5 wt.%, the content of V exceeds 0.1 wt.% or the content of Ti exceeds 0.05 wt.%, oxides such as Cr 2 O 3 , MoO 2 , V 2 O 3 and TiO 2 reside in the electric-resistance welded portion, which deteriorates the mechanical properities of the electric-resistance welded portion or decreases the impact resistance of a steel pipe. When the content of B exceeds 0.005 wt.%, workability of steel deteriorates. The martensitic phase ratio is desired to be from 60 to 100 %. When the martensitic phase ratio is less than 60%, the tensile strength of steel is rapidly decreased. The tensile strength cannot be increased to 120 kgf/mm 2 or more. Example The present inventors produced a steel pipe having a wall thickness of 2.1 mm and a diameter of 34.0 mm, which has elements and structures as shown in Table 1 (A) and (B). Nos.1 to 14, Nos.17 and 18 show Examples of the present invention. Nos.15, 16 and 19 show a Comparisons. A slab having composition as shown in Table 1 (A) was used. Nos.1 to 4 have the composition of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, the balance being Fe and inevitable impurities. Nos.5 to 9, 14, 17 and 18 have the composition of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, one selected from the group consisting of 0.05 wt.% Nb or less, 1.2 wt.% Cr or less, 0.5 wt.% Mo or less, 0.1 wt.% V or less, 0.05 wt.% Ti or less and 0.005 wt.% B or less, the balance being Fe and inevitable impurituies. No. 10 has the composition of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, 1.2 wt.% Cr or less and 0.5 wt.% Mo or less. No. 11 has the composition of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, 0.05 wt.% Ti or less and 0.005 wt. % B or less. No.12 has the composition of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, 1.2 wt.% Cr or less, 0.5 wt. % Mo or less and 0.1 wt.% V or less. No.13 has the composition of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, 0.05 wt.% Nb or less, 1.2 wt.% Cr or less, 0.5 wt.% Mo or less, 0:1 wt.% V or less, 0.05 wt.% Ti or less and 0.005 wt.% B. A steel sheet produced by hot-rolling said slab or a steel sheet produced by hot-rolling and cold-rolling said slab was continuously annealed. Treatments such as soaking in the continuous annealing, continuous water quenching and heat treatment were carried out under the following conditions: The soaking treatment was carried out at a temperature of 870°C. The temperature within a range of from 800 to 900 °C was appropriately used. The water quenching was carried out at a line speed of 50 to 120 m/min and at a cooling rate of 800 to 1500°C /sec. The heat treatment was carried out at a temperature of 100 to 300 °C for ten minutes. Mechanical properties of manufactured steel pipe are shown in Table 1 (B). The steel pipe according to the Example of the present invention had a high strength of 122 to 168 kgf/mm 2 , The yield point of the steel pipe was 85 to 151 kgf/mm 2 . The maximum load at the time of the three-point bending test was 1150 kgf or more. Except for No.18, the maximum load was 1200 kgf or more. The maximum load at the time of the three-point bending test according to the Comparison was 1100 kgf or less. As a result, it was confirmed that the steel pipe according to the present invention had a desirable properties for an impact bar for doors of automobils. A method for measuring the maximum load at the time of the three-point bending test is shown In Figure 1. A specimen 1 was put on a bending span of 822 mm. A bending jig 2 was pressed on the midpoint of the specimen 1. A distribution of hardnesses of steel of the present invention adjacent to the electric-resistance welded portion of the steel pipe is shown in Figure 2. Although the hardnesses of steel adjacent to the electric-resistance welded portion of the steel pipe were somewhat decreased as shown in Figure 2, a portion having a high hardness was formed again at the center of the electric-resistance welded portion. It is clearly seen from a high tensile strength and high maximum load that a formation of the portion having a high hardness at the center of the electric-resistance welded portion realizes properties desirable as an impact bar. According to the present invention as described above, an electric-resitance welded steel pipe having a high strength and other properties desirable for a impact bar for doors of automobiles and other vehicles and being able to secure safety of men in a car can be obtained, which produces an industrially great effect. No Chemical Composition ( wt.% ) Remarks C Si Mn Nb Cr Mo V Ti B 1 0.20 0.33 0.80 Example 2 0.15 0.33 1.76 Example 3 0.20 0.40 1.76 Example 4 0.27 0.33 0.80 Example 5 0.20 0.40 1.76 0.015 Example 6 0.15 0.33 1.60 1.0 Example 7 0.15 0.33 1.60 0.3 Example 8 0.15 0.33 1.60 0.060 Example 9 0.15 0.33 1.60 0.020 Example 10 0.15 0.33 1.60 1.1 0.4 Example 11 0.15 0.33 1.60 0.010 0.001 Example 12 0.15 0.33 1.60 0.6 0.2 0.042 Example 13 0.07 0.10 0.65 0.005 0.4 0.3 0.044 0.008 0.001 Example 14 0.15 0.33 1.60 0.010 Example 15 0.03 0.10 0.80 0.005 0.4 0.3 0.044 0.008 0.001 Comparison 16 0.07 0.10 0.55 0.005 0.4 0.3 0.044 0.008 0.001 Comparison 17 0.15 0.33 1.60 0.010 Example 18 0.15 0.33 1.60 0.010 Example 19 0.15 0.33 1.60 0.010 Comparison No. Yield Strength Kgf/mm 2 Tensile Strength Kgf/mm 2 Martensitic Phase Ratio % Maximum Load Kgf Remarks 1 118 125 100 1202 Example 2 130 135 100 1297 Example 3 122 162 100 1558 Example 4 142 154 100 1483 Example 5 145 168 100 1618 Example 6 139 153 100 1475 Example 7 141 158 100 1522 Example 8 137 147 76 1413 Example 9 127 132 80 1272 Example 10 138 152 100 1461 Example 11 143 154 100 1481 Example 12 151 159 100 1532 Example 13 115 125 65 1205 Example 14 134 140 100 1346 Example 15 107 118 58 1098 Comparison 16 95 105 52 1011 Comparison 17 105 131 70 1261 Example 18 85 122 62 1174 Example 19 109 115 50 1087 Comparison

Claims: Process for making an electric-resistance-welded steel pipe with high strength comprising the steps: a) hot rolling a steel slab to produce a sheet, b) optionally cold rolling said steel sheet, c) continously annealing said obtained steel sheet, and d) electro-resistance welding said steel sheet to form a pipe, e) wherein said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, the balance being Fe and inevitable impurities and has a martensitic phase ratio of 60 to 100%, and f) said steel pipe has a tensile strength of 120 to 170 kgf/mm 2 . Process for making an electric-resistance-welded steel pipe with high strength comprising the steps: a) hot rolling a steel slab to produce a sheet, b) optionally cold rolling said steel sheet, c) continously annealing said obtained steel sheet, and d) electro-resistance welding said sheet sheet to form a pipe, e) wherein said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si and at least one element selected from the group consisting of 0.05 wt.% Nb or less, 1.2 wt.% Cr or less, 0.5 wt.% Mo or less, 0.1 wt.% V or less, 0.05 wt.% Ti or less and 0.005 wt.% B or less, the balance being Fe and inevitable impurities and has a martensitic phase ratio of 60 to 100%, and f) said steel pipe has a tensile strength of 120 to 170 kgf/mm 2 . Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si and 0.05 wt.% Nb or less. Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si and 1.2 wt.% Cr or less. Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si and 0.5 wt.% Mo or less. Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si and 0.1 wt.% V or less. Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si and 0.05 wt.% Ti or less. Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, 1.2 wt.% Cr or less and 0.5 wt.% Mo or less. Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, 0.05 wt.% Ti or less and 0.005 wt.% B or less. Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, 1.2 wt.% Cr or less, 0.5 wt.% Mo or less and 0.1 wt.% V or less. Process for making an electric-resistance-welded steel pipe according to claim 2, characterized in that said steel pipe consists of 0.05 to 0.30 wt.% C, 0.6 to 2.0 wt.% Mn, 0.03 to 0.70 wt.% Si, 0.05 wt.% Nb or less, 1.2 wt.% Cr or less, 0.5 wt.% Mo or less, 0.1 wt.% V or less, 0.05 wt.% Ti or less and 0.005 wt.% B or less.

Inventors: ISHIZAWA YOSHIICHI, MAEDA TATSUO, MORIMOTO MITOSHI, NISHIMURA FUMIHIRO, SUGAYOSHI TETSURO, SUGIMOTO YUJI, TAKAMURA TOSHIHIRO, ISHIZAWA, YOSHIICHI, MAEDA, TATSUO, MORIMOTO, MITOSHI, NISHIMURA, FUMIHIRO, SUGAYOSHI, TETSURO, SUGIMOTO, YUJI, TAKAMURA, TOSHIHIRO, Ishizawa, Yoshiichi, c/o Patent and License Dept., Maeda, Tatsuo, c/o Patent and License Dept., Morimoto, Mitoshi, c/o Patent and License Dept., Nishimura, Fumihiro, c/o Patent and License Dept., Sugayoshi, Tetsuro, c/o Patent and License Dept., Sugimoto, Yuji, c/o Patent and License Dept., Takamura, Toshihiro, c/o Patent and License Dept.
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1991
Main classification: C22C38
IPC codes: B23K13, C21D9, C22C38, F16L9, C21D8, C21D6