JP4777112B2 - Manufacturing method of high strength wire rod with excellent weldability - Google Patents

Description

  The present invention is suitable for products that require high weldability while having high strength for use, and is used for high-strength pareto using PC steel, automobile seat frames, automobile wheel rings, and welded wire mesh. The present invention relates to a method for producing a high-strength wire.

  A high strength steel wire or the like is usually obtained by applying a wire drawing process using a JIS G3521 hard steel wire or a JIS G3522 piano wire. In this case, since it becomes a component close to eutectoid steel and welding performance is extremely inferior, generally, when welding performance is required, JIS G3505 mild steel wire was mostly used after drawing. . However, in this case, since the amount of carbon is low, application is difficult when high strength is required.

  The demand for such high strength and excellent weldability must satisfy the individual characteristics of hard steel wire and mild steel wire, that is, the high strength of hard steel wire and the welding performance of mild steel wire at the same time. In order to achieve both weldability, it is necessary to perform a component system and heat treatment different from the conventional ones. From such a view, as disclosed in Patent Document 1, using the wire material with the C level appropriately controlled, after adjusting to austenite, the adjustment cooling is performed again, and after the tempering process, the true strain is 1.5. A method has been proposed in which a predetermined structure is obtained by drawing up to ~ 2.2 to achieve both high strength and weldability.

Japanese Patent Application Laid-Open No. 08-176736

  However, this method requires a further cost reduction from the viewpoint of production cost and productivity because the process heat treatment process is long, such as performing austenite again after performing wire rod rolling, and then tempering, etc. until commercialization. It is growing. The demand is getting stronger, especially in the current booming market.

  The greatest feature of the present invention is to obtain high strength efficiently by applying in-line heat treatment with molten salt at 500 ° C. to 550 ° C. at the time of wire pressure, and the austenite crystal grains are finer than when reheating. Therefore, a material having better ductility than the conventional one can be obtained. The gist is mass%, C: 0.15 to 0.20% or less, Si: 0.8 to 1.0% or less, Mn: 1.4 to 1.6%, P: 0.01% or less. , S: 0.01% or less, Cr: 0.3-0.5%, Ti: 0.005-0.100%, B: 0.0010-0.0025%, the balance Fe and inevitable After hot rolling a wire having a steel component composed of impurities, it is wound at 850 ° C. to 900 ° C., held at a temperature of 470 to 550 ° C. for 50 to 90 seconds by in-line heat treatment, and then 25 to 100 ° C./sec. And producing a high strength wire with excellent weldability having a strength of 1250 MPa or more, characterized in that the metal structure is a mixed structure of ferrite, pearlite, bainite, martensite and each 2% or more. It is.

  In the present invention, since a wire having high strength and excellent weldability can be produced by direct in-line heat treatment, it has become possible to produce the wire at a lower cost than the conventional heat treatment method by reheating.

The details are described below.
First, the reasons for limitation of the present invention will be described.
The upper limit of C is limited to 0.20% because addition of more than this deteriorates the ductility of the steel and promotes the propagation of cracks, leading to a decrease in strength. The reason why the lower limit is limited to 0.15% is that if it is less than this, the target high strength of 1250 Mpa or more cannot be satisfied.

  The reason why the upper limit of Si is set to 1.0% is that addition of more than this is not preferable in terms of structure control because the deviation from the heat treatment temperature becomes larger than the contribution to the increase in strength. The reason why the lower limit is set to 0.8% is that it is difficult to secure TS for high strength.

  The reason why the upper limit of Mn is 1.6% is that when it is added more than this, the quenching performance is increased more than necessary and the ductility is deteriorated. The reason why the lower limit is limited to 1.4% is an amount necessary for ensuring the strength and properly securing the hardenability.

  The upper limit of P was limited to 0.01% because the addition of more than this would lead to a decrease in the ductility of the steel, and was feared to cause early destruction. The smaller the lower limit, the better the ductility. However, there is no particular restriction because an extremely low P leads to an increase in steel melting costs.

  The reason why the upper limit of S is set to 0.01% or less is that when it is added more than this, red heat embrittlement or the like is caused by MnS and the surface quality during hot rolling is lowered.

  In addition to being used for increasing the hardenability and obtaining strength, Cr has an effect of suppressing softening due to heat input during welding, and therefore plays an important role in the present invention in the same manner as Si. The upper limit was set to 0.5% because the quenching performance was saturated with addition beyond this limit because of economic reasons. The reason why the lower limit is limited to 0.3% is that if it is less than this, the desired high strength cannot be obtained due to a decrease in quenching performance.

  The addition of Ti promotes the fixation of N, and TiN is used to secure the free B necessary for hardenability by suppressing the precipitation of BN. The upper limit is set to 0.100% because addition beyond this amount is too much for N fixation, resulting in an adverse effect of excess Ti. This is because if the amount is 0.005% or less of the lower limit, the fixation of N becomes insufficient and BN precipitates, making it difficult to effectively use B.

The upper limit of B is set to 0.0025 because when B is excessive, precipitates such as Fe ₂₃ (CB) ₆ are precipitated in a large amount at the ferrite grain boundaries and the ductility is deteriorated. The reason why the lower limit is set to 0.0010% is that if it is less than this, the effect of improving the hardenability of B is lost.

Next, the reason for the wire cutting temperature will be described.
The upper limit of the scraping temperature is limited to 900 ° C., because the growth thickness of the secondary scale increases until it is put into the in-line heat treatment equipment. This is because scale deposition tends to occur because the amount of scale peeling increases in the heat treatment bath. The reason why the lower limit temperature of the coiling temperature is set to 850 ° C. is that when the temperature is equal to or lower than this temperature, the transformation proceeds until the in-line inlet, and thus a predetermined structure cannot be stably obtained.

  The reason why the upper limit of the in-line heat treatment temperature is set to 550 ° C. is that if the temperature is higher than this, a stable heat transfer coefficient cannot be obtained due to decomposition of the molten salt used in the heat treatment, and stable heat treatment is impossible. The reason why the lower limit temperature is set to 470 ° C. is that it becomes difficult to obtain a predetermined composite structure (ferrite, pearlite, bainite, martensite) at temperatures lower than this. If the respective structures of ferrite, pearlite, bainite, and martensite are present in an amount of 2% or more, the strength can be maintained at 1250 MPa or more. If the value is less than this, it is difficult to ensure the strength.

  In addition, the holding time of the in-line heat treatment needs to be 50 seconds or more for the reason of generating a predetermined structure. If it exceeds 90 seconds, no martensite is generated in the wire after the heat treatment, so the strength cannot be increased to 1250 MP or more. The cooling rate after in-line heat treatment needs to be 25-100 ° C / sec. The reason for the lower limit is that at 25 ° C / sec or less, it becomes a structure mainly composed of ferrite and pearlite and an increase in strength cannot be expected. The reason for setting the temperature to 100 ° C./sec is the restriction of the cooling capacity of the refrigerant to be used.

All the examples shown in Table 1 were melted in a 250-ton converter, continuously cast into a 500 mm × 300 mm bloom, and then a 122 mm square billet was formed in the lump process. Thereafter, a wire rod having a diameter of 5.5 mm or 11.5 mm was obtained through a wire heating furnace, rolling, and direct heat treatment process. Pcm in Table 1 represents a weld cracking sensitivity index, and the calculation method was obtained by the following equation (1). It represents that welding performance improves, so that this value is small.
Pcm = C + 1 / 30Si + 1 / 20Mn + 1 / 20Cu + 1 / 60Ni + 1 / 20Cr + 1 / 15Mo + 1 / 10V + 10 × B (1) formula

  In addition, spot weldability is indicated by ◯ when butt welding is performed and then the overhanging portion of the welded portion is shaved and a tensile test is performed with a number n of 5 and the average value indicates a value of 70% or more of the base material strength. The case of less than 70% is indicated by a cross.

  Steel type A uses a mild steel wire based on JIS G3505 as a comparison. In this case, the weldability shows very good characteristics, but the strength is low and the high strength which is a requirement of the present invention is not satisfied. On the other hand, steel type B uses a hard steel wire conforming to JIS G3506 as a comparison, and in this case, the strength does not satisfy the high strength of the invention requirement, and it is understood that the spot weldability is also deteriorated.

  As described above, in the case of steel type A mainly composed of ferrite and steel type B mainly composed of pearlite, it is difficult to realize both weldability and strength. Even if the weldability is good as in steel type A, if the strength is low, the amount of C is increased in order to increase the strength, but in this method the weldability deteriorates as in steel type B. That is, it can be seen that characteristics satisfying both strength and weldability cannot be obtained. Steel types C and D show comparative methods. Steel types E to L show examples of 5.5 mmφ wire rods using the method of the present invention. Steel types J to L show examples of the method of the present invention when the wire is manufactured at 11.5 mmφ.

  In any case, it is understood that the spot weldability is good when the value of Pcm is around 0.3. Regarding strength, it can be seen that Examples C to K of the present invention can secure a strength of 1250 MPa. These are all subjected to in-line heat treatment, which is a feature of the present invention, and the structure contains 2% or more of ferrite, pearlite, bainite, and martensite.

Claims (1)

  In mass%, C: 0.15 to 0.20% or less, Si: 0.8 to 1.0% or less, Mn: 1.4 to 1.6%, P: 0.01% or less, S: 0 0.01% or less, Cr: 0.3 to 0.5%, Ti: 0.005 to 0.100%, B: 0.0010 to 0.0025%, and the balance Fe and unavoidable impurities After hot-rolling the wire rod having the components, it is wound at 850 ° C. to 900 ° C., held at a temperature of 470 to 550 ° C. for 50 to 90 seconds by in-line heat treatment, and then at a cooling rate of 25 to 100 ° C./sec. A method for producing a high-strength wire rod having excellent weldability and having a strength of 1250 MPa or more, characterized by cooling and making the metal structure a mixed structure of ferrite, pearlite, bainite, and martensite at 2% or more.