JPH0570684B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing hot rolled thick steel plates having high strength and high toughness. [Prior Art] Conventionally, hot-rolled thick steel sheets generally contain (a) weight% (hereinafter, % indicates weight%), C: 0.05 to 0.15%, Si: 0.001 to 0.05%, Mn: 0.8 to After heating a steel ingot containing 2%, sol.Al: 0.001-0.06%, Nb: 0.005-0.1%, N: 0.002-0.01% to a temperature of 1000℃ or higher, (b) 950℃ or higher (c) After being air-cooled to adjust the rolling temperature within a temperature range of around 900°C, (d) finish rolling at a temperature of 700°C or higher, and (e) then rolling to a temperature of 500°C or lower. It is well known that it is manufactured by hot rolling, which consists of a series of steps (a) to (e), including rapid cooling. [Problems to be Solved by the Invention] On the other hand, in recent years, due to the demand for energy saving and weight reduction, there is a strong desire to produce hot rolled steel sheets with higher strength and toughness. [Means for Solving the Problems] Therefore, from the above-mentioned viewpoint, the present inventors have developed a hot-rolled steel plate that has higher strength and toughness than the hot-rolled thick steel plate manufactured by the above-mentioned conventional method. As a result of research into manufacturing thick steel plates, we found that in the conventional manufacturing method for hot-rolled thick steel plates, as mentioned above, after rough rolling and before finish rolling, a temperature range of around 900℃ is used to adjust the rolling temperature. Air cooling was used (usually the cooling rate is 0.1°C/sec or less), but when this was rapidly cooled at a cooling rate of 0.5°C/sec or more, carbonitriding occurred because the cooling rate was relatively slow with air cooling. Most of the niobium (hereinafter referred to as NbCN) was precipitated, and almost no NbCN was precipitated in the rapid cooling after finish rolling.On the other hand, the grains, especially the ferrite, were coarsened and no improvement was seen in the strength and toughness. On the other hand, if the precipitation of NbCN is suppressed by rapid cooling after rough rolling as described above, part of the Nb that is in solid solution at this point will precipitate as fine NbCN during finish rolling, and this fine NbCN will During rapid cooling after finish rolling, the structure becomes finer because it becomes the nucleus for the formation of ferrite that transforms and begins to precipitate, resulting in high toughness. The research results showed that Nb has the effect of increasing the hardenability of steel during rapid cooling after finish rolling, and that Nb itself precipitates as fine NbCN, resulting in high strength. Therefore, this invention was made based on the above research results, and includes C: 0.05 to 0.15%, Si: 0.001 to 0.05%, Mn: 0.8 to 2%, sol.Al: 0.001 to 0.06. %, Nb: 0.005 to 0.1%, N: 0.002 to 0.01%, when producing a hot rolled thick steel plate by hot rolling from a steel ingot containing: (a) rough rolling (b) At least the temperature range of 870 to 930°C, which is the precipitation temperature range of NbCN, before finish rolling is reduced by 0.5
Rapidly cooling at a cooling rate of ℃/sec or more to suppress the precipitation of NbCN, (c) then finish rolling, (d) successively rapid cooling to precipitate fine NbCN, above (a) to (d) This method is characterized by a method for manufacturing a hot rolled thick steel plate having high strength and high toughness by performing a series of steps. Next, in the method of the present invention, the reason why the content of the constituent components of the steel and the cooling rate after rough rolling are limited as described above will be explained. A Constituent component content of steel (a) C C component has the effect of improving the strength of steel sheets, but if its content is less than 0.05%, the desired strength cannot be secured; If it exceeds 0.15%, weldability will deteriorate, so the content should be reduced from 0.05% to
It was set at 0.15%. (b) Si The Si component has a deoxidizing effect and is used as a deoxidizing agent when making steel.
A content of 0.001% or more is unavoidable, and if the content exceeds 0.5%, weldability will deteriorate, so the content should be reduced from 0.001% to 0.001%.
It was set at 0.5%. (c) Mn The Mn component has the effect of increasing the strength of the steel sheet and improving the toughness during rolling, but if its content is less than 0.8%, the desired effect cannot be obtained; If the content exceeds 2%, the weld heat-affected zone hardens and weldability deteriorates, so the content was set at 0.8 to 2%. (d) sol.Al Al component has a strong deoxidizing effect, so
Like Si, it is used as a deoxidizer, so
A content of 0.002% or more is unavoidable, while if the content exceeds 0.06%, AlN is formed,
On the other hand, since the formation of NbCN decreases and it becomes impossible to secure the desired strength and toughness, its content is reduced to 0.001 to 0.06% in sol.Al.
It was determined that (e) Nb and N Some of these components, together with the C component, precipitate as fine NbCN during finish rolling, which refines the ferrite grains generated by γ to α transformation during quenching after finish rolling. This has the effect of improving the toughness of the steel sheet, and furthermore, even after finish rolling, a relatively large amount of Nb remains in solid solution.
Nb itself has the effect of increasing the hardenability during quenching after finish rolling, and also precipitates as fine NbCN during quenching after finish rolling to improve strength, but its content is 0.005%.
If the content exceeds Nb: 0.1% and N: 0.01%, the toughness will conversely decrease. From, its content is Nb: 0.005~0.1%,
N: determined to be 0.002 to 0.01%. B. Cooling rate after rough rolling As mentioned above, in this type of steel, NbCN precipitation is observed in the temperature range of 870 to 930°C. Therefore, NbCN precipitation occurs even below 870°C and above 930°C temperature becomes slow, so at least 870~930℃
It is necessary to suppress the precipitation of NbCN by rapidly cooling the temperature range. For this purpose, it is necessary to rapidly cool at least the temperature range of 870 to 930°C, which is the temperature range where NbCN precipitation becomes noticeable, at a cooling rate of 0.5°C/sec or more, and at a cooling rate of less than 0.5°C/sec. When this happens, the precipitation of NbCN becomes noticeable, making it impossible to obtain the desired strength and toughness during rapid cooling after finish rolling. [Example] Next, the method of the present invention will be specifically explained with reference to Examples. Ingot A with cross-sectional dimensions of 250 mm x 2100 mm is obtained by preparing molten steel having the composition shown in Table 1 using a normal melting method and casting it.

【table】

〔Effect of the invention〕

From the results shown in Table 2, hot-rolled thick steel plates manufactured by methods 1 to 9 of the present invention all have high strength and high toughness, whereas as seen in comparative methods 1 to 4, It is clear that if the cooling conditions between rough rolling and finish rolling are outside the scope of the present invention, only a hot rolled thick steel plate with poor strength and/or toughness will be obtained. Note that the comparative method corresponds to the above-mentioned conventional method. As described above, according to the method of the present invention, it is possible to produce a hot rolled thick steel plate with high strength and high toughness by suppressing the precipitation of NbCN between rough rolling and finish rolling.

Claims (1)

[Claims] 1% by weight: C: 0.05-0.15%, Si: 0.001-0.05%, Mn: 0.8-2%, sol.Al: 0.001-0.06%, Nb: 0.005-0.1%, N: 0.002 to 0.01%, when producing a hot rolled thick steel plate by hot rolling from a steel ingot containing The temperature range of 870 to 930℃, which is the precipitation temperature range of niobium nitride, is
Rapid cooling is performed at a cooling rate of 0.5°C/sec or more to suppress the precipitation of niobium carbonitride, (c) then finish rolling, (d) successive rapid cooling is performed to precipitate fine niobium carbonitride. 1. A method for producing a hot-rolled thick steel plate having high strength and high toughness, characterized by carrying out a series of steps from ) to (d).