EP0043866A1

EP0043866A1 - Process for producing a high-toughness steel

Info

Publication number: EP0043866A1
Application number: EP80104117A
Authority: EP
Inventors: Yasumitsu Onoe; Shinogu Tamukai; Masaki Umeno
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1980-07-15
Filing date: 1980-07-15
Publication date: 1982-01-20

Abstract

Process for producing a high-toughness steel with uniform hardness distribution in the thickness direction, . which process comprises heating the steel to a temperature higher than 1000°C, rolling the heated steel with a finishing temperature ranging from the Ar₃ temperature to a temperature lower than 1000°C, and rapidly cooling the rolled steel.

Description

The present invention relates to a process for producing high-toughness steel uniform hardness distribution in the plate thickness direction.
Steels having excellent weldability or toughness have been conventionally produced by a process wherein a steel having an adjusted composition is rapidly cooled from the austenite condition as disclosed by Japanese laid-open patent specification Sho 52-27087, or by a process including cooling a steel through a specific temperature zone at a controlled cooling rate after rolling, as disclosed by Japanese laid-open patent specification Sho 51-26615.
These prior art processes have been found to be effective in improving the weldability or toughness of steel materials, but show the following disadvantages in commercial application to mass-production, although in theory they can be used to produce the steel materials.
Thus when the steel is subjected to the rapid cooling treatment which is equivalent to quenching, there are considerable differences in hardness in the thickness direction, so that the resultant sted varies considerably in quality and shape.
Besides, when the steel is cooled through a specific temperature zone at a controlled cooling rate, it is difficult to control temperature and cooling rate.
Therefore, one of the objects of the present invention is to provide a process for producing steel with uniform hardness distribution in the thickness direction and excellent shape in spite of forced rapid cooling. It is not the main object of the present invention to provide a steel with excellent strenght or toughness. When steels are produced by a process including quenching and tempering, no substantial problems appeart from the aspects of quality and shape because even if a non-uniform hardness distribution in the thickness direction is caused quenching, this non-uniform hardness distribution can be ameliorated to a uniform hardness distribution by the subsequent tempering. However, when the steel is used in the as-rapidly-cooled condition, serious problems such as residual stress, differences in quality and undesirable shape arise from the non-uniform hardness distribution in the thickness direction.
Apparently, this non-uniform hardness distribution is attributable to the difference in the cooling rate between the steel surface and its core portion.
The present inventors have conducted various extensive studies in their search for a process by which uniform hardness distribution can be obtained, and have discovered that even if the steel is hardened by rapid cooling the difference in the hardness between the steel surface and the steel core can be reduced so as to be practically negligible if the steel is rolled at a relatively low temperature, for example not higher than 1000°C depending on the steel composition, at a reduction rate of, for example, not less than 10 % (a higher reduction rate is preferable), so as to make the grains in the surface portion (several millimeters or less from the surface) finer than the grains in the core portion, preferably maintaining the grains in the surface portion in a non-recrystallized state, thus providing more transformation sites which are useful at the time of austenite/ferrite transformation of the surface portion, and rendering the surface portion less hardenable. In this way it is possible to produce a steel plate of excellent shape with less differences in quality and less bending.

Fig. 1 shows the hardness distribution in the thickness direction as obtained in connection with Steel H1 of 22 mm in thickness shown in Table 1.
Fig. 2 shows the hardness distribution in the thickness direction as obtained in connection with Steel H1 of 50 mm and 80 mm in thickness shown in Table 1.
Fig. 3 shows the hardness distribution in the thickness direction as obtained in connection with Steel H2 of 22 mm in thickness shown in Table 1.

The present invention is described in detail with reference to the attached drawings.
In the steel composition used in the present invention carbon, silicon and manganese are all essential in order to obtain the desired strength of the steel, but excessive contents of these elements cause deterioration of the toughness and weldability of the resultant steel. Therefore, these elements are limited to 0.3 % max. C, 1.0 % max. Si and 2.5 % max. Mn, respectively.
As regards aluminium, the range of 0.005 % to 0.1 % Al is essential and sufficient to achieve the desired state of deoxidation of the steel.
Especially when higher strength is desired, carbon, silicon and manganese are not sufficient and it is desirable to add one or more of the elements nickel, chromium and molybdenum, each in an amount of not more than 1 %, as the case may require. The upper limit of each of these elements is 1 % because if they are added in an amount higher than 1 %, the toughness and weldability of the resultant steel are deteriorated.
One or more of the elements niobium, vanadium, calcium, titanium and boron may be added for the purpose of refining the grains immediately after rolling and for preventing recrystallization of the surface portion. As these elements are effective when added in very small amounts, the upper limit of these elements has been set at 0.1 %.
Though improving the weather resistance of the steel, copper tends to embrittle the steel when added in an amount exceeding 0.5 %.
Herein below is a description of the heating before rolling and rapid cooling after rolling according to the present invention.
The reason why the steel is heated to a temperature higher than 1000°C is that it is difficult to maintain the desired quenching temperature especially if the . steel plate is thin, and that the steels before rolling are not satisfactorily uniform in their composition, temperature, etc.
Steel slabs used in the present invention may be produced by an ordinary ingot-making process or an ordinary continuous casting process, furthermore hot slabs, hot ingots, cold slabs, cold ingots and broken- down slabs may also be used.
However, when a continuously cast slab is used, it is particularly important to ensure its uniformity in composition, temperature, etc. prior to rolling.
The upper limit of the heating temperature, can be selected within a range suitable for the unification, but the temperature is preferably maintained at 1300°C and more preferably at 1250°C.
After heating, the steel slab is rolled into the desired thickness. In the present invention, the finishing temperature of rolling is important, and the temperature is fixed below 1000°C, because if the finishing temperature is higher than 1000°C, the recovery and the recrystallization of the rolled steel take place prior to the commencement of the rapid cooling, resulting in an almost uniform structure all through the plate, both in the surface portion and the inner portion, which is disadvantageous for obtaining a uniform hardness distribution in the thickness direction.
The lower limit of the heating temperature depends upon the steel composition. However, it should be noted that if the ferrite transformation begins before commencement of the rapid cooling, the hardening effect and the strength improving effect are decreased. Therefore, it is necessary to commence the cooling at a temperature not lower than the temperature (Ar₃) at which the ferrite transformation in the core portion starts. For example, in the case of an ordinary steel containing carbon, silicon and manganese in amounts sufficient to give a tensile strength in the order of 40 kg/mm² under as-rolled conditions, it is desirable that the finishing temperature be higher than 8009C.
The cooling rate which can increase the hardness while maintaining a uniform hardness distribution in the thickness direction varies depending on the thickness or the steel composition, and falls within the range which can be achieved by an amount of cooling water ranging from 0.2 to 2.0 m³/min.m². For example, in the case of a thickness of 22 mm, the average cooling rate in the thickness direction is preferably 20°C - 30°C/s, which is achieved by applying 0.4 - 1.0 m³/min.m² of cooling water. With extremely increased thickness, the hardness in the surface portion becomes slightly higher than that of the case portion, which effects the uniformity (or flatness) of the hardness distribution in the thickness direction. Therefore, in the case of very thick steel materials it is recommended to apply softer cooling (for instance a mixture of water and gas) so as to obtain a more uniform (flatter) hardness distribution.
The present invention will be better understood from the following example.
Example:

Steels having various compositions as shown in Table 1 were subjected to various treatments as shown in Table 2. The results are also shown in Table 2. The numerical data marked with a rectangle show reference conditions outside the scope of the present invention.

As clearly shown in Figs. 1 to 3, the steels H1-3, H1-5, H1-6, H2-2, H2-3 which were treated according to the present invention show a uniform hardness distribution at a high hardness level in the thickness direction as compared with the reference steels.

Claims

Claim 1. A process for producing high-toughness steel with uniform hardness distribution in the thickness direction, comprising heating a steel to a temperature higher than 1000°C, rolling the heated steel with a finishing temperature lower than 1000°C but not higher than the Ar₃temperature and rapidly cooling the rolled steel.
Claim 2. Process according to Claim 1 in which the steel contains not more than 0.3 % carbon, not more than 1.0 % silicon, not more than 2.5 % manganese and 0.005 to 0.1 % A1, with the balance being essentially iron and unavoidable impurities.
Claim 3. Process according to Claim 2 in which the steel further contains at least one of the elements nickel, chromium and molybdenum each in an amount of not more than 1 %.
Claim 4. Process according to Claim 2 in which the steel further contains at least one of the elements niobium, vanadium, calcium, titanium and boron, each in an amount of not more than 0.1 % and copper in an amount of not more than 0.5 %.
Claim 5 Process according to Claim 2 in which the steel further contains at least one of the elements nickel, chromium and molybdenum, each in an amount of not more than 1 %, at least one of the elements niobium, vanadium, calcium, titanium, boron each in an amount of not more than 0.1 % and copper in an amount of not more than 0.5 %.