JP2009242849A - Method for producing high toughness steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel excellent in a low temperature toughness without increasing the adding quantity of micro-alloy. <P>SOLUTION: A method for producing the high toughness steel includes the following process. A steel blank having composition composed of, by mass%, 0.01-0.20% C, 0.01-0.80% Si, 0.20-2.0% Mn, ≤0.020% P, ≤0.0070% S, 0.003-0.100% sol.Al and the balance Fe with inevitable impurities is heated to the temperature in the austenite temperature range and ≤1200°C, and after rolling the blank in the austenite-crystallizing temperature range, and when the rolling is performed in the temperature range of non-recrystallizing upper limit temperature or lower to Ar<SB>3</SB>point or higher, the rolling is divided into two or more stages, and before the secondary and thereafter rolling is performed, the rapid-heating of 2°C/sec, is applied by an induction-heating apparatus near the rolling-mill to perform the temperature compensation, and the accumulated rolling-reduction at ≥70% in the temperature range of the non-recrystallizing upper limit temperature or lower to the Ar<SB>3</SB>point or higher, is applied and the accelerated cooling from the Ar<SB>3</SB>temperature or higher to ≤600°C, is performed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a high toughness steel by online machining heat treatment, and particularly relates to a method for producing a high toughness steel plate.

  In recent years, the requirements regarding the characteristics of steel have become more and more demanding, and further enhancement of strength and toughness is desired. To meet such demands, there has been a conventional heat treatment technology that combines controlled rolling and controlled cooling. Widely done. The basic idea of thermomechanical processing technology is to obtain a fine ferrite or bainite structure by optimizing the steel composition, heating conditions, rolling conditions, and cooling conditions. It is intended to be illustrated.

  The basic idea is that in rolling in the low temperature range of austenite where austenite does not recrystallize (hereinafter referred to as the non-recrystallization temperature range), the cumulative rolling reduction is ensured as high as possible to expand the austenite grains. This is to increase the degree, introduce a large number of deformation bands, and refine the ferrite or bainite by adjusting the austenite / ferrite conversion ratio in the subsequent controlled cooling.

  However, in conventional controlled rolling, if the finish plate thickness and finishing temperature are determined, the temperature drop behavior during rolling is uniquely determined, and the most important factor in controlled rolling is the maximum in the austenite non-recrystallization temperature range. Cumulative rolling reduction is determined. Therefore, it has been difficult to perform rolling to obtain a high cumulative reduction ratio of 70% or more in the austenite non-recrystallization temperature range.

  Therefore, as a method for expanding the austenite non-recrystallization temperature range and increasing the cumulative reduction rate in this region, a method of increasing the maximum non-recrystallization temperature by adding a microalloy such as Nb or Ti is known. ing. Currently 0.06% or less of Nb or 0.10% or less of Ti is added, and the non-recrystallization temperature range is expanded to about 50 ° C. on the high temperature side, which is widely used industrially. It is.

On the other hand, as a technique that does not depend on the addition of alloy elements, the techniques of Patent Document 1 and Patent Document 2 are disclosed. In both technologies, as in the present invention, an on-line heat retention system is utilized to increase the toughness of steel by rolling to ensure a predetermined cumulative rolling reduction in a specific temperature range.
Japanese Patent Laid-Open No. 5-43934 JP-A-5-295432

  However, in the method of adding microalloys such as Nb and Ti to increase the upper limit temperature of non-recrystallization, the effect is saturated even if the amount of microalloy added is increased more than the current state, and welding is also performed. There is a problem that the performance is significantly reduced.

  On the other hand, in the technique disclosed in Patent Document 1 which is a method that does not depend on the addition of alloy elements, the cooling rate of the steel sheet after rolling is about 10 to 10 ° C./second, and the purpose of the controlled cooling is high. Strengthening. Accordingly, there is a problem that even if the cooling rate is changed, the change in toughness does not occur, and it is impossible to achieve both high strength and high toughness with the component system as in the present invention.

  Also, in the technique disclosed in Patent Document 2, accelerated rolling is performed after rolling while suppressing temperature drop in the austenite non-recrystallization temperature range as in the present invention, but unlike the present invention, Since rapid heating is not performed during rolling, recovery of unrecrystallized austenite is likely to proceed during heating, and the effect of controlled rolling and refinement of ferrite grain nucleation from processed austenite are fully exhibited. There is a problem that can not be.

The inventors have conducted extensive research to solve the problems of the prior art as described above, and have reached the present invention. The present inventors have conducted hot rolling of carbon steel and low alloy steel, In hot rolling, rapid heating by an induction heating device close to the rolling mill compensates for the temperature drop of the steel during rolling, while applying a high cumulative reduction below the non-recrystallization upper limit temperature, above the Ar ₃ point, and further rolling. It was succeeded in greatly improving the strength and toughness of the steel by accelerating cooling from the temperature of the rear Ar ₃ point or higher to 600 ° C. or lower. The summary is as follows.

1st invention is the mass%, C: 0.01-0.20%, Si: 0.01-0.80%, Mn: 0.20-2.0%, P: 0.020% or less , S: 0.0070% or less, sol. A steel material having a composition containing Al: 0.003 to 0.100% and the balance being Fe and inevitable impurities is heated to 1200 ° C. or less in the austenite temperature range, and after rolling in the austenite recrystallization temperature range When rolling in the temperature range below the upper limit of non-recrystallization temperature and ₃ points of Ar or more, the rolling is divided into two or more processes, and induction heating close to the rolling mill before the second and subsequent rolling. By performing rapid heating at 2 ° C./sec or more with an apparatus to compensate for temperature, a cumulative reduction of 70% or more is applied in a temperature range below the non-recrystallization upper limit temperature, Ar ₃ points or more, and Ar ₃ points or more It is a method for producing high toughness steel, characterized by accelerated cooling from temperature to 600 ° C. or less.

  According to the second aspect of the present invention, the steel composition further includes, in mass%, Cu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Cr: 0.01 to 2.0%, Mo : 0.01-1.0%, Nb: 0.003-0.1%, V: 0.003-0.5%, Ti: 0.005-0.10%, B: 0.0005-0 The method for producing high-toughness steel according to the first invention, characterized by containing one or more selected from .0040%.

According to the present invention, in the production of carbon steel and low alloy steel, in particular, thick steel sheets, the proper components, heating temperature, non-recrystallization temperature or less, and accumulation of 70% or more in a temperature range of Ar ₃ points or more. By performing accelerated cooling from a rolling reduction, a temperature of ₃ or more rolling Ars, steel materials, in particular thick steel plates, with significantly improved toughness compared to conventional methods can be produced appropriately. It is an invention that has a great effect.

  As described above, in the conventional technique, the emphasis was placed on expanding the austenite non-recrystallization temperature range by adding microalloys such as Nb and Ti. In order to solve the problems, studies have been made from a completely different point of view, and the component composition and production conditions for obtaining the effects of the present invention will be described in detail below.

1. About component composition The reason for limitation of the component composition of the high toughness steel which concerns on this invention is as follows. In addition, in this specification,% showing the component of steel means mass% altogether.

C: 0.01 to 0.20%
In order to ensure the strength of the steel sheet, C needs to be at least 0.01%, and if added over 0.20%, the toughness is significantly deteriorated and weldability is lowered. 01% or more and 0.20% or less.

Si: 0.01-0.80%
Si is an element necessary for deoxidation, but the effect is small if it is less than 0.01%, and if added over 0.80%, the weldability and the base metal toughness are remarkably lowered. 0.01% or more and 0.80% or less.

Mn: 0.20 to 2.0%
Similar to C, Mn is necessary to ensure the strength of the steel sheet, and if added excessively, weldability is impaired, so the amount of Mn is set to 0.20% or more and 2.0% or less.

P: 0.020% or less S: 0.0070% or less P and S are elements inevitably contained in the steel as impurities, and deteriorate the toughness of the steel base material and the weld heat affected zone. In consideration of economic efficiency, it is preferable to reduce the amount as much as possible. The upper limit of the P amount is 0.020%, and the upper limit of the S amount is 0.0070%.

sol. Al: 0.003 to 0.100%
Al is a deoxidizing element, and its effect is not sufficient if it is less than 0.003%, and if added in excess, it causes deterioration of toughness. The Al content is 0.003% or more and 0.100% or less.

  The basic component composition of the present invention is as described above. When further improving properties such as strength and toughness, one or more selected from Cu, Ni, Cr, Mo, Nb, V, Ti, and B are used. Can be added as a selective element.

Cu: 0.01 to 2.0%
Cu is an element for increasing the strength. When it is added in an amount of 0.01% or more and added over 2.0%, the steel surface deteriorates due to hot brittleness. Is in the range of 0.01 to 2.0%.

Ni: 0.01 to 2.0%
Ni is an element that can improve the toughness while increasing the strength of the base metal. It is effective at 0.01% or more, and if it exceeds 2.0%, the effect is saturated and the economy is impaired. If so, the content is made 0.01 to 2.0%.

Cr: 0.01 to 2.0%
Cr is an element effective for increasing the strength, exhibits its effect at 0.01% or more, and deteriorates toughness when added over 2.0%. The range is 2.0%.

Mo: 0.01 to 1.0%
Mo is an element effective for increasing the strength. The effect is exhibited at 0.01% or more, and if added over 1.0%, the toughness is remarkably deteriorated and the economical efficiency is impaired. In the case, it is made 0.01 to 1.0% of range.

Nb: 0.003 to 0.1%
V: 0.003-0.5%
Nb and V are elements that improve the strength and toughness of the base material, and exhibit an effect when added in an amount of 0.003% or more. Further, if it exceeds 0.1% and 0.5%, respectively, the toughness may be lowered. Therefore, when added, Nb: 0.003-0.1%, V: 0.003-0.5 % Range.

Ti: 0.005-0.10%
Ti is an effective element from the viewpoint of ensuring the toughness of the base metal and ensuring the toughness in the heat affected zone, but if added over 0.10%, the toughness is significantly reduced. The range is 005 to 0.10%.

B: 0.0005 to 0.0040%
B is an element that can increase the strength by improving the hardenability. This effect becomes prominent at 0.0005% or more, and the effect is saturated even if added over 0.0040%. Therefore, when added, the content is made 0.0005 to 0.0040%.

  In addition, unless the effect of this invention is impaired, you may contain elements, such as Ca, Mg, and REM, besides the above-mentioned component. These elements have a function of fixing S in steel and improving the toughness of the steel sheet, and are effective when added in an amount of 0.0001% or more. However, if added over 0.0060%, 0.0060%, and 0.0200%, respectively, the amount of inclusions in the steel increases and deteriorates the toughness. Therefore, when adding these elements, it is set as the range of Ca: 0.0001-0.0060%, Mg: 0.0001-0.0060%, REM: 0.0001-0.0200%.

  In addition, the remainder other than the above-mentioned component of the steel of this invention consists of Fe and an unavoidable impurity.

2. Manufacturing conditions Molten steel having the above composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and steel such as a slab by a conventional method, such as a continuous casting method or an ingot-bundling method. It is preferable to use a raw material. In addition, the melting method and the casting method are not limited to the above-described methods, and all known methods can be applied. Thereafter, rolling, heating, and cooling are performed under the following conditions.

(1) About slab heating After casting, after the slab temperature is lowered to room temperature or in a high temperature state, it is inserted into a heating furnace and heated to an austenite temperature range. The heating temperature of the slab is preferably lower from the viewpoint of securing toughness, and the upper limit of the heating temperature is to make the finally obtained structure as fine as possible ferrite or homogeneous fine bainite. In order to suppress this, it shall be 1200 degrees C or less. On the other hand, when the heating temperature is less than 1000 ° C., there is a possibility that an uncompressed zack at the center part of the slab thickness may remain, and the performance of the steel sheet thickness 1/2 t part may be deteriorated, and Nb, V, etc. are added. In such a case, since these elements are not sufficiently dissolved, the slab heating temperature is preferably 1000 ° C. or higher.

(2) About Hot Rolling First, austenite recrystallization zone rolling is necessary for uniformly refining austenite grains during heating to a certain degree, and a reduction of 1% or more, preferably 20% or more is performed cumulatively. You may wait by air cooling from the austenite recrystallization zone rolling to the start of the non-recrystallization zone rolling, but the time until the non-recrystallization zone rolling is cooled by water cooling during or after the austenite recrystallization zone rolling. It is more efficient to reduce the length than air cooling, and it has the effect of suppressing the growth of recrystallized austenite, which is more effective for refining the structure.

  Next, in the present invention, the cumulative rolling reduction in the non-recrystallization temperature region is set to 70% or more. The continuous improvement of strength and toughness in the high cooling rate region as the cumulative rolling reduction in the non-recrystallization temperature region increases is due to the following action.

  That is, the extent of austenite grains and the density of deformation bands increase with increasing cumulative rolling reduction, and the area per unit volume of austenite grain boundaries and deformation bands that can be nucleation sites during transformation (effective interfacial area: Sv) This is because the texture increases and the structure after transformation is refined. However, if the cumulative rolling reduction in the non-recrystallization temperature region is not 70% or more, the structure after transformation obtained after accelerated cooling is not sufficiently fine, and the toughness is not significantly improved.

In order to ensure a cumulative reduction ratio of 70% or more in the non-recrystallization temperature range, the temperature drop of the steel sheet during rolling is compensated by rapid heating at 2 ° C./sec or more by an induction heating device close to the rolling mill. Specifically, after rolling in the non-recrystallized region where the cumulative rolling reduction is about 40% or more, the temperature does not fall below the Ar ₃ point, and the non-recrystallized temperature region is 2 ° C / Heat at a heating rate of sec or more. There is no need to perform holding or the like after heating.

If the heating start temperature is lower than the Ar ₃ point, ferrite transformation occurs, and austenite is refined by reverse transformation at the time of reheating, but the heating temperature cost at the time of subsequent heating becomes large and the efficiency and economy are impaired. Precipitation and coarsening of carbide and the like are promoted, and a mixed grain structure is likely to be formed and toughness is reduced. Therefore, the temperature rise is started from a temperature of Ar ₃ or higher.

  Further, when the rate of temperature rise is 2 ° C./sec or less, recovery of the austenite processed structure and processing-induced precipitation of carbides such as Nb and Ti occur and the toughness is deteriorated, so that it is 2 ° C./sec or more. Although holding after heating may be performed, since recovery of the austenite processed structure occurs, holding more than necessary should not be performed, and a short time is preferable.

(3) Accelerated cooling Accelerated cooling is performed on a steel sheet that has been subjected to a cumulative reduction of not less than the non-recrystallization temperature and not less than Ar ₃ points and 70% or more, and is performed from the temperature not less than Ar ₃ points to a temperature of 600 ° C. or less. .

When accelerated cooling is performed from a temperature below Ar ₃ points, ferrite is partially generated before accelerated cooling, and thus a predetermined strength cannot be obtained. The same applies when cooling is stopped at 600 ° C. or higher.

  The cooling rate requires a cooling rate higher than that of air cooling, and strong cooling of 10 ° C./sec is preferable. The cooling method is not particularly limited, but cooling by water cooling is preferable.

  Here, the steel material temperature in this invention has shown the average temperature of the surface and center part of steel materials.

The recrystallization temperature and the Ar ₃ point vary depending on the components, and the recrystallization temperature (temperature at which recrystallization occurs) is generally in the range of 800 to 950 ° C.

Ar ₃ points can also be obtained by the following equation (1). However, in each formula, each element shows content (mass% display).
Ar ₃ = 910-310C-80Mn-55Ni-15Cr-80Mo-20Cu
(1)
The present invention is applicable to steel products having various shapes such as thick steel plates, section steels, and steel bars. The “thick steel plate” refers to a steel plate having a thickness of 6 mm or more.

  Various steels having the composition shown in Table 1 were melted. Using the obtained steel piece, a thick steel plate having a thickness of 12 mm was produced under the production conditions shown in Table 2.

  A sample for investigating the mechanical properties of the thick steel plate thus obtained was collected and subjected to a tensile test and a Charpy test using the test pieces described below.

  The tensile strength was determined by a tensile test in which a 1A full thickness test piece defined in JISZ2201 was taken in the plate width direction.

As for toughness, in accordance with JISZ2202, the Charpy impact test piece was sampled from the center of the plate thickness so that the longitudinal axis direction of the test piece was parallel to the rolling direction, and a V-notch Charpy impact test was performed. The ductile brittle transition temperature (referred to as vTrs) was evaluated and determined.
In the present invention, the target value of toughness is that the ductile brittle transition temperature (vTrs) is less than −140 ° C.

  The results of the tensile test and Charpy impact test are also shown in Table 2.

  The components and production methods are within the scope of the present invention. 1-No. It can be seen that all the steel sheets No. 5 have a ductile brittle transition temperature (vTrs) of less than −140 ° C. and excellent toughness.

On the other hand, No. C addition amount exceeded the scope of the present invention. In the steel plate No. 6, the toughness deteriorated. No. whose heating temperature is higher than the range of the present invention (1200 ° C. or less). In the steel plate No. 7, since the finally obtained structure was not sufficiently refined, the toughness deteriorated. No. In the steel plate No. 8, since the rapid heating for ensuring the cumulative reduction amount in the non-recrystallization temperature range is not performed, the cumulative reduction rate in the non-recrystallization temperature range becomes smaller than the range of the present invention, and the toughness Deteriorated. The heating rate during rolling was slower than the range of the present invention, and the recovery of processed austenite became remarkable. No. 9 in which the heating temperature during rolling was higher than the range of the present invention and austenite was recrystallized. In all ten steel plates, the toughness deteriorated. In addition, the rolling end temperature is lower than the Ar ₃ point, and it is applied to the ferrite / austenite two-phase region. No. 11 steel, No. 11 whose cooling start temperature is in the ferrite / austenite two-phase region. No. 12 steel, and No. 1 with a high cooling stop temperature. About the steel plate of 13, the coarse ferrite existed and toughness deteriorated.

  Since the steel according to the present invention is excellent in strength and low temperature toughness, it can be applied to steel structures that require high material uniformity and low temperature specifications.

Claims (2)

In mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.0%, P: 0.020% or less, S: 0.0070 % Or less, sol. A steel material having a composition containing Al: 0.003 to 0.100% and the balance being Fe and inevitable impurities is heated to 1200 ° C. or less in the austenite temperature range, and after rolling in the austenite recrystallization temperature range When rolling in the temperature range below the upper limit of non-recrystallization temperature and at ₃ or more points of Ar, the rolling is divided into two or more processes, and induction heating close to the rolling mill before the second and subsequent rolling. By performing rapid heating at 2 ° C./sec or more with an apparatus to compensate for temperature, a cumulative reduction of 70% or more is applied in a temperature range below the non-recrystallization upper limit temperature, Ar ₃ points or more, and Ar ₃ points or more A method for producing high toughness steel, characterized by accelerated cooling from temperature to 600 ° C. or lower.   In addition to steel composition, Cu: 0.01-2.0%, Ni: 0.01-2.0%, Cr: 0.01-2.0%, Mo: 0.01-1 0.0%, Nb: 0.003-0.1%, V: 0.003-0.5%, Ti: 0.005-0.10%, B: 0.0005-0.0040% The method for producing a high toughness steel according to claim 1, comprising one or more selected.