JPH08232016A - Production of high tensile strength steel plate - Google Patents

Abstract

PURPOSE: To produce a high tensile strength steel plate reduced in difference in tensile strength by hot-rolling a steel of specific composition at specific finishing temp., subjecting the resulting steel plate to rapid cooling from specific temp. down to a temp. right above the martensitic transformation initiating temp. and to slow cooling down to room temp., and then tempering it at a temp. not higher than the Ac1 point. CONSTITUTION: A steel, which has a composition consisting of, by weight, 0.03-0.20% C, 0.05-0.50% Si, 0.30-2.50% Mn, 0.01-0.10% Al, <=0.007% N, and the balance Fe with inevitable impurities and further containing 0.05-1.30% Cu, 0.10-10% Ni, 0.05-1.05% Cr, 0.03-0.50% Mo, 0.01-0.15% V, and one kind selected from the group consisting of small amounts of Nb, B, Ca, and rare earth elements, is used. This steel is hot-rolled at a finishing temp. not lower than the Ar3 point, and the resulting steel plate is cooled rapidly from the temp. down to a temp. right above the martensitic transformation initiating temp. and cooled slowly down to room temp. to undergo transformation into bainitic structure, followed by tempering at a temp. not higher than the Ac1 point. By this method, tempering characteristic can be practically uniformized even if plate thickness is dissimilar.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is applied to the production of plate thickness taper high tensile steel plates having a tensile strength of 600 MPa or more used for bridges, construction, shipbuilding, pressure vessels, tanks, etc. The present invention relates to a suitable method for manufacturing a high-tensile steel plate.

[0002]

2. Description of the Related Art Conventionally, many techniques for producing heat-treated steel having high tensile strength have been proposed. As an example of these techniques, JP-A-57-89424 discloses a technique for producing a heat-treated steel having excellent strength and toughness by limiting cooling conditions from an austenitic state. There is. Further, Japanese Patent Application Laid-Open No. 58-120720 discloses a technique for producing a tempered steel of high strength and constant quality by dispersing and precipitating a carbide in the steel and effectively utilizing the carbide. Further, Japanese Patent Application Laid-Open No. 2-141528 discloses a technique of transforming fine-grained austenite into martensite to give an extremely thick material having a thickness of 100 mm or more excellent strength and toughness in the thickness direction. ing.

By the way, in recent years, a plate thickness taper high-tensile steel plate whose plate thickness changes in the longitudinal direction of the steel plate has been widely used in the shipbuilding industry and the like. In general, when manufacturing a high-strength steel plate with a tapered thickness, the cooling rate at the time of quenching is different between parts having different plate thicknesses. Therefore, in order to obtain the same strength with the same chemical composition, for example, the parameter P = (T + 273 ) (20 + logt) (T: tempering temperature (° C), t: tempering holding time (H
It is necessary to change r)) depending on the plate thickness. Therefore, when the conventional technique for producing heat-treated steel having high tensile strength is applied to the production of a taper taper high-strength steel sheet, a strength difference occurs between a thick portion and a thin portion. As a technique for solving this problem, Japanese Patent Laying-Open No. 62-166013 discloses that the cooling stop temperature is controlled by changing the cooling time of the front end and the tail end of the steel plate in the accelerated cooling device, and the material in the longitudinal direction of the steel plate is controlled. A technique for reducing the change is disclosed. In this technology, by controlling the cooling stop temperature to a predetermined temperature,
Although an attempt is made to reduce the difference in material characteristics due to the difference in steel plate thickness, the difference in cooling rate due to the changing plate thickness still occurs. Therefore, there is a problem that it is difficult to keep the material characteristics constant in a material having a large cooling rate dependency.

Further, as described above, when the plate thickness is different,
It is necessary to change the tempering parameter P according to the plate thickness. For this reason, when heat-treating a plurality of steel plates having different plate thicknesses, it is necessary to change the heat treatment conditions according to the plate thicknesses, and the heat treatment efficiency decreases.

[0005]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention keeps constant the material properties of a high-strength steel plate having a taper thickness and a plurality of high-strength steel plates having different thicknesses heat-treated in the same lot. An object of the present invention is to provide a method for producing a high-strength steel sheet that can be manufactured.

[0006]

A method for producing a high-strength steel sheet according to the present invention for achieving the above object is C: 0.03 to 0.20 wt% Si: 0.05 to 0.50 wt% Mn: 0. 30 to 2.50 wt% Al: 0.01 to 0.10 wt% N: 0.007 wt% or less, and further Cu: 0.05 to 1.30 wt% Ni: 0.10 to 10.0 wt% Cr: 0.05 to 1.50 wt% Mo: 0.03 to 0.50 wt% V: 0.01 to 0.15 wt% Nb: 0.005 to 0.06 wt% B: 0.0003 to 0.0020 wt% Ca: 0.0005 to 0.0040 wt% Rare earth element: 0.001 to 0.020 wt% Steel containing at least one element selected from the group consisting of the balance Fe and unavoidable impurities, Ar ₃
Hot rolling at a finishing temperature of not less than the point, rapid cooling from the temperature of the Ar ₃ point or more to just above the martensite transformation start temperature and gradual cooling to room temperature to transform into a bainite structure, and Ac
It is characterized by tempering to a temperature of ₁ point or less.

Here, C: 0.05 to 0.15 wt% Si: 0.10 to 0.40 wt% Mn: 1.0 to 2.0 wt% Al: 0.020 to 0.050 wt% preferable.

[0008]

First, the reasons for limiting the chemical components will be described.
C is an element necessary to secure hardenability and strength, and for this purpose, it is necessary to add 0.03 wt% or more. On the other hand, if the addition amount exceeds 0.20 wt%, the base material toughness and the HAZ (welding heat affected zone) toughness deteriorate, so the addition amount is made 0.20 wt% as the upper limit. Here, when C is in the range of 0.05 to 0.15 wt%, the hardenability and strength are further improved, and the base metal / HAZ toughness is further improved.

Si is an element effective in promoting deoxidation and improving strength, so 0.05% by weight or more is added, but excessive addition causes deterioration of base material toughness and HAZ toughness, so the addition amount is Si. The upper limit is 0.50 wt%. Where S
When i is in the range of 0.10 to 0.40 wt%, deoxidation is further promoted to improve the strength, and the base metal / HAZ
The toughness is further improved.

Mn is effective for improving the strength without impairing the toughness, and for this purpose, it is necessary to add at least 0.30 wt% or more. However, excessive addition deteriorates workability, so the content was limited to 2.50 wt% or less. Here, when Mn is in the range of 1.00 to 2.00 wt%, the deterioration of toughness is further prevented and the strength is further improved.

Al needs to be added in an amount of at least 0.01 wt% for deoxidizing steel and refining the structure.
If added more than necessary, a large amount of oxide-based inclusions are generated in the steel and the toughness of the steel is significantly deteriorated, so the upper limit was made 0.10 wt%. Here, Al is 0.020 to
Within the range of 0.050 wt%, deoxidation of steel is further promoted, and refinement of the structure is further promoted.

[0012] N combines with Al to become AlN, which has an effect of preventing coarsening of crystal grains during heating of a slab. However, if a large amount of N is contained, the HAZ toughness deteriorates, so the N content was made 0.007 wt% or less. Cu is
It is effective for solid solution strengthening and precipitation strengthening. In order to obtain the former effect, addition of 0.05 wt% or more is necessary, while in order to obtain the latter effect, addition of 0.5 wt% or more is necessary. However, when it exceeds 1.30 wt%, the further effect is small in any strengthening and it is not economical.

Ni has the effect of significantly improving toughness and shifts the ductile / brittle transition temperature to the low temperature side, and is therefore effective in the production of low temperature steel. In order to obtain those effects, it is necessary to add 0.10 wt% or more. But 1
A useful low temperature steel cannot be obtained even if an amount exceeding 0.0 wt% is added. Cr is an element useful for improving hardenability,
Therefore, it is necessary to add 0.05 wt% or more. However, addition of more than 1.50 wt% causes deterioration of weldability.

Mo is an element effective for improving hardenability and temper softening resistance. In order to exert these effects, it is necessary to add 0.03 wt% or more. However, even if added in excess of 0.50 wt%, the cost increases greatly and it is not economical. V is an element effective in increasing the strength by precipitation strengthening. In order to exert this effect, 0.
Addition of 01 wt% or more is required. But 0.15
Addition in excess of wt% deteriorates weldability and HAZ toughness.

Nb is an element effective in preventing coarsening of austenite grains during heating of a slab, refining grains during rolling, and strengthening precipitation during tempering. In order to exert these effects, addition of 0.005 wt% or more is required. However, addition of more than 0.06 wt% deteriorates HAZ toughness. B has the effect of improving the hardenability by adding a trace amount. In order to exert this effect,
It is necessary to add 0.0003 wt% or more, but 0.0
If it exceeds 020 wt%, the toughness is deteriorated by the B compound.

Ca is formed by spheroidizing MnS.
Improve toughness. In order to exert this effect,
It is necessary to add 0.0005 wt% or more, but 0.0
If it exceeds 040 wt%, an increase of oxidative inclusions is caused, resulting in deterioration of toughness. REM (rare earth element) is Ca
Improves toughness by a mechanism similar to. In order to exert this effect, it is necessary to add 0.001 wt% or more, but if it exceeds 0.020 wt%, oxidative inclusions are increased and toughness is deteriorated.

Next, the reasons for limitation such as heat treatment conditions will be described. The slab heating temperature is preferably in the temperature range of 1000 to 1300 ° C. The reason for this is that a temperature of 1000 ° C. or higher is necessary to sufficiently form a solid solution with the added components,
On the other hand, if the temperature exceeds 1300 ° C., the austenite grains become coarse, and the coarsening is not promoted even in the subsequent rolling, and the toughness deteriorates. Therefore, 1000 to 1300
It is preferable to heat within the temperature range of ° C.

Further, in hot rolling, it is preferable to introduce working strain in order to obtain high strength and high toughness.
Therefore, in order to effectively introduce the working strain, it is preferable that the rolling reduction at 900 ° C. or less and at Ar ₃ point or more is 40% or more, and the upper limit of the rolling finishing temperature is 900 ° C. The steel sheet that has undergone the above rolling process is transformed into a bainite structure by being rapidly cooled to just above the martensite transformation start temperature (Ms point) and then gradually cooled to room temperature. This process is performed by, for example, M
It is performed by water cooling to just above the s point and then air cooling to room temperature.

The inventors of the present invention investigated the tempering characteristics of a steel sheet which was hot-rolled under the above-mentioned conditions using a steel having the chemical composition shown in Table 1, after which water cooling was started and water cooling was stopped immediately above the Ms point.
It has been found that the strength is less sensitive to the tempering condition as compared with the conventional one in which water cooling is stopped below the Ms point. Examples thereof are shown in FIGS. FIG. 1 is a graph showing the tempering characteristics of the steel whose water cooling was stopped below the Ms point, and FIG. 2 is a graph showing the tempering characteristics of the steel whose water cooling was stopped just above the Ms point. As can be seen from FIGS. 1 and 2, by stopping the water cooling just above the Ms point in the taper steel plate, the tempering characteristics of the thin part having a high cooling rate and the tempering property of the thick part can be obtained. The properties can be made close to each other, and the tensile strength after tempering of each part of the tapered steel plate can be made substantially uniform. In addition, for steel plates with different plate thicknesses, the cooling rate is fast, and the tempering characteristics of thin steel plates are
It is possible to approximate the tempering characteristics of a thick steel plate, and even if the thin steel plate and the thick steel plate are tempered under the same heat treatment conditions, the tensile strength after tempering can be made substantially equal. Here, it is preferable that the cooling stop temperature is just above the Ms point at which the martensitic transformation does not start, from the viewpoint of the strength guarantee, but in consideration of the plate temperature control technique in the actual operation, the Ms point to the Ms point. It is desirable to set it within the range of + (50 to 100 ° C.).

[0020]

[Table 1]

As described above, according to the present invention, when directly quenching a steel sheet which is held at a high temperature after hot rolling, M
By stopping the quenching immediately above the s point, the quenching process is terminated in the state of the bainite transformation structure. Therefore, even if the conditions (temperature and holding time) in the tempering process after the quenching process are variously changed, the change in tensile strength after tempering will be small, and the tempering characteristics of thick and thin parts will be almost uniform. You can Therefore, it becomes possible to make the tensile strength of the tapered steel plate after tempering substantially uniform. Further, as described above, the tempering characteristics of the thin steel sheet having a high cooling rate can be brought close to the tempering characteristics of the thick steel sheet, so that both can be tempered under the same heat treatment condition. . For this reason, it becomes possible to temper the steel sheets having the same chemical composition and different thicknesses in the same lot, and chance-free in the tempering operation is achieved and the heat treatment efficiency is improved. The toughness equivalent to or better than the conventional method can be obtained by using the control technology applied to ordinary untempered material (TMCP steel), that is, the technology of low temperature heating or powder recrystallization zone rolling + double layer zone rolling. Can be obtained.

[0022]

EXAMPLE An example of the method for producing a high-strength steel sheet according to the present invention will be described below. [First Example] Tapered steel plates having the respective component compositions shown in Table 1 above were subjected to slab heating, hot rolling and quenching according to the conditions shown in Table 2. Test pieces were taken from each of the thus obtained taper steel plates having different plate thicknesses and subjected to a tensile test. The results are also shown in Table 2.

[0023]

[Table 2]

As shown in Table 2, the difference in tensile strength between the portions having a plate thickness of 15 mm and the portion having a plate thickness of 30 mm is 50 to 80 MPa in the comparative example, whereas the deviation is 30 MPa in the steel sheet according to the present invention. Uniform properties were obtained. As described above, in the above example, since the predetermined thickness heat treatment was applied to the plate thickness taper steel plate with limited chemical composition,
Even if the tempering conditions after tempering (temperature and holding time) are variously changed, the strength change after tempering is small, and the tempering characteristics of thick and thin parts can be made almost uniform.
It is possible to manufacture a high-strength steel plate having a taper of plate thickness with a small difference in tensile strength between portions having different plate thicknesses. [Second Example] A steel sheet having each component composition shown in Table 1 was subjected to slab heating, hot rolling and direct quenching according to each condition shown in Table 3, and then tempered under the same heat treatment condition for each steel type. Carried out. Test pieces were taken from each of the steel plates thus obtained and subjected to a tensile test. The results are also shown in Table 3. In Table 3, the test No. 1 and test N
o. 2 combination, test no. 3 and test No. Like the combination of 4, the cooling rate and the plate thickness are different for each combination in ascending order of the numbers.

[0025]

[Table 3]

As shown in Table 3, the plate thickness of the comparative example is 15 mm.
The difference in tensile strength between the steel plate of No. 3 and the steel plate of 30 mm thickness is 50 to 8
While the difference was 0 Mpa, the difference was 30 Mpa for the steel sheets under the same conditions in the examples, and it was found that the difference in tensile strength was small even under the same heat treatment conditions. As described above, since the predetermined heat treatment is applied to the steel plates having different chemical compositions and different thicknesses, the sensitivity of the tensile strength of each steel plate to the tempering condition can be made dull. For this reason, the tempering characteristics of the thin steel sheet can be made close to the tempering characteristics of the thick steel sheet, and both can be tempered under the same heat treatment conditions. As a result, steel plates of the same composition with different plate thickness,
It is possible to carry out tempering treatment in the same lot, a chance-free operation in tempering operation is achieved, and heat treatment efficiency is improved.

In the above embodiment, 600 Mpa
Although the case of steel has been described, the present invention is not limited to this, and it goes without saying that the present invention can be applied to 700 Mpa steel and 800 Mpa steel.

[0028]

As described above, according to the method for producing a high-strength steel sheet of the present invention, since the steel having a limited chemical composition is subjected to the predetermined heat treatment, the tempering conditions (temperature and holding time after the quenching are performed. ), The change in tensile strength after tempering becomes small, and the tempering characteristics can be made almost uniform even if the sheet thickness differs, and a high-strength steel sheet with a small difference in tensile strength can be manufactured.

[Brief description of drawings]

FIG. 1 is a graph showing the tempering characteristics of steel whose water cooling is stopped below the Ms point.

FIG. 2 is a graph showing the tempering characteristics of steel whose water cooling has been stopped immediately above the Ms point.

Claims (1)

[Claims] 1. C: 0.03 to 0.20 wt% Si: 0.05 to 0.50 wt% Mn: 0.30 to 2.50 wt% Al: 0.01 to 0.10 wt% N: 0.007 wt % Or less, and further, Cu: 0.05 to 1.30 wt% Ni: 0.10 to 10.0 wt% Cr: 0.05 to 1.50 wt% Mo: 0.03 to 0.50 wt% V: 0.01 to 0.15 wt% Nb: 0.005 to 0.06 wt% B: 0.0003 to 0.0020 wt% Ca: 0.0005 to 0.0040 wt% Rare earth element: 0.001 to 0.020 wt% made containing at least one element selected from the group, the steel and the balance Fe and unavoidable impurities, hot rolling at a temperature finish _three or more points Ar, martensitic transformation starting from Ar ₃ point or more temperature Quench immediately above temperature It is transformed into bainite structure gradually cooled to room temperature Te, the method of producing a high-tensile steel plate, characterized in that tempered to a temperature below _a point Ac.