JPH0645821B2 - Method for producing non-heat treated low temperature steel excellent in brittle crack propagation arresting property - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a non-heat treated low temperature steel excellent in brittle crack propagation arresting property, and more specifically, for ensuring the safety of a steel structure. In particular, the present invention relates to a method for producing a non-heat treated low temperature steel for use in, for example, an LPG tank or the like, which requires excellent brittle crack propagation stopping properties.

[Prior Art] Conventionally, a 50 kgf / mm ² class low temperature steel has been manufactured by performing heat treatment such as normalizing or quenching and tempering depending on the toughness level of the steel. The increase in manufacturing cost associated with heat treatment has become a problem.

With the recent progress in steel sheet manufacturing technology, low temperature steel with excellent low temperature toughness as rolled can be industrialized by utilizing the strength and toughness improvement by controlled rolling or controlled cooling, and the above problems can be solved. became.

In particular, controlled cooling that is performed immediately after controlled rolling can significantly improve strength and toughness without increasing the carbon equivalent and the amount of alloying elements, and this controlled cooling technology will become the main manufacturing technology for low temperature steel in the future. Conceivable.

However, when controlled cooling is performed, residual stress occurs in the steel sheet due to thermal stress during cooling. In general, this residual stress exhibits compressive stress on the surface side of the steel sheet, tensile stress near the center of the steel sheet, and when uneven cooling exists, compressive or tensile residual stress alternates in the width direction of the steel sheet. Occurs in. Therefore, the controlled-cooled steel sheet shows a complicated residual stress distribution in which the residual stress in the sheet thickness direction and the residual stress in the sheet width direction are superimposed.

Then, when these residual stresses are present in the steel sheet, in particular, when a large tensile residual stress close to the yield point of the base material is present, the development of brittle cracks is promoted by this tensile residual stress, and thus the brittleness of the steel sheet. There is a problem that the crack propagation stop characteristic value is lowered.

In order to prevent the deterioration of this brittle crack propagation stopping property, after controlled cooling aiming to reduce the residual stress of the steel sheet,
Although stress relieving annealing or tempering treatment is performed,
This causes a rise in manufacturing cost, and omitting these heat treatments is a major technical issue.

[Problems to be Solved by the Invention] In the present invention, in view of the above-described problems of the conventional technology, as a result of intensive research by the present inventor, as a result of performing special light reduction rolling after controlled cooling, the controlled cooling is performed. By alleviating the residual stress that has occurred, we have developed a method for manufacturing non-heat treated low temperature steel that has excellent brittle crack propagation arrest properties as-rolled without the need for heat treatment after controlled cooling.

[Means for Solving Problems] A method for producing a non-heat treated low temperature use steel excellent in brittle crack propagation arresting property according to the present invention is (1) C0.02 to 0.10 wt%, Si0.03 to 0.60 wt% %, Mn 0.7 to 2.0 wt%, Al 0.005 to 0.08 wt%, Ti 0.005 to 0.03 wt%, with the balance being Fe and unavoidable impurities.
After heating to 900 to 1150 ° C., a cumulative rolling reduction of 50% or more is performed in the non-recrystallization temperature range, and Ar _{3 to} Ar ₃ +
Rolling is completed in the temperature range of 40 ° C., immediately thereafter cooling is performed to a temperature of 2 to 20 ° C./sec at a cooling rate of 600 ° C. or less, and after cooling is stopped, once in a temperature range of less than 600 ° C. to 200 ° C. or more. A first invention is a method for producing a non-heat treated low temperature use steel excellent in brittle crack propagation arresting characteristics, characterized by performing a reduction of 0.5 to 2% and a cumulative reduction of 1 to 5%. (2) C0.02 to 0.10 wt%, Si0.03 to 0.60 wt%, Mn0.7 to 2.0 wt%, Al0.005 to 0.08 wt%, Ti0.005 to 0.03 wt%, and Nb0. One or more selected from 05wt% or less, V0.1wt% or less, Cu0.5wt% or less, Ni1.0wt% or less, Cr0.5wt% or less, Mo0.5wt% or less, B0.003wt% or less Containing steel and the balance Fe and unavoidable impurities,
After heating to 900 to 1150 ° C., a cumulative rolling reduction of 50% or more is performed in the non-recrystallization temperature range, and Ar _{3 to} Ar ₃ +
Rolling is completed in the temperature range of 40 ° C., immediately thereafter, it is cooled to a temperature of 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec, and after cooling is stopped, once in a temperature range of less than 600 ° C. to 200 ° C. or more. And a reduction ratio of 0.5 to 2% and a cumulative reduction ratio of 1 to 5%.
(3) C0.02 to 0.10 wt%, Si0.03 to 0.60 wt%, Mn0.7 to 2.0 wt%, Al0.005 to 0.08 wt%, Ti0.005 to 0.03 wt%, Furthermore, a steel slab containing one or more selected from Ca 0.0005 to 0.005 wt% and REM 0.0005 to 0.01 wt% with the balance Fe and unavoidable impurities,
After heating to 900 to 1150 ° C., a cumulative rolling reduction of 50% or more is performed in the non-recrystallization temperature range, and Ar _{3 to} Ar ₃ +
Rolling is completed in the temperature range of 40 ° C., then immediately cooled to a temperature of 600 ° C. or lower at a cooling temperature of 2 to 20 ° C./sec, and after cooling is stopped, once in a temperature range of less than 600 ° C. to 200 ° C. or higher. And a reduction ratio of 0.5 to 2% and a cumulative reduction ratio of 1 to 5%.
(4) C0.02 to 0.10 wt%, Si0.03 to 0.60 wt%, Mn0.7 to 2.0 wt%, Al0.005 to 0.08 wt%, Ti0.005 to 0.03 wt%, Furthermore, one selected from Nb0.05wt% or less, V0.1wt% or less, Cu0.5wt% or less, Ni1.0wt% or less, Cr0.5wt% or less, Mo0.5wt% or less, B0.003wt% or less Alternatively, a steel slab containing two or more kinds and one or more kinds selected from Ca 0.0005 to 0.005 wt% and REM 0.0005 to 0.01 wt% with the balance Fe and inevitable impurities is contained. ,
After heating to 900 to 1150 ° C., a cumulative rolling reduction of 50% or more is performed in the non-recrystallization temperature range, and Ar _{3 to} Ar ₃ +
Rolling is completed in the temperature range of 40 ° C., immediately thereafter, it is cooled to a temperature of 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec, and after cooling is stopped, once in a temperature range of less than 600 ° C. to 200 ° C. or more. And a reduction ratio of 0.5 to 2% and a cumulative reduction ratio of 1 to 5%.
The invention consists of four inventions.

The method for producing the non-heat treated low temperature use steel excellent in the brittle crack propagation stopping property according to the present invention will be described in detail below.

First, a description will be given of contained components and component ratios of steel used in the method for producing a non-heat treated low temperature use steel excellent in brittle crack propagation arresting property according to the present invention (hereinafter sometimes simply referred to as the production method according to the present invention). To do.

When the content of C is less than 0.02 wt%, the strength of the steel sheet is lowered, the softening of the weld heat affected zone becomes large, and when the content of C exceeds 0.10 wt%, the toughness of the matrix deteriorates. Therefore, the C content is 0.02 to 0.10 wt%.

At least 0.03 wt% of Si must be contained in the steel for deoxidation, and if it exceeds 0.60 wt%, the toughness of the steel sheet and the toughness of the heat-affected zone of the weld are reduced. Therefore, the Si content is 0.03 to 0.60 wt%.

If the content of Mn is less than 0.7 wt%, the strength and toughness of the steel sheet will decrease, and if it exceeds 2.0 wt%, the toughness of the weld heat affected zone will deteriorate. Therefore, the Mn content is 0.7 to 2.0 wt.
%.

At least 0.005 wt% of Al must be contained in the steel for deoxidation, and if it exceeds 0.08 wt%, the toughness of the weld metal and weld heat affected zone deteriorates. Therefore, the Al content is set to 0.005 to 0.08 wt%.

Ti is an element effective in improving the low temperature toughness of the steel sheet and the toughness of the heat affected zone of welding, and if the content is less than 0.005 wt%, this effect is small, and if it exceeds 0.03 wt%, it is welded. The toughness of the heat-affected zone decreases. Therefore, the Ti content is
0.005 to 0.03 wt%

In addition to these essential components, in order to improve strength, Nb,
One or more selected from V, Cu, Ni, Cr, Mo and B can be contained.

Nb is an element that improves the strength and toughness of the steel sheet, and if the content exceeds 0.05 wt%, the toughness of the weld heat affected zone deteriorates. Therefore, the Nb content is 0.05 wt% or less.

V is an element that improves the strength and toughness of the steel sheet, and if the content exceeds 0.1 wt%, the toughness of the weld heat affected zone deteriorates. Therefore, the V content is 0.1 wt% or less.

Cu is an element that improves the strength and corrosion resistance of steel sheets,
If the content exceeds 0.5 wt%, cracks are likely to occur during hot rolling. Therefore, the Cu content is 0.5 wt%
Below.

Ni is an element that improves the strength and toughness of the steel plate without adversely affecting the toughness of the heat-affected zone of welding, but it is expensive and in the sense of suppressing manufacturing cost increase, it is contained as a content necessary to achieve the purpose. It should be 1.0 wt% or less.
Therefore, the Ni content should be 1.0 wt% or less.

Cr is an element that improves the strength and corrosion resistance of the steel sheet,
If the content exceeds 0.5 wt%, the toughness of the weld heat affected zone deteriorates. Therefore, the Cr content is 0.5 wt% or less.

Mo is an element that improves the strength and toughness of steel sheets,
Since it is an expensive element, it is preferable that the content is 0.5 wt% or less as a content necessary for achieving the purpose in order to suppress an increase in manufacturing cost. Therefore, the Mo content is 0.5 wt% or less.

B is an element contained for increasing the strength of the steel sheet, and if the content exceeds 0.003 wt%, the toughness of the weld heat affected zone deteriorates. Therefore, the B content is 0.003 wt% or less.

Furthermore, in order to improve toughness, one or two selected from Ca and REM can be contained.

Ca is an element that is included to control the morphology of MnS and to improve the toughness in the C direction of the steel sheet. If the content is less than 0.0005 wt%, it is less effective, and if it exceeds 0.005 wt%, it is contained in the steel. The number of non-metallic inclusions increases and causes internal defects. Therefore, the Ca content is 0.0005 to 0.005 wt%.

REM is an element that gives the same effect as Ca, and if the content is less than 0.0005 wt%, this effect is small, and 0.01 wt%
%, The content of non-metallic inclusions in steel increases,
It causes internal defects. Therefore, the REM content is 0.0005-
0.01 wt%

Next, heat treatment conditions in the manufacturing method according to the present invention will be described.

It is 11 that the heating temperature of the billet is 900 to 1150 ° C.
When the temperature exceeds 50 ° C, the austenite grains during heating become large, and even if controlled rolling is performed, the grain size cannot be reduced, the toughness of the steel sheet is low, and Nb, etc. at temperatures below 900 ° C. This is because the solid solution amount of is reduced, and it becomes difficult to obtain a predetermined strength. Therefore, the heating temperature is 900-1
The temperature is 500 ° C.

The reason why the cumulative rolling reduction is 50% or more in the non-recrystallization temperature region is to improve the toughness of the steel sheet, and the rolling reduction is 5%.
If it is less than 0%, coarse bainite and martensite are mixed in the microstructure after controlled cooling, and the toughness is significantly deteriorated.

To the end temperature of rolling and Ar ₃ ~Ar ₃ + 40 ℃ begins the transformation from some austenite to ferrite is less than Ar _3, subsequent strength increasing effect be subjected to controlled cooling of will not be sufficiently exhibited, Further, the toughness of the steel sheet is good up to Ar ₃ + 40 ° C., but if it exceeds this temperature, coarse bainite is generated by the controlled cooling thereafter, and the toughness is remarkably lowered.

The cooling rate of the controlled cooling performed immediately after this controlled rolling is 2 to
20 ° C./sec means that the strength increasing effect can hardly be expected when the cooling rate is less than 2 ° C./sec.
The reason is that if it exceeds, most of the structure becomes bainite or martensite, and the toughness of the steel sheet is significantly reduced.

The cooling stop temperature is set to 600 ° C. or lower in order to maximize the strength increasing effect by the controlled cooling. Above this temperature, it is necessary to contain an expensive alloying element to secure the strength.

After stopping the cooling, the rolling reduction is 0.5 to 2% once in the temperature range of less than 600 ° C. to 200 ° C. or more, and the cumulative rolling reduction is 1
As shown in FIG. 1, it is possible to significantly reduce the residual stress of the steel sheet generated by the controlled cooling by performing the reduction of up to 5%. By reducing this residual stress, the residual stress of the steel sheet can be reduced. The brittle crack propagation stopping property can be significantly improved.

The temperature range of less than 600 ° C. to 200 ° C. or more is effective in reducing the residual stress by performing the reduction at a temperature of less than 600 ° C. immediately after the controlled cooling.
This is because reduction at a temperature of less than 0 ° C. lowers the internal quality of the steel sheet, that is, the defects due to hydrogen in the steel are likely to occur, and the flatness of the steel sheet deteriorates.

The rolling reduction in one time in this temperature range is 0.5 to 2%, as shown in FIG. 2, when the rolling reduction is less than 0.5%, the residual stress reducing effect is small, and the rolling reduction is When it exceeds 2%, this effect is saturated, and the flatness of the steel sheet becomes worse at a rolling reduction higher than that.

Further, the cumulative rolling reduction is set to 1 to 5%, as shown in FIG. 3, when the cumulative rolling reduction is less than 1%, the residual stress reducing effect is small, and when the cumulative rolling reduction exceeds 5%, the effect is reduced. Is saturated, and if the rolling reduction is further increased, the toughness of the steel sheet is significantly reduced due to work hardening. Therefore, the cumulative rolling reduction is set to 1 to 5%.

[Example] An example of a method for producing a non-heat treated low temperature use steel excellent in brittle crack propagation arresting property according to the present invention will be described.

Examples Table 1 shows steels (A, B, and B) used in the manufacturing method according to the present invention.
C, D, E, F, G, H) and steels (I,
The components and the component ratios of J) are shown below.

Table 2 shows the heating → rolling → cooling → special light reduction conditions, the mechanical properties of the base metal, and the brittle crack propagation arresting property (Kca value) of the sample steel having a plate thickness of 30 mm produced by the usual melting method. This brittle crack propagation stopping property was judged by conducting a temperature gradient type double tensile test.

The following is clear from Table 2.

Comparing the steel sheets of A1 and A2, A2, which has a high heating temperature and is out of the conditions specified in the manufacturing method according to the present invention, has large crystal grains and deteriorates in toughness and Kca value.

Comparing the steel sheets of B1 and B2, the reduction ratio in the non-recrystallization temperature range was small, and B2 which was out of the conditions specified in the manufacturing method according to the present invention contained coarse bainite in the microstructure after controlled cooling, and thus toughness and The Kca value has deteriorated.

Comparing the steel sheets of C1 and C2, C2, which has a high rolling end temperature and which is out of the conditions specified in the manufacturing method according to the present invention, has coarse toughness and
The Kca value has deteriorated.

Comparing the steel sheets of D1, D2, and D3, D2 not subjected to controlled cooling has a low strength, TS is less than 50 kgf / mm ² , and D3 has a high residual stress in the steel sheet, and thus has a low Kca value.

Comparing the steel sheets of E1 and E2, the cumulative reduction ratio under the special light reduction is high, and E2, which is out of the conditions specified in the manufacturing method according to the present invention, has high strength and deteriorates toughness, and thus the Kca value is deteriorated.

In the steel sheets of F1 and F2, Kca value of F2 which is not subjected to special light reduction is low.

Both the G and H steel sheets were manufactured under the conditions specified in the manufacturing method according to the present invention, and had good toughness and Kca values, and in particular, contained KCa and REM which are morphology control elements for inclusions. Therefore, the toughness is good.

Both I and J are out of the steel composition specified in the manufacturing method according to the present invention, and have extremely low toughness and KCa value.

[Effects of the Invention] As described above, the method for producing a non-heat treated low temperature use steel excellent in brittle crack propagation arresting property according to the present invention has the above-mentioned constitution. By performing a specific treatment, a steel sheet having excellent toughness and brittle crack propagation arresting properties can be produced without performing an off-line heat treatment.

[Brief description of drawings]

Fig. 1 is a diagram showing the effect of reducing the residual stress in the steel sheet by the special light reduction, and Fig. 2 shows the relationship between the reduction ratio of one time of the special light reduction and the reduction ratio of the tensile residual stress at the center of the plate thickness. Figure, third
The figure shows the relationship between the cumulative rolling reduction of less than 600 ° C. and the reduction ratio of the tensile residual stress at the center of the plate thickness under special light rolling.

Claims (4)

[Claims] 1. C0.02-0.10wt%, Si0.03-0.60wt%, Mn0.7-2.0wt%, Al0.005-0.08wt%, Ti0.005-0.03wt%, balance Fe And a steel slab consisting of inevitable impurities,
After heating to 900 to 1150 ° C., a cumulative rolling reduction of 50% or more is performed in the non-recrystallization temperature range, and Ar _{3 to} Ar ₃ +
Rolling is completed in the temperature range of 40 ° C., immediately thereafter, it is cooled to a temperature of 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec, and after cooling is stopped, once in a temperature range of less than 600 ° C. to 200 ° C. or more. A rolling reduction of 0.5 to 2% and a cumulative rolling reduction of 1 to 5% are performed. 2. C0.02-0.10wt%, Si0.03-0.60wt%, Mn0.7-2.0wt%, Al0.005-0.08wt%, Ti0.005-0.03wt%, and One or two selected from Nb0.05wt% or less, V0.1wt% or less, Cu0.5wt% or less, Ni1.0wt% or less, Cr0.5wt% or less, Mo0.5wt% or less, B0.003wt% or less A steel slab containing at least one of the above, with the balance Fe and unavoidable impurities,
After heating to 900 to 1150 ° C., a cumulative rolling reduction of 50% or more is performed in the non-recrystallization temperature range, and Ar _{3 to} Ar ₃ +
Rolling is completed in the temperature range of 40 ° C., immediately thereafter, it is cooled to a temperature of 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec, and after cooling is stopped, once in a temperature range of less than 600 ° C. to 200 ° C. or more. A rolling reduction of 0.5 to 2% and a cumulative rolling reduction of 1 to 5% are performed. 3. C0.02-0.10wt%, Si0.03-0.60wt%, Mn0.7-2.0wt%, Al0.005-0.08wt%, Ti0.005-0.03wt%, and further, A steel slab containing one or more selected from 0.0005 to 0.005 wt% Ca and 0.0005 to 0.01 wt% REM, and the balance Fe and unavoidable impurities,
After heating to 900 to 1150 ° C., a cumulative rolling reduction of 50% or more is performed in the non-recrystallization temperature range, and Ar _{3 to} Ar ₃ +
Rolling is completed in the temperature range of 40 ° C., immediately thereafter, it is cooled to a temperature of 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec, and after cooling is stopped, once in a temperature range of less than 600 ° C. to 200 ° C. or more. A rolling reduction of 0.5 to 2% and a cumulative rolling reduction of 1 to 5% are performed. 4. C0.02-0.10wt%, Si0.03-0.60wt%, Mn0.7-2.0wt%, Al0.005-0.08wt%, Ti0.005-0.03wt%, and further, One or two selected from Nb0.05wt% or less, V0.1wt% or less, Cu0.5wt% or less, Ni1.0wt% or less, Cr0.5wt% or less, Mo0.5wt% or less, B0.003wt% or less A steel slab containing at least one kind, and containing at least one selected from Ca 0.0005 to 0.005 wt% and REM 0.0005 to 0.01 wt% and the balance Fe and unavoidable impurities,
After heating to 900 to 1150 ° C., a cumulative rolling reduction of 50% or more is performed in the non-recrystallization temperature range, and Ar _{3 to} Ar ₃ +
Rolling is completed in the temperature range of 40 ° C., immediately thereafter, it is cooled to a temperature of 600 ° C. or less at a cooling rate of 2 to 20 ° C./sec, and after cooling is stopped, once in a temperature range of less than 600 ° C. to 200 ° C. or more. A rolling reduction of 0.5 to 2% and a cumulative rolling reduction of 1 to 5% are performed.