CH640569A5 - PROCESS FOR THE MANUFACTURE OF STEEL LAMINATES, APPLICATION OF THIS PROCESS FOR THE MANUFACTURE OF CONCRETE ROUND. - Google Patents

Description

The present invention relates to a steel laminate process, intended to have good weldability, a high yield strength, as well as resilience at very low temperatures. These steel laminates can typically be concrete bars

It is well known that concrete rods of current manufacture with an elastic limit of the order of 400 N / mm2 have only a very low toughness. Thus their transition temperature for the Charpy V impact test at 35 J / cm2 is of the order of + 20 ° C. It follows that these products offer only a weak brittle resistance at low temperatures.

In the past a Charpy V resilience at 35 J / cm2 and at temperatures of the order of - 196 ° C was not required for s common uses made of rolled steel such as concrete reinforcing bars. However, recent developments in the technology for manufacturing and especially for the storage of liquefied gases have led to the need to have fairly large quantities of rolled steel resistant to intense cold. For example, it is planned, for safety reasons, to provide the liquefied gas storage tanks with a reinforced concrete envelope in analogy to the safety measures applied in the field of nuclear reactors.

To be usable for the construction of the framework of 15 such envelopes of liquefied gas tanks which are exposed to temperatures of - 50 ° C to - 196 ° C the concrete reinforcing bars must present in addition to an adequate resilience at heart , satisfactory weldability, which implies carbon contents of less than 0.2%. However, the concrete rods 20connus, which have 0.16-0.2% C, are subjected to a cold twisting, which on the one hand has the effect of giving them a satisfactory elastic limit but this which, on the other hand, results in low tenacity, especially at low temperatures.

An attempt has also been made, for example, during the course of the manufacture of the concrete reinforcing bars with a carbon content of at most 0.20%, to a treatment comprising intense cooling on the surface at the outlet of the rolling mill,

as well as a subsequent self-income. This made it possible to obtain weldable and tenacious concrete rounds without these rounds, however, having Charpy V-35 J / cm2 transition temperatures significantly better than - 50 ° C.

So far only the concrete rods made from a 9% Ni alloy steel, which have undergone a double normalization followed by tempering, or else a temperature followed by tempering, have a resilience Charpy V- 35 J / cm2 at minimum at -196 ° C.

Thus, the object of the invention is to propose a process for the manufacture of laminates meeting the criteria mentioned in the first paragraph, this process being feasible still hot 40 at the outlet of the rolling, without intermediate reheating, preferably from a steel containing a minimum of expensive alloying elements, thus reducing the cost price of the laminates produced.

According to the invention, this object is achieved by the presence of the characters set out in the appended claim 1.

It is understood that the concentrations of the various elements present or added, as well as the temperatures of the various treatment phases, will only vary within certain limits.

50 Thus the chemical composition of the steel used is chosen according to the experiences collected during numerous tests which revealed that the carbon content is less than 0.20%

will be all the lower the lower the transition temperature. For example, carbon is preferably limited to 0.08% max. for a transition temperature Charpy V-35 J / cm2 at -140 "C.

A maganese content of the order of 1.7% gives the desired resilience to the steel while improving its toughness, while a silicon content of the order of 0.3% is favorable for reinforcing the resistance.

In addition, it is important to calm the fine grain steel to the aluminum to improve the weldability and considerably reduce the tendency to aging. The refinement of the ft5grain also raises the elastic limit, as well as the toughness.

The Nb and possibly the V and / or the Mo will guarantee a high elastic limit especially for concrete rounds of large diameters.

3

640,569

According to the invention the product is subjected to the particular thermomechanical cycle described during which the temperature of the product is controlled for all the operations before, during and after rolling, the total reduction rate obtained during these passes must be significant c ' that is to say preferably greater than 20%.

It should be emphasized that the purpose of the particular thermo-mechanical cycle, which is part of the process according to the invention, is to obtain an extremely fine grain structure at the heart of the product, since the resilience at very low temperatures must be guaranteed at the heart of the laminated product.

To achieve this result, the temperature of the oven is carefully chosen on one side according to the rules of the art so as to avoid a grain enlargement on the semi-finished product. On the other hand, the temperature at the start of rolling will be such that a considerable grain refinement is obtained from the start of rolling and that too great recrystallization is avoided.

Before the last three rolling passes, the steel undergoes rapid cooling in a cooling ramp to a temperature close to the transformation point Ar3.

The cooling treatment at the outlet of the rolling mill consists of another vigorous cooling of the product, to a sufficiently low core temperature, in order to avoid any recrystallization.

The idea which is the basis of the present invention therefore consists in combining the beneficial effects achievable separately thanks to the judicious choice of the chemical elements incorporated in determined contents in steel with the effects of a directed interaction between the evolution of the temperature of the product during manufacture and the reduction rates applied during rolling.

The result obtained is materialized in particular by an extremely fine grain size of the finished product.

To more easily produce products with transition temperatures below at least - 140 ° C, it is possible according to the invention to use a steel which has nickel contents of the order of 5%, but less than 10% . Such steel, rolled according to the process according to the invention, has a Charpy V-35 J / cm2 resilience at - 196 ° C.

The advantages of the process according to the invention emerge clearly from the six experiments described below:

1. The rolling of naturally hard steel for concrete reinforcing bars (C = 0.35% approximately) results in a product having satisfactory tensile characteristics, including in particular an elastic limit exceeding 400 MPa.

However, the transition temperature for the Charpy V test at an energy level of 35 J / cm2 is only + 20 C. This steel therefore exhibits no toughness at low temperature. Its weldability is poor.

2. A steel with a carbon content limited to 0.8%, which undergoes the treatment according to the invention before, during and after rolling, also offers satisfactory tensile characteristics. A clear improvement compared to Example 1 is made from the point of view of elongation and in particular of the transition temperature, which drops to -60 ° C. The steel turns out to be weldable.

3. The use of a steel having the chemical composition defined in the present application, but not undergoing treatment according to the invention, leads to mechanical characteristics which are insufficient from the point of view of elasticity and resistance limit. The elongation is high. The transition temperature, even without treatment, is however already approximately at the same level as that of Example 2, where the treatment according to the invention was applied before, during and after rolling.

4. The combination of the chemical composition and the treatments before, during and after the rolling according to the invention make it possible to achieve the desired double improvement, namely:

- the satisfactory mechanical characteristics and the high length,

- the transition temperature falling to a very low temperature of at least - 140 ° C.

5. A 9% Ni steel, in the normal hot rolling state, achieves a transition temperature Charpy V-35 J / cm2 at

io - 50 ° C.

6. The same 9% Ni steel, whose resilience at -196 ° C, is normally only achievable using an expensive heat treatment (double normalization + tempering or quenching + tempering), allows arrive at a transition temperature of

15-196 ° C when applying the recommended treatment before, during and after rolling.

The 6 examples are briefly summarized in table 1 in the appendix.

For the process according to the invention, the importance of the chemical zocompositon of the steel as well as that of the temperature control during the manufacturing operations and of the rapid post-rolling cooling is demonstrated by the following six experiments:

1. A naturally hard steel for concrete reinforcing bars 25 (C = 0.35% approximately) treated according to the process of the invention is hardened to the core. This steel has a high yield strength, but the ductility properties are very low.

For the Charpy V test, even at room temperature, the energy level does not exceed 35 J / cm2.

2. The use of a steel showing the chemical composition determined in the present application, but without application of the treatment according to the invention leads to mechanical characteristics which are insufficient from the point of view of elasticity and resistance limit. The transition temperature of the Charpy V re-35silience is 60 ° C.

3. The same rolled steel without thermomechanical treatment, but with post-rolling cooling has an elastic limit and a resistance significantly higher than for example 2. The transition temperature (- 75 ° C) is

40 also improved compared to Example 2.

4. By introducing into the implementation diagram of Example 3 still a thermomechanical rolling, but only for the last passes the elasticity limit and the resistance are further improved. It is the same for the

45 transition temperature (—100 ° C).

5. By carrying out a thermomechanical treatment for all the rolling operations, but without post-rolling cooling, the elastic limit and the resistance are lowered compared with examples 3 and 4. The cutting temperature

50 sition however is further improved (—115 ° C).

6. The best results concerning the mechanical properties of the steel showing the chemical composition determined in the present application are obtained by the treatment according to the invention, before, during and after rolling, at

55 know:

- satisfactory mechanical characteristics and high elongation.

- a very low Charpy V resilience transition temperature.

The abovementioned results are shown in above-mentioned form in the attached Table 2.

Although the present description is focused on the production of concrete bars, the process according to the invention can just as easily be applied to other merchant steels such as 65 smooth, flat, square, angle bars, to sections and to sheets , as long as one wishes to combine the properties of weldability, high yield strength and resilience at very low temperatures in the same product.

Example No. Type of steel

Treatment according to the invention before, during and after rolling

Yield strength (MPa)

Resistance (MPa)

Elongation 10 d (%)

Temp. temperature ° C, Charpy V resilience at 35 J / cm2

N "example

Type of steel

Oven temperature

Start of rolling temperature

Intermediate rapid cooling

End of rolling temperature

Post-rolling rapid cooling

Temperature after post-rolling cooling

Yield strength (MPa)

Resistance (MPa)

Elongation (5 d)%

Transition temperature (° C)

for Charpy V resilience at 35 J / cm2

1

C = 0.35% semi-calmed

no

440 650 13

+ 20 1

C = 0.35% semi-calmed

controlled controlled yes controlled yes controlled

980 980 5%

+ 2 ° C

C = 0.18% semi-calmed

Yes

470 570 25 -60

C = 0.05% subsided Al

1,200 ° C

not controlled not not controlled no

320 480 34% -60 ° C

VS

C = 0.08% calmed

no

C = 0.08% calmed

Yes

9% Neither calmed

no

9% Neither calmed

Yes

320 500 25 -60

490 570 30

-140

890 1010 9

-50

710 940 13

-196

C = 0.05% subsided Al

1200 ° C

not controlled no not controlled yes controlled

C = 0.05% subsided Al

1200 ° C

not controlled yes controlled yes controlled

C = 0.05% calmed Al controlled controlled yes controlled no

C = 0.05% calmed Al controlled controlled yes controlled yes controlled

430 530 32% -75 ° C

470 550 31% -100 ° C

380 465 36% —115 ° C

490 580 32% -140 ° C

Claims (9)

640,569 CLAIMS 1. A method of manufacturing rolled steel, intended to have good weldability, a high yield strength, as well as resilience at very low temperatures, characterized in that a steel containing carbon C, manganese Mn, silicon Si, aluminum Al and nio-bium Nb, that this steel is rolled while ensuring that the total reduction rate, achieved during the last three passes, is greater than 20% and that the temperature T1 for the operations before rolling is adjusted, the temperature T2 for the operations during the rolling, and the temperature T3 for the operations after the rolling of the product having a certain diameter D, so as to ensure that the yield strength LE and the resilience KCV at —120 ° C reach the desired high values, expressed respectively by LE = 1035 +510 C + 192 Mn +2270 Nb - 0.21 T1 - 0.40 T2- 0.48 T3 - 3.51 D and KCV = 2202 - 2066 C +23.20 Mn - 2064 Nb - 0.77 T1 - 1.24 T2 - 0.23 T3 - 1.98 D formulas in which C is the carbon content, Mn is the maganese content, Nb is the niobium content, D being expressed in mm, LE being expressed in MPa and KCV being expressed in Joules / cm2. 2. Method according to claim 1, characterized in that a lower carbon content of 0.20% is chosen, the lower the lower the transition temperature. 3. Method according to claim 1, characterized in that the elastic limit of the steel is raised by operating a fine grain cal-mage with aluminum at a content of at least 0.03%. 4. Method according to claim 1, characterized in that a nickel content of less than 10% is chosen, the higher the lower the transition temperature. 5. Method according to claim 1, characterized in that one regulates the rolling start temperature by creating in the oven a controlled temperature, less than 1200 ° C. 6. Method according to claims 1-5, characterized in that one proceeds before the last three rolling passes to rapid cooling of the steel to a temperature corresponding to the transformation point Ar3. 7. Method according to claims 1 to 6, characterized in that one proceeds to the end of rolling vigorous cooling of the steel to a sufficiently low core temperature to avoid recrystallization. 8. Application of the method according to claim 1 to the manufacture of concrete reinforcing bars. 9. Application according to claim 8 for the manufacture of concrete reinforcing bars according to the process defined in one of claims 2 to 7.