FR2481321A1 - PROCESS FOR MANUFACTURING STEEL LAMINATES HAVING GOOD WELDABILITY, HIGH LIMIT OF ELASTICITY AND RESILIENCE AT VERY LOW TEMPERATURES - Google Patents

Abstract

THE SUBJECT OF THE PRESENT INVENTION IS A PROCESS FOR THE MANUFACTURING OF STEEL ROLLS, IN PARTICULAR CONCRETE ROUNDS, PRESENTING A GOOD WELDABILITY, A HIGH LIMIT OF ELASTICITY, AS WELL AS RESILIENCE TO VERY LOW TEMPERATURES. THE PROCESS IS CHARACTERIZED IN THAT WE USE A STEEL CONTAINING CARBON C, MANGANESE MN, SILICON SI, ALUMINUM AL AND NIOBIUM NB, WHETHER THIS STEEL IS LAMINATED ENSURING THAT THE RATE TOTAL REDUCTION, ACHIEVED DURING THE LAST THREE LAST PASSES, IS GREATER THAN 20 AND THAT THE TEMPERATURES ARE ADJUSTED FOR OPERATIONS BEFORE T1, DURING T2 AND AFTER T3 THE ROLLING OF THE PRODUCT WITH A DIAMETER D, SO AS TO ENSURE THAT THE LIMIT OF ELASTICITY AND RESILIENCE AT -120C KCV, REACH THE HIGH VALUES DESIRED. IT CONCERNS A PROCESS FOR MANUFACTURING STEEL LAMINATES SHOWING GOOD WELDABILITY, A HIGH LIMIT OF ELASTICITY AND RESILIENCE TO VERY LOW TEMPERATURES.

Description

- 2481321

  PROCESS FOR PRODUCING STEEL LAMINATES HAVING A GOOD

  WELDABILITY, A HIGH LIMIT OF ELASTICITY AND RESILIENCE TO

VERY LOW TEMPERATURES.

  The present invention relates to a process for the manufacture of steel laminates, in particular concrete reinforcing bars, having a good reliability, a high yield strength, as well as resilience to

very low temperatures.

  It is well known that concrete reinforcing bars

  rante with a yield strength of the order of 400 N / mm 2 have only a very low toughness. Thus, their transition temperature for the Charpy V resilience test at 35 J / cm 2 is of the order of + 20 ° C. he

  as a result, these products offer little resistance to rup-

  fragile at low temperatures.

  In the past a Charpy V resilience at 35 J / cm2 and time

  temperatures of about -196VC were not required for

  current operations made from steel rolled products such as rebars. However, the recent evolution of manufacturing technology and especially of

  storage of liquefied gases led to the need for quan-

  rather large quantities of cold-hardened steel laminates.

  For example, for reasons of safety, provision is made for liquefied gas storage tanks to be provided with a reinforced concrete envelope, as provided by the safety measures applied in

field of nuclear reactors.

  To be used for the constitution of the frame of such envelopes of liquefied gas tanks which are exposed

  at temperatures of -50OC to -196 ° C, concrete reinforcing bars should be

  in addition to adequate resilience, weldability

  satisfactory, which implies carbon contents of less than 0.2%.

  However, known concrete reinforcing bars, which contain 0.16 to 0.2% C, are

  to a cold twist, which has the effect of their 2 48s3 2> confer a satisfactory yield strength but which leads to

  on the other hand a low tenacity, especially at low temperatures.

  It has also been tried, for example, to subject even in the course of the manufacture of reinforcing bars with a carbon content of at most 0.20% to a treatment comprising an intense surface cooling at the exit of the rolling mill. as well as a subsequent autorevenu. This has made it possible to obtain weldable and stubborn concrete grids without these rounds, however, having Charpy V-35 transition temperatures.

  J / cm2 significantly better than - 50OC.

  So far only concrete reinforcing bars made from a 9% Ni alloy steel, which have undergone a double standardization followed

  income, or tempering followed by income,

  Charpy silience V-35 J / cm2 minimum at -1960C.

  Thus, the present invention relates to a method of

  laminates meeting the criteria mentioned in the first

  paragraph, this process being feasible ex hot rolling, preferably

  from a steel containing a minimum of alloying elements

  expensive, thus reducing the cost price of rolled products.

  The process according to the invention is characterized in that a steel containing carbon (C), manganese (Mn), silicon (Si), aluminum (Al) and niobium (Nb) is used. ), that this steel be laminated by ensuring that the total reduction rate realized during the last three passes is greater than 20% and that the temperatures are set for operations before (Tl) during ( T2) and after (T3) the rolling of the product having a diameter (D), so as to ensure that the elasticity limit (LE) and the resilience -1200C (KCV), expressed by LE = 1035 + 510C + 192Mn + 2270Nb- 0.21T1- 0.42T2- 0.48T3- 3.51D and KCV = 2202- 2066C + 23.2OMn- 2064Nb-0.77T1- 1.24T2- 0.23T3- 1.98D

  reach the desired high values.

3 2 48 132 1

  It is understood that the concentrations of the various elements present or added, as well as the temperatures of the different

  treatment phases, will vary only within certain limits.

  Thus, the chemical composition of the steel used is chosen according to the experiments collected during numerous tests which have been revealed that the carbon content of less than 0.20% will be even lower if a temperature of lower transition. For example, the carbon is preferably limited to 0.08% max. for a

  Charpy transition temperature V-35 J / cm 2 at-140 ° C.

  A manganese content of about 1.7% gives the resi-

  lience to steel while improving its toughness, while a silicon content of about 0.3% is conducive to enhance strength. In addition, it is important to temper fine-grained steel to aluminum to improve weldability and significantly reduce the aging trend. Grain refining is also

  the yield strength, as well as toughness.

  The Nb and possibly the V and / or the Mo will guarantee a

  yield strength, especially for large diameter concrete reinforcing

very.

  According to the invention, the product is subject. to the thermomene cycle

  particular case during which the temperature of the product is controlled for all operations before, during and after rolling,

  the total reduction rate obtained during these passes must be significant.

  so, that is to say preferably greater than 20%.

  It should be emphasized that the thermo-mechanical cycle

  the purpose of the process according to the invention is to ob-

  a very fine grain structure at the heart of the product, since resilience at very low temperatures must be ensured at

core of the rolled product.

  To achieve this result we choose on one side the temperature

  of the oven judiciously following the rules of the art so as to avoid

  a magnification of the grain on the half-product. On the other hand, the temperature

  the beginning of rolling will be such that we obtain from the beginning of

  considerable grain refinement and avoid recrystallization.

too important.

  Before the last three rolling passes, the steel undergoes a

4 2 48 1321

  rapid cooling in a cooling ramp up to a

  temperature close to the transformation point Ar3.

  The cooling treatment at the exit of the rolling mill

  another energetic cooling of the product until a tem-

  at low enough temperature, to avoid any recrystallization. - The idea underlying the present invention is therefore to combine the beneficial effects achievable separately through the

  judicious choice of chemical elements incorporated into specific levels

  steel with the effects of a directed interaction between the evolution of

  the temperature of the product being manufactured and the rates of

  reduction applied during rolling.

  The result obtained is materialized in particular by a size

  extremely fine grain of the finished product.

  To make it easier to produce products with tempera-

  At least 1400 ° C., according to the invention, it is possible to use a steel which has nickel contents of the order of 5% but less than 10%. Such a steel, laminated according to the process according to the invention, has a Charpy V-35 J / cm2 resilience.

at - 1960C.

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

Example 1

  The rolling of a naturally hard steel for concrete reinforcing bars (C = approximately 0.35%) results in a product with characteristics

  satisfactory tensile strength, including a yield strength exceeding

health 400 LaPa.

  However the transition temperature for the Charpy V test

  at an energy level of 35 J / cm2 is only + 20'C. This steel does not

  so feels no toughness at low temperatures. Its weldability is

diocre.

Example 2

  A carbon-limited steel of 0.18%, which undergoes the treatment according to the invention before, during and after rolling, also offers satisfactory tensile characteristics. A net

24 8 3 2

  improvement compared to Example 1 consists of the aspect of elongation and in particular the transition temperature, which goes down

  at - 60OC. The steel turns out to be weldable.

Example 3

  The use of a steel having the chemical composition

  defined in this application, but not subject to

  according to the invention, leads to insufficient mechanical characteristics.

  in terms of yield strength and strength. The elongation is high. The transition temperature, even without treatment, is however already at approximately the same level as that of Example 2, where the treatment according to the invention has been applied before, during and after rolling.

Example 4

  The combination of chemical composition and treatments

  before, during and after the rolling according to the invention make it possible to

  to the double improvement coveted to know:

  the satisfactory mechanical characteristics and the

  high; the transition temperature down to a temperature

very low of at least -140'C.

Example 5

  9% Ni steel in the normal hot rolling state

  at a Charpy V transition temperature - 35J / cm2 - 50% C.

Example 6

  The same 9% Ni steel, whose resilience at - 1960C is normally achievable only with the help of an expensive heat treatment (double normalization + tempering or tempering ± income), makes it possible to reach a temperature of transition of - 196oC when applying treatment

  recommended before, during and after rolling.

  The 6 examples are succinctly summarized in the

I annexed.

  For the process according to the invention, the importance of the composition

  the chemical control of steel as well as that of temperature control during manufacturing operations and rapid post-mill cooling is evidenced by the following six experiments: 6 2 48 132e

Example 7

  Naturally hard steel for concrete reinforcing bars (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

  calf energy does not exceed 35 J / cm2.

Example 8

  The implementation of a steel with the chemical composition determined in the present application, but without application of the treatment

  according to the invention leads to insufficient mechanical characteristics.

  in terms of yield strength and strength. Temperature

  Transpy Charpy V is at 60C.

Example 9

  The same rolled steel without thermomechanical treatment, but with a post-mill cooling has a limit of elasticity

  and a much higher resistance than for Example 8. The temperature

  transition (-750C) is also improved compared to

example 8.

  EXAMPLE 10 By introducing in the embodiment of Example 9 still a thermomechanical rolling, but only for the last passes, the elastic limit and the resistance are further improved. he

  the same is true for the transition temperature (-100 ° C).

  EXAMPLE 11 By carrying out a thermomechanical treatment for all the rolling operations, but without post-rolling cooling, the yield strength and the resistance are lowered compared to

  Examples 9 and 10. However, the transition temperature is still

lioree (-115 ° C).

Example 12

  The best results concerning the mechanical properties of steel with the chemical composition determined in this paper

TABLE I

  N example 1 2 3 4 5 6 Type of steel C = 0.35% C = 0.18% C = 0.08% C = 0.08% 9% Ni 9% Ni semi-calmed semi-calmed calmed calmed calmed Treatment according to the invention before, no yes no yes no yes during and after rolling Yield of elasticity (MPa) 440 470 320 490 890 710 Resistance (MPa) 650 570 500 570 1010 940 Elongation 10 d (%) 13 25 25 30 9 13 Temp. transition C, Resilience + 20 - 60 - 60 - 140 - 50 - 196 Charpy V to 35 J / cm2 Co ru

TABLE II

  N example 7 8 9 10 12 Type of steel C = 0.35% C = 0.05% C = 0.05% C = 0.05% C = 0.05% C = 0.05% semi-calming calmed AL calmed Al calmed Al calmed Al alleviated Al Temperature controlled furnace 1200 OC 1200 C 12000 controlled controlled, temperature controlled rolling start uncontrolled uncontrolled controlled non controlled controlled cooling. fast intermediate or no no yes yes Controlled end of rolling temperature uncontrolled controlled non controlled controlled controlled

Cooling down

  post-milling pide yes no. yes yes no yes Temperature after post-mill cooling controlled. - controlled controlled - controlled Yield strength (MPa) 980 320 430 470 380 490 Strength (MPa) 980 480 530 550 465 580 Elongation (5d) o% 5% 34% 32% 31% 36% 32%

Transition temperature

  sition (C) for the> + 2 C- 60 C 75 C - 100 C - 115 C - 140 C Charpy V resiliency at 35 J / cm2 ro () 4 1>

2 481321

  are obtained by the treatment according to the invention, before, during and after rolling, namely:

  satisfactory mechanical characteristics and an allon-

  high level; a very low Charpy V resiliency transition temperature.

  The above results are represented in a succes-

in annexed Table II.

  This example illustrates non-limiting conditions for

of the process of the invention.

  The proportions of carbon, manganese and niobium in the steel used are as follows:

C = 0.08

  Mn = 1.60% Nb = 0 and the following temperature parameters Tl = 1 0000C

T2 = 800 ° C

T3 = 650 ° C

  the following elastic limit values (LE) and resilience at -120 ° C (KCV) are obtained for a diameter D = 16 mm. LE = 469 MPa KCV = 130 Joules at -20 ° C

  Although this description focuses on production

  from concrete reinforcing bars, the method according to the invention can equally well be applied to other commercial steels such as smooth round bars, flat bars, square bars, angles, profiles and sheets, as long as it is desired to combine the properties weldability, high yield strength and

  Resilience at very low temperature in the same product.

  Of course, the present invention is not limited to

  embodiments described and shown; it is likely to

  Many variants accessible to those skilled in the art, depending on the applications

  contemplated and without departing from the spirit of the invention.

l 2 48 13 21

Claims (7)

R E V E N D I C A T I 0 N S   1. Process for the manufacture of steel laminates, in particular   concrete reinforcing bars, with good weldability, a high limit of   elasticity and resilience at very low temperatures,   in that a steel containing carbon (C), manganese (Mn), silicon (Si), aluminum (Al) and niobium (Nb) is used, that steel is laminated ensuring that the total reduction rate realized during the last three passes is greater than 20%   and set temperatures for operations before (T1),   (T2) and after (T3) the rolling of the product having a diameter (D), so as to ensure that the elasticity limit (LE) and the resilience at 120 ° C (KCV), expressed by LE = 1035+ 510C + 192Mn + 2270Nb-0.21T1-0.42T2-0, 48T3-3.51D and KVC = 2202-2066C + 23.20Mn-2064Nb-0.77T1-1.24T2-0.23T3-1.98D   reach the desired high values.   2. Method according to claim 1, characterized in that one chooses a carbon content of less than 0.20% all the lower   that we aim for a lower transition temperature.   3. A method according to claim 1 or 2, characterized in that one increases the elastic limit of the steel by performing a calming   fine grain with aluminum at a minimum of 0.03%.   4. Process according to any one of claims 1 to 3,   It is characterized in that a nickel content of less than 10% is chosen which is all the higher as a lower transition temperature is aimed for.   5. Process according to any one of claims 1 to 4,   characterized in that the rolling start temperature is set   creating in the oven a controlled temperature, less than 1200 ° C.   6. Process according to any one of claims 1 to 5,   characterized in that prior to the last three rolling passes the steel is cooled rapidly to a temperature   corresponding to transformation point Ar3.   7. Process according to any one of claims 1 to 6,   characterized in that the rolling is terminated by vigorous cooling of the steel to a sufficiently low core temperature   to avoid any recrystallization.