WO2008058410A1

WO2008058410A1 - Method for the continuous production of steel wire or bar

Info

Publication number: WO2008058410A1
Application number: PCT/CH2007/000558
Authority: WO
Inventors: Lotfi Chabbi; Ulrich Urlau
Original assignee: Swiss Steel Ag
Priority date: 2006-11-17
Filing date: 2007-11-12
Publication date: 2008-05-22
Also published as: EP2089552A1; ES2623430T3; EP2089552B1; PL2089552T3

Abstract

The invention relates to a method for the continuous production of steel wire or bar, wherein a first hot-forming is carried out by rolling at a temperature above the recrystallization temperature, and wherein the overall degree of forming during the first hot-forming is at least 60%. Next, the rolled steel is cooled to below the recrystallization temperature, and then a final forming takes place at a temperature in the range of the non-recrystallized austenite. The cooldown is carried out in a manner that inhibits a potential growth of grains in the austenite rolled steel before the final forming. The overall degree of forming during the final forming is at least 30%. Finally, the rolled steel is cooled down to a predetermined heat holding temperature at the Ar1 temperature and left at said temperature for a predetermined heat holding time.

Description

Process for the continuous production of wire or bar steel

Technical area

The invention relates to a method for the continuous production of wire or bar steel.

State of the art

In the production of wire and bar steels there is a continuing need for improved processes, which are optimized in particular in terms of cost, logistics but also with respect to the properties of the products.

An important approach to process optimization is to shorten the production process, which can be achieved by, for example, saving heat treatment (see Ball, J. et al., "Process Reduction by Saving Heat Treatments in the Production of Wire and Rod"). Stahl and Eisen 11 (1997) No. 4, pp. 59-67). In particular, it has been shown that temperature-controlled rolling in certain steels can save the heat treatment for GKZ annealing or that a better quality annealing structure is available, and that the tempering treatment can be omitted for certain steels (see Ball, loc. Cit).

In CN 1088265 A a generic method is described in which steels with a medium high carbon content of 0.26 to 0.5 wt .-% are processed as follows:

a) first, a first hot working by rolling is carried out at a temperature between Ac ₃ + 100 ⁰ C and Ac ₃ , the Gesamtumformgrad is at least 50%; b) then the rolling stock is cooled to a temperature between Ar ₃ + 10O ⁰ C and Ar ₃ and a second hot working carried out, the Gesamtumformgrad is at least 20%;

c) thereafter the rolling stock is cooled to a temperature between Ar ₃ and Ar-j and a third hot working is carried out to the final dimension, the total degree of deformation being at least 25%;

d) Finally, the rolling stock is placed in an oven with a temperature between Ac-] and Ac-I - 100 ⁰ C and held there for at least 30 minutes, then to be cooled to room temperature.

The above method is characterized in that the final forming always takes place in the two-phase region and that reheating or, at best, holding at the final rolling temperature is always necessary after final shaping. The transition to the heat retention temperature is therefore not effected by the cooling process (compare transition from (6) to (7) in FIG. 2 of CN 1088265 A).

DESCRIPTION OF THE INVENTION The object of the invention is to provide an improved process for the continuous production of rod or wire steel, which permits a good control of the properties of the process products and at the same time simplifies the process control.

This object is achieved by the method defined in claim 1.

The process according to the invention comprises the following process stages:

a) in a first process step, a steel with a weight fraction of 0.02 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.5% Manganese, up to 0.035% phosphorus, up to 0.004% sulfur, up to 0.5% molybdenum, up to 1.7% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel and up to 0.25% vanadium as well further admixtures usually contained in steel to 1 O50 to 1,200 ⁰ C heated, then a first hot deformation by rolling at a temperature above the recrystallization is carried out, wherein the cumulative reduction ratio is in the first hot working at least 60%;

b) in a second process step, the rolling stock is cooled below the recrystallization temperature and then a final deformation at a

Temperature is carried out in the region of the non-recrystallized austenite, wherein the cooling is carried out in such a way that possible grain growth of the austenite is prevented in the rolling stock before the final shaping and the total degree of deformation in the final deformation amounts to at least 30%; and

c) in a third process step, the rolling stock is cooled without reheating to a predetermined holding temperature by the Ar ^ temperature and left for a predetermined holding time at the said warming temperature.

Surprisingly, it has been found that the products produced by the process according to the invention after cooling to room temperature are suitable for further cold processing without annealing. Unlike the conventional methods, it is therefore not necessary to reheat the wire or bar steel produced. In addition, it has been found that the heat retention time to be applied in the third method step of the method according to the invention is significantly shorter than the duration of the subsequent heat treatment in the case of the conventional methods. As a result, the process duration and the process chain in the process according to the invention are thus significantly shorter than in the conventional processes. - A -

In contrast to the method described in CN 1088265 A, the method according to the invention requires only two instead of three forming steps. This first of all ensures that only one controlled intermediate cooling is required instead of two such cooling operations between the forming steps. This simplifies the process and the required facility. Moreover, in the process according to the invention, the transition to the holding temperature takes place directly in the course of the cooling process. Reheating to the holding temperature is therefore not required, which not only simplifies the process, but also allows energy savings.

Preferred embodiments of the method are defined in the dependent claims.

In particular, the method according to the invention is suitable on the one hand for steels with a comparatively low proportion by weight of 0.02 to 0.2% carbon and up to 0.25% molybdenum and up to 0.008% boron; On the other hand, it is also suitable for steels with a medium to high weight content of 0.2 to 0.65% carbon and 0.25 to 0.50% molybdenum and up to 1.7% chromium. The holding temperature must be selected as explained below.

In the preferred process of claim 2, the steel has a weight fraction of 0.02 to 0.2% carbon and up to 0.15% silicon, 0.25 to 1.0% manganese, up to 0.025% phosphorus, up to 0.004% sulfur, up to 0.25% molybdenum, bis to 0.1% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel and up to 0.1% vanadium, the heat hold temperature being substantially at Ar ₁ ⁶ .

In contrast, in the preferred process according to claim 3, the steel has a weight fraction of 0.2 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.0% manganese, up to 0.035% phosphorus, up to 0.004% sulfur,

0.25 to 0.50% molybdenum, up to 1.7% chromium, up to 0.25% copper, up to 0.2% nickel and up to 0.1% vanadium, wherein the holding temperature substantially in the range of Ar ₁ ¹³ to Ar _| ^e lies.

Here, Ar ₁ ¹⁵ and Ar ₁ ⁶ denote in a known manner the beginning (upper index "b") and the end (upper index "e") of the austenite-perlite transformation in the cooling direction (where "r" stands for "refroidissement" ).

According to a preferred embodiment, the final deformation is carried out at a temperature above Ar ₃ . In particular, according to claim 4, the final transformation is carried out at most Ar ₃ + 6O ⁰ C. As a result, the required holding time can be reduced to about half compared to the known rolling process.

According to a further embodiment, the final deformation is carried out at a temperature just below Ar ₃ . In particular, according to claim 5, the final deformation is carried out at a temperature in the range of Aη to Ar ₃ , wherein the Endumformtemperatur 690 ⁰ C does not fall below. This allows a further shortening of the required holding time to about one-eighth of the known rolling process.

Brief description of the drawings

Exemplary embodiments of the invention will be described in greater detail below with reference to the drawings, in which:

Fig. 1 shows an apparatus for the continuous production of wire or

Bar steel, in a schematic representation, in plan view;

Fig. 2 is a temperature-time profile for a rolling process according to the prior art; 3 shows a temperature-time profile for a first embodiment of the rolling method according to the invention;

4 shows a temperature-time profile for a second embodiment of the rolling method according to the invention; and

Fig. 5 structure of quenched samples after the last stitch as a function of the final rolling temperature (steel C).

Ways to carry out the invention

The rolling apparatus shown in Fig. 1 has not shown driving means for a steel strand 2, which convey it in a direction of advance V through the device. As will be explained in more detail below, certain assemblies of the rolling apparatus are used only for certain final dimensions, or depending on the final dimension, a certain function is taken over by one or the other assembly. First, however, the rolling process will be described generally.

Starting overall of an oven 4 with a temperature of from 1O50 to 1,200 ⁰ C reaches the steel strand 2 to a first hot deformation device 6, where a first hot deformation by rolling at a temperature above the recrystallization takes place. Depending on the final dimension of the first hot deformation is carried out by six to twenty stitches at a temperature above 950 ⁰ C.

Subsequently, the steel strand passes through a first cooling device 8 (for bar steel) or 8a (for wire), by means of which it is cooled below the recrystallization temperature. Thereafter, the steel strand passes into a second hot working device 10 (for bar steel) or 10a (for wire), where a final reforming is carried out at a temperature in the region of the non-recrystallized austenite. Finally, the beam strand is optionally supplied to a wire station 14 or to a bar station 16. These stations are equipped with second cooling devices 18 (for wire) and 20 (for bar steel) as well as with unheated holding devices for the cut-to-length rolled product in the form of wire rings or rods.

In the production of bar steel, the first hot rolling device 6 is formed by a first stand group 22. In the case of a comparatively thick steel bar, the first cooling device 8 is a direct water cooling system 24 in the area of the first stand group 22. For comparatively thin bar steel, a waiting loop 26 with continuous cooling 28 is used as the first cooling device 8. If necessary, both cooling variants can be used. The second hot rolling apparatus 10 may be formed by a downstream second stand group 30 as in the arrangement shown here. Subsequently, the rolling stock reaches the bar station 16, which as mentioned includes a second cooling device 20.

In the production of wire with a diameter of, for example, up to 22 mm, the first hot rolling device 6 comprises the first stand group 22 and the second stand group 30. The first cooling device 8a is configured as a downstream waiting loop 32, which advantageously comprises a water cooling 34. The second hot rolling apparatus 10 a is formed by a downstream third skeleton group 36. Subsequently, the rolling stock reaches the wire station 14, which includes a second cooling device 18 as mentioned.

2 to 4 show on the basis of the relevant temperature-time profiles a comparison between a previously known rolling process (FIG. 2) and two variants of the process according to the invention (FIGS. 3 and 4). When prior art process according to FIG. 2 of the steel strand is initially heated to about 1,150 ⁰ C. Thereafter, a hot working is carried out by rolling at a temperature above 950 ⁰ C, which is above the recrystallization temperature. In the example shown this is done by 6 stitches. Subsequently, the rolling stock, depending on the final dimension or rolling station, within 5 to

10 s to 820 to 920 ⁰ C and then cooled to room temperature with a cooling rate of 0.3 to 1.9 ° C / s. The product thus obtained is not suitable for cold processing, but it requires a heat treatment lasting several hours. For example, the product must be reheated to a temperature of 680 to 720 ^° C. and typically held at that temperature for about 14 hours. The product treated in this way can finally be cooled to room temperature and is then available for cold further processing.

In the methods of FIGS. 3 and 4, first a first hot working is carried out above the recrystallization temperature, wherein the degree of deformation in this process step is at least 60%. Subsequently, however, the rolling stock is - unlike in the previously described method of Fig. 2 - initially cooled below the recrystallization temperature and then made a final deformation at a temperature in the region of the non-recrystallized austenite. The total degree of deformation in the final forming is at least 30% and is accomplished in the examples shown by two stitches. It is essential that the cooling below the recrystallization temperature is carried out in such a way that no grain growth of the austenite takes place in the rolling stock before the final shaping.

In the variant of Figure 3, the final forming takes place at a temperature which is above Ar ₃ ; in the variant of Fig. 4, the final rolling temperature is even lower and is just below Ar ₃ in the range between Ar ^ and Ar ₃ . As schematically indicated in FIGS. 3 and 4, the final forming could Depending on the rolling station, dimension or rolling speed cause re-heating of the rolling stock. This circumstance must be taken into account, ie it is necessary to avoid that the temperature temporarily rises again above the desired final rolling temperature.

In a third process step, the rolling stock is substantially on the Ar ₁

Temperature, for example, at 680 to 72O ⁰ C, cooled and then left at this holding temperature for a certain holding time. A rewarming process is accordingly not required. Finally, the product is cooled to room temperature.

The keep-warm time is determined in preliminary tests. It is chosen so that the properties of the products after cooling to room temperature are comparable to those after conventional rolling and additional heat treatment. This ensures that the product is cold processed without further heat treatment.

As can be seen from the comparison of FIGS. 3 and 4, by shortening the final rolling temperature combined with a specific modified cooling process, a shortening of the required holding time can be achieved, which in the case of FIG. 3 is approximately 7 hours and in the case of FIG. 4 only 2 to 4 hours.

It is understood that to set the conditions described, the WaIz device must be dimensioned or set up accordingly. In particular, the controlled cooling processes are realized by means of water cooling and possibly by means of waiting loops. In addition, depending on the type and dimension of the product to be produced (ie wire steel, steel bars) different numbers and positions of the rolling stands are selected. Keeping warm takes place - also depending on the type of product - in suitable holding facilities. Basically, the method described for steels with a weight fraction of 0.02 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.5% manganese, up to 0.035% phosphorus, up to 0.004% sulfur, up to 0.5% molybdenum, up Use 1.7% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel and up to 0.25% vanadium and other common steel admixtures. Three examples of such compositions are given in Table 1.

Table 1: Steel compositions in% by weight

Examples

The following examples were carried out on a pilot plant. In all cases, the minimum required heat treatment time or holding time was determined in preliminary tests, which leads to a cold processable product. However, since at the end no wire rings or long bars are laid down on the test facility, the necessary heat treatment times or holding times in the present examples are significantly shorter than in a corresponding production facility. Accordingly, the following heat treatment times or holding times are about 3.5 times shorter than on a production line.

Comparative Example 1 (Prior Art)

A steel with the chemical composition "A" according to Table 1 was heated to 1'150 ⁰ C and then rolled in 6 passes, the initial diameter was 38 mm and the final diameter 17.2 mm. This matches with a total degree of deformation of 80%. In this case, the final rolling temperature at about 980 ⁰ C. was Ansch read send the rolled stock has been s (depending on the rolling station) cooled within 6 seconds at 820-960 ⁰ C and then at a cooling rate of 0.3 to 1.9 ° C / room temperature. The product thus obtained was then subjected to a heat treatment at 680 ^° C. for 4 hours.

example 1

A steel having the same composition as in Comparative Example 1 was heated to 1'15O ⁰ C and then rolled into 4 stitches, wherein the initial diameter of 38 mm and the final diameter was 23.8 mm. This corresponds to a total degree of deformation at the first hot deformation of 60%. The temperature was after the 4th stitch at about 1O20 ° C. Subsequently, the

Rolling stock after intensive cooling within 13 seconds in another 4

Stitches rolled from 23.8 to 17.2 mm. This corresponds to a total conversion rate of 47% for the final forming. The final rolling temperature was

88O ⁰ C, which is about 20 ⁰ C above Ar ₃ . After the last stitch that became

Rolled material cooled to 680 ⁰ C within 6 seconds, which is substantially Ar-) ^e and left for 2 hours at this temperature.

Example 2

A steel having the same composition as in Comparative Example 1 was heated to 1'15O ⁰ C and then rolled into 4 stitches, wherein the initial diameter of 38 mm and the final diameter was 23.8 mm. It was the

Temperature after the 4th stitch at about 1'020 ⁰ C. The mixture was then rolled WaIz- the well after an intensive cooling within 13 seconds in a further 2 stitches of 23.8 to 17.2 mm. The final rolling temperature was 85O ⁰ C which is just about 10 ⁰ C below Ar ₃ . After the last pass, the rolling stock was cooled to 68O ⁰ C within 6 seconds, which was essentially Ar ^e and leave at this temperature for 1 hour. The mechanical properties of steel A rolled products are summarized in Table 2. It can clearly be seen that targeted level conversion to Ar ₃ combined with controlled cooling by Ar ₁ without prior reheating and holding at that temperature can achieve a level of properties comparable to that after conventional forming followed by expensive heat treatment. The required holding time was reduced to half to one quarter of the usual annealing time. The product can be processed cold in this condition.

Table 2: Mechanical properties of steel rolled products A

Comparative Example 2 (prior art)

A steel having the chemical composition according to "C" in Table 1 was heated to 1,150 ⁰ C and then rolled in 6 passes, wherein the initial diameter was 17.2 mm and the final diameter of 38 mm. This corresponds to a Gesamtumformgrad of 80%. The final rolling temperature was approx.

980 ⁰ C. Subsequently, the rolling stock was cooled in 5 seconds to about 820 to 97O ⁰ C and then with a cooling rate of 0.3 to 1.9 ° C / s to room temperature. The product thus obtained was then subjected to a heat treatment at 680 ⁰ C for 4 hours.

Example 3

A steel having the same composition as in Comparative Example 2 was heated to 1,150 ⁰ C and then rolled into 4 stitches, wherein the initial diameter of 38 mm and the final diameter was 23.8 mm. This matches with a total degree of deformation at the first hot deformation of 60%. The temperature was after the 4th stitch at about 1'020 ° C. After intensive cooling, the rolling stock was then rolled within 13 seconds in another 2 passes from 23.8 to 17.2 mm. This corresponds to a total conversion rate of 47% for the final forming. In this case, the final rolling temperature 800 ⁰ C, which is above Ar ₃ was. After the last pass, the rolling stock was cooled to 680 ⁰ C within 6 seconds, which was between Ar | | ^b and Ar-) ⁶ and left at this temperature for 2 hours.

Example 4

A steel having the same composition as in Comparative Example 2 was heated to 1,150 ⁰ C and then rolled into 4 stitches, wherein the initial diameter of 38 mm and the final diameter was 23.8 mm. The temperature was after the 4th stitch at about 1'020 ⁰ C. The mixture was then rolled WaIz- the well after an intensive cooling within 13 seconds in a further 2 stitches of 23.8 to 17.2 mm. The final rolling temperature was 690 ⁰ C, which is below Ar ₃ . After the last stitch, the rolling stock was within

Cooled to 680 ⁰ C for 6 seconds, which is between Ar- ₎ ^b and Ar ^ ^e , and left for 0.5 hours at this temperature.

Fig. 5 shows the structure of quenched samples after the last pass as a function of the final rolling temperature. It can be seen that the final forming took place in metastable austenite in Comparative Example 2 (see Fig. 5a) and in Example 3 (see Fig. 5b) , In contrast, this was done in Example 4 (see Fig. 5c) in the (γ-α) -Zweiphasengebiet.

The mechanical properties of steel C rolled products are summarized in Table 3. Table 3: Mechanical properties of steel rolled products C

Closing remarks

The above comparison measurements show that the continuous rolling with targeted setting of the final rolling temperature by Ar ₃ combined with a specific modified cooling process and a deliberately selected holding temperature leads to products whose mechanical properties are comparable to those after a conventional heat treatment. In particular, the products can be processed cold without annealing.

Claims

Patentansorüche

1. A process for the continuous production of wire or bar steel, which comprises the following process steps:

a) in a first process step, a steel with a weight fraction of 0.02 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.5% manganese, up to 0.035% phosphorus, up to 0.004% sulfur, up to 0.5% molybdenum, up to 1.7% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel and up to 0.25% vanadium and other common admixtures heated to 1'050 to 1'200 ⁰ C, thereafter a first Hot working performed by rolling at a temperature above the recrystallization temperature, wherein the Gesamtumformgrad at the first hot working is at least 60%;

b) in a second process step, the rolling stock is cooled below the recrystallization temperature and then a final deformation at a temperature in the region of the non-recrystallized austenite is carried out, wherein the cooling is carried out such that in

Rolling stock is prevented from austenite grain growth before final forming and the total degree of transformation in the final forming is at least 30%; and

c) in a third process step, the rolling stock without re-heating up to a predetermined holding temperature around the Ar ₁

Temperature is cooled and left for a predetermined holding time at said holding temperature.

2. The method according to claim 1, characterized in that the steel has a weight fraction of 0.02 to 0.2% carbon and up to 0.15% silicon, 0.25 to 1.0% manganese, up to 0.025% phosphorus, up to 0.004% sulfur, up to 0.25% Molybdenum, up to 0.1% chromium, up to 0.25% copper, up to 0.008% boron, up to 0.2% nickel and up to 0.1% vanadium, and that the keep-warm temperature is substantially at Ar ₁ ⁶ .

3. The method according to claim 1, characterized in that the steel has a weight fraction of 0.2 to 0.65% carbon and up to 0.15% silicon, 0.25 to 1.0% manganese, up to 0.035% phosphorus, up to 0.004% sulfur,

Has 0.25 to about 12:50% molybdenum, up to 1.7% chromium, up to 0.25% copper, up to 0.2% nickel and up to 0.1% vanadium, and that the holding temperature in the range of Ar _j ^b is ₁ to Ar. ⁶

4. The method according to any one of claims 1 to 3, characterized in that the final deformation is carried out at a temperature of at most AF ₃ + 6O ⁰ C.

5. The method according to any one of claims 1 to 3, characterized in that the final deformation is carried out at a temperature in the range of Aη to Ar ₃ , with the proviso that the Endumformtemperatur

690 ⁰ C does not fall below.